Imaging callose at plasmodesmata using aniline blue: quantitative confocal microscopy

C-Type LECTIN receptor-like kinase 1 and ACTIN DEPOLYMERIZING FACTOR 3 are key components of plasmodesmata callose modulation

Plasmodesmata (PDs) are intercellular organelles carrying multiple membranous nanochannels that allow the trafficking of cellular signalling molecules. The channel regulation of PDs occurs dynamically and is required in various developmental and physiological processes. It is well known that callose is a critical component in regulating PD permeability or symplasmic connectivity, but the understanding of the signalling pathways and mechanisms of its regulation is limited. Here, we used the reverse genetic approach to investigate the role of C-type lectin receptor-like kinase 1 (CLRLK1) in the aspect of PD callose-modulated symplasmic continuity. Here, we found that loss-of-function mutations in CLRLK1 resulted in excessive PD callose deposits and reduced symplasmic continuity, resulting in an accelerated gravitropic response. The protein interactome study also found that CLRLK1 interacted with actin depolymerizing factor 3 (ADF3) in vitro and in plants. Moreover, mutations in ADF3 result in elevated PD callose deposits and faster gravitropic response. Our results indicate that CLRLK1 and ADF3 negatively regulate PD callose accumulation, contributing to fine-tuning symplasmic opening apertures. Overall, our studies identified two key components involved in the deposits of PD callose and provided new insights into how symplasmic connectivity is maintained by the control of PD callose homoeostasis.

A phase-separated protein hub modulates resistance to Fusarium head blight in wheat

Fusarium head blight (FHB) is a devastating wheat disease. Fhb1, the most widely applied genetic locus for FHB resistance, is conferred by TaHRC of an unknown mode of action. Here, we show that TaHRC alleles distinctly drive liquid-liquid phase separation (LLPS) within a proteinaceous complex, determining FHB susceptibility or resistance. TaHRC-S (susceptible) exhibits stronger LLPS ability than TaHRC-R (resistant), and this distinction is further intensified by fungal mycotoxin deoxynivalenol, leading to opposing FHB symptoms. TaHRC recruits a protein class with intrinsic LLPS potentials, referred to as an "HRC-containing hub." TaHRC-S drives condensation of hub components, while TaHRC-R comparatively suppresses hub condensate formation. The function of TaSR45a splicing factor, a hub member, depends on TaHRC-driven condensate state, which in turn differentially directs alternative splicing, switching between susceptibility and resistance to wheat FHB. These findings reveal a mechanism for FHB spread within a spike and shed light on the roles of complex condensates in controlling plant disease.

Comparing Methods for Detection and Quantification of Plasmodesmal Callose in Nicotiana benthamiana Leaves During Defense Responses

Callose, a β-(1,3)-d-glucan polymer, is essential for regulating intercellular trafficking via plasmodesmata (PD). Pathogens manipulate PD-localized proteins to enable intercellular trafficking by removing callose at PD or, conversely, by increasing callose accumulation at PD to limit intercellular trafficking during infection. Plant defense hormones like salicylic acid regulate PD-localized proteins to control PD and intercellular trafficking during immune defense responses such as systemic acquired resistance. Measuring callose deposition at PD in plants has therefore emerged as a popular parameter for assessing likely intercellular trafficking activity during plant immunity. Despite the popularity of this metric, there is no standard for how these measurements should be made. In this study, three commonly used methods for identifying and quantifying plasmodesmal callose by aniline blue staining were evaluated to determine the most effective in the Nicotiana benthamiana leaf model. The results reveal that the most reliable method used aniline blue staining and fluorescence microscopy to measure callose deposition in fixed tissue. Manual or semiautomated workflows for image analysis were also compared and found to produce similar results, although the semiautomated workflow produced a wider distribution of data points. [Formula: see text] Copyright © 2024 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Arabidopsis PDLP7 modulated plasmodesmata function is related to BG10-dependent glucosidase activity required for callose degradation

The microdomains of plasmodesmata, specialized cell-wall channels responsible for communications between neighboring cells, are composed of various plasmodesmata-located proteins (PDLPs) and lipids. Here, we found that, among all PDLP or homologous proteins in Arabidopsis thaliana genome, PDLP5 and PDLP7 possessed a C-terminal sphingolipid-binding motif, with the latter being the only member that was significantly upregulated upon turnip mosaic virus and cucumber mosaic virus infections. pdlp7 mutant plants exhibited significantly reduced callose deposition, larger plasmodesmata diameters, and faster viral transmission. These plants exhibited increased glucosidase activity but no change in callose synthase activity. PDLP7 interacted specifically with glucan endo-1,3-β-glucosidase 10 (BG10). Consistently, higher levels of callose deposition and slower virus transmission in bg10 mutants were observed. The interaction between PDLP7 and BG10 was found to depend on the presence of the Gnk2-homologous 1 (GnK2-1) domain at the N terminus of PDLP7 with Asp-35, Cys-42, Gln-44, and Leu-116 being essential. In vitro supplementation of callose was able to change the conformation of the GnK2-1 domain. Our data suggest that the GnK2-1 domain of PDLP7, in conjunction with callose and BG10, plays a key role in plasmodesmata opening and closure, which is necessary for intercellular movement of various molecules.

Imaging plant cell walls using fluorescent stains: The beauty is in the details

Plants continuously face various environmental stressors throughout their lifetime. To be able to grow and adapt in different environments, they developed specialized tissues that allowed them to maintain a protected yet interconnected body. These tissues undergo specific primary and secondary cell wall modifications that are essential to ensure normal plant growth, adaptation and successful land colonization. The composition of cell walls can vary among different plant species, organs and tissues. The ability to remodel their cell walls is fundamental for plants to be able to cope with multiple biotic and abiotic stressors. A better understanding of the changes taking place in plant cell walls may help identify and develop new strategies as well as tools to enhance plants' survival under environmental stresses or prevent pathogen attack. Since the invention of microscopy, numerous imaging techniques have been developed to determine the composition and dynamics of plant cell walls during normal growth and in response to environmental stimuli. In this review, we discuss the main advances in imaging plant cell walls, with a particular focus on fluorescent stains for different cell wall components and their compatibility with tissue clearing techniques. Lay Description: Plants are continuously subjected to various environmental stresses during their lifespan. They evolved specialized tissues that thrive in different environments, enabling them to maintain a protected yet interconnected body. Such tissues undergo distinct primary and secondary cell wall alterations essential to normal plant growth, their adaptability and successful land colonization. Cell wall composition may differ among various plant species, organs and even tissues. To deal with various biotic and abiotic stresses, plants must have the capacity to remodel their cell walls. Gaining insight into changes that take place in plant cell walls will help identify and create novel tools and strategies to improve plants' ability to withstand environmental challenges. Multiple imaging techniques have been developed since the introduction of microscopy to analyse the composition and dynamics of plant cell walls during growth and in response to environmental changes. Advancements in plant tissue cleaning procedures and their compatibility with cell wall stains have significantly enhanced our ability to perform high-resolution cell wall imaging. At the same time, several factors influence the effectiveness of cleaning and staining plant specimens, as well as the time necessary for the process, including the specimen's size, thickness, tissue complexity and the presence of autofluorescence. In this review, we will discuss the major advances in imaging plant cell walls, with a particular emphasis on fluorescent stains for diverse cell wall components and their compatibility with tissue clearing techniques. We hope that this review will assist readers in selecting the most appropriate stain or combination of stains to highlight specific cell wall components of interest.

Stem-pitting caused by Citrus tristeza virus is associated with increased phloem occlusion

Stem-pitting (SP) disease results from disruption of normal phloem and xylem development. In citrus, a characteristic manifestation of SP caused by Citrus tristeza virus (CTV) is phloem regeneration. We hypothesized that phloem regeneration occurs due to reduced functionality of CTV infected phloem cells. To examine phloem cell occlusions in CTV-SP, we analyzed callose and phloem-protein (PP) accumulation in Citrus macrophylla trees infected with CTV mutants exhibiting different SP phenotypes from very mild (CTVΔp13) to severe (CTVΔp33), in addition to full-length CTV and healthy plants. CTV infection was accompanied by callose and PP accumulation in the phloem. With the increase in the SP symptoms from very mild to severe, there was a constant increase in the levels of callose and PP, accompanied by an increase in PHLOEM-PROTEIN 2 and a decrease in BETA-1,3-GLUCANASE gene expression levels. These results indicate that SP symptom development is associated with increased phloem occlusion.

Localization of Arabidopsis Glucan Synthase-Like 5, 8, and 12 to plasmodesmata and the GSL8-dependent role of PDLP5 in regulating plasmodesmal permeability

Cell-to-cell communication via membranous channels called plasmodesmata (PD) plays critical roles during plant development and in response to biotic and abiotic stresses. Several enzymes and receptor-like proteins (RLPs), including Arabidopsis thaliana glucan synthase-likes (GSLs), also known as callose synthases (CALSs), and PD-located proteins (PDLPs), have been implicated in plasmodesmal permeability regulation and intercellular communication. Localization of PDLPs to punctate structures at the cell periphery and their receptor-like identity have raised the hypothesis that PDLPs are involved in the regulation of symplastic trafficking during plant development and in response to endogenous and exogenous signals. Indeed, it was shown that PDLP5 could limit plasmodesmal permeability through inducing an increase in callose accumulation at PD. However, mechanistically, how this is achieved remains to be elucidated. To address this key issue in understanding the regulation of PD, physical and functional interactions between PDLPs and GSLs (using the PDLP5-GSL8/CALS10 pair as a model) were investigated. Our results show that GSL8/CALS10 plays essential roles and is required for the function and plasmodesmal localization of PDLP5. Furthermore, it was demonstrated that the localization of PDLP5 to PD and its function in inducing callose deposition are GSL8-dependent. Importantly, our transgenic study shows that three key members of the GSL family, i.e., GSL5/CALS12, GSL8/CALS10, and GSL12/CALS3, localize to PD and co-localize with PDLP5, suggesting that GSL8/CALS10 might not be the only callose synthase with the determining role in PD regulation. These findings, together with our previous observation showing the direct interaction of GSL8/CALS10 with PDLP5, indicate the pivotal role of the GSL8/CALS10-PDLP5 interplay in regulating PD permeability. Future work is needed to investigate whether the PDLP5 functionality and localization are also disrupted in gsl5 and gsl12, or it is just gsl8-specific.

An elastic proteinaceous envelope encapsulates the early Arabidopsis embryo

Plant external surfaces are often covered by barriers that control the exchange of molecules, protect from pathogens and offer mechanical integrity. A key question is when and how such surface barriers are generated. Post-embryonic surfaces have well-studied barriers, including the cuticle, and it has been previously shown that the late Arabidopsis thaliana embryo is protected by an endosperm-derived sheath deposited onto a primordial cuticle. Here, we show that both cuticle and sheath are preceded by another structure during the earliest stages of embryogenesis. This structure, which we named the embryonic envelope, is tightly wrapped around the embryonic surface but can be physically detached by cell wall digestion. We show that this structure is composed primarily of extensin and arabinogalactan O-glycoproteins and lipids, which appear to form a dense and elastic crosslinked embryonic envelope. The envelope forms in cuticle-deficient mutants and in a mutant that lacks endosperm. This embryo-derived envelope is therefore distinct from previously described cuticle and sheath structures. We propose that it acts as an expandable diffusion barrier, as well as a means to mechanically confine the embryo to maintain its tensegrity during early embryogenesis.

Plasmodesmata callose binding protein 2 contributes to the regulation of cambium/phloem formation and auxin response during the tissue reunion process in incised Arabidopsis stem

Plants are exposed to a variety of biotic and abiotic stresses, including wounding at the stem. The healing process (tissue reunion) begins immediately after stem wounding. The plant hormone auxin plays an important role during tissue reunion. In decapitated stems, auxin transport from the shoot apex is reduced and tissue reunion does not occur but is restored by application of indole-3-acetic acid (IAA). In this study, we found that plasmodesmata callose binding protein 2 (PDCB2) affects the expansion of the cambium/phloem region via changes in auxin response during the process of tissue reunion. PDCB2 was expressed in the cortex and endodermis on the incised side of stems 1-3 days after incision. PDCB2-knockout plants showed reduced callose deposition at plasmodesmata and DR5::GUS activity in the endodermis/cortex in the upper region of the incision accompanied by an increase in size of the cambium/phloem region during tissue reunion. In addition, PIN(PIN-FORMED)3, which is involved in lateral auxin transport, was induced by auxin in the cambium/phloem and endodermis/cortex in the upper part of the incision in wild type, but its expression of PIN3 was decreased in pdcb2 mutant. Our results suggest that PDCB2 contributes to the regulation of cambium/phloem development via auxin response.

Reliable detection and quantification of plasmodesmal callose in Nicotiana benthamiana leaves during defense responses

Callose, a beta-(1,3)-D-glucan polymer, is essential for regulating intercellular trafficking via plasmodesmata (PD). Pathogens manipulate PD-localized proteins to enable intercellular trafficking by removing callose at PD, or conversely by increasing callose accumulation at PD to limit intercellular trafficking during infection. Plant defense hormones like salicylic acid regulate PD-localized proteins to control PD and intercellular trafficking during innate immune defense responses such as systemic acquired resistance. Measuring callose deposition at PD in plants has therefore emerged as a popular parameter for assessing the intercellular trafficking activity during plant immunity. Despite the popularity of this metric there is no standard for how these measurements should be made. In this study, three commonly used methods for identifying and quantifying PD callose by aniline blue staining were evaluated to determine the most effective in the Nicotiana benthamiana leaf model. The results reveal that the most reliable method used aniline blue staining and fluorescent microscopy to measure callose deposition in fixed tissue. Manual or semi-automated workflows for image analysis were also compared and found to produce similar results although the semi-automated workflow produced a wider distribution of data points.

Epigenetic silencing of callose synthase by VIL1 promotes bud-growth transition in lily bulbs

In plants, restoring intercellular communication is required for cell activity in buds during the growth transition from slow to fast growth after dormancy release. However, the epigenetic regulation of this phenomenon is far from understood. Here we demonstrate that lily VERNALIZATION INSENSITIVE 3-LIKE 1 (LoVIL1) confers growth transition by mediating plasmodesmata opening via epigenetic repression of CALLOSE SYNTHASE 3 (LoCALS3). Moreover, we found that a novel transcription factor, NUCLEAR FACTOR Y, SUBUNIT A7 (LoNFYA7), is capable of recruiting the LoVIL1-Polycomb Repressive Complex 2 (PRC2) and enhancing H3K27me3 at the LoCALS3 locus by recognizing the CCAAT cis-element (Cce) of its promoter. The LoNFYA7-LoVIL1 module serves as a key player in orchestrating the phase transition from slow to fast growth in lily bulbs. These studies also indicate that LoVIL1 is a suitable marker for the bud-growth-transition trait following dormancy release in lily cultivars.

Nicotiana benthamiana Class 1 Reversibly Glycosylated Polypeptides Suppress Tobacco Mosaic Virus Infection

Reversibly glycosylated polypeptides (RGPs) have been identified in many plant species and play an important role in cell wall formation, intercellular transport regulation, and plant-virus interactions. Most plants have several RGP genes with different expression patterns depending on the organ and developmental stage. Here, we report on four members of the RGP family in N. benthamiana. Based on a homology search, NbRGP1-3 and NbRGP5 were assigned to the class 1 and class 2 RGPs, respectively. We demonstrated that NbRGP1-3 and 5 mRNA accumulation increases significantly in response to tobacco mosaic virus (TMV) infection. Moreover, all identified class 1 NbRGPs (as distinct from NbRGP5) suppress TMV intercellular transport and replication in N. benthamiana. Elevated expression of NbRGP1-2 led to the stimulation of callose deposition at plasmodesmata, indicating that RGP-mediated TMV local spread could be affected via a callose-dependent mechanism. It was also demonstrated that NbRGP1 interacts with TMV movement protein (MP) in vitro and in vivo. Therefore, class 1 NbRGP1-2 play an antiviral role by impeding intercellular transport of the virus by affecting plasmodesmata callose and directly interacting with TMV MP, resulting in the reduced viral spread and replication.

D, Noriega A, Drakakaki G, Sylvester AW, Rasmussen CG. Subcellular positioning during cell division and cell plate formation in maize

Introduction

During proliferative plant cell division, the new cell wall, called the cell plate, is first built in the middle of the cell and then expands outward to complete cytokinesis. This dynamic process requires coordinated movement and arrangement of the cytoskeleton and organelles.

Methods

Here we use live-cell markers to track the dynamic reorganization of microtubules, nuclei, endoplasmic reticulum, and endomembrane compartments during division and the formation of the cell plate in maize leaf epidermal cells.

Results

The microtubule plus-end localized protein END BINDING1 (EB1) highlighted increasing microtubule dynamicity during mitosis to support rapid changes in microtubule structures. The localization of the cell-plate specific syntaxin KNOLLE, several RAB-GTPases, as well as two plasma membrane localized proteins was assessed after treatment with the cytokinesis-specific callose-deposition inhibitor Endosidin7 (ES7) and the microtubule-disrupting herbicide chlorpropham (CIPC). While ES7 caused cell plate defects in Arabidopsis thaliana, it did not alter callose accumulation, or disrupt cell plate formation in maize. In contrast, CIPC treatment of maize epidermal cells occasionally produced irregular cell plates that split or fragmented, but did not otherwise disrupt the accumulation of cell-plate localized proteins.

Discussion

Together, these markers provide a robust suite of tools to examine subcellular trafficking and organellar organization during mitosis and cell plate formation in maize.

Cold stress induces malformed tomato fruits by breaking the feedback loops of stem cell regulation in floral meristem

Fruit malformation is a major constrain in fruit production worldwide resulting in substantial economic losses. The farmers for decades noticed that the chilling temperature before blooming often caused malformed fruits. However, the molecular mechanism underlying this phenomenon is unclear. Here we examined the fruit development in response to cold stress in tomato, and demonstrated that short-term cold stress increased the callose accumulation in both shoot apical and floral meristems, resulting in the symplastic isolation and altered intercellular movement of WUS. In contrast to the rapidly restored SlWUS transcription during the recovery from cold stress, the callose removal was delayed due to obstructed plasmodesmata. The delayed reinstatement of cell-to-cell transport of SlWUS prevented the activation of SlCLV3 and TAG1, causing the interrupted feedback inhibition of SlWUS expression, leading to the expanded stem cell population and malformed fruits. We further showed that the callose dynamics in response to short-term cold stress presumably exploits the mechanism of bud dormancy during the seasonal growth, involving two antagonistic hormones, abscisic acid and gibberellin. Our results provide a novel insight into the cold stress regulated malformation of fruit.

ROS-mediated plasmodesmal regulation requires a network of an Arabidopsis receptor-like kinase, calmodulin-like proteins, and callose synthases

Plasmodesmata (PD) play a critical role in symplasmic communication, coordinating plant activities related to growth & development, and environmental stress responses. Most developmental and environmental stress signals induce reactive oxygen species (ROS)-mediated signaling in the apoplast that causes PD closure by callose deposition. Although the apoplastic ROS signals are primarily perceived at the plasma membrane (PM) by receptor-like kinases (RLKs), such components involved in PD regulation are not yet known. Here, we show that an Arabidopsis NOVEL CYS-RICH RECEPTOR KINASE (NCRK), a PD-localized protein, is required for plasmodesmal callose deposition in response to ROS stress. We identified the involvement of NCRK in callose accumulation at PD channels in either basal level or ROS-dependent manner. Loss-of-function mutant (ncrk) of NCRK induces impaired callose accumulation at the PD under the ROS stress resembling a phenotype of the PD-regulating GLUCAN SYNTHASE-LIKE 4 (gsl4) knock-out plant. The overexpression of transgenic NCRK can complement the callose and the PD permeability phenotypes of ncrk mutants but not kinase-inactive NCRK variants or Cys-mutant NCRK, in which Cys residues were mutated in Cys-rich repeat ectodomain. Interestingly, NCRK mediates plasmodesmal permeability in mechanical injury-mediated signaling pathways regulated by GSL4. Furthermore, we show that NCRK interacts with calmodulin-like protein 41 (CML41) and GSL4 in response to ROS stress. Altogether, our data indicate that NCRK functions as an upstream regulator of PD callose accumulation in response to ROS-mediated stress signaling pathways.

Nicotiana benthamiana Kunitz peptidase inhibitor-like protein involved in chloroplast-to-nucleus regulatory pathway in plant-virus interaction

Plant viruses use a variety of strategies to infect their host. During infection, viruses cause symptoms of varying severity, which are often associated with altered leaf pigmentation due to structural and functional damage to chloroplasts that are affected by viral proteins. Here we demonstrate that Nicotiana benthamiana Kunitz peptidase inhibitor-like protein (KPILP) gene is induced in response to potato virus X (PVX) infection. Using reverse genetic approach, we have demonstrated that KPILP downregulates expression of LHCB1 and LHCB2 genes of antenna light-harvesting complex proteins, HEMA1 gene encoding glutamyl-tRNA reductase, which participates in tetrapyrrole biosynthesis, and RBCS1A gene encoding RuBisCO small subunit isoform involved in the antiviral immune response. Thus, KPILP is a regulator of chloroplast retrograde signaling system during developing PVX infection. Moreover, KPILP was demonstrated to affect carbon partitioning: reduced glucose levels during PVX infection were associated with KPILP upregulation. Another KPILP function is associated with plasmodesmata permeability control. Its ability to stimulate intercellular transport of reporter 2xGFP molecules indicates that KPILP is a positive plasmodesmata regulator. Moreover, natural KPILP glycosylation is indispensable for manifestation of this function. During PVX infection KPILP increased expression leads to the reduction of plasmodesmata callose deposition. These results could indicate that KPILP affects plasmodesmata permeability via callose-dependent mechanism. Thus, virus entering a cell and starting reproduction triggers KPILP expression, which leads to downregulation of nuclear-encoded chloroplast genes associated with retrograde signaling, reduction in photoassimilates accumulation and increase in intercellular transport, creating favorable conditions for reproduction and spread of viral infection.

Sandwich Enzyme-Linked Immunosorbent Assay for Quantification of Callose

The existing methods of callose quantification include epifluorescence microscopy and fluorescence spectrophotometry of aniline blue-stained callose particles, immuno-fluorescence microscopy and indirect assessment of both callose synthase and β-(1,3)-glucanase enzyme activities. Some of these methods are laborious, time consuming, not callose-specific, biased and require high technical skills. Here, we describe a method of callose quantification based on Sandwich Enzyme-Linked Immunosorbent Assay (S-ELISA). Tissue culture-derived banana plantlets were inoculated with Xanthomonas campestris pv. musacearum (Xcm) bacteria as a biotic stress factor inducing callose production. Banana leaf, pseudostem and corm tissue samples were collected at 14 days post-inoculation (dpi) for callose quantification. Callose levels were significantly different in banana tissues of Xcm-inoculated and control groups except in the pseudostems of both banana genotypes. The method described here could be applied for the quantification of callose in different plant species with satisfactory level of specificity to callose, and reproducibility. Additionally, the use of 96-well plate makes this method suitable for high throughput callose quantification studies with minimal sampling and analysis biases. We provide step-by-step detailed descriptions of the method.

A comparative meta-proteomic pipeline for the identification of plasmodesmata proteins and regulatory conditions in diverse plant species

BACKGROUND

A major route for cell-to-cell signalling in plants is mediated by cell wall-embedded pores termed plasmodesmata forming the symplasm. Plasmodesmata regulate the plant development and responses to the environment; however, our understanding of what factors or regulatory cues affect their structure and permeability is still limited. In this paper, a meta-analysis was carried out for the identification of conditions affecting plasmodesmata transport and for the in silico prediction of plasmodesmata proteins in species for which the plasmodesmata proteome has not been experimentally determined.

RESULTS

Using the information obtained from experimental proteomes, an analysis pipeline (named plasmodesmata in silico proteome 1 or PIP1) was developed to rapidly generate candidate plasmodesmata proteomes for 22 plant species. Using the in silico proteomes to interrogate published transcriptomes, gene interaction networks were identified pointing to conditions likely affecting plasmodesmata transport capacity. High salinity, drought and osmotic stress regulate the expression of clusters enriched in genes encoding plasmodesmata proteins, including those involved in the metabolism of the cell wall polysaccharide callose. Experimental determinations showed restriction in the intercellular transport of the symplasmic reporter GFP and enhanced callose deposition in Arabidopsis roots exposed to 75-mM NaCl and 3% PEG (polyethylene glycol). Using PIP1 and transcriptome meta-analyses, candidate plasmodesmata proteins for the legume Medicago truncatula were generated, leading to the identification of Medtr1g073320, a novel receptor-like protein that localises at plasmodesmata. Expression of Medtr1g073320 affects callose deposition and the root response to infection with the soil-borne bacteria rhizobia in the presence of nitrate.

CONCLUSIONS

Our study shows that combining proteomic meta-analysis and transcriptomic data can be a valuable tool for the identification of new proteins and regulatory mechanisms affecting plasmodesmata function. We have created the freely accessible pipeline PIP1 as a resource for the screening of experimental proteomes and for the in silico prediction of PD proteins in diverse plant species.

Structural basis of NPR1 in activating plant immunity

NPR1 is a master regulator of the defence transcriptome induced by the plant immune signal salicylic acid1-4. Despite the important role of NPR1 in plant immunity5-7, understanding of its regulatory mechanisms has been hindered by a lack of structural information. Here we report cryo-electron microscopy and crystal structures of Arabidopsis NPR1 and its complex with the transcription factor TGA3. Cryo-electron microscopy analysis reveals that NPR1 is a bird-shaped homodimer comprising a central Broad-complex, Tramtrack and Bric-à-brac (BTB) domain, a BTB and carboxyterminal Kelch helix bundle, four ankyrin repeats and a disordered salicylic-acid-binding domain. Crystal structure analysis reveals a unique zinc-finger motif in BTB for interacting with ankyrin repeats and mediating NPR1 oligomerization. We found that, after stimulation, salicylic-acid-induced folding and docking of the salicylic-acid-binding domain onto ankyrin repeats is required for the transcriptional cofactor activity of NPR1, providing a structural explanation for a direct role of salicylic acid in regulating NPR1-dependent gene expression. Moreover, our structure of the TGA32-NPR12-TGA32 complex, DNA-binding assay and genetic data show that dimeric NPR1 activates transcription by bridging two fatty-acid-bound TGA3 dimers to form an enhanceosome. The stepwise assembly of the NPR1-TGA complex suggests possible hetero-oligomeric complex formation with other transcription factors, revealing how NPR1 reprograms the defence transcriptome.

An RNA exosome subunit mediates cell-to-cell trafficking of a homeobox mRNA via plasmodesmata

Messenger RNAs (mRNAs) function as mobile signals for cell-to-cell communication in multicellular organisms. The KNOTTED1 (KN1) homeodomain family transcription factors act non–cell autonomously to control stem cell maintenance in plants through cell-to-cell movement of their proteins and mRNAs through plasmodesmata; however, the mechanism of mRNA movement is largely unknown. We show that cell-to-cell movement of a KN1 mRNA requires ribosomal RNA–processing protein 44A (AtRRP44A), a subunit of the RNA exosome that processes or degrades diverse RNAs in eukaryotes. AtRRP44A can interact with plasmodesmata and mediates the cell-to-cell trafficking of KN1 mRNA, and genetic analysis indicates that AtRRP44A is required for the developmental functions of SHOOT MERISTEMLESS, an Arabidopsis KN1 homolog. Our findings suggest that AtRRP44A promotes mRNA trafficking through plasmodesmata to control stem cell–dependent processes in plants.

The E3 Ubiquitin Ligase ATL9 Affects Expression of Defense Related Genes, Cell Death and Callose Deposition in Response to Fungal Infection

Plants use diverse strategies to defend themselves from biotic stresses in nature, which include the activation of defense gene expression and a variety of signal transduction pathways. Previous studies have shown that protein ubiquitination plays a critical role in plant defense responses, however the details of its function remain unclear. Our previous work has shown that increasing expression levels of ATL9, an E3 ubiquitin ligase in Arabidopsis thaliana, increased resistance to infection by the fungal pathogen, Golovinomyces cichoracearum. In this study, we demonstrate that the defense-related proteins PDF1.2, PCC1 and FBS1 directly interact with ATL9 and are targeted for degradation to the proteasome by ATL9. The expression levels of PDF1.2, PCC1 and FBS1 are decreased in T-DNA insertional mutants of atl9 and T-DNA insertional mutants of pdf1.2, pcc1 and fbs1 are more susceptible to fungal infection. In addition, callose is more heavily deposited at infection sites in the mutants of atl9, fbs1, pcc1 and pdf1.2. Overexpression of ATL9 and of mutants in fbs1, pcc1 and pdf1.2 showed increased levels of cell death during infection. Together these results indicate that ubiquitination, cell death and callose deposition may work together to enhance defense responses to fungal pathogens.

In Vivo Aniline Blue Staining and Semiautomated Quantification of Callose Deposition at Plasmodesmata

The deposition and turnover of callose (beta-1,3 glucan polymer) in the cell wall surrounding the neck regions of plasmodesmata (PD) controls the cell-to-cell diffusion rate of molecules and, therefore, plays an important role in the regulation of intercellular communication in plants.Here we describe a simple and fast in vivo staining procedure for the imaging and quantification of callose at PD. We also introduce calloseQuant, a plug-in for semiautomated image analysis and non-biased quantification of callose levels at PD using ImageJ.

Studying Protein-Protein Interactions at Plasmodesmata by Measuring Förster Resonance Energy Transfer

Plasmodesmata (PD) provide interconnectivity between plant cells to enable the intercellular transport and communication that is requisite to multicellularity. Being at the interface of the apoplast, plasma membrane (PM), endoplasmic reticulum (ER), and symplast, PD are uniquely positioned to integrate exogenously and endogenously derived signals with plant developmental and physiological responses. The distinct membrane curvature and composition of PD allow them to function as microdomains to facilitate dynamic protein-protein interactions. Förster resonance energy transfer (FRET) combined with fluorescence lifetime imaging microscopy (FLIM) and fluorescence anisotropic decay measurements provides valuable tools to analyze these interactions in vivo and in planta. Here we describe a detailed methodology to perform FRET-FLIM and fluorescence anisotropy measurements to analyze protein-protein interactions at PD in a transient expression system using Nicotiana benthamiana; however this can be adapted to other plant species and subcellular compartments.

Measuring Plasmodesmata Density on Cell Interfaces of Monocot Leaves Using 3D Immunolocalization and Scanning Electron Microscopy

Quantification of plasmodesmata density on cell interfaces of plant tissues, particularly of leaves, has been a long-standing challenge. Using electron microscopy alone to quantify plasmodesmata is difficult because of the limited surface area coverage per image and hence the need to examine large numbers of sections for robust quantification. Fluorescence microscopy provides the larger surface area coverage per image but can only visualize pit fields and not individual plasmodesma. Moreover, in pigmented tissue like leaves, imaging cell interfaces beyond the epidermal layer would also require accurate sectioning. The advent of tissue clearing techniques such as PEA-CLARITY provided the opportunity to capture all pit fields within the leaf without resorting to sectioning. This paved the way toward the development of a more robust and precise plasmodesmata density quantification method by combining the three-dimensional immunolocalization fluorescence microscopy with scanning electron microscopy (SEM). Here, I describe a protocol to quantify plasmodesmata density on cell interfaces between mesophyll and bundle sheath in C₃ and C₄ monocot leaves.

Spatiotemporal Specific Blocking of Plasmodesmata by Callose Induction

Plasmodesmata are nanoscale cell wall channels connecting neighboring cells in plants. Intercellular trafficking of molecules via plasmodesmata plays important roles in various developmental processes and stress responses. The turnover of callose, a β-1,3-glucan polysaccharide depositing in the cell wall around plasmodesmata, controls the plasmodesmal permeability and symplasmic transport. Here, we describe a protocol for the spatiotemporally controlled induction of callose synthesis and plasmodesmata closure using the cals3m system. In this system, cals3m, a mutant CALLOSE SYNTHASE 3 (CALS3) gene, is driven by inducible tissue-specific promoters of interest. After appropriate induction by 17-β-estradiol, callose is overproduced within the corresponding specific domains, resulting in temporal closure of plasmodesmata at the cell-cell interfaces. This approach can be used to validate and dissect the function of plasmodesmata-mediated symplasmic communications.

Callose Detection and Quantification at Plasmodesmata in Bryophytes

In bryophytes (i.e., mosses, liverworts, and hornworts), extant representatives of early land plants, plasmodesmata have been described in a wide range of tissues. Although their contribution to bryophyte morphogenesis remains largely unexplored, several recent studies have suggested that the deposition of callose around plasmodesmata might regulate developmental and physiological responses in mosses. In this chapter, we provide a protocol to image and quantify callose levels in the filamentous body of the model moss Physcomitrium (Physcomitrella) patens and discuss possible alternatives and pitfalls. More generally, this protocol establishes a framework to explore the distribution of callose in other bryophytes.

Approaches for investigating plasmodesmata and effective communication

Plasmodesmata (PD) are integral plant cell wall components that provide routes for intercellular communication, signaling, and resource sharing. They are therefore essential for plant growth and survival. Much effort has been put forth to understand how PD are generated and their structure is refined for function and to determine how they regulate intercellular trafficking. This review provides an overview of some of the approaches that have been used to study PD structure and function, highlighting those that may be more widely adopted to address questions of PD cell biology and function. Extending our focus on the importance of communication, we address how effective communication strategies can increase diversity and accessibility in the research laboratory, focusing on challenges faced by our deaf/hard-of-hearing colleagues, and highlight successful approaches to including them in the research laboratory.

Light-triggered and phosphorylation-dependent 14-3-3 association with NON-PHOTOTROPIC HYPOCOTYL 3 is required for hypocotyl phototropism

NON-PHOTOTROPIC HYPOCOTYL 3 (NPH3) is a key component of the auxin-dependent plant phototropic growth response. We report that NPH3 directly binds polyacidic phospholipids, required for plasma membrane association in darkness. We further demonstrate that blue light induces an immediate phosphorylation of a C-terminal 14-3-3 binding motif in NPH3. Subsequent association of 14-3-3 proteins is causal for the light-induced release of NPH3 from the membrane and accompanied by NPH3 dephosphorylation. In the cytosol, NPH3 dynamically transitions into membraneless condensate-like structures. The dephosphorylated state of the 14-3-3 binding site and NPH3 membrane recruitment are recoverable in darkness. NPH3 variants that constitutively localize either to the membrane or to condensates are non-functional, revealing a fundamental role of the 14-3-3 mediated dynamic change in NPH3 localization for auxin-dependent phototropism. This regulatory mechanism might be of general nature, given that several members of the NPH3-like family interact with 14-3-3 via a C-terminal motif.

NPH3 is required for auxin-dependent plant phototropism. Here Reuter et al. show that NPH3 is a plasma membrane-bound phospholipid-binding protein and that in response to blue light, NPH3 is phosphorylated and associates with 14-3-3 proteins which leads to dissociation from the plasma membrane.

Regulation of plasmodesmata in Arabidopsis leaves: ATP, NADPH and chlorophyll b levels matter

In mature leaves, cell-to-cell transport via plasmodesmata between mesophyll cells links the production of assimilates by photosynthesis with their export to sink organs. This study addresses the question of how signals derived from chloroplasts and photosynthesis influence plasmodesmata permeability. Cell-to-cell transport was analyzed in leaves of the Arabidopsis chlorophyll b-less ch1-3 mutant, the same mutant complemented with a cyanobacterial CAO gene (PhCAO) overaccumulating chlorophyll b, the trxm3 mutant lacking plastidial thioredoxin m3, and the ntrc mutant lacking functional NADPH:thioredoxin reductase C. The regulation of plasmodesmata permeability in these lines could not be traced back to the reduction state of the thioredoxin system or the types and levels of reactive oxygen species produced in chloroplasts; however, it could be related to chloroplast ATP and NADPH production. The results suggest that light enables plasmodesmata closure via an increase in the ATP and NADPH levels produced in photosynthesis, providing a control mechanism for assimilate export based on the rate of photosynthate production in the Calvin-Benson cycle. The level of chlorophyll b influences plasmodesmata permeability via as-yet-unidentified signals. The data also suggest a role of thioredoxin m3 in the regulation of cyclic electron flow around photosystem I.

A simple and efficient agroinfiltration method for transient gene expression in Citrus

Microwounding pre-treatment facilitates agroinfiltration and transient gene expression in hard-to-agroinfiltrate citrus varieties. Agrobacterium infiltration is a widely used method for transient expression studies in plants, but this method is not used extensively in citrus because of its low efficiency. In this study, we developed an easy, cheap, and reliable agroinfiltration method for transient gene expression in citrus. A microneedle roller was used to create microscopic wounds in the leaf epidermis to facilitate agroinfiltration. Several optimization parameters were explored in this study, including the density of wounds per cm² of abaxial leaf area, the leaf maturity grade, the effect of the Agrobacterium strain, and the length of the incubation period. Increasing the density of wounds on the leaf surface had a positive effect on transient expression. Higher transient expression levels were observed in well-expanded young leaves in comparison with older leaves. The Agrobacterium strain GV2260 was the most suitable to express a large amount of recombinant protein, and an eight- to ten-day incubation period resulted in the highest expression. Endoplasmic reticulum and cytoskeleton-targeted GFP were both successfully localized, confirming that this protocol can be used for protein subcellular localization in citrus. Finally, up to 100 ng of GFP per milligram of agroinfiltrated leaf tissue was estimated to be expressed using this method. This protocol was tested for GFP expression in five different citrus varieties with no significant statistical differences among them. This simple and easy method can speed up functional genomic studies in citrus and may be applied to other recalcitrant species with extensive epidermal cuticular wax.

The Fungal Effector Mlp37347 Alters Plasmodesmata Fluxes and Enhances Susceptibility to Pathogen

Melampsora larici-populina (Mlp) is a devastating pathogen of poplar trees, causing the defoliating poplar leaf rust disease. Genomic studies have revealed that Mlp possesses a repertoire of 1184 small secreted proteins (SSPs), some of them being characterized as candidate effectors. However, how they promote virulence is still unclear. This study investigates the candidate effector Mlp37347’s role during infection. We developed a stable Arabidopsis transgenic line expressing Mlp37347 tagged with the green fluorescent protein (GFP). We found that the effector accumulated exclusively at plasmodesmata (PD). Moreover, the presence of the effector at plasmodesmata favors enhanced plasmodesmatal flux and reduced callose deposition. Transcriptome profiling and a gene ontology (GO) analysis of transgenic Arabidopsis plants expressing the effector revealed that the genes involved in glucan catabolic processes are up-regulated. This effector has previously been shown to interact with glutamate decarboxylase 1 (GAD1), and in silico docking analysis supported the strong binding between Mlp37347 and GAD1 in this study. In infection assays, the effector promoted Hyalonoperospora arabidopsidis growth but not bacterial growth. Our investigation suggests that the effector Mlp37347 targets PD in host cells and promotes parasitic growth.

Evaluating molecular movement through plasmodesmata

Plasmodesmata are membrane-lined cytoplasmic passageways that facilitate the movement of nutrients and various types of molecules between cells in the plant. They are highly dynamic channels, opening or closing in response to physiological and developmental stimuli or environmental challenges such as biotic and abiotic stresses. Accumulating evidence supports the idea that such dynamic controls occur through integrative cellular mechanisms. Currently, a few fluorescence-based methods are available that allow monitoring changes in molecular movement through plasmodesmata. In this chapter, following a brief introduction to those methods, we provide a detailed step-by-step protocol for the Drop-ANd-See (DANS) assay, which is advantageous when it is desirable to measure plasmodesmal permeability non-invasively, in situ and in real-time. We discuss the experimental conditions one should consider to produce reliable and reproducible DANS results along with troubleshooting ideas.

Formation of NPR1 Condensates Promotes Cell Survival during the Plant Immune Response

In plants, pathogen effector-triggered immunity (ETI) often leads to programmed cell death, which is restricted by NPR1, an activator of systemic acquired resistance. However, the biochemical activities of NPR1 enabling it to promote defense and restrict cell death remain unclear. Here we show that NPR1 promotes cell survival by targeting substrates for ubiquitination and degradation through formation of salicylic acid-induced NPR1 condensates (SINCs). SINCs are enriched with stress response proteins, including nucleotide-binding leucine-rich repeat immune receptors, oxidative and DNA damage response proteins, and protein quality control machineries. Transition of NPR1 into condensates is required for formation of the NPR1-Cullin 3 E3 ligase complex to ubiquitinate SINC-localized substrates, such as EDS1 and specific WRKY transcription factors, and promote cell survival during ETI. Our analysis of SINCs suggests that NPR1 is centrally integrated into the cell death or survival decisions in plant immunity by modulating multiple stress-responsive processes in this quasi-organelle.

Quantitative Imaging Reveals Distinct Contributions of SnRK2 and ABI3 in Plasmodesmatal Permeability in Physcomitrella patens

Cell-to-cell communication is tightly regulated in response to environmental stimuli in plants. We previously used a photoconvertible fluorescent protein Dendra2 as a model reporter to study this process. This experiment revealed that macromolecular trafficking between protonemal cells in Physcomitrella patens is suppressed in response to abscisic acid (ABA). However, it remains unknown which ABA signaling components contribute to this suppression and how. Here, we show that ABA signaling components SUCROSE NON-FERMENTING 1-RELATED PROTEIN KINASE 2 (PpSnRK2) and ABA INSENSITIVE 3 (PpABI3) play roles as an essential and promotive factor, respectively, in regulating ABA-induced suppression of Dendra2 diffusion between cells (ASD). Our quantitative imaging analysis revealed that disruption of PpSnRK2 resulted in defective ASD onset itself, whereas disruption of PpABI3 caused an 81-min delay in the initiation of ASD. Live-cell imaging of callose deposition using aniline blue staining showed that, despite this onset delay, callose deposition on cross walls remained constant in the PpABI3 disruptant, suggesting that PpABI3 facilitates ASD in a callose-independent manner. Given that ABA is an important phytohormone to cope with abiotic stresses, we further explored cellular physiological responses. We found that the acquisition of salt stress tolerance is promoted by PpABI3 in a quantitative manner similar to ASD. Our results suggest that PpABI3-mediated ABA signaling may effectively coordinate cell-to-cell communication during the acquisition of salt stress tolerance. This study will accelerate the quantitative study for ABA signaling mechanism and function in response to various abiotic stresses.

An Avirulent Strain of Soybean Mosaic Virus Reverses the Defensive Effect of Abscisic Acid in a Susceptible Soybean Cultivar

In soybean cultivar L29, the Rsv3 gene is responsible for extreme resistance (ER) against the soybean mosaic virus avirulent strain G5H, but is ineffective against the virulent strain G7H. Part of this ER is attributed to the rapid increase in abscisic acid (ABA) and callose, and to the rapid induction of several genes in the RNA-silencing pathway. Whether these two defense mechanisms are correlated or separated in the ER is unknown. Here, we found that ABA treatment of L29 plants increased the expression of several antiviral RNA-silencing genes as well as the PP2C3a gene, which was previously shown to increase callose accumulation; as a consequence, ABA increased the resistance of L29 plants to G7H. The effect of ABA treatment on these genes was weaker in the rsv3-null cultivar (Somyungkong) than in L29. Besides, G5H-infection of Somyungkong plants subverted the effect of ABA leading to reduced callose accumulation and decreased expression of several RNA-silencing genes, which resulted in increased susceptibility to G5H infection. ABA treatment, however, still induced some resistance to G7H in Somyungkong, but only AGO7b was significantly induced. Our data suggest that Rsv3 modulates the effect of ABA on these two resistance mechanisms, i.e., callose accumulation and the antiviral RNA-silencing pathway, and that in the absence of Rsv3, some strains can reverse the effect of ABA and thereby facilitate their replication and spread.

Juxta-membrane S-acylation of plant receptor-like kinases is likely fortuitous and does not necessarily impact upon function

S-acylation is a common post-translational modification of membrane protein cysteine residues with many regulatory roles. S-acylation adjacent to transmembrane domains has been described in the literature as affecting diverse protein properties including turnover, trafficking and microdomain partitioning. However, all of these data are derived from mammalian and yeast systems. Here we examine the role of S-acylation adjacent to the transmembrane domain of the plant pathogen perceiving receptor-like kinase FLS2. Surprisingly, S-acylation of FLS2 adjacent to the transmembrane domain is not required for either FLS2 trafficking or signalling function. Expanding this analysis to the wider plant receptor-like kinase family we find that S-acylation adjacent to receptor-like kinase domains is common, affecting ~25% of Arabidopsis receptor-like kinases, but poorly conserved between orthologues through evolution. This suggests that S-acylation of receptor-like kinases at this site is likely the result of chance mutation leading to cysteine occurrence. As transmembrane domains followed by cysteine residues are common motifs for S-acylation to occur, and many S-acyl transferases appear to have lax substrate specificity, we propose that many receptor-like kinases are fortuitously S-acylated once chance mutation has introduced a cysteine at this site. Interestingly some receptor-like kinases show conservation of S-acylation sites between orthologues suggesting that S-acylation has come to play a role and has been positively selected for during evolution. The most notable example of this is in the ERECTA-like family where S-acylation of ERECTA adjacent to the transmembrane domain occurs in all ERECTA orthologues but not in the parental ERECTA-like clade. This suggests that ERECTA S-acylation occurred when ERECTA emerged during the evolution of angiosperms and may have contributed to the neo-functionalisation of ERECTA from ERECTA-like proteins.

Abscisic Acid Acts as a Regulator of Molecular Trafficking through Plasmodesmata in the Moss Physcomitrella patens

In multi-cellular organisms, cell-to-cell communication is crucial for adapting to changes in the surrounding environment. In plants, plasmodesmata (PD) provide a unique pathway for cell-to-cell communication. PD interconnect most cells and generate a cytoplasmic continuum, allowing the trafficking of various micro- and macromolecules between cells. This molecular trafficking through PD is dynamically regulated by altering PD permeability dependent on environmental changes, thereby leading to an appropriate response to various stresses; however, how PD permeability is dynamically regulated is still largely unknown. Moreover, studies on the regulation of PD permeability have been conducted primarily in a limited number of angiosperms. Here, we studied the regulation of PD permeability in the moss Physcomitrella patens and report that molecular trafficking through PD is rapidly and reversibly restricted by abscisic acid (ABA). Since ABA plays a key role in various stress responses in the moss, PD permeability can be controlled by ABA to adapt to surrounding environmental changes. This ABA-dependent restriction of PD trafficking correlates with a reduction in PD pore size. Furthermore, we also found that the rate of macromolecular trafficking is higher in an ABA-synthesis defective mutant, suggesting that the endogenous level of ABA is also important for PD-mediated macromolecular trafficking. Thus, our study provides compelling evidence that P. patens exploits ABA as one of the key regulators of PD function.

A conserved RxLR effector interacts with host RABA-type GTPases to inhibit vesicle-mediated secretion of antimicrobial proteins

Plant pathogens of the oomycete genus Phytophthora produce virulence factors, known as RxLR effector proteins that are transferred into host cells to suppress disease resistance. Here, we analyse the function of the highly conserved RxLR24 effector of Phytophthora brassicae. RxLR24 was expressed early in the interaction with Arabidopsis plants and ectopic expression in the host enhanced leaf colonization and zoosporangia formation. Co-immunoprecipitation (Co-IP) experiments followed by mass spectrometry identified different members of the RABA GTPase family as putative RxLR24 targets. Physical interaction of RxLR24 or its homologue from the potato pathogen Phytophthora infestans with different RABA GTPases of Arabidopsis or potato, respectively, was confirmed by reciprocal Co-IP. In line with the function of RABA GTPases in vesicular secretion, RxLR24 co-localized with RABA1a to vesicles and the plasma membrane. The effect of RxLR24 on the secretory process was analysed with fusion constructs of secreted antimicrobial proteins with a pH-sensitive GFP tag. PATHOGENESIS RELATED PROTEIN 1 (PR-1) and DEFENSIN (PDF1.2) were efficiently exported in control tissue, whereas in the presence of RxLR24 they both accumulated in the endoplasmic reticulum. Together our results imply a virulence function of RxLR24 effectors as inhibitors of RABA GTPase-mediated vesicular secretion of antimicrobial PR-1, PDF1.2 and possibly other defence-related compounds.

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.

Plasmodesmata at a glance

Plasmodesmata are cytoplasmic communication channels that are vital for the physiology and development of all plants. They facilitate the intercellular movement of various cargos - ranging from small molecules, such as sugars, ions and other essential nutrients and chemicals, to large complex molecules, such as proteins and different types of RNA species - by bridging neighboring cells across their cell walls. Structurally, an individual channel consists of the cytoplasmic sleeve that is formed between the endoplasmic reticulum and the plasma membrane leaflets. Plasmodesmata are highly versatile channels; they vary in number and structure, and undergo constant adjustments to their permeability in response to many internal and external cues. In this Cell Science at a Glance article and accompanying poster, we provide an overview of plasmodesmata form and function, with highlights on their development and variation, associated components and mobile factors. In addition, we present methodologies that are currently used to study plasmodesmata-mediated intercellular communication.