Synaptotagmin 5 Controls SYP132-VAMP721/722 Interaction for Arabidopsis Immunity to Pseudomonas syringae pv tomato DC3000

Where R-SNAREs like to roam - the vesicle-associated membrane proteins VAMP721 & VAMP722 in trafficking hotspots

VAMP721 and VAMP722, play crucial roles in membrane fusion at post-Golgi compartments. They are involved in cell plate formation, recycling, endocytosis, and secretion. While individual SNARE actors and regulators exhibit significant overlap, specificity is achieved through distinct combinations of these components. Cytokinesis-related SNAREs traffic as preformed CIS-complexes, which require disassembly by the NSF/αSNAP chaperoning complex to facilitate subsequent homotypic fusion at the cell plate. Recent findings suggest a similar mechanism may operate during secretion. Regulation of VAMP721 activity involves interactions with tethers, GTPases, and Sec1/Munc18 proteins, along with a newly discovered phosphorylation at Tyrosine residue 57. These advances provide valuable insights into the fascinating world of cellular trafficking and membrane fusion.

Functional and Structural Analysis Reveals Distinct Biological Roles of Plant Synaptotagmins in Response to Environmental Stress

Endoplasmic reticulum-plasma membrane contact sites (ER-PM CSs) are evolutionarily conserved membrane domains found in all eukaryotes, where the ER closely interfaces with the PM. This short distance is achieved in plants through the action of tether proteins such as synaptotagmins (SYTs). Arabidopsis comprises five SYT members (SYT1-SYT5), but whether they possess overlapping or distinct biological functions remains elusive. SYT1, the best-characterized member, plays an essential role in the resistance to abiotic stress. This study reveals that the functionally redundant SYT1 and SYT3 genes, but not SYT5, are involved in salt and cold stress resistance. We also show that, unlike SYT5, SYT1 and SYT3 are not required for Pseudomonas syringae resistance. Since SYT1 and SYT5 interact in vivo via their SMP domains, the distinct functions of these proteins cannot be caused by differences in their localization. Interestingly, structural phylogenetic analysis indicates that the SYT1 and SYT5 clades emerged early in the evolution of land plants. We also show that the SYT1 and SYT5 clades exhibit different structural features in their SMP and Ca2+ binding of their C2 domains, rationalizing their distinct biological roles.

SYNAPTOTAGMIN 4 is expressed mainly in the phloem and participates in abiotic stress tolerance in Arabidopsis

Plant synaptotagmins structurally resemble animal synaptotagmins and extended-synaptotagmins. Animal synaptotagmins are well-characterized calcium sensors in membrane trafficking, and extended-synaptotagmins mediate lipid transfer at the endoplasmic reticulum-plasma membrane contact sites. Here, we characterize SYNAPTOTAGMIN 4 (SYT4), which belongs to the six-member family in Arabidopsis. Fluorometric GUS assay showed that the SYT4 promoter was strongest in roots and the least active in rosettes and cauline leaves, which was confirmed by qPCR. In seedlings, promoter activity was influenced by several factors, such as plant growth regulators, mannitol, sucrose, polyethylene glycol and cold. GUS histochemistry revealed SYT4 promoter activity in the phloem of all organs and even almost exclusively in sieve element precursors and differentiating sieve elements. Accordingly, the SYT-GFP fusion protein also accumulated in these cells with maximal abundance in sieve element precursors. The protein formed a network in the cytoplasm, but during sieve tube differentiation, it deposited at the cell periphery and disappeared from mature tubes. Using photoconvertible fluorescence technology, we showed that a high abundance of SYT4 protein in meristematic protophloem cells was due to its extensive synthesis. SYT4 protein synthesis was interrupted in differentiating sieve elements, but protein degradation was also reduced. In addition to phloem, the fusion protein was detected in shoot and root stem cell niche as early as the late heart stage of the embryo. We isolated and molecularly and biologically characterized five syt4 T-DNA insertion alleles and subjected them to phenotype analysis. The allele with the C2B domain interrupted by an T-DNA insertion exhibits increased sensitivity to factors such as auxins, osmotics, salicylic acid, sodium chloride, and the absence of sucrose in the root growth test.

Interplay of EXO70 and MLO proteins modulates trichome cell wall composition and susceptibility to powdery mildew

Exocyst component of 70-kDa (EXO70) proteins are constituents of the exocyst complex implicated in vesicle tethering during exocytosis. MILDEW RESISTANCE LOCUS O (MLO) proteins are plant-specific calcium channels and some MLO isoforms enable fungal powdery mildew pathogenesis. We here detected an unexpected phenotypic overlap of Arabidopsis thaliana exo70H4 and mlo2 mlo6 mlo12 triple mutant plants regarding the biogenesis of leaf trichome secondary cell walls. Biochemical and Fourier transform infrared spectroscopic analyses corroborated deficiencies in the composition of trichome cell walls in these mutants. Transgenic lines expressing fluorophore-tagged EXO70H4 and MLO exhibited extensive colocalization of these proteins. Furthermore, mCherry-EXO70H4 mislocalized in trichomes of the mlo triple mutant and, vice versa, MLO6-GFP mislocalized in trichomes of the exo70H4 mutant. Expression of GFP-marked PMR4 callose synthase, a known cargo of EXO70H4-dependent exocytosis, revealed reduced cell wall delivery of GFP-PMR4 in trichomes of mlo triple mutant plants. In vivo protein-protein interaction assays in plant and yeast cells uncovered isoform-preferential interactions between EXO70.2 subfamily members and MLO proteins. Finally, exo70H4 and mlo6 mutants, when combined, showed synergistically enhanced resistance to powdery mildew attack. Taken together, our data point to an isoform-specific interplay of EXO70 and MLO proteins in the modulation of trichome cell wall biogenesis and powdery mildew susceptibility.

The SYP123-VAMP727 SNARE complex delivers secondary cell wall components for root hair shank hardening in Arabidopsis

The extended tubular shape of root hairs is established by tip growth and concomitant hardening. Here, we demonstrate that a syntaxin of plants (SYP)123-vesicle-associated membrane protein (VAMP)727-dependent secretion system delivers secondary cell wall components for hardening the subapical zone and shank of Arabidopsis (Arabidopsis thaliana) root hairs. We found increased SYP123 localization at the plasma membrane (PM) of the subapical and shank zones compared with the tip region in elongating root hairs. Inhibition of phosphatidylinositol (PtdIns)(3,5)P2 production impaired SYP123 localization at the PM and SYP123-mediated root hair shank hardening. Moreover, root hair elongation in the syp123 mutant was insensitive to a PtdIns(3,5)P2 synthesis inhibitor. SYP123 interacts with both VAMP721 and VAMP727. syp123 and vamp727 mutants exhibited reduced shank cell wall stiffness due to impaired secondary cell wall component deposition. Based on these results, we conclude that SYP123 is involved in VAMP721-mediated conventional secretion for root hair elongation as well as in VAMP727-mediated secretory functions for the delivery of secondary cell wall components to maintain root hair tubular morphology.

A bacterial type III effector targets plant vesicle-associated membrane proteins

The soilborne bacterial pathogen Ralstonia solanacearum is one of the most destructive plant pathogens worldwide, and its infection process involves the manipulation of numerous plant cellular functions. In this work, we found that the R. solanacearum effector protein RipD partially suppressed different levels of plant immunity triggered by R. solanacearum elicitors, including specific responses triggered by pathogen-associated molecular patterns and secreted effectors. RipD localized in different subcellular compartments in plant cells, including vesicles, and its vesicular localization was enriched in cells undergoing R. solanacearum infection, suggesting that this specific localization may be particularly relevant during infection. Among RipD-interacting proteins, we identified plant vesicle-associated membrane proteins (VAMPs). We also found that overexpression of Arabidopsis thaliana VAMP721 and VAMP722 in Nicotiana benthamiana leaves promoted resistance to R. solanacearum, and this was abolished by the simultaneous expression of RipD, suggesting that RipD targets VAMPs to contribute to R. solanacearum virulence. Among proteins secreted in VAMP721/722-containing vesicles, CCOAOMT1 is an enzyme required for lignin biosynthesis, and mutation of CCOAOMT1 enhanced plant susceptibility to R. solanacearum. Altogether our results reveal the contribution of VAMPs to plant resistance against R. solanacearum and their targeting by a bacterial effector as a pathogen virulence strategy.

Biological roles of plant synaptotagmins

Plant synaptotagmins (SYTs) are resident proteins of the endoplasmic reticulum (ER). They are characterized by an N-terminal transmembrane region and C2 domains at the C-terminus, which tether the ER to the plasma membrane (PM). In addition to their tethering role, SYTs contain a lipid-harboring SMP domain, essential for shuttling lipids between the ER and the PM. There is now abundant literature on Arabidopsis SYT1, the best-characterized family member, which link it to biotic and abiotic responses as well as to ER morphology. Here, we review the current knowledge of SYT members, focusing on their role in stress, and discuss how these roles can be related to their tethering and lipid transport functions. Finally, we contextualize this information about SYTs with their homologs, the yeast tricalbins and the mammalian extended synaptotagmins.

Genome-Wide Analysis of the SNARE Family in Cultivated Peanut (Arachis hypogaea L.) Reveals That Some Members Are Involved in Stress Responses

The superfamily of soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) proteins mediates membrane fusion during vesicular transport between endosomes and the plasma membrane in eukaryotic cells, playing a vital role in plant development and responses to biotic and abiotic stresses. Peanut (Arachis hypogaea L.) is a major oilseed crop worldwide that produces pods below ground, which is rare in flowering plants. To date, however, there has been no systematic study of SNARE family proteins in peanut. In this study, we identified 129 putative SNARE genes from cultivated peanut (A. hypogaea) and 127 from wild peanut (63 from Arachis duranensis, 64 from Arachis ipaensis). We sorted the encoded proteins into five subgroups (Qa-, Qb-, Qc-, Qb+c- and R-SNARE) based on their phylogenetic relationships with Arabidopsis SNAREs. The genes were unevenly distributed on all 20 chromosomes, exhibiting a high rate of homolog retention from their two ancestors. We identified cis-acting elements associated with development, biotic and abiotic stresses in the promoters of peanut SNARE genes. Transcriptomic data showed that expression of SNARE genes is tissue-specific and stress inducible. We hypothesize that AhVTI13b plays an important role in the storage of lipid proteins, while AhSYP122a, AhSNAP33a and AhVAMP721a might play an important role in development and stress responses. Furthermore, we showed that three AhSNARE genes (AhSYP122a, AhSNAP33a and AhVAMP721) enhance cold and NaCl tolerance in yeast (Saccharomyces cerevisiae), especially AhSNAP33a. This systematic study provides valuable information about the functional characteristics of AhSNARE genes in the development and regulation of abiotic stress responses in peanut.

TaSYP137 and TaVAMP723, the SNAREs Proteins from Wheat, Reduce Resistance to Blumeria graminis f. sp. tritici

SNARE protein is an essential factor driving vesicle fusion in eukaryotes. Several SNAREs have been shown to play a crucial role in protecting against powdery mildew and other pathogens. In our previous study, we identified SNARE family members and analyzed their expression pattern in response to powdery mildew infection. Based on quantitative expression and RNA-seq results, we focused on TaSYP137/TaVAMP723 and hypothesized that they play an important role in the interaction between wheat and Blumeria graminis f. sp. Tritici (Bgt). In this study, we measured the expression patterns of TaSYP132/TaVAMP723 genes in wheat post-infection with Bgt and found that the expression pattern of TaSYP137/TaVAMP723 was opposite in resistant and susceptible wheat samples infected by Bgt. The overexpression of TaSYP137/TaVAMP723 disrupted wheat's defense against Bgt infection, while silencing these genes enhanced its resistance to Bgt. Subcellular localization studies revealed that TaSYP137/TaVAMP723 are present in both the plasma membrane and nucleus. The interaction between TaSYP137 and TaVAMP723 was confirmed using the yeast two-hybrid (Y2H) system. This study offers novel insights into the involvement of SNARE proteins in the resistance of wheat against Bgt, thereby enhancing our comprehension of the role of the SNARE family in the pathways related to plant disease resistance.

Secretory membrane traffic in plant-microbe interactions

Plant defense responses include the extracellular release of defense-related molecules, such as pathogenesis-related proteins and secondary metabolites, as well as cell wall materials. This primarily depends on the trafficking of secretory vesicles to the plasma membrane, where they discharge their contents into the apoplastic space via soluble N-ethylmaleimide sensitive factor attachment protein receptor-assisted exocytosis. However, some pathogenic and symbiotic microbes have developed strategies to manipulate host plant exocytic pathways. Here, we discuss the mechanisms by which plant exocytic pathways function in immunity and how microbes have evolved to manipulate those pathways.

Synaptotagmin 4 and 5 additively contribute to Arabidopsis immunity to Pseudomonas syringae DC3000

Soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs) are essential for vesicle trafficking in plants. Vesicle-associated membrane protein 721 and 722 (VAMP721/722) are secretory vesicle-localized R-SNAREs, which are involved in a variety of biological processes in plants. Compared to VAMP721/722, a VAMP721/722-interacting plasma membrane (PM)-localized Qa-SNARE is engaged in a rather specific physiological process. This indicates that an in vivo regulator controls an interaction between a Qa-SNARE and VAMP721/722 for a specific cellular activity. We previously reported that synaptotagmin 5 (SYT5) modulates the interaction between SYP132 PM Qa-SNARE and VAMP721/722 for Arabidopsis resistance to Pseudomonas syringae DC3000. In this study, we show that defense against P. syringae DC3000 is compromised in SYT4-lacking plants, which belongs to the same subclade as SYT5. Further elevation of bacterial growth in syt4 syt5-2 plants compared to either syt4 or syt5-2 single mutant suggests that SYT4 and SYT5 play additive roles in Arabidopsis immunity to P. syringae DC3000.

Elicitation of Innate Immunity by a Bacterial Volatile 2-Nonanone at Levels below Detection Limit in Tomato Rhizosphere

Bacterial volatile compounds (BVCs) exert beneficial effects on plant protection both directly and indirectly. Although BVCs have been detected in vitro, their detection in situ remains challenging. The purpose of this study was to investigate the possibility of BVCs detection under in situ condition and estimate the potentials of in situ BVC to plants at below detection limit. We developed a method for detecting BVCs released by the soil bacteria Bacillus velezensis strain GB03 and Streptomyces griseus strain S4-7 in situ using solid-phase microextraction coupled with gas chromatography-mass spectrometry (SPME-GC-MS). Additionally, we evaluated the BVC detection limit in the rhizosphere and induction of systemic immune response in tomato plants grown in the greenhouse. Two signature BVCs, 2-nonanone and caryolan-1-ol, of GB03 and S4-7 respectively were successfully detected using the soil-vial system. However, these BVCs could not be detected in the rhizosphere pretreated with strains GB03 and S4-7. The detection limit of 2-nonanone in the tomato rhizosphere was 1 µM. Unexpectedly, drench application of 2-nonanone at 10 nM concentration, which is below its detection limit, protected tomato seedlings against Pseudomonas syringae pv. tomato. Our finding highlights that BVCs, including 2-nonanone, released by a soil bacterium are functional even when present at a concentration below the detection limit of SPME-GC-MS.

RAB GTPases and SNAREs at the trans-Golgi network in plants

Membrane traffic is a fundamental cellular system to exchange proteins and membrane lipids among single membrane-bound organelles or between an organelle and the plasma membrane in order to keep integrity of the endomembrane system. RAB GTPases and SNARE proteins, the key regulators of membrane traffic, are conserved broadly among eukaryotic species. However, genome-wide analyses showed that organization of RABs and SNAREs that regulate the post-Golgi transport pathways is greatly diversified in plants compared to other model eukaryotes. Furthermore, some organelles acquired unique properties in plant lineages. Like in other eukaryotic systems, the trans-Golgi network of plants coordinates secretion and vacuolar transport; however, uniquely in plants, it also acts as a platform for endocytic transport and recycling. In this review, we focus on RAB GTPases and SNAREs that function at the TGN, and summarize how these regulators perform to control different transport pathways at the plant TGN. We also highlight the current knowledge of RABs and SNAREs' role in regulation of plant development and plant responses to environmental stimuli.

SNAREs Regulate Vesicle Trafficking During Root Growth and Development

SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor) proteins assemble to drive the final membrane fusion step of membrane trafficking. Thus, SNAREs are essential for membrane fusion and vesicular trafficking, which are fundamental mechanisms for maintaining cellular homeostasis. In plants, SNAREs have been demonstrated to be located in different subcellular compartments and involved in a variety of fundamental processes, such as cytokinesis, cytoskeleton organization, symbiosis, and biotic and abiotic stress responses. In addition, SNAREs can also contribute to the normal growth and development of Arabidopsis. Here, we review recent progress in understanding the biological functions and signaling network of SNAREs in vesicle trafficking and the regulation of root growth and development in Arabidopsis.