Emerging models on the regulation of intercellular transport by plasmodesmata-associated callose

Host cell wall composition and localized microenvironment implicated in resistance to basal stem degradation by lettuce drop (Sclerotinia minor)

BACKGROUND

Sclerotinia spp. are generalist fungal pathogens, infecting over 700 plant hosts worldwide, including major crops. While host resistance is the most sustainable and cost-effective method for disease management, complete resistance to Sclerotinia diseases is rare. We recently identified soft basal stem as a potential susceptibility factor to Sclerotinia minor infection in lettuce (Lactuca sativa) under greenhouse conditions.

RESULTS

Analysis of stem and root cell wall composition in five L. sativa and one L. serriola accessions with varying growth habits and S. minor resistance levels revealed strong association between hemicellulose constituents, lignin polymers, disease phenotypes, and basal stem mechanical strength. Accessions resistant to basal stem degradation consistently exhibited higher levels of syringyl, guaiacyl, and xylose, but lower levels of fucose in stems. These findings suggest that stem cell wall polymers recalcitrant to breakdown by lignocellulolytic enzymes may contribute to stem strength-mediated resistance against S. minor.

CONCLUSIONS

The lignin content, particularly guaiacyl and syringyl, along with xylose could potentially serve as biomarkers for identifying more resistant lettuce accessions and breeding lines. Basal stem degradation by S. minor was influenced by localized microenvironment conditions around the stem base of the plants.

Plasmodesmata dynamics in bryophyte model organisms: secondary formation and developmental modifications of structure and function

MAIN CONCLUSION

Developing bryophytes differentially modify their plasmodesmata structure and function. Secondary plasmodesmata formation via twinning appears to be an ancestral trait. Plasmodesmata networks in hornwort sporophyte meristems resemble those of angiosperms. All land-plant taxa use plasmodesmata (PD) cell connections for symplasmic communication. In angiosperm development, PD networks undergo an extensive remodeling by structural and functional PD modifications, and by postcytokinetic formation of additional secondary PD (secPD). Since comparable information on PD dynamics is scarce for the embryophyte sister groups, we investigated maturating tissues of Anthoceros agrestis (hornwort), Physcomitrium patens (moss), and Marchantia polymorpha (liverwort). As in angiosperms, quantitative electron microscopy revealed secPD formation via twinning in gametophytes of all model bryophytes, which gives rise to laterally adjacent PD pairs or to complex branched PD. This finding suggests that PD twinning is an ancient evolutionary mechanism to adjust PD numbers during wall expansion. Moreover, all bryophyte gametophytes modify their existing PD via taxon-specific strategies resembling those of angiosperms. Development of type II-like PD morphotypes with enlarged diameters or formation of pit pairs might be required to maintain PD transport rates during wall thickening. Similar to angiosperm leaves, fluorescence redistribution after photobleaching revealed a considerable reduction of the PD permeability in maturating P. patens phyllids. In contrast to previous reports on monoplex meristems of bryophyte gametophytes with single initials, we observed targeted secPD formation in the multi-initial basal meristems of A. agrestis sporophytes. Their PD networks share typical features of multi-initial angiosperm meristems, which may hint at a putative homologous origin. We also discuss that monoplex and multi-initial meristems may require distinct types of PD networks, with or without secPD formation, to control maintenance of initial identity and positional signaling.

Haplotype-resolved genomes of octoploid species in Phyllanthaceae family reveal a critical role for polyploidization and hybridization in speciation

The Phyllanthaceae family comprises a diverse range of plants with medicinal, edible, and ornamental value, extensively cultivated worldwide. Polyploid species commonly occur in Phyllanthaceae. Due to the rather complex genomes and evolutionary histories, their speciation process has been still lacking in research. In this study, we generated chromosome-scale haplotype-resolved genomes of two octoploid species (Phyllanthus emblica and Sauropus spatulifolius) in Phyllanthaceae family. Combined with our previously reported one tetraploid (Sauropus androgynus) and one diploid species (Phyllanthus cochinchinensis) from the same family, we explored their speciation history. The three polyploid species were all identified as allopolyploids with subgenome A/B. Each of their two distinct subgenome groups from various species was uncovered to independently share a common diploid ancestor (Ancestor-AA and Ancestor-BB). Via different evolutionary routes, comprising various scenarios of bifurcating divergence, allopolyploidization (hybrid polyploidization), and autopolyploidization, they finally evolved to the current tetraploid S. androgynus, and octoploid S. spatulifolius and P. emblica, respectively. We further discuss the variations in copy number of alleles and the potential impacts within the two octoploids. In addition, we also investigated the fluctuation of metabolites with medical values and identified the key factor in its biosynthesis process in octoploids species. Our study reconstructed the evolutionary history of these Phyllanthaceae species, highlighting the critical roles of polyploidization and hybridization in their speciation processes. The high-quality genomes of the two octoploid species provide valuable genomic resources for further research of evolution and functional genomics.

A coordinated switch in sucrose and callose metabolism enables enhanced symplastic unloading in potato tubers

One of the early changes upon tuber induction is the switch from apoplastic to symplastic unloading. Whether and how this change in unloading mode contributes to sink strength has remained unclear. In addition, developing tubers also change from energy to storage-based sucrose metabolism. Here, we investigated the coordination between changes in unloading mode and sucrose metabolism and their relative role in tuber sink strength by looking into callose and sucrose metabolism gene expression combined with a model of apoplastic and symplastic unloading. Gene expression analysis suggests that callose deposition in tubers is decreased by lower callose synthase expression. Furthermore, changes in callose and sucrose metabolism are strongly correlated, indicating a well-coordinated developmental switch. Modelling indicates that symplastic unloading is not the most efficient unloading mode per se. Instead, it is the concurrent metabolic switch that provides the physiological conditions necessary to potentiate symplastic transport and thereby enhance tuber sink strength .

APP1/NTL9-CalS8 module ensures proper phloem differentiation by stabilizing callose accumulation and symplastic communication

Phloem sieve elements (PSE), the primary conduits collaborating with neighboring phloem pole pericycle (PPP) cells to facilitate unloading in Arabidopsis roots, undergo a series of developmental stages before achieving maturation and functionality. However, the mechanism that maintains the proper progression of these differentiation stages remains largely unknown. We identified a gain-of-function mutant altered phloem pole pericycle 1 Dominant (app1D), producing a truncated, nuclear-localized active form of NAC with Transmembrane Motif 1-like (NTL9). This mutation leads to ectopic expression of its downstream target CALLOSE SYNTHASE 8 (CalS8), thereby inducing callose accumulation, impeding SE differentiation, impairing phloem transport, and inhibiting root growth. The app1D phenotype could be reproduced by blocking the symplastic channels of cells within APP1 expression domain in wild-type (WT) roots. The WT APP1 is primarily membrane-tethered and dormant in the root meristem cells but entries into the nucleus in several cells in PPP near the unloading region, and this import is inhibited by blocking the symplastic intercellular transport in differentiating SE. Our results suggest a potential maintenance mechanism involving an APP1-CalS8 module, which induces CalS8 expression and modulates symplastic communication, and the proper activation of this module is crucial for the successful differentiation of SE in the Arabidopsis root.

Control of phloem unloading and root development

Root growth and development need proper carbon partitioning between sources and sinks. Photosynthesis products are unloaded from the phloem and enter the root meristem cell by cell. While sugar transporters play a major role in phloem loading, phloem unloading occurs via the plasmodesmata in growing root tips. The aperture and permeability of plasmodesmata strongly influence symplastic unloading. Recent research has dissected the symplastic path for phloem unloading and identified several genes that regulate phloem unloading in the root. Callose turnover and membrane lipid composition alter the shape of plasmodesmata, allowing fine-tuning to adapt phloem unloading to the environmental and developmental conditions. Unloaded sugars act both as an energy supply and as signals to coordinate root growth and development. Increased knowledge of how phloem unloading is regulated enhances our understanding of carbon allocation in plants. In the future, it may be possible to modulate carbon allocation between sources and sinks in a manner that would contribute to increased plant biomass and carbon fixation.

Effect of gibberellin on crown root development in the mutant of the rice plasmodesmal Germin-like protein OsGER4

Rice is an essential but highly stress-susceptible crop, whose root system plays an important role in plant development and stress adaptation. The rice root system architecture is controlled by gene regulatory networks involving different phytohormones including auxin, jasmonate, and gibberellin. Gibberellin is generally known as a molecular clock that interacts with different pathways to regulate root meristem development. The exogenous treatment of rice plantlets with Gibberellin reduced the number of crown roots, whilst the exogenous jasmonic acid treatment enhanced them by involving a Germin-like protein OsGER4. Due to those opposite effects, this study aims to investigate the effect of Gibberellin on crown root development in the rice mutant of the plasmodesmal Germin-like protein OsGER4. Under exogenous gibberellin treatment, the number of crown roots significantly increased in osger4 mutant lines and decreased in the OsGER4 overexpressed lines. GUS staining showed that OsGER4 was strongly expressed in rice root systems, particularly crown and lateral roots under GA3 application. Specifically, OsGER4 was strongly expressed from the exodermis, epidermis, sclerenchyma to the endodermis layers of the crown root, along the vascular bundle and throughout LR primordia. The plasmodesmal protein OsGER4 is suggested to be involved in crown root development by maintaining hormone homeostasis, including Gibberillin.

Distributing Plant Developmental Regulatory Proteins via Plasmodesmata

During plant development, mobile proteins, including transcription factors, abundantly serve as messengers between cells to activate transcriptional signaling cascades in distal tissues. These proteins travel from cell to cell via nanoscopic tunnels in the cell wall known as plasmodesmata. Cellular control over this intercellular movement can occur at two likely interdependent levels. It involves regulation at the level of plasmodesmata density and structure as well as at the level of the cargo proteins that traverse these tunnels. In this review, we cover the dynamics of plasmodesmata formation and structure in a developmental context together with recent insights into the mechanisms that may control these aspects. Furthermore, we explore the processes involved in cargo-specific mechanisms that control the transport of proteins via plasmodesmata. Instead of a one-fits-all mechanism, a pluriform repertoire of mechanisms is encountered that controls the intercellular transport of proteins via plasmodesmata to control plant development.

PbPDCB16-mediated callose deposition affects the plasmodesmata blockage and reduces lignification in pear fruit

Stone cell, a type of lignified cell, is a unique trait in pear and one of the key factors affects pear fruit quality and economic value. The transmissibility of cell lignification process has been proven to exist, however the effects of callose on the permeability of plasmodesmata (PD) and how to influence cell lignification processes are still unknown. In this study, the genome-wide analysis of PD callose binding proteins (PDCB) gene family in pear genome was performed, and 25 PbPDCB genes were identified and divided into four branches. Similar intron/exon structural patterns were observed in the same branch, strongly supporting their close evolutionary relationship. The expression of PbPDCB16 was negatively correlated with lignin accumulation through qRT-PCR analysis. With transient expression in pear fruit and stable expression in pear calli, the increased callose content accompanied by decreased lignin content was further observed. Besides, compared with wild type Arabidopsis, the transgenic plants grew slowly, and cell walls in the stem were thinner, while fewer PDs were observed on the cell walls, and the interspore filaments were also blocked in transgenic Arabidopsis through the transmission electron microscope (TEM). In summary, overexpression of PbPDCB16 could promote accumulation of callose at PD to affect the PD-mediated intercellular connectivity, and inhibit the intercellular communication. This study will provide new insight in reducing the lignin content through callose deposition, and also provide the theoretical basis for further exploration of lignin metabolism and cell wall lignification to form stone cells in pear fruit.

Breaking Boundaries: The Perpetual Interplay Between Tobamoviruses and Plant Immunity

Plant viruses of the genus Tobamovirus cause significant economic losses in various crops. The emergence of new tobamoviruses such as the tomato brown rugose fruit virus (ToBRFV) poses a major threat to global agriculture. Upon infection, plants mount a complex immune response to restrict virus replication and spread, involving a multilayered defense system that includes defense hormones, RNA silencing, and immune receptors. To counter these defenses, tobamoviruses have evolved various strategies to evade or suppress the different immune pathways. Understanding the interactions between tobamoviruses and the plant immune pathways is crucial for the development of effective control measures and genetic resistance to these viruses. In this review, we discuss past and current knowledge of the intricate relationship between tobamoviruses and host immunity. We use this knowledge to understand the emergence of ToBRFV and discuss potential approaches for the development of new resistance strategies to cope with emerging tobamoviruses.

Leaf hydraulic maze: Abscisic acid effects on bundle sheath, palisade, and spongy mesophyll conductance

Leaf hydraulic conductance (Kleaf) facilitates the supply of water, enabling continual CO2 uptake while maintaining plant water status. We hypothesized that bundle sheath and mesophyll cells play key roles in regulating the radial flow of water out of the xylem by responding to abscisic acid (ABA). Thus, we generated transgenic Arabidopsis thaliana plants that are insensitive to ABA in their bundle sheath (BSabi) and mesophyll (MCabi) cells. We also introduced tissue-specific fluorescent markers to distinguish between cells of the palisade mesophyll, spongy mesophyll, and bundle sheath. Both BSabi and MCabi plants showed greater Kleaf and transpiration under optimal conditions. MCabi plants had larger stomatal apertures, higher stomatal index, and greater vascular diameter and biomass relative to the wild-type (WT) and BSabi plants. In response to xylem-fed ABA, both transgenic and WT plants reduced their Kleaf and transpiration. The membrane osmotic water permeability (Pf) of the WT's spongy mesophyll was higher than that of the WT's palisade mesophyll. While the palisade mesophyll maintained a low Pf in response to high ABA, the spongy mesophyll Pf was reduced. Compared to the WT, BSabi bundle sheath cells had a higher Pf, but MCabi spongy mesophyll had an unexpected lower Pf. These results suggest that tissue-specific regulation of Pf by ABA may be confounded by whole-leaf hydraulics and transpiration. ABA increased the symplastic permeability, but its contribution to Kleaf was negligible. We suggest that the bundle sheath spongy mesophyll pathway dynamically responds to the fluctuations in water availability, while the palisade mesophyll serves as a hydraulic buffer.

Plasmodesmal endoplasmic reticulum proteins regulate intercellular trafficking of cucumber mosaic virus in Arabidopsis

Plasmodesmata (PD) are plasma membrane-lined cytoplasmic nanochannels that mediate cell-to-cell communication across the cell wall. A range of proteins are embedded in the PD plasma membrane and endoplasmic reticulum (ER), and function in regulating PD-mediated symplasmic trafficking. However, knowledge of the nature and function of the ER-embedded proteins in the intercellular movement of non-cell-autonomous proteins is limited. Here, we report the functional characterization of two ER luminal proteins, AtBiP1/2, and two ER integral membrane proteins, AtERdj2A/B, which are located within the PD. These PD proteins were identified as interacting proteins with cucumber mosaic virus (CMV) movement protein (MP) in co-immunoprecipitation studies using an Arabidopsis-derived plasmodesmal-enriched cell wall protein preparation (PECP). The AtBiP1/2 PD location was confirmed by TEM-based immunolocalization, and their AtBiP1/2 signal peptides (SPs) function in PD targeting. In vitro/in vivo pull-down assays revealed the association between AtBiP1/2 and CMV MP, mediated by AtERdj2A, through the formation of an AtBiP1/2-AtERdj2-CMV MP complex within PD. The role of this complex in CMV infection was established, as systemic infection was retarded in bip1/bip2w and erdj2b mutants. Our findings provide a model for a mechanism by which the CMV MP mediates cell-to-cell trafficking of its viral ribonucleoprotein complex.

A novel cellular factor of Nicotiana benthamiana susceptibility to tobamovirus infection

Viral infection, which entails synthesis of viral proteins and active reproduction of the viral genome, effects significant changes in the functions of many intracellular systems in plants. Along with these processes, a virus has to suppress cellular defense to create favorable conditions for its successful systemic spread in a plant. The virus exploits various cellular factors of a permissive host modulating its metabolism as well as local and systemic transport of macromolecules and photoassimilates. The Nicotiana benthamiana stress-induced gene encoding Kunitz peptidase inhibitor-like protein (KPILP) has recently been shown to be involved in chloroplast retrograde signaling regulation and stimulation of intercellular transport of macromolecules. In this paper we demonstrate the key role of KPILP in the development of tobamovius infection. Systemic infection of N. benthamiana plants with tobacco mosaic virus (TMV) or the closely related crucifer-infecting tobamovirus (crTMV) induces a drastic increase in KPILP mRNA accumulation. KPILP knockdown significantly reduces the efficiency of TMV and crTMV intercellular transport and reproduction. Plants with KPILP silencing become partially resistant to tobamovirus infection. Therefore, KPILP could be regarded as a novel proviral factor in the development of TMV and crTMV infection in N. benthamiana plants.

Apoplastic and Symplasmic Markers of Somatic Embryogenesis

Somatic embryogenesis (SE) is a process that scientists have been trying to understand for many years because, on the one hand, it is a manifestation of the totipotency of plant cells, so it enables the study of the mechanisms regulating this process, and, on the other hand, it is an important method of plant propagation. Using SE in basic research and in practice is invaluable. This article describes the latest, but also historical, information on changes in the chemical composition of the cell wall during the transition of cells from the somatic to embryogenic state, and the importance of symplasmic communication during SE. Among wall chemical components, different pectic, AGP, extensin epitopes, and lipid transfer proteins have been discussed as potential apoplastic markers of explant cells during the acquisition of embryogenic competence. The role of symplasmic communication/isolation during SE has also been discussed, paying particular attention to the formation of symplasmic domains within and between cells that carry out different developmental processes. Information about the number and functionality of plasmodesmata (PD) and callose deposition as the main player in symplasmic isolation has also been presented.

Xylem development and phloem conductivity in relation to the stem mechanical strength of Paeonia lactiflora

The quality of cut Paeonia lactiflora flowers is limited by their low stem mechanical strength, but the underlying mechanism of this low strength is poorly understood. In this study, two P. lactiflora cultivars with distinct stem mechanical strengths (Chui Touhong with low stem mechanical strength and Da Fugui with high stem mechanical strength) were used as test materials. The xylem development was examined at the cellular level, and the phloem conductivity was analyzed by evaluating phloem geometry. The results showed that the secondary cell wall formation of the xylem of Chui Touhong was affected primarily in fiber cells but was affected little in vessel cells. The formation of the secondary cell walls in the xylem fiber cells of Chui Touhong was delayed, resulting in longer and thinner fiber cells with a lack of cellulose and S-lignin in the secondary cell walls. Moreover, the phloem conductivity of Chui Touhong was lower than that of Da Fugui, and more callose was accumulated in the lateral walls of the phloem sieve elements of Chui Touhong. Consequently, the delayed deposition of the secondary cell walls of the xylem fiber cells was the main factor leading to the low stem mechanical strength of Chui Touhong, and the low stem mechanical strength was closely related to the low conductivity of sieve tubes and extensive callose accumulation in the phloem. These findings provide a new perspective on enhancing P. lactiflora stem mechanical strength by targeting single cell level, and lay the foundation for future works on the correlation between phloem long-distance transport and stem mechanical strength.

Disruption of BG14 results in enhanced callose deposition in developing seeds and decreases seed longevity and seed dormancy in Arabidopsis

Seed longevity is an important trait for agriculture and the conservation of genetic resources. β-1,3-Glucanases were first recognized as pathogenesis-related proteins involved in plant defense, but their roles in seeds are largely unknown. Here, we report a glycosylphosphatidylinositol-anchored β-1,3-glucanase, BG14, that degrades callose in seed embryos and functions in seed longevity and dormancy in Arabidopsis. The loss of function of BG14 significantly decreased seed longevity, whereas functional reversion (RE) and overexpression (OE) lines reversed and increased the impaired phenotype, respectively. The loss of function of BG14 enhanced callose deposition in the embryos of mature seeds, confirmed by quantitative determination and the decreased callose degrading ability in bg14. The drop-and-see (DANS) assay revealed that the fluorescence signal in bg14 was significantly lower than that observed in the other three genotypes. BG14 is located on the periphery of the cell wall and can completely merge with callose at the plasmodesmata of epidermal cells. BG14 was highly expressed in developing seeds and was induced by aging and abscisic acid (ABA). The loss of function of BG14 led to a variety of phenotypes related to ABA, including reduced seed dormancy and reduced responses to treatment with ABA or pacolblltrazol, whereas OE lines showed the opposite phenotype. The reduced ABA response is because of the decreased level of ABA and the lowered expression of ABA synthesis genes in bg14. Taken together, this study demonstrated that BG14 is a bona fide BG that mediates callose degradation in the plasmodesmata of embryo cells, transcriptionally influences ABA synthesis genes in developing seeds, and positively affects seed longevity and dormancy in Arabidopsis.

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.

The maize callose synthase SLM1 is critical for a normal growth by controlling the vascular development

Callose, mainly deposited at the cell plate and in the newly formed cell wall at a very low level, is critical for cell activity and growth in plants. The genetic control and function of callose synthases, responsible for the synthesis of callose, are largely unknown in maize. In this study, we cloned a maize callose synthase, SLM1 (Seedling Lethal Mutant1) encoding for a GLUCAN SYNTHASE-LIKE (GSL) gene, from a seedling lethal mutant. Three different point mutations confirmed the key role of SLM1 to maintain maize normal growth. SLM1 was specifically expressed in immature leaf vascular with an enrichment in phloem of developing vasculature. Consistently, slm1 had severe defects in vasculature and leaf development, and terminated growth about 2 weeks after germination. Thus, SLM1 is a key gene to maintain normal growth by controlling leaf vascular development and cell activities. Loss of SLM1 function interrupted severely the important signaling pathways in which cell cyclin and histone related genes are involved. Our study reveals the critical function of a maize GSL gene and also its downstream signaling to maintain a normal growth of maize.

Supplementary information

The online version contains supplementary material available at 10.1007/s11032-022-01350-4.

The Germin-like protein OsGER4 is involved in promoting crown root development under exogenous jasmonic acid treatment in rice

In rice (Oryza sativa L.), crown roots (CRs) have many important roles in processes such as root system expansion, water and mineral uptake, and adaptation to environmental stresses. Phytohormones such as auxin, cytokinin, and ethylene are known to control CR initiation and development in rice. However, the role of jasmonic acid (JA) in CR development remained elusive. Here, we report that JA promotes CR development by regulating OsGER4, a rice Germin-like protein. Root phenotyping analysis revealed that exogenous JA treatment induced an increase in CR number in a concentration-dependent manner. A subsequent genome-wide association study and gene expression analyses pinpointed a strong association between the Germin-like protein OsGER4 and the increase in CR number under exogenous JA treatment. The ProGER4::GUS reporter line showed that OsGER4 is a hormone-responsive gene involved in various stress responses, mainly confined to epidermal and vascular tissues during CR primordia development and to vascular bundles of mature crown and lateral roots. Notable changes in OsGER4 expression patterns caused by the polar auxin transport inhibitor NPA support its connection to auxin signaling. Phenotyping experiments with OsGER4 knockout mutants confirmed that this gene is required for CR development under exogenous JA treatment. Overall, our results provide important insights into JA-mediated regulation of CR development in rice.

Enzymatic fingerprinting reveals specific xyloglucan and pectin signatures in the cell wall purified with primary plasmodesmata

Plasmodesmata (PD) pores connect neighbouring plant cells and enable direct transport across the cell wall. Understanding the molecular composition of these structures is essential to address their formation and later dynamic regulation. Here we provide a biochemical characterisation of the cell wall co-purified with primary PD of Arabidopsis thaliana cell cultures. To achieve this result we combined subcellular fractionation, polysaccharide analyses and enzymatic fingerprinting approaches. Relative to the rest of the cell wall, specific patterns were observed in the PD fraction. Most xyloglucans, although possibly not abundant as a group, were fucosylated. Homogalacturonans displayed short methylated stretches while rhamnogalacturonan I species were remarkably abundant. Full rhamnogalacturonan II forms, highly methyl-acetylated, were also present. We additionally showed that these domains, compared to the broad wall, are less affected by wall modifying activities during a time interval of days. Overall, the protocol and the data presented here open new opportunities for the study of wall polysaccharides associated with PD.

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.

Long-Distance Transported RNAs: From Identity to Function

There is now a wealth of data, from different plants and labs and spanning more than two decades, which unequivocally demonstrates that RNAs can be transported over long distances, from the cell where they are transcribed to distal cells in other tissues. Different types of RNA molecules are transported, including micro- and messenger RNAs. Whether these RNAs are selected for transport and, if so, how they are selected and transported remain, in general, open questions. This aspect is likely not independent of the biological function and relevance of the transported RNAs, which are in most cases still unclear. In this review, we summarize the experimental data supporting selectivity or nonselectivity of RNA translocation and review the evidence for biological functions. After discussing potential issues regarding the comparability between experiments, we propose criteria that need to be critically evaluated to identify important signaling RNAs.

High-Throughput Phenotyping Accelerates the Dissection of the Phenotypic Variation and Genetic Architecture of Shank Vascular Bundles in Maize (Zea mays L.)

The vascular bundle of the shank is an important ‘flow’ organ for transforming maize biological yield to grain yield, and its microscopic phenotypic characteristics and genetic analysis are of great significance for promoting the breeding of new varieties with high yield and good quality. In this study, shank CT images were obtained using the standard process for stem micro-CT data acquisition at resolutions up to 13.5 μm. Moreover, five categories and 36 phenotypic traits of the shank including related to the cross-section, epidermis zone, periphery zone, inner zone and vascular bundle were analyzed through an automatic CT image process pipeline based on the functional zones. Next, we analyzed the phenotypic variations in vascular bundles at the base of the shank among a group of 202 inbred lines based on comprehensive phenotypic information for two environments. It was found that the number of vascular bundles in the inner zone (IZ_VB_N) and the area of the inner zone (IZ_A) varied the most among the different subgroups. Combined with genome-wide association studies (GWAS), 806 significant single nucleotide polymorphisms (SNPs) were identified, and 1245 unique candidate genes for 30 key traits were detected, including the total area of vascular bundles (VB_A), the total number of vascular bundles (VB_N), the density of the vascular bundles (VB_D), etc. These candidate genes encode proteins involved in lignin, cellulose synthesis, transcription factors, material transportation and plant development. The results presented here will improve the understanding of the phenotypic traits of maize shank and provide an important phenotypic basis for high-throughput identification of vascular bundle functional genes of maize shank and promoting the breeding of new varieties with high yield and good quality.

Integrative Physiological and Transcriptomic Analysis Reveals the Transition Mechanism of Sugar Phloem Unloading Route in Camellia oleifera Fruit

Sucrose phloem unloading plays a vital role in photoassimilate distribution and storage in sink organs such as fruits and seeds. In most plants, the phloem unloading route was reported to shift between an apoplasmic and a symplasmic pattern with fruit development. However, the molecular transition mechanisms of the phloem unloading pathway still remain largely unknown. In this study, we applied RNA sequencing to profile the specific gene expression patterns for sucrose unloading in C. oleifera fruits in the apo- and symplasmic pathways that were discerned by CF fluoresce labelling. Several key structural genes were identified that participate in phloem unloading, such as PDBG11, PDBG14, SUT8, CWIN4, and CALS10. In particular, the key genes controlling the process were involved in callose metabolism, which was confirmed by callose staining. Based on the co-expression network analysis with key structural genes, a number of transcription factors belonging to the MYB, C2C2, NAC, WRKY, and AP2/ERF families were identified to be candidate regulators for the operation and transition of phloem unloading. KEGG enrichment analysis showed that some important metabolism pathways such as plant hormone metabolism, starch, and sucrose metabolism altered with the change of the sugar unloading pattern. Our study provides innovative insights into the different mechanisms responsible for apo- and symplasmic phloem unloading in oil tea fruit and represents an important step towards the omics delineation of sucrose phloem unloading transition in crops.

Comparing Machine Learning and Binary Thresholding Methods for Quantification of Callose Deposits in the Citrus Phloem

Callose is a polysaccharide that can be fluorescently stained to study many developmental and immune functions in plants. High-throughput methods to accurately gather quantitative measurements of callose from confocal images are useful for many applications in plant biology. Previous callose quantification methods relied upon binary local thresholding, which had the disadvantage of not being able to differentiate callose in conditions with low contrast from background material. Here, a measurement approach that utilizes the Ilastik supervised machine learning imagery data collection software is described. The Ilastik software method provided superior efficiency for acquiring counts of callose deposits. We also determined the accuracy of these methods as compared to manual counts. We demonstrate that the automated software methods are both good predictors of manual counts, but that the Ilastik counts are significantly closer. Researchers can use this information to guide their choice of method to quantify callose in their work.

Heat Stress Reduces Root Meristem Size via Induction of Plasmodesmal Callose Accumulation Inhibiting Phloem Unloading in Arabidopsis

The intercellular transport of sugars, nutrients, and small molecules is essential for plant growth, development, and adaptation to environmental changes. Various stresses are known to affect the cell-to-cell molecular trafficking modulated by plasmodesmal permeability. However, the mechanisms of plasmodesmata modification and molecules involved in the phloem unloading process under stress are still not well understood. Here, we show that heat stress reduces the root meristem size and inhibits phloem unloading by inducing callose accumulation at plasmodesmata that connect the sieve element and phloem pole pericycle. Furthermore, we identify the loss-of-function of CALLOSE SYNTHASE 8 (CalS8), which is expressed specifically in the phloem pole pericycle, decreasing the plasmodesmal callose deposition at the interface between the sieve element and phloem pole pericycle and alleviating the suppression at root meristem size by heat stress. Our studies indicate the involvement of callose in the interaction between root meristem growth and heat stress and show that CalS8 negatively regulates the thermotolerance of Arabidopsis roots.

Phloem transport limitation in Huanglongbing-affected sweet orange is dependent on phloem-limited bacteria and callose

Huanglongbing (HLB), caused by Candidatus `Liberibacter asiaticus' (CLas), is a phloem-limited disease that disrupts citrus production in affected areas. In HLB-affected plants, phloem sieve plate pores accumulate callose, and leaf carbohydrate export is reduced. However, whether HLB causes a reduction in carbohydrate phloem translocation speed and the quantitative relationships among callose, CLas population and phloem translocation are still unknown. In this work, a procedure was developed to concurrently measure sugar transport, callose deposition and relative pathogen population at different locations throughout the stem. Increasing quantities of CLas genetic material were positively correlated with quantity and density of callose deposits and negatively correlated with phloem translocation speed. Callose deposit quantity was position and rootstock dependent and was negatively correlated with phloem translocation speed, suggesting a localized relationship. Remarkably, callose accumulation and phloem translocation disruption in the scion were dependent on rootstock genotype. Regression results suggested that the interaction of Ct values and number of phloem callose depositions, but not their size or density, explained the effects on translocation speed. Sucrose, starch and sink 14C label allocation data support the interpretation of a transport pathway limitation by CLas infection. This work shows that the interaction of local accumulation of callose and CLas affects phloem transport. Furthermore, the extent of this accumulation is attenuated by the rootstock and provides important information about the disease mechanism of phloem-inhabiting bacteria. Together, these results constitute the first example of a demonstrated transport limitation of phloem function by a microbial infection.

Fruit transcriptional profiling of the contrasting genotypes for shelf life reveals the key candidate genes and molecular pathways regulating post-harvest biology in cucumber

Cucumber fruits are perishable in nature and become unfit for market within 2-3 days of harvesting. A natural variant, DC-48 with exceptionally high shelf life was developed and used to dissect the genetic architecture and molecular mechanism for extended shelf life through RNA-seq for first time. A total of 1364 DEGs were identified and cell wall degradation, chlorophyll and ethylene metabolism related genes played key role. Polygalacturunase (PG), Expansin (EXP) and xyloglucan were down regulated determining fruit firmness and retention of fresh green colour was mainly attributed to the low expression level of the chlorophyll catalytic enzymes (CCEs). Gene regulatory networks revealed the hub genes and cross-talk associated with wide variety of the biological processes. Large number of SSRs (21524), SNPs (545173) and InDels (126252) identified will be instrumental in cucumber improvement. A web genomic resource, CsExSLDb developed will provide a platform for future investigation on cucumber post-harvest biology.

Plasmodesmata Structural Components and Their Role in Signaling and Plant Development

Plasmodesmata are plant intercellular channels that mediate the transport of small and large molecules including RNAs and transcription factors (TFs) that regulate plant development. In this review, we present current research on plasmodesmata form and function and discuss the main regulatory pathways. We show the progress made in the development of approaches and tools to dissect the plasmodesmata proteome in diverse plant species and discuss future perspectives and challenges in this field of research.

Immunofluorescence Detection of Callose in Plant Tissue Sections

The accumulation of the cell wall component callose at plasmodesmata (PD) is crucial for the regulation of symplastic intercellular transport in plants. Here we describe protocols to fluorescently image callose in sectioned plant tissue using monoclonal antibodies. This protocol achieves high-resolution images by the fixation, embedding, and sectioning of plant material to expose internal cell walls. By using this protocol in combination with high-resolution confocal microscopy, we can detect PD callose in a variety of plant tissues and species.

Plasmodesmata and their role in the regulation of phloem unloading during fruit development

Fruit consumption is fundamental to a balanced diet. The contemporary challenge of maintaining a steady food supply to meet the demands of a growing population is driving the development of strategies to improve the production and nutritional quality of fruit. Plasmodesmata, the structures that mediate symplasmic transport between plant cells, play an important role in phloem unloading and distribution of sugars and signalling molecules into developing organs. Targeted modifications to the structures and functioning of plasmodesmata have the potential to improve fruit development; however, knowledge on the mechanisms underpinning plasmodesmata regulation in this context is scarce. In this review, we have compiled current knowledge on plasmodesmata and their structural characterisation during the development of fruit organs. We discuss key questions on phloem unloading, including the pathway shift from symplasmic to apoplastic that takes place during the onset of ripening as potential targets for improving fruit quality.

Mechanisms underlying the phytotoxicity and genotoxicity of aluminum and their alleviation strategies: A review

Aluminum (Al) is considered as a potential limiting factor for plant growth in acidic environment. At lower concentration, Al promotes plant growth by facilitating the phosphorous availability, while, at higher concentration, it causes rhizotoxicity by inhibiting the nutrient transportation system. Cellular membrane is identified as the first site of Al toxicity, which is consequent to Al-induced reactive oxygen species prompted lipid catabolism. Among all the soluble forms, the trivalent cationic form (Al³⁺) of Al is most toxic. Though, the ability to ascribe Al-tolerance is very complex, exclusion is an extensively established process contributing to Al³⁺ detoxification. Alteration in pH at root apex/rhizosphere, exudation of chelating agents, cell wall immobilization, and Al efflux have been recognized as probable methods for exclusion of Al, which is highly dependent on concentrations of organic acids, and plant species. Additionally, exogenous applications of boron, silicon, calcium, etc., in Al-stressed plant species can form a conjugate with it, thereby reducing its bioavailability/toxicity. Moreover, nanoparticles (NPs) are emerging tools in agricultural sector, which are found to be relatively more effective in mitigation of metal stress compared to their bulk materials. This review exhibits the fundamental approaches of Al phytotoxicity and endows with a comprehensive knowledge of the cellular and metabolic processes underlying toxic impacts along with ameliorative efficiencies of various potential agents including NPs. Additionally, it also elucidates the molecular mechanisms, future research prospects and challenges in effective alleviation mechanisms for enhancing plant Al-tolerance, to improve the growth and yields of susceptible-species on acidic soil.

Auxin fluxes through plasmodesmata

Characterising the processes that control auxin dynamics is essential to understanding how auxin regulates plant development. Over recent years, several studies have investigated auxin diffusion through plasmodesmata, characterising this cell-to-cell diffusion and demonstrating that it affects auxin distributions. Furthermore, studies have shown that plasmodesmatal auxin diffusion affects developmental processes, including phototropism, lateral root emergence and leaf hyponasty. This short Tansley Insight review describes how these studies have contributed to our understanding of auxin dynamics and discusses potential future directions in this area.

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.

Plasmodesmata-Involved Battle Against Pathogens and Potential Strategies for Strengthening Hosts

Plasmodesmata (PD) are membrane-lined pores that connect adjacent cells to mediate symplastic communication in plants. These intercellular channels enable cell-to-cell trafficking of various molecules essential for plant development and stress responses, but they can also be utilized by pathogens to facilitate their infection of hosts. Some pathogens or their effectors are able to spread through the PD by modifying their permeability. Yet plants have developed various corresponding defense mechanisms, including the regulation of PD to impede the spread of invading pathogens. In this review, we aim to illuminate the various roles of PD in the interactions between pathogens and plants during the infection process. We summarize the pathogenic infections involving PD and how the PD could be modified by pathogens or hosts. Furthermore, we propose several hypothesized and promising strategies for enhancing the disease resistance of host plants by the appropriate modulation of callose deposition and plasmodesmal permeability based on current knowledge.

Similarities and Differences in the GFP Movement in the Zygotic and Somatic Embryos of Arabidopsis

Intercellular signaling during embryo patterning is not well understood and the role of symplasmic communication has been poorly considered. The correlation between the symplasmic domains and the development of the embryo organs/tissues during zygotic embryogenesis has only been described for a few examples, including Arabidopsis. How this process occurs during the development of somatic embryos (SEs) is still unknown. The aim of these studies was to answer the question: do SEs have a restriction in symplasmic transport depending on the developmental stage that is similar to their zygotic counterparts? The studies included an analysis of the GFP distribution pattern as expressed under diverse promoters in zygotic embryos (ZEs) and SEs. The results of the GFP distribution in the ZEs and SEs showed that 1/the symplasmic domains between the embryo organs and tissues in the SEs was similar to those in the ZEs and 2/the restriction in symplasmic transport in the SEs was correlated with the developmental stage and was similar to the one in their zygotic counterparts, however, with the spatio-temporal differences and different PDs SEL value between these two types of embryos.

Variability, Functions and Interactions of Plant Virus Movement Proteins: What Do We Know So Far?

Of the various proteins encoded by plant viruses, one of the most interesting is the movement protein (MP). MPs are unique to plant viruses and show surprising structural and functional variability while maintaining their core function, which is to facilitate the intercellular transport of viruses or viral nucleoprotein complexes. MPs interact with components of the intercellular channels, the plasmodesmata (PD), modifying their size exclusion limits and thus allowing larger particles, including virions, to pass through. The interaction of MPs with the components of PD, the formation of transport complexes and the recruitment of host cellular components have all revealed different facets of their functions. Multitasking is an inherent property of most viral proteins, and MPs are no exception. Some MPs carry out multitasking, which includes gene silencing suppression, viral replication and modulation of host protein turnover machinery. This review brings together the current knowledge on MPs, focusing on their structural variability, various functions and interactions with host proteins.

Regulation and Function of Defense-Related Callose Deposition in Plants

Plants are constantly exposed to a wide range of potential pathogens and to protect themselves, have developed a variety of chemical and physical defense mechanisms. Callose is a β-(1,3)-D-glucan that is widely distributed in higher plants. In addition to its role in normal growth and development, callose plays an important role in plant defense. Callose is deposited between the plasma membrane and the cell wall at the site of pathogen attack, at the plasmodesmata, and on other plant tissues to slow pathogen invasion and spread. Since it was first reported more than a century ago, defense-related callose deposition has been extensively studied in a wide-spectrum of plant-pathogen systems. Over the past 20 years or so, a large number of studies have been published that address the dynamic nature of pathogen-induced callose deposition, the complex regulation of synthesis and transport of defense-related callose and associated callose synthases, and its important roles in plant defense responses. In this review, we summarize our current understanding of the regulation and function of defense-related callose deposition in plants and discuss both the progresses and future challenges in addressing this complex defense mechanism as a critical component of a plant immune system.

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.

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.

A Phytophthora effector protein promotes symplastic cell-to-cell trafficking by physical interaction with plasmodesmata-localised callose synthases

Pathogen effectors act as disease promoting factors that target specific host proteins with roles in plant immunity. Here, we investigated the function of the RxLR3 effector of the plant-pathogen Phytophthora brassicae. Arabidopsis plants expressing a FLAG-RxLR3 fusion protein were used for co-immunoprecipitation followed by liquid chromatography-tandem mass spectrometry to identify host targets of RxLR3. Fluorescently labelled fusion proteins were used for analysis of subcellular localisation and function of RxLR3. Three closely related members of the callose synthase family, CalS1, CalS2 and CalS3, were identified as targets of RxLR3. RxLR3 co-localised with the plasmodesmal marker protein PDLP5 (PLASMODESMATA-LOCALISED PROTEIN 5) and with plasmodesmata-associated deposits of the β-1,3-glucan polymer callose. In line with a function as an inhibitor of plasmodesmal callose synthases (CalS) enzymes, callose depositions were reduced and cell-to-cell trafficking was promoted in the presence of RxLR3. Plasmodesmal callose deposition in response to infection was compared with wild-type suppressed in RxLR3-expressing Arabidopsis lines. Our results implied a virulence function of the RxLR3 effector as a positive regulator of plasmodesmata transport and provided evidence for competition between P. brassicae and Arabidopsis for control of cell-to-cell trafficking.

The Tobamoviral Movement Protein: A "Conditioner" to Create a Favorable Environment for Intercellular Spread of Infection

During their evolution, viruses acquired genes encoding movement protein(s) (MPs) that mediate the intracellular transport of viral genetic material to plasmodesmata (Pd) and initiate the mechanisms leading to the increase in plasmodesmal permeability. Although the current view on the role of the viral MPs was primarily formed through studies on tobacco mosaic virus (TMV), the function of its MP has not been fully elucidated. Given the intercellular movement of MPs independent of genomic viral RNA (vRNA), this characteristic may induce favorable conditions ahead of the infection front for the accelerated movement of the vRNA (i.e. the MP plays a role as a "conditioner" of viral intercellular spread). This idea is supported by (a) the synthesis of MP from genomic vRNA early in infection, (b) the Pd opening and the MP transfer to neighboring cells without formation of the viral replication complex (VRC), and (c) the MP-mediated movement of VRCs beyond the primary infected cell. Here, we will consider findings that favor the TMV MP as a "conditioner" of enhanced intercellular virus movement. In addition, we will discuss the mechanism by which TMV MP opens Pd for extraordinary transport of macromolecules. Although there is no evidence showing direct effects of TMV MP on Pd leading to their dilatation, recent findings indicate that MPs exert their influence indirectly by modulating Pd external and structural macromolecules such as callose and Pd-associated proteins. In explaining this phenomenon, we will propose a mechanism for TMV MP functioning as a conditioner for virus movement.

Diverse regulation of plasmodesmal architecture facilitates adaptation to phloem translocation

The long-distance translocation of nutrients and mobile molecules between different terminals is necessary for plant growth and development. Plasmodesmata-mediated symplastic trafficking plays an important role in accomplishing this task. To facilitate intercellular transport, plants have evolved diverse plasmodesmata with distinct internal architecture at different cell-cell interfaces along the trafficking route. Correspondingly, different underlying mechanisms for regulating plasmodesmal structures have been gradually revealed. In this review, we highlight recent studies on various plasmodesmal architectures, as well as relevant regulators of their de novo formation and transition, responsible for phloem loading, transport, and unloading specifically. We also discuss the interesting but unaddressed questions relating to, and potential studies on, the adaptation of functional plasmodesmal structures.

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.

Chloroplast-to-nucleus retrograde signalling controls intercellular trafficking via plasmodesmata formation

The signalling pathways that regulate intercellular trafficking via plasmodesmata (PD) remain largely unknown. Analyses of mutants with defects in intercellular trafficking led to the hypothesis that chloroplasts are important for controlling PD, probably by retrograde signalling to the nucleus to regulate expression of genes that influence PD formation and function, an idea encapsulated in the organelle-nucleus-PD signalling (ONPS) hypothesis. ONPS is supported by findings that point to chloroplast redox state as also modulating PD. Here, we have attempted to further elucidate details of ONPS. Through reverse genetics, expression of select nucleus-encoded genes with known or predicted roles in chloroplast gene expression was knocked down, and the effects on intercellular trafficking were then assessed. Silencing most genes resulted in chlorosis, and the expression of several photosynthesis and tetrapyrrole biosynthesis associated nuclear genes was repressed in all silenced plants. PD-mediated intercellular trafficking was changed in the silenced plants, consistent with predictions of the ONPS hypothesis. One striking observation, best exemplified by silencing the PNPase homologues, was that the degree of chlorosis of silenced leaves was not correlated with the capacity for intercellular trafficking. Finally, we measured the distribution of PD in silenced leaves and found that intercellular trafficking was positively correlated with the numbers of PD. Together, these results not only provide further support for ONPS but also point to a genetic mechanism for PD formation, clarifying a longstanding question about PD and intercellular trafficking. This article is part of the theme issue 'Retrograde signalling from endosymbiotic organelles'.

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.

Dare to change, the dynamics behind plasmodesmata-mediated cell-to-cell communication

Plasmodesmata pores control the entry and exit of molecules at cell-to-cell boundaries. Hundreds of pores perforate the plant cell wall, connecting cells together and establishing direct cytosolic and membrane continuity. This ability to connect cells in such a way is a hallmark of plant physiology and is thought to have allowed sessile multicellularity in Plantae kingdom. Indeed, plasmodesmata-mediated cell-to-cell signalling is fundamental to many plant-related processes. In fact, there are so many facets of plant biology under the control of plasmodesmata that it is hard to conceive how such tiny structures can do so much. While they provide 'open doors' between cells, they also need to guarantee cellular identities and territories by selectively transporting molecules. Although plasmodesmata operating mode remains difficult to grasp, little by little plant scientists are divulging their secrets. In this review, we highlight novel functions of cell-to-cell signalling and share recent insights into how plasmodesmata structural and molecular signatures confer functional specificity and plasticity to these unique cellular machines.

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

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

Viruses Reveal the Secrets of Plasmodesmal Cell Biology

Plasmodesmata (PD) are essential for intercellular trafficking of molecules required for plant life, from small molecules like sugars and ions to macromolecules including proteins and RNA molecules that act as signals to regulate plant development and defense. As obligate intracellular pathogens, plant viruses have evolved to manipulate this communication system to facilitate the initial cell-to-cell and eventual systemic spread in their plant hosts. There has been considerable interest in how viruses manipulate the PD that connect the protoplasts of neighboring cells, and viruses have yielded invaluable tools for probing the structure and function of PD. With recent advances in biochemistry and imaging, we have gained new insights into the composition and structure of PD in the presence and absence of viruses. Here, we first discuss viral strategies for manipulating PD for their intercellular movement and examine how this has shed light on our understanding of native PD function. We then address the controversial role of the cytoskeleton in trafficking to and through PD. Finally, we address how viruses could alter PD structure and consider possible mechanisms of the phenomenon described as 'gating'. This discussion supports the significance of virus research in elucidating the properties of PD, these persistently enigmatic plant organelles.