The cytoskeleton is disrupted by the bacterial effector HrpZ, but not by the bacterial PAMP flg22, in tobacco BY-2 cells
XinGuan
1
GntherBuchholz
0
PeterNick
1
0
RLP AgroScience/AlPlanta - Institute for Plant Research
, Breitenweg 71, D-67435, Neustadt an der Weinstrae,
Germany
1
Molecular Cell Biology, Botanical Institute, Karlsruhe Institute of Technology
, Kaiserstr. 2, D-76128 Karlsruhe,
Germany
Plant innate immunity is composed of two layers. Basal immunity is triggered by pathogen-associated molecular patterns (PAMPs) such as the flagellin-peptide flg22 and is termed PAMP-triggered immunity (PTI). In addition, effector-triggered immunity (ETI) linked with programmed cell death and cytoskeletal reorganization can be induced by pathogen-derived factors, such as the Harpin proteins originating from phytopathogenic bacteria. To get insight into the link between cytoskeleton and PTI or ETI, this study followed the responses of actin filaments and microtubules to flg22 and HrpZ in vivo by spinning-disc confocal microscopy in GFP-tagged marker lines of tobacco BY-2. At a concentration that clearly impairs mitosis, flg22 can induce only subtle cytoskeletal responses. In contrast, HrpZ causes a rapid and massive bundling of actin microfilaments (completed in ~20 min, i.e. almost simultaneously with extracellular alkalinization), which is followed by progressive disintegration of actin cables and cytoplasmic microtubules, a loss of cytoplasmic structure, and vacuolar disintegration. Cytoskeletal disruption is proposed as an early event that discriminates HrpZ-triggered ETI-like defence from flg22-triggered PTI.
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Plants lack the somatic adaptive immune system based on
mobile defence cells characteristic for animal immunity.
Plant defence, in contrast, is based upon an innate
immunity of each individual cell (Jones and Dangl, 2006). This
innate immunity comprises two distinct layers. The basal
layer is evolutionarily ancient and triggered by conserved
pathogen structures termed pathogen-associated molecular
patterns (PAMPs). These PAMPs, such as flagellin, the
subunit building the filament of bacterial flagellum, bind to
specific receptors in the plasma membrane triggering so-called
PAMP-triggered immunity (PTI). This basal layer of broad
immunity is often accompanied by a more advanced and
strain-specific immunity termed effector-triggered
immunity (ETI), which is triggered by pathogen effectors that
have to enter the cytoplasm of the host cell. The reason for
this complexity is linked to coevolution between host and
pathogen: PTI would be expected to select for pathogens,
where the eliciting PAMPs are lost. However, since PAMPs
are essential for the lifecycle of the pathogen, this
evolutionary strategy does not work a bacterial intruder lacking
the PAMP flagellin would not elicit a PTI response, but it
would also not be able to move. This dilemma stimulated,
during a second round of hostpathogen warfare, the
development of microbial effector proteins. These effectors are
secreted into the cytoplasm of the host and suppress PTI (for
review, see Tsuda and Katagiri, 2010). In response to these
pathogen effectors, the host plant has evolved additional
pathogen-specific receptors (encoded by so-called R genes)
that specifically recognize the effectors in the cytoplasm and
trigger the second layer of immunity, ETI (Boller and He,
2009). In many cases, ETI culminates in a plant-specific
version of programmed cell death, the hypersensitive response,
often followed by systemic acquired resistance of the host.
The conceptual dichotomy between PTI and ETI has been
very valuable to interpret and classify the huge variety of
plant defence responses, but this concept is presently on the
move again. Recent studies show that the difference between
PAMPs and effectors is more gradual than previously
conceived (Thomma etal., 2011). Moreover, PTI and ETI share
numerous common events (Tsuda and Katagiri, 2010). Thus,
the apparent dichotomy might be a question of signal
quantity rather than quality. In addition, plants can discriminate
different pathogens and activate different responses that are
appropriate for the respective pathogen. Therefore, at
present, the PTI-ETI concept is extended towards a
signaturebased model (for review, see Aslam etal., 2009).
The archetypal elicitor of PTI is bacterial flagellin, which
triggers defence responses in various plants (Gmez-Gmez
and Boller, 2002). Asynthetic 22-amino-acid peptide (flg22)
from a conserved flagellin domain is sufficient to induce most
of the cellular responses (Felix etal., 1999). Agenetic screen
in Arabidopsis thaliana using flg22 identified the Arabidopsis
leucine-rich repeat receptor kinase FLS2, which binds flg22
(for review, see Chinchilla etal., 2006). Upon binding of the
ligand, FLS2 is internalized by a receptor-mediated endocytic
process that presumably has regulatory functions (Jones and
Dangl, 2006).
To trigger ETI-like programmed cell death, Harpin
proteins have been used. These bacterial proteins, first discovered
in Erwinia amylovora, a phytopathogenic bacterium causing
t (...truncated)