The Role of Plasmodesmata-Associated Receptor in Plant Development and Environmental Response

Over the last decade, plasmodesmata (PD) symplasmic nano-channels were reported to be involved in various cell biology activities to prop up within plant growth and development as well as environmental stresses. Indeed, this is highly influenced by their native structure, which is lined with the plasma membrane (PM), conferring a suitable biological landscape for numerous plant receptors that correspond to signaling pathways. However, there are more than six hundred members of Arabidopsis thaliana membrane-localized receptors and over one thousand receptors in rice have been identified, many of which are likely to respond to the external stimuli. This review focuses on the class of plasmodesmal-receptor like proteins (PD-RLPs)/plasmodesmal-receptor-like kinases (PD-RLKs) found in planta. We summarize and discuss the current knowledge regarding RLPs/RLKs that reside at PD-PM channels in response to plant growth, development, and stress adaptation.

On the road to C4 rice: advances and perspectives

The international C4 rice consortium aims to introduce into rice a high capacity photosynthetic mechanism, the C4 pathway, to increase yield. The C4 pathway is characterised by a complex combination of biochemical and anatomical specialisation that ensures high CO2 partial pressure at RuBisCO sites in bundle sheath (BS) cells. Here we report an update of the progress of the C4 rice project. Since its inception in 2008 there has been an exponential growth in synthetic biology and molecular tools. Golden Gate cloning and synthetic promoter systems have facilitated gene building block approaches allowing multiple enzymes and metabolite transporters to be assembled and expressed from single gene constructs. Photosynthetic functionalisation of the BS in rice remains an important step and there has been some success overexpressing transcription factors in the cytokinin signalling network which influence chloroplast volume. The C4 rice project has rejuvenated the research interest in C4 photosynthesis. Comparative anatomical studies now point to critical features essential for the design. So far little attention has been paid to the energetics. C4 photosynthesis has a greater ATP requirement, which is met by increased cyclic electron transport in BS cells. We hypothesise that changes in energy statues may drive this increased capacity for cyclic electron flow without the need for further modification. Although increasing vein density will ultimately be necessary for high efficiency C4 rice, our modelling shows that small amounts of C4 photosynthesis introduced around existing veins could already provide benefits of increased photosynthesis on the road to C4 rice.

Interplay between turgor pressure and plasmodesmata during plant development

Plasmodesmata traverse cell walls, generating connections between neighboring cells. They allow intercellular movement of molecules such as transcription factors, hormones, and sugars, and thus create a symplasmic continuity within a tissue. One important factor that determines plasmodesmal permeability is their aperture, which is regulated during developmental and physiological processes. Regulation of aperture has been shown to affect developmental events such as vascular differentiation in the root, initiation of lateral roots, or transition to flowering. Extensive research has unraveled molecular factors involved in the regulation of plasmodesmal permeability. Nevertheless, many plant developmental processes appear to involve feedbacks mediated by mechanical forces, raising the question of whether mechanical forces and plasmodesmal permeability affect each other. Here, we review experimental data on how one of these forces, turgor pressure, and plasmodesmal permeability may mutually influence each other during plant development, and we discuss the questions raised by these data. Addressing such questions will improve our knowledge of how cellular patterns emerge during development, shedding light on the evolution of complex multicellular plants.

Phytosphinganine Affects Plasmodesmata Permeability via Facilitating PDLP5-Stimulated Callose Accumulation in Arabidopsis

Plant plasmodesmata (PDs) are specialized channels that enable communication between neighboring cells. The intercellular permeability of PDs, which affects plant development, defense, and responses to stimuli, must be tightly regulated. However, the lipid compositions of PD membrane and their impact on PD permeability remain elusive. Here, we report that the Arabidopsis sld1 sld2 double mutant, lacking sphingolipid long-chain base 8 desaturases 1 and 2, displayed decreased PD permeability due to a significant increase in callose accumulation. PD-located protein 5 (PDLP5) was significantly enriched in the leaf epidermal cells of sld1 sld2 and showed specific binding affinity to phytosphinganine (t18:0), suggesting that the enrichment of t18:0-based sphingolipids in sld1 sld2 PDs might facilitate the recruitment of PDLP5 proteins to PDs. The sld1 sld2 double mutant seedlings showed enhanced resistance to the fungal-wilt pathogen Verticillium dahlia and the bacterium Pseudomonas syringae pv. tomato DC3000, which could be fully rescued in sld1 sld2 pdlp5 triple mutant. Taken together, these results indicate that phytosphinganine might regulate PD functions and cell-to-cell communication by modifying the level of PDLP5 in PD membranes.

Creating Contacts Between Replication and Movement at Plasmodesmata - A Role for Membrane Contact Sites in Plant Virus Infections?

To infect their hosts and cause disease, plant viruses must replicate within cells and move throughout the plant both locally and systemically. RNA virus replication occurs on the surface of various cellular membranes, whose shape and composition become extensively modified in the process. Membrane contact sites (MCS) can mediate non-vesicular lipid-shuttling between different membranes and viruses co-opt components of these structures to make their membrane environment suitable for replication. Whereas animal viruses exit and enter cells when moving throughout their host, the rigid wall of plant cells obstructs this pathway and plant viruses therefore move between cells symplastically through plasmodesmata (PD). PD are membranous channels connecting nearly all plant cells and are now viewed to constitute a specialized type of endoplasmic reticulum (ER)-plasma membrane (PM) MCS themselves. Thus, both replication and movement of plant viruses rely on MCS. However, recent work also suggests that for some viruses, replication and movement are closely coupled at ER-PM MCS at the entrances of PD. Movement-coupled replication at PD may be distinct from the main bulk of replication and virus accumulation, which produces progeny virions for plant-to-plant transmission. Thus, MCS play a central role in plant virus infections, and may provide a link between two essential steps in the viral life cycle, replication and movement. Here, we provide an overview of plant virus-MCS interactions identified to date, and place these in the context of the connection between viral replication and cell-to-cell movement.

From plasmodesma geometry to effective symplasmic permeability through biophysical modelling

Regulation of molecular transport via intercellular channels called plasmodesmata (PDs) is important for both coordinating developmental and environmental responses among neighbouring cells, and isolating (groups of) cells to execute distinct programs. Cell-to-cell mobility of fluorescent molecules and PD dimensions (measured from electron micrographs) are both used as methods to predict PD transport capacity (i.e., effective symplasmic permeability), but often yield very different values. Here, we build a theoretical bridge between both experimental approaches by calculating the effective symplasmic permeability from a geometrical description of individual PDs and considering the flow towards them. We find that a dilated central region has the strongest impact in thick cell walls and that clustering of PDs into pit fields strongly reduces predicted permeabilities. Moreover, our open source multi-level model allows to predict PD dimensions matching measured permeabilities and add a functional interpretation to structural differences observed between PDs in different cell walls.

Maize Carbohydrate Partitioning Defective33 Encodes an MCTP Protein and Functions in Sucrose Export from Leaves

To sustain plant growth, development, and crop yield, sucrose must be transported from leaves to distant parts of the plant, such as seeds and roots. To identify genes that regulate sucrose accumulation and transport in maize (Zea mays), we isolated carbohydrate partitioning defective33 (cpd33), a recessive mutant that accumulated excess starch and soluble sugars in mature leaves. The cpd33 mutants also exhibited chlorosis in the leaf blades, greatly diminished plant growth, and reduced fertility. Cpd33 encodes a protein containing multiple C2 domains and transmembrane regions. Subcellular localization experiments showed the CPD33 protein localized to plasmodesmata (PD), the plasma membrane, and the endoplasmic reticulum. We also found that a loss-of-function mutant of the CPD33 homolog in Arabidopsis, QUIRKY, had a similar carbohydrate hyperaccumulation phenotype. Radioactively labeled sucrose transport assays showed that sucrose export was significantly lower in cpd33 mutant leaves relative to wild-type leaves. However, PD transport in the adaxial-abaxial direction was unaffected in cpd33 mutant leaves. Intriguingly, transmission electron microscopy revealed fewer PD at the companion cell-sieve element interface in mutant phloem tissue, providing a possible explanation for the reduced sucrose export in mutant leaves. Collectively, our results suggest that CPD33 functions to promote symplastic transport into sieve elements.

Arabidopsis glucosinolate storage cells transform into phloem fibres at late stages of development

The phloem cap of Arabidopsis thaliana accumulates glucosinolates that yield toxic catabolites upon damage-induced hydrolysis. These defence compounds are stored in high concentrations in millimetre long S-cells. At early stages of development, S-cells initiate a process indicative of programmed cell death. How these cells are maintained in a highly turgescent state following this process is currently unknown. Here, we show that S-cells undergo substantial morphological changes during early differentiation. Vacuolar collapse and rapid clearance of the cytoplasm did not occur until senescence. Instead, smooth endoplasmic reticulum, Golgi bodies, vacuoles, and undifferentiated plastids were observed. Lack of chloroplasts indicates that S-cells depend on metabolite supply from neighbouring cells. Interestingly, TEM revealed numerous plasmodesmata between S-cells and neighbouring cells. Photoactivation of a symplasmic tracer showed coupling with neighbouring cells that are involved in glucosinolate synthesis. Hence, symplasmic transport might contribute to glucosinolate storage in S-cells. To investigate the fate of S-cells, we traced them in flower stalks from the earliest detectable stages to senescence. At late stages, S-cells were shown to deposit thick secondary cell walls and transform into phloem fibres. Thus, phloem fibres in the herbaceous plant Arabidopsis pass a pronounced phase of chemical defence during early stages of development.

Multiple C2 domains and transmembrane region proteins (MCTPs) tether membranes at plasmodesmata

In eukaryotes, membrane contact sites (MCS) allow direct communication between organelles. Plants have evolved a unique type of MCS, inside intercellular pores, the plasmodesmata, where endoplasmic reticulum (ER)-plasma membrane (PM) contacts coincide with regulation of cell-to-cell signalling. The molecular mechanism and function of membrane tethering within plasmodesmata remain unknown. Here, we show that the multiple C2 domains and transmembrane region protein (MCTP) family, key regulators of cell-to-cell signalling in plants, act as ER-PM tethers specifically at plasmodesmata. We report that MCTPs are plasmodesmata proteins that insert into the ER via their transmembrane region while their C2 domains dock to the PM through interaction with anionic phospholipids. A Atmctp3/Atmctp4 loss of function mutant induces plant developmental defects, impaired plasmodesmata function and composition, while MCTP4 expression in a yeast Δtether mutant partially restores ER-PM tethering. Our data suggest that MCTPs are unique membrane tethers controlling both ER-PM contacts and cell-to-cell signalling.

m5C Methylation Guides Systemic Transport of Messenger RNA over Graft Junctions in Plants

In plants, transcripts move to distant body parts to potentially act as systemic signals regulating development and growth. Thousands of messenger RNAs (mRNAs) are transported across graft junctions via the phloem to distinct plant parts. Little is known regarding features, structural motifs, and potential base modifications of transported transcripts and how these may affect their mobility. We identified Arabidopsis thaliana mRNAs harboring the modified base 5-methylcytosine (m⁵C) and found that these are significantly enriched in mRNAs previously described as mobile, moving over graft junctions to distinct plant parts. We confirm this finding with graft-mobile methylated mRNAs TRANSLATIONALLY CONTROLLED TUMOR PROTEIN 1 (TCTP1) and HEAT SHOCK COGNATE PROTEIN 70.1 (HSC70.1), whose mRNA transport is diminished in mutants deficient in m⁵C mRNA methylation. Together, our results point toward an essential role of cytosine methylation in systemic mRNA mobility in plants and that TCTP1 mRNA mobility is required for its signaling function.

Plasmodesmata-Related Structural and Functional Proteins: The Long Sought-After Secrets of a Cytoplasmic Channel in Plant Cell Walls

Plant cells are separated by cellulose cell walls that impede direct cell-to-cell contact. In order to facilitate intercellular communication, plant cells develop unique cell-wall-spanning structures termed plasmodesmata (PD). PD are membranous channels that link the cytoplasm, plasma membranes, and endoplasmic reticulum of adjacent cells to provide cytoplasmic and membrane continuity for molecular trafficking. PD play important roles for the development and physiology of all plants. The structure and function of PD in the plant cell walls are highly dynamic and tightly regulated. Despite their importance, plasmodesmata are among the few plant cell organelles that remain poorly understood. The molecular properties of PD seem largely elusive or speculative. In this review, we firstly describe the general PD structure and its protein composition. We then discuss the recent progress in identification and characterization of PD-associated plant cell-wall proteins that regulate PD function, with particular emphasis on callose metabolizing and binding proteins, and protein kinases targeted to and around PD.

Control of Meristem Size

A fascinating feature of plant growth and development is that plants initiate organs continually throughout their lifespan. The ability to do this relies on specialized groups of pluripotent stem cells termed meristems, which allow for the elaboration of the shoot, root, and vascular systems. We now have a deep understanding of the genetic networks that control meristem initiation and stem cell maintenance, including the roles of receptors and their ligands, transcription factors, and integrated hormonal and chromatin control. This review describes these networks and discusses how this knowledge is being applied to improve crop productivity by increasing fruit size and seed number.

Lipids or Proteins: Who Is Leading the Dance at Membrane Contact Sites?

Understanding the mode of action of membrane contact sites (MCSs) across eukaryotic organisms at the near-atomic level to infer function at the cellular and tissue levels is a challenge scientists are currently facing. These peculiar systems dedicated to inter-organellar communication are perfect examples of cellular processes where the interplay between lipids and proteins is critical. In this mini review, we underline the link between membrane lipid environment, the recruitment of proteins at specialized membrane domains and the function of MCSs. More precisely, we want to give insights on the crucial role of lipids in defining the specificity of plant endoplasmic reticulum (ER)-plasma membrane (PM) MCSs and we further propose approaches to study them at multiple scales. Our goal is not so much to go into detailed description of MCSs, as there are numerous focused reviews on the subject, but rather try to pinpoint the critical elements defining those structures and give an original point of view by considering the subject from a near-atomic angle with a focus on lipids. We review current knowledge as to how lipids can define MCS territories, play a role in the recruitment and function of the MCS-associated proteins and in turn, how the lipid environment can be modified by proteins.

Sieve Plate Pores in the Phloem and the Unknowns of Their Formation

Sieve pores of the sieve plates connect neighboring sieve elements to form the conducting sieve tubes of the phloem. Sieve pores are critical for phloem function. From the 1950s onwards, when electron microscopes became increasingly available, the study of their formation had been a pillar of phloem research. More recent work on sieve elements instead has largely focused on sieve tube hydraulics, phylogeny, and eco-physiology. Additionally, advanced molecular and genetic tools available for the model species Arabidopsis thaliana helped decipher several key regulatory mechanisms of early phloem development. Yet, the downstream differentiation processes which form the conductive sieve tube are still largely unknown, and our understanding of sieve pore formation has only moderately progressed. Here, we summarize our current knowledge on sieve pore formation and present relevant recent advances in related fields such as sieve element evolution, physiology, and plasmodesmata formation.

The essential chloroplast ribosomal protein uL15c interacts with the chloroplast RNA helicase ISE2 and affects intercellular trafficking through plasmodesmata

Chloroplasts retain part of their ancestral genomes and the machinery for expression of those genomes. The nucleus-encoded chloroplast RNA helicase INCREASED SIZE EXCLUSION LIMIT2 (ISE2) is required for chloroplast ribosomal RNA processing and chloro-ribosome assembly. To further elucidate ISE2's role in chloroplast translation, two independent approaches were used to identify its potential protein partners. Both a yeast two-hybrid screen and a pull-down assay identified plastid ribosomal protein L15, uL15c (formerly RPL15), as interacting with ISE2. The interaction was confirmed in vivo by co-immunoprecipitation. Interestingly, we found that rpl15 null mutants do not complete embryogenesis, indicating that RPL15 is an essential gene for autotrophic growth of Arabidopsis thaliana. Arabidopsis and Nicotiana benthamiana plants with reduced expression of RPL15 developed chlorotic leaves, had reduced photosynthetic capacity and exhibited defective chloroplast development. Processing of chloroplast ribosomal RNAs and assembly of ribosomal subunits were disrupted by reduced expression of RPL15. Chloroplast translation was also decreased, reducing accumulation of chloroplast-encoded proteins, in such plants compared to wild-type plants. Notably, knockdown of RPL15 expression increased intercellular trafficking, a phenotype also observed in plants with reduced ISE2 expression. This finding provides further evidence for chloroplast function in modulating intercellular trafficking via plasmodesmata.

A Tree Ortholog of SHORT VEGETATIVE PHASE Floral Repressor Mediates Photoperiodic Control of Bud Dormancy

Perennials in boreal and temperate ecosystems display seasonally synchronized growth. In many tree species, prior to the advent of winter, exposure to photoperiods shorter than a critical threshold for growth (short days; SDs) induces growth cessation, culminating in the formation of an apical bud that encloses the shoot apical meristem and arrested leaf primordia [1-4]. Following growth cessation, subsequent exposure to SDs induces transition to dormancy in the shoot apex [5]. Establishment of dormancy is crucial for winter survival and is characterized by the inability of the shoot meristem to respond to growth-promotive signals [6]. Recently, SDs were shown to induce bud dormancy by activating the abscisic acid (ABA) pathway. ABA upregulates expression of CALLOSE SYNTHASE 1 (CALS1) and suppresses glucanases that break down callose to induce the blockage of intracellular conduits (plasmodesmata; PDs) with callosic plugs called "dormancy sphincters" that by restricting access to growth-promotive signals promote dormancy [7]. However, components downstream of ABA in dormancy regulation remain largely unknown, and thus there are significant gaps in our understanding of photoperiodic control of bud dormancy. Here we demonstrate that SVL, orthologous to Arabidopsis floral repressor SHORT VEGETATIVE PHASE (SVP), is a mediator of photoperiodic control of dormancy downstream of the ABA pathway in hybrid aspen. SVL downregulation impairs dormancy, whereas SVL overexpression suppresses dormancy defects resulting from ABA insensitivity. Downstream, SVL induces callose synthase expression and negatively regulates the gibberellic acid (GA) pathway to promote dormancy, thus revealing the regulatory module mediating photoperiodic control of dormancy by ABA.

Citrus Psorosis Virus Movement Protein Contains an Aspartic Protease Required for Autocleavage and the Formation of Tubule-Like Structures at Plasmodesmata

Plant virus cell-to-cell movement is an essential step in viral infections. This process is facilitated by specific virus-encoded movement proteins (MPs), which manipulate the cell wall channels between neighboring cells known as plasmodesmata (PD). Citrus psorosis virus (CPsV) infection in sweet orange involves the formation of tubule-like structures within PD, suggesting that CPsV belongs to "tubule-forming" viruses that encode MPs able to assemble a hollow tubule extending between cells to allow virus movement. Consistent with this hypothesis, we show that the MP of CPsV (MPCPsV) indeed forms tubule-like structures at PD upon transient expression in Nicotiana benthamiana leaves. Tubule formation by MPCPsV depends on its cleavage capacity, mediated by a specific aspartic protease motif present in its primary sequence. A single amino acid mutation in this motif abolishes MPCPsV cleavage, alters the subcellular localization of the protein, and negatively affects its activity in facilitating virus movement. The amino-terminal 34-kDa cleavage product (34KCPsV), but not the 20-kDa fragment (20KCPsV), supports virus movement. Moreover, similar to tubule-forming MPs of other viruses, MPCPsV (and also the 34KCPsV cleavage product) can homooligomerize, interact with PD-located protein 1 (PDLP1), and assemble tubule-like structures at PD by a mechanism dependent on the secretory pathway. 20KCPsV retains the protease activity and is able to cleave a cleavage-deficient MPCPsV in trans Altogether, these results demonstrate that CPsV movement depends on the autolytic cleavage of MPCPsV by an aspartic protease activity, which removes the 20KCPsV protease and thereby releases the 34KCPsV protein for PDLP1-dependent tubule formation at PD.IMPORTANCE Infection by citrus psorosis virus (CPsV) involves a self-cleaving aspartic protease activity within the viral movement protein (MP), which results in the production of two peptides, termed 34KCPsV and 20KCPsV, that carry the MP and viral protease activities, respectively. The underlying protease motif within the MP is also found in the MPs of other members of the Aspiviridae family, suggesting that protease-mediated protein processing represents a conserved mechanism of protein expression in this virus family. The results also demonstrate that CPsV and potentially other ophioviruses move by a tubule-guided mechanism. Although several viruses from different genera were shown to use this mechanism for cell-to-cell movement, our results also demonstrate that this mechanism is controlled by posttranslational protein cleavage. Moreover, given that tubule formation and virus movement could be inhibited by a mutation in the protease motif, targeting the protease activity for inactivation could represent an important approach for ophiovirus control.

Evolution of root apical meristem structures in vascular plants: plasmodesmatal networks

PREMISE OF THE STUDY

The apical meristem generates indeterminate apical growth of the stem and root of vascular plants. Our previous examination showed that shoot apical meristems (SAMs) can be classified into two types based on plasmodesmatal networks (PNs), which are important elements in symplasmic signaling pathways within the apical meristem. Here, we examined the PNs of root apical meristems (RAMs) in comparison with those of SAMs.

METHODS

Root apical meristems of 18 families and 22 species of lycophytes and euphyllophytes were analyzed. Plasmodesmata (PD) in cell walls in median longitudinal sections of RAMs were enumerated using transmission electron micrographs, and the PD density per 1 μm² of each cell wall was calculated.

KEY RESULTS

Root apical meristems with prominent apical cells of monilophytes (euphyllophytes) and Selaginellaceae (lycophytes) had high PD densities, while RAMs with plural initial cells of gymnosperms and angiosperms (euphyllophytes), and of Lycopodiaceae and Isoetaceae (lycophytes) had low PD densities. This correlation between structures of apical meristems and PD densities is identical to that in SAMs already described.

CONCLUSIONS

Irrespective of their diversified structures, the RAMs of vascular plants can be classified into two types with respect to PNs: the fern (monilophyte) type, which has a lineage-specific PN with only primary PD, and the seed-plant type, which has an interspecific PN with secondary PD in addition to primary PD. PNs may have played a key role in the evolution of apical meristems in vascular plants.

Ultrastructural Analysis of Prune DwarfVirus Intercellular Transport and Pathogenesis

Prune dwarf virus (PDV) is an important viral pathogen of plum, sweet cherry, peach, and many herbaceous test plants. Although PDV has been intensively investigated, mainly in the context of phylogenetic relationship of its genes and proteins, many gaps exist in our knowledge about the mechanism of intercellular transport of this virus. The aim of this work was to investigate alterations in cellular organelles and the cell-to-cell transport of PDV in Cucumis sativus cv. Polan at ultrastructural level. To analyze the role of viral proteins in local transport, double-immunogold assays were applied to localize PDV coat protein (CP) and movement protein (MP). We observe structural changes in chloroplasts, mitochondria, and cellular membranes. We prove that PDV is transported as viral particles via MP-generated tubular structures through plasmodesmata. Moreover, the computer-run 3D modeling reveals structural resemblances between MPs of PDV and of Alfalfa mosaic virus (AMV), implying similarities of transport mechanisms for both viruses.

Arabidopsis formin 2 regulates cell-to-cell trafficking by capping and stabilizing actin filaments at plasmodesmata

Here, we demonstrate that Arabidopsis thaliana Formin 2 (AtFH2) localizes to plasmodesmata (PD) through its transmembrane domain and is required for normal intercellular trafficking. Although loss-of-function atfh2 mutants have no overt developmental defect, PD's permeability and sensitivity to virus infection are increased in atfh2 plants. Interestingly, AtFH2 functions in a partially redundant manner with its closest homolog AtFH1, which also contains a PD localization signal. Strikingly, targeting of Class I formins to PD was also confirmed in rice, suggesting that the involvement of Class I formins in regulating actin dynamics at PD may be evolutionarily conserved in plants. In vitro biochemical analysis showed that AtFH2 fails to nucleate actin assembly but caps and stabilizes actin filaments. We also demonstrate that the interaction between AtFH2 and actin filaments is crucial for its function in vivo. These data allow us to propose that AtFH2 regulates PD's permeability by anchoring actin filaments to PD.

Dynamical Patterning Modules, Biogeneric Materials, and the Evolution of Multicellular Plants

Comparative analyses of developmental processes across a broad spectrum of organisms are required to fully understand the mechanisms responsible for the major evolutionary transitions among eukaryotic photosynthetic lineages (defined here as the polyphyletic algae and the monophyletic land plants). The concepts of dynamical patterning modules (DPMs) and biogeneric materials provide a framework for studying developmental processes in the context of such comparative analyses. In the context of multicellularity, DPMs are defined as sets of conserved gene products and molecular networks, in conjunction with the physical morphogenetic and patterning processes they mobilize. A biogeneric material is defined as mesoscale matter with predictable morphogenetic capabilities that arise from complex cellular conglomerates. Using these concepts, we outline some of the main events and transitions in plant evolution, and describe the DPMs and biogeneric properties associated with and responsible for these transitions. We identify four primary DPMs that played critical roles in the evolution of multicellularity (i.e., the DPMs responsible for cell-to-cell adhesion, identifying the future cell wall, cell differentiation, and cell polarity). Three important conclusions emerge from a broad phyletic comparison: (1) DPMs have been achieved in different ways, even within the same clade (e.g., phycoplastic cell division in the Chlorophyta and phragmoplastic cell division in the Streptophyta), (2) DPMs had their origins in the co-option of molecular species present in the unicellular ancestors of multicellular plants, and (3) symplastic transport mediated by intercellular connections, particularly plasmodesmata, was critical for the evolution of complex multicellularity in plants.

The Plasmodesmal Localization Signal of TMV MP Is Recognized by Plant Synaptotagmin SYTA

Plant viruses cross the barrier of the plant cell wall by moving through intercellular channels, termed plasmodesmata, to invade their hosts. They accomplish this by encoding movement proteins (MPs), which act to alter plasmodesmal gating. How MPs target to plasmodesmata is not well understood. Our recent characterization of the first plasmodesmal localization signal (PLS) identified in a viral MP, namely, the MP encoded by the Tobamovirus Tobacco mosaic virus (TMV), now provides the opportunity to identify host proteins that recognize this PLS and may be important for its plasmodesmal targeting. One such candidate protein is Arabidopsis synaptotagmin A (SYTA), which is required to form endoplasmic reticulum (ER)-plasma membrane contact sites and regulates the MP-mediated trafficking of begomoviruses, tobamoviruses, and potyviruses. In particular, SYTA interacts with, and regulates the cell-to-cell transport of, both TMV MP and the MP encoded by the Tobamovirus Turnip vein clearing virus (TVCV). Using in planta bimolecular fluorescence complementation (BiFC) and yeast two-hybrid assays, we show here that the TMV PLS interacted with SYTA. This PLS sequence was both necessary and sufficient for interaction with SYTA, and the plasmodesmal targeting activity of the TMV PLS was substantially reduced in an Arabidopsis syta knockdown line. Our findings show that SYTA is one host factor that can recognize the TMV PLS and suggest that this interaction may stabilize the association of TMV MP with plasmodesmata.IMPORTANCE Plant viruses use their movement proteins (MPs) to move through host intercellular connections, plasmodesmata. Perhaps one of the most intriguing, yet least studied, aspects of this transport is the MP signal sequences and their host recognition factors. Recently, we have described the plasmodesmal localization signal (PLS) of the Tobacco mosaic virus (TMV) MP. Here, we identified the Arabidopsis synaptotagmin A (SYTA) as a host factor that recognizes TMV MP PLS and promotes its association with the plasmodesmal membrane. The significance of these findings is two-fold: (i) we identified the TMV MP association with the cell membrane at plasmodesmata as an important PLS-dependent step in plasmodesmal targeting, and (ii) we identified the plant SYTA protein that specifically recognizes PLS as a host factor involved in this step.

Guard cells in fern stomata are connected by plasmodesmata, but control cytosolic Ca2+ levels autonomously

Recent studies have revealed that some responses of fern stomata to environmental signals differ from those of their relatives in seed plants. However, it is unknown whether the biophysical properties of guard cells differ fundamentally between species of both clades. Intracellular micro-electrodes and the fluorescent Ca²⁺ reporter FURA2 were used to study voltage-dependent cation channels and Ca²⁺ signals in guard cells of the ferns Polypodium vulgare and Asplenium scolopendrium. Voltage clamp experiments with fern guard cells revealed similar properties of voltage-dependent K⁺ channels as found in seed plants. However, fluorescent dyes moved within the fern stomata, from one guard cell to the other, which does not occur in most seed plants. Despite the presence of plasmodesmata, which interconnect fern guard cells, Ca²⁺ signals could be elicited in each of the cells individually. Based on the common properties of voltage-dependent channels in ferns and seed plants, it is likely that these key transport proteins are conserved in vascular plants. However, the symplastic connections between fern guard cells in mature stomata indicate that the biophysical mechanisms that control stomatal movements differ between ferns and seed plants.

Exposure to heavy metal stress triggers changes in plasmodesmatal permeability via deposition and breakdown of callose

Both plants and animals must contend with changes in their environment. The ability to respond appropriately to these changes often underlies the ability of the individual to survive. In plants, an early response to environmental stress is an alteration in plasmodesmatal permeability with accompanying changes in cell to cell signaling. However, the ways in which plasmodesmata are modified, the molecular players involved in this regulation, and the biological significance of these responses are not well understood. Here, we examine the effects of nutrient scarcity and excess on plasmodesmata-mediated transport in the Arabidopsis thaliana root and identify two CALLOSE SYNTHASES and two β-1,3-GLUCANASES as key regulators of these processes. Our results suggest that modification of plasmodesmata-mediated signaling underlies the ability of the plant to maintain root growth and properly partition nutrients when grown under conditions of excess nutrients.

Peeking at a plant through the holes in the wall - exploring the roles of plasmodesmata

Plasmodesmata (PD) are membrane-lined pores that connect neighbouring plant cells and allow molecular exchange via the symplast. Past studies have revealed the basic structure of PD, some of the transport mechanisms for molecules through PD, and a variety of physiological processes in which they function. Recently, with the help of newly developed technologies, several exciting new features of PD have been revealed. New PD structures were observed during early formation of PD and between phloem sieve elements and phloem pole pericycle cells in roots. Both observations challenge our current understanding of PD structure and function. Research into novel physiological responses, which are regulated by PD, indicates that we have not yet fully explored the potential contribution of PD to overall plant function. In this Viewpoint article, we summarize some of the recent advances in understanding the structure and function of PD and propose the challenges ahead for the community.