Remorin, a solanaceae protein resident in membrane rafts and plasmodesmata, impairs potato virus X movement

Review: Membrane tethers control plasmodesmal function and formation

Cell-to-cell communication is crucial in coordinating diverse biological processes in multicellular organisms. In plants, communication between adjacent cells occurs via nanotubular passages called plasmodesmata (PD). The PD passage is composed of an appressed endoplasmic reticulum (ER) internally, and plasma membrane (PM) externally, that traverses the cell wall, and associates with the actin-cytoskeleton. The coordination of the ER, PM and cytoskeleton plays a potential role in maintaining the architecture and conductivity of PD. Many data suggest that PD-associated proteins can serve as tethers that connect these structures in a functional PD, to regulate cell-to-cell communication. In this review, we summarize the organization and regulation of PD activity via tethering proteins, and discuss the importance of PD-mediated cell-to-cell communication in plant development and defense against environmental stress.

Engineering of Plants for Efficient Production of Therapeutics

Plants are becoming useful platforms for recombinant protein production at present time. With the advancement of efficient molecular tools of genomics, proteomics, plants are now being used as a biofactory for production of different life saving therapeutics. Plant-based biofactory is an established production system with the benefits of cost-effectiveness, high scalability, rapid production, enabling post-translational modification, and being devoid of harmful pathogens contamination. This review introduces the main challenges faced by plant expression system: post-translational modifications, protein stability, biosafety concern and regulation. It also summarizes essential factors to be considered in engineering plants, including plant expression system, promoter, post-translational modification, codon optimization, and fusion tags, protein stabilization and purification, subcellular targeting, and making vaccines in an edible way. This review will be beneficial and informative to scholars and readers in the field of plant biotechnology.

Salicylic acid regulates PIN2 auxin transporter hyperclustering and root gravitropic growth via Remorin-dependent lipid nanodomain organisation in Arabidopsis thaliana

To adapt to the diverse array of biotic and abiotic cues, plants have evolved sophisticated mechanisms to sense changes in environmental conditions and modulate their growth. Growth-promoting hormones and defence signalling fine tune plant development antagonistically. During host-pathogen interactions, this defence-growth trade-off is mediated by the counteractive effects of the defence hormone salicylic acid (SA) and the growth hormone auxin. Here we revealed an underlying mechanism of SA regulating auxin signalling by constraining the plasma membrane dynamics of PIN2 auxin efflux transporter in Arabidopsis thaliana roots. The lateral diffusion of PIN2 proteins is constrained by SA signalling, during which PIN2 proteins are condensed into hyperclusters depending on REM1.2-mediated nanodomain compartmentalisation. Furthermore, membrane nanodomain compartmentalisation by SA or Remorin (REM) assembly significantly suppressed clathrin-mediated endocytosis. Consequently, SA-induced heterogeneous surface condensation disrupted asymmetric auxin distribution and the resultant gravitropic response. Our results demonstrated a defence-growth trade-off mechanism by which SA signalling crosstalked with auxin transport by concentrating membrane-resident PIN2 into heterogeneous compartments.

Genome Editing for Plasmodesmal Biology

Plasmodesmata (PD) are cytoplasmic canals that facilitate intercellular communication and molecular exchange between adjacent plant cells. PD-associated proteins are considered as one of the foremost factors in regulating PD function that is critical for plant development and stress responses. Although its potential to be used for crop engineering is enormous, our understanding of PD biology was relatively limited to model plants, demanding further studies in crop systems. Recently developed genome editing techniques such as Clustered Regularly Interspaced Short Palindromic Repeats/CRISPR associate protein (CRISPR/Cas) might confer powerful approaches to dissect the molecular function of PD components and to engineer elite crops. Here, we assess several aspects of PD functioning to underline and highlight the potential applications of CRISPR/Cas that provide new insight into PD biology and crop improvement.

Diversity of Plant Virus Movement Proteins: What Do They Have in Common?

The modern view of the mechanism of intercellular movement of viruses is based largely on data from the study of the tobacco mosaic virus (TMV) 30-kDa movement protein (MP). The discovered properties and abilities of TMV MP, namely, (a) in vitro binding of single-stranded RNA in a non-sequence-specific manner, (b) participation in the intracellular trafficking of genomic RNA to the plasmodesmata (Pd), and (c) localization in Pd and enhancement of Pd permeability, have been used as a reference in the search and analysis of candidate proteins from other plant viruses. Nevertheless, although almost four decades have passed since the introduction of the term “movement protein” into scientific circulation, the mechanism underlying its function remains unclear. It is unclear why, despite the absence of homology, different MPs are able to functionally replace each other in trans-complementation tests. Here, we consider the complexity and contradictions of the approaches for assessment of the ability of plant viral proteins to perform their movement function. We discuss different aspects of the participation of MP and MP/vRNA complexes in intra- and intercellular transport. In addition, we summarize the essential MP properties for their functioning as “conditioners”, creating a favorable environment for viral reproduction.

Natural variation in Arabidopsis thaliana rosette area unveils new genes involved in plant development

Growth is a complex trait influenced by multiple genes that act at different moments during the development of an organism. This makes it difficult to spot its underlying genetic mechanisms. Since plant growth is intimately related to the effective leaf surface area (ELSA), identifying genes controlling this trait will shed light on our understanding of plant growth. To find new genes with a significant contribution to plant growth, here we used the natural variation in Arabidopsis thaliana to perform a genome-wide association study of ELSA. To do this, the projected rosette area of 710 worldwide distributed natural accessions was measured and analyzed using the genome-wide efficient mixed model association algorithm. From this analysis, ten genes were identified having SNPs with a significant association with ELSA. To validate the implication of these genes into A. thaliana growth, six of them were further studied by phenotyping knock-out mutant plants. It was observed that rem1.2, orc1a, ppd1, and mcm4 mutants showed different degrees of reduction in rosette size, thus confirming the role of these genes in plant growth. Our study identified genes already known to be involved in plant growth but also assigned this role, for the first time, to other genes.

Intercellular trafficking via plasmodesmata: molecular layers of complexity

Plasmodesmata are intercellular pores connecting together most plant cells. These structures consist of a central constricted form of the endoplasmic reticulum, encircled by some cytoplasmic space, in turn delimited by the plasma membrane, itself ultimately surrounded by the cell wall. The presence and structure of plasmodesmata create multiple routes for intercellular trafficking of a large spectrum of molecules (encompassing RNAs, proteins, hormones and metabolites) and also enable local signalling events. Movement across plasmodesmata is finely controlled in order to balance processes requiring communication with those necessitating symplastic isolation. Here, we describe the identities and roles of the molecular components (specific sets of lipids, proteins and wall polysaccharides) that shape and define plasmodesmata structural and functional domains. We highlight the extensive and dynamic interactions that exist between the plasma/endoplasmic reticulum membranes, cytoplasm and cell wall domains, binding them together to effectively define plasmodesmata shapes and purposes.

Sphingolipids Modulate Secretion of Glycosylphosphatidylinositol-Anchored Plasmodesmata Proteins and Callose Deposition

Plasma membranes encapsulated in the symplasmic nanochannels of plasmodesmata (PD) contain abundant lipid rafts, which are enriched with sphingolipids (SLs) and sterols. Reduction of sterols has highlighted the role played by lipid raft integrity in the intercellular trafficking of glycosylphosphatidylinositol (GPI)-anchored PD proteins, particularly in affecting callose enhancement. The presence of callose at PD is strongly attributed to the regulation of callose accumulation and callose degradation by callose synthases and β-1,3-glucanases (BGs), respectively. SLs are implicated in signaling and membrane protein trafficking; however, the underlying processes linking SL composition to the control of symplasmic apertures remain unknown. The wide variety of SLs in plants prompted us to investigate which SL molecules are important for regulating symplasmic apertures in Arabidopsis (Arabidopsis thaliana). We introduced several potential SL pathway inhibitors and genetically modified SL contents using two independent SL pathway mutants. We were able to modulate callose deposition to control symplasmic connectivity through perturbations of SL metabolism. Alteration in glucosylhydroxyceramides or related SL composition particularly disturbed the secretory machinery for the GPI-anchored PdBG2 protein, resulting in an overaccumulation of callose. Moreover, our results revealed that SL-enriched lipid rafts link symplasmic channeling to PD callose homeostasis by controlling the targeting of GPI-anchored PdBG2. This study elevates our understanding of the molecular linkage underlying intracellular trafficking and precise targeting of GPI-anchored PD proteins incorporating glucosyl SLs.

Rapid identification of an Arabidopsis NLR gene as a candidate conferring susceptibility to Sclerotinia sclerotiorum using time-resolved automated phenotyping

The broad host range necrotrophic fungus Sclerotinia sclerotiorum is a devastating pathogen of many oil and vegetable crops. Plant genes conferring complete resistance against S. sclerotiorum have not been reported. Instead, plant populations challenged by S. sclerotiorum exhibit a continuum of partial resistance designated as quantitative disease resistance (QDR). Because of their complex interplay and their small phenotypic effect, the functional characterization of QDR genes remains limited. How broad host range necrotrophic fungi manipulate plant programmed cell death is for instance largely unknown. Here, we designed a time-resolved automated disease phenotyping pipeline enabling high-throughput disease lesion measurement with high resolution, low footprint at low cost. We could accurately recover contrasted disease responses in several pathosystems using this system. We used our phenotyping pipeline to assess the kinetics of disease symptoms caused by seven S. sclerotiorum isolates on six A. thaliana natural accessions with unprecedented resolution. Large effect polymorphisms common to the most resistant A. thaliana accessions identified highly divergent alleles of the nucleotide-binding site leucine-rich repeat gene LAZ5 in the resistant accessions Rubezhnoe and Lip-0. We show that impaired LAZ5 expression in laz5.1 mutant lines and in A. thaliana Rub natural accession correlate with enhanced QDR to S. sclerotiorum. These findings illustrate the value of time-resolved image-based phenotyping for unravelling the genetic bases of complex traits such as QDR. Our results suggest that S. sclerotiorum manipulates plant sphingolipid pathways guarded by LAZ5 to trigger programmed cell death and cause disease.

What determines the composition of the phloem sap? Is there any selectivity filter for macromolecules entering the phloem sieve elements?

In view of recent findings, it is still a matter of debate whether the composition of the phloem sap of higher plants is specific and based on a plasmodesmal selectivity filter for macromolecular transport, or whether simply related to size, abundance and half-life of the macromolecules within the phloem sap. A range of reports indicates specific function of phloem-mobile signaling molecules such as the florigen making it indispensable to discriminate specific macromolecules entering the phloem from others which cannot cross this selectivity filter. Nevertheless, several findings have discussed for a non-selective transport via plasmodesmata, or contamination of the phloem sap by degradation products coming from immature still developing young sieve elements undergoing differentiation. Here, we discuss several possibilities, and raise the question how selectivity of the phloem sap composition could be achieved thereby focusing on mobility and dynamics of sucrose transporter mRNA and proteins.

SlREM1 Triggers Cell Death by Activating an Oxidative Burst and Other Regulators

Programmed cell death (PCD), a highly regulated feature of the plant immune response, involves multiple molecular players. Remorins (REMs) are plant-specific proteins with varied biological functions, but their function in PCD and plant defense remains largely unknown. Here, we report a role for remorin in disease resistance, immune response, and PCD regulation. Overexpression of tomato (Solanum lycopersicum) REMORIN1 (SlREM1) increased susceptibility of tomato to the necrotrophic fungus Botrytis cinerea and heterologous expression of this gene triggered cell death in Nicotiana benthamiana leaves. Further investigation indicated that amino acids 173 to 187 and phosphorylation of SlREM1 played key roles in SlREM1-triggered cell death. Intriguingly, multiple tomato REMs induced cell death in N benthamiana leaves. Yeast two-hybrid, split luciferase complementation, and coimmunoprecipitation assays all demonstrated that remorin proteins could form homo- and heterocomplexes. Using isobaric tags for relative and absolute quantitative proteomics, we identified that some proteins related to cell death regulation, as well as N benthamiana RESPIRATORY BURST OXIDASE HOMOLOG B (which is essential for reactive oxygen species production), were notably upregulated in SlREM1-expressing leaves. Heterologous expression of SlREM1 increased reactive oxygen species accumulation and triggered other cell death regulators. Our findings indicate that SlREM1 is a positive regulator of plant cell death and provide clues for understanding the PCD molecular regulatory network in plants.

Populus euphratica remorin 6.5 activates plasma membrane H+-ATPases to mediate salt tolerance

Remorins (REMs) play an important role in the ability of plants to adapt to adverse environments. PeREM6.5, a protein of the REM family in Populus euphratica (salt-resistant poplar), was induced by NaCl stress in callus, roots and leaves. We cloned the full-length PeREM6.5 from P. euphratica and transformed it into Escherichia coli and Arabidopsis thaliana. PeREM6.5 recombinant protein significantly increased the H+-ATPase hydrolytic activity and H+ transport activity in P. euphratica plasma membrane (PM) vesicles. Yeast two-hybrid assay showed that P. euphratica REM6.5 interacted with RPM1-interacting protein 4 (PeRIN4). Notably, the PeREM6.5-induced increase in PM H+-ATPase activity was enhanced by PeRIN4 recombinant protein. Overexpression of PeREM6.5 in Arabidopsis significantly improved salt tolerance in transgenic plants in terms of survival rate, root growth, electrolyte leakage and malondialdehyde content. Arabidopsis plants overexpressing PeREM6.5 retained high PM H+-ATPase activity in both in vivo and in vitro assays. PeREM6.5-transgenic plants had reduced accumulation of Na+ due to the Na+ extrusion promoted by the H+-ATPases. Moreover, the H+ pumps caused hyperpolarization of the PM, which reduced the K+ loss mediated by the depolarization-activated channels in the PM of salinized roots. Therefore, we conclude that PeREM6.5 regulated H+-ATPase activity in the PM, thus enhancing the plant capacity to maintain ionic homeostasis under salinity.

Patterns of Sequence and Expression Diversification Associate Members of the PADRE Gene Family With Response to Fungal Pathogens

Pathogen infection triggers extensive reprogramming of the plant transcriptome, including numerous genes the function of which is unknown. Due to their wide taxonomic distribution, genes encoding proteins with Domains of Unknown Function (DUFs) activated upon pathogen challenge likely play important roles in disease. In Arabidopsis thaliana, we identified thirteen genes harboring a DUF4228 domain in the top 10% most induced genes after infection by the fungal pathogen Sclerotinia sclerotiorum. Based on functional information collected through homology and contextual searches, we propose to refer to this domain as the pathogen and abiotic stress response, cadmium tolerance, disordered region-containing (PADRE) domain. Genome-wide and phylogenetic analyses indicated that PADRE is specific to plants and diversified into 10 subfamilies early in the evolution of Angiosperms. PADRE typically occurs in small single-domain proteins with a bipartite architecture. PADRE N-terminus harbors conserved sequence motifs, while its C-terminus includes an intrinsically disordered region with multiple phosphorylation sites. A pangenomic survey of PADRE genes expression upon S. sclerotiorum inoculation in Arabidopsis, castor bean, and tomato indicated consistent expression across species within phylogenetic groups. Multi-stress expression profiling and co-expression network analyses associated AtPADRE genes with the induction of anthocyanin biosynthesis and responses to chitin and to hypoxia. Our analyses reveal patterns of sequence and expression diversification consistent with the evolution of a role in disease resistance for an uncharacterized family of plant genes. These findings highlight PADRE genes as prime candidates for the functional dissection of mechanisms underlying plant disease resistance to fungi.

Contributions and Limitations of Biophysical Approaches to Study of the Interactions between Amphiphilic Molecules and the Plant Plasma Membrane

Some amphiphilic molecules are able to interact with the lipid matrix of plant plasma membranes and trigger the immune response in plants. This original mode of perception is not yet fully understood and biophysical approaches could help to obtain molecular insights. In this review, we focus on such membrane-interacting molecules, and present biophysically grounded methods that are used and are particularly interesting in the investigation of this mode of perception. Rather than going into overly technical details, the aim of this review was to provide to readers with a plant biochemistry background a good overview of how biophysics can help to study molecular interactions between bioactive amphiphilic molecules and plant lipid membranes. In particular, we present the biomimetic membrane models typically used, solid-state nuclear magnetic resonance, molecular modeling, and fluorescence approaches, because they are especially suitable for this field of research. For each technique, we provide a brief description, a few case studies, and the inherent limitations, so non-specialists can gain a good grasp on how they could extend their toolbox and/or could apply new techniques to study amphiphilic bioactive compound and lipid interactions.

Functions of Anionic Lipids in Plants

Anionic phospholipids, which include phosphatidic acid, phosphatidylserine, and phosphoinositides, represent a small percentage of membrane lipids. They are able to modulate the physical properties of membranes, such as their surface charges, curvature, or clustering of proteins. Moreover, by mediating interactions with numerous membrane-associated proteins, they are key components in the establishment of organelle identity and dynamics. Finally, anionic lipids also act as signaling molecules, as they are rapidly produced or interconverted by a set of dedicated enzymes. As such, anionic lipids are major regulators of many fundamental cellular processes, including cell signaling, cell division, membrane trafficking, cell growth, and gene expression. In this review, we describe the functions of anionic lipids from a cellular perspective. Using the localization of each anionic lipid and its related metabolic enzymes as starting points, we summarize their roles within the different compartments of the endomembrane system and address their associated developmental and physiological consequences.

The Nanoscale Organization of the Plasma Membrane and Its Importance in Signaling: A Proteolipid Perspective

Plasma membranes provide a highly selective environment for a large number of transmembrane and membrane-associated proteins. Whereas lateral movement of proteins in this lipid bilayer is possible, it is rather limited in turgid and cell wall-shielded plant cells. However, membrane-resident signaling processes occur on subsecond scales that cannot be explained by simple diffusion models. Accordingly, several receptors and other membrane-associated proteins are organized and functional in membrane nanodomains. Although the general presence of membrane nanodomains has become widely accepted as fact, fundamental functional aspects, the roles of individual lipid species and their interplay with proteins, and aspects of nanodomain maintenance and persistence remain poorly understood. Here, we review the current knowledge of nanodomain organization and function, with a particular focus on signaling processes involving proteins, lipids, and their interactions. Furthermore, we propose new and hypothetical aspects of plant membrane biology that we consider important for future research.

Biological and Cellular Functions of the Microdomain-Associated FWL/CNR Protein Family in Plants

Membrane microdomains/nanodomains are sub-compartments of the plasma membrane enriched in sphingolipids and characterized by their unique protein composition. They play important roles in regulating plant development and plant-microbe interactions including mutualistic symbiotic interactions. Several protein families are associated with the microdomain fraction of biological membranes such as flotillins, prohibitins, and remorins. More recently, GmFWL1, a FWL/CNR protein exclusively expressed in the soybean nodule, was functionally characterized as a new microdomain-associated protein. Interestingly, GmFWL1 is homologous to the tomato FW2-2 protein, a major regulator of tomato fruit development. In this review, we summarize the knowledge gained about the biological, cellular, and physiological functions of members of the FWL/CNR family across various plant species. The role of the FWL/CNR proteins is also discussed within the scope of their evolution and transcriptional regulation.

Remorin interacting with PCaP1 impairs Turnip mosaic virus intercellular movement but is antagonised by VPg

Group 1 Remorins (REMs) are extensively involved in virus trafficking through plasmodesmata (PD). However, their roles in Potyvirus cell-to-cell movement are not known. The plasma membrane (PM)-associated Ca²⁺ binding protein 1 (PCaP1) interacts with the P3N-PIPO of Turnip mosaic virus (TuMV) and is required for TuMV cell-to-cell movement, but the underlying mechanism remains elusive. The mutant plants with overexpression or knockout of REM1.2 were used to investigate its role in TuMV cell-to-cell movement. Arabidopsis thaliana complementary mutants of pcap1 were used to investigate the role of PCaP1 in TuMV cell-to-cell movement. Yeast-two-hybrid, bimolecular fluorescence complementation, co-immunoprecipitation and RT-qPCR assays were employed to investigate the underlying molecular mechanism. The results show that TuMV-P3N-PIPO recruits PCaP1 to PD and the actin filament-severing activity of PCaP1 is required for TuMV intercellular movement. REM1.2 negatively regulates the cell-to-cell movement of TuMV via competition with PCaP1 for binding actin filaments. As a counteractive response, TuMV mediates REM1.2 degradation via both 26S ubiquitin-proteasome and autophagy pathways through the interaction of VPg with REM1.2 to establish systemic infection in Arabidopsis. This work unveils the actin cytoskeleton and PM nanodomain-associated molecular events underlying the cell-to-cell movement of potyviruses.

Deviating from the Beaten Track: New Twists in Brassinosteroid Receptor Function

A key feature of plants is their plastic development tailored to the environmental conditions. To integrate environmental signals with genetic growth regulatory programs, plants rely on a number of hormonal pathways, which are intimately connected at multiple levels. Brassinosteroids (BRs), a class of plant sterol hormones, are perceived by cell surface receptors and trigger responses instrumental in tailoring developmental programs to environmental cues. Arguably, BR signalling is one of the best-characterized plant signalling pathways, and the molecular composition of the core signal transduction cascade seems clear. However, BR research continues to reveal new twists to re-shape our view on this key signalling circuit. Here, exciting novel findings pointing to the plasma membrane as a key site for BR signalling modulation and integration with other pathways are reviewed and new inputs into the BR signalling pathway and emerging “non-canonical” functions of the BR receptor complex are highlighted. Together, this new evidence underscores the complexity of plant signalling integration and serves as a reminder that highly-interconnected signalling pathways frequently comprise non-linear aspects which are difficult to convey in classical conceptual models.

Enzyme assembly guided by SPFH-induced functional inclusion bodies for enhanced cascade biocatalysis

Enzyme clustering into compact agglomerates could accelerate the processing of intermediates to enhance metabolic pathway flux. However, enzyme clustering is still a challenging task due to the lack of universal assembly strategy applicable to all enzymes. Therefore, we proposed an alternative enzyme assembly strategy based on functional inclusion bodies. First, functional inclusion bodies in cells were formed by the fusion expression of stomatin/prohibitin/flotillin/HflK/C (SPFH) domain and enhanced green fluorescent protein, as observed visually and by transmission electron microscopy. The formation of SPFH-induced functional inclusion bodies enhanced intermolecular polymerization as revealed by further analysis combined with Förster resonance energy transfer and bimolecular fluorescent complimentary. Finally, the functional inclusion bodies significantly improved the enzymatic catalysis in living cells, as proven by the examples with whole-cell biocatalysis of phenyllactic acid by Escherichia coli, and the production of N-acetylglucosamine by Bacillus subtilis. Our findings suggest that SPFH-induced functional inclusion bodies can enhance the cascade reaction of enzymes, to serve as a potential universal strategy for the construction of efficient microbial cell factories.

Dynamics of Candidatus Liberibacter asiaticus Movement and Sieve-Pore Plugging in Citrus Sink Cells

Citrus greening or Huanglongbing (HLB) is caused by the phloem-limited intracellular Gram-negative bacterium Candidatus Liberibacter asiaticus (CLas). HLB-infected citrus phloem cells undergo structural modifications that include cell wall thickening, callose and phloem protein induction, and cellular plugging. However, very little is known about the intracellular mechanisms that take place during CLas cell-to-cell movement. Here, we show that CLas movement through phloem pores of sweet orange (Citrus sinensis) and grapefruit (Citrus paradisi) is carried out by the elongated form of the bacteria. The round form of CLas is too large to move, but can change its morphology to enable its movement. CLas cells adhere to the plasma membrane of the phloem cells specifically adjacent to the sieve pores. Remarkably, CLas was present in both mature sieve element cells and nucleated nonsieve element cells. The sieve plate plugging structures of host plants were shown to have different composition in different citrus tissues. Callose deposition was the main plugging mechanism in the HLB-infected flush, where it reduced the open space of the pores. In the roots, pores were surrounded by dark extracellular material, with very little accumulation of callose. The expression of CALLOSE SYNTHASE7 and PHLOEM PROTEIN2 genes was upregulated in the shoots, but downregulated in root tissues. In seed coats, no phloem occlusion was observed, and CLas accumulated to high levels. Our results provide insight into the cellular mechanisms of Gram-negative bacterial cell-to-cell movement in plant phloem.

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

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

Organization and dynamics of functional plant membrane microdomains

Plasma membranes are heterogeneous and laterally compartmentalized into distinct microdomains. These membrane microdomains consist of special lipids and proteins and are thought to act as signaling platforms. In plants, membrane microdomains have been detected by super-resolution microscopy, and there is evidence that they play roles in several biological processes. Here, we review current knowledge about the lipid and protein components of membrane microdomains. Furthermore, we summarize the dynamics of membrane microdomains in response to different stimuli. We also explore the biological functions associated with membrane microdomains as signal integration hubs. Finally, we outline challenges and questions for further studies.

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.

Cucumber mosaic virus resistance: Comparative proteomics of contrasting Cucumis sativus cultivars after long-term infection

Plant viruses are a significant threat to a wide range of host species, causing substantial losses in agriculture. Particularly, Cucumber mosaic virus (CMV) evokes severe symptoms, thus dramatically limiting yield. Activation of plant defense reactions is associated with changes in the cellular proteome to ensure virus resistance. Herein, we studied two cultivars of cucumber (Cucumis sativus) resistant host Heliana and susceptible host Vanda. Plant cotyledons were mechanically inoculated with CMV isolate PK1, and systemic leaves were harvested at 33 days post-inoculation. Proteome was profiled by ultrahigh-performance liquid chromatography and comprehensively quantified by ion mobility enhanced mass spectrometry. From 1516 reproducibly quantified proteins using a label-free approach, 133 were differentially abundant among cultivars or treatments by strict statistic and effect size criteria. Pigments and hydrogen peroxide measurements corroborated proteomic findings. Comparison of both cultivars in the uninfected state highlighted more abundant photosynthetic and development-related proteins in resistant cucumber cultivar. Long-term CMV infection caused worse preservation of energy processes and less robust translation in the susceptible cultivar. Contrary, compatible plants had numerous more abundant stress and defense-related proteins. We proposed promising targets for functional validation in transgenic lines: A step toward durable virus resistance in cucurbits and other crops. SIGNIFICANCE: Sustainable production of crops requires an understanding of natural mechanisms of resistance/susceptibility to ubiquitous viral infections. We report original findings of comparative analysis of plant genotypes exposed to CMV. Deep discovery proteomics of resistant and susceptible cucumber cultivars, inoculated with widespread phytovirus, allowed to suggest several novel molecular targets for functional testing in plant protection strategies.

The functional diversity of structural disorder in plant proteins

Structural disorder in proteins is a widespread feature distributed in all domains of life, particularly abundant in eukaryotes, including plants. In these organisms, intrinsically disordered proteins (IDPs) perform a diversity of functions, participating as integrators of signaling networks, in transcriptional and post-transcriptional regulation, in metabolic control, in stress responses and in the formation of biomolecular condensates by liquid-liquid phase separation. Their roles impact the perception, propagation and control of various developmental and environmental cues, as well as the plant defense against abiotic and biotic adverse conditions. In this review, we focus on primary processes to exhibit a broad perspective of the relevance of IDPs in plant cell functions. The information here might help to incorporate this knowledge into a more dynamic view of plant cells, as well as open more questions and promote new ideas for a better understanding of plant life.

Plasmodesmata Conductivity Regulation: A Mechanistic Model

Plant cells form a multicellular symplast via cytoplasmic bridges called plasmodesmata (Pd) and the endoplasmic reticulum (ER) that crosses almost all plant tissues. The Pd proteome is mainly represented by secreted Pd-associated proteins (PdAPs), the repertoire of which quickly adapts to environmental conditions and responds to biotic and abiotic stresses. Although the important role of Pd in stress-induced reactions is universally recognized, the mechanisms of Pd control are still not fully understood. The negative role of callose in Pd permeability has been convincingly confirmed experimentally, yet the roles of cytoskeletal elements and many PdAPs remain unclear. Here, we discuss the contribution of each protein component to Pd control. Based on known data, we offer mechanistic models of mature leaf Pd regulation in response to stressful effects.

Plant Hormones Differentially Control the Sub-Cellular Localization of Plasma Membrane Microdomains during the Early Stage of Soybean Nodulation

Phytohormones regulate the mutualistic symbiotic interaction between legumes and rhizobia, nitrogen-fixing soil bacteria, notably by controlling the formation of the infection thread in the root hair (RH). At the cellular level, the formation of the infection thread is promoted by the translocation of plasma membrane microdomains at the tip of the RH. We hypothesize that phytohormones regulate the translocation of plasma membrane microdomains to regulate infection thread formation. Accordingly, we treated with hormone and hormone inhibitors transgenic soybean roots expressing fusions between the Green Fluorescent Protein (GFP) and GmFWL1 or GmFLOT2/4, two microdomain-associated proteins translocated at the tip of the soybean RH in response to rhizobia. Auxin and cytokinin treatments are sufficient to trigger or inhibit the translocation of GmFWL1 and GmFLOT2/4 to the RH tip independently of the presence of rhizobia, respectively. Unexpectedly, the application of salicylic acid, a phytohormone regulating the plant defense system, also promotes the translocation of GmFWL1 and GmFLOT2/4 to the RH tip regardless of the presence of rhizobia. These results suggest that phytohormones are playing a central role in controlling the early stages of rhizobia infection by regulating the translocation of plasma membrane microdomains. They also support the concept of crosstalk of phytohormones to control nodulation.

Mechanisms governing subcompartmentalization of biological membranes

Membranes show a tremendous variety of lipids and proteins operating biochemistry, transport and signalling. The dynamics and the organization of membrane constituents are regulated in space and time to execute precise functions. Our understanding of the molecular mechanisms that shape and govern membrane subcompartmentalization and inter-organelle contact sites still remains limited. Here, we review some reported mechanisms implicated in regulating plant membrane domains including those of plasma membrane, plastids, mitochondria and endoplasmic reticulum. Finally, we discuss several state-of-the-art methods that allow nowadays researchers to decipher the architecture of these structures at the molecular and atomic level.

Comparative proteomics combined with analyses of transgenic plants reveal ZmREM1.3 mediates maize resistance to southern corn rust

Southern corn rust (SCR), which is a destructive disease caused by Puccinia polysora Underw. (P. polysora), commonly occurs in warm-temperate and tropical regions. To identify candidate proteins related to SCR resistance and characterize the molecular mechanisms underlying the maize-P. polysora interaction, a comparative proteomic analysis of susceptible and resistant maize lines was performed. Statistical analyses revealed 1489 differentially abundant proteins in the resistant line, as well as 1035 differentially abundant proteins in the susceptible line. After the P. polysora infection, the abundance of one remorin protein (ZmREM1.3) increased in the resistant genotype, but decreased in the susceptible genotype. Plant-specific remorins are important for responses to microbial infections as well as plant signalling processes. In this study, transgenic maize plants overexpressing ZmREM1.3 exhibited enhanced resistance to the biotrophic P. polysora. In contrast, homozygous ZmREM1.3 UniformMu mutant plants were significantly more susceptible to P. polysora than wild-type plants. Additionally, the ZmREM1.3-overexpressing plants accumulated more salicylic acid (SA) and jasmonic acid (JA). Moreover, the expression levels of defence-related genes were higher in ZmREM1.3-overexpressing maize plants than in non-transgenic control plants in response to the P. polysora infection. Overall, our results provide evidence that ZmREM1.3 positively regulates maize defences against P. polysora likely via SA/JA-mediated defence signalling pathways. This study represents the first large-scale proteomic analysis of the molecular mechanisms underlying the maize-P. polysora interaction. This is also the first report confirming the remorin protein family affects plant resistance to SCR.

Nanodomain Clustering of the Plant Protein Remorin by Solid-State NMR

Nanodomains are dynamic membrane subcompartments, enriched in specific lipid, and protein components that act as functional platforms to manage an abundance of cellular processes. The remorin protein of plants is a well-established nanodomain marker and widely serves as a paradigm to study nanodomain clustering. Located at the inner leaflet of the plasma membrane, remorins perform essential functions during signaling. Using deuterium and phosphorus solid-state NMR, we inquire on the molecular determinants of the lipid-protein and protein-protein interactions driving nanodomain clustering. By monitoring thermotropism properties, lipid acyl chain order and membrane thickness, we report the effects of phosphoinositides and sterols on the interaction of various remorin peptides and protein constructs with the membrane. We probed several critical residues involved in this interaction and the involvement of the coiled-coil homo-oligomerisation domain into the formation of remorin nanodomains. We trace the essential role of the pH in nanodomain clustering based on anionic lipids such as phosphoinositides. Our results reveal a complex interplay between specific remorin residues and domains, the environmental pH and their resulting effects on the lipid dynamics for phosphoinositide-enriched membranes.

Salicylic acid-mediated plasmodesmal closure via Remorin-dependent lipid organization

Plasmodesmata (PD) create cytoplasmic and membrane continuities between adjacent cells to facilitate cell–cell communication and virus movement. Plant cells have evolved diverse mechanisms to regulate PD plasticity against plant pathogens, including the accumulation of the defense hormone, salicylic acid (SA). However, the mechanism of how this occurs is not well understood. Here, we uncover a mechanism by which SA triggers Remorin-dependent membrane lipid nanodomain assembly, leading to enhancement of the liquid-ordered phase. The higher-ordered lipids, which are particularly enriched at PD membrane, decreased PD membrane plasticity, and thus restricted PD opening and impeded virus spreading. Our findings address a knowledge gap in plant defense mechanisms at the membrane level that rely on SA-controlled lipid order and PD closure.

Plasmodesmata (PD) are plant-specific membrane-lined channels that create cytoplasmic and membrane continuities between adjacent cells, thereby facilitating cell–cell communication and virus movement. Plant cells have evolved diverse mechanisms to regulate PD plasticity in response to numerous environmental stimuli. In particular, during defense against plant pathogens, the defense hormone, salicylic acid (SA), plays a crucial role in the regulation of PD permeability in a callose-dependent manner. Here, we uncover a mechanism by which plants restrict the spreading of virus and PD cargoes using SA signaling by increasing lipid order and closure of PD. We showed that exogenous SA application triggered the compartmentalization of lipid raft nanodomains through a modulation of the lipid raft-regulatory protein, Remorin (REM). Genetic studies, superresolution imaging, and transmission electron microscopy observation together demonstrated that Arabidopsis REM1.2 and REM1.3 are crucial for plasma membrane nanodomain assembly to control PD aperture and functionality. In addition, we also found that a 14-3-3 epsilon protein modulates REM clustering and membrane nanodomain compartmentalization through its direct interaction with REM proteins. This study unveils a molecular mechanism by which the key plant defense hormone, SA, triggers membrane lipid nanodomain reorganization, thereby regulating PD closure to impede virus spreading.

A Remorin from Nicotiana benthamiana Interacts with the Pseudomonas Type-III Effector Protein HopZ1a and is Phosphorylated by the Immune-Related Kinase PBS1

The plasma membrane (PM) is at the interface of plant-pathogen interactions and, thus, many bacterial type-III effector (T3E) proteins target membrane-associated processes to interfere with immunity. The Pseudomonas syringae T3E HopZ1a is a host cell PM-localized effector protein that has several immunity-associated host targets but also activates effector-triggered immunity in resistant backgrounds. Although HopZ1a has been shown to interfere with early defense signaling at the PM, no dedicated PM-associated HopZ1a target protein has been identified until now. Here, we show that HopZ1a interacts with the PM-associated remorin protein NbREM4 from Nicotiana benthamiana in several independent assays. NbREM4 relocalizes to membrane nanodomains after treatment with the bacterial elicitor flg22 and transient overexpression of NbREM4 in N. benthamiana induces the expression of a subset of defense-related genes. We can further show that NbREM4 interacts with the immune-related receptor-like cytoplasmic kinase avrPphB-susceptible 1 (PBS1) and is phosphorylated by PBS1 on several residues in vitro. Thus, we conclude that NbREM4 is associated with early defense signaling at the PM. The possible relevance of the HopZ1a-NbREM4 interaction for HopZ1a virulence and avirulence functions is discussed.Copyright © 2019 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Controlling intercellular flow through mechanosensitive plasmodesmata nanopores

In plants, plasmodesmata (PD) are nanopores that serve as channels for molecular cell-to-cell transport. Precise control of PD permeability is essential to regulate processes such as growth and tissue patterning, photoassimilate distribution and defense against pathogens. Callose deposition modulates PD transport but little is known of the rapid events that lead to PD closure in response to tissue damage or osmotic shock. We propose a mechanism of PD closure as a result of mechanosensing. Pressure forces acting on the dumbbell-shaped ER-desmotubule complex cause it to be displaced from its equilibrium position, thus closing the PD aperture. The filamentous protein tethers that link the plasma membrane to the ER-desmotubule complex play a key role in determining the selectivity of the PD pore. This model of PD control compares favorably with experimental data on the pressure-generated closure of PD.

Key checkpoints in the movement of plant viruses through the host

Plant viruses cannot exploit any of the membrane fusion-based routes of entry described for animal viruses. In addition, one of the distinctive structures of plant cells, the cell wall, acts as the first barrier against the invasion of pathogens. To overcome the rigidity of the cell wall, plant viruses normally take advantage of the way of life of different biological vectors. Alternatively, the physical damage caused by environmental stresses can facilitate virus entry. Once inside the cell and taking advantage of the characteristic symplastic continuity of plant cells, viruses need to remodel and/or modify the restricted pore size of the plasmodesmata (channels that connect plant cells). In a successful interaction for the virus, it can reach the vascular tissue to systematically invade the plant. The connections between the different cell types in this path are not designed to allow the passage of molecules with the complexity of viruses. During this process, viruses face different cell barriers that must be overcome to reach the distal parts of the plant. In this review, we highlight the current knowledge about how plant RNA viruses enter plant cells, move between them to reach vascular cells and overcome the different physical and cellular barriers that the phloem imposes. Finally, we update the current research on cellular organelles as key regulator checkpoints in the long-distance movement of plant viruses.

The cell wall regulates dynamics and size of plasma-membrane nanodomains in Arabidopsis

Plant plasma-membrane (PM) proteins are involved in several vital processes, such as detection of pathogens, solute transport, and cellular signaling. For these proteins to function effectively there needs to be structure within the PM allowing, for example, proteins in the same signaling cascade to be spatially organized. Here we demonstrate that several proteins with divergent functions are located in clusters of differing size in the membrane using subdiffraction-limited Airyscan confocal microscopy. Single particle tracking reveals that these proteins move at different rates within the membrane. Actin and microtubule cytoskeletons appear to significantly regulate the mobility of one of these proteins (the pathogen receptor FLS2) and we further demonstrate that the cell wall is critical for the regulation of cluster size by quantifying single particle dynamics of proteins with key roles in morphogenesis (PIN3) and pathogen perception (FLS2). We propose a model in which the cell wall and cytoskeleton are pivotal for regulation of protein cluster size and dynamics, thereby contributing to the formation and functionality of membrane nanodomains.

Genome-Wide Identification and Characterization of the Wheat Remorin (TaREM) Family during Cold Acclimation

Remorins (REMs) are plant-specific proteins that play an essential role in plant-microbe interactions. However, their roles in vernalization and abiotic stress responses remain speculative. Most remorins have a variable proline-rich -half and a more conserved -half that is predicted to form coils. A search of the wheat ( L.) database revealed the existence of 20 different genes, which we classified into six groups on the basis of whether they shared a common phylogenetic and structural origin. Analysis of the physical genomic distributions demonstrated that genes are dispersed in the wheat genome and have one to seven introns. Promoter analysis of genes revealed the presence of putative -elements related to diverse functions like development, hormonal regulation, and biotic and abiotic stress responsiveness. Expression levels of genes were measured in plants grown under field and controlled conditions and in response to hormone treatment. Our analyses revealed that 12 members of the REM family are regulated during cold acclimation in wheat in four different tissues (roots, crowns, stems, and leaves), with the highest expression in roots. Differential gene expression was found between wheat cultivars with contrasting degrees of cold tolerance, suggesting the implication of genes in cold response and tolerance. Additionally, eight genes were induced in response to abscisic acid and methyl jasmonate treatment. This genome-wide analysis of genes provides valuable resources for functional analysis aimed at understanding their role in stress adaptation.

Manipulating Endoplasmic Reticulum-Plasma Membrane Tethering in Plants Through Fluorescent Protein Complementation

The bimolecular fluorescence complementation (BiFC) assay has been widely used to examine interactions between integral and peripheral proteins within putative plasma membrane (PM) microdomains. In the course of using BiFC assays to examine the co-localization of plasma membrane (PM) targeted receptor-like kinases (RLKs), such as FLS2, with PM micro-domain proteins such as remorins, we unexpectedly observed heterogeneous distribution patterns of fluorescence on the PM of Nicotiana benthamiana leaf cortical cells. These patterns appeared to co-localize with the endoplasmic reticulum (ER) and with ER-PM contact sites, and closely resembled patterns caused by over-expression of the ER-PM tether protein Synaptotagmin1 (SYT1). Using domain swap experiments with SYT1, we inferred that non-specific dimerization between FLS2-VenusN and VenusC-StRem1.3 could create artificial ER-PM tether proteins analogous to SYT1. The same patterns of ER-PM tethering were produced when a representative set of integral membrane proteins were partnered in BiFC complexes with PM-targeted peripheral membrane proteins, including PtdIns(4)P-binding proteins. We inferred that spontaneous formation of mature fluorescent proteins caused the BiFC complexes to trap the integral membrane proteins in the ER during delivery to the PM, producing a PM-ER tether. This phenomenon could be a useful tool to deliberately manipulate ER-PM tethering or to test protein membrane localization. However, this study also highlights the risk of using the BiFC assay to study membrane protein interactions in plants, due to the possibility of alterations in cellular structures and membrane organization, or misinterpretation of protein-protein interactions. A number of published studies using this approach may therefore need to be revisited.

Phytophthora sojae Effector PsAvh240 Inhibits Host Aspartic Protease Secretion to Promote Infection

Plants secrete defense molecules into the extracellular space (the apoplast) to combat attacking microbes. However, the mechanisms by which successful pathogens subvert plant apoplastic immunity remain poorly understood. In this study, we show that PsAvh240, a membrane-localized effector of the soybean pathogen Phytophthora sojae, promotes P. sojae infection in soybean hairy roots. We found that PsAvh240 interacts with the soybean-resistant aspartic protease GmAP1 in planta and suppresses the secretion of GmAP1 into the apoplast. By solving its crystal structure we revealed that PsAvh240 contain six α helices and two WY motifs. The first two α helices of PsAvh240 are responsible for its plasma membrane-localization and are required for PsAvh240's interaction with GmAP1. The second WY motifs of two PsAvh240 molecules form a handshake arrangement resulting in a handshake-like dimer. This dimerization is required for the effector's repression of GmAP1 secretion. Taken together, these data reveal that PsAvh240 localizes at the plasma membrane to interfere with GmAP1 secretion, which represents an effective mechanism by which effector proteins suppress plant apoplastic immunity.

Identification of Candidate Ergosterol-Responsive Proteins Associated with the Plasma Membrane of Arabidopsis thaliana

The impact of fungal diseases on crop production negatively reflects on sustainable food production and overall economic health. Ergosterol is the major sterol component in fungal membranes and regarded as a general elicitor or microbe-associated molecular pattern (MAMP) molecule. Although plant responses to ergosterol have been reported, the perception mechanism is still unknown. Here, Arabidopsis thaliana protein fractions were used to identify those differentially regulated following ergosterol treatment; additionally, they were subjected to affinity-based chromatography enrichment strategies to capture and categorize ergosterol-interacting candidate proteins using liquid chromatography coupled with tandem mass spectrometry (LC-MS/MS). Mature plants were treated with 250 nM ergosterol over a 24 h period, and plasma membrane-associated fractions were isolated. In addition, ergosterol was immobilized on two different affinity-based systems to capture interacting proteins/complexes. This resulted in the identification of defense-related proteins such as chitin elicitor receptor kinase (CERK), non-race specific disease resistance/harpin-induced (NDR1/HIN1)-like protein, Ras-related proteins, aquaporins, remorin protein, leucine-rich repeat (LRR)- receptor like kinases (RLKs), G-type lectin S-receptor-like serine/threonine-protein kinase (GsSRK), and glycosylphosphatidylinositol (GPI)-anchored protein. Furthermore, the results elucidated unknown signaling responses to this MAMP, including endocytosis, and other similarities to those previously reported for bacterial flagellin, lipopolysaccharides, and fungal chitin.

Review: Plant-pathogen interactions through the plasmodesma prism

Plasmodesmata (PD) allow membrane and cytoplasmic continuity between plant cells, and they are essential for intercellular communication and signaling in addition to metabolite partitioning. Plant pathogens have evolved a variety of mechanisms to subvert PD to facilitate their infection of plant hosts. PD are implicated not only in local spread around infection sites but also in the systemic spread of pathogens and pathogen-derived molecules. In turn, plants have developed strategies to limit pathogen spread via PD, and there is increasing evidence that PD may also be active players in plant defense responses. The last few years have seen important advances in understanding the roles of PD in plant-pathogen infection. Nonetheless, several critical areas remain to be addressed. Here we highlight some of these, focusing on the need to consider the effects of pathogen-PD interaction on the trafficking of endogenous molecules, and the involvement of chloroplasts in regulating PD during pathogen defense. By their very nature, PD are recalcitrant to most currently used investigative techniques, therefore answering these questions will require creative imaging and novel quantification approaches.

Dynamic regulation of plasmodesmatal permeability and its application to horticultural research

Effective cell-to-cell communication allows plants to fine-tune their developmental processes in accordance with the prevailing environmental stimuli. Plasmodesmata (PD) are intercellular channels that span the plant cell wall and serve as cytoplasmic bridges to facilitate efficient exchange of signaling molecules between neighboring cells. The identification of PD-associated proteins and the subsequent elucidation of the regulation of PD structure have provided vital insights into the role of PD architecture in enforcing crucial cellular processes, including callose deposition, ER-Golgi-based secretion, cytoskeleton dynamics, membrane lipid raft organization, chloroplast metabolism, and cell wall formation. In this review, we summarize the emerging discoveries from recent studies that elucidated the regulatory mechanisms involved in PD biogenesis and the dynamics of PD opening-closure. Retrospectively, PD-mediated cell-to-cell communication has been implicated in diverse cellular and physiological processes that are fundamental for the development of horticultural plants. The potential application of PD biotechnological engineering represents a powerful approach for improving agronomic traits in horticultural crops in the future.

Autophagy in crop plants: what's new beyond Arabidopsis?

Autophagy is a major degradation and recycling pathway in plants. It functions to maintain cellular homeostasis and is induced by environmental cues and developmental stimuli. Over the past decade, the study of autophagy has expanded from model plants to crop species. Many features of the core machinery and physiological functions of autophagy are conserved among diverse organisms. However, several novel functions and regulators of autophagy have been characterized in individual plant species. In light of its critical role in development and stress responses, a better understanding of autophagy in crop plants may eventually lead to beneficial agricultural applications. Here, we review recent progress on understanding autophagy in crops and discuss potential future research directions.

Callose balancing at plasmodesmata

In plants, communication and molecular exchanges between different cells and tissues are dependent on the apoplastic and symplastic pathways. Symplastic molecular exchanges take place through the plasmodesmata, which connect the cytoplasm of neighboring cells in a highly controlled manner. Callose, a β-1,3-glucan polysaccharide, is a plasmodesmal marker molecule that is deposited in cell walls near the neck zone of plasmodesmata and controls their permeability. During cell differentiation and plant development, and in response to diverse stresses, the level of callose in plasmodesmata is highly regulated by two antagonistic enzymes, callose synthase or glucan synthase-like and β-1,3-glucanase. The diverse modes of regulation by callose synthase and β-1,3-glucanase have been uncovered in the past decades through biochemical, molecular, genetic, and omics methods. This review highlights recent findings regarding the function of plasmodesmal callose and the molecular players involved in callose metabolism, and provides new insight into the mechanisms maintaining plasmodesmal callose homeostasis.

Plant lipids: Key players of plasma membrane organization and function

The plasma membrane (PM) is the biological membrane that separates the interior of all cells from the outside. The PM is constituted of a huge diversity of proteins and lipids. In this review, we will update the diversity of molecular species of lipids found in plant PM. We will further discuss how lipids govern global properties of the plant PM, explaining that plant lipids are unevenly distributed and are able to organize PM in domains. From that observation, it emerges a complex picture showing a spatial and multiscale segregation of PM components. Finally, we will discuss how lipids are key players in the function of PM in plants, with a particular focus on plant-microbe interaction, transport and hormone signaling, abiotic stress responses, plasmodesmata function. The last chapter is dedicated to the methods that the plant membrane biology community needs to develop to get a comprehensive understanding of membrane organization in plants.

REM1.3's phospho-status defines its plasma membrane nanodomain organization and activity in restricting PVX cell-to-cell movement

Plants respond to pathogens through dynamic regulation of plasma membrane-bound signaling pathways. To date, how the plant plasma membrane is involved in responses to viruses is mostly unknown. Here, we show that plant cells sense the Potato virus X (PVX) COAT PROTEIN and TRIPLE GENE BLOCK 1 proteins and subsequently trigger the activation of a membrane-bound calcium-dependent kinase. We show that the Arabidopsis thaliana CALCIUM-DEPENDENT PROTEIN KINASE 3-interacts with group 1 REMORINs in vivo, phosphorylates the intrinsically disordered N-terminal domain of the Group 1 REMORIN REM1.3, and restricts PVX cell-to-cell movement. REM1.3's phospho-status defines its plasma membrane nanodomain organization and is crucial for REM1.3-dependent restriction of PVX cell-to-cell movement by regulation of callose deposition at plasmodesmata. This study unveils plasma membrane nanodomain-associated molecular events underlying the plant immune response to viruses.

Viruses propagate in plants through membranous channels, called plasmodesmata, linking each cell to its neighboring cell. In this work, we challenge the role of the plasma membrane in the regulation of virus propagation. By studying the dynamics and the activation of a plant-specific protein called REMORIN, we found that the way this protein is organized inside the membrane is crucial to fulfill its function in the immunity of plants against viruses.

Transcriptome Analysis of Intrusively Growing Flax Fibers Isolated by Laser Microdissection

The intrusive growth, a type of plant cell elongation occurring in the depths of plant tissues, is characterized by the invasion of a growing cell between its neighbours due to a higher rate of elongation. In order to reveal the largely unknown molecular mechanisms of intrusive growth, we isolated primary flax phloem fibers specifically at the stage of intrusive growth by laser microdissection. The comparison of the RNA-Seq data from several flax stem parts enabled the characterization of those processes occurring specifically during the fiber intrusive elongation. The revealed molecular players are summarized as those involved in the supply of assimilates and support of turgor pressure, cell wall enlargement and modification, regulation by transcription factors and hormones, and responses to abiotic stress factors. The data obtained in this study provide a solid basis for developing approaches to manipulate fiber intrusive elongation, which is of importance both for plant biology and the yield of fiber crops.