Cell-to-cell communication via plasmodesmata during Arabidopsis embryogenesis

The primary function of Six5 of Fusarium oxysporum is to facilitate Avr2 activity by together manipulating the size exclusion limit of plasmodesmata

Pathogens produce effector proteins to manipulate their hosts. While most effectors act autonomously, some fungal effectors act in pairs and rely on each other for function. During the colonization of the plant vasculature, the root-infecting fungus Fusarium oxysporum (Fo) produces 14 so-called Secreted in Xylem (SIX) effectors. Two of these effector genes, Avr2 (Six3) and Six5, form a gene pair on the pathogenicity chromosome of the tomato-infecting Fo strain. Avr2 has been shown to suppress plant defense responses and is required for full pathogenicity. Although Six5 and Avr2 together manipulate the size exclusion limit of plasmodesmata to facilitate cell-to-cell movement of Avr2, it is unclear whether Six5 has additional functions as well. To investigate the role of Six5, we generated transgenic Arabidopsis lines expressing Six5. Notably, increased susceptibility during the early stages of infection was observed in these Six5 lines, but only to Fo strains expressing Avr2 and not to wild-type Arabidopsis-infecting Fo strains lacking this effector gene. Furthermore, neither PAMP-triggered defense responses, such as ROS accumulation and callose deposition upon treatment with Flg22, necrosis and ethylene-inducing peptide 1-like protein (NLP), or chitosan, nor susceptibility to other plant pathogens, such as the bacterium Pseudomonas syringae or the fungus Verticilium dahlia, were affected by Six5 expression. Further investigation of the ability of the Avr2/Six5 effector pair to manipulate plasmodesmata (PD) revealed that it not only permits cell-to-cell movement of Avr2, but also facilitates the movement of two additional effectors, Six6 and Six8. Moreover, although Avr2/Six5 expands the size exclusion limit of plasmodesmata (i.e., gating) to permit the movement of a 2xFP fusion protein (53 kDa), a larger variant, 3xFP protein (80 kDa), did not move to the neighboring cells. The PD manipulation mechanism employed by Avr2/Six5 did not involve alteration of callose homeostasis in these structures. In conclusion, the primary function of Six5 appears to function together with Avr2 to increase the size exclusion limit of plasmodesmata by an unknown mechanism to facilitate cell-to-cell movement of Fo effectors.

Sphingolipids at Plasmodesmata: Structural Components and Functional Modulators

Plasmodesmata (PD) are plant-specific channels connecting adjacent cells to mediate intercellular communication of molecules essential for plant development and defense. The typical PD are organized by the close apposition of the plasma membrane (PM), the desmotubule derived from the endoplasmic reticulum (ER), and spoke-like elements linking the two membranes. The plasmodesmal PM (PD-PM) is characterized by the formation of unique microdomains enriched with sphingolipids, sterols, and specific proteins, identified by lipidomics and proteomics. These components modulate PD to adapt to the dynamic changes of developmental processes and environmental stimuli. In this review, we focus on highlighting the functions of sphingolipid species in plasmodesmata, including membrane microdomain organization, architecture transformation, callose deposition and permeability control, and signaling regulation. We also briefly discuss the difference between sphingolipids and sterols, and we propose potential unresolved questions that are of help for further understanding the correspondence between plasmodesmal structure and function.

Emerging Roles of β-Glucanases in Plant Development and Adaptative Responses

Plant β-glucanases are enzymes involved in the synthesis, remodelling and turnover of cell wall components during multiple physiological processes. Based on the type of the glycoside bond they cleave, plant β-glucanases have been grouped into three categories: (i) β-1,4-glucanases degrade cellulose and other polysaccharides containing 1,4-glycosidic bonds to remodel and disassemble the wall during cell growth. (ii) β-1,3-glucanases are responsible for the mobilization of callose, governing the symplastic trafficking through plasmodesmata. (iii) β-1,3-1,4-glucanases degrade mixed linkage glucan, a transient wall polysaccharide found in cereals, which is broken down to obtain energy during rapid seedling growth. In addition to their roles in the turnover of self-glucan structures, plant β-glucanases are crucial in regulating the outcome in symbiotic and hostile plant-microbe interactions by degrading non-self glucan structures. Plants use these enzymes to hydrolyse β-glucans found in the walls of microbes, not only by contributing to a local antimicrobial defence barrier, but also by generating signalling glucans triggering the activation of global responses. As a counterpart, microbes developed strategies to hijack plant β-glucanases to their advantage to successfully colonize plant tissues. This review outlines our current understanding on plant β-glucanases, with a particular focus on the latest advances on their roles in adaptative responses.

Plasmodesmata and their role in assimilate translocation

During multicellularization, plants evolved unique cell-cell connections, the plasmodesmata (PD). PD of angiosperms are complex cellular domains, embedded in the cell wall and consisting of multiple membranes and a large number of proteins. From the beginning, it had been assumed that PD provide passage for a wide range of molecules, from ions to metabolites and hormones, to RNAs and even proteins. In the context of assimilate allocation, it has been hypothesized that sucrose produced in mesophyll cells is transported via PD from cell to cell down a concentration gradient towards the phloem. Entry into the sieve element companion cell complex (SECCC) is then mediated on three potential routes, depending on the species and conditions, - either via diffusion across PD, after conversion to raffinose via PD using a polymer trap mechanism, or via a set of transporters which secrete sucrose from one cell and secondary active uptake into the SECCC. Multiple loading mechanisms can likely coexist. We here review the current knowledge regarding photoassimilate transport across PD between cells as a prerequisite for translocation from leaves to recipient organs, in particular roots and developing seeds. We summarize the state-of-the-art in protein composition, structure, transport mechanism and regulation of PD to apprehend their functions in carbohydrate allocation. Since many aspects of PD biology remain elusive, we highlight areas that require new approaches and technologies to advance our understanding of these enigmatic and important cell-cell connections.

Qualitative and quantitative analyses of the plasmodesmata that accompany cell fate changes during the somatic embryogenesis of Arabidopsis thaliana

Plasmodesmata (PD) are cytoplasmic and membrane-lined microchannels that enable symplasmic communication in plants, which is involved in the regulation of cell differentiation. The presented results emphasise the qualitative and quantitative analyses of PD, which are the basis of the symplasmic communication. The cells that initiate various development programmes create symplasmic domains that are characterised by different degrees of symplasmic communication. Changes in symplasmic communication are caused by the presence or absence of PD and/or the ability of signals to move through them. In the presented studies, somatic embryogenesis was used to describe the characteristics of the PD within and between the symplasmic domains in explants of the Arabidopsis thaliana (L.) Heynh ecotype Columbia-0 and 35S:BBM transgenic line. Transmission electron microscopy was used to describe the cells that regain totipotency/pluripotency during somatic embryogenesis, as well as the number and shape of the PD in the different symplasmic domains of the explants and somatic embryos. Array tomography was used to create a 3D reconstruction of the protodermal cells of the somatic embryos with particular emphasis on the PD distribution in the cell walls. The results showed that there were different frequencies of the PD within and between the symplasmic domain that emerges during somatic embryogenesis and between the Col-0 and 35S:BBM somatic embryos with regard to the differences in the shape of the PD.

Function of Plasmodesmata in the Interaction of Plants with Microbes and Viruses

Plasmodesmata (PD) are gated plant cell wall channels that allow the trafficking of molecules between cells and play important roles during plant development and in the orchestration of cellular and systemic signaling responses during interactions of plants with the biotic and abiotic environment. To allow gating, PD are equipped with signaling platforms and enzymes that regulate the size exclusion limit (SEL) of the pore. Plant-interacting microbes and viruses target PD with specific effectors to enhance their virulence and are useful probes to study PD functions.

Cell Wall Composition and Structure Define the Developmental Fate of Embryogenic Microspores in Brassica napus

Microspore cultures generate a heterogeneous population of embryogenic structures that can be grouped into highly embryogenic structures [exine-enclosed (EE) and loose bicellular structures (LBS)] and barely embryogenic structures [compact callus (CC) and loose callus (LC) structures]. Little is known about the factors behind these different responses. In this study we performed a comparative analysis of the composition and architecture of the cell walls of each structure by confocal and quantitative electron microscopy. Each structure presented specific cell wall characteristics that defined their developmental fate. EE and LBS structures, which are responsible for most of the viable embryos, showed a specific profile with thin walls rich in arabinogalactan proteins (AGPs), highly and low methyl-esterified pectin and callose, and a callose-rich subintinal layer not necessarily thick, but with a remarkably high callose concentration. The different profiles of EE and LBS walls support the development as suspensorless and suspensor-bearing embryos, respectively. Conversely, less viable embryogenic structures (LC) presented the thickest walls and the lowest values for almost all of the studied cell wall components. These cell wall properties would be the less favorable for cell proliferation and embryo progression. High levels of highly methyl-esterified pectin are necessary for wall flexibility and growth of highly embryogenic structures. AGPs seem to play a role in cell wall stiffness, possibly due to their putative role as calcium capacitors, explaining the positive relationship between embryogenic potential and calcium levels.

Locally restricted glucose availability in the embryonic hypocotyl determines seed germination under abscisic acid treatment

Abiotic stresses affect plant growth and development by causing cellular damage and/or restricting resources. Plants often respond to stresses through abscisic acid (ABA) signaling. Exogenous ABA application can therefore be used to mimic stress responses, which can be overridden by glucose (Glc) addition during seed germination. It remains unclear whether ABA-mediated germination inhibition is due to regional or global suppression of Glc availability in germinating Arabidopsis seeds. We used a genetically engineered Förster resonance energy transfer (FRET) sensor to ascertain whether ABA affects the spatiotemporal distribution of Glc, ¹⁴ C-Glc uptake assays to track potential effects of ABA on sugar import, and transcriptome and mutant analyses to identify genes associated with Glc availability that are involved in ABA-inhibited seed germination. Abscisic acid limits Glc in the hypocotyl largely by suppressing sugar allocation as well as altering sugar metabolism. Mutant plants carrying loss-of-function ABA-inducible sucrose-phosphate synthase (SPS) genes accumulated more Glc, leading to ABA-insensitive germination. We reveal that Glc antagonizes ABA by globally counteracting the ABA influence at the transcript level, including expansin (EXP) family genes suppressed by ABA. This study presents a new perspective on how ABA affects Glc distribution, which likely reflects what occurs when seeds are subjected to abiotic stresses such as drought and salt stress.

Time-course observation of the reconstruction of stem cell niche in the intact root

The stem cell niche (SCN) is critical in maintaining continuous postembryonic growth of the plant root. During their growth in soil, plant roots are often challenged by various biotic or abiotic stresses, resulting in damage to the SCN. This can be repaired by the reconstruction of a functional SCN. Previous studies examining the SCN's reconstruction often introduce physical damage including laser ablation or surgical excision. In this study, we performed a time-course observation of the SCN reconstruction in pWOX5:icals3m roots, an inducible system that causes non-invasive SCN differentiation upon induction of estradiol on Arabidopsis (Arabidopsis thaliana) root. We found a stage-dependent reconstruction of SCN in pWOX5:icals3m roots, with division-driven anatomic reorganization in the early stage of the SCN recovery, and cell fate specification of new SCN in later stages. During the recovery of the SCN, the local accumulation of auxin was coincident with the cell division pattern, exhibiting a spatial shift in the root tip. In the early stage, division mostly occurred in the neighboring stele to the SCN position, while division in endodermal layers seemed to contribute more in the later stages, when the SCN was specified. The precise re-positioning of SCN seemed to be determined by mutual antagonism between auxin and cytokinin, a conserved mechanism that also regulates damage-induced root regeneration. Our results thus provide time-course information about the reconstruction of SCN in intact Arabidopsis roots, which highlights the stage-dependent re-patterning in response to differentiated quiescent center.

Do Plasmodesmata Play a Prominent Role in Regulation of Auxin-Dependent Genes at Early Stages of Embryogenesis?

Plasmodesmata form intercellular channels which ensure the transport of various molecules during embryogenesis and postembryonic growth. However, high permeability of plasmodesmata may interfere with the establishment of auxin maxima, which are required for cellular patterning and the development of distinct tissues. Therefore, diffusion through plasmodesmata is not always desirable and the symplastic continuum must be broken up to induce or accomplish some developmental processes. Many data show the role of auxin maxima in the regulation of auxin-responsive genes and the establishment of various cellular patterns. However, still little is known whether and how these maxima are formed in the embryo proper before 16-cell stage, that is, when there is still a nonpolar distribution of auxin efflux carriers. In this work, we focused on auxin-dependent regulation of plasmodesmata function, which may provide rapid and transient changes of their permeability, and thus take part in the regulation of gene expression.

Plasmodesmata-Dependent Intercellular Movement of Bacterial Effectors

Pathogenic microorganisms deliver protein effectors into host cells to suppress host immune responses. Recent findings reveal that phytopathogens manipulate the function of plant cell-to-cell communication channels known as plasmodesmata (PD) to promote diseases. Several bacterial and filamentous pathogen effectors have been shown to regulate PD in their host cells. A few effectors of filamentous pathogens have been reported to move from the infected cells to neighboring plant cells through PD; however, it is unclear whether bacterial effectors can traffic through PD in plants. In this study, we determined the intercellular movement of Pseudomonas syringae pv. tomato (Pst) DC3000 effectors between adjoining plant cells in Nicotiana benthamiana. We observed that at least 16 Pst DC3000 effectors have the capacity to move from transformed cells to the surrounding plant cells. The movement of the effectors is largely dependent on their molecular weights. The expression of PD regulators, Arabidopsis PD-located protein PDLP5 and PDLP7, leads to PD closure and inhibits the PD-dependent movement of a bacterial effector in N. benthamiana. Similarly, a 22-amino acid peptide of bacterial flagellin (flg22) treatment induces PD closure and suppresses the movement of a bacterial effector in N. benthamiana. Among the mobile effectors, HopAF1 and HopA1 are localized to the plasma membrane (PM) in plant cells. Interestingly, the PM association of HopAF1 does not negatively affect the PD-dependent movement. Together, our findings demonstrate that bacterial effectors are able to move intercellularly through PD in plants.

Plasmodesmata-Dependent Intercellular Movement of Bacterial Effectors

Pathogenic microorganisms deliver protein effectors into host cells to suppress host immune responses. Recent findings reveal that phytopathogens manipulate the function of plant cell-to-cell communication channels known as plasmodesmata (PD) to promote diseases. Several bacterial and filamentous pathogen effectors have been shown to regulate PD in their host cells. A few effectors of filamentous pathogens have been reported to move from the infected cells to neighboring plant cells through PD; however, it is unclear whether bacterial effectors can traffic through PD in plants. In this study, we determined the intercellular movement of Pseudomonas syringae pv. tomato (Pst) DC3000 effectors between adjoining plant cells in Nicotiana benthamiana. We observed that at least 16 Pst DC3000 effectors have the capacity to move from transformed cells to the surrounding plant cells. The movement of the effectors is largely dependent on their molecular weights. The expression of PD regulators, Arabidopsis PD-located protein PDLP5 and PDLP7, leads to PD closure and inhibits the PD-dependent movement of a bacterial effector in N. benthamiana. Similarly, a 22-amino acid peptide of bacterial flagellin (flg22) treatment induces PD closure and suppresses the movement of a bacterial effector in N. benthamiana. Among the mobile effectors, HopAF1 and HopA1 are localized to the plasma membrane (PM) in plant cells. Interestingly, the PM association of HopAF1 does not negatively affect the PD-dependent movement. Together, our findings demonstrate that bacterial effectors are able to move intercellularly through PD in plants.

Symplasmic Isolation Contributes to Somatic Embryo Induction and Development in the Tree Fern Cyathea delgadii Sternb

In this report, we describe studies on symplasmic communication and cellular rearrangement during direct somatic embryogenesis (SE) in the tree fern Cyathea delgadii. We analyzed changes in the symplasmic transport of low-molecular-weight fluorochromes, such as 8-hydroxypyrene-1,3,6-trisulfonic acid, trisodium salt (HPTS) and fluorescein (delivered to cells as fluorescein diacetate, FDA), within stipe explants and somatic embryos originating from single epidermal cells and developing during 16-d long culture. Induction of SE is preceded by a restriction in fluorochrome distribution between certain explant cells. Microscopic analysis showed a series of cellular changes like a decrease in vacuole size, increase in vacuole numbers, and increased density of cytoplasm and deposition of electron-dense material in cell walls that may be related with embryogenic transition. In somatic embryos, the limited symplasmic communication between cells was observed first in linear tri-cellular embryos. Further development of the fern embryo was associated with the formation of symplasmic domains corresponding to the four segments of the plant body. Using symplasmic tracers, we provided evidence that the changes in plasmodesmata permeability are corelated with somatic-to-embryogenic transition and somatic embryo development.

Symplasmic isolation marks cell fate changes during somatic embryogenesis

Cell-to-cell signalling is a major mechanism controlling plant morphogenesis. Transport of signalling molecules through plasmodesmata is one way in which plants promote or restrict intercellular signalling over short distances. Plasmodesmata are membrane-lined pores between cells that regulate the intercellular flow of signalling molecules through changes in their size, creating symplasmic fields of connected cells. Here we examine the role of plasmodesmata and symplasmic communication in the establishment of plant cell totipotency, using somatic embryo induction from Arabidopsis explants as a model system. Cell-to-cell communication was evaluated using fluorescent tracers, supplemented with histological and ultrastructural analysis, and correlated with expression of a WOX2 embryo reporter. We showed that embryogenic cells are isolated symplasmically from non-embryogenic cells regardless of the explant type (immature zygotic embryos or seedlings) and inducer system (2,4-dichlorophenoxyacetic acid or the BABY BOOM (BBM) transcription factor), but that the symplasmic domains in different explants differ with respect to the maximum size of molecule capable of moving through the plasmodesmata. Callose deposition in plasmodesmata preceded WOX2 expression in future sites of somatic embryo development, but later was greatly reduced in WOX2-expressing domains. Callose deposition was also associated with a decrease DR5 auxin response in embryogenic tissue. Treatment of explants with the callose biosynthesis inhibitor 2-deoxy-D-glucose supressed somatic embryo formation in all three systems studied, and also blocked the observed decrease in DR5 expression. Together these data suggest that callose deposition at plasmodesmata is required for symplasmic isolation and establishment of cell totipotency in Arabidopsis.

First evidence showing that Pepper vein yellows virus P4 protein is a movement protein

BACKGROUND

Plant viruses move through plasmodesmata (PD) to infect new cells. To overcome the PD barrier, plant viruses have developed specific protein(s) to guide their genomic RNAs or DNAs to path through the PD.

RESULTS

In the present study, we analyzed the function of Pepper vein yellows virus P4 protein. Our bioinformatic analysis using five commonly used algorithms showed that the P4 protein contains an transmembrane domain, encompassing the amino acid residue 117-138. The subcellular localization of P4 protein was found to target PD and form small punctates near walls. The P4 deletion mutant or the substitution mutant constructed by overlap PCR lost their function to produce punctates near the walls inside the fluorescent loci. The P4-YFP fusion was found to move from cell to cell in infiltrated leaves, and P4 could complement Cucumber mosaic virus movement protein deficiency mutant to move between cells.

CONCLUSION

Taking together, we consider that the P4 protein is a movement protein of Pepper vein yellows virus.

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.

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.

Sphingolipid biosynthesis modulates plasmodesmal ultrastructure and phloem unloading

During phloem unloading, multiple cell-to-cell transport events move organic substances to the root meristem. Although the primary unloading event from the sieve elements to the phloem pole pericycle has been characterized to some extent, little is known about post-sieve element unloading. Here, we report a novel gene, PHLOEM UNLOADING MODULATOR (PLM), in the absence of which plasmodesmata-mediated symplastic transport through the phloem pole pericycle-endodermis interface is specifically enhanced. Increased unloading is attributable to a defect in the formation of the endoplasmic reticulum-plasma membrane tethers during plasmodesmal morphogenesis, resulting in the majority of pores lacking a visible cytoplasmic sleeve. PLM encodes a putative enzyme required for the biosynthesis of sphingolipids with very-long-chain fatty acid. Taken together, our results indicate that post-sieve element unloading involves sphingolipid metabolism, which affects plasmodesmal ultrastructure. They also raise the question of how and why plasmodesmata with no cytoplasmic sleeve facilitate molecular trafficking.

The development of a hairless phenotype in barley roots treated with gold nanoparticles is accompanied by changes in the symplasmic communication

Uptake of water and nutrients by roots affects the ontogenesis of the whole plant. Nanoparticles, e.g. gold nanoparticles, have a broad range of applications in many fields which leads to the transfer of these materials into the environment. Thus, the understanding of their impact on the growth and development of the root system is an emerging issue. During our studies on the effect of positively charged gold nanoparticles on the barley roots, a hairless phenotype was found. We investigated whether this phenotype correlates with changes in symplasmic communication, which is an important factor that regulates, among others, differentiation of the rhizodermis into hair and non-hair cells. The results showed no restriction in symplasmic communication in the treated roots, in contrast to the control roots, in which the trichoblasts and atrichoblasts were symplasmically isolated during their differentiation. Moreover, differences concerning the root morphology, histology, ultrastructure and the cell wall composition were detected between the control and the treated roots. These findings suggest that the harmful effect of nanoparticles on plant growth may, among others, consist in disrupting the symplasmic communication/isolation, which leads to the development of a hairless root phenotype, thus limiting the functioning of the roots.

A Molecular Framework for the Embryonic Initiation of Shoot Meristem Stem Cells

The establishment of pluripotent stem cells is a key event during plant and animal embryogenesis, but the underlying mechanisms remain enigmatic. We show that in the flowering plant Arabidopsis thaliana, expression of the shoot meristem stem cell marker CLV3 becomes detectable in transition stage embryos. Surprisingly, the key regulator of stem cell homeostasis WUSCHEL (WUS) is expressed but dispensable for stem cell initiation. Rather, the WUS paralog WOX2, a regulator of embryo patterning initiated in the zygote, functions in this process by shielding stem cell progenitors from differentiation. WOX2 upregulates HD-ZIP III transcription factors required for shoot identity and balances cytokinin versus auxin hormone pathways, revealing that classical plantlet regeneration procedures recapitulate the natural induction mechanism. Our findings link transcriptional regulation of early embryo patterning to hormonal control of stem cell initiation and suggest that similar strategies have evolved in plant and animal stem cell formation.

Architecture and permeability of post-cytokinesis plasmodesmata lacking cytoplasmic sleeves

Plasmodesmata are remarkable cellular machines responsible for the controlled exchange of proteins, small RNAs and signalling molecules between cells. They are lined by the plasma membrane (PM), contain a strand of tubular endoplasmic reticulum (ER), and the space between these two membranes is thought to control plasmodesmata permeability. Here, we have reconstructed plasmodesmata three-dimensional (3D) ultrastructure with an unprecedented level of 3D information using electron tomography. We show that within plasmodesmata, ER-PM contact sites undergo substantial remodelling events during cell differentiation. Instead of being open pores, post-cytokinesis plasmodesmata present such intimate ER-PM contact along the entire length of the pores that no intermembrane gap is visible. Later on, during cell expansion, the plasmodesmata pore widens and the two membranes separate, leaving a cytosolic sleeve spanned by tethers whose presence correlates with the appearance of the intermembrane gap. Surprisingly, the post-cytokinesis plasmodesmata allow diffusion of macromolecules despite the apparent lack of an open cytoplasmic sleeve, forcing the reassessment of the mechanisms that control plant cell-cell communication.

Lipid Raft, Regulator of Plasmodesmal Callose Homeostasis

Abstract: The specialized plasma membrane microdomains known as lipid rafts are enriched by sterols and sphingolipids. Lipid rafts facilitate cellular signal transduction by controlling the assembly of signaling molecules and membrane protein trafficking. Another specialized compartment of plant cells, the plasmodesmata (PD), which regulates the symplasmic intercellular movement of certain molecules between adjacent cells, also contains a phospholipid bilayer membrane. The dynamic permeability of plasmodesmata (PDs) is highly controlled by plasmodesmata callose (PDC), which is synthesized by callose synthases (CalS) and degraded by β-1,3-glucanases (BGs). In recent studies, remarkable observations regarding the correlation between lipid raft formation and symplasmic intracellular trafficking have been reported, and the PDC has been suggested to be the regulator of the size exclusion limit of PDs. It has been suggested that the alteration of lipid raft substances impairs PDC homeostasis, subsequently affecting PD functions. In this review, we discuss the substantial role of membrane lipid rafts in PDC homeostasis and provide avenues for understanding the fundamental behavior of the lipid raft-processed PDC.

Identification of symplasmic domains in the embryo and seed of Sedum acre L. (Crassulaceae)

MAIN CONCLUSION

Our study demonstrated that symplasmic communication between Sedum acre seed compartments and the embryo proper is not uniform. The presence of plasmodesmata (PD) constitutes the structural basis for information exchange between cells, and symplasmic communication is involved in the regulation of cell differentiation and plant development. Most recent studies concerning an analysis of symplasmic communication between seed compartments and the embryo have been predominantly performed on Arabidopsis thaliana. The results presented in this paper describe the analysis of symplasmic communication on the example of Sedum acre seeds, because the ultrastructure of the seed compartments and the embryo proper, including the PD, have already been described, and this species represents an embryonic type of development different to Arabidopsis. Moreover, in this species, an unusual electron-dense dome associated with plasmodesmata on the border between the basal cell/chalazal suspensor cells and the basal cell/the endosperm has been described. This prompted the question as to whether these plasmodesmata are functional. Thus, the aim of this study was to describe the movement of symplasmic transport fluorochromes between different Sedum seed compartments, with particular emphasis on the movement between the basal cell and the embryo proper and endosperm, to answer the following questions: (1) are seeds divided into symplasmic domains; (2) if so, are they stable or do they change with the development? The results have shown that symplasmic tracers movement: (a) from the external integument to internal integument is restricted; (b) from the basal cell to the other part of the embryo proper and from the basal cell to the endosperm is also restricted;

Regulatory function of homeodomain-leucine zipper (HD-ZIP) family proteins during embryogenesis

Homeodomain-leucine zipper proteins (HD-ZIPs) form a plant-specific family of transcription factors functioning as homo- or heterodimers. Certain members of all four classes of this family are involved in embryogenesis, the focus of this review. They support auxin biosynthesis, transport and response, which are in turn essential for the apical-basal patterning of the embryo, radicle formation and outgrowth of the cotyledons. They transcriptionally regulate meristem regulators to maintain the shoot apical meristem once it is initiated. Some members are specific to the protoderm, the outermost layer of the embryo, and play a role in shoot apical meristem function. Within classes, homeodomain-leucine zippers tend to act redundantly during embryo development, and there are many examples of regulation within and between classes of homeodomain-leucine zippers. This indicates a complex network of regulation that awaits future experiments to uncover.

The ER-Membrane Transport System Is Critical for Intercellular Trafficking of the NSm Movement Protein and Tomato Spotted Wilt Tospovirus

Plant viruses move through plasmodesmata to infect new cells. The plant endoplasmic reticulum (ER) is interconnected among cells via the ER desmotubule in the plasmodesma across the cell wall, forming a continuous ER network throughout the entire plant. This ER continuity is unique to plants and has been postulated to serve as a platform for the intercellular trafficking of macromolecules. In the present study, the contribution of the plant ER membrane transport system to the intercellular trafficking of the NSm movement protein and Tomato spotted wilt tospovirus (TSWV) is investigated. We showed that TSWV NSm is physically associated with the ER membrane in Nicotiana benthamiana plants. An NSm-GFP fusion protein transiently expressed in single leaf cells was trafficked into neighboring cells. Mutations in NSm that impaired its association with the ER or caused its mis-localization to other subcellular sites inhibited cell-to-cell trafficking. Pharmacological disruption of the ER network severely inhibited NSm-GFP trafficking but not GFP diffusion. In the Arabidopsis thaliana mutant rhd3 with an impaired ER network, NSm-GFP trafficking was significantly reduced, whereas GFP diffusion was not affected. We also showed that the ER-to-Golgi secretion pathway and the cytoskeleton transport systems were not involved in the intercellular trafficking of TSWV NSm. Importantly, TSWV cell-to-cell spread was delayed in the ER-defective rhd3 mutant, and this reduced viral infection was not due to reduced replication. On the basis of robust biochemical, cellular and genetic analysis, we established that the ER membrane transport system serves as an important direct route for intercellular trafficking of NSm and TSWV.

The cytosol must flow: intercellular transport through plasmodesmata

Plant cells are connected across cell walls by nanoscopic channels called plasmodesmata (PD), which allow plant cells to share resources and exchange signaling molecules. Several protein components of PD membranes have been identified, and recent advances in superresolution live-cell microscopy are illuminating PD ultrastructure. Restricting transport through PD is crucial for morphogenesis, since hormones and hundreds of transcription factors regularly move through PD, and this transport must stop to allow cells to begin differentiating. Chloroplasts and mitochondria regulate PD function through signal transduction networks that coordinate plant physiology and development. Recent discoveries on the relationships of land plants and their algal relatives suggest that PD have evolved independently in several lineages, emphasizing the importance of cytosolic bridges in multicellular biology.

The production, localization and spreading of reactive oxygen species contributes to the low vitality of long-term stored common beech (Fagus sylvatica L.) seeds

The common beech (Fagus sylvatica L.) is propagated by seeds, but the seed set is irregular with five to ten years in between crops. It is therefore necessary to store the seeds. However, beech seeds lose germinability during long-term storage. In this study, beech seeds were stored at -10°C under controlled conditions for 2, 5, 8, 11 and 13 years. Our results show that beech seeds lose germinability during storage in proportion to the duration of storage. The decrease in germinability correlated with increased electrolyte leakage and accumulation of superoxide anion radicals, hydrogen peroxide and hydroxyl radicals. Furthermore, a strong positive correlation was observed among the releases of superoxide anion radicals, hydrogen peroxide and hydroxyl radicals. In situ localization showed that superoxide anion radicals and hydrogen peroxide were first detectable in root cap cells. When the seed storage time was extended, the reactive oxygen species fluorescence expanded to more areas of the radicle, reaching the root apical meristem. A storage time-dependent decrease in catalase activity, observed in both embryonic axes and cotyledons, was also positively correlated with germinability. DNA fragmentation was observed in beech seeds during storage and occurred predominantly in embryonic axes stored for 5 years and more. Altogether, these results suggest that the loss of germinability in beech seeds during long-term storage depends on several factors, including strong of reactive oxygen species accumulation accompanied by reduced catalase activity as well as membrane injury and DNA alternations, which may be aging-related and ROS-derived. We suggest that the accumulating reactive oxygen species that spread to the root apical meristem are key factors that affect seed germinability after long-term storage.

A model system for analyzing intercellular communication through plasmodesmata using moss protonemata and leaves

Plant growth, development, and environmental responses require the proper regulation of intercellular movement of signals and nutrients. For this, plants have specialized cytoplasmic channels, the plasmodesmata (PD), which allow the symplasmic movement of micro- and macromolecules between neighboring cells. Internal and external signals spatio-temporally regulate the movement of molecules through the PD to control plant development and environmental responses. Although some aspects of targeted movement of molecules have been revealed, the mechanisms of non-targeted, diffusible flow of molecules through PD, and its regulation and function, remain poorly understood, particularly at the cellular level. Previously, we developed a system to quantitatively analyze non-targeted movement of a photoconvertible fluorescent protein, Dendra2, at the single-cell level in the filamentous protonemata tissue of the moss Physcomitrella patens. In protonemata, one-dimensional intercellular communication can be easily observed and quantitatively analyzed at the cellular level. In this review, we describe how protonemata and leaves of P. patens can be used to study symplasmic movement through PD, and discuss how this system can help improve our understanding of PD regulation and function in development and environmental responses in plants.

Plasmodesmata: channels for intercellular signaling during plant growth and development

Plants have evolved strategies for short- and long-distance communication to coordinate plant development and to adapt to changing environmental conditions. Plasmodesmata (PD) are intercellular nanochannels that provide an effective pathway for both selective and nonselective movement of various molecules that function in diverse biological processes. Numerous non-cell-autonomous proteins (NCAP) and small RNAs have been identified that have crucial roles in cell fate determination and organ patterning during development. Both the density and aperture size of PD are developmentally regulated, allowing formation of spatial symplastic domains for establishment of tissue-specific developmental programs. The PD size exclusion limit (SEL) is controlled by reversible deposition of callose, as well as by some PD-associated proteins. Although a large number of PD-associated proteins have been identified, many of their functions remain unknown. Despite the fact that PD are primarily membranous structures, surprisingly very little is known about their lipid composition. Thus, future studies in PD biology will provide deeper insights into the high-resolution structure and tightly regulated functions of PD and the evolution of PD-mediated cell-to-cell communication in plants.

Diffusion or bulk flow: how plasmodesmata facilitate pre-phloem transport of assimilates

Assimilates synthesized in the mesophyll of mature leaves move along the pre-phloem transport pathway to the bundle sheath of the minor veins from which they are loaded into the phloem. The present review discusses the most probable driving force(s) for the pre-phloem pathway, diffusion down the concentration gradient or bulk flow along a pressure gradient. The driving force seems to depend on the mode of phloem loading. In a majority of plant species phloem loading is a thermodynamically active process, involving the activity of membrane transporters in the sieve-element companion cell complex. Since assimilate movement includes an apoplasmic step, this mode is called apoplasmic loading. Well established is also the polymer-trap loading mode, where the phloem-transport sugars are raffinose-family oligomers in herbaceous plants. Also this mode depends on the investment of energy, here for sugar oligomerization, and leads to a high sugar accumulation in the phloem, even though the phloem is not symplasmically isolated, but well coupled by plasmodesmata (PD). Hence the mode polymer-trap mode is also designated active symplasmic loading. For woody angiosperms and gymnosperms an alternate loading mode is currently matter of discussion, called passive symplasmic loading. Based on the limited material available, this review compares the different loading modes and suggests that diffusion is the driving force in apoplasmic loaders, while bulk flow plays an increasing role in plants having a continuous symplasmic pathway from mesophyll to sieve elements. Crucial for the driving force is the question where water enters the pre-phloem pathway. Surprisingly, the role of PD in water movement has not been addressed so far appropriately. Modeling of assimilate and water fluxes indicates that in symplasmic loaders a considerable part of water flux happens through the PD between bundle sheath and phloem.

On the track of transfer cell formation by specialized plant-parasitic nematodes

Transfer cells are ubiquitous plant cells that play an important role in plant development as well as in responses to biotic and abiotic stresses. They are highly specialized and differentiated cells playing a central role in the acquisition, distribution and exchange of nutrients. Their unique structural traits are characterized by augmented ingrowths of invaginated secondary wall material, unsheathed by an amplified area of plasma membrane enriched in a suite of solute transporters. Similar morphological features can be perceived in vascular root feeding cells induced by sedentary plant-parasitic nematodes, such as root-knot and cyst nematodes, in a wide range of plant hosts. Despite their close phylogenetic relationship, these obligatory biotrophic plant pathogens engage different approaches when reprogramming root cells into giant cells or syncytia, respectively. Both nematode feeding-cells types will serve as the main source of nutrients until the end of the nematode life cycle. In both cases, these nematodes are able to remarkably maneuver and reprogram plant host cells. In this review we will discuss the structure, function and formation of these specialized multinucleate cells that act as nutrient transfer cells accumulating and synthesizing components needed for survival and successful offspring of plant-parasitic nematodes. Plant cells with transfer-like functions are also a renowned subject of interest involving still poorly understood molecular and cellular transport processes.

Increased symplasmic permeability in barley root epidermal cells correlates with defects in root hair development

It is well known that the process of plant cell differentiation depends on the symplasmic isolation of cells. Before starting the differentiation programme, the individual cell or group of cells should restrict symplasmic communication with neighbouring cells. We tested the symplasmic communication between epidermal cells in the different root zones of parental barley plants Hordeum vulgare L., cv. 'Karat' with normal root hair development, and two root hairless mutants (rhl1.a and rhl1.b). The results clearly show that symplasmic communication was limited during root hair differentiation in the parental variety, whereas in both root hairless mutants epidermal cells were still symplasmically connected in the corresponding root zone. This paper is the first report on the role of symplasmic isolation in barley root cell differentiation, and additionally shows that a disturbance in the restriction of symplasmic communication is present in root hairless mutants.

Identification of evolutionarily conserved amino acid residues in homeodomain of KNOX proteins for intercellular trafficking

Maize knotted (KN1) homeodomain (HD) protein is a well-known mobile transcription factor crucial for stem cell maintenance. Recent studies have revealed that the trihelical HD of knotted1-like homeobox (KNOX) proteins is necessary and sufficient for selective cell-to-cell trafficking. Also, the efficient trafficking ability for HD is likely to be acquired during the evolution of early nonvascular land plants. Here, using the point-mutated HD of KN1 and shoot meristemless (STM) in the trichome rescue system, together with molecular structure modeling, we have found the evolutionarily conserved amino acid residues, such as arginine in helix α1 and leucine in helix α3, which are essential for intercellular trafficking. Our studies provided important clues for the 3-dimensional protein structure required for cell-to-cell movement of non-cell-autonomous transcription factors.

Embryo and endosperm, partners in seed development

Angiosperm seeds are the major source of human calories, generating a pressing need to understand the underlying processes governing seed growth and development. They are composed of the two fertilization products, embryo and endosperm surrounded by the maternally derived seed coat. The successful interaction of all three seed components is a requirement for seeds to complete their development and to produce viable embryos that are competent to establish a new sporophytic generation. Here, we review recent reports investigating signal exchange between embryo and endosperm, focusing in particular on the transport of metabolites and small RNAs between both fertilization products.

Importance of symplasmic communication in cell differentiation

Symplasmic communication via plasmodesmata (PD) is part of the system of information exchange between plant cells. Molecules that pass through the PD include ions, some hormones, minerals, amino acids, and sugars but also proteins, transcription factors, and different classes of RNA, and as such PD can participate in the coordination of plant growth and development. This review summarizes the current literature on this subject and the role of PD in signal exchange, the importance of symplasmic communication and symplasmic domains in plant cell differentiation, and highlights the future prospective in the exploration of PD functions in plants. Moreover, this review also describes the potential use of barley root epidermis and non-zygotic embryogenesis in study of symplasmic communication during cell differentiation.

Arabidopsis mutants in sphingolipid synthesis as tools to understand the structure and function of membrane microdomains in plasmodesmata

Plasmodesmata-intercellular channels that communicate adjacent cells-possess complex membranous structures. Recent evidences indicate that plasmodesmata contain membrane microdomains. In order to understand how these submembrane regions collaborate to plasmodesmata function, it is necessary to characterize their size, composition and dynamics. An approach that can shed light on these microdomain features is based on the use of Arabidopsis mutants in sphingolipid synthesis. Sphingolipids are canonical components of microdomains together with sterols and some glycerolipids. Moreover, sphingolipids are transducers in pathways that display programmed cell death as a defense mechanism against pathogens. The study of Arabidopsis mutants would allow determining which structural features of the sphingolipids are important for the formation and stability of microdomains, and if defense signaling networks using sphingoid bases as second messengers are associated to plasmodesmata operation. Such studies need to be complemented by analysis of the ultrastructure and the use of protein probes for plasmodesmata microdomains and may constitute a very valuable source of information to analyze these membrane structures.

Callose homeostasis at plasmodesmata: molecular regulators and developmental relevance

Plasmodesmata are membrane-lined channels that are located in the plant cell wall and that physically interconnect the cytoplasm and the endoplasmic reticulum (ER) of adjacent cells. Operating as controllable gates, plasmodesmata regulate the symplastic trafficking of micro- and macromolecules, such as endogenous proteins [transcription factors (TFs)] and RNA-based signals (mRNA, siRNA, etc.), hence mediating direct cell-to-cell communication and long distance signaling. Besides this physiological role, plasmodesmata also form gateways through which viral genomes can pass, largely facilitating the pernicious spread of viral infections. Plasmodesmatal trafficking is either passive (e.g., diffusion) or active and responses both to developmental and environmental stimuli. In general, plasmodesmatal conductivity is regulated by the controlled build-up of callose at the plasmodesmatal neck, largely mediated by the antagonistic action of callose synthases (CalSs) and β-1,3-glucanases. Here, in this theory and hypothesis paper, we outline the importance of callose metabolism in PD SEL control, and highlight the main molecular factors involved. In addition, we also review other proteins that regulate symplastic PD transport, both in a developmental and stress-responsive framework, and discuss on their putative role in the modulation of PD callose turn-over. Finally, we hypothesize on the role of structural sterols in the regulation of (PD) callose deposition and outline putative mechanisms by which this regulation may occur.

Biophysical and size-dependent perspectives on plant evolution

Physical laws and processes have profoundly influenced plant evolution. Their effects are invariably size dependent and thus subject to scaling as well as biophysical analyses even though these effects differ depending upon the fluid (water or air) in which plants evolve. Although organisms cannot obviate the effects of physical laws and processes, the consequences of these effects can be altered by ontogenetic or phylogenetic alterations in geometry, shape, or orientation as well as in body size. These assertions are examined using theoretical insights and empirical data drawn from extant and fossil plants pertinent to four evolutionary transitions: (1) the evolution of multicellularity, (2) the transition from an aquatic to an aerial habitat, (3) the evolution of vascular tissues, and (4) the evolution of secondary growth by the independent acquisition of cambia. This examination shows how physical laws limit phenotypic expression, but how they also simultaneously provide alternative, potentially adaptive possibilities.

Asymmetric growth of root epidermal cells is related to the differentiation of root hair cells in Hordeum vulgare (L.)

The root epidermis of most vascular plants harbours two cell types, namely trichoblasts (capable of producing a root hair) and atrichoblasts. Here, in vivo analysis, confocal laser-scanning microscopy, transmission electron microscopy, histological analysis, and three-dimensional reconstruction were used to characterize the cell types present in the barley root epidermis and their distribution in the tissue. Both trichoblasts and atrichoblasts were present in the wild-type cultivars and could be distinguished from one another at an early stage. Trichoblast/atrichoblast differentiation depended on asymmetric cell expansion after a period of symmetrical cell division. After asymmetric growth, only the shorter epidermal cells could produce root hairs, whereas the longer cells became atrichoblasts. Moreover, the root epidermis did not develop root hairs at all if the epidermal cells did not differentiate into two asymmetric cell types. The root hairless phenotype of bald root barley (brb) and root hairless 1.b (rhl1.b) mutants was caused by a mutation in a gene related to the asymmetric expansion of the root epidermal cells. Additionally, the results showed that the mechanism of trichoblast/atrichoblast differentiation is not evolutionally conserved across the subfamilies of the Poaceae; in the Pooideae subfamily, both asymmetric division and asymmetric cell expansion have been observed.

Quantitative imaging of directional transport through plasmodesmata in moss protonemata via single-cell photoconversion of Dendra2

Cell-to-cell transport of molecules in plants must be properly regulated for plant growth and development. One specialized mechanism that plants have evolved involves transport through plasmodesmata (PD), but when and how transport of molecules via PD is regulated among individual cells remains largely unknown, particularly at the single-cell level. Here, we developed a tool for quantitatively analyzing cell-to-cell transport via PD at a single-cell level using protonemata of Physcomitrella patens and a photoconvertible fluorescent protein, Dendra2. In the filamentous protonemal tissues, one-dimensional intercellular communication can be observed easily. Using this system, we found that Dendra2 was directionally transported toward the apex of the growing protonemata. However, this directional transport could be eliminated by incubation in the dark or treatment with a metabolic inhibitor. Thus, we propose that directional transport of macromolecules can occur via PD in moss protonemata, and may be affected by the photosynthetic and metabolic activity of cells.

A comprehensive expression analysis of the Arabidopsis MICRORNA165/6 gene family during embryogenesis reveals a conserved role in meristem specification and a non-cell-autonomous function

One of the most fundamental events in plant ontogeny is the specification of the shoot and root apical meristem (SAM and RAM) in embryogenesis. In Arabidopsis, the restricted expression of class III homeodomain leucine zipper (HD-ZIP III) transcription factors (TFs) at the central-apical domain of early embryos is required for the correct specification of the SAM and RAM. Because the expression of HD-ZIP III TFs is suppressed by microRNA165/166 (miR165/6), elucidation of the sites of miR165/6 production and their activity range is a key to understanding the molecular basis of SAM and RAM specification in embryogenesis. Here, we present a comprehensive reporter analysis of all nine Arabidopsis MICRORNA165/166 (MIR165/6) genes during embryogenesis. We show that five MIR165/6 genes are transcribed in a largely conserved pattern in embryos, with their expression being preferentially focused at the basal-peripheral region of embryos. Our analysis also indicated that MIR165/6 transcription does not depend on SCARECROW (SCR) function in early embryos, in contrast to its requirement in post-embryonic roots. Furthermore, by observing the expression pattern of the miR-resistant PHBmu-GFP (green fluorescent protein) reporter, in either the presence or absence of the MIR165Amu transgene, which targets PHBmu-GFP, we obtained data that indicate a non-cell-autonomous function for miR165 in early embryos. These results suggest that miR165, and possibly miR166 as well, has the capacity to act as a positional cue from the basal-peripheral region of early embryos, and remotely controls SAM and RAM specification with their non-cell-autonomous function.

Are there symplastic connections between the endosperm and embryo in some angiosperms?--a lesson from the Crassulaceae family

It is believed that there is symplastic isolation between the embryo (new sporophyte) and the endosperm (maternal-parental origin tissue, which nourishes the embryo) in angiosperms. However, in embryological literature there are rare examples in which plasmodesmata between the embryo suspensor and endosperm cells have been recorded (three species from Fabaceae). This study was undertaken in order to test the hypothesis that plasmodesmata between the embryo suspensor and the endosperm are not so rare but also occur in other angiosperm families; in order to check this, we used the Crassulaceae family because embryogenesis in Crassulaceae has been studied extensively at an ultrastructure level recently and also we tread members of this family as model for suspensor physiology and function studies. These plasmodesmata even occurred between the basal cell of the two-celled proembryo and endosperm cells. The plasmodesmata were simple at this stage of development. During the development of the embryo proper and the suspensor, the structure of plasmodesmata changes. They were branched and connected with electron-dense material. Our results suggest that in Crassulaceae with plasmodesmata between the endosperm and suspensor, symplastic connectivity at this cell-cell boundary is still reduced or blocked at a very early stage of embryo development (before the globular stage). The occurrence of plasmodesmata between the embryo suspensor and endosperm cells suggests possible symplastic transport between these different organs, at least at a very early stage of embryo development. However, whether this transport actually occurs needs to be proven experimentally. A broader analysis of plants from various families would show whether the occurrence of plasmodesmata between the embryo suspensor and the endosperm are typical embryological characteristics and if this is useful in discussions about angiosperm systematic and evolution.

Regulation of plasmodesmatal permeability and stomatal patterning by the glycosyltransferase-like protein KOBITO1

The differentiation of stomata provides a convenient model for studying pattern formation in plant tissues. Stomata formation is induced by a set of basic helix-loop-helix transcription factors and inhibited by a signal transduction pathway initiated by TOO MANY MOUTHS (TMM) and ERECTA family (ERf) receptors. The formation of a proper stomata pattern is also dependent upon the restriction of symplastic movement of basic helix-loop-helix transcription factors into neighboring cells, especially in the backgrounds where the function of the TMM/ERf signaling pathway is compromised. Here, we describe a novel mutant of KOBITO1 in Arabidopsis (Arabidopsis thaliana). The kob1-3 mutation leads to the formation of stomata clusters in the erl1 erl2 background but not in the wild type. Cell-to-cell mobility assays demonstrated an increase in intercellular protein trafficking in kob1-3, including increased diffusion of SPEECHLESS, suggesting that the formation of stomata clusters is due to an escape of cell fate-specifying factors from stomatal lineage cells. While plasmodesmatal permeability is increased in kob1-3, we did not detect drastic changes in callose accumulation at the neck regions of the plasmodesmata. Previously, KOBITO1 has been proposed to function in cellulose biosynthesis. Our data demonstrate that disruption of cellulose biosynthesis in the erl1 erl2 background does not lead to the formation of stomata clusters, indicating that cellulose biosynthesis is not a major determining factor for regulating plasmodesmatal permeability. Analysis of KOBITO1 structure suggests that it is a glycosyltransferase-like protein. KOBITO1 might be involved in a carbohydrate metabolic pathway that is essential for both cellulose biosynthesis and the regulation of plasmodesmatal permeability.

Chaperonins facilitate KNOTTED1 cell-to-cell trafficking and stem cell function

Cell-to-cell communication in plants includes the selective trafficking of transcription factors and other signals through plasmodesmata. The KNOTTED1 (KN1) homeobox (KNOX) family transcription factors, which use this pathway, are essential for stem cell establishment and/or maintenance. Here we show that KN1 trafficking requires the chaperonin complex, which belongs to a group of cytosolic chaperones that fold specific substrate proteins. Genetic and physical interaction data show a functional relevance for chaperonins in KNOX family-dependent stem cell maintenance. Furthermore, tissue-specific complementation assays indicate a mechanistic basis for chaperonin function during the posttranslocational refolding process. Our study shows that chaperonins are essential for the cell-to-cell trafficking of a subset of mobile transcription factors and demonstrates the importance of chaperonin-dependent protein trafficking for plant stem cell function.

Organogenesis from stem cells in planta: multiple feedback loops integrating molecular and mechanical signals

In multicellular organisms, the coordination of cell behaviors largely relies on biochemical and biophysical signals. Understanding how such signals control development is often challenging, because their distribution relies on the activity of individual cells and, in a feedback loop, on tissue behavior and geometry. This review focuses on one of the best-studied structures in biology, the shoot apical meristem (SAM). This tissue is responsible for the production of all the aerial parts of a plant. In the SAM, a population of stem cells continuously produces new cells that are incorporated in lateral organs, such as leaves, branches, and flowers. Organogenesis from stem cells involves a tight regulation of cell identity and patterning as well as large-scale morphogenetic events. The gene regulatory network controlling these processes is highly coordinated in space by various signals, such as plant hormones, peptides, intracellular mobile factors, and mechanical stresses. Many crosstalks and feedback loops interconnecting these pathways have emerged in the past 10 years. The plant hormone auxin and mechanical forces have received more attention recently and their role is more particularly detailed here. An integrated view of these signaling networks is also presented in order to help understanding how robust shape and patterning can emerge from these networks.

Histology and symplasmic tracer distribution during development of barley androgenic embryos

The present study concerns three aspects of barley androgenesis: (1) the morphology and histology of the embryos during their development, (2) the time course of fluorescent symplasmic tracers' distribution, and (3) the correlation between symplasmic communication and cell differentiation. The results indicate that barley embryos, which are developing via an androgenic pathway, resemble their zygotic counterparts with respect to their developmental stages, morphology and histology. Analysis of the distribution of the symplasmic tracers, HPTS, and uncaged fluorescein indicates the symplasmic isolation of (1) the protodermis from the underlying cells of the late globular stage onwards, and (2) the embryonic organs at the mature stage of development.

Plasmodesmata: the battleground against intruders

Plasmodesmata are intercellular channels that establish a symplastic communication pathway between neighboring cells in plants. Owing to this role, opportunistic microbial pathogens have evolved to exploit plasmodesmata as gateways to spread infection from cell to cell within the plant. However, although these pathogens have acquired the capacity to breach the plasmodesmal trafficking pathway, plants are unlikely to relinquish control over a structure essential for their survival so easily. In this review, we examine evidence that suggests plasmodesmata play an active role in plant immunity against viral, fungal and bacterial pathogens. We discuss how these pathogens differ in their lifestyles and infection modes, and present the defense strategies that plants have adopted to prevent the intercellular spread of an infection.

Regulation of solute flux through plasmodesmata in the root meristem

Plasmodesmata permit solutes to move between cells nonspecifically and without having to cross a membrane. This symplastic connectivity, while straightforward to observe using fluorescent tracers, has proven difficult to quantify. We use fluorescence recovery after photobleaching, combined with a mathematical model of symplastic diffusion, to assay plasmodesmata-mediated permeability in the Arabidopsis (Arabidopsis thaliana) root meristem in wild-type and transgenic lines, and under selected chemical treatments. The permeability measured for the wild type is nearly 10-times greater than previously reported. Plamodesmal permeability remains constant in seedlings treated with auxin (30 mM indoleacetic acid for 2 and 24 h; 100 nm indoleacetic acid for 2 h); however, permeability is diminished in two lines previously reported to have impaired plasmodesmal function as well as in wild-type seedlings treated for 24 h with 0.6 mM tryptophan. Moreover, plasmodesmal permeability is strongly altered by applied hydrogen peroxide within 2 h of treatment, being approximately doubled at a low concentration (0.6 mM) and nearly eliminated at a higher one (6 mM). These results reveal that the plasmodesmata in the root meristem carry a substantial flux of small molecules and that this flux is subject to rapid regulation.

Redox regulation of intercellular transport

Plant cells communicate with each other via plasmodesmata (PDs) in order to orchestrate specific responses to environmental and developmental cues. At the same time, environmental signals regulate this communication by promoting changes in PD structure that modify symplastic permeability and, in extreme cases, isolate damaged cells. Reactive oxygen species (ROS) are key messengers in plant responses to a range of biotic and abiotic stresses. They are also generated during normal metabolism, and mediate signaling pathways that modulate plant growth and developmental transitions. Recent research has suggested the participation of ROS in the regulation of PD transport. The study of several developmental and stress-induced processes revealed a co-regulation of ROS and callose (a cell wall polymer that regulates molecular flux through PDs). The identification of Arabidopsis mutants simultaneously affected in cell redox homeostasis and PD transport, and the histological detection of hydrogen peroxide and peroxidases in the PDs of the tomato vascular cambium provide new information in support of this novel regulatory mechanism. Here, we describe the evidence that supports a role for ROS in the regulation of callose deposition and/or in the formation of secondary PD, and discuss the potential importance of this mechanism during plant growth or defense against environmental stresses.