Symplastic intercellular transport from a developmental perspective
Journal of Experimental Botany
Symplastic intercellular transport from a developmental perspective
Yoselin Benitez-Alfonso 0
0 Centre for Plant Sciences, School of Biology, University of Leeds , Leeds LS2 9JT , UK
Plant cells have channel-like structures named plasmodesmata that allow for the symplastic molecular transport between neighbouring cells. The importance of plasmodesmata in whole plant development is well acknowledged. They mediate the cell-to-cell and vascular loading and unloading of metabolites, proteins, and other signalling molecules. However, it is still not clear how, mechanistically, these channels are regulated in response to developmental and environmental cues. This review aims to bring together knowledge acquired in recent years on plasmodesmata composition, regulation, and function. Progress in the discovery of factors that regulate symplastic transport and plant development in particular are discussed. This will hopefully highlight the challenges faced by the scientific community to unveil the mechanisms controlling symplastic communication during the formation and maintenance of plant meristems.
Intercellular communication; meristem development; plasmodesmata; plasmodesmata proteins; plasmadesmata regulation; symplastic transport
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of complex genetic and proteomic screens in different
laboratoriess (Faulkner and Maule, 2011). New information has also
been gained on the function of PD proteins, intracellular and
extracellular regulators, and of mobile factors in the
regulation of plant growth, organ patterning, and stress response in
model and non-model organisms. This review compiles part
of this information, focusing on recent evidence supporting
or clarifying the role of symplastic transport in the initiation
and maintenance of primary and secondary meristems.
Identification of plasmodesmata proteins
The proteomic analysis of digested cell walls represented
a significant step towards the elucidation of PD molecular
composition (Faulkner and Maule, 2011; Fernandez-Calvino
et al., 2011; Salmon and Bayer, 2012). A number of proteins
identified using this approach have been confirmed to target
PDs in stable transgenic lines. These include
PlasmoDesmataLocated Proteins (PDLPs), PlasmoDesmata Callose Binding
proteins (PDCBs), Glycosyl Hydrolases family 17 (GHL17;
BG), Receptor Like Kinases (RLK), etc. Some of these
proteins (or families of proteins) have been characterized and
their role in PD regulation and plant development has been
studied (Thomas et al., 2008; Simpson et al., 2009;
BenitezAlfonso et al., 2013; Faulkner et al., 2013). So far, no common
molecular signature has been identified among PD proteins
and, therefore, it is difficult to distinguish them (based on
their sequence) from other secreted or plasma
membranetargeted proteins, and/or from other family members playing
redundant and non-redundant functions in the cell.
Genetic and functional redundancy between PD
proteins may have hindered their identification through genetic
screens. Almost simultaneously, two forward genetic
approaches were pursued to isolate PD mutants (Kobayashi
et al., 2007; Benitez-Alfonso et al., 2009; Stonebloom et al.,
2009; Xu et al., 2011, 2012). In both instances, intracellular
regulators rather than PD-linked components were identified
(see below). Similarly, an enhancer genetic screen using as the
starter line the erl1erl2 mutant (defective in stomata
differentiation) identified KOBITO1 as a regulator of PD
permeability in epidermal pavement cells (Kong et al., 2012).
Highlighting the importance of PD in vascular transport,
screening for mutants in phloem transport successfully
identified gain-of-function mutations in the PD protein Callose
Synthase 3 (CALS3). CALS3 acts in the phloem where it
controls symplastic connectivity and vascular development
(Vaten et al., 2011).
Alternative proteomic strategies have enriched the list of
confirmed PD proteins in model and non-model species. For
example, affinity purification of interactors, using as bait the
non-cell autonomous protein CmPP16, identified PD-located
Germin like proteins (Ham et al., 2012). Similar methodology,
in this case using as bait PD-associated calreticulin, revealed
PD-localization for a member of the glycosyltransferase-like
family (Zalepa-King and Citovsky, 2013).
Comparative genomic approaches have been useful for the
identification of PD proteins in other species. Orthologues of
Arabidopsis PD-located β-1,3 glucanases were identified in
poplar. Localization and expression studies of these proteins
revealed a novel role for members of this family in bud
dormancy release (Rinne et al., 2011).
In summary, despite major challenges in the isolation of
PDs from cell walls, the combined use of genetic, proteomic,
and cell biology approaches has led to the ide (...truncated)