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Commentary
Gated communities: apoplastic
and symplastic signals converge at
plasmodesmata to control cell fates
Yvonne Stahl* and Rüdiger Simon*
Institute of Developmental Genetics, Heinrich Heine University,
D-40225 Düsseldorf, Germany
* To whom correspondence should be addressed.
E-mail: [email protected] or [email protected]
Journal of Experimental Botany, Vol. 64, No. 17, pp. 5237–5241, 2013
doi:10.1093/jxb/ert245
Received 7 June 2013; Revised 7 June 2013; Accepted 2 July 2013
Advance Access publication 24 August, 2013
Abstract
Due to their rigid cell walls, plant cells can only communicate with each other either by symplastic transport
of diverse non-cell autonomous signalling molecules
via plasmodesmata (PDs) or by endo- and exocytosis
of signalling molecules via the extracellular apoplastic
space. PDs are plasma membrane-lined channels spanning the cell wall between neighbouring cells, allowing
the exchange of molecules by symplastic movement
through them. This review focuses on developmental
decisions that are coordinated by short- and long-distance communication of cells via PDs. We propose a
model combining both apoplastic and symplastic signalling events via secreted ligands and their PD-localized
receptor kinases which gate the symplastic transport of
information molecules through PDs. Cell communities
can thus coordinate cell-fate decisions non-cell autonomously by connecting or disconnecting symplastic subdomains. Here we concentrate on the establishment of
such subdomains in the plant’s primary meristems that
serve to maintain long-lasting stem cell populations in
the shoot and root apical meristems, and discuss how
apoplastic signalling via transport of information molecules through PDs is integrated with symplastic feedback signalling events.
Key words: Apoplast, peptide ligands, plasmodesmata,
receptors, signalling, symplast.
Introduction
Plant cells, in contrast to animal cells, possess a rigid cell
wall and remain fixed at a given position after cell division.
Therefore they are highly dependent on positional cues to
determine their identity. Laser ablation experiments in shoot
and root meristems have elegantly proved the dependency
of plant cells on positional cues and that cell identities are
reversible (van den Berg et al., 1995, 1997; Reinhardt et al.,
2003). Therefore, intercellular signalling processes are needed
to exchange information between cells and are achieved either
by apoplastic signalling events which involve ligands and
plasma membrane-localized receptors or by symplastic information molecules moving through PDs.
PDs form narrow channels that interconnect plant cells
with each other in order to allow intercellular movement of
nutrients, but also signalling molecules in different symplastic
subdomains (Epel, 1994; Rinne and van der Schoot, 1998).
Primary PDs are already formed during cell divisions in the
newly forming cell plate, whereas secondary PDs are formed in
already established cell walls in localized breaks. Both classes
of PDs are plasma membrane-lined channels through the cell
walls of neighbouring cells building a central symplastically
connected channel between two adjacent cells. They contain
a membranous tube, the desmotubule, connected to the endoplasmic reticulum of both cells (Fig. 1). Both primary and
secondary PDs can form higher-order structures by branching and generally allow for transport of transcription factors,
metabolites, and even small RNAs needed for the necessary
communication for developmental decisions, plant defence
upon virus infection, and assimilate transport.
Mobile signals can theoretically pass through PDs in four
different ways: (i) by travelling through the cytoplasmic
sleeve; (ii) by entering the lumen of the endoplasmic reticulum-derived desmotubule; (iii) by lateral diffusion in the
plasma membrane that lines the PD; or (iv) within the membrane of the desmotubule (Wu and Gallagher, 2012) (Fig. 1).
Although PDs form pores between cells across the rigid
physical barrier of the cell wall, their potential for letting
molecules of different sizes pass is dynamic and dependent on the production and/or degradation of callose (β-1,3glucan) in the neck region of PDs. PD pores have a so-called
size exclusion limit, allowing passage only of those molecules
whose size falls below that limit (Crawford and Zambryski,
2000). The activity of β-1,3-glucan synthases and glucanases
regulates callose deposition and degradation and thereby
affects the symplastic connectivity between plant cells.
Furthermore, redox-potential and callose-binding proteins
© The Author 2013. Published by Oxford University Press on behalf of the Society for Experimental Biology. All rights reserved.
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5238 | Stahl and Simon
Fig. 1. Different ways of molecule movement through
plasmodesmata (PDs). Mobile molecules can pass through PDs
by (i) travelling through the cytoplasmic sleeve (red pathway),
(ii) entering the lumen of the desmotubule (blue pathway),
(iii) lateral diffusion in the plasma membrane that lines the PD
(green pathway) or (iv) lateral diffusion in the membrane of the
desmotubule (yellow pathway).
have been proposed to play a role in the PD callose modulation (Benitez-Alfonso et al., 2009; Simpson et al., 2009).
However, it is still largely unknown which mechanisms and
players serve to regulate traffic at PDs and select specific molecules for passage, while others are excluded from access to
adjacent cells.
Regulation of meristematic fate decisions
by symplastic movement of transcription
factors and RNA through PDs
Plant transcription factors (TFs) have been reported to move
between cells through PDs, and one of the first described
cases is the TF KNOTTED1 (KN1). KN1 RNA is synthesized in the L2 layer of the shoot apical meristem (SAM) in
maize, but the KN1 protein can be found also in the epidermal L1 layer (for SAM layer organization, see Fig. 2). It was
shown that KN1 can interact with PDs to increase their size
exclusion limit and mediate its own cell-to-cell transport.
Furthermore, it was shown that KN1 can also transport its
own sense RNA from cell to cell (Lucas et al., 1995). This
report suggested for the first time that plant development
and cell fate specification exploit intracellular communication by protein and RNA movement through PDs. Later it
was shown that for KN1 movement, a chaperonin complex
unfolding KN1 while passing through PDs is needed, giving first insights into the mechanism of protein movement
through PDs (Xu et al., 2011).
Fig. 2. WUS movement in the shoot apical meristem. Red
shading indicates expression of CLV3 (in the central zone); blue
shading indicates expression of WUS (in the organizing centre);
blue dots indicate WUS protein and its movement (blue arrow).
The red blocked line indicates negative regulation by CLV3 on
WUS expression. L1, L2, L3 = cell layers 1, 2, 3.
Another example of a mobile TF is the homeodomain TF
WUSCHEL (WUS), which is expressed in the organizing
centre of the SAM and maintains the overlying stem cells in
the central zone (Fig. 2). WUS, after being transcribed in the
organizing centre cells, migrates most likely through PDs to the
central zone cells and activates its negative regulator, the stem
cell-expressed small signalling peptide CLAVATA3 (CLV3)
(Yadav et al., 2011). Through this feedback mechanism, stem
cell homeostasis in the shoot is maintained. However, the cues
that guide the mobile WUS protein preferentially towards
the L1 and L2 layers of the SAM are not known. One could
hypothesize that the abundance of WUS protein, regulated by
the negative feedback with CLV3, and the direction of WUS
protein movement, regulated by PD apertures and selectivity,
can lead to the formation of symplastic subdomains within
the SAM, which are necessary to regulate meristem size and
stem cell maintenance (Fig. 2).
The TF SHORTROOT is expressed in the stele cells of the
root and migrates to the immediately adjacent cell layer (the
endodermis, the cortex/endodermis initial, or the quiescent
centre. Here, it controls endodermis and stem cell maintenance together with the TF SCARECROW (Helariutta et al.,
2000; Nakajima et al., 2001). The symplastic movement of
the TF SHORTROOT from the stele to the endodermis and
of microRNA165 in the opposite direction is PD dependent
and can be regulated by CALLOSE SYNTHASE3, a bona
fide callose synthase which belongs to a small gene family
that can affect the size exclusion limit of PDs and therefore
molecular trafficking between cells when misexpressed (Vatén
et al., 2011).
The given examples all describe PD-dependent movement
of TFs and RNA over a short distance to locally affect developmental decisions. However, there is also long-range movement of TFs, a prominent example being FLOWERING
LOCUS T (FT). FT movement is a key regulator for the
Coordination of developmental decisions at plasmodesmata | 5239
transition from vegetative to reproductive growth in plants.
FT is produced in the phloem companion cells of leaves
and cotyledons (Kotake et al., 2003) and moves to the SAM
where it promotes the transition to flowering by interaction
with the FLOWERING LOCUS D protein to ultimately
promote the expression of LEAFY (Corbesier et al., 2007;
Jaeger and Wigge, 2007; Mathieu et al., 2007). Transport of
FT requires the activity of FT INTERACTING PROTEIN1
(FTIP1) to load FT from companion cells into sieve elements.
FTIP1 localizes to the endoplasmic reticulum and PDs and is
a putative membrane spanning protein that directly interacts
with FT (Liu et al., 2012). It is assumed that FTIP1 associates
with the desmotubule and by physical interaction guides FT
through the lumen of the desmotubule or through the cytoplasmic sleeve of the PD (Liu et al., 2012).
Thus, the spectrum of different TFs and other information molecules found to move through PDs is expanding, but
how short- and long-distance trafficking of these molecules is
regulated at the site of PDs is not yet understood.
How is PD trafficking regulated?
It is known for many years that plant viruses use movement
proteins for spreading viral nucleic acids throughout the
plant. Here, movement proteins build complexes with the
viral nucleic acids and by interaction with endogenous proteins can increase the size exclusion limit of PDs to facilitate
the spread of the infection from cell to cell. The transport of
movement proteins from cell to cell via PDs can be regulated
by PD-associated protein kinases, which change the phosphorylation status of the movement proteins and thereby
limit/allow for passage through PDs (Waigmann et al., 2000;
Lee and Lucas, 2001).
Recently, receptor kinases localized at PDs have been
shown to regulate stem cell maintenance in the root meristem.
Although the architecture of the root stem cell niche is clearly
distinct from the SAM, they share similar regulatory modules
for their maintenance. In the root meristem, the four cells of
the quiescent centre maintain the adjacent single layer of dedicated stem cells giving rise to all the different cell layers of the
root. The quiescent centre cells express the WUS homologue
WUSCHEL RELATED HOMEOBOX5 (WOX5), which
maintains the surrounding stem cells (Doerner, 1998; Sarkar
et al., 2007). The fate of columella stem cells (CSCs), which
form cell layers immediately distal to the quiescent centre, are
regulated by a feedback mechanism, similar to the CLV pathway operating in the SAM. Here, the CLV3 homologue CLE40
(CLAVATA3/EMBRYO SURROUNDING REGION40)
acts as a signal from differentiated descendants, the columella
cells (CCs), to restrict WOX5 expression in the quiescent centre (Stahl et al., 2009) (Fig. 3). The CLE40 signal is perceived
by the receptor kinases ARABIDOPSIS CRINKLY4 (ACR4)
and CLAVATA1 (CLV1), a key receptor kinase in SAM maintenance, and acts to restrict CSC fate in the Arabidopsis root
meristem. Both CLV1 and ACR4 overlap in their expression
domains in the distal root meristem where they localize to
the plasma membrane and PDs. Here, ACR4 preferentially
Fig. 3. Distal stem cell regulation in the root meristem. Red
shading indicates expression of CLE40 (in differentiated
columella cells); blue shading indicates expression of WOX5 (in
the quiescent centre). The red blocked line indicates negative
regulation by CLE40 on WOX5 expression.
accumulates (Stahl et al., 2013). ACR4 and CLV1 interact
directly and can assemble into complexes that differ in their
composition and their subcellular localization. Importantly,
PD-localized complexes appear to comprise more ACR4 molecules than those at the plasma membrane (Stahl et al., 2013).
We now introduce a new conceptual model that indicates
how these receptors integrate apoplastic signalling of the
secreted peptide CLE40 with symplastic signalling through
PDs. We propose that a yet-unknown mobile stemness factor (e.g. a transcription factor or another mobile information
molecule) can move from the quiescent centre cells through
PDs to the next cell layers. This movement could be controlled by the PD-localized receptor kinase complexes consisting of ACR4 and CLV1. At the cell wall shared between
the quiescent centre and CSCs, only few or no CLE40 peptide is available from the apoplastic space to bind to and activate the receptor complexes; therefore, the stemness factor
remains unphosphorylated and can move further to the CSCs
to maintain their stem cell character. CLE40 concentration in
the apoplast is higher between CSC and CCs, and the ACR4/
CLV1 complexes are activated by the CLE40 ligand. Thus,
the stemness factor will be phosphorylated when it diffuses
towards the PDs either by direct interaction with the receptor kinase domains or by other downstream signalling components, restricting its further movement towards the CCs.
Lacking a sufficient amount of stemness factor, these cells will
now differentiate (Fig. 4). Noteworthy, a somewhat similar
mechanism that allows controlling movement of information
molecules via selective phosphorylation has been described
for viral infections. Here, phosphorylation of movement
5240 | Stahl and Simon
pathways to achieve this: either apoplastic and/or symplastic
signalling. PDs play a major role in allowing and regulating
transport of information molecules that serve to establish
functional cell communities with different developmental
fates. In this review, we have proposed a novel conceptual
model integrating both ways of communication, by focusing
on the example of distal root stem cell maintenance. Here,
PD-localized receptor kinases could serve to bridge the gap
between apoplastic signalling molecules and mobile information molecules, thereby establishing the necessary symplastic
subdomains. In contrast to the classical peptide-triggered signalling pathway controlling transcription of target genes, we
propose here that stem cell signalling in the root depends on
the differential mobility of stemness factors. Future research
on PD-localized receptor kinases and the identification of
mobile targets will allow the addition of more detail to this
only just emerging picture.
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Fig. 4. Model of distal stem cell regulation in the root meristem
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