Activity determinants and functional specialization of Arabidopsis PEN1 syntaxin in innate immunity

LKS4-mediated SYP121 phosphorylation participates in light-induced stomatal opening in Arabidopsis

By modulating stomatal opening and closure, plants control gas exchange, water loss, and photosynthesis in response to various environmental signals. During light-induced stomatal opening, the transport of ions and solutes across the plasma membrane (PM) of the surrounding guard cells results in an increase in turgor pressure, leading to cell swelling. Simultaneously, vesicles for exocytosis are delivered via membrane trafficking to compensate for the enlarged cell surface area and maintain an appropriate ion-channel density in the PM. In eukaryotic cells, soluble N-ethylmaleimide-sensitive factor adaptor protein receptors (SNAREs) mediate membrane fusion between vesicles and target compartments by pairing the cognate glutamine (Q)- and arginine (R)-SNAREs to form a core SNARE complex. Syntaxin of plants 121 (SYP121) is a known Q-SNARE involved in stomatal movement, which not only facilitates the recycling of K+ channels to the PM but also binds to the channels to regulate their activity. In this study, we found that the expression of a receptor-like cytoplasmic kinase, low-K+ sensitive 4/schengen 1 (LKS4/SGN1), was induced by light; it directly interacted with SYP121 and phosphorylated T270 within the SNARE motif. Further investigation revealed that LKS4-dependent phosphorylation of SYP121 facilitated the interaction between SYP121 and R-SNARE vesicle-associated membrane protein 722 (VAMP722), promoting the assembly of the SNARE complex. Our findings demonstrate that the phosphorylation of SNARE proteins is an important strategy adopted by plants to regulate the SNARE complex assembly as well as membrane fusion. Additionally, we discovered the function of LKS4/SGN1 in light-induced stomatal opening via the phosphorylation of SYP121.

Plasma Membrane-Associated Proteins Identified in Arabidopsis Wild Type, lbr2-2 and bak1-4 Mutants Treated with LPSs from Pseudomonas syringae and Xanthomonas campestris

Plants recognise bacterial microbe-associated molecular patterns (MAMPs) from the environment via plasma membrane (PM)-localised pattern recognition receptor(s) (PRRs). Lipopolysaccharides (LPSs) are known as MAMPs from gram-negative bacteria that are most likely recognised by PRRs and trigger defence responses in plants. The Arabidopsis PRR(s) and/or co-receptor(s) complex for LPS and the associated defence signalling remains elusive. As such, proteomic identification of LPS receptors and/or co-receptor complexes will help to elucidate the molecular mechanisms that underly LPS perception and defence signalling in plants. The Arabidopsis LPS-binding protein (LBP) and bactericidal/permeability-increasing protein (BPI)-related-2 (LBR2) have been shown to recognise LPS and trigger defence responses while brassinosteroid insensitive 1 (BRI1)-associated receptor kinase 1 (BAK1) acts as a co-receptor for several PRRs. In this study, Arabidopsis wild type (WT) and T-DNA knock out mutants (lbr2-2 and bak1-4) were treated with LPS chemotypes from Pseudomonas syringae pv. tomato DC3000 (Pst) and Xanthomonas campestris pv. campestris 8004 (Xcc) over a 24 h period. The PM-associated protein fractions were separated by liquid chromatography and analysed by tandem mass spectrometry (LC-MS/MS) followed by data analysis using Byonic^TM software. Using Gene Ontology (GO) for molecular function and biological processes, significant LPS-responsive proteins were grouped according to defence and stress response, perception and signalling, membrane transport and trafficking, metabolic processes and others. Venn diagrams demarcated the MAMP-responsive proteins that were common and distinct to the WT and mutant lines following treatment with the two LPS chemotypes, suggesting contributions from differential LPS sub-structural moieties and involvement of LBR2 and BAK1 in the LPS-induced MAMP-triggered immunity (MTI). Moreover, the identification of RLKs and RLPs that participate in other bacterial and fungal MAMP signalling proposes the involvement of more than one receptor and/or co-receptor for LPS perception as well as signalling in Arabidopsis defence responses.

Arabidopsis SYP121 acts as an ROP2 effector in the regulation of root hair tip growth

Tip growth is an extreme form of polarized cell expansion that occurs in all eukaryotic kingdoms to generate highly elongated tubular cells with specialized functions, including fungal hyphae, animal neurons, plant pollen tubes, and root hairs (RHs). RHs are tubular structures that protrude from the root epidermis to facilitate water and nutrient uptake, microbial interactions, and plant anchorage. RH tip growth requires polarized vesicle targeting and active exocytosis at apical growth sites. However, how apical exocytosis is spatially and temporally controlled during tip growth remains elusive. Here, we report that the Qa-Soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) SYP121 acts as an effector of Rho of Plants 2 (ROP2), mediating the regulation of RH tip growth. We show that active ROP2 promotes SYP121 targeting to the apical plasma membrane. Moreover, ROP2 directly interacts with SYP121 and promotes the interaction between SYP121 and the R-SNARE VAMP722 to form a SNARE complex, probably by facilitating the release of the Sec1/Munc18 protein SEC11, which suppresses the function of SYP121. Thus, the ROP2-SYP121 pathway facilitates exocytic trafficking during RH tip growth. Our study uncovers a direct link between an ROP GTPase and vesicular trafficking and a new mechanism for the control of apical exocytosis, whereby ROP GTPase signaling spatially regulates SNARE complex assembly and the polar distribution of a Q-SNARE.

Plant SYP12 syntaxins mediate an evolutionarily conserved general immunity to filamentous pathogens

Filamentous fungal and oomycete plant pathogens that invade by direct penetration through the leaf epidermal cell wall cause devastating plant diseases. Plant preinvasive immunity toward nonadapted filamentous pathogens is highly effective and durable. Pre- and postinvasive immunity correlates with the formation of evolutionarily conserved and cell-autonomous cell wall structures, named papillae and encasements, respectively. Yet, it is still unresolved how papillae/encasements are formed and whether these defense structures prevent pathogen ingress. Here, we show that in Arabidopsis the two closely related members of the SYP12 clade of syntaxins (PEN1 and SYP122) are indispensable for the formation of papillae and encasements. Moreover, loss-of-function mutants were hampered in preinvasive immunity toward a range of phylogenetically distant nonadapted filamentous pathogens, underlining the versatility and efficacy of this defense. Complementation studies using SYP12s from the early diverging land plant, Marchantia polymorpha, showed that the SYP12 clade immunity function has survived 470 million years of independent evolution. These results suggest that ancestral land plants evolved the SYP12 clade to provide a broad and durable preinvasive immunity to facilitate their life on land and pave the way to a better understanding of how adapted pathogens overcome this ubiquitous plant defense strategy.

Gene Gun-Mediated Transient Gene Expression for Functional Studies in Plant Immunity

One major threat to plant cultivation are fungal pathogens, which can cause substantial yield losses in agriculture. As an example, cereal powdery mildew fungi such as the barley (Hordeum vulgare) pathogen, Blumeria graminis f. sp. hordei (Bgh), are among the ten most relevant fungal plant pathogens in molecular plant pathology and can lead to yield losses of up to 30%. Plant Mildew resistance Locus O (MLO) genes are required for successful colonization of plants by powdery mildew fungi. Accordingly, loss-of-function mlo mutants confer durable resistance against powdery mildew fungi in many plant species. In the case of barley, mlo-based resistance has been used for more than 40 years in agriculture without powdery mildew fungi effectively overcoming this kind of immunity. However, the molecular basis of mlo resistance and function(s) of the transmembrane Mlo protein(s) are still incompletely understood. The generation of transgenic barley plants to study the plant immune response and the involvement of Mlo therein is time-consuming and challenging. Therefore, transient gene expression via gene gun-mediated particle bombardment became a popular, easy, and efficient tool to investigate different aspects of plant defense responses in barley. Since Bgh fails to penetrate leaf epidermal cells of mlo mutants, single-cell complementation upon biolistic transformation resulting in (over-)expression of Mlo can be used to characterize the Mlo protein functionally in vivo. In this chapter, we describe in detail the gene gun-mediated transient expression of Mlo in barley leaf epidermal cells followed by powdery mildew inoculation and the subsequent microscopic evaluation. However, gene gun-mediated transient gene expression may be also used to address other research questions or to transform the epidermal tissues of other plant organs and/or species.

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

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

ShNPSN11, a vesicle-transport-related gene, confers disease resistance in tomato to Oidium neolycopersici

Tomato powdery mildew, caused by Oidium neolycopersici, is a fungal disease that results in severe yield loss in infected plants. Herein, we describe the function of a class of proteins, soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs), which play a role in vesicle transport during defense signaling. To date, there have been no reports describing the function of tomato SNAREs during resistance signaling to powdery mildew. Using a combination of classical plant pathology-, genetics-, and cell biology-based approaches, we evaluate the role of ShNPSN11 in resistance to the powdery mildew pathogen O. neolycopersici. Quantitative RT-PCR analysis of tomato SNAREs revealed that ShNPSN11 mRNA accumulation in disease-resistant varieties was significantly increased following pathogen, compared with susceptible varieties, suggesting a role during induced defense signaling. Using in planta subcellular localization, we demonstrate that ShNPSN11 was primarily localized at the plasma membrane, consistent with the localization of SNARE proteins and their role in defense signaling and trafficking. Silencing of ShNPSN11 resulted in increased susceptibility to O. neolycopersici, with pathogen-induced levels of H2O2 and cell death elicitation in ShNPSN11-silenced lines showing a marked reduction. Transient expression of ShNPSN11 did not result in the induction of a hypersensitive cell death response or suppress cell death induced by BAX. Taken together, these data demonstrate that ShNPSNl11 plays an important role in defense activation and host resistance to O. neolycopersici in tomato LA1777.

Interaction Between the SNARE SYP121 and the Plasma Membrane Aquaporin PIP2;7 Involves Different Protein Domains

Plasma membrane intrinsic proteins (PIPs) are channels facilitating the passive diffusion of water and small solutes. Arabidopsis PIP2;7 trafficking occurs through physical interaction with SNARE proteins including the syntaxin SYP121, a plasma membrane Qa-SNARE involved in membrane fusion. To better understand the interaction mechanism, we aimed at identifying the interaction motifs in SYP121 and PIP2;7 using ratiometric bimolecular fluorescence complementation assays in Nicotiana benthamiana. SYP121 consists of four regions, N, H, Q, and C, and sequential deletions revealed that the C region, containing the transmembrane domain, as well as the H and Q regions, containing the Habc and Qa-SNARE functional domains, interact with PIP2;7. Neither the linker between the Habc and the Qa-SNARE domains nor the H or Q regions alone could fully restore the interaction with PIP2;7, suggesting that the interacting motif depends on the conformation taken by the HQ region. When investigating the interacting motif(s) in PIP2;7, we observed that deletion of the cytosolic N- and/or C- terminus led to a significant decrease in the interaction with SYP121. Shorter deletions revealed that at the N-terminal amino acid residues 18-26 were involved in the interaction. Domain swapping experiments between PIP2;7 and PIP2;6, a PIP isoform that does not interact with SYP121, showed that PIP2;7 N-terminal part up to the loop C was required to restore the full interaction signal, suggesting that, as it is the case for SYP121, the interaction motif(s) in PIP2;7 depend on the protein conformation. Finally, we also showed that PIP2;7 physically interacted with other Arabidopsis SYP1s and SYP121 orthologs.

Mass Spectrometry Untangles Plant Membrane Protein Signaling Networks

Plasma membranes (PMs) act as primary cellular checkpoints for sensing signals and controlling solute transport. Membrane proteins communicate with intracellular processes through protein interaction networks. Deciphering these signaling networks provides crucial information for elucidating in vivo cellular regulation. Large-scale proteomics enables system-wide characterization of the membrane proteome, identification of ligand-receptor pairs, and elucidation of signals originating at membranes. In this review we assess recent progress in the development of mass spectrometry (MS)-based proteomic pipelines for determining membrane signaling pathways. We focus in particular on current techniques for the analysis of membrane protein phosphorylation and interaction, and how these proteins may be connected to downstream changes in gene expression, metabolism, and physiology.

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.

Vesicle trafficking in plant immunity

To defend against extracellular pathogens, plants primarily depend on cell-autonomous innate immunity due to the lack of the circulatory immune system including mobile immune cells. To extracellularly restrict or kill the pathogens, plant cells dump out antimicrobials. However, since antimicrobials are also toxic to plant cells themselves, they have to be safely delivered to the target sites in a separate vesicular compartment. In addition, because immune responses often requires energy otherwise used for the other metabolic processes, it is very important to properly control the duration and strength of immune responses depending on pathogen types. This can be achieved by regulating the sensing of immune signals and the delivery/discharge of extracellular immune molecules, all of which are controlled by membrane trafficking in plant cells. Soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs) are now considered as the minimal factors that can merge two distinct membranes of cellular compartments. Hence, in this review, known and potential immune functions of SNAREs as well as regulatory proteins will be discussed.

Components of the SNARE-containing regulon are co-regulated in root cells undergoing defense

The term regulon has been coined in the genetic model plant Arabidopsis thaliana, denoting a structural and physiological defense apparatus defined genetically through the identification of the penetration (pen) mutants. The regulon is composed partially by the soluble N-ethylmaleimide-sensitive fusion protein attachment protein receptor (SNARE) syntaxin PEN1. PEN1 has homology to a Saccharomyces cerevisae gene that regulates a Secretion (Sec) protein, Suppressor of Sec 1 (Sso1p). The regulon is also composed of the β-glucosidase (PEN2) and an ATP binding cassette (ABC) transporter (PEN3). While important in inhibiting pathogen infection, limited observations have been made regarding the transcriptional regulation of regulon genes until now. Experiments made using the model agricultural Glycine max (soybean) have identified co-regulated gene expression of regulon components. The results explain the observation of hundreds of genes expressed specifically in the root cells undergoing the natural process of defense. Data regarding additional G. max genes functioning within the context of the regulon are presented here, including Sec 14, Sec 4 and Sec 23. Other examined G. max homologs of membrane fusion genes include an endosomal bromo domain-containing protein1 (Bro1), syntaxin6 (SYP6), SYP131, SYP71, SYP8, Bet1, coatomer epsilon (ϵ-COP), a coatomer zeta (ζ-COP) paralog and an ER to Golgi component (ERGIC) protein. Furthermore, the effectiveness of biochemical pathways that would function within the context of the regulon ave been examined, including xyloglucan xylosyltransferase (XXT), reticuline oxidase (RO) and galactinol synthase (GS). The experiments have unveiled the importance of the regulon during defense in the root and show how the deposition of callose relates to the process.

Functional Characterization of a Syntaxin Involved in Tomato (Solanum lycopersicum) Resistance against Powdery Mildew

Specific syntaxins, such as Arabidopsis AtPEN1 and its barley ortholog ROR2, play a major role in plant defense against powdery mildews. Indeed, the impairment of these genes results in increased fungal penetration in both host and non-host interactions. In this study, a genome-wide survey allowed the identification of 21 tomato syntaxins. Two of them, named SlPEN1a and SlPEN1b, are closely related to AtPEN1. RNAi-based silencing of SlPEN1a in a tomato line carrying a loss-of-function mutation of the susceptibility gene SlMLO1 led to compromised resistance toward the tomato powdery mildew fungus Oidium neolycopersici. Moreover, it resulted in a significant increase in the penetration rate of the non-adapted powdery mildew fungus Blumeria graminis f. sp. hordei. Codon-based evolutionary analysis and multiple alignments allowed the detection of amino acid residues that are under purifying selection and are specifically conserved in syntaxins involved in plant-powdery mildew interactions. Our findings provide both insights on the evolution of syntaxins and information about their function which is of interest for future studies on plant-pathogen interactions and tomato breeding.

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

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

Protein trafficking during plant innate immunity

Plants have evolved a sophisticated immune system to fight against pathogenic microbes. Upon detection of pathogen invasion by immune receptors, the immune system is turned on, resulting in production of antimicrobial molecules including pathogenesis-related (PR) proteins. Conceivably, an efficient immune response depends on the capacity of the plant cell's protein/membrane trafficking network to deploy the right defense-associated molecules in the right place at the right time. Recent research in this area shows that while the abundance of cell surface immune receptors is regulated by endocytosis, many intracellular immune receptors, when activated, are partitioned between the cytoplasm and the nucleus for induction of defense genes and activation of programmed cell death, respectively. Vesicle transport is an essential process for secretion of PR proteins to the apoplastic space and targeting of defense-related proteins to the plasma membrane or other endomembrane compartments. In this review, we discuss the various aspects of protein trafficking during plant immunity, with a focus on the immunity proteins on the move and the major components of the trafficking machineries engaged.

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

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

Effector biology during biotrophic invasion of plant cells

Several obligate biotrophic phytopathogens, namely oomycetes and fungi, invade and feed on living plant cells through specialized structures known as haustoria. Deploying an arsenal of secreted proteins called effectors, these pathogens balance their parasitic propagation by subverting plant immunity without sacrificing host cells. Such secreted proteins, which are thought to be delivered by haustoria, conceivably reprogram host cells and instigate structural modifications, in addition to the modulation of various cellular processes. As effectors represent tools to assist disease resistance breeding, this short review provides a bird’s eye view on the relationship between the virulence function of effectors and their subcellular localization in host cells.

Binding of SEC11 indicates its role in SNARE recycling after vesicle fusion and identifies two pathways for vesicular traffic to the plasma membrane

SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor) proteins drive vesicle fusion in all eukaryotes and contribute to homeostasis, pathogen defense, cell expansion, and growth in plants. Two homologous SNAREs, SYP121 (=SYR1/PEN1) and SYP122, dominate secretory traffic to the Arabidopsis thaliana plasma membrane. Although these proteins overlap functionally, differences between SYP121 and SYP122 have surfaced, suggesting that they mark two discrete pathways for vesicular traffic. The SNAREs share primary cognate partners, which has made separating their respective control mechanisms difficult. Here, we show that the regulatory protein SEC11 (=KEULE) binds selectively with SYP121 to affect secretory traffic mediated by this SNARE. SEC11 rescued traffic block by dominant-negative (inhibitory) fragments of both SNAREs, but only in plants expressing the native SYP121. Traffic and its rescue were sensitive to mutations affecting SEC11 interaction with the N terminus of SYP121. Furthermore, the domain of SEC11 that bound the SYP121 N terminus was itself able to block secretory traffic in the wild type and syp122 but not in syp121 mutant Arabidopsis. Thus, SEC11 binds and selectively regulates secretory traffic mediated by SYP121 and is important for recycling of the SNARE and its cognate partners.

Strategies for improving potassium use efficiency in plants

Potassium is a macronutrient that is crucial for healthy plant growth. Potassium availability, however, is often limited in agricultural fields and thus crop yields and quality are reduced. Therefore, improving the efficiency of potassium uptake and transport, as well as its utilization, in plants is important for agricultural sustainability. This review summarizes the current knowledge on the molecular mechanisms involved in potassium uptake and transport in plants, and the molecular response of plants to different levels of potassium availability. Based on this information, four strategies for improving potassium use efficiency in plants are proposed; 1) increased root volume, 2) increasing efficiency of potassium uptake from the soil and translocation in planta, 3) increasing mobility of potassium in soil, and 4) molecular breeding new varieties with greater potassium efficiency through marker assisted selection which will require identification and utilization of potassium associated quantitative trait loci.

Plant exocytic secretion of toxic compounds for defense

In contrast to animals, plants do not have a circulatory system as well as mobile immune cells that allow them to protect themselves against pathogens. Instead, plants exclusively depend on the innate immune system to defend against pathogens. As typically observed in the animal innate immunity, plant immune responses are composed of pathogen detection, defense signaling which includes transcriptional reprogramming, and secretion of antimicrobial compounds. Although knowledge on recognition and subsequent signaling of pathogen-derived molecules called elicitors is now expanding, the mechanisms of how these immune molecules are excreted are yet poorly understood. Therefore, current understandings of how plants secrete defense products especially via exocytosis will be discussed in this review.

Syntaxin 31 functions in Glycine max resistance to the plant parasitic nematode Heterodera glycines

A Glycine max syntaxin 31 homolog (Gm-SYP38) was identified as being expressed in nematode-induced feeding structures known as syncytia undergoing an incompatible interaction with the plant parasitic nematode Heterodera glycines. The observed Gm-SYP38 expression was consistent with prior gene expression analyses that identified the alpha soluble NSF attachment protein (Gm-α-SNAP) resistance gene because homologs of these genes physically interact and function together in other genetic systems. Syntaxin 31 is a protein that resides on the cis face of the Golgi apparatus and binds α-SNAP-like proteins, but has no known role in resistance. Experiments presented here show Gm-α-SNAP overexpression induces Gm-SYP38 transcription. Overexpression of Gm-SYP38 rescues G. max [Williams 82/PI 518671], genetically rhg1 (-/-), by suppressing H. glycines parasitism. In contrast, Gm-SYP38 RNAi in the rhg1 (+/+) genotype G. max [Peking/PI 548402] increases susceptibility. Gm-α-SNAP and Gm-SYP38 overexpression induce the transcriptional activity of the cytoplasmic receptor-like kinase BOTRYTIS INDUCED KINASE 1 (Gm-BIK1-6) which is a family of defense proteins known to anchor to membranes through a 5' MGXXXS/T(R) N-myristoylation sequence. Gm-BIK1-6 had been identified previously by RNA-seq experiments as expressed in syncytia undergoing an incompatible reaction. Gm-BIK1-6 overexpression rescues the resistant phenotype. In contrast, Gm-BIK1-6 RNAi increases parasitism. The analysis demonstrates a role for syntaxin 31-like genes in resistance that until now was not known.

Syntaxin of plant proteins SYP123 and SYP132 mediate root hair tip growth in Arabidopsis thaliana

Root hairs are fast-growing tubular protrusions on root epidermal cells that play important roles in water and nutrient uptake in plants. The tip-focused polarized growth of root hairs is accomplished by the secretion of newly synthesized materials to the tip via the polarized membrane trafficking mechanism. Here, we report the function of two different types of plasma membrane (PM) Qa-SNAREs (soluble N-ethylmaleimide-sensitive factor attachment protein receptors), SYP123 and SYP132, in the growth of root hair in Arabidopsis. We found that SYP123, but not SYP132, localizes in the tip region of root hairs by recycling between the brefeldin A (BFA)-sensitive endosomes and the PM of the expanding tip in an F-actin-dependent manner. The vesicle-associated membrane proteins VAMP721/722/724 also exhibited tip-focused localization in root hairs and formed ternary SNARE complexes with both SYP123 and SYP132. These results demonstrate that SYP123 and SYP132 act in a coordinated fashion to mediate tip-focused membrane trafficking for root hair tip growth.

Plasma membrane protein trafficking in plant-microbe interactions: a plant cell point of view

In order to ensure their physiological and cellular functions, plasma membrane (PM) proteins must be properly conveyed from their site of synthesis, i.e., the endoplasmic reticulum, to their final destination, the PM, through the secretory pathway. PM protein homeostasis also relies on recycling and/or degradation, two processes that are initiated by endocytosis. Vesicular membrane trafficking events to and from the PM have been shown to be altered when plant cells are exposed to mutualistic or pathogenic microbes. In this review, we will describe the fine-tune regulation of such alterations, and their consequence in PM protein activity. We will consider the formation of intracellular perimicrobial compartments, the PM protein trafficking machinery of the host, and the delivery or retrieval of signaling and transport proteins such as pattern-recognition receptors, producers of reactive oxygen species, and sugar transporters.

Transcytosis shuts the door for an unwanted guest

Penetration resistance is a well-described plant defense process, in which SOLUBLE N-ETHYLMALEIMIDE-SENSITIVE-FACTOR ATTACHMENT RECEPTOR (SNARE) proteins have essential roles in membrane fusion processes. Strong focal accumulation of these proteins at the site of attack by powdery mildew fungi has been considered important for their function. However, recent insight indicates that transcytosis, leading to the formation of exosomes, has an important role in this defense and, furthermore, that strong accumulation of these SNARE proteins with the exosomes is biologically irrelevant. These findings alter the established function of SNAREs in penetration resistance; therefore, in this opinion, we propose that PEN1 and its SNARE partners function on an endosome in their control of penetration resistance.

Host cell entry of powdery mildew is correlated with endosomal transport of antagonistically acting VvPEN1 and VvMLO to the papilla

Challenge by a nonadapted powdery mildew fungal pathogen leads to the formation of a local cell-wall apposition (papilla) beneath the point of attempted penetration. Several plasma membrane (PM) proteins with opposing roles in powdery mildew infection, including Arabidopsis thaliana PENETRATION1 (PEN1) and barley (Hordeum vulgare) MILDEW RESISTANCE LOCUS O (MLO), are localized to the site of powdery mildew attack. PEN1 contributes to penetration resistance to nonadapted powdery mildews, whereas MLO is a susceptibility factor required by adapted powdery mildew pathogens for host cell entry. Our previous studies have demonstrated that the vesicle and endosomal trafficking inhibitors, brefeldin A and wortmannin, have opposite effects on the penetration rates of adapted and nonadapted powdery mildews on grapevine. These findings prompted us to study the pathogen-induced intracellular trafficking of grapevine variants of MLO and PEN1. We first identified grapevine (Vitis vinifera) VvPEN1 and VvMLO orthologs that rescue Arabidopsis Atpen1 and Atmlo2 mlo6 mlo12 null mutants, respectively. By using endomembrane trafficking inhibitors in combination with fluorescence microscopy, we demonstrate that VvMLO3/VvMLO4 and VvPEN1 are co-trafficked together from the PM to the site of powdery mildew challenge. This focal accumulation of VvMLO3/VvMLO4 and VvPEN1 to the site of attack seems to be required for their opposing functions during powdery mildew attack, because their subcellular localization is correlated with the outcome of attempted powdery mildew penetration.

Requirement of vesicle-associated membrane protein 721 and 722 for sustained growth during immune responses in Arabidopsis

Extracellular immune responses to ascomycete and oomycete pathogens in Arabidopsis are dependent on vesicle-associated secretion mediated by the SNARE proteins PEN1 syntaxin, SNAP33 and endomembrane-resident VAMP721/722. Continuous movement of functional GFP-VAMP722 to and from the plasma membrane in non-stimulated cells reflects the second proposed function of VAMP721/722 in constitutive secretion during plant growth and development. Application of the bacterium-derived elicitor flg22 stabilizes VAMP721/722 that are otherwise constitutively degraded via the 26S proteasome pathway. Depletion of VAMP721/722 levels by reducing VAMP721/722 gene dosage enhances flg22-induced seedling growth inhibition in spite of elevated VAMP721/722 abundance. We therefore propose that plants prioritize the deployment of the corresponding secretory pathway for defense over plant growth. Interstingly, VAMP721/722 specifically interact in vitro and in vivo with the plasma membrane syntaxin SYP132 that is required for plant growth and resistance to bacteria. This suggests that the plant growth/immunity-involved VAMP721/722 form SNARE complexes with multiple plasma membrane syntaxins to discharge cue-dependent cargo molecules.

Tight interconnection and multi-level control of Arabidopsis MYB44 in MAPK cascade signalling

Abiotic stress poses a huge, ever-increasing problem to plants and agriculture. The dissection of signalling pathways mediating stress tolerance is a prerequisite to develop more resistant plant species. Mitogen-activated protein kinase (MAPK) cascades are universal signalling modules. In Arabidopsis, the MAPK MPK3 and its upstream regulator MAPK kinase MKK4 initiate the adaptation response to numerous abiotic and biotic stresses. Yet, molecular steps directly linked with MKK4-MPK3 activation are largely unknown. Starting with a yeast-two-hybrid screen for interacting partners of MKK4, we identified a transcription factor, MYB44. MYB44 is controlled at multiple levels by and strongly inter-connected with MAPK signalling. As we had shown earlier, stress-induced expression of the MYB44 gene is regulated by a MPK3-targeted bZIP transcription factor VIP1. At the protein level, MYB44 interacts with MPK3 in vivo. MYB44 is phosphorylated by MPK3 in vitro at a single residue, Ser145. Although replacement of Ser145 by a non-phosphorylatable (S145A) or phosphomimetic (S145D) residue did not alter MYB44 subcellular localisation, dimerization behaviour nor DNA-binding characteristics, abiotic stress tolerance tests in stable transgenic Arabidopsis plants clearly related S145 phosphorylation to MYB44 function: Compared to Arabidopsis wild type plants, MYB44 overexpressing lines exhibit an enhanced tolerance to osmotic stress and are slightly more sensitive to abscisic acid. Interestingly, overexpression of the S145A variant revealed that impaired phosphorylation does not render the MYB44 protein non-functional. Instead, S145A lines are highly sensitive to abiotic stress, and thereby remarkably similar to mpk3-deficient plants. Its in vivo interaction with the nuclear sub-pools of both MPK3 and MKK4 renders MYB44 the first plant transcription factor to have a second function as putative MAPK cascade scaffolding protein.

Recycling of Arabidopsis plasma membrane PEN1 syntaxin

Penetration resistance against powdery mildews is one of the best-studied processes of plant innate immunity. One vital component is the plant syntaxin, PEN1, which is required for timely deposition of callose and extracellular membrane material, as well as PEN1 itself, at the attack sites. Recently, we reported that the ARF-GEF GNOM also is required for penetration resistance, mediating transport of recycled material, including PEN1, to the site of attack. The close relative of PEN1, SYP122, does not accumulate at the sites of attack nor does it affect penetration resistance. In support of this, we show here that in contrast to PEN1, SYP122 does not continuously recycle. Furthermore, by using a PEN1 transgene that is only transcribed in dividing cells, we show that papillary PEN1 accumulation is not dependent on de-novo protein synthesis. This emphasizes the involvement of recycling in penetration resistance, which possibly relates to the differences in function of the two syntaxins.

The expression of a naturally occurring, truncated allele of an α-SNAP gene suppresses plant parasitic nematode infection

Transcriptional mapping experiments of the major soybean cyst nematode resistance locus, rhg1, identified expression of the vesicular transport machinery component, α soluble NSF attachment protein (α-SNAP), occurring during defense. Sequencing the α-SNAP coding regions from the resistant genotypes G. max ([Peking/PI 548402]) and G. max ([PI 437654]) revealed they are identical, but differ from the susceptible G. max ([Williams 82/PI 518671]) by the presence of several single nucleotide polymorphisms. Using G. max ([Williams 82/PI 518671]) as a reference, a G → T(2,822) transversion in the genomic DNA sequence at a functional splice site of the α-SNAP([Peking/PI 548402]) allele produced an additional 17 nucleotides of mRNA sequence that contains an in-frame stop codon caused by a downstream G → A(2,832) transition. The G. max ([Peking/PI 548402]) genotype has cell wall appositions (CWAs), structures identified as forming as part of a defense response by the activity of the vesicular transport machinery. In contrast, the 17 nt α-SNAP([Peking/PI 548402]) mRNA motif is not found in G. max ([PI 88788]) that exhibits defense to H. glycines, but lack CWAs. The α-SNAP([PI 88788]) promoter contains sequence elements that are nearly identical to the α-SNAP([Peking/PI 548402]) allele, but differs from the G. max ([Williams 82/PI 518671]) ortholog. Overexpressing the α-SNAP([Peking/PI 548402]) allele in the susceptible G. max ([Williams 82/PI 518671]) genotype suppressed H. glycines infection. The experiments indicate a role for the vesicular transport machinery during infection of soybean by the soybean cyst nematode. However, increased GmEREBP1, PR1, PR2, PR5 gene activity but suppressed PR3 expression accompanied the overexpression of the α-SNAP([Peking/PI 548402]) allele prior to infection.

Protocol: optimised electrophyiological analysis of intact guard cells from Arabidopsis

Genetic resources available for Arabidopsis thaliana make this species particularly attractive as a model for molecular genetic studies of guard cell homeostasis, transport and signalling, but this facility is not matched by accessible tools for quantitative analysis of transport in the intact cell. We have developed a reliable set of procedures for voltage clamp analysis of guard cells from Arabidopsis leaves. These procedures greatly simplify electrophysiological recordings, extending the duration of measurements and scope for analysis of the predominant K+ and anion channels of intact stomatal guard cells to that achieved previously in work with Vicia and tobacco guard cells.

Activation of defense against Phytophthora infestans in potato by down-regulation of syntaxin gene expression

The oomycete Phytophthora infestans is the causal agent of late blight, the most devastating disease of potato. The importance of vesicle fusion processes and callose deposition for defense of potato against Phytophthora infestans was analyzed. Transgenic plants were generated, which express RNA interference constructs targeted against plasma membrane-localized SYNTAXIN-RELATED 1 (StSYR1) and SOLUBLE N-ETHYLMALEIMIDE-SENSITIVE FACTOR ADAPTOR PROTEIN 33 (StSNAP33), the potato homologs of Arabidopsis AtSYP121 and AtSNAP33, respectively. Phenotypically, transgenic plants grew normally, but showed spontaneous necrosis and chlorosis formation at later stages. In response to infection with Phytophthora infestans, increased resistance of StSYR1-RNAi plants, but not StSNAP33-RNAi plants, was observed. This increased resistance correlated with the constitutive accumulation of salicylic acid and PR1 transcripts. Aberrant callose deposition in Phytophthora infestans-infected StSYR1-RNAi plants coincided with decreased papilla formation at penetration sites. Resistance against the necrotrophic fungus Botrytis cinerea was not significantly altered. Infiltration experiments with bacterial solutions of Agrobacterium tumefaciens and Escherichia coli revealed a hypersensitive phenotype of both types of RNAi lines. The enhanced defense status and the reduced growth of Phytophthora infestans on StSYR1-RNAi plants suggest an involvement of syntaxins in secretory defense responses of potato and, in particular, in the formation of callose-containing papillae.

Quantitative proteomics reveals dynamic changes in the plasma membrane during Arabidopsis immune signaling

The plant plasma membrane is a crucial mediator of the interaction between plants and microbes. Understanding how the plasma membrane proteome responds to diverse immune signaling events will lead to a greater understanding of plant immunity and uncover novel targets for crop improvement. Here we report the results from a large scale quantitative proteomics study of plasma membrane-enriched fractions upon activation of the Arabidopsis thaliana immune receptor RPS2. More than 2300 proteins were identified in total, with 1353 proteins reproducibly identified across multiple replications. Label-free spectral counting was employed to quantify the relative protein abundance between different treatment samples. Over 20% of up-regulated proteins have known roles in plant immune responses. Significantly changing proteins include those involved in calcium and lipid signaling, membrane transport, primary and secondary metabolism, protein phosphorylation, redox homeostasis, and vesicle trafficking. A subset of differentially regulated proteins was independently validated during bacterial infection. This study presents the largest quantitative proteomics data set of plant immunity to date and provides a framework for understanding global plasma membrane proteome dynamics during plant immune responses.

The trafficking protein SYP121 of Arabidopsis connects programmed stomatal closure and K⁺ channel activity with vegetative growth

The vesicle-trafficking protein SYP121 (SYR1/PEN1) was originally identified in association with ion channel control at the plasma membrane of stomatal guard cells, although stomata of the Arabidopsis syp121 loss-of-function mutant close normally in ABA and high Ca²⁺. We have now uncovered a set of stomatal phenotypes in the syp121 mutant that reduce CO₂ assimilation, slow vegetative growth and increase water use efficiency in the whole plant, conditional upon high light intensities and low relative humidity. Stomatal opening and the rise in stomatal transpiration of the mutant was delayed in the light and following Ca²⁺-evoked closure, consistent with a constitutive form of so-called programmed stomatal closure. Delayed reopening was observed in the syp121, but not in the syp122 mutant lacking the homologous gene product; the delay was rescued by complementation with wild-type SYP121 and was phenocopied in wild-type plants in the presence of the vesicle-trafficking inhibitor Brefeldin A. K⁺ channel current that normally mediates K⁺ uptake for stomatal opening was suppressed in the syp121 mutant and, following closure, its recovery was slowed compared to guard cells of wild-type plants. Evoked stomatal closure was accompanied by internalisation of GFP-tagged KAT1 K⁺ channels in both wild-type and syp121 mutant guard cells, but their subsequently recycling was slowed in the mutant. Our findings indicate that SYP121 facilitates stomatal reopening and they suggest that K⁺ channel traffic and recycling to the plasma membrane underpins the stress memory phenomenon of programmed closure in stomata. Additionally, they underline the significance of vesicle traffic for whole-plant water use and biomass production, tying SYP121 function to guard cell membrane transport and stomatal control.

Cell biology of the plant-powdery mildew interaction

Powdery mildew fungi represent a paradigm for obligate biotrophic parasites, which only propagate in long-lasting intimate interactions with living host cells. These highly specialized phytopathogens induce re-organization of host cell architecture and physiology for their own demands. This probably includes the corruption of basal host cellular functions for successful fungal pathogenesis. Recent studies revealed secretory processes by both interaction partners as key incidents of the combat at the plant-fungus interface. The analysis of cellular events during plant-powdery mildew interactions may not only lead to a better understanding of plant pathological features, but may also foster novel discoveries in the area of plant cell biology.

High-throughput confocal imaging of intact live tissue enables quantification of membrane trafficking in Arabidopsis

Membrane compartmentalization and trafficking within and between cells is considered an essential cellular property of higher eukaryotes. We established a high-throughput imaging method suitable for the quantitative detection of membrane compartments at subcellular resolution in intact epidermal tissue. Whole Arabidopsis (Arabidopsis thaliana) cotyledon leaves were subjected to quantitative confocal laser microscopy using automated image acquisition, computational pattern recognition, and quantification of membrane compartments. This revealed that our method is sensitive and reliable to detect distinct endomembrane compartments. We applied quantitative confocal laser microscopy to a transgenic line expressing GFP-2xFYVE as a marker for endosomal compartments during biotic or abiotic stresses, and detected markedly quantitative adaptations in response to changing environments. Using a transgenic line expressing the plasma membrane-resident syntaxin GFP-PEN1, we quantified the pathogen-inducible extracellular accumulation of this fusion protein at fungal entry sites. Our protocol provides a platform to study the quantitative and dynamic changes of endomembrane trafficking, and potential adaptations of this machinery to physiological stress.

Expression, localization and interaction of SNARE proteins in Arabidopsis are selectively altered by the dark

A method to detect binary interactions among SNAREs, membrane proteins mediating vesicle fusion, in Arabidopsis cells was established. In this method, a pair of recombinant SNAREs is first expressed within Arabidopsis protoplasts at levels similar to their endogenous proteins in 96-well plates. Changes of the interaction are then detected by luminescence. Here, we report that the interaction of SYP122 and VAMP721, a SNARE pair mediating exocytosis, is enhanced when Arabidopsis protoplasts are incubated in the dark. Microscopic observation of plants expressing GFP-SYP122 by the syp122 promoter suggests SYP122 is expressed in the root tip when the seedlings are grown in the dark but not in the light. In the identical dark-grown condition, the subcellular localization of SYP111/KNOLLE, specifically expressed in dividing cells, is altered. Together with our previous report, we hypothesize that expression, localization, and interaction of SNAREs are selectively altered by light conditions to regulate cargo transports in Arabidopsis.

A ubiquitin-10 promoter-based vector set for fluorescent protein tagging facilitates temporal stability and native protein distribution in transient and stable expression studies

Fluorescent tagging of proteins and confocal imaging techniques have become methods of choice in analysing the distributions and dynamic characteristics of proteins at the subcellular level. In common use are a number of strategies for transient expression that greatly reduce the preparation time in advance of imaging, but their applications are limited in success outside a few tractable species and tissues. We previously developed a simple method to transiently express fluorescently-tagged proteins in Arabidopsis root epidermis and root hairs. We describe here a set of Gateway-compatable vectors with fluorescent tags incorporating the ubiqutin-10 gene promoter (P(UBQ10) ) of Arabidopsis that gives prolonged expression of the fluorescently-tagged proteins, both in tobacco and Arabidopsis tissues, after transient transformation, and is equally useful in generating stably transformed lines. As a proof of principle, we carried out transformations with fluorescent markers for the integral plasma membrane protein SYP121, a member of the SNARE family of vesicle-trafficking proteins, and for DHAR1, a cytosolic protein that facilitates the scavenging of reactive oxygen species. We also carried out transformations with SYP121 and its interacting partner, the KC1 K(+) channel, to demonstrate the utility of the methods in bimolecular fluorescence complementation (BiFC). Transient transformations of Arabidopsis using Agrobacterium co-cultivation methods yielded expression in all epidermal cells, including root hairs and guard cells. Comparative studies showed that the P(UBQ10) promoter gives similar levels of expression to that driven by the native SYP121 promoter, faithfully reproducing the characteristics of protein distributions at the subcellular level. Unlike the 35S-driven construct, expression under the P(UBQ10) promoter remained elevated for periods in excess of 2 weeks after transient transformation. This toolbox of vectors and fluorescent tags promises significant advantages for the study of membrane dynamics and cellular development, as well as events associated with environmental stimuli in guard cells and nutrient acquisition in roots.

A novel motif essential for SNARE interaction with the K(+) channel KC1 and channel gating in Arabidopsis

The SNARE (for soluble N-ethylmaleimide-sensitive factor protein attachment protein receptor) protein SYP121 (=SYR1/PEN1) of Arabidopsis thaliana facilitates vesicle traffic, delivering ion channels and other cargo to the plasma membrane, and contributing to plant cell expansion and defense. Recently, we reported that SYP121 also interacts directly with the K(+) channel subunit KC1 and forms a tripartite complex with a second K(+) channel subunit, AKT1, to control channel gating and K(+) transport. Here, we report isolating a minimal sequence motif of SYP121 prerequisite for its interaction with KC1. We made use of yeast mating-based split-ubiquitin and in vivo bimolecular fluorescence complementation assays for protein-protein interaction and of expression and electrophysiological analysis. The results show that interaction of SYP121 with KC1 is associated with a novel FxRF motif uniquely situated within the first 12 residues of the SNARE sequence, that this motif is the minimal requirement for SNARE-dependent alterations in K(+) channel gating when heterologously expressed, and that rescue of KC1-associated K(+) current of the root epidermis in syp121 mutant Arabidopsis plants depends on expression of SNARE constructs incorporating this motif. These results establish the FxRF sequence as a previously unidentified motif required for SNARE-ion channel interactions and lead us to suggest a mechanistic framework for understanding the coordination of vesicle traffic with transmembrane ion transport.

Entry mode-dependent function of an indole glucosinolate pathway in Arabidopsis for nonhost resistance against anthracnose pathogens

When faced with nonadapted fungal pathogens, Arabidopsis thaliana mounts nonhost resistance responses, which typically result in the termination of early pathogenesis steps. We report that nonadapted anthracnose fungi engage two alternative entry modes during pathogenesis on leaves: turgor-mediated invasion beneath melanized appressoria, and a previously undiscovered hyphal tip-based entry (HTE) that is independent of appressorium formation. The frequency of HTE is positively regulated by carbohydrate nutrients and appears to be subject to constitutive inhibition by the fungal mitogen-activated protein kinase (MAPK) cascade of MAPK ESSENTIAL FOR APPRESSORIUM FORMATION1. The same MAPK cascade is essential for appressorium formation. Unexpectedly, the Arabidopsis indole glucosinolate pathway restricts entry of the nonadapted anthracnose fungi only when these pathogens employ HTE. Arabidopsis mutants defective in indole glucosinolate biosynthesis or metabolism support the initiation of postinvasion growth of nonadapted Colletotrichum gloeosporioides and Colletotrichum orbiculare. However, genetic disruption of Colletotrichum appressorium formation does not permit HTE on host plants. Thus, Colletotrichum appressoria play a critical role in the suppression of preinvasion plant defenses, in addition to their previously described role in turgor-mediated plant cell invasion. We also show that HTE is the predominant morphogenetic response of Colletotrichum at wound sites. This implies the existence of a fungal sensing system to trigger appropriate morphogenetic responses during pathogenesis at wound sites and on intact leaf tissue.

Combined bimolecular fluorescence complementation and Forster resonance energy transfer reveals ternary SNARE complex formation in living plant cells

Various fluorophore-based microscopic methods, comprising Förster resonance energy transfer (FRET) and bimolecular fluorescence complementation (BiFC), are suitable to study pairwise interactions of proteins in living cells. The analysis of interactions between more than two protein partners using these methods, however, remains difficult. In this study, we report the successful application of combined BiFC-FRET-fluorescence lifetime imaging microscopy and BiFC-FRET-acceptor photobleaching measurements to visualize the formation of ternary soluble N-ethylmaleimide-sensitive factor attachment receptor complexes in leaf epidermal cells. This method expands the repertoire of techniques to study protein-protein interactions in living plant cells by a procedure capable of visualizing simultaneously interactions between three fluorophore-tagged polypeptide partners.

Lipopolysaccharides and plant innate immunity

Plants posses an innate immune system that has many parallels with those found in mammals and insects. A range of molecules of microbial origin called Microbe Associated Molecular Patterns (MAMPs) act to trigger basal defense responses in plants. These elicitors include lipopolysaccharides (LPS) from diverse Gram-negative bacteria. Both core oligosaccharide and the lipid A moieties of LPS as well as synthetic O-antigen oligosaccharides have activity in inducing defense responses in the model plant Arabidopsis thaliana. Very little is known of the mechanism of LPS perception by plants, although plant receptors for other MAMPs such as flagellin have been described. Recent work has implicated the Arabidopsis syntaxin PEN1 as a potential actor in LPS induction of plant defenses, which may suggest a role for vesicle trafficking in the signalling process.

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

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

[Plant SNAREs and their biological functions]

The signal communication between various organelles is essential for cells of eukaryotic organisms. Vesicle trafficking is an important pathway for this kind of communication. Most of the membrane fusion is mediated by SNAREs (Soluble N-ethyl-maleimide-sensitive fusion protein attachment protein receptors), which are highly conserved from various species. Compared with genomes of other eukaryotes, plant genome encodes an even higher number of SNAREs. Accumulating evidences support that plant SNAREs is a multifunctional protein family, which is involved in variety of biological processes. We review the recent advances on molecular mechanism and biological functions of plant SNAREs.

Extracellular transport and integration of plant secretory proteins into pathogen-induced cell wall compartments

Many fungal parasites enter plant cells by penetrating the host cell wall and, thereafter, differentiate specialized intracellular feeding structures, called haustoria, by invagination of the plant's plasma membrane. Arabidopsis PEN gene products are known to act at the cell periphery and function in the execution of apoplastic immune responses to limit fungal entry. This response underneath fungal contact sites is tightly linked with the deposition of plant cell wall polymers, including PMR4/GSL5-dependent callose, in the paramural space, thereby producing localized wall thickenings called papillae. We show that powdery mildew fungi specifically induce the extracellular transport and entrapment of the fusion protein GFP-PEN1 syntaxin and its interacting partner monomeric yellow fluorescent protein (mYFP)-SNAP33 within the papillary matrix. Remarkably, PMR4/GSL5 callose, GFP-PEN1, mYFP-SNAP33, and the ABC transporter GFP-PEN3 are selectively incorporated into extracellular encasements surrounding haustoria of the powdery mildew Golovinomyces orontii, suggesting that the same secretory defense responses become activated during the formation of papillae and haustorial encasements. This is consistent with a time-course analysis of the encasement process, indicating that these extracellular structures are generated through the extension of papillae. We show that PMR4/GSL5 callose accumulation in papillae and haustorial encasements occurs independently of PEN1 syntaxin. We propose a model in which exosome biogenesis/release serves as a common transport mechanism by which the proteins PEN1 and PEN3, otherwise resident in the plasma membrane, together with membrane lipids, become stably incorporated into both pathogen-induced cell wall compartments.