Setting SNAREs in a different wood

A century journey of organelles research in the plant endomembrane system

We are entering an exciting century in the study of the plant organelles in the endomembrane system. Over the past century, especially within the past 50 years, tremendous advancements have been made in the complex plant cell to generate a much clearer and informative picture of plant organelles, including the molecular/morphological features, dynamic/spatial behavior, and physiological functions. Importantly, all these discoveries and achievements in the identification and characterization of organelles in the endomembrane system would not have been possible without: (1) the innovations and timely applications of various state-of-art cell biology tools and technologies for organelle biology research; (2) the continuous efforts in developing and characterizing new organelle markers by the plant biology community; and (3) the landmark studies on the identification and characterization of the elusive organelles. While molecular aspects and results for individual organelles have been extensively reviewed, the development of the techniques for organelle research in plant cell biology is less appreciated. As one of the ASPB Centennial Reviews on "organelle biology," here we aim to take a journey across a century of organelle biology research in plants by highlighting the important tools (or landmark technologies) and key scientists that contributed to visualize organelles. We then highlight the landmark studies leading to the identification and characterization of individual organelles in the plant endomembrane systems.

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.

Genome-wide transcriptome analysis uncovers gene networks regulating fruit quality and volatile compounds in mango cultivar 'Tainong' during postharvest

Mango is one of the most economically important fruit; however, the gene regulatory mechanism associated with ripening and quality changes during storage remains largely unclear. This study explored the relationship between transcriptome changes and postharvest mango quality. Fruit quality patterns and volatile components were obtained using headspace gas chromatography and ion-mobility spectrometry (HS-GC-IMS). The changes in mango peel and pulp transcriptome were analyzed during four stages (pre-harvesting, harvesting, maturity, and overripe stages). Based on the temporal analysis, multiple genes involved in the biosynthesis of secondary metabolites were upregulated in both the peel and pulp during the mango ripening process. Moreover, cysteine and methionine metabolism related to ethylene synthesis were upregulated in the pulp over time. Weighted gene co-expression network analysis (WGCNA) further showed that the pathways of pyruvate metabolism, citrate cycle, propionate metabolism, autophagy, and SNARE interactions in vesicular transport were positively correlated with the ripening process. Finally, a regulatory network of important pathways from pulp to peel was constructed during the postharvest storage of mango fruit. The above findings provide a global insight into the molecular regulation mechanisms of postharvest mango quality and flavor changes.

A conserved clathrin-coated vesicle component, OsSCYL2, regulates plant innate immunity in rice

Clathrin-mediated vesicle trafficking (CMVT) is a fundamental process in all eukaryotic species, and indispensable to organism's growth and development. Recently, it has been suggested that CMVT also plays important roles in the regulation of plant immunity. However, the molecular link between CMVT and plant immunity is largely unknown. SCY1-LIKE2 (SCYL2) is evolutionally conserved among the eukaryote species. Loss-of-function of SCYL2 in Arabidopsis led to severe growth defects. Here, we show that mutation of OsSCYL2 in rice gave rise to a novel phenotype-hypersensitive response-like (HR) cell death in a light-dependent manner. Although mutants of OsSCYL2 showed additional defects in the photosynthetic system, they exhibited enhanced resistance to bacterial pathogens. Subcellular localisation showed that OsSCYL2 localized at Golgi, trans-Golgi network and prevacuolar compartment. OsSCYL2 interacted with OsSPL28, subunit of a clathrin-associated adaptor protein that is known to regulate HR-like cell death in rice. We further showed that OsSCYL2-OsSPL28 interaction is mediated by OsCHC1. Collectively, we characterized a novel component of the CMVT pathway in the regulation of plant immunity. Our work also revealed unidentified new functions of the very conserved SCYL2. It thus may provide new breeding targets to achieve both high yield and enhanced resistance in crops.

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.

Genome-Scale Identification, in Silico Characterization and Interaction Study Between Wheat SNARE and NPSN Gene Families Involved in Vesicular Transport

Wheat is an important cereal crop grown worldwide but it's yield is severely affected by various biotic and abiotic stresses. SNAREs are key regulators of vesicle trafficking and are present in abundance in higher plant species suggesting their prominence in growth and development. Novel Plant SNAREs (NPSN) are found exclusively in plants. Hence, a comprehensive analysis of these two gene families in wheat genome was accomplished in this study. We report here 27 SNAREs and eight NPSN genes. These genes and their respective proteins were investigated for gene structure, physiochemical properties, domain and motif architecture, phylogeny, chromosomal localization and possible interactions. Phylogenetic and motif analysis confirmed SNARE domain in all the proteins. Functional annotation revealed participation in biological process like vesicle fusion, exocytosis, protein targeting to vacuole and SNAP receptor activity. At subcellular level, SNAREs were localized in multiple organelles whereas NPSN proteins were localized in cytoplasm where they regulate vesicle fusion. The 3-D structures built with Modeller proved the presence of SNARE motifs in the identified proteins. Possible protein-protein interactions between SNARE and NPSN proteins were determined and docking was performed. The results augmented our understanding about molecular function, evolutionary relation, location inside the cell and their interactions.

Sticking With It: ER-PM Membrane Contact Sites as a Coordinating Nexus for Regulating Lipids and Proteins at the Cell Cortex

Membrane contact sites between the cortical endoplasmic reticulum (ER) and the plasma membrane (PM) provide a direct conduit for small molecule transfer and signaling between the two largest membranes of the cell. Contact is established through ER integral membrane proteins that physically tether the two membranes together, though the general mechanism is remarkably non-specific given the diversity of different tethering proteins. Primary tethers including VAMP-associated proteins (VAPs), Anoctamin/TMEM16/Ist2p homologs, and extended synaptotagmins (E-Syts), are largely conserved in most eukaryotes and are both necessary and sufficient for establishing ER-PM association. In addition, other species-specific ER-PM tether proteins impart unique functional attributes to both membranes at the cell cortex. This review distils recent functional and structural findings about conserved and species-specific tethers that form ER-PM contact sites, with an emphasis on their roles in the coordinate regulation of lipid metabolism, cellular structure, and responses to membrane stress.

Evaluation of the effects of humic acids on maize root architecture by label-free proteomics analysis

Humic substances have been widely used as plant growth promoters to improve the yield of agricultural crops. However, the mechanisms underlying this effect remain unclear. Root soluble protein profiles in plants 11 days after planting and cultivated with and without humic acids (HA, 50 mg CL^-1), were analyzed using the label-free quantitative proteomic approach. Cultivation of maize with HA resulted in higher fresh weight of roots than in untreated plants (control). Plants treated with HA showed increased number, diameter and length of roots. In the proteomics analysis, differences were detected in the following categories: energy metabolism, cytoskeleton, cellular transport, conformation and degradation of proteins, and DNA replication. Thirty-four proteins were significantly more abundant in the seedlings treated with HA, whereas only nine proteins were abundant in the control. The effects on root architecture, such as the induction of lateral roots and biomass increase were accompanied by changes in the energy metabolism-associated proteins. The results show that the main effect of HA is protective, mainly associated with increased expression of the 2-cys peroxidase, putative VHS/GAT, and glutathione proteins. Indeed, these proteins had the highest fold-difference. Overall, these results improve our understanding of the molecular mechanisms of HA-promoted plant growth.

Evaluation of the effects of humic acids on maize root architecture by label-free proteomics analysis

Humic substances have been widely used as plant growth promoters to improve the yield of agricultural crops. However, the mechanisms underlying this effect remain unclear. Root soluble protein profiles in plants 11 days after planting and cultivated with and without humic acids (HA, 50 mg CL^-1), were analyzed using the label-free quantitative proteomic approach. Cultivation of maize with HA resulted in higher fresh weight of roots than in untreated plants (control). Plants treated with HA showed increased number, diameter and length of roots. In the proteomics analysis, differences were detected in the following categories: energy metabolism, cytoskeleton, cellular transport, conformation and degradation of proteins, and DNA replication. Thirty-four proteins were significantly more abundant in the seedlings treated with HA, whereas only nine proteins were abundant in the control. The effects on root architecture, such as the induction of lateral roots and biomass increase were accompanied by changes in the energy metabolism-associated proteins. The results show that the main effect of HA is protective, mainly associated with increased expression of the 2-cys peroxidase, putative VHS/GAT, and glutathione proteins. Indeed, these proteins had the highest fold-difference. Overall, these results improve our understanding of the molecular mechanisms of HA-promoted plant growth.

Communications Between the Endoplasmic Reticulum and Other Organelles During Abiotic Stress Response in Plants

To adapt to constantly changing environmental conditions, plants have evolved sophisticated tolerance mechanisms to integrate various stress signals and to coordinate plant growth and development. It is well known that inter-organellar communications play important roles in maintaining cellular homeostasis in response to environmental stresses. The endoplasmic reticulum (ER), extending throughout the cytoplasm of eukaryotic cells, is a central organelle involved in lipid metabolism, Ca²⁺ homeostasis, and synthesis and folding of secretory and transmembrane proteins crucial to perceive and transduce environmental signals. The ER communicates with the nucleus via the highly conserved unfolded protein response pathway to mitigate ER stress. Importantly, recent studies have revealed that the dynamic ER network physically interacts with other intracellular organelles and endomembrane compartments, such as the Golgi complex, mitochondria, chloroplast, peroxisome, vacuole, and the plasma membrane, through multiple membrane contact sites between closely apposed organelles. In this review, we will discuss the signaling and metabolite exchanges between the ER and other organelles during abiotic stress responses in plants as well as the ER-organelle membrane contact sites and their associated tethering complexes.

Proteomic and ecophysiological responses of soybean (Glycine max L.) root nodules to Pb and hg stress

BACKGROUND

Lead (Pb) and mercury (Hg) are persistent hazardous metals in industrially polluted soils which can be toxic in low quantities. Metal toxicity can cause changes at cellular and molecular level which should be studied for better understanding of tolerance mechanism in plants. Soybean (Glycine max L.) is an important oilseed crop of the world including India. Indian soils growing soybean are often contaminated by Pb and Hg. The aim of this study was to explore how soybean root nodule responds to Pb and Hg through proteomic and ecophysiological alterations in order to enhance tolerance to metal stress.

RESULTS

Soybean plants were exposed to Pb (30 ppm PbCl2) and Hg (0.5 ppm HgCl2) to study histological, histochemical, biochemical and molecular response of N2-fixing symbiotic nodules. Both Pb and Hg treatment increased the level of oxidative stress in leaves and nodules. Chlorosis in leaves and morphological/anatomical changes in nodules were observed. Activities of ascorbate peroxidase, glutathione reductase and catalase were also modulated. Significant changes were observed in abundance of 76 proteins by Pb and Hg. Pb and Hg influenced abundance of 33 proteins (17 up and 16 down) and 43 proteins (33 up and 10 down), respectively. MS/MS ion search identified 55 proteins which were functionally associated with numerous cellular functions. Six crucial proteins namely catalase (CAT), allene oxide synthase (AOS), glutathione S-transferase (GST), calcineurin B like (CBL), calmodulin like (CML) and rapid alkalinisation factor (RAF) were selected for transcript abundance estimation. The qRT-PCR based real time expression exhibited a positive correlation with proteomics expression except for GST and RAF.

CONCLUSION

Soybean root nodule responds to metal stress by increased abundance of defence, development and repair related proteins. An efficient proteomic modulation might lead to metal-induced stress tolerance in N2-fixing nodules. Although concentrations of Pb and Hg used in the study cannot be considered equimolar, yet Hg seems to induce more changes in nodule proteomic profile, and higher damage to both bacteroides and root anatomy.

Nudge-nudge, WNK-WNK (kinases), say no more?

Contents Summary 35 I Overview of animal and plant WNK kinases 35 II. Structure: domains and topology 36 III. Phylogeny-evolutionary relationships 41 IV. Plant WNK kinase distribution and regulation of WNK expression and activity 41 V. Functions of WNK family members in physiology and development 41 VI. Say no more? Still many questions to be answered 45 Acknowledgements 46 References 46 SUMMARY: WITH NO LYSINE (WNK) kinases are serine/threonine kinases uniquely characterized by an anomalous placement of a catalytic lysine, hence their moniker. In animals, WNK protein kinases play critical roles in protein trafficking of components that mediate renal ion transport processes and regulate osmoregulation of cell volume. In plants, the WNK kinase gene family is larger and more diverse. Recent studies revealed WNK kinase roles in orchestrating the trafficking of an ion channel, a lipid kinase complex in animals, and a heterotrimeric G protein signaling component in plants that is necessary for signal transduction. For this reason, new attention is geared toward investigating the mechanisms adopted by WNK kinases to nudge intracellular proteins to their destinations. In this review, the functions of WNK kinases in protein trafficking are derived from what we have learned from the model organism Arabidopsis thaliana. To place this new idea in context, we provide the predicted WNK kinase structures, their predicted expression patterns, a speculated evolutionary pathway, and the regulatory roles of plant WNKs in transport processes and other physiologies. We brazenly predict that the WNK kinases in both plants and animals will soon be recognized as a nexus for trafficking-based signal transduction.

GsSNAP33, a novel Glycine soja SNAP25-type protein gene: Improvement of plant salt and drought tolerances in transgenic Arabidopsis thaliana

The N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) superfamily, specifically the SNAP25-type proteins and t-SNAREs, have been proposed to regulate cellular processes and plant resistance mechanisms. However, little is known about the role of SNAP25-type proteins in combating abiotic stresses, specifically in wild soybean. In the current study, the isolation and functional characterization of the putative synaptosomal-associated SNAP25-type protein gene GsSNAP33 from wild soybean (Glycine soja) were performed. GsSNAP33 has a molecular weight of 33,311 Da and comprises 300 amino acid residues along with Qb-Qc SNARE domains. Multiple sequence alignment revealed the highest similarity of the GsSNAP33 protein to GmSNAP33 (91%), VrSNAP33 (89%), PvSNAP33 (86%) and AtSNAP33 (63%). Phylogenetic studies revealed the abundance of SNAP33 proteins mostly in dicotyledons. Quantitative real-time PCR assays confirmed that GsSNAP33 expression can be induced by salt, alkali, ABA and PEG treatments and that GsSNAP33 is highly expressed in the pods, seeds and roots of Glycine soja. Furthermore, the overexpression of the GsSNAP33 gene in WT Arabidopsis thaliana resulted in increased germination rates, greater root lengths, improved photosynthesis, lower electrolyte leakage, higher biomass production and up-regulated expression levels of various stress-responsive marker genes, including KINI, COR15A, P5Cs, RAB18, RD29A and COR47 in transgenic lines compared with those in WT lines. Subcellular localization studies revealed that the GsSNAP33-eGFP fusion protein was localized to the plasma membrane, while eGFP was distributed throughout whole cytoplasm of onion epidermal cells. Collectively, our findings suggest that GsSNAP33, a novel plasma membrane protein gene of Glycine soja, might be involved in improving plant responses to salt and drought stresses in Arabidopsis.

Commandeering Channel Voltage Sensors for Secretion, Cell Turgor, and Volume Control

Control of cell volume and osmolarity is central to cellular homeostasis in all eukaryotes. It lies at the heart of the century-old problem of how plants regulate turgor, mineral and water transport. Plants use strongly electrogenic H⁺-ATPases, and the substantial membrane voltages they foster, to drive solute accumulation and generate turgor pressure for cell expansion. Vesicle traffic adds membrane surface and contributes to wall remodelling as the cell grows. Although a balance between vesicle traffic and ion transport is essential for cell turgor and volume control, the mechanisms coordinating these processes have remained obscure. Recent discoveries have now uncovered interactions between conserved subsets of soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) proteins that drive the final steps in secretory vesicle traffic and ion channels that mediate in inorganic solute uptake. These findings establish the core of molecular links, previously unanticipated, that coordinate cellular homeostasis and cell expansion.

Arabidopsis SYT1 maintains stability of cortical endoplasmic reticulum networks and VAP27-1-enriched endoplasmic reticulum-plasma membrane contact sites

Arabidopsis synaptotagmin 1 (SYT1) is localized on the endoplasmic reticulum-plasma membrane (ER-PM) contact sites in leaf and root cells. The ER-PM localization of Arabidopsis SYT1 resembles that of the extended synaptotagmins (E-SYTs) in animal cells. In mammals, E-SYTs have been shown to regulate calcium signaling, lipid transfer, and endocytosis. Arabidopsis SYT1 was reported to be essential for maintaining cell integrity and virus movement. This study provides detailed insight into the subcellular localization of SYT1 and VAP27-1, another ER-PM-tethering protein. SYT1 and VAP27-1 were shown to be localized on distinct ER-PM contact sites. The VAP27-1-enriched ER-PM contact sites (V-EPCSs) were always in contact with the SYT1-enriched ER-PM contact sites (S-EPCSs). The V-EPCSs still existed in the leaf epidermal cells of the SYT1 null mutant; however, they were less stable than those in the wild type. The polygonal networks of cortical ER disassembled and the mobility of VAP27-1 protein on the ER-PM contact sites increased in leaf cells of the SYT1 null mutant. These results suggest that SYT1 is responsible for stabilizing the ER network and V-EPCSs.

Rice OsVAMP714, a membrane-trafficking protein localized to the chloroplast and vacuolar membrane, is involved in resistance to rice blast disease

Membrane trafficking plays pivotal roles in many cellular processes including plant immunity. Here, we report the characterization of OsVAMP714, an intracellular SNARE protein, focusing on its role in resistance to rice blast disease caused by the fungal pathogen Magnaporthe oryzae. Disease resistance tests using OsVAMP714 knockdown and overexpressing rice plants demonstrated the involvement of OsVAMP714 in blast resistance. The overexpression of OsVAMP7111, whose product is highly homologous to OsVAMP714, did not enhance blast resistance to rice, implying a potential specificity of OsVAMP714 to blast resistance. OsVAMP714 was localized to the chloroplast in mesophyll cells and to the cellular periphery in epidermal cells of transgenic rice plant leaves. We showed that chloroplast localization is critical for the normal OsVAMP714 functioning in blast resistance by analyzing the rice plants overexpressing OsVAMP714 mutants whose products did not localize in the chloroplast. We also found that OsVAMP714 was located in the vacuolar membrane surrounding the invasive hyphae of M. oryzae. Furthermore, we showed that OsVAMP714 overexpression promotes leaf sheath elongation and that the first 19 amino acids, which are highly conserved between animal and plant VAMP7 proteins, are crucial for normal rice plant growths. Our studies imply that the OsVAMP714-mediated trafficking pathway plays an important role in rice blast resistance as well as in the vegetative growth of rice.

Functional Identification and Characterization of Genes Cloned from Halophyte Seashore Paspalum Conferring Salinity and Cadmium Tolerance

Salinity-affected and heavy metal-contaminated soils limit the growth of glycophytic plants. Identifying genes responsible for superior tolerance to salinity and heavy metals in halophytes has great potential for use in developing salinity- and Cd-tolerant glycophytes. The objective of this study was to identify salinity- and Cd-tolerance related genes in seashore paspalum (Paspalum vaginatum), a halophytic perennial grass species, using yeast cDNA expression library screening method. Based on the Gateway-compatible vector system, a high-quality entry library was constructed, which contained 9.9 × 10(6) clones with an average inserted fragment length of 1.48 kb representing a 100% full-length rate. The yeast expression libraries were screened in a salinity-sensitive and a Cd-sensitive yeast mutant. The screening yielded 32 salinity-tolerant clones harboring 18 salinity-tolerance genes and 20 Cd-tolerant clones, including five Cd-tolerance genes. qPCR analysis confirmed that most of the 18 salinity-tolerance and five Cd-tolerance genes were up-regulated at the transcript level in response to salinity or Cd stress in seashore paspalum. Functional analysis indicated that salinity-tolerance genes from seashore paspalum could be involved mainly in photosynthetic metabolism, antioxidant systems, protein modification, iron transport, vesicle traffic, and phospholipid biosynthesis. Cd-tolerance genes could be associated with regulating pathways that are involved in phytochelatin synthesis, HSFA4-related stress protection, CYP450 complex, and sugar metabolism. The 18 salinity-tolerance genes and five Cd-tolerance genes could be potentially used as candidate genes for genetic modification of glycophytic grass species to improve salinity and Cd tolerance and for further analysis of molecular mechanisms regulating salinity and Cd tolerance.

The Qb-SNARE Memb11 interacts specifically with Arf1 in the Golgi apparatus of Arabidopsis thaliana

The SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor) proteins are critical for the function of the secretory pathway. The SNARE Memb11 is involved in membrane trafficking at the ER-Golgi interface. The aim of the work was to decipher molecular mechanisms acting in Memb11-mediated ER-Golgi traffic. In mammalian cells, the orthologue of Memb11 (membrin) is potentially involved in the recruitment of the GTPase Arf1 at the Golgi membrane. However molecular mechanisms associated to Memb11 remain unknown in plants. Memb11 was detected mainly at the cis-Golgi and co-immunoprecipitated with Arf1, suggesting that Arf1 may interact with Memb11. This interaction of Memb11 with Arf1 at the Golgi was confirmed by in vivo BiFC (Bimolecular Fluorescence Complementation) experiments. This interaction was found to be specific to Memb11 as compared to either Memb12 or Sec22. Using a structural bioinformatic approach, several sequences in the N-ter part of Memb11 were hypothesized to be critical for this interaction and were tested by BiFC on corresponding mutants. Finally, by using both in vitro and in vivo approaches, we determined that only the GDP-bound form of Arf1 interacts with Memb11. Together, our results indicate that Memb11 interacts with the GDP-bound form of Arf1 in the Golgi apparatus.

Arabidopsis Qc-SNARE gene AtSFT12 is involved in salt and osmotic stress responses and Na(+) accumulation in vacuoles

AtSFT12, an Arabidopsis Qc-SNARE protein, is localized to Golgi organelles and is involved in salt and osmotic stress responses via accumulation of Na (+) in vacuoles. To reduce the detrimental effects of environmental stresses, plants have evolved many defense mechanisms. Here, we identified an Arabidopsis Qc-SNARE gene, AtSFT12, involved in salt and osmotic stress responses using an activation-tagging method. Both activation-tagged plants and overexpressing transgenic plants (OXs) of the AtSFT12 gene were tolerant to high concentrations of NaCl, LiCl, and mannitol, whereas loss-of-function mutants were sensitive to NaCl, LiCl, and mannitol. AtSFT12 transcription increased under NaCl, ABA, cold, and mannitol stresses but not MV treatment. GFP-fusion AtSFT12 protein was juxtaposed with Golgi marker, implying that its function is associated with Golgi-mediated transport. Quantitative measurement of Na(+) using induced coupled plasma atomic emission spectroscopy revealed that AtSFT12 OXs accumulated significantly more Na(+) than WT plants. In addition, Na(+)-dependent fluorescence analysis of Sodium Green showed comparatively higher Na(+) accumulation in vacuoles of AtSFT12 OX cells than in those of WT plant cells after salt treatments. Taken together, our findings suggest that AtSTF12, a Golgi Qc-SNARE protein, plays an important role in salt and osmotic stress responses and functions in the salt stress response via sequestration of Na(+) in vacuoles.

A VAMP-associated protein, PVA31 is involved in leaf senescence in Arabidopsis

VAMP-associated proteins (VAPs) are highly conserved among eukaryotes. Here, we report a functional analysis of one of the VAPs, PVA31, and demonstrate its novel function on leaf senescence in Arabidopsis. The expression of PVA31 is highly induced in senescence leaves, and localizes to the plasma membrane as well as the ARA7-positive endosomes. Yeast two-hybrid analysis demonstrates that PVA31 is interacted with the plasma membrane localized-VAMP proteins, VAMP721/722/724 but not with the endosome-localized VAMPs, VAMP711 and VAMP727, indicating that PVA31 is associated with VAMP721/722/724 on the plasma membrane. Strong constitutive expression of PVA31 under the control of the Cauliflower mosaic virus 35S promoter induces the typical symptom of leaf senescence earlier than WT in normal growth and an artificially induced senescence conditions.　In addition, the marker genes for the SA-mediated signaling pathways, PR-1, is promptly expressed with elicitor application. These data indicate that PVA31-overexpressing plants exhibit the early senescence phenotype in their leaves, and suggest that PVA31 is involved in the SA-mediated programmed cell death process during leaf senescence and PR-protein secretion during pathogen infection in Arabidopsis.

The complexity of vesicle transport factors in plants examined by orthology search

Vesicle transport is a central process to ensure protein and lipid distribution in eukaryotic cells. The current knowledge on the molecular components and mechanisms of this process is majorly based on studies in Saccharomyces cerevisiae and Arabidopsis thaliana, which revealed 240 different proteinaceous factors either experimentally proven or predicted to be involved in vesicle transport. In here, we performed an orthologue search using two different algorithms to identify the components of the secretory pathway in yeast and 14 plant genomes by using the 'core-set' of 240 factors as bait. We identified 4021 orthologues and (co-)orthologues in the discussed plant species accounting for components of COP-II, COP-I, Clathrin Coated Vesicles, Retromers and ESCRTs, Rab GTPases, Tethering factors and SNAREs. In plants, we observed a significantly higher number of (co-)orthologues than yeast, while only 8 tethering factors from yeast seem to be absent in the analyzed plant genomes. To link the identified (co-)orthologues to vesicle transport, the domain architecture of the proteins from yeast, genetic model plant A. thaliana and agriculturally relevant crop Solanum lycopersicum has been inspected. For the orthologous groups containing (co-)orthologues from yeast, A. thaliana and S. lycopersicum, we observed the same domain architecture for 79% (416/527) of the (co-)orthologues, which documents a very high conservation of this process. Further, publically available tissue-specific expression profiles for a subset of (co-)orthologues found in A. thaliana and S. lycopersicum suggest that some (co-)orthologues are involved in tissue-specific functions. Inspection of localization of the (co-)orthologues based on available proteome data or localization predictions lead to the assignment of plastid- as well as mitochondrial localized (co-)orthologues of vesicle transport factors and the relevance of this is discussed.

Arabidopsis Sec1/Munc18 protein SEC11 is a competitive and dynamic modulator of SNARE binding and SYP121-dependent vesicle traffic

The Arabidopsis thaliana Qa-SNARE SYP121 (=SYR1/PEN1) drives vesicle traffic at the plasma membrane of cells throughout the vegetative plant. It facilitates responses to drought, to the water stress hormone abscisic acid, and to pathogen attack, and it is essential for recovery from so-called programmed stomatal closure. How SYP121-mediated traffic is regulated is largely unknown, although it is thought to depend on formation of a fusion-competent SNARE core complex with the cognate partners VAMP721 and SNAP33. Like SYP121, the Arabidopsis Sec1/Munc18 protein SEC11 (=KEULE) is expressed throughout the vegetative plant. We find that SEC11 binds directly with SYP121 both in vitro and in vivo to affect secretory traffic. Binding occurs through two distinct modes, one requiring only SEC11 and SYP121 and the second dependent on assembly of a complex with VAMP721 and SNAP33. SEC11 competes dynamically for SYP121 binding with SNAP33 and VAMP721, and this competition is predicated by SEC11 association with the N terminus of SYP121. These and additional data are consistent with a model in which SYP121-mediated vesicle fusion is regulated by an unusual "handshaking" mechanism of concerted SEC11 debinding and rebinding. They also implicate one or more factors that alter or disrupt SEC11 association with the SYP121 N terminus as an early step initiating SNARE complex formation.

Overexpression of the Qc-SNARE gene OsSYP71 enhances tolerance to oxidative stress and resistance to rice blast in rice (Oryza sativa L.)

OsSYP71 is an oxidative stress and rice blast response gene that encodes a Qc-SNARE protein in rice. Qc-SNARE proteins belong to the superfamily of SNAREs (soluble N-ethylmaleimide-sensitive factor attachment protein receptors), which function as important components of the vesicle trafficking machinery in eukaryotic cells. In this paper, 12 Qc-SNARE genes were isolated from rice, and expression patterns of 9 genes were detected in various tissues and in seedlings challenged with oxidative stresses and inoculated with rice blast. The expression of OsSYP71 was clearly up-regulated under these stresses. Overexpression of OsSYP71 in rice showed more tolerance to oxidative stress and resistance to rice blast than wild-type plants. These results indicate that Qc-SNAREs play an important role in rice response to environmental stresses, and OsSYP71 is useful in engineering crop plants with enhanced tolerance to oxidative stress and resistance to rice blast.

The pollen-specific R-SNARE/longin PiVAMP726 mediates fusion of endo- and exocytic compartments in pollen tube tip growth

The growing pollen tube apex is dedicated to balancing exo- and endocytic processes to form a rapidly extending tube. As perturbation of either tends to cause a morphological phenotype, this system provides tractable model for studying these processes. Vesicle-associated membrane protein 7s (VAMP7s) are members of the soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) family that mediate cognate membrane fusion but their role in pollen tube growth has not been investigated. This manuscript identifies PiVAMP726 of Petunia inflata as a pollen-specific VAMP7 that localizes to the inverted cone of transport vesicles at the pollen tube tip. The endocytic marker FM4-64 was found to colocalize with yellow fluorescent protein (YFP)-PiVAMP726, which is consistent with PiVAMP726 containing an amino-acid motif implicated in endosomal localization, At high overexpression levels, YFP- PiVAMP726 inhibited growth and caused the formation of novel membrane compartments within the pollen tube tip. Functional dissection of PiVAMP726 implicated the N-terminal longin domain in negative regulation of the SNARE activity, but not localization of PiVAMP726. Expression of the constitutively active C-terminal SNARE domain alone, in pollen tubes, generated similar phenotypes to the full-length protein, but the truncated domain was more potent than the wild-type protein at both inhibiting growth and forming the novel membrane compartments. Both endo- and exocytic markers localized to these compartments in addition to YFP-PiVAMP726, leading to the speculation that PiVAMP726 might be involved in the recycling of endocytic vesicles in tip growth.

News and Views into the SNARE Complexity in Arabidopsis

Secretory organelles are engaged in a continuous flux of membranes, which is believed to occur mostly via transport vesicles. Being critical in maintaining several cellular functions, transport vesicles are membrane-enclosed sacs that temporarily store and then deliver membrane lipids, protein, and polysaccharides. SNAREs have a crucial role in vesicle traffic by driving membrane fusion and conferring fidelity through the formation of specific SNARE complexes. Additionally, specific roles of SNAREs in growth and development implicate that they are versatile components for the life of a plant. Here, we summarize the recent progress on the understanding of the role of SNAREs and highlight some of the questions that are still unsolved.

Arabidopsis R-SNARE proteins VAMP721 and VAMP722 are required for cell plate formation

Background

Cell plate formation during plant cytokinesis is facilitated by SNARE complex-mediated vesicle fusion at the cell-division plane. However, our knowledge regarding R-SNARE components of membrane fusion machinery for cell plate formation remains quite limited.

Methodology/Principal Findings

We report the in vivo function of Arabidopsis VAMP721 and VAMP722, two closely sequence-related R-SNAREs, in cell plate formation. Double homozygous vamp721vamp722 mutant seedlings showed lethal dwarf phenotypes and were characterized by rudimentary roots, cotyledons and hypocotyls. Furthermore, cell wall stubs and incomplete cytokinesis were frequently observed in vamp721vamp722 seedlings. Confocal images revealed that green fluorescent protein-tagged VAMP721 and VAMP722 were preferentially localized to the expanding cell plates in dividing cells. Drug treatments and co-localization analyses demonstrated that punctuate organelles labeled with VAMP721 and VAMP722 represented early endosomes overlapped with VHA-a1-labeled TGN, which were distinct from Golgi stacks and prevacuolar compartments. In addition, protein traffic to the plasma membrane, but not to the vacuole, was severely disrupted in vamp721vamp722 seedlings by subcellular localization of marker proteins.

Conclusion/Significance

These observations suggest that VAMP721 and VAMP722 are involved in secretory trafficking to the plasma membrane via TGN/early endosomal compartment, which contributes substantially to cell plate formation during plant cytokinesis.

Ion transport, membrane traffic and cellular volume control

Throughout their development, plants balance cell surface area and volume with ion transport and turgor. This balance lies at the core of cellular homeostatic networks and is central to the capacity to withstand abiotic as well as biotic stress. Remarkably, very little is known of its mechanics, notably how membrane traffic is coupled with osmotic solute transport and its control. Here we outline recent developments in the understanding of so-called SNARE proteins that form part of the machinery for membrane vesicle traffic in all eukaryotes. We focus on SNAREs active at the plasma membrane and the evidence for specialisation in enhanced, homeostatic and stress-related traffic. Recent studies have placed a canonical SNARE complex associated with the plasma membrane in pathogen defense, and the discovery of the first SNARE as a binding partner with ion channels has demonstrated a fundamental link to inorganic osmotic solute uptake. Work localising the channel binding site has now identified a new and previously uncharacterised motif, yielding important clues to a plausible mechanism coupling traffic and transport. We examine the evidence that this physical interaction serves to balance enhanced osmotic solute uptake with membrane expansion through mutual control of the two processes. We calculate that even during rapid cell expansion only a minute fraction of SNAREs present at the membrane need be engaged in vesicle traffic at any one time, a number surprisingly close to the known density of ion channels at the plant plasma membrane. Finally, we suggest a framework of alternative models coupling transport and traffic, and approachable through direct, experimental testing.

Involvement of the Arabidopsis HIT1/AtVPS53 tethering protein homologue in the acclimation of the plasma membrane to heat stress

Arabidopsis thaliana hit1-1 is a heat-intolerant mutant. The HIT1 gene encodes a protein that is homologous to yeast Vps53p, which is a subunit of the Golgi-associated retrograde protein (GARP) complex that is involved in retrograde membrane trafficking to the Golgi. To investigate the correlation between the cellular role of HIT1 and its protective function in heat tolerance in plants, it was verified that HIT1 was co-localized with AtVPS52 and AtVPS54, the other putative subunits of GARP, in the Golgi and post-Golgi compartments in Arabidopsis protoplasts. A bimolecular fluorescence complementation assay showed that HIT1 interacted with AtVPS52 and AtVPS54, which indicated their assembly into a protein complex in vivo. Under heat stress conditions, the plasma membrane of hit1-1 was less stable than that of the wild type, as determined by an electrolyte leakage assay, and enhanced leakage occurred before peroxidation injury to the membrane. In addition, the ability of hit1-1 to survive heat stress was not influenced by exposure to light, which suggested that the heat intolerance of hit-1 was a direct outcome of reduced membrane thermostability rather than heat-induced oxidative stress. Furthermore, hit1-1 was sensitive to the duration (sustained high temperature stress at 37 °C for 3 d) but not the intensity (heat shock at 44 °C for 30 min) of exposure to heat. Collectively, these results imply that HIT1 functions in the membrane trafficking that is involved in the thermal adaptation of the plasma membrane for tolerance to long-term heat stress in plants.

A molecular framework for coupling cellular volume and osmotic solute transport control

Eukaryotic cells expand using vesicle traffic to increase membrane surface area. Expansion in walled eukaryotes is driven by turgor pressure which depends fundamentally on the uptake and accumulation of inorganic ions. Thus, ion uptake and vesicle traffic must be controlled coordinately for growth. How this coordination is achieved is still poorly understood, yet is so elemental to life that resolving the underlying mechanisms will have profound implications for our understanding of cell proliferation, development, and pathogenesis, and will find applications in addressing the mineral and water use by plants in the face of global environmental change. Recent discoveries of interactions between trafficking and ion transport proteins now open the door to an entirely new approach to understanding this coordination. Some of the advances to date in identifying key protein partners in the model plant Arabidopsis and in yeast at membranes vital for cell volume and turgor control are outlined here. Additionally, new evidence is provided of a wider participation among Arabidopsis Kv-like K(+) channels in selective interaction with the vesicle-trafficking protein SYP121. These advances suggest some common paradigms that will help guide further exploration of the underlying connection between ion transport and membrane traffic and should transform our understanding of cellular homeostasis in eukaryotes.

Reduced expression of the v-SNAREs AtVAMP71/AtVAMP7C gene family in Arabidopsis reduces drought tolerance by suppression of abscisic acid-dependent stomatal closure

Stomatal closure during water stress is a major plant mechanism for reducing the loss of water through leaves. The opening and closure of stomata are mediated by endomembrane trafficking. The role of the vacuolar trafficking pathway, that involves v-SNAREs of the AtVAMP71 family (formerly called AtVAMP7C) in stomatal movements, was analysed. Expression of AtVAMP711-14 genes was manipulated in Arabidopsis plants with sense or antisense constructs by transformation of the AtVAMP711 gene. Antisense plants exhibited decreased stomatal closure during drought or after treatment with abscisic acid (ABA), resulting in the rapid loss of leaf water and tissue collapse. No improvement was seen in plants overexpressing the AtVAMP711 gene, suggesting that wild-type levels of AtVAMP711 expression are sufficient. ABA treatment induced the production of reactive oxygen species (ROS) in guard cells of both wild-type and antisense plants, indicating that correct hormone sensing is maintained. ROS were detected in nuclei, chloroplasts, and vacuoles. ABA treatment caused a significant increase in ROS-containing small vacuoles and also in plastids and nuclei of neighbouring epidermal and mesophyll cells. Taken together, our results show that VAMP71 proteins play an important role in the localization of ROS, and in the regulation of stomatal closure by ABA treatment. The paper also describes a novel aspect of ROS signalling in plants: that of ROS production in small vacuoles that are dispersed in the cytoplasm.

Characterization of the Arabidopsis thaliana exocyst complex gene families by phylogenetic, expression profiling, and subcellular localization studies

*The exocyst is a complex of eight proteins (Sec3p, Sec5p, Sec6p, Sec8p, Sec10p, Sec15p, Exo70p and Exo84p) involved in tethering vesicles to the plasma membrane during regulated or polarized secretion. Here, the plant exocyst complex was explored in phylogenetic, expression, and subcellular localization studies. *Evolutionary relationships of predicted exocyst subunits were examined in the complete genomes of Arabidopsis thaliana, Oryza sativa, Populus trichocarpa and Physcomitrella patens. Furthermore, detailed expression profiling of the A. thaliana microarray databases was performed and subcellular localization patterns were studied. *Several plant exocyst subunit genes appear to have undergone gene expansion in a common ancestor and subsequent duplication events in independent plant lineages. Expression profiling revealed that the A. thaliana Exo70 gene family exhibits dynamic expression patterns, while the remaining exocyst subunit genes displayed more static profiles. Subcellular localization patterns for A. thaliana exocyst subunits ranged from cytosolic to endosomal compartments (with enrichment in the early endosomes and the trans-Golgi network). Interestingly, two endosomal-localized AtExo70 proteins also recruited other exocyst subunits to these compartments. *Overall subcellular localization patterns were observed that were also found in yeast and animal cells, and this, coupled with the evolutionary relationships, suggests that the exocyst may perform similar conserved functions in plants.

A tripartite SNARE-K+ channel complex mediates in channel-dependent K+ nutrition in Arabidopsis

A few membrane vesicle trafficking (SNARE) proteins in plants are associated with signaling and transmembrane ion transport, including control of plasma membrane ion channels. Vesicle traffic contributes to the population of ion channels at the plasma membrane. Nonetheless, it is unclear whether these SNAREs also interact directly to affect channel gating and, if so, what functional impact this might have on the plant. Here, we report that the Arabidopsis thaliana SNARE SYP121 binds to KC1, a regulatory K(+) channel subunit that assembles with different inward-rectifying K(+) channels to affect their activities. We demonstrate that SYP121 interacts preferentially with KC1 over other Kv-like K(+) channel subunits and that KC1 interacts specifically with SYP121 but not with its closest structural and functional homolog SYP122 nor with another related SNARE SYP111. SYP121 promoted gating of the inward-rectifying K(+) channel AKT1 but only when heterologously coexpressed with KC1. Mutation in any one of the three genes, SYP121, KC1, and AKT1, selectively suppressed the inward-rectifying K(+) current in Arabidopsis root epidermal protoplasts as well as K(+) acquisition and growth in seedlings when channel-mediated K(+) uptake was limiting. That SYP121 should be important for gating of a K(+) channel and its role in inorganic mineral nutrition demonstrates an unexpected role for SNARE-ion channel interactions, apparently divorced from signaling and vesicle traffic. Instead, it suggests a role in regulating K(+) uptake coordinately with membrane expansion for cell growth.

The targeting of the oxysterol-binding protein ORP3a to the endoplasmic reticulum relies on the plant VAP33 homolog PVA12

In plants, sterols play fundamental roles as membrane constituents in the biosynthesis of steroid hormones, and act as precursors for cell wall deposition. Sterols are synthesized in the endoplasmic reticulum (ER), but mainly accumulate in the plasma membrane. How sterols are trafficked in plant cells is largely unknown. In non-plant systems, oxysterol-binding proteins have been involved in sterol trafficking and homeostasis. There are at least twelve homologs of oxysterol-binding proteins in the Arabidopsis genome, but the biology of these proteins remains for the most part obscure. Here, we report our analysis of the targeting requirements and the sterol-binding properties of a small Arabidopsis oxysterol-binding protein, ORP3a. We have determined that ORP3a is a bona fide sterol-binding protein with sitosterol-binding properties. Live-cell imaging analyses revealed that ORP3a is localized at the ER, and that binding to this organelle depends on a direct interaction with PVA12, a member of the largely uncharacterized VAP33 family of plant proteins. Molecular modeling analyses and site-directed mutagenesis led to the identification of a novel protein domain that is responsible for the PVA12-ORP3a interaction. Disruption of the integrity of this domain caused redistribution of ORP3a to the Golgi apparatus, suggesting that ORP3a may cycle between the ER and the Golgi. These results represent new insights into the biology of sterol-binding proteins in plant cells, and elucidate a hitherto unknown relationship between members of oxysterol-binding protein and VAP33 families of plant proteins in the early plant secretory pathway.

SNAREs--molecular governors in signalling and development

SNARE (soluble N-ethylmaleimide-sensitive factor protein attachment protein receptor) proteins drive membrane fusion and contribute to membrane and protein targeting and delivery in all eukaryotic cells. SNAREs are essential to the mechanics of cell growth and development, and they facilitate a number of homeostatic and evoked responses in plants, from hormone signalling to pathogen defence. Additionally, there is now unambiguous evidence that SNAREs play roles in anchoring other membrane proteins and in facilitating ion channel gating through direct, physical interaction with channel proteins. What is the physiological significance of these additional features of plant SNAREs? We explore possible interpretations and suggest functions as scaffolds for effective signal transmission between proteins and, by analogy with a mechanical device invented by James Watt, as molecular governors to coordinate solute transport with cell expansion and growth.

The secretory system of Arabidopsis

Over the past few years, a vast amount of research has illuminated the workings of the secretory system of eukaryotic cells. The bulk of this work has been focused on the yeast Saccharomyces cerevisiae, or on mammalian cells. At a superficial level, plants are typical eukaryotes with respect to the operation of the secretory system; however, important differences emerge in the function and appearance of endomembrane organelles. In particular, the plant secretory system has specialized in several ways to support the synthesis of many components of the complex cell wall, and specialized kinds of vacuole have taken on a protein storage role-a role that is intended to support the growing seedling, but has been co-opted to support human life in the seeds of many crop plants. In the past, most research on the plant secretory system has been guided by results in mammalian or fungal systems but recently plants have begun to stand on their own as models for understanding complex trafficking events within the eukaryotic endomembrane system.

The PRA1 gene family in Arabidopsis

Prenylated Rab acceptor 1 (PRA1) domain proteins are small transmembrane proteins that regulate vesicle trafficking as receptors of Rab GTPases and the vacuolar soluble N-ethylmaleimide-sensitive factor attachment receptor protein VAMP2. However, little is known about PRA1 family members in plants. Sequence analysis revealed that higher plants, compared with animals and primitive plants, possess an expanded family of PRA1 domain-containing proteins. The Arabidopsis (Arabidopsis thaliana) PRA1 (AtPRA1) proteins were found to homodimerize and heterodimerize in a manner corresponding to their phylogenetic distribution. Different AtPRA1 family members displayed distinct expression patterns, with a preference for vascular cells and expanding or developing tissues. AtPRA1 genes were significantly coexpressed with Rab GTPases and genes encoding vesicle transport proteins, suggesting an involvement in the vesicle trafficking process similar to that of their animal counterparts. Correspondingly, AtPRA1 proteins were localized in the endoplasmic reticulum, Golgi apparatus, and endosomes/prevacuolar compartments, hinting at a function in both secretory and endocytic intracellular trafficking pathways. Taken together, our data reveal a high functional diversity of AtPRA1 proteins, probably dealing with the various demands of the complex trafficking system.

Functional interaction of the SNARE protein NtSyp121 in Ca2+ channel gating, Ca2+ transients and ABA signalling of stomatal guard cells

There is now growing evidence that membrane vesicle trafficking proteins, especially of the superfamily of SNAREs, are critical for cellular signalling in plants. Work from this laboratory first demonstrated that a soluble, inhibitory (dominant-negative) fragment of the SNARE NtSyp121 blocked K+ and Cl- channel responses to the stress-related hormone abscisic acid (ABA), but left open a question about functional impacts on signal intermediates, especially on Ca2+-mediated signalling events. Here, we report one mode of action for the SNARE mediated directly through alterations in Ca2+ channel gating and its consequent effects on cytosolic-free [Ca2+] ([Ca2+]i) elevation. We find that expressing the same inhibitory fragment of NtSyp121 blocks ABA-evoked stomatal closure, but only partially suppresses stomatal closure in the presence of the NO donor, SNAP, which promotes [Ca2+]i elevation independently of the plasma membrane Ca2+ channels. Consistent with these observations, Ca2+ channel gating at the plasma membrane is altered by the SNARE fragment in a manner effective in reducing the potential for triggering a rise in [Ca2+]i, and we show directly that its expression in vivo leads to a pronounced suppression of evoked [Ca2+]i transients. These observations offer primary evidence for the functional coupling of the SNARE with Ca2+ channels at the plant cell plasma membrane and, because [Ca2+]i plays a key role in the control of K+ and Cl- channel currents in guard cells, they underscore an important mechanism for SNARE integration with ion channel regulation during stomatal closure.

The POK/AtVPS52 protein localizes to several distinct post-Golgi compartments in sporophytic and gametophytic cells

The organization and dynamics of the plant endomembrane system require both universal and plant-specific molecules and compartments. The latter, despite the growing wealth of information, remains poorly understood. From the study of an Arabidopsis thaliana male gametophytic mutant, it was possible to isolate a gene named POKY POLLEN TUBE (POK) essential for pollen tube tip growth. The similarity between the predicted POK protein sequence and yeast Vps52p, a subunit from the GARP/VFT complex which is involved in the docking of vesicles from the prevacuolar compartment to the Golgi apparatus, suggested that the POK protein plays a role in plant membrane trafficking. Genetic analysis of Arabidopsis mutants affecting AtVPS53 or AtVPS54 genes which encode putative POK partners shows a transmission defect through the male gametophyte for all lines, which is similar to the pok mutant. Using a combination of biochemical approaches and specific antiserum it has been demonstrated that the POK protein is present in phylogenetically divergent plant species, associated with membranes and belongs to a high molecular weight complex. Combination of immunolocalization studies and pharmacological approaches in different plant cells revealed that the POK protein associates with Golgi and post-Golgi compartments. The role of POK in post-Golgi endomembrane trafficking and as a member of a putative plant GARP/VFT complex is discussed.

The role of Rab GTPases in cell wall metabolism

The synthesis and modification of the cell wall must involve the production of new cell wall polymers and enzymes. Their targeted secretion to the apoplast is one of many potential control points. Since Rab GTPases have been strongly implicated in the regulation of vesicle trafficking, a review of their involvement in cell wall metabolism should throw light on this possibility. Cell wall polymer biosynthesis occurs mainly in the Golgi apparatus, except for cellulose and callose, which are made at the plasma membrane by an enzyme complex that cycles through the endomembrane system and which may be regulated by this cycling. Several systems, including the growth of root hairs and pollen tubes, cell wall softening in fruit, and the development of root nodules, are now being dissected. In these systems, secretion of wall polymers and modifying enzymes has been documented, and Rab GTPases are highly expressed. Reverse genetic experiments have been used to interfere with these GTPases and this is revealing their importance in regulation of trafficking to the wall. The role of the RabA (or Rab11) GTPases is particularly exciting in this respect.

SNARE-Ware: The Role of SNARE-Domain Proteins in Plant Biology

In yeast and animal cells, members of the superfamily of N-ethylmaleimide-sensitive factor adaptor protein receptor (SNARE)-domain-containing proteins are key players in vesicle-associated membrane fusion events during transport processes between individual compartments of the endomembrane system, including exocytosis and endocytosis. Compared with genomes of other eukaryotes, genomes of monocotyledonous and dicotyledonous plants encode a surprisingly high number of SNARE proteins, suggesting vital roles for this protein class in higher plant species. Although to date it remains elusive whether plant SNARE proteins function like their yeast and animal counterparts, genetic screens have recently begun to unravel the variety of biological tasks in which plant SNAREs are involved. These duties involve fundamental processes such as cytokinesis, shoot gravitropism, pathogen defense, symbiosis, and abiotic stress responses, suggesting that SNAREs contribute essentially to many facets of plant biology.

Split luciferase complementation assay to study protein-protein interactions in Arabidopsis protoplasts

We developed a split luciferase complementation assay to study protein-protein interactions in Arabidopsis protoplasts. In this assay, the N- and C-terminal fragments of Renilla reniforms luciferase are translationally fused to bait and prey proteins, respectively. When the proteins interact, split luciferase becomes activated and emits luminescence that can be measured by a microplate luminometer. Split luciferase activity was measured by first transforming protoplasts with a DNA vector in a 96-well plate. DNA vector expressing both bait and prey genes was constructed through two independent in vitro DNA recombinant reactions, Gateway and Cre-loxP. As proof of concept, we detected the protein-protein interactions between the nuclear histones 2A and 2B, as well as between membrane proteins SYP (syntaxin of plant) 51 and SYP61, in Arabidopsis protoplasts.

An elaborate classification of SNARE proteins sheds light on the conservation of the eukaryotic endomembrane system

Proteins of the SNARE (soluble N-ethylmalemide-sensitive factor attachment protein receptor) family are essential for the fusion of transport vesicles with an acceptor membrane. Despite considerable sequence divergence, their mechanism of action is conserved: heterologous sets assemble into membrane-bridging SNARE complexes, in effect driving membrane fusion. Within the cell, distinct functional SNARE units are involved in different trafficking steps. These functional units are conserved across species and probably reflect the conservation of the particular transport step. Here, we have systematically analyzed SNARE sequences from 145 different species and have established a highly accurate classification for all SNARE proteins. Principally, all SNAREs split into four basic types, reflecting their position in the four-helix bundle complex. Among these four basic types, we established 20 SNARE subclasses that probably represent the original repertoire of a eukaryotic cenancestor. This repertoire has been modulated independently in different lines of organisms. Our data are in line with the notion that the ur-eukaryotic cell was already equipped with the various compartments found in contemporary cells. Possibly, the development of these compartments is closely intertwined with episodes of duplication and divergence of a prototypic SNARE unit.

Selective targeting of plasma membrane and tonoplast traffic by inhibitory (dominant-negative) SNARE fragments

Vesicle traffic underpins cell homeostasis, growth and development in plants, and is facilitated by a superfamily of proteins known as SNAREs [soluble NSF (N-ethylmaleimide-sensitive factor) attachment protein receptors] that interact to draw vesicle and target membrane surfaces together for fusion. Structural homologies, biochemical and genetic analyses have yielded information about the localization and possible roles of these proteins. However, remarkably little evidence is yet available that speaks directly to the functional specificities of these proteins in selected trafficking pathways in vivo. Previously, we found that expressing a cytosolic (so-called Sp2) fragment of one plasma membrane SNARE from tobacco and Arabidopsis had severe effects on growth, tissue development and secretory traffic to the plasma membrane. We have explored this dominant-negative approach further to examine the specificity and overlaps in Sp2 activity by generating a toolbox of truncated SNARE constructs and antibodies for transient expression and analysis. Using a quantitative ratiometric approach with secreted green fluorescent protein (secGFP), we report here that traffic to the plasma membrane is suppressed selectively by Sp2 fragments of plasma membrane SNAREs AtSYP121 and AtSYP122, but not of the closely related SNARE AtSYP111 nor of the SNARE AtSYP21 that resides at the pre-vacuolar compartment (PVC). By contrast, traffic of the YFP-tagged aquaporin fusion protein TIP1;1-YFP to the tonoplast was blocked (leading to its accumulation in the PVC) when co-expressed with the Sp2 fragment of AtSYP21, but not when co-expressed with that of AtSYP121. Export of secGFP was also sensitive to the Sp2 fragment of the novel, plant-specific SNARE AtSYP71 that was recently found to be present in detergent-resistant, plasma membrane fractions. Co-incubation analyses of the plasma membrane SNAREs with the regulatory subdomain included within the Sp2 fragments showed activity in destabilizing protein complexes, but only with the complementary SNAREs. We conclude that the Sp2 fragment action accurately reflects the known specificity and targeting of these SNAREs, implies functional overlaps that are of potential physiological interest, and underscores the use of a dominant-negative strategy in functional studies of a major subfamily of SNAREs in plants.

Abscisic Acid Triggers the Endocytosis of the Arabidopsis KAT1 K+ Channel and Its Recycling to the Plasma Membrane

Membrane vesicle traffic to and from the plasma membrane is essential for cellular homeostasis in all eukaryotes. In plants, constitutive traffic to and from the plasma membrane has been implicated in maintaining the population of integral plasma-membrane proteins and its adjustment to a variety of hormonal and environmental stimuli. However, direct evidence for evoked and selective traffic has been lacking. Here, we report that the hormone abscisic acid (ABA), which controls ion transport and transpiration in plants under water stress, triggers the selective endocytosis of the KAT1 K+ channel protein in epidermal and guard cells. Endocytosis of the K+ channel from the plasma membrane initiates in concert with changes in K+ channel activities evoked by ABA and leads to sequestration of the K+ channel within an endosomal membrane pool that recycles back to the plasma membrane over a period of hours. Selective K+ channel endocytosis, sequestration, and recycling demonstrates a tight and dynamic control of the population of K+ channels at the plasma membrane as part of a key plant signaling and response mechanism, and the observations point to a role for channel traffic in adaptive changes in the capacity for osmotic solute flux of stomatal guard cells.

Increases in the number of SNARE genes parallels the rise of multicellularity among the green plants

The green plant lineage is the second major multicellular expansion among the eukaryotes, arising from unicellular ancestors to produce the incredible diversity of morphologies and habitats observed today. In the unicellular ancestors, secretion of material through the endomembrane system was the major mechanism for interacting and shaping the external environment. In a multicellular organism, the external environment can be made of other cells, some of which may have vastly different developmental fates, or be part of different tissues or organs. In this context, a given cell must find ways to organize its secretory pathway at a level beyond that of the unicellular ancestor. Recently, sequence information from many green plants have become available, allowing an examination of the genomes for the machinery involved in the secretory pathway. In this work, the SNARE proteins of several green plants have been identified. While little increase in gene number was seen in the SNAREs of the early secretory system, many new SNARE genes and gene families have appeared in the multicellular green plants with respect to the unicellular plants, suggesting that this increase in the number of SNARE genes may have some relation to the rise of multicellularity in green plants.