Interactions between syntaxins identify at least five SNARE complexes within the Golgi/prevacuolar system of the Arabidopsis cell

ZIP genes encode proteins involved in membrane trafficking of the TGN-PVC/vacuoles

The Arabidopsis zigzgag (zig) is a loss-of-function mutant of Qb-SNARE VTI11 which is involved in vesicle trafficking between the trans-Golgi network (TGN) and vacuoles. zig-1 exhibits abnormality in both shoot gravitropism and morphology. To elucidate the molecular network of the post-Golgi membrane trafficking in plant cells, we have isolated the suppressor mutants of zig. Here we report zig suppressor 2 (zip2) and zip4 that are recessive mutants and partially suppress abnormality in both gravitropism and morphology. ZIP2 encodes AtVPS41/AtVAM2 protein that is thought to be an Arabidopsis ortholog of yeast Vps41p/Vam2p, which is involved in protein sorting to vacuoles as a subunit of the tethering complex HOPS. Yeast Vps41p is also proposed to function in budding of adaptor protein (AP)-3-coated vesicles from the Golgi. The zip2 mutation is a missense mutation in a conserved amino acid of a putative clathrin heavy chain repeat (CHCR) domain. AtVPS41 is a single-copy gene in the Arabidopsis genome and the T-DNA insertion mutant appears to be lethal, whereas the zip2 single mutant showed no obvious phenotype. On the other hand, zip4 is a loss-of-function mutant of a putative ortholog of the yeast AP-3 mu subunit. In addition, loss-of-function mutants of other subunits of AP-3, ap-3beta and ap-3delta, also exhibit a suppressive effect on the zig-1 phenotype. Although these genes are also single-copy genes in the genome, the loss-of-function mutants of AP-3 grow normally. Our results suggest that AtVPS41 and AP-3 play roles in the proper function of the post-Golgi trafficking network and support membrane trafficking to vacuoles.

Regulation of membrane trafficking and organ separation by the NEVERSHED ARF-GAP protein

Cell separation, or abscission, is a highly specialized process in plants that facilitates remodeling of their architecture and reproductive success. Because few genes are known to be essential for organ abscission, we conducted a screen for mutations that alter floral organ shedding in Arabidopsis. Nine recessive mutations that block shedding were found to disrupt the function of an ADP-ribosylation factor-GTPase-activating protein (ARF-GAP) we have named NEVERSHED (NEV). As predicted by its homology to the yeast Age2 ARF-GAP and transcriptional profile, NEV influences other aspects of plant development, including fruit growth. Co-localization experiments carried out with NEV-specific antiserum and a set of plant endomembrane markers revealed that NEV localizes to the trans-Golgi network and endosomes in Arabidopsis root epidermal cells. Interestingly, transmission electron micrographs of abscission zone regions from wild-type and nev flowers reveal defects in the structure of the Golgi apparatus and extensive accumulation of vesicles adjacent to the cell walls. Our results suggest that NEV ARF-GAP activity at the trans-Golgi network and distinct endosomal compartments is required for the proper trafficking of cargo molecules required for cell separation.

Medicago N2-fixing symbiosomes acquire the endocytic identity marker Rab7 but delay the acquisition of vacuolar identity

Rhizobium bacteria form N(2)-fixing organelles, called symbiosomes, inside the cells of legume root nodules. The bacteria are generally thought to enter the cells via an endocytosis-like process. To examine this, we studied the identity of symbiosomes in relation to the endocytic pathway. We show that in Medicago truncatula, the small GTPases Rab5 and Rab7 are endosomal membrane identity markers, marking different (partly overlapping) endosome populations. Although symbiosome formation is considered to be an endocytosis-like process, symbiosomes do not acquire Rab5 at any stage during their development, nor do they accept the trans-Golgi network identity marker SYP4, presumed to mark early endosomes in plants. By contrast, the endosomal marker Rab7 does occur on symbiosomes from an early stage of development when they have stopped dividing up to the senescence stage. However, the symbiosomes do not acquire vacuolar SNAREs (SYP22 and VTI11) until the onset of their senescence. By contrast, symbiosomes acquire the plasma membrane SNARE SYP132 from the start of symbiosome formation throughout their development. Therefore, symbiosomes appear to be locked in a unique SYP132- and Rab7-positive endosome stage and the delay in acquiring (lytic) vacuolar identity (e.g., vacuolar SNAREs) most likely ensures their survival and maintenance as individual units.

Cellular pathways regulating responses to compatible and self-incompatible pollen in Brassica and Arabidopsis stigmas intersect at Exo70A1, a putative component of the exocyst complex

In the Brassicaceae, compatible pollen-pistil interactions result in pollen adhesion to the stigma, while pollen grains from unrelated plant species are largely ignored. There can also be an additional layer of recognition to prevent self-fertilization, the self-incompatibility response, whereby self pollen grains are distinguished from nonself pollen grains and rejected. This pathway is activated in the stigma and involves the ARM repeat-containing 1 (ARC1) protein, an E3 ubiquitin ligase. In a screen for ARC1-interacting proteins, we have identified Brassica napus Exo70A1, a putative component of the exocyst complex that is known to regulate polarized secretion. We show through transgenic studies that loss of Exo70A1 in Brassica and Arabidopsis thaliana stigmas leads to the rejection of compatible pollen at the same stage as the self-incompatibility response. A red fluorescent protein:Exo70A1 fusion rescues this stigmatic defect in Arabidopsis and is found to be mobilized to the plasma membrane concomitant with flowers opening. By contrast, increased expression of Exo70A1 in self-incompatible Brassica partially overcomes the self pollen rejection response. Thus, our data show that the Exo70A1 protein functions at the intersection of two cellular pathways, where it is required in the stigma for the acceptance of compatible pollen in both Brassica and Arabidopsis and is negatively regulated by Brassica self-incompatibility.

Rapid, combinatorial analysis of membrane compartments in intact plants with a multicolor marker set

Plant membrane compartments and trafficking pathways are highly complex, and are often distinct from those of animals and fungi. Progress has been made in defining trafficking in plants using transient expression systems. However, many processes require a precise understanding of plant membrane trafficking in a developmental context, and in diverse, specialized cell types. These include defense responses to pathogens, regulation of transporter accumulation in plant nutrition or polar auxin transport in development. In all of these cases a central role is played by the endosomal membrane system, which, however, is the most divergent and ill-defined aspect of plant cell compartmentation. We have designed a new vector series, and have generated a large number of stably transformed plants expressing membrane protein fusions to spectrally distinct, fluorescent tags. We selected lines with distinct subcellular localization patterns, and stable, non-toxic expression. We demonstrate the power of this multicolor 'Wave' marker set for rapid, combinatorial analysis of plant cell membrane compartments, both in live-imaging and immunoelectron microscopy. Among other findings, our systematic co-localization analysis revealed that a class of plant Rab1-homologs has a much more extended localization than was previously assumed, and also localizes to trans-Golgi/endosomal compartments. Constructs that can be transformed into any genetic background or species, as well as seeds from transgenic Arabidopsis plants, will be freely available, and will promote rapid progress in diverse areas of plant cell biology.

Endocytosis and endosomal regulation of the S-receptor kinase during the self-incompatibility response in Brassica oleracea

Intracellular trafficking of plant receptor kinases (PRKs) is a key step in regulation of cellular signaling. Our current knowledge in this field is based on systems that address signaling pathways affecting the whole cell. There are, however, signaling phenomena that add a further layer of complexity. In the Brassica self-incompatibility response, a single cell can adequately respond to two opposite stimuli: accepting cross-pollen and rejecting self-pollen simultaneously. To understand how PRK signaling can influence the coexistence of two seemingly exclusive states of the cell, we investigated the subcellular localization and internalization of the S-receptor kinase (SRK) involved in the self-incompatibility response of Brassica oleracea. Here, we describe the unusual subcellular distribution of SRK3, which localizes predominantly to intracellular compartments and to a much lesser extent to the plasma membrane. Using an anti-SRK antibody that fully substitutes for the natural ligand, we demonstrate that the interaction with the receptor takes place at the plasma membrane and is followed by SRK internalization in endosomes that are enriched in the SRK negative regulator Thioredoxin-h-like1.

Post-Golgi traffic in plants

Secretory and endocytic traffic through the post-Golgi endomembrane system regulates the abundance of plasma-membrane proteins such as receptors, transporters and ion channels, modulating the ability of a cell to communicate with its neighbours and to adapt to a changing environment. The major post-Golgi compartments are numerous and appear to be similar to their counterparts in animals. However, endosomes are rather ill defined morphologically but seem to be involved in specific trafficking pathways. Many plasma-membrane proteins cycle constitutively via endosomal compartments. The trans-Golgi network (TGN) appears to be an early endosome where secretory and endocytic traffic meet. Endocytosed proteins that are to be degraded are targeted to the vacuole via the multivesiculate prevacuolar compartment (PVC) whereas cycling proteins pass through recycling endosomes. The trafficking machinery involves the same classes of proteins as in other eukaryotes. However, there are modifications that match the specifics of post-Golgi traffic in plants. Although plants lack epithelia, some plasma-membrane proteins are located on specific faces of the cell which reflects polarized traffic and influences the physiological performance of the tissue. Plants also differentiate highly polarized tip-growing cells in which post-Golgi traffic is adapted to very high rates of targeted exocytosis, endocytosis and recycling.

AtVPS45 is a positive regulator of the SYP41/SYP61/VTI12 SNARE complex involved in trafficking of vacuolar cargo

We report a functional characterization of AtVPS45 (for vacuolar protein sorting 45), a protein from the Sec1/Munc18 family in Arabidopsis (Arabidopsis thaliana) that interacts at the trans-Golgi network (TGN) with the SYP41/SYP61/VTI12 SNARE complex. A null allele of AtVPS45 was male gametophytic lethal, whereas stable RNA interference lines with reduced AtVPS45 protein levels had stunted growth but were viable and fertile. In the silenced lines, we observed defects in vacuole formation that correlated with a reduction in cell expansion and with autophagy-related defects in nutrient turnover. Moreover, transport of vacuolar cargo with carboxy-terminal vacuolar sorting determinants was blocked in the silenced lines, suggesting that AtVPS45 functions in vesicle trafficking to the vacuole. These trafficking defects are similar to those observed in vti12 mutants, supporting a functional relationship between AtVPS45 and VTI12. Consistent with this, we found a decrease in SYP41 protein levels coupled to the silencing of AtVPS45, pointing to instability and malfunction of the SYP41/SYP61/VTI12 SNARE complex in the absence of its cognate Sec1/Munc18 regulator. Based on its localization on the TGN, we hypothesized that AtVPS45 could be involved in membrane fusion of retrograde vesicles recycling vacuolar trafficking machinery. Indeed, in the AtVPS45-silenced plants, we found a striking alteration in the subcellular fractionation pattern of vacuolar sorting receptors, which are required for sorting of carboxy-terminal vacuolar sorting determinant-containing cargo. We propose that AtVPS45 is essential for recycling of the vacuolar sorting receptors back to the TGN and that blocking this step underlies the defects in vacuolar cargo trafficking observed in the silenced lines.

Chemical dissection of endosomal pathways

Membrane trafficking and associated signal transduction pathways are critical for plant development and responses to environment. These transduction pathways, including those for brassinosteroids and auxins, require endocytosis to endosomes and recycling back to the plasma membrane. A major challenge toward understanding these processes and their biological roles has been the highly dynamic nature of endomembrane trafficking. To effectively study endocytosis and recycling, which occur in a time frame of minutes, bioactive chemicals provide a powerful and exacting tool. Pharmacological inhibitors such as Brefeldin A (BFA) and the newly identified Endosidin 1 (ES1) have been used to define endosome compartments. ES1 is a clear example of the ability of chemicals to dissect even distinct subpopulations of endosomes involved in trafficking and signal transduction. The ability to characterize and dissect such highly dynamic pathways in a temporal and spatial manner is possible only using pharmacological reagents which can act rapidly and reversibly.

Identification of new up-regulated genes under drought stress in soybean nodules

Legumes/rhizobium biological N(2) fixation (BNF) is dramatically affected under abiotic stress such as drought, salt, cold and heavy metal stresses. Nodule response to drought stress at the molecular level was analysed using soybean (Glycine max) and Bradyrhizobium japonicum as a model, since this symbiotic partnership is extremely sensitive to this stress. To gain insight into molecular mechanisms involved in drought-induced BNF inhibition, we have constructed a SSH (Suppression Subtractive Hybridisation) cDNA library from nodular tissue of plants irrigated at field capacity or plants water deprived for 5 days. Sequence analysis of the first set of 128 non redundant ESTs using protein databases and the Blastx program indicated that 70% of ESTs could be classified into putative known functions. Using reverse northern hybridization, 56 ESTs were validated as up-regulated genes in response to drought. Interestingly, only a few of them had been previously described as involved in plant response to drought, therefore most of the ESTs could be considered as new markers of drought stress. Here we discuss the potential role of some of these up-regulated genes in response to drought. Our analysis focused on two genes, encoding respectively a ferritin and a metallothionein, which are known to be involved in homeostasis and detoxification of metals and in response to oxidative stress. Their spatiotemporal expression patterns showed a high accumulation of transcripts restricted to infected cells of nodules in response to drought.

The functions of Rab GTPases in plant membrane traffic

Rab GTPases are important determinants of membrane identity and membrane targeting. Higher plants have evolved a unique set of Rab GTPases that presumably reflects the specific demands of plant cell trafficking. In recent years, significant progress has been made in identifying Rab GTPases involved in endosome organisation, cytokinesis and in post-Golgi traffic to the plasma membrane and vacuoles. These include members of the Rab-F1, Rab-F2, Rab-A1, Rab-A2 and Rab-A4 subclasses. Some important regulators or effectors have also been identified for Rab-F, Rab-A1 and Rab-A4 proteins. However, uncertainties remain about the trafficking pathways that connect the compartments in the trans-Golgi/prevacuolar/endosomal system and there is still little or no insight into the functions of several major subclasses within the Rab GTPase family.

A SNARE complex unique to seed plants is required for protein storage vacuole biogenesis and seed development of Arabidopsis thaliana

The SNARE complex is a key regulator of vesicular traffic, executing membrane fusion between transport vesicles or organelles and target membranes. A functional SNARE complex consists of four coiled-coil helical bundles, three of which are supplied by Q-SNAREs and another from an R-SNARE. Arabidopsis thaliana VAMP727 is an R-SNARE, with homologs only in seed plants. We have found that VAMP727 colocalizes with SYP22/ VAM3, a Q-SNARE, on a subpopulation of prevacuolar compartments/endosomes closely associated with the vacuolar membrane. Genetic and biochemical analyses, including examination of a synergistic interaction of vamp727 and syp22 mutations, histological examination of protein localization, and coimmunoprecipitation from Arabidopsis lysates indicate that VAMP727 forms a complex with SYP22, VTI11, and SYP51 and that this complex plays a crucial role in vacuolar transport, seed maturation, and vacuole biogenesis. We suggest that the VAMP727 complex mediates the membrane fusion between the prevacuolar compartment and the vacuole and that this process has evolved as an essential step for seed development.

An inositolphosphorylceramide synthase is involved in regulation of plant programmed cell death associated with defense in Arabidopsis

The Arabidopsis thaliana resistance gene RPW8 triggers the hypersensitive response (HR) to restrict powdery mildew infection via the salicylic acid-dependent signaling pathway. To further understand how RPW8 signaling is regulated, we have conducted a genetic screen to identify mutations enhancing RPW8-mediated HR-like cell death (designated erh). Here, we report the isolation and characterization of the Arabidopsis erh1 mutant, in which the At2g37940 locus is knocked out by a T-DNA insertion. Loss of function of ERH1 results in salicylic acid accumulation, enhanced transcription of RPW8 and RPW8-dependent spontaneous HR-like cell death in leaf tissues, and reduction in plant stature. Sequence analysis suggests that ERH1 may encode the long-sought Arabidopsis functional homolog of yeast and protozoan inositolphosphorylceramide synthase (IPCS), which converts ceramide to inositolphosphorylceramide. Indeed, ERH1 is able to rescue the yeast aur1 mutant, which lacks the IPCS, and the erh1 mutant plants display reduced ( approximately 53% of wild type) levels of leaf IPCS activity, indicating that ERH1 encodes a plant IPCS. Consistent with its biochemical function, the erh1 mutation causes ceramide accumulation in plants expressing RPW8. These data reinforce the concept that sphingolipid metabolism (specifically, ceramide accumulation) plays an important role in modulating plant programmed cell death associated with defense.

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.

Evolution of specificity in the eukaryotic endomembrane system

Two hundred years after Darwin's birth, our understanding of genetic mechanisms and cell biology has advanced to a level unimaginable in the 19th century. We now know that eukaryotic cells contain a huge variety of internal compartments, each with their own function, identity and history. For the compartments that together form the membrane-trafficking system, one of the central questions is how that identity is encoded and how it evolved. Here we review the key components involved in membrane-trafficking events, including SNAREs, Rabs, vesicle coats, and tethers and what is known about their evolutionary history. Our current understanding suggests a possible common mechanism by which the membrane-trafficking organelles might have evolved. This model of increased organellar complexity by gene duplication and co-evolution of multiple, interacting, specificity-encoding proteins could well be applicable to other non-endosymbiotic organelles as well. The application of basic evolutionary principles well beyond their original scope has been exceedingly powerful not only in reconstructing the history of cellular compartments, but for medical and applied research as well, and underlines the contributions of Darwin's ideas in modern biology.

Endosidin1 defines a compartment involved in endocytosis of the brassinosteroid receptor BRI1 and the auxin transporters PIN2 and AUX1

Although it is known that proteins are delivered to and recycled from the plasma membrane (PM) via endosomes, the nature of the compartments and pathways responsible for cargo and vesicle sorting and cellular signaling is poorly understood. To define and dissect specific recycling pathways, chemical effectors of proteins involved in vesicle trafficking, especially through endosomes, would be invaluable. Thus, we identified chemicals affecting essential steps in PM/endosome trafficking, using the intensely localized PM transport at the tips of germinating pollen tubes. The basic mechanisms of this localized growth are likely similar to those of non-tip growing cells in seedlings. The compound endosidin 1 (ES1) interfered selectively with endocytosis in seedlings, providing a unique tool to dissect recycling pathways. ES1 treatment induced the rapid agglomeration of the auxin translocators PIN2 and AUX1 and the brassinosteroid receptor BRI1 into distinct endomembrane compartments termed "endosidin bodies"; however, the markers PIN1, PIN7, and other PM proteins were unaffected. Endosidin bodies were defined by the syntaxin SYP61 and the V-ATPase subunit VHA-a1, two trans-Golgi network (TGN)/endosomal proteins. Interestingly, brassinosteroid (BR)-induced gene expression was inhibited by ES1 and treated seedlings displayed a brassinolide (BL)-insensitive phenotype similar to a bri1 loss-of-function mutant. No effect was detected in auxin signaling. Thus, PIN2, AUX1, and BRI1 use interactive pathways involving an early SYP61/VHA-a1 endosomal compartment.

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.

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.

Post-Golgi protein traffic in the plant secretory pathway

The Golgi apparatus in plants is organized as a multitude of individual stacks that are motile in the cytoplasm and in close association with the endoplasmic reticulum (ER) (Boevink et al. in Plant J 15:441-447, 1998). These stacks operate as a sorting centre for cargo molecules, providing modification and redirection to other organelles as appropriate. In the post-Golgi direction, these include vacuole and plasma membrane, and specialized transport routes to each are required to prevent mislocalization. Recent evidence in plant cells points to the existence of post-Golgi organelles that function as intermediate stations for efficient protein traffic, as well as to the influence of small GTPases such as Rabs and ARFs on post-Golgi trafficking. This review focuses on the latest findings on post-Golgi trafficking routes and on the involvement of GTPases and their effectors on the trafficking of proteins in the plant secretory pathway.

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.

Molecular Identification of 26 Syntaxin Genes and their Assignment to the Different Trafficking Pathways in Paramecium

SNARE proteins have been classified as vesicular (v)- and target (t)-SNAREs and play a central role in the various membrane interactions in eukaryotic cells. Based on the Paramecium genome project, we have identified a multigene family of at least 26 members encoding the t-SNARE syntaxin (PtSyx) that can be grouped into 15 subfamilies. Paramecium syntaxins match the classical build-up of syntaxins, being 'tail-anchored' membrane proteins with an N-terminal cytoplasmic domain and a membrane-bound single C-terminal hydrophobic domain. The membrane anchor is preceded by a conserved SNARE domain of approximately 60 amino acids that is supposed to participate in SNARE complex assembly. In a phylogenetic analysis, most of the Paramecium syntaxin genes were found to cluster in groups together with those from other organisms in a pathway-specific manner, allowing an assignment to different compartments in a homology-dependent way. However, some of them seem to have no counterparts in metazoans. In another approach, we fused one representative member of each of the syntaxin isoforms to green fluorescent protein and assessed the in vivo localization, which was further supported by immunolocalization of some syntaxins. This allowed us to assign syntaxins to all important trafficking pathways in Paramecium.

Divergent functions of VTI12 and VTI11 in trafficking to storage and lytic vacuoles in Arabidopsis

The protein storage vacuole (PSV) is a plant-specific organelle that accumulates reserve proteins, one of the main agricultural products obtained from crops. Despite the importance of this process, the cellular machinery required for transport and accumulation of storage proteins remains largely unknown. Interfering with transport to PSVs has been shown to result in secretion of cargo. Therefore, secretion of a suitable marker could be used as an assay to identify mutants in this pathway. CLV3, a negative regulator of shoot stem cell proliferation, is an extracellular ligand that is rendered inactive when targeted to vacuoles. We devised an assay where trafficking mutants secrete engineered vacuolar CLV3 and show reduced meristems, a phenotype easily detected by visual inspection of plants. We tested this scheme in plants expressing VAC2, a fusion of CLV3 to the vacuolar sorting signal from the storage protein barley lectin. In this way, we determined that trafficking of VAC2 requires the SNARE VTI12 but not its close homologue, the conditionally redundant VTI11 protein. Furthermore, a vti12 mutant is specifically altered in transport of storage proteins, whereas a vti11 mutant is affected in transport of a lytic vacuole marker. These results demonstrate the specialization of VTI12 and VTI11 in mediating trafficking to storage and lytic vacuoles, respectively. Moreover, they validate the VAC2 secretion assay as a simple method to isolate genes that mediate trafficking to the PSV.

Overexpression of the Arabidopsis syntaxin PEP12/SYP21 inhibits transport from the prevacuolar compartment to the lytic vacuole in vivo

Golgi-mediated transport to the lytic vacuole involves passage through the prevacuolar compartment (PVC), but little is known about how vacuolar proteins exit the PVC. We show that this last step is inhibited by overexpression of Arabidopsis thaliana syntaxin PEP12/SYP21, causing an accumulation of soluble and membrane cargo and the plant vacuolar sorting receptor BP80 in the PVC. Anterograde transport proceeds normally from the endoplasmic reticulum to the Golgi and the PVC, although export from the PVC appears to be compromised, affecting both anterograde membrane flow to the vacuole and the recycling route of BP80 to the Golgi. However, Golgi-mediated transport of soluble and membrane cargo toward the plasma membrane is not affected, but a soluble BP80 ligand is partially mis-sorted to the culture medium. We also observe clustering of individual PVC bodies that move together and possibly fuse with each other, forming enlarged compartments. We conclude that PEP12/SYP21 overexpression specifically inhibits export from the PVC without affecting the Golgi complex or compromising the secretory branch of the endomembrane system. The results provide a functional in vivo assay that confirms PEP12/SYP21 involvement in vacuolar sorting and indicates that excess of this syntaxin in the PVC can be detrimental for further transport from this organelle.

Setting SNAREs in a different wood

Vesicle traffic is essential for cell homeostasis, growth and development in plants, as it is in other eukaryotes, and is facilitated by a superfamily of proteins known as soluble N-ethylmaleimide-sensitive fusion protein attachment protein receptors (SNAREs). Although SNAREs are well-conserved across phylla, genomic analysis for two model angiosperm species available to date, rice and Arabidopsis, highlights common patterns of divergence from other eukaryotes. These patterns are associated with the expansion of some gene subfamilies of SNAREs, the absence of others and the appearance of new proteins that show no significant homologies to SNAREs of mammals, yeast or Drosophila. Recent findings indicate that the functions of these plant SNAREs also extend beyond the conventional 'housekeeping' activities associated with vesicle trafficking. A number of SNAREs have been implicated in environmental responses as diverse as stomata movements and gravisensing as well as sensitivity to salt and drought. These proteins are essential for signal transduction and response and, in most cases, appear also to maintain additional roles in membrane trafficking. One common theme to this added functionality lies in control of non-SNARE proteins, notably ion channels. Other examples include interactions between the SNAREs and scaffolding or other structural components within the plant cell.

Comparative EST analysis provides insights into the basal aquatic fungus Blastocladiella emersonii

Background

Blastocladiella emersonii is an aquatic fungus of the Chytridiomycete class, which is at the base of the fungal phylogenetic tree. In this sense, some ancestral characteristics of fungi and animals or fungi and plants could have been retained in this aquatic fungus and lost in members of late-diverging fungal species. To identify in B. emersonii sequences associated with these ancestral characteristics two approaches were followed: (1) a large-scale comparative analysis between putative unigene sequences (uniseqs) from B. emersonii and three databases constructed ad hoc with fungal proteins, animal proteins and plant unigenes deposited in Genbank, and (2) a pairwise comparison between B. emersonii full-length cDNA sequences and their putative orthologues in the ascomycete Neurospora crassa and the basidiomycete Ustilago maydis.

Results

Comparative analyses of B. emersonii uniseqs with fungi, animal and plant databases through the two approaches mentioned above produced 166 B. emersonii sequences, which were identified as putatively absent from other fungi or not previously described. Through these approaches we found: (1) possible orthologues of genes previously identified as specific to animals and/or plants, and (2) genes conserved in fungi, but with a large difference in divergence rate in B. emersonii. Among these sequences, we observed cDNAs encoding enzymes from coenzyme B₁₂-dependent propionyl-CoA pathway, a metabolic route not previously described in fungi, and validated their expression in Northern blots.

Conclusion

Using two different approaches involving comparative sequence analyses, we could identify sequences from the early-diverging fungus B. emersonii previously considered specific to animals or plants, and highly divergent sequences from the same fungus relative to other fungi.

Protein transport in the plant secretory pathwayThis review is one of a selection of papers published in the Special Issue on Plant Cell Biology

The study of the plant secretory pathway is a relatively new field, developing rapidly over the last 30 years. Many exciting discoveries have already been made in this area, but as old questions are answered new ones become apparent. Our understanding of the functions and mechanisms of the plant secretory pathway is constantly expanding, in part because of the development of new technologies, mainly in bioimaging. The increasing accessibility of these new tools in combination with more established methods provides an ideal way to increase knowledge of the secretory pathway in plants. In this review we discuss recent developments in understanding protein transport between organelles in the plant secretory pathway.

Vacuolar H+-ATPase activity is required for endocytic and secretory trafficking in Arabidopsis

In eukaryotic cells, compartments of the highly dynamic endomembrane system are acidified to varying degrees by the activity of vacuolar H(+)-ATPases (V-ATPases). In the Arabidopsis thaliana genome, most V-ATPase subunits are encoded by small gene families, thus offering potential for a multitude of enzyme complexes with different kinetic properties and localizations. We have determined the subcellular localization of the three Arabidopsis isoforms of the membrane-integral V-ATPase subunit VHA-a. Colocalization experiments as well as immunogold labeling showed that VHA-a1 is preferentially found in the trans-Golgi network (TGN), the main sorting compartment of the secretory pathway. Uptake experiments with the endocytic tracer FM4-64 revealed rapid colocalization with VHA-a1, indicating that the TGN may act as an early endosomal compartment. Concanamycin A, a specific V-ATPase inhibitor, blocks the endocytic transport of FM4-64 to the tonoplast, causes the accumulation of FM4-64 together with newly synthesized plasma membrane proteins, and interferes with the formation of brefeldin A compartments. Furthermore, nascent cell plates are rapidly stained by FM4-64, indicating that endocytosed material is redirected into the secretory flow after reaching the TGN. Together, our results suggest the convergence of the early endocytic and secretory trafficking pathways in the TGN.

AtVAM3 is required for normal specification of idioblasts, myrosin cells

Myrosin cells in Capparales plants are idioblasts that accumulate thioglucoside glucohydrolase (TGG, also called myrosinase), which hydrolyzes glucosinolates to produce toxic compounds for repelling pests. Here, we show that AtVAM3 is involved in development of myrosin cells. It has been shown that yeast VAM3 is a Q(a)-SNARE that is involved in vesicle transport of vacuolar proteins and vacuolar assembly. We found that two Arabidopsis atvam3 alleles, atvam3-3 and atvam3-4/ssm, accumulate large amounts of TGG1 and TGG2 that are enzymatically active. An immunogold analysis revealed that TGGs were specifically localized in the vacuole of myrosin cells in atvam3 mutants. This result indicates that TGGs are normally transported to vacuoles in these mutants and that AtVAM3 is not essential for vacuolar transport of the proteins. We developed a staining method with Coomassie brilliant blue that detects myrosin cells in whole leaves by their high TGG content. This method showed that atvam3 leaves have a larger number of myrosin cells than do wild-type leaves. Myrosin cells were scattered along leaf veins in wild-type leaves, while they were abnormally distributed in atvam3 leaves. The mutants developed a network of myrosin cells throughout the leaves: myrosin cells were not only distributed continuously along leaf veins, but were also observed independent of leaf veins. The excess of myrosin cells in atvam3 mutants might be responsible for the abnormal abundance of TGGs and the reduction of elongation of inflorescence stems and leaves in these mutants. Our results suggest that AtVAM3 has a plant-specific function in development of myrosin cells.

Crossing the divide--transport between the endoplasmic reticulum and Golgi apparatus in plants

The transport of proteins between the endoplasmic reticulum (ER) and the Golgi apparatus in plants is an exciting and constantly expanding topic, which has attracted much attention in recent years. The study of protein transport within the secretory pathway is a relatively new field, dating back to the 1970s for mammalian cells and considerably later for plants. This may explain why COPI- and COPII-mediated transport between the ER and the Golgi in plants is only now becoming clear, while the existence of these pathways in other organisms is relatively well documented. We summarize current knowledge of these protein transport routes, as well as highlighting key differences between those of plant systems and those of mammals and yeast. These differences have necessitated the study of plant-specific aspects of protein transport in the early secretory pathway, and this review discusses recent developments in this area. Advances in live-cell-imaging technology have allowed the observation of protein movement in vivo, giving a new insight into many of the processes involved in vesicle formation and protein trafficking. The use of these new technologies has been combined with more traditional methods, such as protein biochemistry and electron microscopy, to increase our understanding of the transport routes in the cell.

Identification of an allele of VAM3/SYP22 that confers a semi-dwarf phenotype in Arabidopsis thaliana

The short stem and midrib (ssm) mutants of Arabidopsis thaliana show both semi-dwarf and wavy leaf phenotypes due to defects in the elongation of the stem internodes and leaves. Moreover, these abnormalities cannot be recovered by exogenous phytohormones. ssm was originally identified as a single recessive mutant of the ecotype Columbia (Col-0), but genetic crossing experiments have revealed that this mutant phenotype is restored by another gene that is functional in the ecotype Landsberg erecta (Ler) and not in Col-0. Map-based cloning of the gene that is defective in ssm mutants has uncovered a small deletion in the sixth intron of a gene encoding a syntaxin, VAM3/SYP22, which has been implicated in vesicle transport to the vacuole. This mutation appears to cause a peptide insertion in the deduced VAM3/SYP22 polypeptide sequence due to defective splicing of the shortened sixth intron. Significantly, when compared with the wild-type Ler genome, the wild-type Col-0 genome has a single base pair deletion causing a frameshift mutation in SYP23, a gene with the highest known homology to VAM3/SYP22. These findings suggest that VAM3/SYP22 and SYP23 have overlapping functions and that the vesicle transport mediated by these syntaxins is important for shoot morphogenesis.

YKT6 is a Core Constituent of Membrane Fusion Machineries at the Arabidopsis trans-Golgi Network

SNARE complex formation is essential for membrane fusion in exocytotic and vacuolar trafficking pathways. Vesicle-associated (v-) SNARE associates with a target membrane (t-) SNARE to form a SNARE complex bridging two membranes, which may facilitate membrane fusion. The Arabidopsis genome encodes a large number of predicted SNARE proteins that might function primarily as fusogens for vesicle transport in endomembrane systems. The SNAREs SYP41, SYP61 and VTI12 reside in the trans-Golgi network and have been proposed to function together in vesicle fusion with this organelle. Here, we use a liposome fusion assay to demonstrate that VTI12 and either SYP41 or SYP61, but not both, are required for membrane fusion. This indicates that SYP41 and SYP61 are likely to function in independent vesicle fusion reactions in Arabidopsis. In addition, we have identified two new functionally interchangeable components, YKT61 and YKT62, that show sequence similarity to the multifunctional yeast SNARE YKT6. Both YKT61 and YKT62 interact with SYP41 and are essential for membrane fusion mediated by either SYP41 or SYP61. These results therefore define the core constituents required for membrane fusion at the Arabidopsis trans-Golgi network.

Identification and characterization of AtCASP, a plant transmembrane Golgi matrix protein

Golgins are a family of coiled-coil proteins that are associated with the Golgi apparatus. They are necessary for tethering events in membrane fusion and may act as structural support for Golgi cisternae. Here we report on the identification of an Arabidopsis golgin which is a homologue of CASP, a known transmembrane mammalian and yeast golgin. Similar to its homologues, the plant CASP contains a long N-terminal coiled-coil region protruding into the cytosol and a C-terminal transmembrane domain with amino acid residues which are highly conserved across species. Through fluorescent protein tagging experiments, we show that plant CASP localizes at the plant Golgi apparatus and that the C-terminus of this protein is sufficient for its localization, as has been shown for its mammalian counterpart. In addition, we demonstrate that the plant CASP is able to localize at the mammalian Golgi apparatus. However, mutagenesis of a conserved tyrosine in the transmembrane domain revealed that it is necessary for ER export and Golgi localization of the Arabidopsis CASP in mammalian cells, but is not required for its correct localization in plant cells. These data suggest that mammalian and plant cells have different mechanisms for concentrating CASP in the Golgi apparatus.

The plant Golgi apparatus--going with the flow

The plant Golgi apparatus is composed of many separate stacks of cisternae which are often associated with the endoplasmic reticulum and which in many cell types are motile. In this review, we discuss the latest data on the molecular regulation of Golgi function. The concept of the Golgi as a distinct organelle is challenged and the possibility of a continuum between the endoplasmic reticulum and Golgi is proposed.

Conversion of Functional Specificity in Qb-SNARE VTI1 Homologues of Arabidopsis

In higher multicellular eukaryotes, highly specialized membrane structures or membrane trafficking events are required for supporting various physiological functions. SNAREs (soluble NSF attachment protein receptors) play an important role in specific membrane fusions. These protein receptors are assigned to subgroubs (Qa-, Qb-, Qc-, and R-SNARE) according to their specific SNARE structural motif. A specific set of Qa-, Qb-, and Qc-SNAREs, located on the target membrane, interact with R-SNARE on the vesicle to form a tight complex, leading to membrane fusion. The zig-1 mutant of Arabidopsis lacking Qb-SNARE VTI11 shows little shoot gravitropism and abnormal stem morphology. VTI11 and its homolog VTI12 exhibit partially overlapping but distinct intracellular localization and have different biological functions in plants. Little is known about how SNAREs are targeted to specific organelles, even though their functions and specific localization are closely linked. Here, we report that a novel mutation in VTI12 (zip1) was found as a dominant suppressor of zig-1. The zip1 mutation gave VTI12 the ability to function as VTI11 by changing both the specificity of SNARE complex formation and its intracellular localization. One amino acid substitution drastically altered VTI12, allowing it to suppress abnormalities of higher order physiological functions such as gravitropism and morphology. The zip1 mutation may be an indication of the flexibility in plant cell function afforded by gene duplication, particularly among the VTI11 genes and their recently diverged orthologs.

The vegetative vacuole proteome of Arabidopsis thaliana reveals predicted and unexpected proteins

Vacuoles play central roles in plant growth, development, and stress responses. To better understand vacuole function and biogenesis we have characterized the vegetative vacuolar proteome from Arabidopsis thaliana. Vacuoles were isolated from protoplasts derived from rosette leaf tissue. Total purified vacuolar proteins were then subjected either to multidimensional liquid chromatography/tandem mass spectrometry or to one-dimensional SDS-PAGE coupled with nano-liquid chromatography/tandem mass spectrometry (nano-LC MS/MS). To ensure maximum coverage of the proteome, a tonoplast-enriched fraction was also analyzed separately by one-dimensional SDS-PAGE followed by nano-LC MS/MS. Cumulatively, 402 proteins were identified. The sensitivity of our analyses is indicated by the high coverage of membrane proteins. Eleven of the twelve known vacuolar-ATPase subunits were identified. Here, we present evidence of four tonoplast-localized soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs), representing each of the four groups of SNARE proteins necessary for membrane fusion. In addition, potential cargo of the N- and C-terminal propeptide sorting pathways, association of the vacuole with the cytoskeleton, and the vacuolar localization of 89 proteins of unknown function are identified. A detailed analysis of these proteins and their roles in vacuole function and biogenesis is presented.

Gravity sensing and signaling

Gravitropism has attracted much attention from plant biologists. Recent studies have provided molecular evidence supporting two long-surviving hypotheses about the mechanism of gravitropism: the starch-statolith hypothesis and the Cholodney-Went hypothesis. Amyloplast movement along the gravity vector within gravity-sensing cells in the root and shoot is the most likely trigger of subsequent intracellular signaling. Several possible events leading from this signaling to differential auxin distribution within the sensing cells have been suggested recently.

Functional differentiation of endosomes in Arabidopsis cells

Endocytosis plays an important role in plant physiology, but how endocytic organelles are organized remains unknown. We present the evidence that endosomes are functionally differentiated in Arabidopsis cells. Two types of Rab5-related GTPases are localized on distinct population of endosomes in a partially overlapping manner. Ara7 and Rha1 are on an early type of endosomes with AtVamp727, where recycling of plasma membrane proteins occurs. In contrast, the plant-unique Rab5, Ara6, resides on distinct endosomes with the prevacuolar SNAREs. Partially overlapping localization of Ara6 and Ara7/Rha1 with reciprocal gradients suggests maturation of endosomes from one to the other.

Structure-function analysis of the presumptive Arabidopsis auxin permease AUX1

We have investigated the subcellular localization, the domain topology, and the amino acid residues that are critical for the function of the presumptive Arabidopsis thaliana auxin influx carrier AUX1. Biochemical fractionation experiments and confocal studies using an N-terminal yellow fluorescent protein (YFP) fusion observed that AUX1 colocalized with plasma membrane (PM) markers. Because of its PM localization, we were able to take advantage of the steep pH gradient that exists across the plant cell PM to investigate AUX1 topology using YFP as a pH-sensitive probe. The YFP-coding sequence was inserted in selected AUX1 hydrophilic loops to orient surface domains on either apoplastic or cytoplasmic faces of the PM based on the absence or presence of YFP fluorescence, respectively. We were able to demonstrate in conjunction with helix prediction programs that AUX1 represents a polytopic membrane protein composed of 11 transmembrane spanning domains. In parallel, a large aux1 allelic series containing null, partial-loss-of-function, and conditional mutations was characterized to identify the functionally important domains and amino acid residues within the AUX1 polypeptide. Whereas almost all partial-loss-of-function and null alleles cluster in the core permease region, the sole conditional allele aux1-7 modifies the function of the external C-terminal domain.

MEMBRANE TRAFFICKING IN PLANTS

Plant membrane trafficking shares many features with other eukaryotic organisms, including the machinery for vesicle formation and fusion. However, the plant endomembrane system lacks an ER-Golgi intermediate compartment, has numerous Golgi stacks and several types of vacuoles, and forms a transient compartment during cell division. ER-Golgi trafficking involves bulk flow and efficient recycling of H/KDEL-bearing proteins. Sorting in the Golgi stacks separates bulk flow to the plasma membrane from receptor-mediated trafficking to the lytic vacuole. Cargo for the protein storage vacuole is delivered from the endoplasmic reticulum (ER), cis-Golgi, and trans-Golgi. Endocytosis includes recycling of plasma membrane proteins from early endosomes. Late endosomes appear identical with the multivesiculate prevacuolar compartment that lies on the Golgi-vacuole trafficking pathway. In dividing cells, homotypic fusion of Golgi-derived vesicles forms the cell plate, which expands laterally by targeted vesicle fusion at its margin, eventually fusing with the plasma membrane.

Structure-function analysis of the presumptive Arabidopsis auxin permease AUX1

We have investigated the subcellular localization, the domain topology, and the amino acid residues that are critical for the function of the presumptive Arabidopsis thaliana auxin influx carrier AUX1. Biochemical fractionation experiments and confocal studies using an N-terminal yellow fluorescent protein (YFP) fusion observed that AUX1 colocalized with plasma membrane (PM) markers. Because of its PM localization, we were able to take advantage of the steep pH gradient that exists across the plant cell PM to investigate AUX1 topology using YFP as a pH-sensitive probe. The YFP-coding sequence was inserted in selected AUX1 hydrophilic loops to orient surface domains on either apoplastic or cytoplasmic faces of the PM based on the absence or presence of YFP fluorescence, respectively. We were able to demonstrate in conjunction with helix prediction programs that AUX1 represents a polytopic membrane protein composed of 11 transmembrane spanning domains. In parallel, a large aux1 allelic series containing null, partial-loss-of-function, and conditional mutations was characterized to identify the functionally important domains and amino acid residues within the AUX1 polypeptide. Whereas almost all partial-loss-of-function and null alleles cluster in the core permease region, the sole conditional allele aux1-7 modifies the function of the external C-terminal domain.

The role of ADP-ribosylation factor and SAR1 in vesicular trafficking in plants

Ras-like small GTP binding proteins regulate a wide variety of intracellular signalling and vesicular trafficking pathways in eukaryotic cells including plant cells. They share a common structure that operates as a molecular switch by cycling between active GTP-bound and inactive GDP-bound conformational states. The active GTP-bound state is regulated by guanine nucleotide exchange factors (GEF), which promote the exchange of GDP for GTP. The inactive GDP-bound state is promoted by GTPase-activating proteins (GAPs) which accelerate GTP hydrolysis by orders of magnitude. Two types of small GTP-binding proteins, ADP-ribosylation factor (Arf) and secretion-associated and Ras-related (Sar), are major regulators of vesicle biogenesis in intracellular traffic and are founding members of a growing family that also includes Arf-related proteins (Arp) and Arf-like (Arl) proteins. The most widely involved small GTPase in vesicular trafficking is probably Arf1, which not only controls assembly of COPI- and AP1, AP3, and AP4/clathrin-coated vesicles but also recruits other proteins to membranes, including some that may be components of further coats. Recent molecular, structural and biochemical studies have provided a wealth of detail of the interactions between Arf and the proteins that regulate its activity as well as providing clues for the types of effector molecules which are controlled by Arf. Sar1 functions as a molecular switch to control the assembly of protein coats (COPII) that direct vesicle budding from ER. The crystallographic analysis of Sar1 reveals a number of structurally unique features that dictate its function in COPII vesicle formation. In this review, I will summarize the current knowledge of Arf and Sar regulation in vesicular trafficking in mammalian and yeast cells and will highlight recent advances in identifying the elements involved in vesicle formation in plant cells. Additionally, I will briefly discuss the similarities and dissimilarities of vesicle traffic in plant, mammalian and yeast cells.

Sorting inhibitors (Sortins): Chemical compounds to study vacuolar sorting in Arabidopsis

Chemical genomics is an interdisciplinary approach that unites the power of chemical screens and genomics strategies to dissect biological processes such as endomembrane trafficking. We have taken advantage of the evolutionary conservation between plants and Saccharomyces cerevisiae to identify such chemicals. Using S. cerevisiae, we screened a library of diverse chemical structures for compounds that induce the secretion of carboxypeptidase Y, which is normally targeted to the vacuole. Among 4,800 chemicals screened, 14 compounds, termed sorting inhibitors (Sortins), were identified that stimulated secretion in yeast. In Arabidopsis seedlings, application of Sortin1 and -2 led to reversible defects in vacuole biogenesis and root development. Sortin1 was found to redirect the vacuolar destination of plant carboxypeptidase Y and other proteins in Arabidopsis suspension cells and cause these proteins to be secreted. Sortin1 treatment of whole Arabidopsis seedlings also resulted in carboxypeptidase Y secretion, indicating that the drug has a similar mode of action in cells and intact plants. We have demonstrated that screening of a simple eukaryote, in which vacuolar biogenesis is not essential, can be a powerful tool to find chemicals that interfere with vacuolar delivery of proteins in plants, where vacuole biogenesis is essential. Our studies were done by using a sublethal dose of Sortin1, demonstrating the powerful ability of the chemical to control the induced phenotype in a manner that would be difficult to achieve using conventional genetics.

The Arabidopsis Rab GTPase RabA4b localizes to the tips of growing root hair cells

Spatial and temporal control of cell wall deposition plays a unique and critical role during growth and development in plants. To characterize membrane trafficking pathways involved in these processes, we have examined the function of a plant Rab GTPase, RabA4b, during polarized expansion in developing root hair cells. Whereas a small fraction of RabA4b cofractionated with Golgi membrane marker proteins, the majority of this protein labeled a unique membrane compartment that did not cofractionate with the previously characterized trans-Golgi network syntaxin proteins SYP41 and SYP51. An enhanced yellow fluorescent protein (EYFP)-RabA4b fusion protein specifically localizes to the tips of growing root hair cells in Arabidopsis thaliana. Tip-localized EYFP-RabA4b disappears in mature root hair cells that have stopped expanding, and polar localization of the EYFP-RabA4b is disrupted by latrunculin B treatment. Loss of tip localization of EYFP-RabA4b was correlated with inhibition of expansion; upon washout of the inhibitor, root hair expansion recovered only after tip localization of the EYFP-RabA4b compartments was reestablished. Furthermore, in mutants with defective root hair morphology, EYFP-RabA4b was improperly localized or was absent from the tips of root hair cells. We propose that RabA4b regulates membrane trafficking through a compartment involved in the polarized secretion of cell wall components in plant cells.

Traffic jams affect plant development and signal transduction

Analysis of the Arabidopsis thaliana endomembrane system has shown that plant cell viability depends on a properly functioning vacuole and intact vesicular trafficking. The endomembrane system is also essential for various aspects of plant development and signal transduction. In this review, we discuss examples of these newly discovered roles for the endomembrane system in plants, and new experimental approaches and technologies that are based on high-throughput screens, which combine chemical genetics and automated confocal microscopy.