Plasmodesmata in integrated cell signalling: insights from development and environmental signals and stresses
Journal of Experimental Botany
Plasmodesmata in integrated cell signalling: insights from development and environmental signals and stresses
Ross Sager 0
Jung-Youn Lee 0
0 Department of Plant and Soil Sciences, Delaware Biotechnology Institute, University of Delaware , Newark, DE 19711 , USA
To survive as sedentary organisms built of immobile cells, plants require an effective intercellular communication system, both locally between neighbouring cells within each tissue and systemically across distantly located organs. Such a system enables cells to coordinate their intracellular activities and produce concerted responses to internal and external stimuli. Plasmodesmata, membrane-lined intercellular channels, are essential for direct cell-to-cell communication involving exchange of diffusible factors, including signalling and information molecules. Recent advances corroborate that plasmodesmata are not passive but rather highly dynamic channels, in that their density in the cell walls and gating activities are tightly linked to developmental and physiological processes. Moreover, it is becoming clear that specific hormonal signalling pathways play crucial roles in relaying primary cellular signals to plasmodesmata. In this review, we examine a number of studies in which plasmodesmal structure, occurrence, and/or permeability responses are found to be altered upon given cellular or environmental signals, and discuss common themes illustrating how plasmodesmal regulation is integrated into specific cellular signalling pathways.
Plasmodesmata; cell-to-cell communication; development; environmental stresses; biotic and abiotic stress; hormones; cell signalling; callose
Throughout their life cycle, plants process an array of
internal and external signals and adapt to environmental
conditions and challenges. These range from abiotic factors such
as daily and seasonal fluctuations in light intensity/duration
and temperature, and availability of water, carbon, nitrogen,
and minerals, to biotic factors such as microbial infection
and insect herbivory. Plant cells are equipped with both
common and specific types of receptors necessary for perception
of endogenous and environmental signals, as well as other
molecular components for downstream signal transduction
cascades. Thus, appropriate signal processing and metabolic
generation of responses occur primarily at the individual cell
level. Within a single cell, signal transduction is often
initiated when chemical ligands are bound by cognate receptors,
triggering an intracellular signalling cascade that
eventually leads to changes in metabolic and/or nuclear activities.
Although these cellular capacities meet the very basic need
for individual cells to grow and survive, coordination of
cellular responses between neighbours is necessary for the
complex responses that affect the wellbeing and developmental
progression of the plant as a whole.
It is now generally accepted that membrane-lined
intercellular bridges known as plasmodesmata are key parts of the
cellular infrastructure that allows plant cells to communicate
with virtually all of adjoining cells as well as distantly located
cells, forming a symplastic network within the organism
(Lee, 2014; Lucas and Lee, 2004; Roberts and Oparka, 2003)
Plasmodesma-mediated communication allows plant cells to
dispatch mobile forms of intracellular information to
adjacent cells within a tissue or organ. Moreover, using
plasmodesmata in conjunction with a phloem-based transport system,
plant cells can deploy supracellular, long-distance signalling.
In essence, one might argue that plant cells are seldom fully
independent from the physiological and biochemical
influences of their neighbouring cells owing to the presence of
plasmodesmata. However, although this state might be
sufficient for photosynthetic organisms with simple body plans
such as green algae, a monotonous cellular synchrony would
not afford the complex lifestyles and body plans found in
land plants (Lee, 2014). Indeed, plasmodesmata in vascular
plants are dynamic channels, as demonstrated by the range in
sizes and types of their molecular cargoes, and by their ability
to undergo spatiotemporal gating and changes in frequency
(Burch-Smith et al., 2011b; Burch-Smith and Zambryski,
2011; Ehlers and Westerloh, 2013; Heinlein, 2002)
can thus regulate cellular autonomy/non-autonomy by
altering plasmodesmal connectivity, allowing them to orchestrate
changes in growth and development at the organismal level
and produce protective responses to environmental stresses.
Plasmodesmata facilitate the movement of
non-cell-autonomous signalling molecules, including transcription factors
and other regulatory molecules such as long and small RNAs,
and viral components. These topics are reviewed in many
excellent articles and will not be covered in the current review;
interested readers are referred to:
Benitez-Alfonso et al., 2010
Ding, 2009; Lee et al., 2011a; Lee and Cui, 2009;
Niehl and Heinlein, 2011; and
Wu and Gallagher, 2012
. In the
current review, our primary focus will be on a less understood
subject: how cellular signalling pathways are integrated into
the regulation of plasmodesmata. The nature of the signals
and signal transduction pathways that bring about changes in
plasmodesmal permeability and the underlying mechanisms
linking these signals to control of plasmodesmata have begun
to be elucidated only recently. To stimulate new thoughts
and discussion on the control of plasmodesmata-mediated
intercellular communication, this review will highlight what
is known about the intra- and extracellular stimuli, and
challenges that lead to changes in plasmodesmal structure and
connectivity, including cytosolic calcium, light, cold, and
oxidative stresses, hormones, and pathogens. Furthermore,
we will discuss various developmental processes that
accompany changes in plasmodesmal frequency and permeability.
Finally, we will present common and recurring mechanistic
themes underlying the integration of cellular signalling
pathways into the control of plasmodesmata.
Regulation of plasmodesmal connectivity and permeability
Plasmodesmata are dynamic channels
A fundamental difference between animal and plant cells in
terms of the biogenesis of intercellular communication
channels is that the formation of plasmodesmata is coupled with
cytokinesis in plant cells. The plasmodesmata formed during
cell division are termed primary plasmodesmata
. These have a simple morphology, and each
primary plasmodesma is composed of a cytoplasmic sleeve
delimited by the plasma membrane (PM) externally, and an
appressed endoplasmic reticulum (ER) cylinder internally
(Fig. 1A). Formation of the primary plasmodesmata involves
insertion or trapping of ER strands in the newly forming cell
plate. Strands of cortical ER stretched across the
phragmoplast become lined by plasma membrane, which leads to the
formation of a cytoplasmic sleeve between daughter cells
(Hepler, 1982; Hepler and Newcomb, 1967)
determines the frequency (i.e. density) of primary plasmodesmata
at the newly built cell wall interface is not understood, but it
is generally assumed that the trapping of ER strands in the
cell plate occurs fairly randomly, and new division walls all
contain similar numbers of plasmodesmata (Faulkner et al.,
2008). Through this direct coupling between sister cells, all
plant cells newly produced by division embark on their
cellular life in a non-cell autonomous manner. However,
plasmodesmal frequency in vascular plants is not a fixed cellular
parameter. As cells expand and differentiate, so-called
secondary plasmodesmata can arise de novo in post-cytokinetic walls
(Ehlers and Kollmann, 2001)
. Both primary and secondary
plasmodesmata are subject to various structural
modifications as well as disintegration, depending on developmental
or physiological cues (Fig. 1).
Degeneration and de novo biogenesis of plasmodesmata
are frequently associated with developmental progression
or cell-type specification
(reviewed in Burch-Smith et al.,
. For example, as guard cells mature and differentiate,
plasmodesmata connecting them to adjacent pavement cells
are lost (Fig. 1B). Similarly, plasmodesmata connecting
pollen to nutritive tissue degenerate as the pollen cells mature.
Interestingly, between the cells within each nurturing layer in
anther, wide-open PM-lined channels similar to sieve plate
(Esau and Thorsch, 1984)
are formed as the internal
structures of the plasmodesmata are removed
(Mamun et al.,
2005b; Steer, 1977; Willemse and van Went, 1984; Willmer
and Sexton, 1979)
(Fig. 1B, C). In addition,
plasmodesmal frequency in the shoot apex can be transiently
modified during floral development
and new plasmodesmata are formed between non-daughter
cells during the establishment of graft unions or heterocystic
(Kollmann and Glockmann, 1991; Vaughn, 2003)
Furthermore, the morphology of both primary and
secondary plasmodesmata is subject to significant modifications. As
cells mature, plasmodesmata often undergo transformation
from simple pores to complex multi-channel connections
(Ehlers and Kollmann, 2001; Ehlers and Westerloh, 2013;
Oparka et al., 1999)
(Fig. 1E). Branched, complex
plasmodesmata often have a reduced size exclusion limit (SEL)
compared with simple plasmodesmata, restricting the
diffusion of larger molecules
(Oparka et al., 1999)
The genetic components involved in degeneration,
secondary biogenesis, and ultrastructural modification of
plasmodesmata remain to be discovered. However, cell
wallmodifying enzymes and their regulators probably play crucial
roles in these processes
(Burch-Smith et al., 2011b; Kollmann
and Glockmann, 1991)
. Whereas these types of
plasmodesmal modifications reflect long-term or permanent cellular
decisions, plant cells can also control plasmodesmal
connectivity rapidly and transiently
(Holdaway-Clarke et al., 2000;
Tucker and Boss, 1996)
. This level of control allows cells to
respond to spatiotemporal changes in physiological and/or
environmental conditions. Most well-understood reversible
plasmodesmal regulation involves enzymatic biosynthesis
and degradation of a polysaccharide called β-1,3-glucan
(callose) at the plasmodesmata (Fig. 1F), which is described in
the following section.
Plasmodesmal gating is modulated by callose levels
Plasmodesmata-mediated molecular flux between cells
seems to be modulated through reversible deposition of
(Zavaliev et al., 2011)
(Fig. 1A and F). Callose deposits
appear at the primary plasmodesmata in newly formed cell
walls, and could be either a remnant of cell plate callose or
resynthesized upon completion of plasmodesmal formation
(Northcote et al., 1989)
. In post-cytokinetic walls, callose is
often found at the neck regions of plasmodesmata,
reflecting a wound response associated with sample preparation
for electron microscopy
(Radford et al., 1998)
. It is currently
unclear whether callose deposited around the neck region
strictly correlates with plasmodesmal closure as callose
accumulating along the length of plasmodesmal channels is not
(Vaten et al., 2011)
. However, it is well established
that callose levels at plasmodesmata can fluctuate, and the
extent of callose accumulation surrounding the channels
negatively correlates with plasmodesmal conductivity
(Lee et al.,
2011b; Rinne et al., 2001)
. The alterations in plasmodesma
callose levels involves two counteracting enzymatic activities,
i.e. β-1,3-glucan synthases and hydrolases. In fact, a recurring
theme in plasmodesmal regulation is that various endogenous
and extracellular signals bring about changes in
transcriptional or enzymatic activities of β-1,3-glucan synthases or
(Dong et al., 2008; Rinne et al., 2011)
In Arabidopsis thaliana, twelve genes are predicted to
encode the catalytic subunit of β-1,3-glucan synthase and are
referred to as CALLOSE SYNTHASE (CALS) or GLUCAN
SYNTHASE-LIKE (GSL) genes
(Saxena and Brown, 2000;
Verma and Hong, 2001)
. Functional studies using genetic
mutants have shown that various CALS family members
are critical for different developmental and
(Barratt et al., 2011; Chen et al., 2009; Dong
et al., 2005; Guseman et al., 2010; Hong et al., 2001; Jacobs
et al., 2003; Vaten et al., 2011; Xie et al., 2011)
CALS3, and CALS7 affect molecular transport across
plasmodesmata or sieve elements. CALS10 (also called GSL8) is
required for proper cell plate establishment during
cytokinesis as well as guard cell and root tissue patterning (Guseman
et al., 2010). Gain-of-function mutations in CALS3 have
been tied to increased deposition of callose at
plasmodesmata and decreased macromolecular trafficking between root
cells, in addition to developmental defects in roots
et al., 2011)
. CALS7 encodes a phloem-specific isoform that
is required for normal deposition of callose in developing
sieve elements and for phloem transport
(Barratt et al., 2011;
Xie et al., 2011)
. Highlighting the critical role of CALS in
restricting plasmodesmal permeability, two novel CALS
family members control basal and induced plasmodesmal closure
(J.-Y. Lee, unpublished data).
With regard to callose degradation, the Arabidopsis
genome encodes approximately fifty β-1,3-GLUCANASE
(BGL) genes, each containing a glycosyl hydrolase family 17
(Doxey et al., 2007; Zavaliev et al., 2011)
. A few
characterized BGL genes affect plasmodesmal callose levels and
are involved in a range of developmental processes
including cotton (Gossypium hirsutum) fibre elongation
et al., 2004)
, lateral root emergence
(Benitez-Alfonso et al.,
, and dormancy breakage
(Rinne et al., 2011)
in transcript levels of specific groups of tobacco (Nicotiana
tabacum) BGLs (BGLs expressed in tobacco are grouped into
five classes according to amino acid sequence identity of the
mature proteins) positively correlate with viral spread both
locally and systemically. For example, the silencing of genes
for class I BGLs in tobacco leaves, which led to increased
accumulation of callose at plasmodesmata, was enough to
significantly delay the systemic movement of several viruses
(Beffa et al., 1996; Iglesias and Meins, 2000)
this effect, overexpressing genes encoding class III or
class I BGLs in transgenic potato (Solanum tuberosum) and
tobacco, respectively, stimulated the spread of potato virus
YNTN (Dobnik et al., 2013) and Tobacco mosaic virus (TMV)
(Bucher et al., 2001). Given the large number of genes in this
family, it would not be surprising to find more members that
translate specific developmental and environmental cues into
increased cell-to-cell permeability through plasmodesmal
callose degradation. Experimental details described in the
sections Forming lateral roots, Elongating cotton fibres, and
Gibberellic acid plays a role in reestablishment of
plasmodesmal connectivity during chilling-induced breakage of dormancy,
below, highlight the theme that specific signalling pathways
function in spatiotemporal induction of particular BGLs,
which in turn reverse restrictions in plasmodesmal flux.
Cytoskeletal components affect plasmodesmal targeting and flux: A callose-independent mechanism for plasmodesmal gating
A number of cytoskeletal elements have been found to
localize to or near plasmodesmata or to have a significant
function in cell-to-cell transport. Actin microfilaments have
been implicated in the transport of viral movement proteins
(MPs) to plasmodesmata, although disruption of actin can
have differing effects on plasmodesmata and viral MP, often
dependent on the cell type. Protein dual-labelling by
fluorescent tagging for co-expression in tobacco leaves showed that
a 126-kDa component of the TMV viral replication
complex colocalized with, and moved along, actin filaments
et al., 2005)
. Similarly, GFP-tagged cauliflower mosaic virus
(CaMV) inclusion body protein P6 trafficked intracellularly
(Harries et al., 2009a)
blocking actin polymerization allowed 10-kDa dextrans to move
between tobacco mesophyll cells (Ding et al., 1996), whereas
a similar treatment had no influence on basal trichome
plasmodesmal function (Christensen et al., 2009). By contrast,
such actin disruption severely reduced plasmodesmal
targeting of the TMV MP in tobacco epidermal cells
et al., 2004; Wright et al., 2007)
, and inhibited CaMV
(Harries et al., 2009a)
Intriguingly, certain viruses may use targeted
destabilization of actin as a means to facilitate their intercellular
movement. For example, treatment of tobacco leaves with the
drug phalloidin, which stabilizes actin filaments, prevented
the cucumber mosaic virus (CMV) MP from increasing the
(Su et al., 2010)
. Further analysis
demonstrated that CMV MP binds to actin filaments in vitro and
can in fact sever them
(Su et al., 2010)
. This led the authors to
speculate that plasmodesmata-targeted CMV MP increases
the SEL by breaking down actin cytoskeletal scaffolding in
the plasmodesmal channel. Endogenous factors using such a
mechanism to manipulate the plasmodesmal SEL are not yet
known. Other viral proteins seem to localize to F-actin and
use it as a scaffold for viral complexes. In infected cells, the
TGB1 protein from Potato virus X (PVX) forms a structure
called the X-body that collects F-actin in a mesh around its
(Tilsner et al., 2012)
. Though the function of the
actin scaffolding is not yet known, PVX requires TGB1 and
intact actin microfilaments to move cell-to-cell
(Harries et al.,
Immunogold labelling has revealed filamentous actin at the
plasmodesmal neck region
(Baluska et al., 2004; White et al.,
, and myosin proteins, such as the unconventional plant
myosin VIII, at the plasma membrane and in clusters near
(Baluska et al., 2004; Radford and White,
1998; Reichelt et al., 1999)
. Myosin VIII seems to be involved
in targeting viral cargoes to plasmodesmata. Overexpression
of a truncated, tail domain-only form of myosin VIII acts
as a dominant-negative mutant, blocking cargo recognition
by normal myosin VIII; overexpression of this domain in
tobacco impairs targeting of the viral protein Hsp70 to
(Avisar et al., 2008)
. Recently, a study showed
that cell-to-cell movement of 1- to 3-kDa dextrans was
increased in staminal hairs of Tradescantia virginiana upon
inhibition of myosin VIII function by treatment with
antimyosin antibodies or the drug 2,3-butanedione monoxime,
which binds myosin and slows its ATPase activity. On the
contrary, permanent binding of myosin to actin induced by
the drug N-ethylmalemide decreased cell-to-cell movement
(Radford and White, 2011)
. As the authors note, the
application of these drugs may cause side-effects in plant cells that
could alter intercellular transport, so further research into
their effects on plasmodesmata is warranted. Furthermore,
in this particular study, inhibition of actin polymerization
with latrunculin B did not disrupt intercellular trafficking, in
contrast to previous reports
(Ding et al., 1996; Wright et al.,
, indicating there may be tissue- or species-specific
differences in how actin affects plasmodesmal function.
Other results support a possible role for myosins in
plasmodesmal transport. In one study using grapevine fanleaf
virus, which forms MP-derived tubules that stretch through
plasmodesmata to allow the virus to move
(Amari et al.,
, blocking the function of myosin XI by
overexpressing its tail domain alone suppressed the formation of viral
tubules, and thereby also inhibited the cell-to-cell movement
of the virus. It has also been shown that certain viruses can
selectively use particular myosin proteins; silencing myosin
XI-2 in tobacco repressed the intercellular spread of turnip
mosaic virus by a factor of ten, whereas silencing myosin
VIII-1, VIII-2, or XI-F did not affect its infection
et al., 2013)
. Similar results were obtained for TMV
et al., 2009b)
. Finally, certain treatments, like calcium, may
result in modulation of plasmodesmal conductivity by
affecting the function of myosin proteins (see Calcium fluxes
rapidly close plasmodesmata, below).
The role of the microtubule-based cytoskeleton in
plasmodesmal targeting remains more contentious
(Niehl et al.,
. Certain viruses seem to use microtubules for
targeting to plasmodesmata, whereas others do not. For instance,
TGB1 of potato mop top virus requires association with
microtubules with its N-terminal domain to localize to
(Shemyakina et al., 2011; Wright et al., 2010)
Treatment of tobacco with the microtubule-disassembling
drug colchicine inhibited targeting of TGB1–GFP to
plasmodesmata, whereas blocking microtubule polymerization
with colchicine or oryzalin had no effect on the targeting
of TMV MP to plasmodesmata in tobacco epidermal cells
(Wright et al., 2007). However, other studies have suggested
that whereas plasmodesmal targeting of TMV MP–GFP
is unaffected, interaction of the MP with microtubules is
required for effective cell-to-cell spread of viral RNA
et al., 2000; Boyko et al., 2007)
. When tobacco plants infected
with TMV were shifted to a high temperature, a stronger
association of TMV MP–GFP with microtubules correlated
highly with greater spread of the viral RNA. Intact
microtubules may also be required for proper movement of
endogenous non-cell autonomous proteins, as shown by trafficking
of GFP-tagged SHORTROOT along microtubules into
endodermal cells of Arabidopsis roots (Wu and Gallagher, 2013).
Plasmodesmata undergo degeneration and structural remodelling during organogenesis, cell growth, and development
Isolating mature guard cells from their neighbouring epidermal cells
Guard cells require cellular autonomy to regulate stomatal
aperture by modulation of their turgor pressure. Several lines
of evidence indicate that this autonomy is achieved by
severing their plasmodesmal connections to surrounding
epidermal cells. Ultrastructural studies using transmission electron
microscopy on Vicia faba, Allium cepa, Beta vulgaris, and
tobacco guard cells revealed that the intact plasmodesmata
between developing guard cells and their neighbouring cells
disintegrate by the time of guard cell maturity
Lucas, 1984; Willmer and Sexton, 1979)
. The deposition of
a thick layer of cell wall material on the guard cell side of
the shared wall results in severed (or halved) plasmodesmata
in the adjacent epidermal cell wall
(Wille and Lucas, 1984;
Willmer and Sexton, 1979)
Consistent with these observed ultrastructural changes to
guard cell plasmodesmata, numerous studies utilizing
microinjection have shown that mature guard cells are
symplasmically isolated from surrounding epidermal cells. Before
maturation, fluorescent dyes can diffuse freely between
immature guard cell pairs and subsidiary cells. For instance, when
the membrane-impermeable fluorescent dye lucifer yellow
carbohydrazide (LYCH) was injected into immature guard
cells and subsidiary cells of A. cepa and Commelina
communis, the probe was mobile between cells
(Palevitz and Hepler,
. However, when the dye was injected into mature
subsidiary cells, the fluorescent signal spread into neighbouring
epidermal cells but was excluded from mature guard cells.
Furthermore, when one of a mature guard cell pair of A. cepa
and Commelina communis was injected, the fluorescent signal
was retained within that single cell, indicating that in these
species each mature guard cell is symplasmically isolated not
only from surrounding epidermal cells but also from its sister
(Palevitz and Hepler, 1985)
. By contrast, injecting one of a
guard cell pair in maize (Zea mays) with the dye
2’,7’-bis-(2carboxyethyl)-5(and 6)carboxyflurescein revealed
cytoplasmic sharing only between mature guard cells
(Mumm et al.,
. This indicates that plasmodesmal connectivity within
the stomatal complex itself may differ somewhat between
Further supporting the idea that immature guard cell pairs
remain symplasmically connected with surrounding
epidermal cells, gene silencing signals penetrate into guard cells
before their maturation
(Voinnet et al., 1998)
. Likewise, free
GFP and fluorescently tagged viral MP were confined when
transiently expressed in the mature guard cells of various
dicots (tobacco, potato, Arabidopsis, and V. faba) through
(Hofius et al., 2004; Isono et al., 1997;
Lee et al., 2005; Myouga et al., 2008)
, which indicates that
there is neither diffusion nor active transport out of mature
Generating pollen, megagametophytes, and zygotes
A developing anther contains multiple cell layers,
including the tapetum which surrounds sporogenous tissue and
provides nutrients to the developing pollen mother cells
(Fig. 2A). Electron microscopy together with
microinjection experiments show that the development of
reproductive tissues accompanies major alterations in plasmodesmal
connectivity. In early stages of pollen development,
plasmodesmal connections exist at the cellular junctions between
tapetum–tapetum and tapetum–pollen mother cell layers
(Radice et al., 2008; Steer, 1977)
. The presence of
plasmodesmata in this tissue is thought to be crucial for the feeding
of the developing pollen mother cells with nutrients and
metabolites by the tapetal cells. Plasmodesmata were also
observed between pre-meiotic pollen mother cells; however,
as pollen mother cells progress into meiotic cell division,
tapetal cells secrete callose that accumulates within the newly
forming cell walls of pollen tetrads
(Mamun et al., 2005a)
The thickening of callose walls coincides with the
disappearance of plasmodesmata at the end of this stage, which leads
to symplasmic isolation of pollen grains
(Mamun et al.,
2005b; Steer, 1977)
(Fig. 2A). During meiosis, the tapetum
layer itself also becomes symplasmically isolated from the
surrounding tissue. This means that soluble nutrients
produced in the inner middle layers must be transported
apoplastically when they enter into and exit from the tapetal
layers before reaching the developing pollens
Audran, 1995; Roschzttardtz et al., 2011)
. It is speculated
that the symplasmic isolation of the tapetum during pollen
development is necessary to protect neighbouring cell
layers from potentially detrimental chemicals produced during
programmed cell death of the tapetum. The cell death of the
tapetum is thought to provide a mechanism to salvage the
energy that should go into supporting the developing pollen
cells (Wu and Cheun, 2000). Consistent with this notion, it
was recently shown that plants contain underdeveloped or
aborted pollens when tapetal cells are prevented from
undergoing programmed cell death in rice
(Niu et al., 2013)
Notably, the plasmodesmal connections remaining among
tapetum cells seem to go through structural modification
during pollen development. Sometimes much larger
connections called cytomictic channels form between tapetum cells
following symplasmic disconnection from the middle layer
(Clement and Audran, 1995; Mursalimov et al., 2010; Perdue
et al., 1992; Vinckier and Smets, 2005)
(see Fig. 1C & 2A).
These large channels (or holes) are thought to facilitate the
movement of organelles and chromatin, producing
syncytia. Furthermore, whereas fully developed pollen grains are
enveloped completely in callose, and thus lack plasmodesmal
connections with each other
(Mamun et al., 2005b; Steer,
, cytomictic channels can form between pollen mother
(Mamun et al., 2005a; Wang et al., 2004)
function of these channels is unknown, but it is conceivable that
they facilitate exchange of essential nutrients or help
coordinate the timing of meiotic division (Wang et al., 2004). It
remains to be determined whether cytomictic channels form
de novo, or through the modification/fusion of existing
plasmodesmata through the help of pit field-localized cellulase
(Henslop-Harrison, 1966; Wang et al., 1998)
Megagametophytes, the female reproductive cells, also
seem to require a symplastic system to fine-tune the timing
of nutrient distribution during development
van Went, 1984)
. Ultrastructural analysis of the chalaza, the
tissue connecting the ovule to the placenta, has shown that
plasmodesmal frequency is higher in the chalazal cells than in
neighbouring cell types
(Russell, 1979; Thijssen, 2003)
feature may have to do with a potential role of the
chalazal tissue in regulating the symplastic flow of nutrients from
the phloem to the egg sacs. In fact, during the tetrad stage
of megagametophyte development, the ovule primordium
excluding the tetrad is symplastically connected to the
(Werner et al., 2011)
(Fig. 2B.1). However, by stage 12
of anthesis during ovule maturation, the plasmodesmata in
chalazal cells apparently become occluded
coinciding with a lack of symplastic dye movement from the
(Werner et al., 2011)
(Fig. 2B.2). Furthermore,
no plasmodesmal connections between the cells of the
nucellus (egg sac tissue) and integuments (outer protective tissue)
are observed, other than within the chalazal zone, in late
stages of ovule development
symplastic dye movement into ovules from the
vasculature resumes by late stage 13, but the tracer can move only
into the outer integument
(Werner et al., 2011)
Studies utilizing electron microscopy, symplastic dye
tracers, and outer/inner integument-specific expression of GFP
have all revealed that plasmodesmal connections are retained
within the nucellus and integument tissues themselves
et al., 2000; Thijssen, 2003; Wang and Fisher, 1994; Werner
et al., 2011)
. For example, examination of symplasmic
domains within late-stage ovules using transcriptional GFP
reporters driven by the promoters of AtGL2 and AtTT12
showed that the outer and inner integuments are isolated
from each other (Fig. 2B.4). However, although AtGL2:GFP
drives expression only within the outermost cell layer of the
outer integument, GFP could move into the inner cell layer
of the outer integument. Similarly, AtTT12:GFP is active
only within the innermost layer of the inner integument,
and GFP could diffuse outward through all inner
integument layers. Collectively, these data suggest that ovules
develop tissue-specific symplasmic and symplastic barriers,
and isolate megagametophytes following a period of initial
development during which photosynthates are directed
symplastically through the chalaza to the growing
megagametophytes. Similar to the pollen development, the symplasmic
isolation of megagametophytes may be required to prevent
cytoplasmic leakage of signalling molecules produced by
neighbouring cells that could interfere with the gameto- promoter, which is active in the hypocotyl epidermis, can
phytic differentiation. diffuse throughout the whole embryo (Stadler et al., 2005)
A similar increase in symplasmic isolation also occurs dur- (Fig. 2C.2). Furthermore, the small transcription factor
ing embryo development. Before fertilization, the egg cell TARGET OF MONOPTEROS 7 (TMO7) is trafficked
interof Torenia fournieri is cytoplasmically connected to the sur- cellularly from embryonic cells to an adjacent suspensor cell
rounding tissue in the embryo sac, as evidenced by movement called the hypophysis, or primary root meristem founder cell
of a fluorescent reporter conjugated to a 10-kDa dextran of the developing seedling
(Schlereth et al., 2010)
from microinjected central cells into the egg cell and syner- of TMO7–GFP was observed in globular and heart stage
(Han et al., 2000)
(see Fig. 2B.2, inset). Starting around embryos in both the hypophysis and adjacent embryonic cells
24 hours post-fertilization, however, movement of even free (Fig. 2C.2), whereas a much larger form of TMO7 fused to
dye reporter into the newly formed zygote is prevented. Early triple GFP could not traffic from the embryo into the
hypoultrastructural imaging of soybean and Arabidopsis showed physis. Another study showed that early in embryogenesis
that embryonic plasmodesmata are disconnected from the and up to the heart stage, fluorescent tracers, e.g. 0.5-kDa
surrounding endosperm except within the suspensor, a spe- 8-hydroxypyrene-1,3,6-trisulfonic acid (HPTS) and 10-kDa
cialized tissue that draws nutrients from maternal tissues into F-dextrans, spread cell-to-cell throughout the whole embryo
(Dute et al., 1989; Mansfield and Briarty, 1991)
. (Kim et al., 2002). By the torpedo stage, however, movement
In Arabidopsis, suspensor-specific expression of diffus- of 10-kDa dextrans was restricted to only a few cells. Use of
ible GFP under the SUC3 promoter revealed movement of multimeric GFPs as fluorescent reporters expressed from the
the protein into the globular stage embryo (Fig. 2C.1), yet embryonic SAM produced similar results
(Kim et al., 2005a;
such cell-to-cell movement was prevented during the heart Kim et al., 2005b)
. In early heart stage embryos, both the
(Stadler et al., 2005)
(Fig. 2C.2). This result is consist- gle and a tandem double GFP could move throughout the
ent with ultrastructural study of Crassulaceae ovules at dif- hypocotyl into embryonic cotyledons and root. However, by
ferent embryonic stages, which revealed that the complexity the late heart stage, double GFP was blocked from moving
of suspensor cell plasmodesmata increases as the embryo into embryonic cotyledons, but not root. A triple GFP was
matures; transitioning from simple, unblocked plasmodes- always restricted to the hypocotyl. GFP driven by the SUC3
mata to complex, branched plasmodesmata that are plugged promoter, which is active in the embryonic root during the
with an electron-dense substance (Kozieradzka-Kiszkurno torpedo stage, spread into a wider symplasmic zone of the
and Plachno, 2012). The authors noted the similarity of embryo through the mid-torpedo stage
(Stadler et al., 2005)
this material to the callose that was accumulated in the plas- (Fig. 2C.3). However, in late torpedo stage, GFP signal was
modesmata of birch (Betula pubescens) shoot apical meris- confined to where the promoter was active, demonstrating
tem (SAM) during dormancy
(Rinne and van der Schoot, the overall decrease in intercellular permeability as embryos
. However, intriguingly, shadows within the plugs have mature (Fig. 2C.4). These studies underscore that
spatiotemcontinuity with the ER, suggesting that the appressed ER of poral regulation of plasmodesmata is probably critical for
suspensor plasmodesmata may have been structurally modi- coordinating the development of distinct tissues and organs
fied to increase the filtration of nutrients or disable transport from the very early stage of embryo formation.
altogether (Kozieradzka-Kiszkurno and Bohdanowicz, 2010;
Kozieradzka-Kiszkurno and Plachno, 2012). Preparing organs for abscission
As zygotes develop, internal signalling factors need to
be contained and directed to the developing areas, which Organ separation during abscission results from dissolution
is apparently aided by changes in plasmodesmal connec- of the cell walls between the organ and the main plant body
tivity and symplasmic zoning. During the heart stage of at specialized areas called abscission zones
(Estornell et al.,
embryo development, GFP expressed under the AtGL2 2013; Sexton and Roberts, 1982)
. Within abscission zones,
layers, though still blocked from the outer integument. (C) Changes in plasmodesmal connectivity during embryo development. 1. Globular stage:
The suspensor (Sus) and embryo (Em) are cytoplasmically connected, as indicated by the diffusion of GFP from the suspensor into the embryo (green
gradient) reported by AtSUC3:GFP. Dashed bracket, expression domain of AtSUC3:GFP; broken brown lines, open plasmodesmata within suspensor.
2. Heart stage: The suspensor becomes cytoplasmically isolated as its plasmodesmata increase in structural complexity and are occluded by a
calloselike substance (filled red boxes), preventing GFP diffusion from suspensor to embryo (green). However, the heart stage embryo itself remains a symplastic
domain, as indicated by the diffusion of GFP throughout the embryo from the hypocotyl-located expression of AtGL2:GFP (yellow). The plasmodesmata
between the hypophysis and the embryo also remain open, as indicated by the movement of GFP signal in AtTMO7:TMO7–GFP from the embryo into
the hypophysis (orange). Dashed bracket, expression domain of AtSUC3:GFP. 3. Mid-Torpedo stage: Plasmodesmata remain open in this embryo stage,
as indicated by the diffusion of GFP from the root tip (dark green) throughout the embryo (green), driven by the AtSUC3 promoter. 4. Late Torpedo stage:
The symplastic connectivity of the torpedo stage embryo becomes restricted as the embryo matures, and GFP does not diffuse out of its expression
domain in the root tip (dark green). At this stage, AtSUC3 is active in other tissues of the embryo, but GFP remains restricted to these cells as well
(green line and dashes). (D) Plasmodesmata undergo structural remodelling and form highly complex channels during abscission. In the abscission zone,
multivesicular (MVB) and paramural bodies (PB) are frequently found nearby or associated with complex plasmodesmata. This occurs almost exclusively
in the proximal tissue of the zone. It is hypothesized that these bodies contain cell wall (CW)-remodelling enzymes, such as cell wall hydrolases (CWH)
and peroxidases (Per), to loosen and degrade the middle lamella (ML) between the main plant body and the abscised organ, allowing it to separate.
Post-separation, these vesicles may also play a role in bringing cell wall material to repair the outer layer and prevent cytoplasmic leakage through
plasmodesmata. PM, plasma membrane; AER, appressed endoplasmic reticulum.
plasmodesmata undergo extensive structural remodelling,
with a very high occurrence of branched plasmodesmata
(Bar-Dror et al., 2011; Jensen and Valdovinos, 1967; Osborne
and Sargent, 1976; Webster, 1973)
. Complex plasmodesmata,
large cavities, and membrane vesicularization are observed
within the middle lamella region of the abscission zone
(BarDror et al., 2011; Osborne and Sargent, 1976)
, indicating that
cell wall separation may preferentially initiate there (Fig. 2D).
Diaminobenzidine staining also revealed high levels of
hydrogen peroxide, which is known to weaken cell wall connections,
within plasmodesmata, cell walls, and Golgi vesicles in the
abscission zones of tobacco flower pedicels
Furthermore, paramural bodies—membrane vesicles that
form between the cell wall and PM—accumulate at complex
plasmodesmata during abscission, with the majority
forming in the proximal side of the zone
(Bar-Dror et al., 2011)
(Fig. 2D). These vesicular bodies have been hypothesized to
contain cell wall-modifying enzymes, such as hydrolases and
peroxidases, brought there to enhance the breakdown of the
cell wall between the strands of complex plasmodesmata,
enabling separation of the distal tissue
(Osborne and Sargent,
. Alternatively, they may contain cell wall material,
deposited to plug the plasmodesmata of the proximal tissue
once the distal tissue separates
(An et al., 2006)
It is noteworthy that, in addition to complex branching of
plasmodesmata, sieve plate pores in the vasculature of
P. vulgaris have significantly reduced levels of callose plugs during
(Scott et al., 1967)
. This parallel modification in
sieve plate pores may facilitate nutrient mobilization from the
distal to the proximal tissue prior to abscission of the former.
All the abscission studies discussed above involved
ethylene treatment. Considering this experimental condition,
it is tempting to speculate that there might be a direct link
between the changes in plasmodesmal structure and the
ethylene signalling pathways. This possibility may be tested by
assessing the effect of ethylene treatment on plasmodesmal
permeability as has been shown for the salicylic acid-induced
(Wang et al., 2013)
Producing larger pores in sieve tubes for phloem mass flow
The formation of sieve plate pores is a fascinating example of
a developmental process in which plasmodesmata at specific
cellular junctions are structurally modified to provide the cells
with enhanced intercellular transport capacity (see Fig. 1C).
Sieve elements are elongated, specialized conducting cells of
the phloem that are alive at maturity. Conjoined end to end
via the sieve plate, these cells form functional sieve tubes, and
mass flow of phloem contents occurs through enlarged pores
within the sieve plate
(Knoblauch and van Bel, 1998; van Bel
et al., 2002)
. During sieve element maturation, the sieve pores
form through major structural modifications of the primary
plasmodesmata that once connected daughter sieve elements.
Initially, hyperaccumulation of callose creates a thick cell
wall layer surrounding sieve plate plasmodesmata. This is
followed by disappearance of the appressed ER and degradation
of the callose, eventually leaving much wider pores that are
lined only with PM
(Esau and Thorsch, 1984)
. The functional
importance of the transient callose accumulation in sieve
pore formation was recently supported by mutant analysis of
the phloem-specific callose synthase Arabidopsis CALS7
et al., 2011)
. Loss-of-function mutation in CALS7 resulted
in poorly developed sieve plates that had greatly decreased
callose accumulation and fewer sieve pores with narrower
openings. Moreover, using 14CO2 labelling, the rate of phloem
transport of photosynthate from source to sink tissues was
found to be decreased in cals7
(Barratt et al., 2011)
Shoot apical meristems undergoing developmental phase change
Symplasmically regulated cellular zoning of SAMs ensures
that signals involved in organ differentiation remain in the
intended groups of cells. In the vegetative SAM of birch,
iontophoretic microinjection of LYCH dye into a single cell
of the central zone tunica led to spread of the dye
throughout only central zone tunica cells (Fig. 3A.2). Similar results
were obtained when a tunica cell within the peripheral zone
(Rinne and van der Schoot, 1998)
SAM/phloem interface is also symplastically regulated
et al., 1999)
. When HPTS dye was loaded into cut petioles of
Arabidopsis leaves, cell-to-cell movement through the
vasculature into younger SAM tissue was restricted until the plants
were about 4 weeks old (Fig. 3A.1). At this point, the dye
diffused into the peripheral zone and tunica of the central zone
except for a region of cells called the inner central zone, which
may be the organization centre
(Wang and Fiers, 2010)
, a site
of high levels of signalling and growth regulation (Fig. 3A.2).
The control of cell-to-cell connectivity is especially
dynamic in the SAM, as seen during developmental
processes like floral transition (Fig. 3A) and dormancy cycling
(Fig. 3B, detailed in Gibberellic acid plays a role in
reestablishment of plasmodesmal connectivity during chilling-induced
breakage of dormancy below). SAMs undergo several massive
changes during reproductive transition, with not only gene
expression profiles, but also zoning patterns being altered
(Chandler, 2012). Arabidopsis requires 1–4 consecutive long
days to commit to flowering, during which time HPTS dye
could unload into the SAM from the vasculature. However,
from the time immediately before floral commitment, no dye
moved into the SAM
(Gisel et al., 1999; Gisel et al., 2002)
(Fig. 3A.3). During and immediately after the floral
transition, there was a period of symplastic isolation, but dye
movement in the SAM eventually returned to normal (Gisel
et al., 1999) (Fig. 3A.4). Surprisingly, however, the time it
took Arabidopsis plants to recover symplastic connectivity
between the SAM and stem vasculature depended upon their
initial growth conditions. When grown in long days from the
start, dye could move from leaves to the SAM almost
immediately after floral transition. By contrast, when plants were
initially grown in short days, this symplastic recovery took
(Gisel et al., 2002)
. This prompts the intriguing
question of how symplastic transport can be altered by initial
photoperiod and light levels. As described below, the addition
of secondary plasmodesmata to cell-cell junctions could be
one aspect of this phenomenon.
The Sinapis alba SAM central zone changes shape and size
upon long day-induced floral transition. By the second long day,
the central zone symplasmic field became more circular, and the
area of dye spread more than tripled, though the sizes of SAM
cells remained unchanged
(Ormenese et al., 2002)
These changes in zoning pattern may be caused by alterations to
plasmodesmal frequency (see Fig. 1D), modulating the
connectivity between cell layers in the SAM (Fig. 3A.3, inset).
A single long-day treatment sufficient to induce floral transition in
Sinapis also induced transient changes in plasmodesmal
formation in the SAM
(Ormenese et al., 2000)
. Serial thin sections of
each Sinapis meristematic layer revealed that starting about 28
hours after floral induction, the plasmodesmal frequency had
increased in all meristematic layers compared with uninduced
plants, peaking at about 4-fold higher. However, at 48 hours
post-long day induction, the frequencies of plasmodesmata in
all SAM layers had returned to normal. The plasmodesmata in
all observed Sinapis SAM tissues were almost always simple,
and their ultrastructure unmodified, in both control and long
(Ormenese et al., 2000)
Consistent with these observations, electron microscopic
analyses of Rudbeckia and Perilla SAMs that were induced
to the reproductive phase by changes in photoperiod also
revealed that plasmodesmal frequency between SAM cells
in all layers increased significantly at first
Intriguingly however, after this initial increase, only the cells
of the SAM central zone retained their higher
plasmodesmal frequency, whereas the number of plasmodesmata in the
medullar zone dropped to below that of non-induced
control plants. In Iris inflorescence meristem, the L2 layer has
the highest plasmodesmal frequencies in comparison to the
L1 and corpus layers (Bergmans et al., 1997). However,
during the development of the inflorescence meristem into a
floral meristem plasmodesmal densities of the L2 cells are
significantly reduced in all interfaces, whereas the number of
plasmodesmata between clonal cells within the L1 and those
within the corpus remain essentially the same. As the L2 layer
is positioned between the inner corpus and the outer L1 in
the SAM, Bergmans et al. (1997) posit that the L2 layer may
play a central role in dictating the symplasmic integration and
thus the developmental fate of meristem in Iris.
Collectively, the studies described above suggest that
transient changes in plasmodesmal frequency or patterning in the
shoot meristem probably reflect dynamic intercellular
signalling, necessary for determining the shift in the developmental
phase and to specify the zones of floral organ differentiation.
Upon floral induction, symplastic transport from the
vasculature is blocked, and plasmodesmal frequencies within the
SAM initially increase, possibly to prevent movement of
unwanted signals from the vasculature into the SAM, while
allowing necessary floral differentiation signals to move from
one SAM zone into another. However, plasmodesmal
frequencies in these tissues eventually either return to
pre-transition levels or decrease permanently, underscoring that cellular
autonomy and non-autonomy are highly plastic parameters
which well reflect developmental status of the plant.
Forming lateral roots
Lateral root primordia are initiated in the xylem pole
pericycle, growing via a series of controlled cell divisions from
a few cells into the new lateral organ, which pushes through
outer cell layers of the primary root
(Malamy and Benfey,
. During development, the lateral root becomes
symplastically isolated from the primary root phloem and,
phloem-loaded symplastic tracer cannot move into the newly
developing lateral root primordium (Fig. 4A). However, this
symplastic isolation is temporary, lasting only until the new
phloem elements within growing lateral root have
differentiated, at which point phloem unloading resumes
et al., 1995)
(Fig. 4A). Transient callose deposition at
plasmodesmata within the lateral root primordium plays a
critical role in the formation and maintenance of this symplastic
(Benitez-Alfonso et al., 2013)
(Fig. 4A). Using GFP
as a reporter, Benitez-Alfonso et al. (2013) showed that cells
of lateral root primordia are symplastically connected to
the pericycle at initial stages of development, but gradually
become isolated. As detected by immunofluorescence, the
timing of symplastic discontinuity correlates with an increase
in callose deposition not only within lateral root primordia,
but also in the underlying vasculature and overlying
endodermal cells. Using a combination of transcriptome data mining
with localization studies, the authors also identified
plasmodesmal-localized β-1,3-glucanase 1 (PdBG1) as an
important player in this process. A lack of PdBG1, and its closely
related isoform PdBG2, resulted in higher callose
accumulation in growing lateral root primordia, reduced GFP
unloading from the phloem, and increased lateral root initiation and
Elongating cotton fibres
Cotton fibres, unique trichomes formed on the surface of
seeds, are single cells that elongate from microns to centimetres
in a matter of weeks
(Lee et al., 2007; Schubert et al., 1973)
Transient inhibition of plasmodesmal connectivity during
cotton fibre elongation correlates with the maintenance of
the high turgor pressure necessary for driving the elongation
(Ruan et al., 2001)
. During the elongation period of
10–16 days after anthesis, movement of fluorescent reporter
from the phloem into the fibre cells was blocked (Fig. 4B).
Ultrastructural imaging revealed that the plasmodesmata
connecting the fibre to the seed coat gradually change from
simple to complex as elongation takes place. Moreover, using
immunogold labelling, callose accumulation in plasmodesmata
was shown to transiently increase at the fibre base during the
elongation phase (Fig. 4B). The reversal in callose
accumulation was correlated with the induction of fibre-specific
β-1,3glucanase named GhGlu1 and plasmodesmal re-opening
et al., 2004)
(Fig. 4B), similar to the role of PdBG1 during
lateral root initiation
(Benitez-Alfonso et al., 2013)
in the cotton mutant line fls, which shows a severe short-fibre
phenotype, fluorescent symplastic dyes consistently moved
into the fibres throughout fibre elongation, concomitant with
an inability to close plasmodesmata during any stage of
(Ruan et al., 2005)
. These studies point to a
well-coordinated signalling pathway integrating spatiotemporal regulation
of plasmodesmal conductivity at the seed coat/cotton fibre cell
interface into developmental programme.
Intra- and extracellular signals alter plasmodesmal permeability
Salicylic acid is critical for plasmodesmal closure during defence
Salicylic acid (SA) is the central hormone in innate immune
responses to biotrophic and hemibiotrophic pathogens. SA
is produced in chloroplasts and triggers nuclear
localization of the SA-response coordinator NPR1 to up-regulate
genes essential for protecting cells from infection
(Mou et al.,
. Our recent studies have shown that SA, and SA
signalling components including NPR1, play a critical role
in plasmodesmal closure through
PLASMODESMATALOCATED PROTEIN 5 (PDLP5)
(Lee et al., 2011b; Wang
et al., 2013)
(Fig. 5, top right). PDLP5 is a type
I transmembrane protein containing a cysteine-rich extracellular domain
and a transmembrane domain with short C-terminal tail, and
belongs to a family of eight members in Arabidopsis
et al., 2008)
. PDLP5 was initially isolated from a biochemical
extraction of plasmodesma-enriched cell walls prepared from
young Arabidopsis seedlings
(Lee et al., 2011b)
. In this study,
immunogold labelling showed that PDLP5 was localized at
the central region of plasmodesmal channels. It was also
shown that PDLP5 was expressed at a very low level in young
plants under normal growth conditions, but was induced in
senescing tissues and upon SA treatment or pathogenic
Loss of PDLP5 function in the pdlp5-1 mutant results in
enhanced basal plasmodesmal permeability, whereas
overexpression of PDLP5 severely restricts plasmodesmal
permeability, underscoring its role in constraining plasmodesmata
(Lee et al., 2011b)
. Interestingly, although no enzymatic
function has been predicted for PDLP5, plasmodesmal callose
deposition is positively correlated with the level of PDLP5.
Moreover, SA- or pathogen-induced plasmodesmal callose
hyperaccumulation and subsequent plasmodesmal closure
require PDLP5, suggesting its key role in controlling
plasmodesmal callose deposition
(Wang et al., 2013)
highlighting the role of PDLP5 in mediating immune
signalling through plasmodesmal regulation, the enhancement of
plasmodesmal permeability in pdlp5-1 is directly correlated
with susceptibility to pathogenic bacteria, whereas restriction
of plasmodesmal permeability in PDLP5-overexpressing
plants correlates with resistance against bacterial pathogens.
Excitingly, plants overexpressing PDLP5 hyperaccumulate
SA, indicating a positive feedback loop between SA-directed
PDLP5 induction and PDLP5-regulated SA accumulation
(Lee et al., 2011b)
. Notably, the SA downstream regulator
NPR1 is critical not only for PDLP5 induction but also for
PDLP5-based activity in plasmodesmal inhibition
et al., 2013)
, underscoring a tight link between SA signalling
and plasmodesmal closure.
Interestingly, SA has also been implicated in enhancing
plasmodesmal complexity. Seedlings treated with SA for
five days had approximately three times more plasmodesmal
branching than did wild-type plants (Fitzgibbon et al., 2013). SA biosynthesis is up-regulated during the basal defence
Given that SA functions also in promoting senescence, this response, and its accumulation is often inextricably linked to
effect on plasmodesmal modification probably reflects over- reactive oxygen species (ROS) production and intracellular
all changes in developmental programming. It is tempting redox changes. Both ROS production and SA biosynthesis
to speculate, based on the known relationship between SA occur in chloroplasts
(Serrano et al., 2013)
, and as described
level and changes in cellular redox states, that such SA effect in the following section, changes in the redox status of the
on plasmodesmal complexity could possibly be explained plastids can affect plasmodesmal permeability and
complexby altered redox status of the cell
(Sager and Lee, 2012)
. ity. Thus, perhaps SA- and ROS-based redox changes in the
plastids and cytoplasm during defence and senescence may
together contribute to the fine-tuning of the structure and
permeability of plasmodesmata.
Gibberellic acid plays a role in reestablishment of plasmodesmal connectivity during chilling-induced breakage of dormancy
Dormancy is a period when plant growth slows to a halt; to
survive cold winters, shoots may develop protective buds over
meristematic zones, and plant cells induce a set of
freezetolerance proteins. A crucial step for enabling dormancy is
the arrest of growth within shoot apical meristems (SAMs),
accomplished in part by sealing the plasmodesmata within
the entire SAM zone and the vasculature of the stem below
it. Microinjection of LYCH into a single cell of the
proliferating SAM of birch plants showed cell-to-cell movement of the
(Rinne and van der Schoot, 1998)
(see Fig. 3B). However,
no dye diffusion was observed in the dormant SAM,
concomitant with the high level of callose deposited within the
plasmodesmata of all cells within the SAM at that stage
(Rinne et al., 2001)
(Fig. 3B.1). However, a chilling
treatment sufficient to break dormancy restored the dye-coupling
patterns found in active SAMs, indicating that dormancy
breakage involves re-establishing the symplastic connection
of the SAM (Fig. 3B.2). Consistently the strong labelling of
plasmodesmata by anti-callose antibodies within dormant
birch SAM tissue was reversed after dormancy was released
by chilling treatment.
As dormancy breakage eliminated most of the accumulated
callose at plasmodesmata, it was hypothesized that subsets of
the family GH17 enzymes may be expressed and localize to
(Rinne et al., 2001)
. Immunoblotting using
antibodies produced against a conserved sequence of GH17
family BGL proteins revealed that some members are present
during all stages of dormancy, whereas others accumulate
only during the release period. Immunogold labelling showed
changes in BGL cellular localization depending on the cycle
(Rinne et al., 2001)
. Normally present in cell
plates and young division walls of active SAM, BGLs in
dormant SAM were found predominantly within the lumen of
cortical ER-associated lipid bodies that became abundant
during short day induction. Importantly, upon chilling to
break dormancy, these lipid bodies labelled with BGL
antibody were often associated with plasmodesmata, while
plasmodesmal orifices were also occasionally labelled. These data
suggest that ER-derived lipid bodies may retain BGLs
during dormancy and act as vehicles for delivery to
plasmodesmata as soon as dormancy is lifted. Using this mechanism,
degradation of plasmodesmal callose, and thereby
unsealing of plasmodesmata, may occur in time to promote
cellto-cell movement of signalling molecules that support the
growth and differentiation of SAM when it is released from
The phytohormone gibberellic acid (GA) has been tied to
dormancy establishment and release
(Saure, 1985; Zanewich
and Rood, 1995)
, while chilling of dormant hybrid aspen
(Populus tremula × Populus tremuloides) buds promotes
up-regulation of GA biosynthesis genes
(Rinne et al., 2011)
The GA3 and GA4 analogues induced specific subsets of
GH17 family BGL genes. Furthermore, transport of the
symplastic tracer dye calcein into the shoot apex was restored
after a chilling period sufficient to release dormancy (8
weeks) or after the addition of GA4, but not GA3, to
(Rinne et al., 2011)
. FLOWERING LOCUS T
(FT) and CENTRORADIALIS-LIKE 1 (CENL1) are
signalling peptides targeting regions in the SAM, and their
expression is regulated by dormancy cycling
(Rinne et al., 2011)
FT transcripts were up-regulated in dormant aspen buds by
800-fold during chilling, but after bud burst were markedly
reduced. By contrast, CENL1 transcripts were low in buds
during chilling, but a shift to long day and higher
temperature conditions greatly induced CENL1 preceding bud burst
(Rinne et al., 2011)
. Both GA analogues had little effect on
FT, but strongly induced CENL1 in buds.
Collectively, these data have led to a model explaining the
role of symplasmic and symplastic reestablishment during
(Rinne et al., 2011)
(Fig. 5, bottom right).
According to this model, chilling up-regulates GA3
biosynthesis and FT expression together in the embryonic leaves of
dormant buds. At this stage, FT cannot move from the leaves
into the SAM because phloem sieve pores are blocked by
callose. GA3 also induces the production of a specific subset
of BGLs that localize to lipid bodies produced during
dormancy. After a chilling period sufficient to break dormancy,
the BGL-containing lipid bodies are mobilized to
plasmodesmata and sieve plate pores, where they release the BGLs to
degrade callose deposits. FT can now move through the
vasculature into the SAM, where it acts to prime SAM cells for full
dormancy breakage and bud burst. Increasing temperature
then triggers GA4 production, which activates another subset
of BGLs that fully degrade dormancy callose accumulated
at the plasmodesmata between SAM cells. This initiates bud
burst and stem elongation in the dormancy-released SAM.
CENL1, induced by GA4, is known to be expressed in the rib
meristem of Populus, but it is speculated that the
degradation of callose in the SAM may allow CENL1 to move into
nearby cells. It would be interesting to determine if CENL1
has a non-cell autonomous function outside the rib meristem
tissue, similarly to its Arabidopsis orthologue TFL1 (Conti
and Bradley, 2007), which affects dormancy release or bud
burst. Also, it would be highly informative to test whether
GA-mediated BGL induction is conserved across plant
species, and if so, how prevalent this singling pathway is for
spatiotemporal control of plasmodesmal permeability.
Changes in redox status modulate plasmodesmal permeability
Reactive oxygen species (ROS), such as hydrogen peroxide
and superoxide, can be utilized as second messengers when
produced in controlled bursts during various physiological,
developmental, and stress responses in plants
(Nanda et al.,
2010; Tian et al., 2013)
. Several studies have shown close
relationships between cellular redox status and regulation of
plasmodesmal trafficking (Fig. 5, top left). ISE1 and ISE2,
encoding a mitochondrion-localized DEAD-box RNA
helicase and a chloroplast-localized DEVH-box RNA helicase,
(Burch-Smith et al., 2011a; Stonebloom et al.,
, were identified from a mutant screen of Arabidopsis
embryos exhibiting increased SEL (Kim et al., 2002).
Mutations in these genes rendered the embryonic
plasmodesmata more open to the movement of large dextrans.
Moreover, whereas wild-type embryos mostly had simple
plasmodesmata at the torpedo stage, ise1 and ise2 contained
more branched and twinned plasmodesmata (Burch-Smith
and Zambryski, 2010). Similarly, silencing ISE1 or ISE2 in
adult tobacco leaves coincided with an increase in frequency
or proportion of branched plasmodesmata, respectively.
Further, silencing ISE1 heightened ROS production in
mitochondria and increased intercellular permeability to TMV
(Stonebloom et al., 2009)
. Strongly oxidizing the
mitochondria or reducing the chloroplasts also increased
(Stonebloom et al., 2012)
producing ROS are known to function in cell wall
(Liu et al., 2014)
, and specific targeting of
such enzymes to plasmodesmata may lead to the enhanced
plasmodesmal structural complexity seen in ise1 and ise2
(Burch-Smith and Zambryski, 2010). Detection of a class of
peroxidases that produce hydroxyl radicals at or near
plasmodesmata in the cambial zone of tomato (Solanum
(Ehlers and van Bel, 2010)
is consistent with the
notion that these enzymes may contribute to the changes in
local redox state, which in turn affects plasmodesmal
permeability and remodelling.
The importance of redox status for control of intercellular
trafficking was further supported by a recent study showing
that mutating the chloroplast-localized antioxidant
thioredoxin m3 (TRX-m3) severely hindered phloem unloading
of GFP in the roots of pSUC2:GFP compared with
(Benitez-Alfonso et al., 2009)
. The mutant,
called gat1, died very early in development, similar to ise1
and ise2, but unlike in those mutants embryonic diffusion of
carboxyfluorescein was unaffected in gat1, suggesting that
defects in symplastic connectivity manifest later in
development. However, diaminobenzidine and aniline blue staining
detected high levels of ROS and callose, respectively, in the
root apices of gat1 and similar gating mutants
(BenitezAlfonso and Jackson, 2009)
. When ectopically overexpressed
in leaf tissues, GAT1/TRX-m3 could increase the movement
of GFP that was transiently expressed through biolistic
(Benitez-Alfonso et al., 2009)
. Overexpression of
GAT1/TRX-m3 in chloroplasts led to a more reduced plastid
environment, whereas the knockout gat1 mutant contained
oxidized plastids, along with an increase and decrease in
plasmodesmal trafficking. These results suggest the existence of
a system by which plant cells detect intracellular redox states
and transmit a signal to modify plasmodesmal structure and
permeability (Fig. 5, top left).
Interestingly, another thioredoxin family member,
TRXh9, was shown to move cell-to-cell through plasmodesmata
(Meng et al., 2010)
. TRX-h9–GFP localized to the plasma
membrane, unlike many of its family members, though it has
no known transmembrane domains. However, analysis of
the TRX-h9 protein sequence revealed N-terminal domain
Gly and Cys residues in a pattern frequently associated
with palmitoylation, the post-translational addition of
fatty acids that can enhance the membrane association of
an otherwise soluble protein
(Meng et al., 2010)
putative palmitoylation site was critical for the intercellular
trafficking of TRX-h9, as when TRX-h9–GFP was expressed
under the root endodermis-specific SCARECROW
promoter, its movement out of the endodermal layer was
halted by mutations in the palmitoylation motif. It is not
yet known whether the intercellular movement of TRX-h9
has any effect on redox-based intercellular signalling and/
or plasmodesmal modulation. It is tantalizing, though, to
speculate that plant cells may utilize intercellular
antioxidant proteins, like Trx-h9, to help prevent oxidative stress
from spreading into neighbouring cells through
plasmodesmata (Fig. 5, top left).
Calcium fluxes rapidly close plasmodesmata
Cytoplasmic calcium is involved in numerous signalling
pathways as a secondary messenger, and has been shown
to affect plasmodesmal permeability (Fig. 5, bottom left).
When the cytosolic free calcium concentration was raised in
staminal hairs of Setcreasea purpurea using microinjected
calcium-loaded calcium chelator, fluorescent dye movement
out of these cells was blocked for several hours, indicating
that the increase in cytosolic free calcium stimulated
closure of plasmodesmata
. A transient closure
of plasmodesmata was also observed when calcium influx
inducers mastoparan and inositol-1,4,5-triphosphate were
microinjected into the staminal hairs
(Tucker and Boss,
. Notably, this plasmodesmal closure response was
suppressed when the hair cells were pre-treated with the calcium
channel blocker La3+, supporting the idea that calcium influx
itself was the signal causing the plasmodesmal change.
Further evidence linking calcium to plasmodesmal
responses, comes from experiments using direct calcium
injection or chilling treatments that induce an increase in
(Holdaway-Clarke et al., 2000)
. The electrical
resistance between sister maize suspension culture cells was
monitored through the use of microelectrodes placed within
the cytoplasm of each cell, assuming an increase in
resistance indicative of plasmodesmal closure (Holdaway-Clarke
et al., 1996). When chilled the culture cells at 0–5 °C,
plasmodesmata were closed and remained restricted until the
temperature rose again, while accompanying a measured 2-fold
increase in cytosolic Ca2+ concentration. However, when cells
were injected, Ca2+ caused only a transient plasmodesmal
closure, and the pores reopened within ten seconds. Assuming
that the cytosolic calcium release induced in these experiments
was physiologically relevant, callose deposition and
degradation, which takes hours, is unlikely to be responsible for the
plasmodesmal closure and reopening in response to calcium
(Holdaway-Clarke et al., 2000; Tucker and Boss, 1996)
Instead, the authors suggested that calcium-dependent
structural protein rearrangement might take place on this short time
scale to alter plasmodesmal permeability (Fig. 5, bottom left).
Intriguingly, several calcium-sensitive proteins have been
found to localize to plasmodesmata. For example,
immunogold labelling detected a 17-kDa centrin-like protein in the
plasmodesmal neck regions of onion (Allium cepa) root tips
and cauliflower (Brassica oleracea) florets
(Blackman et al.,
. Centrins are a major part of filamentous scaffolds that
contract upon binding to calcium in algal centrosomes and
the basal regions of flagella
(Geimer and Melkonian, 2005;
Hu et al., 2004; Sanders and Salisbury, 1989)
perhaps a centrin-containing scaffold configured across
the plasmodesmal entranceway could quickly contract the
pore neck region in response to intracellular calcium flux.
Immunodetection also found calreticulin and myosin VIII
at plasmodesmata in maize root cells
(Baluska et al., 1999;
Reichelt et al., 1999; Baluska et al., 2001)
. Calreticulin is a
calcium-binding protein with diverse roles in plants,
including calcium signalling and sequestration (Jia et al., 2009).
Myosin VIII contains within its neck region potential
binding domains for calmodulins (Knight and Kendrick-Jones,
1993), a calcium-binding family of intermediate messengers
mediating transduction of calcium signalling
(Ranty et al.,
. Finding that these proteins, along with actin
et al., 1994)
, localize at plasmodesmata attracts the
hypothesis that an acto-myosin and centrin-filamentous system
within the neck of plasmodesma translates the fluctuation
of calcium flux exerted by calcium channels and
calreticulin into rapid but transient contractions of the pore (Fig. 5,
Osmotic stress transiently increases plasmodesmal flux
Plant root tips were shown to increase plasmodesmal flux
temporarily and enhance sugar transport to osmotically
(Schulz, 1994, 1995)
. Electron microscopy
of pea (Pisum sativum) root tip plasmodesmal ultrastructure
showed that the diameter of the cytoplasmic sleeves in the
cortical cells increased markedly, along with a loss in
plasmodesmal neck constriction, after one hour of osmotic stress
imposed using 350 mM mannitol (Schulz, 1995). However,
this plasmodesmal response to mannitol was also transient,
thereby the morphological alterations in root tip cortical
plasmodesmata were reversed within a few hours. This result
is reminiscent of previous observations that pea root tips
subjected to osmotic stress displayed a transient up-regulation in
the phloem unloading of 14C-sucrose into the root tip
cortical cells after two hours
. The author suggests
that root tip plasmodesmal dilation during the initial osmotic
stress response may increase the symplastic unloading of
sugars into the stressed cells, to lower their water potential and
help prevent plasmolysis. It is possible that the underlying
mechanism involves callose degradation and re-deposition
at plasmodesmata between root cells. Alternatively, calcium
influx and/or ROS burst, both of which can be quickly and
highly induced in response to mechanical stress of the plasma
membrane during osmotic shock
(Kurusu et al., 2013; Xiong
and Zhu, 2002)
, may play a role in regulating plasmodesmal
aperture in a callose-independent manner.
Cold and high light induce plasmodesmal remodelling
The onset of cold temperatures not only slows overall plant
growth but also can reduce photoassimilate transport rates.
A change in plasmodesmal ultrastructure or frequency is
thought to relieve this stress by increasing sugar movement
from source to sink tissues
(Bilska and Sowinski, 2010)
Analyses of the plasmodesmal ultrastructure at the interface
between Kranz mesophyll (KMS), bundle sheath (BS), and
vascular parenchyma (VP) cells in cold-tolerant (CT) and
cold-sensitive (CS) maize subjected to chilling temperatures
revealed that CT maize could better maintain a state of open
plasmodesmata compared with the CS lines. Intriguingly,
a previous study comparing CS and CT maize found that
plasmodesmal frequencies between KMS, BS, and VP cells
increased by the greatest amount in the CT line grown at
suboptimal temperatures, compared with CS plants. Under
such conditions, the rate of photoassimilate movement into
the transport path also increased in the CT line
et al., 2003)
. These data suggest that enhancing
plasmodesmal permeability allows the plants to better survive the cold
Plasmodesmata of the KMS/BS junction typically contain
an internal ring of an unknown globular element called the
(Bilska and Sowinski, 2010)
with control CS maize grown in normal temperatures,
CS lines subjected to chilling responded with a swelling
of the internal sphincter. Following four hours of
treatment, KMS/BS plasmodesmata of the CS lines had about
25% higher incidence of sphincter enlargement, compared
with CT maize. The cross-area of the cytoplasmic sleeves
in BS/BS and VP/VP also shrank significantly in the
coldtreated CS maize, but remained unaffected in the CT lines.
Immunogold labelling showed that callose and calreticulin
in the plasmodesmal neck regions of KMS/BS and BS/VP
interfaces greatly increased in cold-treated CS lines, but not
in the CT lines. Finally, 14C transport capacity post-chilling
was much reduced in the CS compared with CT maize, even
after only 1 hour of chilling
(Bilska and Sowinski, 2010)
These results indicate that CS maize respond very rapidly
to chilling by blocking plasmodesmata. Although callose
deposition probably accounts for the long-term sealing of
plasmodesmata in the CS lines, cold-induced calcium flux
has previously been shown to rapidly close plasmodesmata
(Holdaway-Clarke et al., 2000)
. The enhanced level of
calreticulin found at the plasmodesmal neck regions in cold
conditions in CS lines may represent a negative feedback
response to sequester free calcium. Elucidation of the
mechanisms used by the CT maize to maintain plasmodesmal
opening and increase their frequency in response to
chilling could be useful for improving cold-stress tolerance in
many crop plants. It is notable that a putative member of the
dehydrin protein family localizes to the plasmodesmal neck
region in a highly cold-tolerant dogwood species (Karlson
et al., 2003). Dehydrins are hypothesized to protect and
stabilize other proteins and membranes against abiotic stresses
caused by high salinity, drought, and cold. It will be
interesting to determine whether these proteins indeed have a role
in maintaining plasmodesmal connectivity in cold-tolerant
High light intensity, a condition that can enhance the rate
of photosynthesis, was also shown to elevate plasmodesmal
density at KMS/BS/VP junctions
(Sowinski et al., 2007)
similar to cold-tolerant maize adaptation to chilling
temperatures. C4 grasses were grown in low, medium, and high
intensity light, and plasmodesmal density between KMS/BS
and BS/VP junctions in each condition was compared, as well
as the rate of photosynthate export. Higher light enhanced
the export of photosynthate through the mesophyll into the
vasculature, as measured using radiolabelled carbon
dioxide (14CO2). Plasmodesmal density increased significantly
between KMS/BS, and to a lesser extent between BS/VP
junctions, as the light levels intensified
(Sowinski et al., 2007)
Collectively, these results indicate that de novo biosynthesis of
plasmodesmata may help alleviate the stresses caused by
certain abiotic stresses by enhancing intercellular nutrient flux.
Toxic metals and wounding trigger plasmodesmal closure
Toxic metals have been shown to increase callose deposition
at plasmodesmata. Treatments with 5–20 µM aluminium
(Al3+) induced closure of plasmodesmata within three hours,
as measured by microinjected LYCH diffusion in wheat
(Triticum aestivum) root epidermal and tobacco mesophyll
(Sivaguru et al., 2000)
. This response is
callose-dependent, as it was preventable by pre-treatment with a callose
synthesis inhibitor, 2-deoxy-d -glucose. Similarly, treatment with
subtoxic levels of cadmium ions (Cd2+) can severely inhibit
the cell-to-cell spread of turnip vein clearing virus, with
10 µM cadmium completely halting viral systemic movement
(Citovsky et al., 1998). Toxic metals other than aluminium
and cadmium have also implicated in plasmodesmal closure.
For example, six hours after treatment with lead cations
(Pb2+), Lemna minor radial root cell walls and plasmodesmata
had a high level of callose accumulation
et al., 2012)
. Similar experiments were performed with
(Pirselova et al., 2012)
, but although aniline blue
staining resulted in punctate signals within root cell walls
in response to arsenic, whether these signals indicated
plasmodesmata was not determined.
Wound stresses have also been shown to induce
accumulation of plasmodesmal callose
(Radford et al., 1998)
Reversal of wound-induced plasmodesmal callose seems to
involve Arabidopsis BETA-1,3-GLUCANASE PUTATIVE
PLASMODESMATA-ASSOCIATED PROTEIN (AtBG_
ppap; Levy et al., 2007). Aniline blue staining revealed that
after wounding, there was higher accumulation of
plasmodesmal callose in mutant leaves lacking AtBG_ppap
compared with wild type. An AtBG_ppap–GFP fusion protein
infiltrated into tobacco leaves confirmed that this glucanase
could localize to both plasma and ER membranes, with a
noticeably brighter punctate fluorescent pattern across cell
perimeters that overlapped with plasmodesmal callose at
cell–cell junctions. Bombardment of GFP into AtBG_ppap
mutant and wild-type Arabidopsis leaves revealed that the
overaccumulation of plasmodesmal callose in the mutant
also blocked the cell-to-cell spread of GFP, supporting its
effect on plasmodesmal permeability
(Levy et al., 2007)
Since plasmodesmata were first described some hundred years
ago, plant biologists have been striving to elucidate their role
in plant biology. Recent discoveries have demonstrated that
plasmodesmata are not just simple “holes in the walls” but
rather are highly dynamic intercellular communication
channels, enabling plant cells to organize into a synchronized
entity. However, it has also become evident that gaining
cellular autonomy by severing or closing plasmodesmata serves
as a crucial mechanism, not only for cell differentiation and
specialization, but also for plant survival under drastic
Challenges ahead include identifying various intracellular
signalling pathways and factors that convert primary intrinsic
and extrinsic signals into alterations in plasmodesmal
structure, formation, and permeability. Given the essential role of
plasmodesmata in cell-to-cell transport, understanding how
plasmodesmal regulation is integrated into specific cellular
signalling pathways that enhance intercellular flux could
provide insights into improving health and fitness of
agriculturally important crops. Furthermore, advancing knowledge
about various signalling factors that regulate plasmodesmata
during environmental stress conditions could help improve
traits such as pathogen resistance, drought and cold
tolerance, and fruit/seed yield, in preparation for imminent climate
changes. In this regard, it is inevitable that we will have to
pay special attention to how plasmodesmal alterations bring
about changes in distribution of the most critical plant
signalling molecules, hormones, and it will not be surprising to
find more crosstalk between plasmodesmata and hormonal
signalling beyond SA and GA. Excitingly, a very recent
finding showed that auxin may regulate callose synthase GSL8 to
prevent its own diffusion out of cells during the phototropic
(Han et al., 2014)
. As our understanding advances,
complex feed-forward and feed-back regulatory circuits may
prove to be a common theme linking hormonal and other
signalling pathways to plasmodesmal control.
Plasmodesmal research is one of the most vital, but
extremely challenging, fields of biology. Plasmodesmata
are recalcitrant to compositional and anatomical studies at
the molecular level, owing to the difficulties associated with
isolating them not only as biochemically intact entities but
also as genetic mutants. However, the refinement of certain
techniques, such as mass spectroscopy-based plasmodesmal
protein identification from plasmodesmata-enriched cell wall
fractions, improved fluorescent-tagging-based localization
studies, and genetic approaches, has provided much-needed
insights into the role of plasmodesmata in developmental
and intercellular signalling processes. Thus, although the
field is still far from grasping the complete picture of the
molecular anatomy and genetic networks of plasmodesmata,
the future is bright. As we move toward fully unlocking the
mystery of these indispensable cellular bridges, we will need
to integrate our own expertise and resources through
collaborations across fields and disciplines, just as we are finding
that plasmodesmata are integrated into all aspects of
development and cellular responses to environmental signals and
The support for this work was provided by the National Science Foundation
(IOS-0954931) and partially by grants from the National Center for Research
Resources (5P30RR031160-03) and the National Institute of General
Medical Sciences (8 P30 GM103519-03) from the National Institutes of
Health. Special thanks go to N. Hofmann for editorial support.
Burch-Smith TM, Zambryski PC. 2010. Loss of INCREASED SIZE
EXCLUSION LIMIT (ISE)1 or ISE2 increases the formation of secondary
plasmodesmata. Current Biology 20, 989–993.
Burch-Smith TM, Zambryski PC. 2011. Plasmodesmata paradigm
shift: Regulation from without versus within. Annual Review of Plant
Chandler JW. 2012. Floral meristem initiation and emergence in plants.
Cellular and Molecular Life Sciences 69, 3807–3818.
Chen XY, Liu L, Lee E, Han X, Rim Y, Chu H, Kim SW, Sack F, Kim
JY. 2009. The Arabidopsis callose synthase gene GSL8 is required for
cytokinesis and cell patterning. Plant Physiology 150, 105–113.
Christensen NM, Faulkner C, Oparka K. 2009. Evidence for
unidirectional flow through plasmodesmata. Plant Physiology 150, 96–104.
Citovsky V, Ghoshroy S, Tsui F, Klessig D. 1998. Non-toxic
concentrations of cadmium inhibit systemic movement of turnip vein
clearing virus by a salicylic acid-independent mechanism. The Plant
Journal 16, 13–20.
Clement C, Audran JC. 1995. Anther wall layers control pollen sugar
nutrition in lilium. Protoplasma 187, 172–181.
Conti L, Bradley D. 2007. TERMINAL FLOWER1 is a mobile signal
controlling Arabidopsis architecture. Plant Cell 19, 767–778.
Ding B. 2009. The biology of viroid–host interactions. Annual Review of
Phytopathology 47, 105–131.
Ding B, Kwon M, Warnberg L. 1996. Evidence that actin filaments
are involved in controlling the permeability of plasmodesmata in tobacco
mesophyll. The Plant Journal 10, 157–164.
Dobnik D, Baebler S, Kogovsek P, Pompe-Novak M, Stebih D,
Panter G, Janez N, Morisset D, Zel J, Gruden K. 2013.
β-1,3glucanase class III promotes spread of PVY and improves protein
production. Plant Biotechnology Report 7, 547–555.
Dong X, Hong Z, Chatterjee J, Kim S, Verma DP. 2008. Expression of
callose synthase genes and its connection with Npr1 signaling pathway
during pathogen infection. Planta 229, 87–98.
Dong X, Hong Z, Sivaramakrishnan M, Mahfouz M, Verma DP.
2005. Callose synthase (CalS5) is required for exine formation during
microgametogenesis and for pollen viability in Arabidopsis. The Plant
Journal 42, 315–328.
Doxey AC, Yaish MW, Moffatt BA, Griffith M, McConkey BJ. 2007.
Functional divergence in the Arabidopsis β-1,3-glucanase gene family
inferred by phylogenetic reconstruction of expression states. Molecular
Biology and Evolution 24, 1045–1055.
Dute RR, Peterson CM, Rushing AE. 1989. Utrastructural changes
of the egg apparatus associated with fertilization and proembryo
development of soybean, Glycine max (Fabaceae). Annals of Botany 64,
result in transient closure of higher plant plasmodesmata. Planta 210,
Holdaway-Clarke TL, Walker NA, Overall RL. 1996. Measurement
of the electrical resistance of plasmodesmata and membranes of corn
suspension-culture cells. Planta 199, 537–544.
Hong Z, Delauney AJ, Verma DP. 2001. A cell plate-specific callose
synthase and its interaction with phragmoplastin. Plant Cell 13, 755–768.
Hu H, Sheehan JH, Chazin WJ. 2004. The mode of action of centrin.
Binding of Ca2+ and a peptide fragment of Kar1p to the C-terminal
domain. The Journal of Biological Chemistry 279, 50895–50903.
Iglesias VA, Meins F. 2000. Movement of plant viruses is delayed in a
β-1,3-glucanase-deficient mutant showing a reduced plasmodesmatal
size exclusion limit and enhanced callose deposition. The Plant Journal 21,
Isono K, Shimizu M, Yoshimoto K, Niwa Y, Satoh K, Yokota A,
Kobayashi H. 1997. Leaf-specifically expressed genes for polypeptides
destined for chloroplasts with domains of σ70 factors of bacterial RNA
polymerases in Arabidopsis thaliana. Proceedings of the National Academy
of Sciences, USA 94, 14948–14953.
Jacobs AK, Lipka V, Burton RA, Panstruga R, Strizhov N,
SchulzeLefert P, Fincher GB. 2003. An Arabidopsis callose synthase, GSL5,
is required for wound and papillary callose formation. Plant Cell 15,
Jensen TE, Valdovinos JG. 1967. Fine structure of abscission zones:
i. abscission zones of the pedicels of tobacco and tomato flowers at
anthesis. Planta 77, 298–318.
Jia XY, He LH, Jing RL, Li RZ. 2009. Calreticulin: conserved protein and
diverse functions in plants. Physiologia Plantarum 136, 127–138.
Karlson DT, Fujino T, Kimura S, Baba K, Itoh T, Ashworth EN. 2003.
Novel plasmodesmata association of dehydrin-like proteins in
coldacclimated Red-osier dogwood (Cornus sericea). Tree Physiology 23,
Kawakami S, Watanabe Y, Beachy RN. 2004. Tobacco mosaic virus
infection spreads cell to cell as intact replication complexes. Proceedings
of the National Academy of Sciences, USA 101, 6291–6296.
Kim I, Cho E, Crawford K, Hempel FD, Zambryski PC. 2005a.
Cell-to-cell movement of GFP during embryogenesis and early seedling
development in Arabidopsis. Proceedings of the National Academy of
Sciences, USA 102, 2227–2231.
Kim I, Hempel FD, Sha K, Pfluger J, Zambryski PC. 2002.
Identification of a developmental transition in plasmodesmatal function
during embryogenesis in Arabidopsis thaliana. Development 129,
Kim I, Kobayashi K, Cho E, Zambryski PC. 2005b. Subdomains for
transport via plasmodesmata corresponding to the apical-basal axis
are established during Arabidopsis embryogenesis. Proceedings of the
National Academy of Sciences, USA 102, 11945–11950.
Knight AE, Kendrick-Jones J. 1993. A myosin-like protein from a higher
plant. Journal of Molecular Biology 231, 148–154.
Knoblauch M, van Bel AJ. 1998. Sieve tubes in action. Plant Cell 10,
Kollmann R, Glockmann C. 1991. Studies on graft unions. Protoplasma
Kozieradzka-Kiszkurno M, Bohdanowicz J. 2010. Unusual
electrondense dome associates with compound plasmodesmata in the
embryosuspensor of genus Sedum (Crassulaceae). Protoplasma 247, 117–120.
Kozieradzka-Kiszkurno M, Plachno BJ. 2012. Are there symplastic
connections between the endosperm and embryo in some angiosperms?
A lesson from the Crassulaceae family. Protoplasma 249, 1081–1089.
Kurusu T, Kuchitsu K, Nakano M, Nakayama Y, Iida H. 2013. Plant
mechanosensing and Ca2+ transport. Trends Plant Science 18, 227–233.
Lee J-Y, Cho S, Sager R. 2011a. Plasmodesmata and non-cell
autonomous signling in plants. In: Murphy AS, ed. The Plant Plasma
Membrane . Berlin: Springer-Verlag, 87–106.
Lee JJ, Woodward AW, Chen ZJ. 2007. Gene expression changes
and early events in cotton fibre development. Annals of Botany 100,
Lee JY. 2014. New and old roles of plasmodesmata in immunity and
parallels to tunneling nanotubes. Plant Science 221–222, 13–20.
Agbeci M , Grangeon R , Nelson RS , Zheng H , Laliberte JF . 2013 .
Contribution of host intracellular transport machineries to intercellular movement of turnip mosaic virus . Plos Pathogens 9 , e1003683 .
Amari K , Lerich A , Schmitt-Keichinger C , Dolja VV , Ritzenthaler C. 2011 . Tubule-guided cell-to-cell movement of a plant virus requires class XI myosin motors . Plos Pathogens 7 , e1002327 .
An Q , Ehlers K , Kogel KH , van Bel AJ , Huckelhoven R. 2006 .
Multivesicular compartments proliferate in susceptible and resistant MLA12-barley leaves in response to infection by the biotrophic powdery mildew fungus . New Phytologist 172 , 563 - 576 .
Avisar D , Prokhnevsky AI , Dolja VV . 2008 . Class VIII myosins are required for plasmodesmatal localization of a closterovirus Hsp70 homolog . Journal of Virology 82 , 2836 - 2843 .
Baluska F , Cvrckova F , Kendrick-Jones J , Volkmann D. 2001 Sink plasmodesmata as gateways for phloem unloading. Myosin VIII and calreticulin as molecular determinants of sink strength? Plant Physiology 126 , 39 - 46 .
2004. Actin-dependent fluid-phase endocytosis in inner cortex cells of maize root apices . Journal of Experimental Botany 55 , 463 - 473 .
Baluska F , Samaj J , Napier R , Volkmann D. 1999 . Maize calreticulin localizes preferentially to plasmodesmata in root apex . The Plant Journal 19 , 481 - 488 .
Bar-Dror T , Dermastia M , Kladnik A et al. 2011 . Programmed cell death occurs asymmetrically during abscission in tomato . Plant Cell 23 , 4146 - 4163 .
Barratt DH , Kolling K , Graf A , Pike M , Calder G , Findlay K , Zeeman SC , Smith AM . 2011 . Callose synthase GSL7 is necessary for normal phloem transport and inflorescence growth in Arabidopsis . Plant Physiology 155 , 328 - 341 .
Beffa RS , Hofer RM , Thomas M , Meins F , Jr . 1996 . Decreased susceptibility to viral disease of β-1,3-glucanase-deficient plants generated by antisense transformation . Plant Cell 8 , 1001 - 1011 .
Benitez-Alfonso Y , Cilia M , San Roman A, Thomas C , Maule A , Hearn S , Jackson D. 2009 . Control of Arabidopsis meristem development by thioredoxin-dependent regulation of intercellular transport . Proceedings of the National Academy of Sciences, USA 106 , 3615 - 3620 .
Benitez-Alfonso Y , Faulkner C , Pendle A , Miyashima S , Helariutta Y , Maule A. 2013 . Symplastic intercellular connectivity regulates lateral root patterning . Developmental Cell 26 , 136 - 147 .
Benitez-Alfonso Y , Faulkner C , Ritzenthaler C , Maule AJ . 2010 .
Plasmodesmata: gateways to local and systemic virus infection . Molecular Plant-Microbe Interactions 23 , 1403 - 1412 .
Benitez-Alfonso Y , Jackson D. 2009 . Redox homeostasis regulates plasmodesmal communication in Arabidopsis meristems . Plant Signaling and Behavior 4 , 655 - 659 .
1997 . The symplasmic coupling of L2-cells diminishes in early floral development of Iris . Planta 203 , 245 - 252 .
Bilska A , Sowinski P. 2010 . Closure of plasmodesmata in maize (Zea mays) at low temperature: a new mechanism for inhibition of photosynthesis . Annals of Botany 106 , 675 - 686 .
Blackman LM , Harper JD , Overall RL . 1999 . Localization of a centrinlike protein to higher plant plasmodesmata . European Journal of Cell Biology 78 , 297 - 304 .
Boyko V , Ferralli J , Ashby J , Schellenbaum P , Heinlein M. 2000 .
Nature Cell Biology 2 , 826 - 832 .
Boyko V , Hu Q , Seemanpillai M , Ashby J , Heinlein M. 2007 . Validation of microtubule-associated Tobacco mosaic virus RNA movement and involvement of microtubule-aligned particle trafficking . The Plant Journal 51 , 589 - 603 .
2001. Local expression of enzymatically active class I β-1, 3-glucanase enhances symptoms of TMV infection in tobacco . The Plant Journal 28 , 361 - 369 .
Burch-Smith TM , Brunkard JO , Choi YG , Zambryski PC . 2011a .
Organelle-nucleus cross-talk regulates plant intercellular communication via plasmodesmata . Proceedings of the National Academy of Sciences, USA 108 , E1451 - 1460 .
Burch-Smith TM , Stonebloom S , Xu M , Zambryski PC . 2011b .
Plasmodesmata during development: re-examination of the importance of primary, secondary, and branched plasmodesmata structure versus function . Protoplasma 248 , 61 - 74 .
Ehlers K , Kollmann R. 2001 . Primary and secondary plasmodesmata: structure, origin, and functioning . Protoplasma 216 , 1 - 30 .
Ehlers K , van Bel AJ. 2010 . Dynamics of plasmodesmal connectivity in successive interfaces of the cambial zone . Planta 231 , 371 - 385 .
Ehlers K , Westerloh MG . 2013 . Developmental control of plasmodesmata frequency, structure, and function . In: Sokolowska K , Sowinski P, eds. Symplasmic transport in vascular plants . New York: Springer Science, 41 - 82 .
Esau K , Thorsch J. 1984 . The sieve plate of Echium (Boraginaceae): Developmental aspects and response of P-protein to protein digestion .
Journal of Ultrastructure Research 86 , 31 - 45 .
Estornell LH , Agusti J , Merelo P , Talon M , Tadeo FR . 2013 .
Elucidating mechanisms underlying organ abscission . Plant Science 199-200 , 48 - 60 .
Faulkner C , Akman OE , Bell K , Jeffree C , Oparka K. 2008 . Peeking into pit fields: A multiple twinning model of secondary plasmodesmata formation in tobacco . Plant Cell 20 , 1504 - 1518 .
2013. A developmental framework for complex plasmodesmata formation revealed by large-scale imaging of the Arabidopsis leaf epidermis . Plant Cell 25 , 57 - 70 .
Geimer S , Melkonian M. 2005 . Centrin scaffold in Chlamydomonas reinhardtii revealed by immunoelectron microscopy . Eukaryotic Cell 4 , 1253 - 1263 .
Gisel A , Barella S , Hempel FD , Zambryski PC . 1999 . Temporal and spatial regulation of symplastic trafficking during development in Arabidopsis thaliana apices . Development 126 , 1879 - 1889 .
Gisel A , Hempel FD , Barella S , Zambryski P. 2002 . Leaf-to-shoot apex movement of symplastic tracer is restricted coincident with flowering in Arabidopsis . Proceedings of the National Academy of Sciences, USA 99 , 1713 - 1717 .
Guseman JM , Lee JS , Bogenschutz NL , Peterson KM , Virata RE , Xie B , Kanaoka MM , Hong Z , Torii KU . 2010 . Dysregulation of cellto-cell connectivity and stomatal patterning by loss-of-function mutation in Arabidopsis chorus (glucan synthase-like 8) . Development 137 , 1731 - 1741 .
Han X , Hyun TK , Zhang M , Kumar R , Koh E , Kang BH , Lucas WJ , Kim JY . 2014 . Auxin-callose-mediated plasmodesmal gating is essential for tropic auxin gradient formation and signaling . Developmental Cell 28 , 132 - 146 .
Han YZ , Huang BQ , Zee SY , Yuan M. 2000 . Symplastic communication between the central cell and the egg apparatus cells in the embryo sac of Torenia fournieri Lind. before and during fertilization . Planta 211 , 158 - 162 .
Harries PA , Palanichelvam K , Yu W , Schoelz JE , Nelson RS . 2009a .
The cauliflower mosaic virus protein P6 forms motile inclusions that traffic along actin microfilaments and stabilize microtubules . Plant Physiology 149 , 1005 - 1016 .
2009b. Differing requirements for actin and myosin by plant viruses for sustained intercellular movement . Proceedings of the National Academy of Sciences, USA 106 , 17594 - 17599 .
Heinlein M. 2002 . Plasmodesmata: dynamic regulation and role in macromolecular cell-to-cell signaling . Current Opinion in Plant Biology 5 , 543 - 552 .
Henry EW . 1979 . Peroxidases in tobacco abscission zone tissue . VI.
Ultrastructural localization in plasmodesmata during ethylene-induced abscission . Cytologia 44 , 135 - 152 .
Henslop-Harrison J. 1966 . Cytoplasmic connexions between angiosperm meiocytes . Annals of Botany 30 , 221 - 222 .
Hepler PK . 1982 . Endoplasmic reticulum in the formation of the cell plate and plasmodesmata . Protoplasma 111 , 121 - 133 .
Hepler PK , Newcomb EH . 1967 . Fine structure of cell plate formation in the apical meristem of Phaseolus roots . Journal of Ultrastructure Research 19 , 498 - 513 .
Hofius D , Hajirezaei MR , Geiger M , Tschiersch H , Melzer M , Sonnewald U. 2004 . RNAi-mediated tocopherol deficiency impairs photoassimilate export in transgenic potato plants . Plant Physiology 135 , 1256 - 1268 .
Holdaway-Clarke TL , Walker NA , Hepler PK , Overall RL . 2000 .
Physiological elevations in cytoplasmic free calcium by cold or ion injection Lee JY , Cui WE . 2009 . Non-cell autonomous RNA trafficking and longdistance signaling . Journal of Plant Biology 52 , 10 - 18 .
Lee JY , Taoka K , Yoo BC , Ben-Nissan G , Kim DJ , Lucas WJ . 2005 .
Plasmodesmal-associated protein kinase in tobacco and Arabidopsis recognizes a subset of non-cell-autonomous proteins . Plant Cell 17 , 2817 - 2831 .
Lee JY , Wang X , Cui W et al. 2011b . A plasmodesmata-localized protein mediates crosstalk between cell-to-cell communication and innate immunity in Arabidopsis . Plant Cell 23 , 3353 - 3373 .
Levy A , Erlanger M , Rosenthal M , Epel BL . 2007 . A plasmodesmataassociated β-1,3-glucanase in Arabidopsis . The Plant Journal 49 , 669 - 682 .
Liu JZ , Blancaflor EB , Nelson RS . 2005 . The tobacco mosaic virus 126-kilodalton protein, a constituent of the virus replication complex, alone or within the complex aligns with and traffics along microfilaments . Plant Physiology 138 , 1853 - 1865 .
Liu N , Lin Z , Guan L , Gaughan G , Lin G. 2014 . Antioxidant enzymes regulate reactive oxygen species during pod elongation in Pisum sativum and Brassica chinensis . PLoS ONE 9 , e87588 .
Lucas WJ . 2006 . Plant viral movement proteins: agents for cell-to-cell trafficking of viral genomes . Virology 344 , 169 - 184 .
Lucas WJ , Lee JY . 2004 . Plasmodesmata as a supracellular control network in plants . Nature Reviews Molecular Cell Biology 5 , 712 - 726 .
Malamy JE , Benfey PN . 1997 . Organization and cell differentiation in lateral roots of Arabidopsis thaliana . Development 124 , 33 - 44 .
Mamun EA , Cantrill LC , Overall RL , Sutton BG . 2005a . Cellular organisation and differentiation of organelles in pre-meiotic rice anthers .
Cell Biology International 29 , 792 - 802 .
Mamun EA , Cantrill LC , Overall RL , Sutton BG . 2005b . Cellular organisation in meiotic and early post-meiotic rice anthers . Cell Biology International 29 , 903 - 913 .
Mansfield SG , Briarty LG . 1991 . Early embryogenesis in Arabidopsis thaliana. 2. The developing embryo . Canadian Journal of Botany-Revue Canadienne De Botanique 69 , 461 - 476 .
Meng L , Wong JH , Feldman LJ , Lemaux PG , Buchanan BB . 2010 . A membrane-associated thioredoxin required for plant growth moves from cell to cell, suggestive of a role in intercellular communication . Proceedings of the National Academy of Sciences, USA 107 , 3900 - 3905 .
Milyaeva EL . 2007 . Presumable role of plasmodesmata in floral signal transduction in shoot apical meristems of rudbeckia and perilla plants .
Russian Journal of Plant Physiology 54 , 498 - 506 .
Mou Z , Fan W , Dong X. 2003 . Inducers of plant systemic acquired resistance regulate NPR1 function through redox changes . Cell 113 , 935 - 944 .
Mumm P , Wolf T , Fromm J , Roelfsema MR , Marten I. 2011 . Cell type-specific regulation of ion channels within the maize stomatal complex .
Plant and Cell Physiology 52 , 1365 - 1375 .
Mursalimov SR , Baiborodin SI , Sidorchuk YV , Shumny VK , Deineko EV . 2010 . Characteristics of the cytomictic channel formation in Nicotiana tabacum L. pollen mother cells . Cytology and Genetics 44 , 14 - 18 .
Myouga F , Hosoda C , Umezawa T , Iizumi H , Kuromori T , Motohashi R , Shono Y , Nagata N , Ikeuchi M , Shinozaki K. 2008 .
A heterocomplex of iron superoxide dismutases defends chloroplast nucleoids against oxidative stress and is essential for chloroplast development in Arabidopsis . Plant Cell 20 , 3148 - 3162 .
Nanda AK , Andrio E , Marino D , Pauly N , Dunand C. 2010 . Reactive oxygen species during plant-microorganism early interactions . Journal of Integrative Plant Biology 52 , 195 - 204 .
Niehl A , Heinlein M. 2011 . Cellular pathways for viral transport through plasmodesmata . Protoplasma 248 , 75 - 99 .
Niehl A , Pena EJ , Amari K , Heinlein M. 2013 . Microtubules in viral replication and transport . The Plant Journal 75 , 290 - 308 .
Niu N , Liang W , Yang X , Jin W , Wilson ZA , Hu J , Zhang D. 2013 .
EAT1 promotes tapetal cell death by regulating aspartic proteases during male reproductive development in rice . Nature Communications 4 , 1445 .
Northcote DH , Davey R , Lay J. 1989 . Use of antisera to localize callose, xylan and arabinogalactan in the cell-plate, primary and secondary walls of plant-cells . Planta 178 , 353 - 366 .
Oparka KJ , Prior DAM , Wright KM . 1995 . Symplastic communication between primary and developing lateral roots of Arabidopsis thaliana .
Journal of Experimental Botany 46 , 187 - 197 .
Oparka KJ , Roberts AG , Boevink P , Santa Cruz S , Roberts I , Pradel KS , Imlau A , Kotlizky G , Sauer N , Epel B. 1999 . Simple, but not branched, plasmodesmata allow the nonspecific trafficking of proteins in developing tobacco leaves . Cell 97 , 743 - 754 .
Ormenese S , Havelange A , Bernier G , van der Schoot C. 2002 . The shoot apical meristem of Sinapis alba L. expands its central symplasmic field during the floral transition . Planta 215 , 67 - 78 .
Ormenese S , Havelange A , Deltour R , Bernier G. 2000 . The frequency of plasmodesmata increases early in the whole shoot apical meristem of Sinapis alba L. during floral transition . Planta 211 , 370 - 375 .
Osborne DJ , Sargent JA. 1976 . The positional differentiation of ethyleneresponsive cells in rachis abscission zones in leaves of Sambucus nigra and their growth and ultrastructural changes at senescence and separation . Planta 130 , 203 - 210 .
Palevitz BA , Hepler PK . 1985 . Changes in dye coupling of stomatal cells of Allium and Commelina demonstrated by microinjection of Lucifer yellow .
Planta 164 , 473 - 479 .
Perdue TD , Loukides CA , Bedinger PA . 1992 . The formation of cytoplasmic channels between tapetal cells in Zea mays . Protoplasma 171 , 75 - 79 .
Pirselova B , Mistrikova V , Libantova J , Moravcikova J , Matusikova I. 2012 . Study on metal-triggered callose deposition in roots of maize and soybean . Biologia 67 , 698 - 705 .
Radford JE , Vesk M , Overall RL . 1998 . Callose deposition at plasmodesmata . Protoplasma 201 , 30 - 37 .
Radford JE , White RG . 1998 . Localization of a myosin-like protein to plasmodesmata . The Plant Journal 14 , 743 - 750 .
Radford JE , White RG . 2011 . Inhibitors of myosin, but not actin, alter transport through Tradescantia plasmodesmata . Protoplasma 248 , 205 - 216 .
Radice S , Ontivero M , Giordani E , Bellini E. 2008 . Anatomical differences on development of fertile and sterile pollen grains of Prunus salicina Lindl . Plant Systemics and Evolution 273 , 63 - 69 .
Ranty B , Aldon D , Galaud JP . 2006 . Plant calmodulins and calmodulinrelated proteins: multifaceted relays to decode calcium signals . Plant Signaling and Behavior 1 , 96 - 104 .
Reichelt S , Knight AE , Hodge TP , Baluska F , Samaj J , Volkmann D , Kendrick-Jones J. 1999 . Characterization of the unconventional myosin VIII in plant cells and its localization at the post-cytokinetic cell wall . The Plant Journal 19 , 555 - 567 .
Rinne PL , Kaikuranta PM , van der Schoot C. 2001 . The shoot apical meristem restores its symplasmic organization during chilling-induced release from dormancy . The Plant Journal 26 , 249 - 264 .
Rinne PL , van der Schoot C. 1998 . Symplasmic fields in the tunica of the shoot apical meristem coordinate morphogenetic events . Development 125 , 1477 - 1485 .
Rinne PL , Welling A , Vahala J , Ripel L , Ruonala R , Kangasjarvi J , van der Schoot C. 2011 . Chilling of dormant buds hyperinduces FLOWERING LOCUS T and recruits GA-inducible 1,3-β-glucanases to reopen signal conduits and release dormancy in Populus . Plant Cell 23 , 130 - 146 .
Roberts AG , Oparka KJ . 2003 . Plasmodesmata and the control of symplastic transport . Plant Cell and Environment 26 , 103 - 124 .
Roschzttardtz H , Seguela-Arnaud M , Briat JF , Vert G , Curie C. 2011 .
The FRD3 citrate effluxer promotes iron nutrition between symplastically disconnected tissues throughout Arabidopsis development . Plant Cell 23 , 2725 - 2737 .
Ruan YL , Llewellyn DJ , Furbank RT . 2001 . The control of singlecelled cotton fiber elongation by developmentally reversible gating of plasmodesmata and coordinated expression of sucrose and K+ transporters and expansin . Plant Cell 13 , 47 - 60 .
Ruan YL , Llewellyn DJ , Furbank RT , Chourey PS. 2005 . The delayed initiation and slow elongation of fuzz-like short fibre cells in relation to altered patterns of sucrose synthase expression and plasmodesmata gating in a lintless mutant of cotton . Journal of Experimental Botany 56 , 977 - 984 .
Ruan YL , Xu SM , White R , Furbank RT . 2004 . Genotypic and developmental evidence for the role of plasmodesmatal regulation in cotton fiber elongation mediated by callose turnover . Plant Physiology 136 , 4104 - 4113 .
Russell SD . 1979 . Fine structure of megagametophyte development in Zea mays . Canadian Journal of Botany 57 .
Sager R , Lee JY . 2012 . To close or not to close: plasmodesmata in defense . Plant Signaling and Behavior 7 , 431 - 436 .
Samardakiewicz S , Krzeslowska M , Bilski H , Bartosiewicz R , Wozny A. 2012 . Is callose a barrier for lead ions entering Lemna minor L. root cells ? Protoplasma 249 , 347 - 351 .
Sanders MA , Salisbury JL . 1989 . Centrin-mediated microtubule severing during flagellar excision in Chlamydomonas reinhardtii . The Journal of Cell Biology 108 , 1751 - 1760 .
Saure MC . 1985 . Dormancy release in deciduous fruit trees . Horticultural Reviews 7 , 239 - 300 .
Saxena IM , Brown RM , Jr. 2000 . Cellulose synthases and related enzymes . Current Opinion in Plant Biology 3 , 523 - 531 .
Schlereth A , Moller B , Liu W , Kientz M , Flipse J , Rademacher EH , Schmid M , Jurgens G , Weijers D. 2010 . MONOPTEROS controls embryonic root initiation by regulating a mobile transcription factor . Nature 464 , 913 - 916 .
Schubert AM , Benedict CR , Berlin JD , Kohel RJ . 1973 . Cotton fiber development-kinetics of cell elongation and secondary wall thickening .
Crop Science 13 , 704 - 709 .
Schulz A. 1994 . Phloem transport and differential unloading in pea seedlings after source and sink manipulations . Planta 192 , 239 - 248 .
Schulz A. 1995 . Plasmodesmal widening accompanies the short-term increase in symplasmic phloem unloading in pea root tips under osmotic stress . Protoplasma 188 , 22 - 37 .
Scott PC , Miller LW , Webster BD , Leopold AC . 1967 . Structural changes during bean leaf abscission . American Journal of Botany 54 , 730 - 734 .
Serrano M , Wang B , Aryal B , Garcion C , Abou-Mansour E , Heck S , Geisler M , Mauch F , Nawrath C , Metraux JP . 2013 . Export of salicylic acid from the chloroplast requires the multidrug and toxin extrusion-like transporter EDS5 . Plant Physiology 162 , 1815 - 1821 .
Sexton R , Roberts JA. 1982 . Cell biology of abscission . Annual Review of Plant Physiology 33 , 133 - 162 .
Shemyakina EA , Solovyev AG , Leonova OG , Popenko VI , Schiemann J , Morozov SY . 2011 . The role of microtubule association in plasmodesmal targeting of potato mop-top virus movement protein TGBp1 . The Open Virology Journal 5 , 1 - 11 .
Sivaguru M , Fujiwara T , Samaj J , Baluska F , Yang Z , Osawa H , Maeda T , Mori T , Volkmann D , Matsumoto H. 2000 . Aluminuminduced 1→3-β-d -glucan inhibits cell-to-cell trafficking of molecules through plasmodesmata. A new mechanism of aluminum toxicity in plants.
Plant Physiology 124 , 991 - 1006 .
Sowinski P , Bilska A , Baranska K , Fronk J , Kobus P. 2007 .
Plasmodesmata density in vascular bundles in leaves of C4 grasses grown at different light conditions in respect to photosynthesis and photosynthate export efficiency . Environmental and Experimental Botany 61 , 74 - 84 .
Sowinski P , Rudzinska-Langwald A , Kobus P. 2003 . Changes in plasmodesmata frequency in vascular bundles of maize seedling leaf induced by growth at sub-optimal temperatures in relation to photosynthesis and assimilate export . Environmental and Experimental Botany 50 , 183 - 196 .
Stadler R , Lauterbach C , Sauer N. 2005 . Cell-to-cell movement of green fluorescent protein reveals post-phloem transport in the outer integument and identifies symplastic domains in Arabidopsis seeds and embryos . Plant Physiology 139 , 701 - 712 .
Steer MW . 1977 . Differentiation of the tapetum in Avena. I. The cell surface . Journal of Cell Science 25 , 125 - 138 .
Stonebloom S , Brunkard JO , Cheung AC , Jiang KN , Feldman L , Zambryski P. 2012 . Redox states of plastids and mitochondria differentially regulate intercellular transport via plasmodesmata . Plant Physiology 158 , 190 - 199 .
Stonebloom S , Burch-Smith T , Kim I , Meinke D , Mindrinos M , Zambryski P. 2009 . Loss of the plant DEAD-box protein ISE1 leads to defective mitochondria and increased cell-to-cell transport via plasmodesmata . Proceedings of the National Academy of Sciences, USA 106 , 17229 - 17234 .
Su S , Liu Z , Chen C , Zhang Y , Wang X , Zhu L , Miao L , Wang XC , Yuan M. 2010 . Cucumber mosaic virus movement protein severs actin filaments to increase the plasmodesmal size exclusion limit in tobacco.
Plant Cell 22 , 1373 - 1387 .
Thijssen MH . 2003 . Ovules, megagametophytes and embryos: Ultrastructural studies after cryofixation . PhD Thesis , Wageningen University.
Thomas CL , Bayer EM , Ritzenthaler C , Fernandez-Calvino L , Maule AJ . 2008 . Specific targeting of a plasmodesmal protein affecting cell-tocell communication . PLoS Biology 6 , e7 .
Tian S , Qin G , Li B. 2013 . Reactive oxygen species involved in regulating fruit senescence and fungal pathogenicity . Plant Molecular Biology 82 , 593 - 602 .
Tilsner J , Linnik O , Wright KM , Bell K , Roberts AG , Lacomme C , Santa Cruz S , Oparka KJ . 2012 . The TGB1 movement protein of Potato virus X reorganizes actin and endomembranes into the X-body, a viral replication factory . Plant Physiology 158 , 1359 - 1370 .
Tucker EB . 1990 . Calcium-loaded 1,2-bis(2-aminophenoxy )ethaneN,N,N',N' -tetraacetic acid blocks cell-to-cell diffusion of carboxyfluorescein in staminal hairs of Setcreasea purpurea . Planta 182 , 34 - 38 .
Tucker EB , Boss WF . 1996 . Mastoparan-induced intracellular Ca2+ fluxes may regulate cell-to-cell communication in plants . Plant Physiology 111 , 459 - 467 .
van Bel AJ , Ehlers K , Knoblauch M. 2002 . Sieve elements caught in the act . Trends Plant Science 7 , 126 - 132 .
Vaten A , Dettmer J , Wu S et al. 2011 . Callose biosynthesis regulates symplastic trafficking during root development . Developmental Cell 21 , 1144 - 1155 .
Vaughn KC . 2003 . Dodder hyphae invade the host: a structural and immunocytochemical characterization . Protoplasma 220 , 189 - 200 .
Verma DP , Hong Z. 2001 . Plant callose synthase complexes . Plant Molecular Biology 47 , 693 - 701 .
Vinckier S , Smets E. 2005 . A histological study of microsporogenesis in Tarenna gracilipes . Grana 44 , 30 - 44 .
Voinnet O , Vain P , Angell S , Baulcombe DC . 1998 . Systemic spread of sequence-specific transgene RNA degradation in plants is initiated by localized introduction of ectopic promoterless DNA . Cell 95 , 177 - 187 .
Wang G , Fiers M. 2010 . CLE peptide signaling during plant development .
Protoplasma 240 , 33 - 43 .
Wang N , Fisher DB . 1994 . The Use of fluorescent tracers to characterize the post-phloem transport pathway in maternal tissues of developing wheat grains . Plant Physiology 104 , 17 - 27 .
Wang X , Sager R , Cui W , Zhang C , Lu H , Lee JY . 2013 . Salicylic acid regulates plasmodesmata closure during innate immune responses in Arabidopsis . Plant Cell 25 , 2315 - 2329 .
Wang XY , Guo GQ , Nie XW , Zheng GC . 1998 . Cytochemical localization of cellulase activity in pollen mother cells of david lily during meiotic prophase i and its relation to secondary formation of plasmodesmata.
Protoplasma 204 , 128 - 138 .
Wang XY , Yu CH , Li X , Wang CY , Zheng GC . 2004 . Ultrastructural aspects and possible origin of cytoplasmic channels providing intercellular connection in vegetative tissues of anthers . Russian Journal of Plant Physiology 51 , 97 - 106 .
Webster BD . 1973 . Ultrastructural studies of abscission in Phaseolus: Ethylene effects on cell walls . American Journal of Botany 60 , 436 - 447 .
Werner D , Gerlitz N , Satdler R. 2011 . A dual switch in phloem unloading during ovule development in Arabidopsis . Protoplasma 248 , 225 - 235 .
White RG , Badelt K , Overall RL , Vesk M. 1994 . Actin associated with plasmodesmata . Protoplasma 180 , 169 - 184 .
Wille AC , Lucas WJ . 1984 . Ultrastructural and histochemical-studies on guard-cells . Planta . 160 , 129 - 142 .
Willemse MTM , van Went JL. 1984 . The female gametophyte . Berlin: Springer-Verlag.
Willmer CM , Sexton R. 1979 . Stomata and plasmodesmata .
Protoplasma 100 , 113 - 124 .
Wright KM , Cowan GH , Lukhovitskaya NI , Tilsner J , Roberts AG , Savenkov EI , Torrance L. 2010 . The N-terminal domain of PMTV TGB1 movement protein is required for nucleolar localization, microtubule association, and long-distance movement . Molecular Plant-Microbe Interactions 23 , 1486 - 1497 .
Wright KM , Wood NT , Roberts AG , Chapman S , Boevink P , Mackenzie KM , Oparka KJ . 2007 . Targeting of TMV movement protein to plasmodesmata requires the actin/ER network: evidence from FRAP .
Traffic 8 , 21 - 31 .
Wu HM , Cheun AY . 2000 . Programmed cell death in plant reproduction .
Plant Molecular Biology 44 , 267 - 281 .
Wu S , Gallagher KL . 2012 . Transcription factors on the move . Current Opinion in Plant Biology 15 , 645 - 651 .
Wu S , Gallagher KL . 2013 . Intact microtubules are required for the intercellular movement of the SHORT-ROOT transcription factor . The Plant Journal 74 , 148 - 159 .
Xie B , Wang X , Zhu M , Zhang Z , Hong Z. 2011 . CalS7 encodes a callose synthase responsible for callose deposition in the phloem . The Plant Journal 65 , 1 - 14 .
Xiong L , Zhu JK . 2002 . Molecular and genetic aspects of plant responses to osmotic stress . Plant, Cell and Environment 25 , 131 - 139 .
Zanewich KP , Rood SB . 1995 . Vernalization and gibberellin physiology of winter canola (endogenous gibberellin (GA) content and metabolism of [3H] GA1 and [3H] GA20 . Plant Physiology 108 , 615 - 621 .
Zavaliev R , Ueki S , Epel BL , Citovsky V. 2011 . Biology of callose (β-1,3-glucan) turnover at plasmodesmata . Protoplasma 248 , 117 - 130 .