Recruitment and interaction dynamics of plant penetration resistance components in a plasma membrane microdomain

The Role and Regulation of Receptor-Like Kinases in Plant Defense

Receptor-like kinases (RLKs) in plants are a large superfamily of proteins that are structurally similar. RLKs are involved in a diverse array of plant responses including development, growth, hormone perception and the response to pathogens. Current studies have focused attention on plant receptor-like kinases as an important class of sentinels acting in plant defense responses. RLKs have been identified that act in both broad-spectrum, elicitor-initiated defense responses and as dominant resistance (R) genes in race-specific pathogen defense. Most defense-related RLKs are of the leucine-rich repeat (LRR) subclass although new data are highlighting other classes of RLKs as important players in defense responses. As our understanding of RLK structure, activation and signaling has expanded, the role of the ubiquitin/proteasome system in the regulation of these receptors has emerged as a central theme.

Cell type-dependent localization of MLO proteins

Mildew resistance locus O (MLO) proteins are transmembrane proteins that mediate cell-cell communication in plants. We recently demonstrated the importance of subcellular localization to MLO function during pollen tube reception. NORTIA (NTA), the MLO protein involved in this process, localizes to the Golgi of the synergid cell before interaction with the pollen tube. MLO proteins that can substitute for NTA's function in this pathway all partially localize with the same Golgi marker in the synergid cell. In this study, we report that MLO subcellular localization is cell type-dependent, with different distributions of some MLOs observed when ectopically expressed in the epidermal cells of tobacco and Arabidopsis compared to synergids. This dependency may be due to co-factors that influence MLO function within a given cell type, providing an intriguing new target for understanding MLO distribution and subsequent function in their respective processes.

MILDEW RESISTANCE LOCUS O Function in Pollen Tube Reception Is Linked to Its Oligomerization and Subcellular Distribution

Sexual reproduction in flowering plants requires communication between synergid cells and a tip-elongating pollen tube (PT) for the successful delivery of sperm cells to the embryo sac. The reception of the PT relies on signaling within the synergid cell that ultimately leads to the degeneration of the receptive synergid and PT rupture, releasing the sperm cells for double fertilization. In Arabidopsis (Arabidopsis thaliana), NORTIA, a member of the MILDEW RESISTANCE LOCUS O (MLO) family of proteins, plays a critical role in the communication processes regulating PT reception. In this study, we determined that MLO function in PT reception is dependent on MLO protein localization into a Golgi-associated compartment before PT arrival, indicating that PT-triggered regulation of the synergid secretory system is important for synergid function during pollination. Additionally, a structure-function analysis revealed that MLO homooligomerization, mediated by the amino-terminal region of the protein, and carboxyl-terminal tail identity both contribute to MLO activity during PT reception.

Exocytosis for endosymbiosis: membrane trafficking pathways for development of symbiotic membrane compartments

During endosymbiosis with arbuscular mycorrhizal fungi or rhizobial bacteria, the microbial symbionts are housed within membrane-bound compartments in root cortex or nodule cells respectively. Their development involves polarized deposition of membrane around the symbionts as they enter the cells and the membranes show functional specialization, including transporters that mediate nutrient transfer between host and symbiont. The cellular changes associated with development of these compartments point to membrane deposition via exocytosis and over the past few years, researchers have uncovered several proteins within the exocytotic pathway that are required for development of endosymbiotic membrane compartments. The emerging theme is that unique membrane trafficking homologs or splice variants have evolved to enable exocytosis during endosymbiosis.

RUN1 and REN1 Pyramiding in Grapevine (Vitis vinifera cv. Crimson Seedless) Displays an Improved Defense Response Leading to Enhanced Resistance to Powdery Mildew (Erysiphe necator)

Fungal pathogens are the cause of the most common diseases in grapevine and among them powdery mildew represents a major focus for disease management. Different strategies for introgression of resistance in grapevine are currently undertaken in breeding programs. For example, introgression of several resistance genes (R) from different sources for making it more durable and also strengthening the plant defense response. Taking this into account, we cross-pollinated P09-105/34, a grapevine plant carrying both RUN1 and REN1 pyramided loci of resistance to Erysiphe necator inherited from a pseudo-backcrossing scheme with Muscadinia rotundifolia and Vitis vinifera 'Dzhandzhal Kara,' respectively, with the susceptible commercial table grape cv. 'Crimson Seedless.' We developed RUN1REN1 resistant genotypes through conventional breeding and identified them by marker assisted selection. The characterization of defense response showed a highly effective defense mechanism against powdery mildew in these plants. Our results reveal that RUN1REN1 grapevine plants display a robust defense response against E. necator, leading to unsuccessful fungal establishment with low penetration rate and poor hypha development. This resistance mechanism includes reactive oxygen species production, callose accumulation, programmed cell death induction and mainly VvSTS36 and VvPEN1 gene activation. RUN1REN1 plants have a great potential as new table grape cultivars with durable complete resistance to E. necator, and are valuable germplasm to be included in grape breeding programs to continue pyramiding with other sources of resistance to grapevine diseases.

Guilt by Association: A Phenotype-Based View of the Plant Phosphoinositide Network

Eukaryotic membranes contain small amounts of phospholipids that have regulatory effects on the physiological functions of cells, tissues, and organs. Phosphoinositides (PIs)-the phosphorylated derivatives of phosphatidylinositol-are one example of such regulatory lipids. Although PIs were described in plants decades ago, their contribution to the regulation of physiological processes in plants is not well understood. In the past few years, evidence has emerged that PIs are essential for plant function and development. Recently reported phenotypes associated with the perturbation of different PIs suggest that some subgroups of PIs influence specific processes. Although the molecular targets of PI-dependent regulation in plants are largely unknown, the effects of perturbed PI metabolism can be used to propose regulatory modules that involve particular downstream targets of PI regulation. This review summarizes phenotypes associated with the perturbation of the plant PI network to categorize functions and suggest possible downstream targets of plant PI regulation.

The conformational state of hERG1 channels determines integrin association, downstream signaling, and cancer progression

Ion channels regulate cell proliferation, differentiation, and migration in normal and neoplastic cells through cell-cell and cell-extracellular matrix (ECM) transmembrane receptors called integrins. K⁺ flux through the human ether-à-go-go-related gene 1 (hERG1) channel shapes action potential firing in excitable cells such as cardiomyocytes. Its abundance is often aberrantly high in tumors, where it modulates integrin-mediated signaling. We found that hERG1 interacted with the β₁ integrin subunit at the plasma membrane of human cancer cells. This interaction was not detected in cardiomyocytes because of the presence of the hERG1 auxiliary subunit KCNE1 (potassium voltage-gated channel subfamily E regulatory subunit 1), which blocked the β₁ integrin-hERG1 interaction. Although open hERG1 channels did not interact as strongly with β₁ integrins as did closed channels, current flow through hERG1 channels was necessary to activate the integrin-dependent phosphorylation of Tyr³⁹⁷ in focal adhesion kinase (FAK) in both normal and cancer cells. In immunodeficient mice, proliferation was inhibited in breast cancer cells expressing forms of hERG1 with impaired K⁺ flow, whereas metastasis of breast cancer cells was reduced when the hERG1/β₁ integrin interaction was disrupted. We conclude that the interaction of β₁ integrins with hERG1 channels in cancer cells stimulated distinct signaling pathways that depended on the conformational state of hERG1 and affected different aspects of tumor progression.

mlo-Based Resistance: An Apparently Universal "Weapon" to Defeat Powdery Mildew Disease

Loss-of-function mutations of one or more of the appropriate Mildew resistance locus o (Mlo) genes are an apparently reliable "weapon" to protect plants from infection by powdery mildew fungi, as they confer durable broad-spectrum resistance. Originally detected as a natural mutation in an Ethiopian barley landrace, this so-called mlo-based resistance has been successfully employed in European barley agriculture for nearly four decades. More recently, mlo-mediated resistance was discovered to be inducible in virtually every plant species of economic or scientific relevance. By now, mlo resistance has been found (as natural mutants) or generated (by induced mutagenesis, gene silencing, and targeted or nontargeted gene knock-out) in a broad range of monocotyledonous and dicotyledonous plant species. Here, we review features of mlo resistance in barley, discuss approaches to identify the appropriate Mlo gene targets to induce mlo-based resistance, and consider the issue of pleiotropic effects often associated with mlo-mediated immunity, which can harm plant yield and quality. We portray mlo-based resistance as an apparently universal and effective weapon to defeat powdery mildew disease in a multitude of plant species.

Why cellular communication during plant reproduction is particularly mediated by CRP signalling

Secreted cysteine-rich peptides (CRPs) represent one of the main classes of signalling peptides in plants. Whereas post-translationally modified small non-CRP peptides (psNCRPs) are mostly involved in signalling events during vegetative development and interactions with the environment, CRPs are overrepresented in reproductive processes including pollen germination and growth, self-incompatibility, gamete activation and fusion as well as seed development. In this opinion paper we compare the involvement of both types of peptides in vegetative and reproductive phases of the plant lifecycle. Besides their conserved cysteine pattern defining structural features, CRPs exhibit hypervariable primary sequences and a rapid evolution rate. As a result, CRPs represent a pool of highly polymorphic signalling peptides involved in species-specific functions during reproduction and thus likely represent key players to trigger speciation in plants by supporting reproductive isolation. In contrast, precursers of psNCRPs are proteolytically processed into small functional domains with high sequence conservation and act in more general processes. We discuss parallels in downstream processes of CRP signalling in both reproduction and defence against pathogenic fungi and alien pollen tubes, with special emphasis on the role of ROS and ion channels. In conclusion we suggest that CRP signalling during reproduction in plants has evolved from ancient defence mechanisms.

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

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

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.

Lipids in plant-microbe interactions

Bacteria and fungi can undergo symbiotic or pathogenic interactions with plants. Membrane lipids and lipid-derived molecules from the plant or the microbial organism play important roles during the infection process. For example, lipids (phospholipids, glycolipids, sphingolipids, sterol lipids) are involved in establishing the membrane interface between the two organisms. Furthermore, lipid-derived molecules are crucial for intracellular signaling in the plant cell, and lipids serve as signals during plant-microbial communication. These signal lipids include phosphatidic acid, diacylglycerol, lysophospholipids, and free fatty acids derived from phospholipase activity, apocarotenoids, and sphingolipid breakdown products such as ceramide, ceramide-phosphate, long chain base, and long chain base-phosphate. Fatty acids are the precursors for oxylipins, including jasmonic acid, and for azelaic acid, which together with glycerol-3-phosphate are crucial for the regulation of systemic acquired resistance. This article is part of a Special Issue titled "Plant Lipid Biology," guest editors Kent Chapman and Ivo Feussner.

Observation of plant-pathogen interaction by simultaneous hyperspectral imaging reflection and transmission measurements

Hyperspectral imaging sensors are valuable tools for plant disease detection and plant phenotyping. Reflectance properties are influenced by plant pathogens and resistance responses, but changes of transmission characteristics of plants are less described. In this study we used simultaneously recorded reflectance and transmittance imaging data of resistant and susceptible barley genotypes that were inoculated with Blumeria graminis f. sp. hordei to evaluate the added value of imaging transmission, reflection and absorption for characterisation of disease development. These datasets were statistically analysed using principal component analysis, and compared with visual and molecular disease estimation. Reflection measurement performed significantly better for early detection of powdery mildew infection, colonies could be detected 2 days before symptoms became visible in RGB images. Transmission data could be used to detect powdery mildew 2 days after symptoms becoming visible in reflection based RGB images. Additionally distinct transmission changes occurred at 580-650nm for pixels containing disease symptoms. It could be shown that the additional information of the transmission data allows for a clearer spatial differentiation and localisation between powdery mildew symptoms and necrotic tissue on the leaf then purely reflectance based data. Thus the information of both measurement approaches are complementary: reflectance based measurements facilitate an early detection, and transmission measurements provide additional information to better understand and quantify the complex spatio-temporal dynamics of plant-pathogen interactions.

Biotrophy at Its Best: Novel Findings and Unsolved Mysteries of the Arabidopsis-Powdery Mildew Pathosystem

It is generally accepted in plant-microbe interactions research that disease is the exception rather than a common outcome of pathogen attack. However, in nature, plants with symptoms that signify colonization by obligate biotrophic powdery mildew fungi are omnipresent. The pervasiveness of the disease and the fact that many economically important plants are prone to infection by powdery mildew fungi drives research on this interaction. The competence of powdery mildew fungi to establish and maintain true biotrophic relationships renders the interaction a paramount example of a pathogenic plant-microbe biotrophy. However, molecular details underlying the interaction are in many respects still a mystery. Since its introduction in 1990, the Arabidopsis-powdery mildew pathosystem has become a popular model to study molecular processes governing powdery mildew infection. Due to the many advantages that the host Arabidopsis offers in terms of molecular and genetic tools this pathosystem has great capacity to answer some of the questions of how biotrophic pathogens overcome plant defense and establish a persistent interaction that nourishes the invader while in parallel maintaining viability of the plant host.

Current understanding of grapevine defense mechanisms against the biotrophic fungus (Erysiphe necator), the causal agent of powdery mildew disease

The most economically important disease of cultivated grapevines worldwide is powdery mildew (PM) caused by the ascomycete fungus Erysiphe necator. The majority of grapevine cultivars used for wine, table grape, and dried fruit production are derived from the Eurasian grape species Vitis vinifera because of its superior aroma and flavor characteristics. However, this species has little genetic resistance against E. necator meaning that grape production is highly dependent on the frequent use of fungicides. The integration of effective genetic resistance into cultivated grapevines would lead to significant financial and environmental benefits and represents a major challenge for viticultural industries and researchers worldwide. This review will outline the strategies being used to increase our understanding of the molecular basis of V. vinifera susceptibility to this fungal pathogen. It will summarize our current knowledge of different resistance loci/genes that have evolved in wild grapevine species to restrict PM infection and assess the potential application of these defense genes in the generation of PM-resistant grapevine germplasm. Finally, it addresses future research priorities which will be important in the rapid identification, evaluation, and deployment of new PM resistance genes which are capable of conferring effective and durable resistance in the vineyard.

AIR12, a b-type cytochrome of the plasma membrane of Arabidopsis thaliana is a negative regulator of resistance against Botrytis cinerea

AIR12 (Auxin Induced in Root culture) is a single gene of Arabidopsis that codes for a mono-heme cytochrome b. Recombinant AIR12 from Arabidopsis accepted electrons from ascorbate or superoxide, and donated electrons to either monodehydroascorbate or oxygen. AIR12 was found associated in vivo to the plasma membrane. Though linked to the membrane by a glycophosphatidylinositol anchor, AIR12 is a hydrophilic and glycosylated protein predicted to be fully exposed to the apoplast. The expression pattern of AIR12 in Arabidopsis is developmentally regulated and correlated to sites of controlled cell separation (e.g. micropilar endosperm during germination, epidermal cells surrounding the emerging lateral root) and cells around wounds. Arabidopsis (Landsberg erecta-0) mutants with altered levels of AIR12 did not show any obvious phenotype. However, AIR12-overexpressing plants accumulated ROS (superoxide, hydrogen peroxide) and lipid peroxides in leaves, indicating that AIR12 may alter the redox state of the apoplast under particular conditions. On the other hand, AIR12-knock out plants displayed a strongly decreased susceptibility to Botrytis cinerea infection, which in turn induced AIR12 expression in susceptible wild type plants. Altogether, the results suggest that AIR12 plays a role in the regulation of the apoplastic redox state and in the response to necrotrophic pathogens. Possible relationships between these functions are discussed.

Investigating protein-protein interactions in the plant endomembrane system using multiphoton-induced FRET-FLIM

Real-time noninvasive fluorescence-based protein assays enable a direct access to study interactions in their natural environment and hence overcome the limitations of other methods that rely on invasive cell disruption techniques. The determination of Förster resonance energy transfer (FRET) by means of fluorescence lifetime imaging microscopy (FLIM) is currently the most advanced method to observe protein-protein interactions at nanometer resolution inside single living cells and in real-time. In the FRET-FLIM approach, the information gained using steady-state FRET between interacting proteins is considerably improved by monitoring changes in the excited-state lifetime of the donor fluorophore where its quenching in the presence of the acceptor is evidence for a direct physical interaction. The combination of confocal laser scanning microscopy with the sensitive advanced technique of time-correlated single photon counting allows the mapping of the spatial distribution of fluorescence lifetimes inside living cells on a pixel-by-pixel basis that is the same as the fluorescence image. Moreover, the use of multiphoton excitation particularly for plant cells provides further advantages such as reduced phototoxicity and photobleaching. In this protocol, we briefly describe the instrumentation and experimental design to study protein interactions within the plant endomembrane system, with a focus on the imaging of plant cells expressing fluorescent proteins and acquisition and analysis of fluorescence lifetime resolved data.

The song of lipids and proteins: dynamic lipid-protein interfaces in the regulation of plant cell polarity at different scales

Successful establishment and maintenance of cell polarity is crucial for many aspects of plant development, cellular morphogenesis, response to pathogen attack, and reproduction. Polar cell growth depends on integrating membrane and cell-wall dynamics with signal transduction pathways, changes in ion membrane transport, and regulation of vectorial vesicle trafficking and the dynamic actin cytoskeleton. In this review, we address the critical importance of protein-membrane crosstalk in the determination of plant cell polarity and summarize the role of membrane lipids, particularly minor acidic phospholipids, in regulation of the membrane traffic. We focus on the protein-membrane interface dynamics and discuss the current state of knowledge on three partially overlapping levels of descriptions. Finally, due to their multiscale and interdisciplinary nature, we stress the crucial importance of combining different strategies ranging from microscopic methods to computational modelling in protein-membrane studies.

ENHANCED DISEASE RESISTANCE4 associates with CLATHRIN HEAVY CHAIN2 and modulates plant immunity by regulating relocation of EDR1 in Arabidopsis

Obligate biotrophs, such as the powdery mildew pathogens, deliver effectors to the host cell and obtain nutrients from the infection site. The interface between the plant host and the biotrophic pathogen thus represents a major battleground for plant-pathogen interactions. Increasing evidence shows that cellular trafficking plays an important role in plant immunity. Here, we report that Arabidopsis thaliana ENHANCED DISEASE RESISTANCE4 (EDR4) plays a negative role in resistance to powdery mildew and that the enhanced disease resistance in edr4 mutants requires salicylic acid signaling. EDR4 mainly localizes to the plasma membrane and endosomal compartments. Genetic analyses show that EDR4 and EDR1 function in the same genetic pathway. EDR1 and EDR4 accumulate at the penetration site of powdery mildew infection, and EDR4 physically interacts with EDR1, recruiting EDR1 to the fungal penetration site. In addition, EDR4 interacts with CLATHRIN HEAVY CHAIN2 (CHC2), and edr4 mutants show reduced endocytosis rates. Taken together, our data indicate that EDR4 associates with CHC2 and modulates plant immunity by regulating the relocation of EDR1 in Arabidopsis.

A cellular backline: specialization of host membranes for defence

In plant-pathogen interactions, the host plasma membrane serves as a defence front for pathogens that invade from the extracellular environment. As such, the lipid bilayer acts as a scaffold that targets and delivers defence responses to the site of attack. During pathogen infection, numerous changes in plasma membrane composition, organization, and structure occur. There is increasing evidence that this facilitates the execution of a variety of responses, highlighting the regulatory role membranes play in cellular responses. Membrane microdomains such as lipid rafts are hypothesized to create signalling platforms for receptor signalling in response to pathogen perception and for callose synthesis. Further, the genesis of pathogen-associated structures such as papillae and the extra-haustorial membrane necessitates polarization of membranes and membrane trafficking pathways. Unlocking the mechanisms by which this occurs will enable greater understanding of how targeted defences, some of which result in resistance, are executed. This review will survey some of the changes that occur in host membranes during pathogen attack and how these are associated with the generation of defence responses.

Membrane nanodomains in plants: capturing form, function, and movement

The plasma membrane is the interface between the cell and the external environment. Plasma membrane lipids provide scaffolds for proteins and protein complexes that are involved in cell to cell communication, signal transduction, immune responses, and transport of small molecules. In animals, fungi, and plants, a substantial subset of these plasma membrane proteins function within ordered sterol- and sphingolipid-rich nanodomains. High-resolution microscopy, lipid dyes, pharmacological inhibitors of lipid biosynthesis, and lipid biosynthetic mutants have been employed to examine the relationship between the lipid environment and protein activity in plants. They have also been used to identify proteins associated with nanodomains and the pathways by which nanodomain-associated proteins are trafficked to their plasma membrane destinations. These studies suggest that plant membrane nanodomains function in a context-specific manner, analogous to similar structures in animals and fungi. In addition to the highly conserved flotillin and remorin markers, some members of the B and G subclasses of ATP binding cassette transporters have emerged as functional markers for plant nanodomains. Further, the glycophosphatidylinositol-anchored fasciclin-like arabinogalactan proteins, that are often associated with detergent-resistant membranes, appear also to have a functional role in membrane nanodomains.

Across the great divide: the plant cell surface continuum

The plant cell wall, plasma membrane and cytoskeleton exist as a cell surface continuum. This interconnection of organelles forms the interface between the plant cell and the external environment and is important for detecting the presence of a diverse range of stimuli. A plethora of plasma membrane microdomains with putative roles in membrane localized enzymatic or signalling processes have been described. While regulation of cell wall composition is defined by proteins within the plasma membrane, the cell wall has been shown to have an anchoring role on plasma membrane proteins which affects their lateral mobility. This interplay between plasma membrane and cell wall components is necessary for plasma membrane microdomain function. Actin and microtubule cytoskeletons are also involved in maintenance and function of the cell surface continuum. Investigation of the interactions between organellar components of this mechanism are important if we are to understand how cells respond to external signals.

Physiological characterization and genetic modifiers of aberrant root thigmomorphogenesis in mutants of Arabidopsis thaliana MILDEW LOCUS O genes

Root architecture and growth patterns are plant features that are still poorly understood. When grown under in vitro conditions, seedlings with mutations in Arabidopsis thaliana genes MLO4 or MLO11 exhibit aberrant root growth patterns upon contact with hard surfaces, exemplified as tight root spirals. We used a set of physiological assays and genetic tools to characterize this thigmomorphogenic defect in detail. We observed that the mlo4/mlo11-associated root curling phenotype is not recapitulated in a set of mutants with altered root growth patterns or architecture. We further found that mlo4/mlo11-conditioned root curling is not dependent upon light and endogenous flavonoids, but is pH-sensitive and affected by exogenous calcium levels. Based upon the latter two characteristics, mlo4-associated root coiling appears to be mechanistically different from the natural strong root curvature of the Arabidopsis ecotype Landsberg erecta. Gravistimulation reversibly overrides the aberrant thigmomorphogenesis of mlo4 seedlings. Mutants with dominant negative defects in α-tubulin modulate the extent and directionality of mlo4/mlo11-conditioned root coils, whereas mutants defective in polar auxin transport (axr4, aux1) or gravitropism (pgm1) completely suppress the mlo4 root curling phenotype. Our data implicate a joint contribution of calcium signalling, pH regulation, microtubular function, polar auxin transport and gravitropism in root thigmomorphogenesis.

OsMLO12, encoding seven transmembrane proteins, is involved with pollen hydration in rice

Hydration is the first step in pollen germination. However, the process is not well understood. OsMLO12 is highly expressed in mature pollen grains; plants containing alleles caused by transfer DNA insertions do not produce homozygous progeny. Reciprocal crosses between wild-type and OsMLO12/osmlo12 plants showed that the mutant alleles were not transmitted through the male gametophyte. Microscopic observations revealed that, although mutant grains became mature pollen with three nuclei, they did not germinate in vitro or in vivo due to a failure in hydration. The OsMLO12 protein has seven transmembrane motifs, with an N-terminal extracellular region and a C-terminal cytosolic region. We demonstrated that the C-terminal region mediates a calcium-dependent interaction with calmodulin. Our findings suggest that pollen hydration is regulated by MLO12, possibly through an interaction with calmodulin in the cytosol.

Magical mystery tour: MLO proteins in plant immunity and beyond

Stable heritable restriction of the ubiquitous powdery mildew disease is a desirable trait for agri and horticulture. In barley (Hordeum vulgare), loss-of-function mutant alleles of the Mildew resistance locus o (Mlo) gene confer broad-spectrum resistance to almost all known isolates of the fungal barley powdery mildew pathogen, Blumeria graminis f.sp. hordei. Despite extensive cultivation of barley mlo genotypes, mlo resistance has been durable in the field. Mlo genes are present as small families in the genomes of all higher plant species. The presumed negative regulatory role of particular members in plant immunity is evolutionarily conserved, as powdery mildew resistant mlo mutants have also been described in Arabidopsis thaliana, tomato(Solanum lycopersicum) and pea (Pisum sativum). Barley Mlo encodes a plasma membrane-localized seven-transmembrane domain protein of unknown biochemical activity. Here, we review the known requirements for mlo-mediated disease resistance in barley and Arabidopsis and reflect current views regarding Mlo function. We discuss additional mlo mutant phenotypes recently discovered in Arabidopsis and present a meta-analysis of the phylogenetic relationships within the Mlo family. Finally, we consider the novel versatile tools for functional analysis and targeted genome modification that can be used to induce mlo-based powdery mildew resistance in virtually any plant species.

Direct purification of detergent-insoluble membranes from Medicago truncatula root microsomes: comparison between floatation and sedimentation

BACKGROUND

Membrane microdomains are defined as highly dynamic, sterol- and sphingolipid-enriched domains that resist to solubilization by non-ionic detergents. In plants, these so-called Detergent Insoluble Membrane (DIM) fractions have been isolated from plasma membrane by using conventional ultracentrifugation on density gradient (G). In animals, a rapid (R) protocol, based on sedimentation at low speed, which avoids the time-consuming sucrose gradient, has also been developed to recover DIMs from microsomes as starting material. In the current study, we sought to compare the ability of the Rapid protocol versus the Gradient one for isolating DIMs directly from microsomes of M. truncatula roots. For that purpose, Triton X-100 detergent-insoluble fractions recovered with the two methods were analyzed and compared for their sterol/sphingolipid content and proteome profiles.

RESULTS

Inferred from sterol enrichment, presence of typical sphingolipid long-chain bases from plants and canonical DIM protein markers, the possibility to prepare DIMs from M. truncatula root microsomes was confirmed both for the Rapid and Gradient protocols. Contrary to sphingolipids, the sterol and protein profiles of DIMs were found to depend on the method used. Namely, DIM fractions were differentially enriched in spinasterol and only shared 39% of common proteins as assessed by GeLC-MS/MS profiling. Quantitative analysis of protein indicated that each purification procedure generated a specific subset of DIM-enriched proteins from Medicago root microsomes. Remarkably, these two proteomes were found to display specific cellular localizations and biological functions. In silico analysis of membrane-associative features within R- and G-enriched proteins, relative to microsomes, showed that the most noticeable difference between the two proteomes corresponded to an increase in the proportion of predicted signal peptide-containing proteins after sedimentation (R) compared to its decrease after floatation (G), suggesting that secreted proteins likely contribute to the specificity of the R-DIM proteome.

CONCLUSIONS

Even though microsomes were used as initial material, we showed that the protein composition of the G-DIM fraction still mostly mirrored that of plasmalemma-originating DIMs conventionally retrieved by floatation. In parallel, the possibility to isolate by low speed sedimentation DIM fractions that seem to target the late secretory pathway supports the existence of plant microdomains in other organelles.

Myosins XI modulate host cellular responses and penetration resistance to fungal pathogens

The rapid reorganization and polarization of actin filaments (AFs) toward the pathogen penetration site is one of the earliest cellular responses, yet the regulatory mechanism of AF dynamics is poorly understood. Using live-cell imaging in Arabidopsis, we show that polarization coupled with AF bundling involves precise spatiotemporal control at the site of attempted penetration by the nonadapted barley powdery mildew fungus, Blumeria graminis f. sp. hordei (Bgh). We further show that the Bgh-triggered AF mobility and organelle aggregation are predominately driven by the myosin motor proteins. Inactivation of myosins by pharmacological inhibitors prevents bulk aggregation of organelles and blocks recruitment of lignin-like compounds to the penetration site and deposition of callose and defensive protein, PENETRATION 1 (PEN1) into the apoplastic papillae, resulting in attenuation of penetration resistance. Using gene knockout analysis, we demonstrate that highly expressed myosins XI, especially myosin XI-K, are the primary contributors to cell wall-mediated penetration resistance. Moreover, the quadruple myosin knockout mutant xi-1 xi-2 xi-i xi-k displays impaired trafficking pathway responsible for the accumulation of PEN1 at the cell periphery. Strikingly, this mutant shows not only increased penetration rate but also enhanced overall disease susceptibility to both adapted and nonadapted fungal pathogens. Our findings establish myosins XI as key regulators of plant antifungal immunity.

S-acylation anchors remorin proteins to the plasma membrane but does not primarily determine their localization in membrane microdomains

Remorins are well-established marker proteins for plasma membrane microdomains. They specifically localize to the inner membrane leaflet despite an overall hydrophilic amino acid composition. Here, we determined amino acids and post-translational lipidations that are required for membrane association of remorin proteins. We used a combination of cell biological and biochemical approaches to localize remorin proteins and truncated variants of those in living cells and determined S-acylation on defined residues in these proteins. S-acylation of cysteine residues in a C-terminal hydrophobic core contributes to membrane association of most remorin proteins. While S-acylation patterns differ between members of this multi-gene family, initial membrane association is mediated by protein-protein or protein-lipid interactions. However, S-acylation is not a key determinant for the localization of remorins in membrane microdomains. Although remorins bind via a conserved mechanism to the plasma membrane, other membrane-resident proteins may be involved in the recruitment of remorins into membrane domains. S-acylation probably occurs after an initial targeting of the proteins to the plasma membrane and locks remorins in this compartment. As S-acylation is a reversible post-translational modification, stimulus-dependent intracellular trafficking of these proteins can be envisioned.

Spatial H2O2 signaling specificity: H2O2 from chloroplasts and peroxisomes modulates the plant transcriptome differentially

Hydrogen peroxide (H2O2) operates as a signaling molecule in eukaryotes, but the specificity of its signaling capacities remains largely unrevealed. Here, we analyzed whether a moderate production of H2O2 from two different plant cellular compartments has divergent effects on the plant transcriptome. Arabidopsis thaliana overexpressing glycolate oxidase in the chloroplast (Fahnenstich et al., 2008; Balazadeh et al., 2012) and plants deficient in peroxisomal catalase (Queval et al., 2007; Inzé et al., 2012) were grown under non-photorespiratory conditions and then transferred to photorespiratory conditions to foster the production of H2O2 in both organelles. We show that H2O2 originating in a specific organelle induces two types of responses: one that integrates signals independently from the subcellular site of H2O2 production and another that is dependent on the H2O2 production site. H2O2 produced in peroxisomes induces transcripts involved in protein repair responses, while H2O2 produced in chloroplasts induces early signaling responses, including transcription factors and biosynthetic genes involved in production of secondary signaling messengers. There is a significant bias towards the induction of genes involved in responses to wounding and pathogen attack by chloroplastic-produced H2O2, including indolic glucosinolates-, camalexin-, and stigmasterol-biosynthetic genes. These transcriptional responses were accompanied by the accumulation of 4-methoxy-indol-3-ylmethyl glucosinolate and stigmasterol.

The Plant Membrane-Associated REMORIN1.3 Accumulates in Discrete Perihaustorial Domains and Enhances Susceptibility to Phytophthora infestans

Filamentous pathogens such as the oomycete Phytophthora infestans infect plants by developing specialized structures termed haustoria inside the host cells. Haustoria are thought to enable the secretion of effector proteins into the plant cells. Haustorium biogenesis, therefore, is critical for pathogen accommodation in the host tissue. Haustoria are enveloped by a specialized host-derived membrane, the extrahaustorial membrane (EHM), which is distinct from the plant plasma membrane. The mechanisms underlying the biogenesis of the EHM are unknown. Remarkably, several plasma membrane-localized proteins are excluded from the EHM, but the remorin REM1.3 accumulates around P. infestans haustoria. Here, we used overexpression, colocalization with reporter proteins, and superresolution microscopy in cells infected by P. infestans to reveal discrete EHM domains labeled by REM1.3 and the P. infestans effector AVRblb2. Moreover, SYNAPTOTAGMIN1, another previously identified perihaustorial protein, localized to subdomains that are mainly not labeled by REM1.3 and AVRblb2. Functional characterization of REM1.3 revealed that it is a susceptibility factor that promotes infection by P. infestans. This activity, and REM1.3 recruitment to the EHM, require the REM1.3 membrane-binding domain. Our results implicate REM1.3 membrane microdomains in plant susceptibility to an oomycete pathogen.

Unconventional protein secretion (UPS) pathways in plants

As in yeast and mammalian cells, novel unconventional protein secretion (UPS) or unconventional membrane trafficking pathways are now known to operate in plants. UPS in plants is generally associated with stress conditions such as pathogen attack, but little is known about its underlying mechanism and function. Here, we present an update on the current knowledge of UPS in the plants in terms of its transport pathways, possible functions and its relationship to autophagy.

Clustering of the K+ channel GORK of Arabidopsis parallels its gating by extracellular K+

GORK is the only outward-rectifying Kv-like K(+) channel expressed in guard cells. Its activity is tightly regulated to facilitate K(+) efflux for stomatal closure and is elevated in ABA in parallel with suppression of the activity of the inward-rectifying K(+) channel KAT1. Whereas the population of KAT1 is subject to regulated traffic to and from the plasma membrane, nothing is known about GORK, its distribution and traffic in vivo. We have used transformations with fluorescently-tagged GORK to explore its characteristics in tobacco epidermis and Arabidopsis guard cells. These studies showed that GORK assembles in puncta that reversibly dissociated as a function of the external K(+) concentration. Puncta dissociation parallelled the gating dependence of GORK, the speed of response consistent with the rapidity of channel gating response to changes in the external ionic conditions. Dissociation was also suppressed by the K(+) channel blocker Ba(2+) . By contrast, confocal and protein biochemical analysis failed to uncover substantial exo- and endocytotic traffic of the channel. Gating of GORK is displaced to more positive voltages with external K(+) , a characteristic that ensures the channel facilitates only K(+) efflux regardless of the external cation concentration. GORK conductance is also enhanced by external K(+) above 1 mm. We suggest that GORK clustering in puncta is related to its gating and conductance, and reflects associated conformational changes and (de)stabilisation of the channel protein, possibly as a platform for transmission and coordination of channel gating in response to external K(+) .

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 Membranes Are Subcompartmentalized into a Plethora of Coexisting and Diverse Microdomains in Arabidopsis and Nicotiana benthamiana

Eukaryotic plasma membranes are highly compartmentalized structures. So far, only a few individual proteins that function in a wide range of cellular processes have been shown to segregate into microdomains. However, the biological roles of most microdomain-associated proteins are unknown. Here, we investigated the heterogeneity of distinct microdomains and the complexity of their coexistence. This diversity was determined in living cells of intact multicellular tissues using 20 different marker proteins from Arabidopsis thaliana, mostly belonging to the Remorin protein family. These proteins associate with microdomains at the cytosolic leaflet of the plasma membrane. We characterized these membrane domains and determined their lateral dynamics by extensive quantitative image analysis. Systematic colocalization experiments with an extended subset of marker proteins tested in 45 different combinations revealed the coexistence of highly distinct membrane domains on individual cell surfaces. These data provide valuable tools to study the lateral segregation of membrane proteins and their biological functions in living plant cells. They also demonstrate that widely used biochemical approaches such as detergent-resistant membranes cannot resolve this biological complexity of membrane compartmentalization in vivo.

The carbon/nitrogen regulator ARABIDOPSIS TOXICOS EN LEVADURA31 controls papilla formation in response to powdery mildew fungi penetration by interacting with SYNTAXIN OF PLANTS121 in Arabidopsis

The carbon/nitrogen (C/N) balance of plants is not only required for growth and development but also plays an important role in basal immunity. However, the mechanisms that link C/N regulation and basal immunity are poorly understood. We previously demonstrated that the Arabidopsis (Arabidopsis thaliana) Arabidopsis Tóxicos en Levadura31 (ATL31) ubiquitin ligase, a regulator of the C/N response, positively regulates the defense response against bacterial pathogens. In this study, we identified the plasma membrane-localized soluble N-ethylmaleimide-sensitive fusion protein attachment protein receptor SYNTAXIN OF PLANTS121 (SYP121) as a novel ATL31 interactor. The syp121-1 loss-of-function mutant showed similar hypersensitivity to C/N stress conditions as the atl31 atl6 double mutant. SYP121 is essential for resistance to penetration by powdery mildew fungus and positively regulates the formation of cell wall appositions (papillae) at fungal entry sites. Microscopic analysis demonstrated that ATL31 was specifically localized around papillae. In addition, ATL31 overexpressors showed accelerated papilla formation, enhancing their resistance to penetration by powdery mildew fungus. Together, these data indicate that ATL31 plays an important role in connecting the C/N response with basal immunity by promoting papilla formation through its association with SYP121.

Modification of plasma membrane organization in tobacco cells elicited by cryptogein

Lipid mixtures within artificial membranes undergo a separation into liquid-disordered and liquid-ordered phases. However, the existence of this segregation into microscopic liquid-ordered phases has been difficult to prove in living cells, and the precise organization of the plasma membrane into such phases has not been elucidated in plant cells. We developed a multispectral confocal microscopy approach to generate ratiometric images of the plasma membrane surface of Bright Yellow 2 tobacco (Nicotiana tabacum) suspension cells labeled with an environment sensitive fluorescent probe. This allowed the in vivo characterization of the global level of order of this membrane, by which we could demonstrate that an increase in its proportion of ordered phases transiently occurred in the early steps of the signaling triggered by cryptogein and flagellin, two elicitors of plant defense reactions. The use of fluorescence recovery after photobleaching revealed an increase in plasma membrane fluidity induced by cryptogein, but not by flagellin. Moreover, we characterized the spatial distribution of liquid-ordered phases on the membrane of living plant cells and monitored their variations induced by cryptogein elicitation. We analyze these results in the context of plant defense signaling, discuss their meaning within the framework of the "membrane raft" hypothesis, and propose a new mechanism of signaling platform formation in response to elicitor treatment.

The use of nanoscale fluorescence microscopic to decipher cell wall modifications during fungal penetration

Plant diseases are one of the most studied subjects in the field of plant science due to their impact on crop yield and food security. Our increased understanding of plant-pathogen interactions was mainly driven by the development of new techniques that facilitated analyses on a subcellular and molecular level. The development of labeling technologies, which allowed the visualization and localization of cellular structures and proteins in live cell imaging, promoted the use of fluorescence and laser-scanning microscopy in the field of plant-pathogen interactions. Recent advances in new microscopic technologies opened their application in plant science and in the investigation of plant diseases. In this regard, in planta Förster/Fluorescence resonance energy transfer has demonstrated to facilitate the measurement of protein-protein interactions within the living tissue, supporting the analysis of regulatory pathways involved in plant immunity and putative host-pathogen interactions on a nanoscale level. Localization microscopy, an emerging, non-invasive microscopic technology, will allow investigations with a nanoscale resolution leading to new possibilities in the understanding of molecular processes.

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.

Callose-mediated resistance to pathogenic intruders in plant defense-related papillae

Plants are exposed to a wide range of potential pathogens, which derive from diverse phyla. Therefore, plants have developed successful defense mechanisms during co-evolution with different pathogens. Besides many specialized defense mechanisms, the plant cell wall represents a first line of defense. It is actively reinforced through the deposition of cell wall appositions, so-called papillae, at sites of interaction with intruding microbial pathogens. The papilla is a complex structure that is formed between the plasma membrane and the inside of the plant cell wall. Even though the specific biochemical composition of papillae can vary between different plant species, some classes of compounds are commonly found which include phenolics, reactive oxygen species, cell wall proteins, and cell wall polymers. Among these polymers, the (1,3)-β-glucan callose is one of the most abundant and ubiquitous components. Whereas the function of most compounds could be directly linked with cell wall reinforcement or an anti-microbial effect, the role of callose has remained unclear. An evaluation of recent studies revealed that the timing of the different papilla-forming transport processes is a key factor for successful plant defense.

Wheat gene TaS3 contributes to powdery mildew susceptibility

Identification of TaS3 as a potential susceptibility gene encoding a protein homologous to ULP1 protease in wheat, which may regulate SUMO function facilitating powdery mildew attack. Some plant genes that are required for susceptibilities to certain pathogens are known as susceptibility genes or susceptibility factors, whose loss-of-function mutations can confer the plants resistances. To identify potential susceptibility genes to powdery mildew in wheat, differentially expressed genes in compatible and incompatible interactions between wheat and powdery mildew were examined by the cDNA chip assay. The genes exclusively expressed in the susceptible cultivar were interfered using biolistic transient transformation in wheat epidermal cells. The suppression of gene TaS3 (Triticum aestivum susceptibility) decreased the pathogen penetration by 19%, and its over-expression increased the disease susceptibility. The deduced protein from TaS3 belongs to the putative ubiquitin-like protease 1 peptidase domain family. Subcellular localization studies revealed that its protein was accumulated in the nucleus. Quantitative real-time polymerase chain reaction analysis revealed that TaS3 transcript was significantly induced in the compatible host. This suggests that TaS3 is a potential susceptible gene and its function may be related to regulate SUMO functions.

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.

Probing plasma membrane dynamics at the single-molecule level

The plant plasma membrane is highly dynamic and changes multiple aspects of its organization in response to environmental and internal factors. A detailed understanding of membrane dynamics in living plant cells has remained obscure because of the limited spatial resolution of conventional optical microscopy. Recently, several single-molecule imaging approaches have been developed and used to provide valuable insights into the fundamental biochemical and biophysical properties of the plant plasma membrane, including the organization of membrane microdomains and the dynamics of single-molecule diffusion. Here we review single-molecule imaging methods, including total internal reflection fluorescence microscopy (TIRFM), fluorescence correlation spectroscopy (FCS), and super-resolution microscopy, and examine their contributions to recent progress in understanding protein dynamics and membrane organization in living plant cells.

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.

Molecular cloning and functional analysis of the Populus deltoides remorin gene PdREM

Remorins play vital roles in signal transduction, energy transformation, ion flow and transport in plants. Upregulation of remorins correlates with dehiscence and cell maturation; however, no studies have been performed to elucidate the function of remorins in tree species. In this study, a Populus deltoides (Marsh.) plasma membrane-binding protein remorin gene (PdREM) was cloned and characterized by investigating its expression pattern and creating transgenic hybrid poplar (P. davidiana Dode × P. bolleana Lauche) lines expressing sense or antisense PdREM. PdREM was specifically expressed in leaf buds, and immature and mature phloem in P. deltoides. Downregulation of PdREM increased plant height, stem diameter, number of leaves, size of the xylem and phloem zones and induced expression of cell wall biosynthesis- and microfibril angle (MFA)-related genes. Overexpression of PdREM retarded vegetative growth. PdREM may negatively regulate vascular growth by inhibiting secondary cell wall expansion in poplar. In addition, antisense PdREM transgenic poplar had a lower MFA, suggesting that PdREM might contribute to sheet strength and wood properties in poplar. This study sheds light on the molecular mechanism of PdREM in P. deltoides growth and development, and lays the foundation for future functional genomics research into wood formation and the genetic engineering of forest trees with improved wood quality traits.

Arabidopsis phospholipase dδ is involved in basal defense and nonhost resistance to powdery mildew fungi

Plants have evolved a complex array of defensive responses against pathogenic microorganisms. Recognition of microbes initiates signaling cascades that activate plant defenses. The membrane lipid phosphatidic acid, produced by phospholipase D (PLD), has been shown to take part in both abiotic and biotic stress signaling. In this study, the involvement of PLD in the interaction between Arabidopsis (Arabidopsis thaliana) and the barley powdery mildew fungus Blumeria graminis f. sp. hordei (Bgh) was investigated. This nonadapted pathogen is normally resisted by a cell wall-based defense, which stops the fungal hyphae from penetrating the epidermal cell wall. Chemical inhibition of phosphatidic acid production by PLD increased the penetration rate of Bgh spores on wild-type leaves. The analysis of transfer DNA knockout lines for all Arabidopsis PLD genes revealed that PLDδ is involved in penetration resistance against Bgh, and chemical inhibition of PLDs in plants mutated in PLDδ indicated that this isoform alone is involved in Bgh resistance. In addition, we confirmed the involvement of PLDδ in penetration resistance against another nonadapted pea powdery mildew fungus, Erysiphe pisi. A green fluorescent protein fusion of PLDδ localized to the plasma membrane at the Bgh attack site, where it surrounded the cell wall reinforcement. Furthermore, in the pldδ mutant, transcriptional up-regulation of early microbe-associated molecular pattern response genes was delayed after chitin stimulation. In conclusion, we propose that PLD is involved in defense signaling in nonhost resistance against powdery mildew fungi and put PLDδ forward as the main isoform participating in this process.

Suppression of DS1 phosphatidic acid phosphatase confirms resistance to Ralstonia solanacearum in Nicotiana benthamiana

Nicotianabenthamiana is susceptible to Ralstonia solanacearum. To analyze molecular mechanisms for disease susceptibility, we screened a gene-silenced plant showing resistance to R. solanacearum, designated as DS1 (Disease suppression 1). The deduced amino acid sequence of DS1 cDNA encoded a phosphatidic acid phosphatase (PAP) 2. DS1 expression was induced by infection with a virulent strain of R. solanacearum in an hrp-gene-dependent manner. DS1 rescued growth defects of the temperature-sensitive ∆lpp1∆dpp1∆pah1 mutant yeast. Recombinant DS1 protein showed Mg(2+)-independent PAP activity. DS1 plants showed reduced PAP activity and increased phosphatidic acid (PA) content. After inoculation with R. solanacearum, DS1 plants showed accelerated cell death, over-accumulation of reactive oxygen species (ROS), and hyper-induction of PR-4 expression. In contrast, DS1-overexpressing tobacco plants showed reduced PA content, greater susceptibility to R. solanacearum, and reduced ROS production and PR-4 expression. The DS1 phenotype was partially compromised in the plants in which both DS1 and NbCoi1 or DS1 and NbrbohB were silenced. These results show that DS1 PAP may affect plant immune responses related to ROS and JA cascades via regulation of PA levels. Suppression of DS1 function or DS1 expression could rapidly activate plant defenses to achieve effective resistance against Ralstonia solanacearum.

Induction and suppression of PEN3 focal accumulation during Pseudomonas syringae pv. tomato DC3000 infection of Arabidopsis

The pleiotropic drug resistance (PDR) proteins belong to the super-family of ATP-binding cassette (ABC) transporters. AtPDR8, also called PEN3, is required for penetration resistance of Arabidopsis to nonadapted powdery mildew fungi. During fungal infection, plasma-membrane-localized PEN3 is concentrated at fungal entry sites, as part of the plant's focal immune response. Here, we show that the pen3 mutant is compromised in resistance to the bacterial pathogen Pseudomonas syringae pv. tomato DC3000. P. syringae pv. tomato DC3000 infection or treatment with a flagellin-derived peptide, flg22, induced strong focal accumulation of PEN3-green fluorescent protein. Interestingly, after an initial induction of PEN3 accumulation, P. syringae pv. tomato DC3000 but not the type-III-secretion-deficient mutant hrcC could suppress PEN3 accumulation. Moreover, transgenic overexpression of the P. syringae pv. tomato DC3000 effector AvrPto was sufficient to suppress PEN3 focal accumulation in response to flg22. Analyses of P. syringae pv. tomato DC3000 effector deletion mutants showed that individual effectors, including AvrPto, appear to be insufficient to suppress PEN3 accumulation when delivered by bacteria, suggesting a requirement for a combined action of multiple effectors. Collectively, our results indicate that PEN3 plays a positive role in plant resistance to a bacterial pathogen and show that focal accumulation of PEN3 protein may be a useful cellular response marker for the Arabidopsis-P. syringae interaction.

A molecular imaging analysis of Cx43 association with Cdo during skeletal myoblast differentiation

Cell-to-cell contacts are crucial for cell differentiation. The promyogenic cell surface protein, Cdo, functions as a component of multiprotein clusters to mediate cell adhesion signaling. Connexin 43, the main connexin forming gap junctions, also plays a key role in myogenesis. At least part of its effects is independent of the intercellular channel function, but the mechanisms underlying are unknown. Here, using multiple optical approaches, we provided the first evidence that Cx43 physically interacts with Cdo to form dynamic complexes during myoblast differentiation, offering clues for considering this interaction a structural basis of the channel-independent function of Cx43.

Perception of conserved pathogen elicitors at the plasma membrane leads to relocalization of the Arabidopsis PEN3 transporter

The Arabidopsis penetration resistance 3 (PEN3) ATP binding cassette transporter participates in nonhost resistance to fungal and oomycete pathogens and is required for full penetration resistance to the barley powdery mildew Blumeria graminis f. sp. hordei. PEN3 resides in the plasma membrane and is recruited to sites of attempted penetration by invading fungal appressoria, where the transporter shows strong focal accumulation. We report that recruitment of PEN3 to sites of pathogen detection is triggered by perception of pathogen-associated molecular patterns, such as flagellin and chitin. PEN3 recruitment requires the corresponding pattern recognition receptors but does not require the BAK1 coreceptor. Pathogen- and pathogen-associated molecular pattern-induced focal accumulation of PEN3 and the penetration resistance 1 (PEN1) syntaxin show differential sensitivity to specific pharmacological inhibitors, indicating distinct mechanisms for recruitment of these defense-associated proteins to the host-pathogen interface. Focal accumulation of PEN3 requires actin but is not affected by inhibitors of microtubule polymerization, secretory trafficking, or protein synthesis, and plasmolysis experiments indicate that accumulation of PEN3 occurs outside of the plasma membrane within papillae. Our results implicate pattern recognition receptors in the recruitment of defense-related proteins to sites of pathogen detection. Additionally, the process through which PEN3 is recruited to the host-pathogen interface is independent of new protein synthesis and BFA-sensitive secretory trafficking events, suggesting that existing PEN3 is redirected through an unknown trafficking pathway to sites of pathogen detection for export into papillae.

Analyses of wrky18 wrky40 plants reveal critical roles of SA/EDS1 signaling and indole-glucosinolate biosynthesis for Golovinomyces orontii resistance and a loss-of resistance towards Pseudomonas syringae pv. tomato AvrRPS4

Simultaneous mutation of two WRKY-type transcription factors, WRKY18 and WRKY40, renders otherwise susceptible wild-type Arabidopsis plants resistant towards the biotrophic powdery mildew fungus Golovinomyces orontii. Resistance in wrky18 wrky40 double mutant plants is accompanied by massive transcriptional reprogramming, imbalance in salicylic acid (SA) and jasmonic acid (JA) signaling, altered ENHANCED DISEASE SUSCEPTIBILITY1 (EDS1) expression, and accumulation of the phytoalexin camalexin. Genetic analyses identified SA biosynthesis and EDS1 signaling as well as biosynthesis of the indole-glucosinolate 4MI3G as essential components required for loss-of-WRKY18 WRKY40-mediated resistance towards G. orontii. The analysis of wrky18 wrky40 pad3 mutant plants impaired in camalexin biosynthesis revealed an uncoupling of pre- from postinvasive resistance against G. orontii. Comprehensive infection studies demonstrated the specificity of wrky18 wrky40-mediated G. orontii resistance. Interestingly, WRKY18 and WRKY40 act as positive regulators in effector-triggered immunity, as the wrky18 wrky40 double mutant was found to be strongly susceptible towards the bacterial pathogen Pseudomonas syringae DC3000 expressing the effector AvrRPS4 but not against other tested Pseudomonas strains. We hypothesize that G. orontii depends on the function of WRKY18 and WRKY40 to successfully infect Arabidopsis wild-type plants while, in the interaction with P. syringae AvrRPS4, they are required to mediate effector-triggered immunity.