Vesicle-associated membrane proteins 721 and 722 are required for unimpeded growth of Arabidopsis under ABA application

Cellular basis of legume-rhizobium symbiosis

The legume-rhizobium symbiosis represents the most important system for terrestrial biological nitrogen fixation on land. Efficient nitrogen fixation during this symbiosis depends on successful rhizobial infection and complete endosymbiosis, which are achieved by complex cellular events including cell-wall remodeling, cytoskeletal reorganizations, and extensive membrane expansion and trafficking. In this review, we explore the dynamic remodeling of the plant-specific cell wall-membrane system-cytoskeleton (WMC) continuum during symbiotic nitrogen fixation. We focus on key processes linked to efficient nitrogen fixation, including rhizobial uptake, infection thread formation and elongation, rhizobial droplet release, cytoplasmic bridge formation, and rhizobial endosymbiosis. Additionally, we discuss the advanced techniques for investigating the cellular basis of root-nodule symbiosis and provide insights into the unsolved mysteries of robust symbiotic nitrogen fixation.

A tyrosine phospho-switch within the Longin domain of VAMP721 modulates SNARE functionality

The final step in secretion is membrane fusion facilitated by SNARE proteins that reside in opposite membranes. The formation of a trans-SNARE complex between one R and three Q coiled-coiled SNARE domains drives the final approach of the membranes providing the mechanical energy for fusion. Biological control of this mechanism is exerted by additional domains within some SNAREs. For example, the N-terminal Longin domain (LD) of R-SNAREs (also called Vesicle-associated membrane proteins, VAMPs) can fold back onto the SNARE domain blocking interaction with other cognate SNAREs. The LD may also determine the subcellular localization via interaction with other trafficking-related proteins. Here, we provide cell-biological and genetic evidence that phosphorylation of the Tyrosine57 residue regulates the functionality of VAMP721. We found that an aspartate mutation mimics phosphorylation, leading to protein instability and subsequent degradation in lytic vacuoles. The mutant SNARE also fails to rescue the defects of vamp721vamp722 loss-of-function lines in spite of its wildtype-like localization within the secretory pathway and the ability to interact with cognate SNARE partners. Most importantly, it imposes a dominant negative phenotype interfering with root growth, normal secretion and cytokinesis in wildtype plants generating large aggregates that mainly contain secretory vesicles. Non-phosphorylatable VAMP721Y57F needs higher gene dosage to rescue double mutants in comparison to native VAMP721 underpinning that phosphorylation modulates SNARE function. We propose a model where short-lived phosphorylation of Y57 serves as a regulatory step to control VAMP721 activity, favoring its open state and interaction with cognate partners to ultimately drive membrane fusion.

Genome-wide identification of R-SNARE gene family in upland cotton and function analysis of GhVAMP72l response to drought stress

Soluble N-ethylmaleimide-sensitive factor attachment protein receptors (R-SNAREs) mainly promoted the assembly of the SNARE complex to drive the final membrane fusion step of membrane transport. Previous research on R-SNAREs has mainly focused on development and growth and has rarely been involved in abiotic stress, especially in cotton. Here, we performed a comprehensive analysis of R-SNARE genes in upland cotton. In total, 51 Gh-R-SNARE genes across six phylogenetic groups were unevenly distributed on 21 chromosomes. Cis elements related to plant growth and response to abiotic stress responses were found in the promoter region of Gh-R-SNAREs. Nine Gh-R-SNARE genes were obviously upregulated under drought stress conditions by RNA-seq and qRT-PCR analysis. Among them, GhVAMP72l might be the key candidate gene contributing to drought stress tolerance in cotton by virus-induced gene silencing (VIGS) assay. These results provide valuable insights for the functional analysis of cotton R-SNAREs in response to drought stress and highlight potential beneficial genes for genetic improvement and breeding in cotton.

Transcriptomic Profiling Highlights the ABA Response Role of BnSIP1-1 in Brassica napus

BnSIP1-1 is the first identified SIP1 (6b Interacting Protein1) subfamily gene of the trihelix transcription factor family from Brassica napus (B. napus). We previously used a reverse genetic method to reveal its abiotic stress response function in endowing plants resistance to drought and salinity, as well as ABA (Abscisic acid). However, the molecular mechanisms of BnSIP1-1 are unclear. In this study, the global transcriptome files of BnSIP1-1-overexpressing transgenic and wildtype B. napus seedlings under ABA treatment were constructed using RNA-seq. A total of 1823 and 5512 DEGs (Differentially Expressed Genes) were identified in OE vs. WT and OE_ABA vs. WT_ABA comparison groups, which included 751 and 2567 up-regulated DEGs, and 1072 and 2945 down-regulated DEGs, separately. The impact of overexpressed BnSIP1-1 on plants was amplified by ABA, indicating BnSIP1-1 was an ABA-conditioned responsive gene. More interestingly, we found the reasons for BnSIP1-1 increasing plants' insensitivity to ABA were not by regulating ABA synthesis and catabolism, but by manipulating ABA transportation, ABA signal perception and transduction, inositol phosphate metabolism, as well as endomembrane trafficking, indirectly suggesting this gene may play roles upstream of the core ABA response pathway. Our results provided new insights into improving the knowledge about the function of BnSIP1-1 and the ABA signaling mechanism in B. napus.

SEC1A and SEC6 synergistically regulate pollen tube polar growth

Pollen tube polar growth is a key physiological activity for angiosperms to complete double fertilization, which is highly dependent on the transport of polar substances mediated by secretory vesicles. The exocyst and Sec1/Munc18 (SM) proteins are involved in the regulation of the tethering and fusion of vesicles and plasma membranes, but the molecular mechanism by which they regulate pollen tube polar growth is still unclear. In this study, we found that loss of function of SEC1A, a member of the SM protein family in Arabidopsis thaliana, resulted in reducing pollen tube growth and a significant increase in pollen tube width. SEC1A was diffusely distributed in the pollen tube cytoplasm, and was more concentrated at the tip of the pollen tube. Through co-immunoprecipitation-mass spectrometry screening, protein interaction analysis and in vivo microscopy, we found that SEC1A interacted with the exocyst subunit SEC6, and they mutually affected the distribution and secretion rate at the tip of the pollen tube. Meanwhile, the functional loss of SEC1A and SEC6 significantly affected the distribution of the SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor) complex member SYP125 at the tip of the pollen tube, and led to the disorder of pollen tube cell wall components. Genetic analysis revealed that the pollen tube-related phenotype of the sec1a sec6 double mutant was significantly enhanced compared with their respective single mutants. Therefore, we speculated that SEC1A and SEC6 cooperatively regulate the fusion of secretory vesicles and plasma membranes in pollen tubes, thereby affecting the length and the width of pollen tubes.

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.

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.

Proteomics and Co-expression Network Analysis Reveal the Importance of Hub Proteins and Metabolic Pathways in Nicotine Synthesis and Accumulation in Tobacco (Nicotiana tabacum L.)

Nicotine is a unique alkaloid present in tobacco that is widely used in cigarettes and in the agricultural, chemical, and pharmaceutical industries. However, the research on nicotine is mostly limited to its synthesis pathways, and only a few studies have explored the effects of other metabolic pathways on nicotine precursors. Regulating the nicotine content in tobacco can greatly promoting the application of nicotine in other fields. In this study, we performed global data-independent acquisition proteomics analysis of four tobacco varieties. Of the four varieties, one had high nicotine content and three had a low nicotine content. A total of 31,259 distinct peptides and 6,018 proteins across two samples were identified. A total of 45 differentially expressed proteins (DEPs) co-existed in the three comparison groups and were mainly involved in the transport and metallic processes of the substances. Most DEPs were enriched in the biosynthesis of secondary metals, glutathione metabolism, carbon metabolism, and glycolysis/gluconeogenesis. In addition, the weighted gene co-expression network analysis identified an expression module closely related to the nicotine content (Brown, r = 0.74, P = 0.006). Gene Ontology annotation and Kyoto Encyclopaedia of Genes and Genomes enrichment analysis showed that the module proteins were mainly involved in the synthesis and metabolism of nicotine precursors such as arginine, ornithine aspartate, proline, and glutathione. The increased levels of these precursors lead to the synthesis and accumulation of nicotine in plants. More importantly, these proteins regulate nicotine synthesis by affecting the formation of putrescine, which is the core intermediate product in nicotine anabolism. Our results provide a reference for tobacco variety selection with a suitable nicotine content and regulation of the nicotine content. Additionally, the results highlight the importance of other precursor metabolism in nicotine synthesis.

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

Vesicle-associated membrane proteins 721 and 722 (VAMP721/722) are secretory vesicle-localized arginine-conserved soluble N-ethylmaleimide-sensitive factor attachment protein receptors (R-SNAREs) to drive exocytosis in plants. They are involved in diverse physiological processes in plants by interacting with distinct plasma membrane (PM) syntaxins. Here, we show that synaptotagmin 5 (SYT5) is involved in plant defense against Pseudomonas syringae pv tomato (Pst) DC3000 by regulating SYP132-VAMP721/722 interactions. Calcium-dependent stimulation of in vitro SYP132-VAMP722 interaction by SYT5 and reduced in vivo SYP132-VAMP721/722 interaction in syt5 plants suggest that SYT5 regulates the interaction between SYP132 and VAMP721/722. We interestingly found that disease resistance to Pst DC3000 bacterium but not to Erysiphe pisi fungus is compromised in syt5 plants. Since SYP132 plays an immune function to bacteria, elevated growth of surface-inoculated Pst DC3000 in VAMP721/722-deficient plants suggests that SYT5 contributes to plant immunity to Pst DC3000 by promoting the SYP132-VAMP721/722 immune secretory pathway.

Wild soybean SNARE proteins BET1s mediate the subcellular localization of the cytoplasmic receptor-like kinases CRCK1s to modulate salt stress responses

Plants have evolved numerous receptor-like kinases (RLKs) that modulate environmental stress responses. However, little is known regarding soybean (Glycine max) RLKs. We have previously identified that Glycine soja Ca2+ /CAM-binding RLK (GsCBRLK) is involved in salt tolerance. Here, we report that soluble NSF attachment protein receptor proteins BET1s mediate subcellular localization of calmodulin-binding receptor-like cytoplasmic kinases CRCK1s to modulate salt stress responses. Direct interaction between GsCBRLK and GsBET11a was initially identified via yeast two-hybrid and bimolecular fluorescence complementation assays. Further analysis demonstrated conserved interaction between BET1s and CRCK1s. GsCBRLK interacted with all BET1 proteins in wild soybean (Glycine soja) and Arabidopsis, and GsBET11a strongly associated with GsCRCK1a-1d, but slightly with AtCRCK1. In addition, GsBET11a interacted with GsCBRLK via its C-terminal transmembrane domain (TMD), where the entire TMD, not the sequence, was critical for the interaction. Moreover, the N-terminal variable domain (VD) of GsCBRLK was responsible for interacting with GsBET11a, and the intensity of interaction between GsCBRLK/AtCRCK1 and GsBET11a was dependent on VD. Furthermore, GsBET11a was able to mediate the GsCBRLK subcellular localization via direct interaction with VD. Additionally, knockout of AtBET11 or AtBET12 individually did not alter GsCBRLK localization, while GsBET11a expression caused partial internalization of GsCBRLK from the plasma membrane (PM). We further suggest the necessity of GsCBRLK VD for its PM localization via N-terminal truncation assays. Finally, GsBET11a was shown to confer enhanced salt stress tolerance when overexpressed in Arabidopsis and soybean. These results revealed the conserved and direct interaction between BET1s and CRCK1s, and suggested their involvement in salt stress responses.

Proteomic Responses to Alkali Stress in Oats and the Alleviatory Effects of Exogenous Spermine Application

Alkali stress limits plant growth and yield more strongly than salt stress and can lead to the appearance of yellow leaves; however, the reasons remain unclear. In this study, we found that (1) the down-regulation of coproporphyrinogen III oxidase, protoporphyrinogen oxidase, and Pheophorbide a oxygenase in oats under alkali stress contributes to the appearance of yellow leaves (as assessed by proteome and western blot analyses). (2) Some oat proteins that are involved in the antioxidant system, root growth, and jasmonic acid (JA) and indole-3-acetic acid (IAA) synthesis are up-regulated in response to alkalinity and help increase alkali tolerance. (3) We added exogenous spermine to oat plants to improve their alkali tolerance, which resulted in higher chlorophyll contents and plant dry weights than in plants subjected to alkaline stress alone. This was due to up-regulation of chitinase and proteins related to chloroplast structure, root growth, and the antioxidant system. Spermine addition increased sucrose utilization efficiency, and promoted carbohydrate export from leaves to roots to increase energy storage in roots. Spermine addition also increased the IAA and JA contents required for root growth.

SNAREs in Plant Biotic and Abiotic Stress Responses

In eukaryotes, membraneous cellular compartmentation essentially requires vesicle trafficking for communications among distinct organelles. A donor organelle-generated vesicle releases its cargo into a target compartment by fusing two distinct vesicle and target membranes. Vesicle fusion, the final step of vesicle trafficking, is driven intrinsically by complex formation of soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs). Although SNAREs are well-conserved across eukaryotes, genomic studies revealed that plants have dramatically increased the number of SNARE genes than other eukaryotes. This increase is attributed to the sessile nature of plants, likely for more sensitive and harmonized responses to environmental stresses. In this review, we therefore try to summarize and discuss the current understanding of plant SNAREs function in responses to biotic and abiotic stresses.

Functions of the Plant Qbc SNARE SNAP25 in Cytokinesis and Biotic and Abiotic Stress Responses

Eukaryotes transport biomolecules between intracellular organelles and between cells and the environment via vesicle trafficking. Soluble N -ethylmaleimide-sensitive factor attachment protein receptors (SNARE proteins) play pivotal roles in vesicle and membrane trafficking. These proteins are categorized as Qa, Qb, Qc, and R SNAREs and form a complex that induces vesicle fusion for targeting of vesicle cargos. As the core components of the SNARE complex, the SNAP25 Qbc SNAREs perform various functions related to cellular homeostasis. The Arabidopsis thaliana SNAP25 homolog AtSNAP33 interacts with Qa and R SNAREs and plays a key role in cytokinesis and in triggering innate immune responses. However, other Arabidopsis SNAP25 homologs, such as AtSNAP29 and AtSNAP30, are not well studied; this includes their localization, interactions, structures, and functions. Here, we discuss three biological functions of plant SNAP25 orthologs in the context of AtSNAP33 and highlight recent findings on SNAP25 orthologs in various plants. We propose future directions for determining the roles of the less well-characterized AtSNAP29 and AtSNAP30 proteins.

CCOAOMT1, a candidate cargo secreted via VAMP721/722 secretory vesicles in Arabidopsis

We previously found that VAMP721/722 SNARE proteins guide secretory vesicles to pathogen-attacking sites during immune responses in Arabidopsis, which suggests that these vesicles should deliver immune molecules. However, the lethality of vamp721 vamp722 double null mutant makes it difficult to understand the nature of cargo transported via VAMP721/722 vesicles. Since VAMP721/722-depleted (VAMP721^+/-VAMP722^-/- and VAMP721^-/-VAMP722^+/-) plants show compromised resistance to extracellular pathogens, we assume that an immune protein secreted through the VAMP721/722-engaged exocytosis would be remained more in VAMP721/722-depleted plants than WT. By comparing intracellular proteins between WT and VAMP721/722-depleted plants, we found caffeoyl-CoA O-methyltransferase 1 (CCOAOMT1) involved in the lignin biosynthesis was more abundantly detected in both VAMP721/722-depleted lines than WT. Plants are well-known to deposit secondary cell walls as physical barriers at pathogen-attempting sites. Therefore, extracellular detection of CCOAOMT1 and impaired resistance to Pseudomonas syringae DC3000 in ccoaomt1 plants suggest that plants secrete cell wall-modifying enzymes at least including CCOAOMT1 to reinforce the secondary cell walls for immunity.

Endoplasmic reticulum stress-induced accumulation of VAMP721/722 requires CALRETICULIN 1 and CALRETICULIN 2 in Arabidopsis

Excessive demand for translation and protein folding in the endoplasmic reticulum (ER) can cause ER stress in plants. Here, we show that CALRETICULIN 1 (CRT1) and CRT2 are critical components in the accumulation of VESICLE-ASSOCIATED MEMBRANE PROTEIN 721 (VAMP721) and VAMP722 during ER stress responses. We show that CRT2 interacts with VAMP722 and that CRT1/2 post-translationally maintain elevated VAMP721/722 levels under ER stress. The greater growth inhibition in VAMP721/722-deficient plants, induced by tunicamycin, suggests that plants under ER stress maintain physiological homeostasis, at least in part, by regulating VAMP721/722 levels, as VAMP721/722 are known to participate in various biological processes.

Regulation of cellular VAMP721/722 abundance in arabidopsis

Sessile plants are continuously threatened by biotic and abiotic environmental stresses. Since stress responses are in general accompanied by growth retardation, plants in nature should tightly control timing and duration of their stress responses for sustained growth. We previously reported that vesicle-associated membrane protein (VAMP) 721 and 722 are required for growth/development and stress responses in plants. It is suggested that plants regulate expression of VAMP721/722 and/or drive VAMP721/722 to form distinct SNARE complexes with different plasma membrane (PM)-residing SNARE proteins in response to distinct stimuli. We here report that immune signaling triggered by the bacterial flg22 elicitor elevates VAMP721/722 levels in calreticulin 1 and 2 (CRT1/2)-lacking plants. Since VAMP721/722 amounts were reported not to be increased by an ER stress inducer, tunicamycin in crt1/2 plants, our results suggest that ER stress and immune signalings distinctly control cellular abundance of VAMP721/722.

Arabidopsis immune secretory pathways to powdery mildew fungi

Innate immune responses in host plants begin with the recognition of pathogen-specific nonself molecules and terminate with the secretion of immune molecules. In the dicotyledonous model plant, Arabidopsis thaliana, two distinct secretory pathways required for disease resistance to powdery mildew fungi have been identified so far. One is an exocytic pathway consisting of PEN1, SNAP33 and VAMP721/722 SNARE proteins, but the other is an efflux-mediated one composed of PEN2 atypical myrosinase and PEN3 ABC transporter. Based on the conservation of the mechanically same exocytic pathway in the monocotyledonous plant barely, the former is regarded as an ancient secretory pathway, whereas the latter is considered as a newly evolved one in the Brassicaceae family including Arabidopsis. We recently identified synaptotagmin 1 as an additional regulator of these two secretory pathways. With current results, we discuss how these two secretory pathways contribute to Arabidopsis immunity depending on fungal adaptedness to Arabidopsis.

Synaptotagmin 1 Negatively Controls the Two Distinct Immune Secretory Pathways to Powdery Mildew Fungi in Arabidopsis

PEN1, one of the plasma membrane (PM) syntaxins, comprises an immune exocytic pathway by forming the soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) complex with SNAP33 and VAMP721/722 in plants. Although this secretory pathway is also involved in plant growth and development, how plants control their exocytic activity is as yet poorly understood. Since constitutive PEN1 cycling between the PM and endocytosed vesicles is critical for its immune activity, we studied here the relationship of PEN1 to synaptotagmin 1 (SYT1) that is known to regulate endocytosis at the PM. Interestingly, syt1 plants showed enhanced disease resistance to the Arabidopsis-adapted Golovinomyces orontii fungus, and elevated protein but not transcript levels of PEN1 Calcium-dependent promotion of PEN1-SYT1 interaction suggests that SYT1 controls defense activities of the PEN1-associated secretory pathway by post-translationally modulating PEN1. Increased PEN1-SYT1 interaction and inhibited PEN1 SNARE complex induction by G. orontii additionally suggest that the adaption of phytopathogens to host plants might partly result from effective suppression of the PEN1-related secretory pathway. Further genetic analyses revealed that SYT1 also regulates the atypical peroxisomal myrosinase PEN2-associated secretory pathway.

Genome-Wide Analysis of MicroRNA Responses to the Phytohormone Abscisic Acid in Populus euphratica

MicroRNA (miRNA) is a type of non-coding small RNA with a regulatory function at the posttranscriptional level in plant growth development and in response to abiotic stress. Previous studies have not reported on miRNAs responses to the phytohormone abscisic acid (ABA) at a genome-wide level in Populus euphratica, a model tree for studying abiotic stress responses in woody plants. Here we analyzed the miRNA response to ABA at a genome-wide level in P. euphratica utilizing high-throughput sequencing. To systematically perform a genome-wide analysis of ABA-responsive miRNAs in P. euphratica, nine sRNA libraries derived from three groups (control, treated with ABA for 1 day and treated with ABA for 4 days) were constructed. Each group included three libraries from three individual plantlets as biological replicate. In total, 151 unique mature sequences belonging to 75 conserved miRNA families were identified, and 94 unique sequences were determined to be novel miRNAs, including 56 miRNAs with miRNA(*) sequences. In all, 31 conserved miRNAs and 31 novel miRNAs response to ABA significantly differed among the groups. In addition, 4132 target genes were predicted for the conserved and novel miRNAs. Confirmed by real-time qPCR, expression changes of miRNAs were inversely correlated with the expression profiles of their putative targets. The Populus special or novel miRNA-target interactions were predicted might be involved in some biological process related stress tolerance. Our analysis provides a comprehensive view of how P. euphratica miRNA respond to ABA, and moreover, different temporal dynamics were observed in different ABA-treated libraries.

The powdery mildew resistance protein RPW8.2 is carried on VAMP721/722 vesicles to the extrahaustorial membrane of haustorial complexes

Plants employ multiple cell-autonomous defense mechanisms to impede pathogenesis of microbial intruders. Previously we identified an exocytosis defense mechanism in Arabidopsis against pathogenic powdery mildew fungi. This pre-invasive defense mechanism depends on the formation of ternary protein complexes consisting of the plasma membrane-localized PEN1 syntaxin, the adaptor protein SNAP33 and closely sequence-related vesicle-resident VAMP721 or VAMP722 proteins. The Arabidopsis thaliana resistance to powdery mildew 8.2 protein (RPW8.2) confers disease resistance against powdery mildews upon fungal entry into host cells and is specifically targeted to the extrahaustorial membrane (EHM), which envelops the haustorial complex of the fungus. However, the secretory machinery involved in trafficking RPW8.2 to the EHM is unknown. Here we report that RPW8.2 is transiently located on VAMP721/722 vesicles, and later incorporated into the EHM of mature haustoria. Resistance activity of RPW8.2 against the powdery mildew Golovinomyces orontii is greatly diminished in the absence of VAMP721 but only slightly so in the absence of VAMP722. Consistent with this result, trafficking of RPW8.2 to the EHM is delayed in the absence of VAMP721. These findings implicate VAMP721/722 vesicles as key components of the secretory machinery for carrying RPW8.2 to the plant-fungal interface. Quantitative fluorescence recovery after photobleaching suggests that vesicle-mediated trafficking of RPW8.2-yellow fluorescent protein (YFP) to the EHM occurs transiently during early haustorial development and that lateral diffusion of RPW8.2-YFP within the EHM exceeds vesicle-mediated replenishment of RPW8.2-YFP in mature haustoria. Our findings imply the engagement of VAMP721/722 in a bifurcated trafficking pathway for pre-invasive defense at the cell periphery and post-invasive defense at the EHM.

Model for regulation of VAMP721/722-mediated secretion: growth vs. stress responses

The PEN1-SNAP33-VAMP721/722 exocytic pathway is a conserved immunity-associated secretory pathway between monocotyledonous barley and dicotyledonous Arabidopsis plants. In Arabidopsis, this secretory pathway plays an additional role in plant growth and development. However, how this pathway can be manipulated to engage in both growth/development and immunity remains to be answered. To understand its regulation, we recently analyzed the expression of VAMP721/722 genes whose products drive secretory vesicles to the target plasma membrane. By investigating their transcript and protein levels, we found that plants distinctly control the activity of this secretory pathway during biotic or abiotic stress responses. Since stress responses are in general accompanied by growth inhibition in plants and since plants in nature are simultaneously threatened by a number of environmental stresses, understanding of this growth/immunity-related secretory pathway would help to generate more efficiently growth/immunity-balancing plants.

SNARE complexes of different composition jointly mediate membrane fusion in Arabidopsis cytokinesis

Membrane fusion is mediated by soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) complexes. Although membrane fusion is required for separating daughter cells in eukaryotic cytokinesis, the SNARE complexes involved are not known. In plants, membrane vesicles targeted to the cell division plane fuse with one another to form the partitioning membrane, progressing from the center to the periphery of the cell. In Arabidopsis, the cytokinesis-specific Qa-SNARE KNOLLE interacts with two other Q-SNAREs, SNAP33 and novel plant-specific SNARE 11 (NPSN11), whose roles in cytokinesis are not clear. Here we show by coimmunoprecipitation that KNOLLE forms two SNARE complexes that differ in composition. One complex is modeled on the trimeric plasma membrane type of SNARE complex and includes, in addition to KNOLLE, the promiscuous Qb,c-SNARE SNAP33 and the R-SNARE vesicle-associated membrane protein (VAMP) 721,722, also involved in innate immunity. In contrast, the other KNOLLE-containing complex is tetrameric and includes Qb-SNARE NPSN11, Qc-SNARE SYP71, and VAMP721,722. Elimination of only one or the other type of KNOLLE complex by mutation, including the double mutant npsn11 syp71, causes a mild or no cytokinesis defect. In contrast, the two double mutants snap33 npsn11 and snap33 syp71 eliminate both types of KNOLLE complexes and display knolle-like cytokinesis defects. Thus the two distinct types of KNOLLE complexes appear to jointly mediate membrane fusion in Arabidopsis cytokinesis.