The expression of the t-SNARE AtSNAP33 is induced by pathogens and mechanical stimulation

Integrated multi-omics and genetic analyses reveal molecular determinants underlying Arabidopsis snap33 mutant phenotype

The secretory pathway is essential for plant immunity, delivering diverse antimicrobial molecules into the extracellular space. Arabidopsis thaliana soluble N-ethylmaleimide-sensitive-factor attachment protein receptor SNAP33 is a key actor of this process. The snap33 mutant displays dwarfism and necrotic lesions, however the molecular determinants of its macroscopic phenotypes remain elusive. Here, we isolated several new snap33 mutants that exhibited constitutive cell death and H2O2 accumulation, further defining snap33 as an autoimmune mutant. We then carried out quantitative transcriptomic and proteomic analyses showing that numerous defense transcripts and proteins were up-regulated in the snap33 mutant, among which genes/proteins involved in defense hormone, pattern-triggered immunity, and nucleotide-binding domain leucine-rich-repeat receptor signaling. qRT-PCR analyses and hormone dosages supported these results. Furthermore, genetic analyses elucidated the diverse contributions of the main defense hormones and some nucleotide-binding domain leucine-rich-repeat receptor signaling actors in the establishment of the snap33 phenotype, emphasizing the preponderant role of salicylic acid over other defense phytohormones. Moreover, the accumulation of pattern-triggered immunity and nucleotide-binding domain leucine-rich-repeat receptor signaling proteins in the snap33 mutant was confirmed by immunoblotting analyses and further shown to be salicylic acid-dependent. Collectively, this study unveiled molecular determinants underlying the Arabidopsis snap33 mutant phenotype and brought new insights into autoimmunity signaling.

Identification of a major QTL and candidate genes analysis for branch angle in rapeseed (Brassica napus L.) using QTL-seq and RNA-seq

Introduction

Branching angle is an essential trait in determining the planting density of rapeseed (Brassica napus L.) and hence the yield per unit area. However, the mechanism of branching angle formation in rapeseed is not well understood.

Methods

In this study, two rapeseed germplasm with extreme branching angles were used to construct an F2 segregating population; then bulked segregant analysis sequencing (BSA-seq) and quantitative trait loci (QTL) mapping were utilized to localize branching anglerelated loci and combined with transcriptome sequencing (RNA-seq) and quantitative real-time PCR (qPCR) for candidate gene mining.

Results and discussion

A branching angle-associated quantitative trait loci (QTL) was mapped on chromosome C3 (C3: 1.54-2.65 Mb) by combining BSA-seq as well as traditional QTL mapping. A total of 54 genes had SNP/Indel variants within the QTL interval were identified. Further, RNA-seq of the two parents revealed that 12 of the 54 genes were differentially expressed between the two parents. Finally, we further validated the differentially expressed genes using qPCR and found that six of them presented consistent differential expression in all small branching angle samples and large branching angles, and thus were considered as candidate genes related to branching angles in rapeseed. Our results introduce new candidate genes for the regulation of branching angle formation in rapeseed, and provide an important reference for the subsequent exploration of its formation mechanism.

Traffic Control: Subversion of Plant Membrane Trafficking by Pathogens

Membrane trafficking pathways play a prominent role in plant immunity. The endomembrane transport system coordinates membrane-bound cellular organelles to ensure that immunological components are utilized effectively during pathogen resistance. Adapted pathogens and pests have evolved to interfere with aspects of membrane transport systems to subvert plant immunity. To do this, they secrete virulence factors known as effectors, many of which converge on host membrane trafficking routes. The emerging paradigm is that effectors redundantly target every step of membrane trafficking from vesicle budding to trafficking and membrane fusion. In this review, we focus on the mechanisms adopted by plant pathogens to reprogram host plant vesicle trafficking, providing examples of effector-targeted transport pathways and highlighting key questions for the field to answer moving forward.

Characterization of Banana SNARE Genes and Their Expression Analysis under Temperature Stress and Mutualistic and Pathogenic Fungal Colonization

SNAREs (soluble N-ethylmaleimide-sensitive-factor attachment protein receptors) are engines for almost all of the membrane fusion and exocytosis events in organism cells. In this study, we identified 84 SNARE genes from banana (Musa acuminata). Gene expression analysis revealed that the expression of MaSNAREs varied a lot in different banana organs. By analyzing their expression patterns under low temperature (4 °C), high temperature (45 °C), mutualistic fungus (Serendipita indica, Si) and fungal pathogen (Fusarium oxysporum f. sp. Cubense Tropical Race 4, FocTR4) treatments, many MaSNAREs were found to be stress responsive. For example, MaBET1d was up-regulate by both low and high temperature stresses; MaNPSN11a was up-regulated by low temperature but down-regulated by high temperature; and FocTR4 treatment up-regulated the expression of MaSYP121 but down-regulated MaVAMP72a and MaSNAP33a. Notably, the upregulation or downregulation effects of FocTR4 on the expression of some MaSNAREs could be alleviated by priorly colonized Si, suggesting that they play roles in the Si-enhanced banana wilt resistance. Foc resistance assays were performed in tobacco leaves transiently overexpressing MaSYP121, MaVAMP72a and MaSNAP33a. Results showed that transient overexpression of MaSYP121 and MaSNPA33a suppressed the penetration and spread of both Foc1 (Foc Race 1) and FocTR4 in tobacco leaves, suggesting that they play positive roles in resisting Foc infection. However, the transient overexpression of MaVAMP72a facilitated Foc infection. Our study can provide a basis for understanding the roles of MaSNAREs in the banana responses to temperature stress and mutualistic and pathogenic fungal colonization.

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

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

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.

An atypical Phytophthora sojae RxLR effector manipulates host vesicle trafficking to promote infection

In plants, the apoplast is a critical battlefield for plant-microbe interactions. Plants secrete defense-related proteins into the apoplast to ward off the invasion of pathogens. How microbial pathogens overcome plant apoplastic immunity remains largely unknown. In this study, we reported that an atypical RxLR effector PsAvh181 secreted by Phytophthora sojae, inhibits the secretion of plant defense-related apoplastic proteins. PsAvh181 localizes to plant plasma membrane and essential for P. sojae infection. By co-immunoprecipitation assay followed by liquid chromatography-tandem mass spectrometry analyses, we identified the soybean GmSNAP-1 as a candidate host target of PsAvh181. GmSNAP-1 encodes a soluble N-ethylmaleimide-sensitive factor (NSF) attachment protein, which associates with GmNSF of the SNARE complex functioning in vesicle trafficking. PsAvh181 binds to GmSNAP-1 in vivo and in vitro. PsAvh181 interferes with the interaction between GmSNAP-1 and GmNSF, and blocks the secretion of apoplastic defense-related proteins, such as pathogenesis-related protein PR-1 and apoplastic proteases. Taken together, these data show that an atypical P. sojae RxLR effector suppresses host apoplastic immunity by manipulating the host SNARE complex to interfere with host vesicle trafficking pathway.

The Exocytosis Associated SNAP25-Type Protein, SlSNAP33, Increases Salt Stress Tolerance by Modulating Endocytosis in Tomato

In plants, vesicular trafficking is crucial for the response and survival to environmental challenges. The active trafficking of vesicles is essential to maintain cell homeostasis during salt stress. Soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs) are regulatory proteins of vesicular trafficking. They mediate membrane fusion and guarantee cargo delivery to the correct cellular compartments. SNAREs from the Qbc subfamily are the best-characterized plasma membrane SNAREs, where they control exocytosis during cell division and defense response. The Solanum lycopersicum gene SlSNAP33.2 encodes a Qbc-SNARE protein and is induced under salt stress conditions. SlSNAP33.2 localizes on the plasma membrane of root cells of Arabidopsis thaliana. In order to study its role in endocytosis and salt stress response, we overexpressed the SlSNAP33.2 cDNA in a tomato cultivar. Constitutive overexpression promoted endocytosis along with the accumulation of sodium (Na⁺) in the vacuoles. It also protected the plant from cell damage by decreasing the accumulation of hydrogen peroxide (H₂O₂) in the cytoplasm of stressed root cells. Subsequently, the higher level of SlSNAP33.2 conferred tolerance to salt stress in tomato plants. The analysis of physiological and biochemical parameters such as relative water content, the efficiency of the photosystem II, performance index, chlorophyll, and MDA contents showed that tomato plants overexpressing SlSNAP33.2 displayed a better performance under salt stress than wild type plants. These results reveal a role for SlSNAP33.2 in the endocytosis pathway involved in plant response to salt stress. This research shows that SlSNAP33.2 can be an effective tool for the genetic improvement of crop plants.

Genome-wide identification, evolution, and expression of the SNARE gene family in wheat resistance to powdery mildew

SNARE proteins mediate eukaryotic cell membrane/transport vesicle fusion and act in plant resistance to fungi. Herein, 173 SNARE proteins were identified in wheat and divided into 5 subfamilies and 21 classes. The number of the SYP1 class type was largest in TaSNAREs. Phylogenetic tree analysis revealed that most of the SNAREs were distributed in 21 classes. Analysis of the genetic structure revealed large differences among the 21 classes, and the structures in the same group were similar, except across individual genes. Excluding the first homoeologous group, the number in the other homoeologous groups was similar. The 2,000 bp promoter region of the TaSNARE genes were analyzed, and many W-box, MYB and disease-related cis-acting elements were identified. The qRT-PCR-based analysis of the SNARE genes revealed similar expression patterns of the same subfamily in one wheat variety. The expression patterns of the same gene in resistant/sensitive varieties largely differed at 6 h after infection, suggesting that SNARE proteins play an important role in early pathogen infection. Here, the identification and expression analysis of SNARE proteins provide a theoretical basis for studies of SNARE protein function and wheat resistance to powdery mildew.

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

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

OsSYP121 Accumulates at Fungal Penetration Sites and Mediates Host Resistance to Rice Blast

Magnaporthe oryzae is a fungal pathogen that causes rice (Oryza sativa) blast. SNAREs (soluble N-ethylmaleimide-sensitive factor attachment protein receptors) are key components in vesicle trafficking in eukaryotic cells and are known to contribute to fungal pathogen resistance. Syntaxin of Plants121 (SYP121), a Qa-SNARE, has been reported to function in nonhost resistance in Arabidopsis (Arabidopsis thaliana). However, the functions of SYP121 in host resistance to rice blast are largely unknown. Here, we report that the rice SYP121 protein, OsSYP121, accumulates at fungal penetration sites and mediates host resistance to rice blast. OsSYP121 is plasma membrane localized and its expression was obviously induced by the rice blast in both the blast-resistant rice landrace Heikezijing and the blast-susceptible landrace Suyunuo (Su). Overexpression of OsSYP121 in Su resulted in enhanced resistance to blast. Knockdown of OsSYP121 expression in Su resulted in a more susceptible phenotype. However, knockdown of OsSYP121 expression in the resistant landrace Heikezijing resulted in susceptibility to the blast fungus. The P_OsSYP121 ::GFP-OsSYP121 accumulated at rice blast penetration sites in transgenic rice, as observed by confocal microscopy. Yeast two-hybrid results showed that OsSYP121 can interact with OsSNAP32 (Synaptosome-associated protein of 32 kD) and Vesicle-associated membrane protein714/724. The interaction between OsSYP121 and OsSNAP32 may contribute to host resistance to rice blast. Our study reveals that OsSYP121 plays an important role in rice blast resistance as it is a key component in vesicle trafficking.

Proteomic Analysis of Plasmodesmata From Populus Cell Suspension Cultures in Relation With Callose Biosynthesis

Plasmodesmata are channels that link adjacent cells in plant tissues through which molecular exchanges take place. They are involved in multiple processes vital to plant cells, such as responses to hormonal signaling or environmental challenges including osmotic stress, wounding and pathogen attack. Despite the importance of plasmodesmata, their proteome is not well-defined. Here, we have isolated fractions enriched in plasmodesmata from cell suspension cultures of Populus trichocarpa and identified 201 proteins that are enriched in these fractions, thereby providing further insight on the multiple functions of plasmodesmata. Proteomics analysis revealed an enrichment of proteins specifically involved in responses to stress, transport, metabolism and signal transduction. Consistent with the role of callose deposition and turnover in the closure and aperture of the plasmodesmata and our proteomic analysis, we demonstrate the enrichment of callose synthase activity in the plasmodesmata represented by several gene products. A new form of calcium-independent callose synthase activity was detected, in addition to the typical calcium-dependent enzyme activity, suggesting a role of calcium in the regulation of plasmodesmata through two forms of callose synthase activities. Our report provides the first proteomic investigation of the plasmodesmata from a tree species and the direct biochemical evidence for the occurrence of several forms of active callose synthases in these structures. Data are available via ProteomeXchange with identifier PXD010692.

GhSNAP33, a t-SNARE Protein From Gossypium hirsutum, Mediates Resistance to Verticillium dahliae Infection and Tolerance to Drought Stress

Soluble N-ethylmaleimide-sensitive fusion protein attachment protein receptor (SNARE) proteins mediate membrane fusion and deliver cargo to specific cellular locations through vesicle trafficking. Synaptosome-associated protein of 25 kDa (SNAP25) is a target membrane SNARE that drives exocytosis by fusing plasma and vesicular membranes. In this study, we isolated GhSNAP33, a gene from cotton (Gossypium hirsutum), encoding a SNAP25-type protein containing glutamine (Q)b- and Qc-SNARE motifs connected by a linker. GhSNAP33 expression was induced by H₂O₂, salicylic acid, abscisic acid, and polyethylene glycol 6000 treatment and Verticillium dahliae inoculation. Ectopic expression of GhSNAP33 enhanced the tolerance of yeast cells to oxidative and osmotic stresses. Virus-induced gene silencing of GhSNAP33 induced spontaneous cell death and reactive oxygen species accumulation in true leaves at a later stage of cotton development. GhSNAP33-deficient cotton was susceptible to V. dahliae infection, which resulted in severe wilt on leaves, an elevated disease index, enhanced vascular browning and thylose accumulation. Conversely, Arabidopsis plants overexpressing GhSNAP33 showed significant resistance to V. dahliae, with reduced disease index and fungal biomass and elevated expression of PR1 and PR5. Leaves from GhSNAP33-transgenic plants showed increased callose deposition and reduced mycelia growth. Moreover, GhSNAP33 overexpression enhanced drought tolerance in Arabidopsis, accompanied with reduced water loss rate and enhanced expression of DERB2A and RD29A during dehydration. Thus, GhSNAP33 positively mediates plant defense against stress conditions and V. dahliae infection, rendering it a candidate for the generation of stress-resistant engineered cotton.

Overexpression of Rice Auxilin-Like Protein, XB21, Induces Necrotic Lesions, up-Regulates Endocytosis-Related Genes, and Confers Enhanced Resistance to Xanthomonas oryzae pv. oryzae

BACKGROUND

The rice immune receptor XA21 confers resistance to the bacterial pathogen, Xanthomonas oryzae pv. oryzae (Xoo). To elucidate the mechanism of XA21-mediated immunity, we previously performed a yeast two-hybrid screening for XA21 interactors and identified XA21 binding protein 21 (XB21).

RESULTS

Here, we report that XB21 is an auxilin-like protein predicted to function in clathrin-mediated endocytosis. We demonstrate an XA21/XB21 in vivo interaction using co-immunoprecipitation in rice. Overexpression of XB21 in rice variety Kitaake and a Kitaake transgenic line expressing XA21 confers a necrotic lesion phenotype and enhances resistance to Xoo. RNA sequencing reveals that XB21 overexpression results in the differential expression of 8735 genes (4939 genes up- and 3846 genes down-regulated) (≥2-folds, FDR ≤0.01). The up-regulated genes include those predicted to be involved in 'cell death' and 'vesicle-mediated transport'.

CONCLUSION

These results indicate that XB21 plays a role in the plant immune response and in regulation of cell death. The up-regulation of genes controlling 'vesicle-mediated transport' in XB21 overexpression lines is consistent with a functional role for XB21 as an auxilin.

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.

GENERAL CONTROL NONREPRESSIBLE4 Degrades 14-3-3 and the RIN4 Complex to Regulate Stomatal Aperture with Implications on Nonhost Disease Resistance and Drought Tolerance

Plants have complex and adaptive innate immune responses against pathogen infections. Stomata are key entry points for many plant pathogens. Both pathogens and plants regulate stomatal aperture for pathogen entry and defense, respectively. Not all plant proteins involved in stomatal aperture regulation have been identified. Here, we report GENERAL CONTROL NONREPRESSIBLE4 (GCN4), an AAA+-ATPase family protein, as one of the key proteins regulating stomatal aperture during biotic and abiotic stress. Silencing of GCN4 in Nicotiana benthamiana and Arabidopsis thaliana compromises host and nonhost disease resistance due to open stomata during pathogen infection. AtGCN4 overexpression plants have reduced H+-ATPase activity, stomata that are less responsive to pathogen virulence factors such as coronatine (phytotoxin produced by the bacterium Pseudomonas syringae) or fusicoccin (a fungal toxin produced by the fungus Fusicoccum amygdali), reduced pathogen entry, and enhanced drought tolerance. This study also demonstrates that AtGCN4 interacts with RIN4 and 14-3-3 proteins and suggests that GCN4 degrades RIN4 and 14-3-3 proteins via a proteasome-mediated pathway and thereby reduces the activity of the plasma membrane H+-ATPase complex, thus reducing proton pump activity to close stomata.

Identification of CkSNAP33, a gene encoding synaptosomal-associated protein from Cynanchum komarovii, that enhances Arabidopsis resistance to Verticillium dahliae

SNARE proteins are essential to vesicle trafficking and membrane fusion in eukaryotic cells. In addition, the SNARE-mediated secretory pathway can deliver diverse defense products to infection sites during exocytosis-associated immune responses in plants. In this study, a novel gene (CkSNAP33) encoding a synaptosomal-associated protein was isolated from Cynanchum komarovii and characterized. CkSNAP33 contains Qb- and Qc-SNARE domains in the N- and C-terminal regions, respectively, and shares high sequence identity with AtSNAP33 from Arabidopsis. CkSNAP33 expression was induced by H₂O₂, salicylic acid (SA), Verticillium dahliae, and wounding. Arabidopsis lines overexpressing CkSNAP33 had longer primary roots and larger seedlings than the wild type (WT). Transgenic Arabidopsis lines showed significantly enhanced resistance to V. dahliae, and displayed reductions in disease index and fungal biomass, and also showed elevated expression of PR1 and PR5. The leaves of transgenic plants infected with V. dahliae showed strong callose deposition and cell death that hindered the penetration and spread of the fungus at the infection site. Taken together, these results suggest that CkSNAP33 is involved in the defense response against V. dahliae and enhanced disease resistance in Arabidopsis.

Phytophthora infestans RXLR Effector AVR1 Interacts with Exocyst Component Sec5 to Manipulate Plant Immunity

Phytophthora infestans secretes numerous RXLR effectors that modulate host defense and thereby pave the way for successful invasion. Here, we show that the RXLR effector AVR1 is a virulence factor that promotes colonization and suppresses callose deposition, a hallmark of basal defense. To identify host targets of AVR1, we performed yeast two-hybrid screens and selected Sec5 as a candidate. Sec5 is a subunit of the exocyst, a protein complex that is involved in vesicle trafficking. AVR1-like (A-L), a close homolog of AVR1, also acts as a virulence factor, but unlike AVR1, A-L does not suppress CRINKLER2 (CRN2)-induced cell death or interact with Sec5. Compared with AVR1, A-L is shorter and lacks the carboxyl-terminal tail, the T-region that is crucial for CRN2-induced cell death suppression and Sec5 interaction. In planta analyses revealed that AVR1 and Sec5 are in close proximity, and coimmunoprecipitation confirmed the interaction. Sec5 is required for secretion of the pathogenesis-related protein PR-1 and callose deposition and also plays a role in CRN2-induced cell death. Our findings show that P. infestans manipulates an exocyst subunit and thereby potentially disturbs vesicle trafficking, a cellular process that is important for basal defense. This is a novel strategy that oomycete pathogens exploit to modulate host defense.

Wheat TaNPSN SNARE homologues are involved in vesicle-mediated resistance to stripe rust (Puccinia striiformis f. sp. tritici)

Subcellular localisation of SNAREs (soluble N-ethylmaleimide-sensitive factor attachment protein receptors) and their ability to form SNARE complexes are critical for determining the specificity of vesicle fusion. NPSN11, a Novel Plant SNARE (NPSN) gene, has been reported to be involved in the delivery of cell wall precursors to the newly formed cell plate during cytokinesis. However, functions of NPSN genes in plant-pathogen interactions are largely unknown. In this study, we cloned and characterized three NPSN genes (TaNPSN11, TaNPSN12, and TaNPSN13) and three plant defence-related SNARE homologues (TaSYP132, TaSNAP34, and TaMEMB12). TaSYP132 showed a highly specific interaction with TaNPSN11 in both yeast two-hybrid and bimolecular fluorescence complementation (BiFC) assays. We hypothesize that this interaction may indicate a partnership in vesicle trafficking. Expressions of the three TaNPSNs and TaSYP132 were differentially induced in wheat leaves when challenged by Puccinia striiformis f. sp. tritici (Pst). In virus-induced gene silencing (VIGS) assays, resistance of wheat (Triticum aestivum) cultivar Xingzi9104 to the Pst avirulent race CYR23 was reduced by knocking down TaNPSN11, TaNPSN13 and TaSYP132, but not TaNPSN12, implying diversified functions of these wheat SNARE homologues in prevention of Pst infection and hyphal elongation. Immuno-localization results showed that TaNPSN11 or its structural homologues were mainly distributed in vesicle structures near cell membrane toward Pst hypha. Taken together, our data suggests a role of TaNPSN11 in vesicle-mediated resistance to stripe rust.

Regulation of protein trafficking: posttranslational mechanisms and the unexplored transcriptional control

Endomembrane protein trafficking assures protein location through the endocytic and secretory routes. Trafficking pathways are diverse, depending on the proteins being trafficked, the final destination as well as their itinerary. Trafficking pathways are operated by machineries composed of a set of coordinately acting factors that transport proteins between compartments. Different machineries participate in each protein trafficking pathway, providing specificity and accuracy. Changes in the activity and abundance of trafficking proteins regulate protein flux. The preponderance of one pathway over another regulates protein location and relocation. Cellular requirements change during different processes and in response to stimuli; modulation of trafficking mechanisms must relocate proteins or alternatively increase/decrease the targeting rate of certain proteins. Conventionally, protein trafficking modulation has been explained as posttranslational modification of components of the relevant trafficking machinery. However, trafficking components are also transcriptionally regulated and several reports support that this regulation can modulate protein trafficking as well. This transcriptional modulation has an impact on plant physiology, and is a critical and fundamental mechanism. This scenario suggests a determinant mechanism that must be considered in the endomembrane protein trafficking research field.

A force of nature: molecular mechanisms of mechanoperception in plants

The ability to sense and respond to a wide variety of mechanical stimuli-gravity, touch, osmotic pressure, or the resistance of the cell wall-is a critical feature of every plant cell, whether or not it is specialized for mechanotransduction. Mechanoperceptive events are an essential part of plant life, required for normal growth and development at the cell, tissue, and whole-plant level and for the proper response to an array of biotic and abiotic stresses. One current challenge for plant mechanobiologists is to link these physiological responses to specific mechanoreceptors and signal transduction pathways. Here, we describe recent progress in the identification and characterization of two classes of putative mechanoreceptors, ion channels and receptor-like kinases. We also discuss how the secondary messenger Ca(2+) operates at the centre of many of these mechanical signal transduction pathways.

An approach to quantify endomembrane dynamics in pollen utilizing bioactive chemicals

Tip growth of pollen tubes and root hairs occurs via rapid polar growth. These rapidly elongating cells require tip-focused endomembrane trafficking for the deposition and recycling of proteins, membranes, and cell wall materials. Most of the image-based data published to date are subjective and non-quantified. Quantitative and comparative descriptors of these highly dynamic processes have been a major challenge, but are highly desirable for genetic and chemical genomics approaches to dissect this biological network. To address this problem, we screened for small molecules that perturbed the localization of a marker for the Golgi Ras-like monomeric G-protein RAB2:GFP expressed in transgenic tobacco pollen. Semi-automated high-throughput imaging and image analysis resulted in the identification of novel compounds that altered pollen tube development and endomembrane trafficking. Six compounds that caused mislocalization and varying degrees of altered movement of RAB2:GFP-labeled endomembrane bodies were used to generate a training set of image data from which to quantify vesicle dynamics. The area, velocity, straightness, and intensity of each body were quantified using semi-automated image analysis tools revealing quantitative differences in the phenotype caused by each compound. A score was then given to each compound enabling quantitative comparisons between compounds. Our results demonstrate that image analysis can be used to quantitatively evaluate dynamic subcellular endomembrane phenotypes induced by bioactive chemicals, mutations, or other perturbing agents as part of a strategy to quantitatively dissect the endomembrane network.

Label-free quantitative proteomic analysis of systemic responses to local wounding and virus infection in Arabidopsis thaliana

Plants are continuously exposed to changing environmental conditions and must, as sessile organisms, possess sophisticated acclimative mechanisms. To gain insight into systemic responses to local virus infection or wounding, we performed comparative LC-MS/MS protein profiling of distal, virus-free leaves four and five days after local inoculation of Arabidopsis thaliana plants with either Oilseed rape mosaic virus (ORMV) or inoculation buffer alone. Our study revealed biomarkers for systemic signaling in response to wounding and compatible virus infection in Arabidopsis, which should prove useful in further addressing the trigger-specific systemic response network and the elusive systemic signals. We observed responses common to ORMV and mock treatment as well as protein profile changes that are specific to local virus infection or mechanical wounding (mock treatment) alone, which provides evidence for the existence of more than one systemic signal to induce these distinct changes. Comparison of the systemic responses between time points indicated that the responses build up over time. Our data indicate stress-specific changes in proteins involved in jasmonic and abscisic acid signaling, intracellular transport, compartmentalization of enzyme activities, protein folding and synthesis, and energy and carbohydrate metabolism. In addition, a virus-triggered systemic signal appears to suppress antiviral host defense.

Lack of the Golgi phosphate transporter PHT4;6 causes strong developmental defects, constitutively activated disease resistance mechanisms and altered intracellular phosphate compartmentation in Arabidopsis

The Golgi-located phosphate exporter PHT4;6 has been described as involved in salt tolerance but further analysis on the physiological impact of PHT4;6 remained elusive. Here we show that PHT4;6-GFP is targeted to the trans-Golgi compartment and that loss of function of this carrier protein has a dramatic impact on plant growth and development. Knockout mutants of pht4;6 exhibit a dwarf phenotype that is complemented by the homologous gene from rice (Oryza sativa). Interestingly, pht4;6 mutants show altered characteristics of several Golgi-related functions, such as an altered abundance of certain N-glycosylated proteins, altered composition of cell-wall hemicelluose, and higher sensitivity to the Golgi α-mannosidase and the retrograde transport inhibitors kifunensine and brefeldin A, respectively. Moreover, pht4;6 mutants exhibit a 'mimic disease' phenotype accompanied by constitutively activated pathogen defense mechanisms and increased resistance against the virulent Pseudomonas syringae strain DC3000. Surprisingly, pht4;6 mutants also exhibit phosphate starvation symptoms, as revealed at the morphological and molecular level, although total Pi levels in wild-type and pht4;6 plants are similar. This suggested that subcellular Pi compartmentation was impaired. By use of nuclear magnetic resonance (NMR), increased Pi concentration was detected in acidic compartments of pht4;6 mutants. We propose that impaired Pi efflux from the trans-Golgi lumen results in accumulation of inorganic phosphate in other internal compartments, leading to low cytoplasmic phosphate levels with detrimental effects on plant performance.

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 role for the exocyst complex subunits Exo70B2 and Exo70H1 in the plant-pathogen interaction

Recently, the octameric vesicle-tethering complex exocyst was found in plants and its importance for Arabidopsis morphogenesis was demonstrated. Exo70 exocyst subunits in plants, unlike in yeasts and mammals, are represented by a multigene family, comprising 23 members in Arabidopsis. For Exo70B2 and Exo70H1 paralogues, transcriptional up-regulation was confirmed on treatment with an elicitor peptide, elf18, derived from the bacterial elongation factor. Their ability to participate in the exocyst complex formation was inferred by the interaction of both the Exo70s with several other exocyst subunits using the yeast two-hybrid system. Arabidopsis plants mutated in these two genes were used to analyse their local reaction upon inoculation with Pseudomonas syringae pv. maculicola and the fungal pathogen Blumeria graminis f. sp. hordei. The Pseudomonas sensitivity test revealed enhanced susceptibility for the two exo70B2 and one H1 mutant lines. After Blumeria inoculation, an increase in the proportion of abnormal papilla formation, with an unusual wide halo made of vesicle-like structures, was found in exo70B2 mutants. Intracellular localization of both Exo70 proteins was studied following a GFP fusion assay and Agrobacterium-mediated transient expression of the constructs in Nicotiana benthamiana leaf epidermis. GFP-Exo70H1 localizes in the vesicle-like structures, while GFP-Exo70B2 is localized mainly in the cytoplasm. It is concluded that both Exo70B2 and Exo70H1 are involved in the response to pathogens, with Exo70B2 having a more important role in cell wall apposition formation related to plant defence.

Molecular cloning and expression analysis of riceOsTVLP1, encoding a protein with similarity to TGF-β receptor interacting proteins and vacuolar assembly Vam6p/Vps39p proteins

We describe the cloning and identification of a rice cDNA, OsTVLP1, encoding a protein with similarity to TGF-beta receptor interacting proteins and vacuolar assembly Vam6p/Vps39p proteins. OsTVLP1 has an open reading frame of 2955 bp, which encodes a 984 amino acid protein, containing a citron homology (CNH) domain at its N-terminal and a clathrin heavy-chain repeat homology (CLH) domain at its C-terminal. The expression of OsTVLP1 was induced by treatments with benzothiadiazole (BTH), a chemical activator of plant disease resistance responses, and by infection of the blast fungus, Magnaporthe grisea. Importantly, the expression of OsTVLP1 was activated specifically in disease resistance response induced by BTH and in an incompatible interaction between rice and the blast fungus. Our observations suggest that OsTVLP1 may play a role in rice disease resistance response against pathogen infection.

Production of cecropin A in transgenic rice plants has an impact on host gene expression

Expression of the cecropin A gene in rice confers resistance to the rice blast fungus Magnaporthe oryzae. In this study, a polymerase chain reaction-based suppression subtractive hybridization approach was used to generate a cDNA macroarray from the elite japonica rice (Oryza sativa L.) cultivar 'Senia'. Gene expression studies revealed that the expression of components of the protein secretory and vesicular transport machinery is co-ordinately activated at the pre-invasive stage of infection of rice by the blast fungus. Comparisons of gene expression between wild-type and cecropin A plants revealed the over-expression of genes involved in protection against oxidative stress in transgenic plants in the absence of the pathogen, which correlated well with the tolerance of these plants to oxidative stress. A subcellular fractionation analysis suggested that cecropin A accumulates in the endoplasmic reticulum in cecropin A rice. Moreover, a large number of genes related to the processes of synthesis, folding and stabilization of proteins that enter into the secretory pathway are over-expressed in cecropin A rice, confirming that these plants constitutively express the unfolded protein response. Transgenic expression of cecropin A in rice has an effect on the transcriptional reprogramming that accompanies plant adaptation to fungal infection. Overall, this study provides evidence for transgene-induced changes in gene expression in cecropin A rice under both optimal growth conditions and stress conditions imposed by fungal infection. The data also indicate that resistance to blast in cecropin A rice may be the consequence of a combination of the antifungal activity of cecropin A and cecropin A-mediated over-expression of rice genes.

The trypanosome transcriptome is remodelled during differentiation but displays limited responsiveness within life stages

BACKGROUND

Trypanosomatids utilise polycistronic transcription for production of the vast majority of protein-coding mRNAs, which operates in the absence of gene-specific promoters. Resolution of nascent transcripts by polyadenylation and trans-splicing, together with specific rates of mRNA turnover, serve to generate steady state transcript levels that can differ in abundance across several orders of magnitude and can be developmentally regulated. We used a targeted oligonucleotide microarray, representing the strongly developmentally-regulated T. brucei membrane trafficking system and approximately 10% of the Trypanosoma brucei genome, to investigate both between-stage, or differentiation-dependent, transcriptome changes and within-stage flexibility in response to various challenges.

RESULTS

6% of the gene cohort are developmentally regulated, including several small GTPases, SNAREs, vesicle coat factors and protein kinases both consistent with and extending previous data. Therefore substantial differentiation-dependent remodeling of the trypanosome transcriptome is associated with membrane transport. Both the microarray and qRT-PCR were then used to analyse transcriptome changes resulting from specific gene over-expression, knockdown, altered culture conditions and chemical stress. Firstly, manipulation of Rab5 expression results in co-ordinate changes to clathrin protein expression levels and endocytotic activity, but no detectable changes to steady-state mRNA levels, which indicates that the effect is mediated post-transcriptionally. Secondly, knockdown of clathrin or the variant surface glycoprotein failed to perturb transcription. Thirdly, exposure to dithiothreitol or tunicamycin revealed no evidence for a classical unfolded protein response, mediated in higher eukaryotes by transcriptional changes. Finally, altered serum levels invoked little transcriptome alteration beyond changes to expression of ESAG6/7, the transferrin receptor.

CONCLUSION

While trypanosomes regulate mRNA abundance to effect the major changes accompanying differentiation, a given differentiated state appears transcriptionally inflexible. The implications of the absence of a transcriptome response in trypanosomes for both virulence and models of life cycle progression are discussed.

Rapid and dynamic subcellular reorganization following mechanical stimulation of Arabidopsis epidermal cells mimics responses to fungal and oomycete attack

BACKGROUND

Plant cells respond to the presence of potential fungal or oomycete pathogens by mounting a basal defence response that involves aggregation of cytoplasm, reorganization of cytoskeletal, endomembrane and other cell components and development of cell wall appositions beneath the infection site. This response is induced by non-adapted, avirulent and virulent pathogens alike, and in the majority of cases achieves penetration resistance against the microorganism on the plant surface. To explore the nature of signals that trigger this subcellular response and to determine the timing of its induction, we have monitored the reorganization of GFP-tagged actin, microtubules, endoplasmic reticulum (ER) and peroxisomes in Arabidopsis plants - after touching the epidermal surface with a microneedle.

RESULTS

Within 3 to 5 minutes of touching the surface of Arabidopsis cotyledon epidermal cells with fine glass or tungsten needles, actin microfilaments, ER and peroxisomes began to accumulate beneath the point of contact with the needle. Formation of a dense patch of actin was followed by focusing of actin cables on the site of contact. Touching the cell surface induced localized depolymerization of microtubules to form a microtubule-depleted zone surrounding a dense patch of GFP-tubulin beneath the needle tip. The concentration of actin, GFP-tubulin, ER and peroxisomes remained focused on the contact site as the needle moved across the cell surface and quickly dispersed when the needle was removed.

CONCLUSION

Our results show that plant cells can detect the gentle pressure of a microneedle on the epidermal cell surface and respond by reorganizing subcellular components in a manner similar to that induced during attack by potential fungal or oomycete pathogens. The results of our study indicate that during plant-pathogen interactions, the basal defence response may be induced by the plant's perception of the physical force exerted by the pathogen as it attempts to invade the epidermal cell surface.

Detection of mechanical and disease stresses in citrus plants by fluorescence spectroscopy

We have investigated the detection of mechanical and disease stresses in citrus plants (Citrus limonia [L.] Osbeck) using laser-induced fluorescence spectroscopy. Due to its economic importance we have chosen to investigate the citrus canker disease, which is caused by the Xanthomonas axonopodis pv. citri bacteria. Mechanical stress was also studied because it plays an important role in the plant's infection by such bacteria. A laser-induced fluorescence spectroscopy system, composed of a spectrometer and a 532 nm 10 mW excitation laser was used to perform fluorescence spectroscopy. The ratio of two chlorophyll fluorescence bands allows us to detect and discriminate between mechanical and disease stresses. This ability to discriminate may have an important application in the field to detect citrus canker infected trees.

Cloning and characterization of three genes encoding Qb-SNARE proteins in rice

Qb-SNARE proteins belong to the superfamily of SNAREs (soluble N-ethylmaleimide-sensitive factor attachment protein receptors) and function as important components of the vesicle trafficking machinery in eukaryotic cells. Here, we report three novel plant SNARE (NPSN) genes isolated from rice and named OsNPSN11, OsNPSN12 and OsNPSN13. They have about 70% nucleotide identity over their entire coding regions and similar genomic organization with ten exons and nine introns in each gene. Multiple alignment of deduced amino acid sequences indicate that the OsNPSNs proteins are homologous to AtNPSNs from Arabidopsis, containing a Qb-SNARE domain and a membrane-spanning domain in the C-terminal region. Semi-quantitative RT-PCR assays showed that the OsNPSNs were ubiquitously and differentially expressed in roots, culms, leaves, immature spikes and flowering spikes. The expression of OsNPSNs was significantly activated in rice seedlings treated with H(2)O(2), but down-regulated under NaCl and PEG6000 stresses. Transient expression method in onion epidermal cells revealed that OsNPSNs were located in the plasma membrane. Transformed yeast cells with OsNPSNs had better growth rates than empty-vector transformants when cultured on either solid or liquid selective media containing various concentrations of H(2)O(2), but more sensitive to NaCl and mannitol stresses. The 35S:OsNPSN11 transgenic tobacco also showed more tolerance to H(2)O(2) and sensitivity to NaCl and mannitol than non-transgenic tobacco. These results indicate that OsNPSNs may be involved in different aspects of the signal transduction in plant and yeast responses to abiotic stresses.

Transport and secretion in plant-microbe interactions

Microbial elicitors and effectors, as well as plant receptors and defence compounds, traffic at the interface of plants and microbes in pathogenic or mutualistic interactions. Net exocytosis appears to be required for surface enlargement of plasma membrane during accommodation of microbes in intact plant cells. By contrast, ligand-induced endocytosis of surface receptors operates in basal defence. The first layer of plant defence appears to depend on polarized transport of small molecules and on local secretion of defence proteins. In return, pathogen effectors target plasma membrane bound and intracellular proteins to inhibit extracellular host defences.

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.

Phytoremediation of chromium using Salix species: Cloning ESTs and candidate genes involved in the Cr response

In this research a differential display based on the detection of cDNA-AFLP markers was used to identify candidate genes potentially involved in the regulation of the response to chromium in four different willow species (Salix alba, Salix eleagnos, Salix fragilis and Salix matsudana) chosen on the basis of their suitability in phytoremediation techniques. Our approach enabled the assay of a large set of mRNA-related fragments and increased the reliability of amplification-based transcriptome analysis. The vast majority of transcript-derived fragments were shared among samples within species and thus attributable to constitutively expressed genes. However, a number of differentially expressed mRNAs were scored in each species and a total of 68 transcripts displaying an altered expression in response to Cr were isolated and sequenced. Public database querying revealed that 44.1% and 4.4% of the cloned ESTs score significant similarity with genes encoding proteins having known or putative function, or with genes coding for unknown proteins, respectively, whereas the remaining 51.5% did not retrieve any homology. Semi-quantitative RT-PCR analysis of seven candidate genes fully confirmed the expression patterns obtained by cDNA-AFLP. Our results indicate the existence of common mechanisms of gene regulation in response to Cr, pathogen attack and senescence-mediated programmed cell death, and suggest a role for the genes isolated in the cross-talk of the signaling pathways governing the adaptation to biotic and abiotic stresses.

Cytoskeleton and cell wall function in penetration resistance

Plants successfully repel the vast majority of potential pathogens that arrive on their surface, with most microorganisms failing to breach the outer epidermal wall. Resistance to penetration at the epidermis is a key component of basal defence against disease and critically depends on fortification of the cell wall at the site of attempted penetration through the development of specialised cell wall appositions rich in antimicrobial compounds. Formation of cell wall appositions is achieved by rapid reorganisation of actin microfilaments, actin-dependent transport of secretory products to the infection site and local activation of callose synthesis. Plants are finely tuned to detect the presence of pathogens on their surface, perceiving both chemical and physical signals of pathogen origin. In the on-going evolution of interaction strategies, plants must continually monitor and out manoeuvre pathogen avoidance or suppression of plant defences in order to preserve the effectiveness of penetration resistance.

Molecular cloning and characterization of a novel SNAP25-type protein gene OsSNAP32 in rice (Oryza sativa L.)

The SNAP25-type proteins belong to the superfamily of the SNAREs (soluble N-ethylmaleimide-sensitive factor attachment protein receptors), and function as important components of the vesical trafficking machinery in eukaryotic cells. In this paper, we report the cloning and expression characterization of OsSNAP32 gene, and the subcellular localization of its encoded protein. The OsSNAP32 gene contains five exons and four introns, and is located between RFLP markers C12276S and S1917 on chromosome 2 in rice. The OsSNAP32 has a molecular weight of 31.3 kD, comprises 283 amino acid residues, and contains Qb-SNARE and Qc-SNARE domains in the N- and C-terminal, respectively. Multiple sequence alignment of the SNARE domains indicates that OsSNAP32 protein is homologous to HvSNAP34 and HvSNAP28 (63% and 55% of amino acid identity respectively) from barley. The transient expression method in onion epidermal cells, revealed that OsSNAP32 is located in the plasma membrane, like other SNAP25-type proteins. Semi-quantitative RT-PCR assay showed that the OsSNAP32 is highly expressed in leaves and culms, and low in roots of rice, while hardly detected in immature spikes and flowering spikes. The expression of OsSNAP32 was significantly activated in rice seedlings treated with H2O2, PEG6000, and low temperature or after inoculation with rice blast (Magnaporthe grisea strain Hoku 1). The results suggest that this gene belongs to a novel member of this gene family encoding SNAP25-type proteins, involved in the rice responses to biotic and abiotic stresses.

Vesicle trafficking in plant immune responses

In plants, perception of pathogen-associated molecular patterns at the surface is the first line of defence in cellular immunity. This review summarizes recent evidence of the involvement of vesicle trafficking in the plant's immune response against pathogens. I first discuss aspects of ligand-stimulated receptor endocytosis. The best-characterized pattern-recognition receptor (PRR), FLS2, is a transmembrane leucine-rich repeat receptor kinase that recognizes bacterial flagellin. FLS2 was recently shown to undergo internalization upon activation with its cognate ligand. An animal PRR, TLR4 that mediates perception of bacterial-derived lipopolysaccharides, similarly exhibits ligand-stimulated endocytosis. The second focus is N-ethylmaleimide-sensitive factor adaptor protein receptor (SNARE)-mediated immunity involving syntaxins and their cognate partners. One of the genes involved in basal immunity in Arabidopsis, PEN1, encodes a syntaxin that focally accumulates at fungal penetration sites, raising the possibility that induced exocytosis is important for active defence. Pathogen-triggered endocytic and exocytic processes have to be balanced to ensure host cell homeostasis. Thus, understanding how phytopathogens have evolved strategies to exploit host cell vesicle trafficking to manipulate immune responses is currently an area of intense study.

Multivesicular compartments proliferate in susceptible and resistant MLA12-barley leaves in response to infection by the biotrophic powdery mildew fungus

There is growing evidence that multivesicular bodies and cell wall-associated paramural bodies participate in the enhanced vesicle trafficking induced by pathogen attack. Here, we performed transmission electron microscopy in combination with cytochemical localization of H2O2 to investigate multivesicular compartments during establishment of compatible interaction in susceptible barley (Hordeum vulgare) and during hypersensitive response in resistant MLA12-barley infected by the barley powdery mildew fungus (Blumeria graminis f. sp. hordei). Multivesicular bodies, intravacuolar vesicle aggregates and paramural bodies proliferated in the penetrated epidermal cell during development of the fungal haustorium. These vesicular structures also proliferated at the periphery of intact cells, which were adjacent to the hypersensitive dying cells and deposited cell wall appositions associated with H2O2 accumulation. All plasmodesmata between intact cells and hypersensitive cells were constricted or blocked by cell wall appositions. These results suggest that multivesicular compartments participate in secretion of building blocks for cell wall appositions not only to arrest fungal penetration but also to contain hypersensitive cell death through blocking plasmodesmata. They may also participate in internalization of damaged membranes, deleterious materials, nutrients, elicitors and elicitor receptors.

Dynamic cellular responses in plant-microbe interactions

Encounters between plant cells and both 'friendly' and 'hostile' microbes (such as those in symbiotic and pathogenic interactions, respectively) trigger a range of highly dynamic plant cellular responses. These include reorganization of the cytoskeleton, organelle translocation, vesicle trafficking, and alterations in subcellular protein localization. Recent progress in this borderland that bridges the fields of plant-microbe interactions and cell biology heralds the transition from descriptive phenomenology to the identification and characterization of key molecules that are involved in these processes. Intriguingly, molecular events that occur in plant cells in response to microbes also take place upon abiotic wounding and during fundamental plant developmental processes, such as the tip growth of pollen, root hairs and trichomes. Thus, elementary 'activity modules' that are required for the generation of cell polarity in plant morphogenesis appear to be re-used in both abiotic and biotic stress response pathways.

Rapid phosphorylation of a syntaxin during the Avr9/Cf-9-race-specific signaling pathway

The tomato (Lycopersicon esculentum) resistance (R) gene Cf-9 is required for resistance to races of the fungal pathogen Cladosporium fulvum expressing the elicitor Avr9 and also confers responsiveness to Avr9 in Cf-9-containing transgenic tobacco (Nicotiana tabacum; Cf9 tobacco). Although protein phosphorylation is required for many early Avr9/Cf-9-signaling events, so far the only phosphorylation targets known in this race-specific signaling pathway are three kinases: the two mitogen-activated protein kinases, wound-induced protein kinase and salicylic acid-induced protein kinase, and the calcium-dependent protein kinase NtCDPK2. Here, we provide evidence that a tobacco syntaxin is rapidly and transiently phosphorylated after Avr9 elicitation. The syntaxin was detected with an antibody against NtSyp121, a plasma membrane-localized syntaxin implicated in abscisic acid responses and secretion. Consistent with the gene-for-gene hypothesis, syntaxin phosphorylation required the presence of both Avr9 and Cf-9. This phosphorylation event occurred either upstream of the pathway leading to reactive oxygen species production or in a parallel pathway. Interestingly, rapid syntaxin phosphorylation was triggered by the race-specific elicitor Avr9 but not by flg22(P.aer), a general elicitor capable of inducing other defense-related signaling events in Cf9 tobacco such as reactive oxygen species production, mitogen-activated protein kinase activation, and PR5 transcript up-regulation. Furthermore, NtSyp121 transcript levels were increased at 24 h after elicitation with Avr9 but not with flg22(P.aer). Because most other previously described Avr9- and flg22(P.aer)-elicited responses are similar, syntaxin phosphorylation and NtSyp121 transcript up-regulation may serve as novel early biochemical and late molecular markers, respectively, to elucidate further differences in the signaling responses between these two elicitors.

Induction of Protein Secretory Pathway Is Required for Systemic Acquired Resistance

In plants, systemic acquired resistance (SAR) is established as a result of NPR1-regulated expression of pathogenesis-related (PR) genes. Using gene expression profiling in Arabidopsis, we found that in addition to controlling the expression of PR genes, NPR1 also directly controls the expression of the protein secretory pathway genes. Up-regulation of these genes is essential for SAR, because mutations in some of them diminished the secretion of PR proteins (for example, PR1), resulting in reduced resistance. We provide evidence that NPR1 coordinately regulates these secretion-related genes through a previously undescribed cis-element. Activation of this cis-element is controlled by a transcription factor that is translocated into the nucleus upon SAR induction.