A renaissance of elicitors: perception of microbe-associated molecular patterns and danger signals by pattern-recognition receptors

Engineering plant resistance by constructing chimeric receptors that recognize damage-associated molecular patterns (DAMPs)

An efficient sensing of danger and a rapid activation of the immune system are crucial for the survival of plants. Conserved pathogen/microbe-associated molecular patterns (PAMPs/MAMPs) and endogenous molecular patterns, which are present only when the tissue is infected or damaged (damage-associated molecular patterns or DAMPs), can act as danger signals and activate the plant immune response. These molecules are recognized by surface receptors that are indicated as pattern recognition receptors (PRRs). In this paper we summarize recent information on oligogalacturonides (OGs), a class of DAMPs that is released from the extracellular matrix of the plant cell during pathogen attack or wounding. We also describe the characteristics of the Arabidopsis Wall-Associated Kinase 1 (WAK1), a PRR recently identified as a receptor of OGs and discuss the use of WAK1, PRRs and chimeric receptors to engineer resistance in crop plants.

Stem-cell-triggered immunity through CLV3p-FLS2 signalling

Stem cells in the shoot apical meristem (SAM) of plants are the self-renewable reservoir for leaf, stem and flower organogenesis1,2. In nature, disease-free plants can be regenerated from SAM despite infections elsewhere, which underlies a horticultural practice for decades3. However, the molecular basis of the SAM immunity remains enigmatic. Here we show a surprising discovery that the CLAVATA3 peptide (CLV3p), expressed and secreted from the stem cells and functioning as a key regulator of stem cell homeostasis in the Arabidopsis SAM1,2,4, can trigger immune signalling and pathogen resistance via the flagellin receptor kinase FLS25,6. CLV3p-FLS2 signalling acts independently from the stem cell signalling pathway mediated through CLV1 and CLV2 receptors 1,2,4, and is uncoupled from the FLS2-mediated growth suppression5,6. Endogenous CLV3p perception in the SAM by a pattern recognition receptor FLS2 for bacterial flagellin breaks the previously defined self and nonself discrimination in innate immunity 6,7. The dual CLV3p perceptions illustrate co-evolution of plant peptide and receptor kinase signalling for both development and immunity. The enhanced immunity in SAM or germ lines may represent a common strategy toward immortal fate in plants and animals1,2,8.

Extracellular leucine-rich repeats as a platform for receptor/coreceptor complex formation

Receptor kinases with leucine-rich repeat (LRR) extracellular domains form the largest family of receptors in plants. In the few cases for which there is mechanistic information, ligand binding in the extracellular domain often triggers the recruitment of a LRR-coreceptor kinase. The current model proposes that this recruitment is mediated by their respective kinase domains. Here, we show that the extracellular LRR domain of BRI1-ASSOCIATED KINASE1 (BAK1), a coreceptor involved in the disparate processes of cell surface steroid signaling and immunity in plants, is critical for its association with specific ligand-binding LRR-containing receptors. The LRRs of BAK1 thus serve as a platform for the molecular assembly of signal-competent receptors. We propose that this mechanism represents a paradigm for LRR receptor activation in plants.

Plasmodesmata: the battleground against intruders

Plasmodesmata are intercellular channels that establish a symplastic communication pathway between neighboring cells in plants. Owing to this role, opportunistic microbial pathogens have evolved to exploit plasmodesmata as gateways to spread infection from cell to cell within the plant. However, although these pathogens have acquired the capacity to breach the plasmodesmal trafficking pathway, plants are unlikely to relinquish control over a structure essential for their survival so easily. In this review, we examine evidence that suggests plasmodesmata play an active role in plant immunity against viral, fungal and bacterial pathogens. We discuss how these pathogens differ in their lifestyles and infection modes, and present the defense strategies that plants have adopted to prevent the intercellular spread of an infection.

Eicosapolyenoic acids: novel MAMPs with reciprocal effect on oomycete-plant defense signaling networks

Thirty years ago arachidonic (AA; 20:4 Δ ( 5,8,11,14) ) and eicosapentaenoic (EPA; 20:5 Δ ( 5,8,11,14,17) ) acids were identified as elicitors from the late blight pathogen, Phytophthora infestans, capable of triggering the dramatic shifts in isoprenoid metabolism, defense reactions, and cell death associated with the hypersensitive response of potato to incompatible races of the pathogen. ( 1) Among plant pathogens, the capacity for eicosapolyenoic acid synthesis appears to be largely restricted to oomycetes, primitive fungi (e.g., zygomycetes and chytrids), and nematodes. AA and EPA, precursors to eicosanoids that mediate inflammatory responses and serve as critical signals for immune and central nervous system functions in mammals, continue to be compelling molecules for study in plants because of what they may reveal about lipid-based signaling and induced immunity in plant-microbe interactions and possible mechanistic parallels as conserved signaling molecules across eukaryotic kingdoms. In spite of the intriguing cross-kingdom connections in AA/EPA signaling, there has been relatively little research to resolve eicosapolyenoic acid perception and action in plants, in part because of experimental limitations of systems where these fatty acids display strong activity. However, this state of affairs may change with our recent discovery that Arabidopsis responds to AA and that plants engineered to express very low levels of eicosapolyenoic acids (EP plants) have remarkably altered phenotypes to biotic challengers. 

A systems biology perspective on plant-microbe interactions: biochemical and structural targets of pathogen effectors

Plants have biochemical defences against stresses from predators, parasites and pathogens. In this review we discuss the interaction of plant defences with microbial pathogens such as bacteria, fungi and oomycetes, and viruses. We examine principles of complex dynamic networks that allow identification of network components that are differentially and predictably sensitive to perturbation, thus making them likely effector targets. We relate these principles to recent developments in our understanding of known effector targets in plant-pathogen systems, and propose a systems-level framework for the interpretation and modelling of host-microbe interactions mediated by effectors. We describe this framework briefly, and conclude by discussing useful experimental approaches for populating this framework.

Developing a model of plant hormone interactions

Plant growth and development is influenced by mutual interactions among plant hormones. The five classical plant hormones are auxins, cytokinins, gibberellins, abscisic acid and ethylene. They are small diffusible molecules that easily penetrate between cells. In addition, newer classes of plant hormones have been identified such as brassinosteroids, jasmonic acid, salicylic acid and various small proteins or peptides. These hormones also play important roles in the regulation of plant growth and development. This review begins with a brief summary of the current findings on plant hormones. Based on this knowledge, a conceptual model about interactions among plant hormones is built so as to link and develop an understanding of the diverse functions of different plant hormones as a whole in plants.

Molecular, cellular, and physiological responses to phosphatidic acid formation in plants

Phosphatidic acid (PA) is an essential phospholipid involved in membrane biosynthesis and signal transduction in all eukaryotes. This review focuses on its role as lipid second messenger during plant stress, metabolism, and development. The contribution of different individual isoforms of enzymes that generate and break down PA will be discussed and the downstream responses highlighted, with particular focus on proteins that bind PA. Through characterization of several of these PA targets, a molecular and genetic basis for PA's role in plant stress and development is emerging.

The lectin receptor kinase LecRK-I.9 is a novel Phytophthora resistance component and a potential host target for a RXLR effector

In plants, an active defense against biotrophic pathogens is dependent on a functional continuum between the cell wall (CW) and the plasma membrane (PM). It is thus anticipated that proteins maintaining this continuum also function in defense. The legume-like lectin receptor kinase LecRK-I.9 is a putative mediator of CW-PM adhesions in Arabidopsis and is known to bind in vitro to the Phytophthora infestans RXLR-dEER effector IPI-O via a RGD cell attachment motif present in IPI-O. Here we show that LecRK-I.9 is associated with the plasma membrane, and that two T-DNA insertions lines deficient in LecRK-I.9 (lecrk-I.9) have a ‘gain-of-susceptibility’ phenotype specifically towards the oomycete Phytophthora brassicae. Accordingly, overexpression of LecRK-I.9 leads to enhanced resistance to P. brassicae. A similar ‘gain-of-susceptibility’ phenotype was observed in transgenic Arabidopsis lines expressing ipiO (35S-ipiO1). This phenocopy behavior was also observed with respect to other defense-related functions; lecrk-I.9 and 35S-ipiO1 were both disturbed in pathogen- and MAMP-triggered callose deposition. By site-directed mutagenesis, we demonstrated that the RGD cell attachment motif in IPI-O is not only essential for disrupting the CW-PM adhesions, but also for disease suppression. These results suggest that destabilizing the CW-PM continuum is one of the tactics used by Phytophthora to promote infection. As countermeasure the host may want to strengthen CW-PM adhesions and the novel Phytophthora resistance component LecRK-I.9 seems to function in this process.

Phytophthora species are notorious plant pathogens which cause a variety of devastating crop diseases. Phytophthora pathogens secrete a plethora of effector proteins, several of which are known to interact with receptors in the host cell thereby either activating or suppressing defense responses. Unlike animals, plants lack an adaptive immune system; however, they are not defenseless and have acquired other mechanisms to withstand pathogens. Receptor proteins play important roles in sensing alterations at the plant cell wall and in mediating responses upon pathogen attack. This paper focuses on the Arabidopsis lectin receptor kinase LecRK-I.9, a mediator of cell wall – plasma membrane (CW-PM) adhesions that is known to bind in vitro to the Phytophthora infestans effector IPI-O via the cell attachment motif RGD. T-DNA mutants deficient in LecRK-I.9 and transgenic Arabidopsis lines expressing ipiO1 were found to behave as phenocopies. Both show a ‘gain-of-susceptibility’ phenotype towards the Arabidopsis pathogen Phytophthora brassicae and are disturbed in callose deposition. Overall, the results suggest that destabilizing the CW-PM continuum is a strategy for Phytophthora to promote infection. As countermeasure, the host may want to strengthen CW-PM adhesions, and the novel resistance component LecRK-I.9 apparently functions in this process.

Enhancement of MAMP signaling by chimeric receptors improves disease resistance in plants

Plants activate defense responses through the recognition of microbe-associated molecular patterns (MAMPs). Recently, several pattern-recognition receptors (PRRs) have been identified in plants, paving the way for manipulating MAMP signaling. CEBiP is a receptor for the chitin elicitor (CE) identified in the rice plasma membrane and XA21 is a member of the receptor-like protein kinase (RLK) family that confers disease resistance to rice bacterial leaf blight expressing the sulfated protein Ax21. To improve resistance to rice blast, the most serious fungal disease of rice, we aimed to create a defense system that combines high affinity of CEBiP for CE and the ability of XA21 to confer disease resistance. Cultured rice cells expressing the chimeric receptor CRXA, which consists of CEBiP and the intracellular region of XA21, induced cell death accompanied by an increased production of reactive oxygen and nitrogen species after exposure to CE. Rice plants expressing the chimeric receptor exhibited more resistance to rice blast. Engineering PRRs may be a new strategy in molecular breeding for achieving disease resistance.

ZmPep1, an ortholog of Arabidopsis elicitor peptide 1, regulates maize innate immunity and enhances disease resistance

ZmPep1 is a bioactive peptide encoded by a previously uncharacterized maize (Zea mays) gene, ZmPROPEP1. ZmPROPEP1 was identified by sequence similarity as an ortholog of the Arabidopsis (Arabidopsis thaliana) AtPROPEP1 gene, which encodes the precursor protein of elicitor peptide 1 (AtPep1). Together with its receptors, AtPEPR1 and AtPEPR2, AtPep1 functions to activate and amplify innate immune responses in Arabidopsis and enhances resistance to both Pythium irregulare and Pseudomonas syringae. Candidate orthologs to the AtPROPEP1 gene have been identified from a variety of crop species; however, prior to this study, activities of the respective peptides encoded by these orthologs were unknown. Expression of the ZmPROPEP1 gene is induced by fungal infection and treatment with jasmonic acid or ZmPep1. ZmPep1 activates de novo synthesis of the hormones jasmonic acid and ethylene and induces the expression of genes encoding the defense proteins endochitinase A, PR-4, PRms, and SerPIN. ZmPep1 also stimulates the expression of Benzoxazineless1, a gene required for the biosynthesis of benzoxazinoid defenses, and the accumulation of 2-hydroxy-4,7-dimethoxy-1,4-benzoxazin-3-one glucoside in leaves. To ascertain whether ZmPep1-induced defenses affect resistance, maize plants were pretreated with the peptide prior to infection with fungal pathogens. Based on cell death and lesion severity, ZmPep1 pretreatment was found to enhance resistance to both southern leaf blight and anthracnose stalk rot caused by Cochliobolis heterostrophus and Colletotrichum graminicola, respectively. We present evidence that peptides belonging to the Pep family have a conserved function across plant species as endogenous regulators of innate immunity and may have potential for enhancing disease resistance in crops.

Long-distance transport of signals during symbiosis: are nodule formation and mycorrhization autoregulated in a similar way?

Legumes enter nodule symbioses with nitrogen-fixing bacteria (rhizobia), whereas most flowering plants establish symbiotic associations with arbuscular mycorrhizal (AM) fungi. Once first steps of symbiosis are initiated, nodule formation and mycorrhization in legumes is negatively controlled by a shoot-derived inhibitor (SDI), a phenomenon termed autoregulation. According to current views, autoregulation of nodulation and mycorrhization in legumes is regulated in a similar way. CLE peptides induced in response to rhizobial nodulation signals (Nod factors) have been proposed to represent the ascending long-distance signals to the shoot. Although not proven yet, these CLE peptides are likely perceived by leucine-rich repeat (LRR) autoregulation receptor kinases in the shoot. Autoregulation of mycorrhization in non-legumes is reminiscent to the phenomenon of "systemic acquired resistance" in plant-pathogen interactions.

Functional importance of BAK1 tyrosine phosphorylation in vivo

The plant receptor kinase BRI1-ASSOCIATED RECEPTOR KINASE 1 (BAK1) is known as a partner of several ligand-binding leucine-rich repeat receptor kinases, including BRASSINOSTEROID INSENSITIVE 1 (BRI1) and the flagellin receptor FLS2. Autophosphorylation of receptor kinases is recognized to be an important process in receptor kinase signaling, and at least with the recombinant protein, BAK1 was shown to autophosphorylate on Tyr residues in addition to numerous Ser/Thr residues documented previously. We recently identified Tyr-610 in the carboxy-terminal domain of BAK1 as a major site of autophosphorylation and showed that phosphorylation of this residue is essential for at least some functions of BAK1 in vivo. In particular, the function of BAK1 as co-receptor with BRI1 in brassinosteroid (BR) signaling is impaired in transgenic plants expressing the BAK1(Y610F)-Flag directed mutant. Recombinant cytoplasmic domains of BRI1 and BAK1 interact and transphosphorylate each other in vitro in a manner that mimics their interaction in vivo; while BAK1(Y610F) binds normally to BRI1 its ability to transphosphorylate and activate the kinase domain of BRI1 is severely compromised. To further elaborate on this earlier model, we present additional results showing that the interaction between BAK1 and BRI1 in vitro is Mg(2+) dependent, suggesting that cytosolic [Mg(2+)] may play some role in receptor kinase signaling in vivo. We also compare the primary structures of BRI1 and BAK1 in terms of the occurrence of Tyr residues in the cytoplasmic domain, and identify differences in which residues are essential for kinase activity. Finally, transgenic plants expressing the BAK1(Y610F) directed mutant have alterations in the transcriptome that extend beyond the genes that are BR regulated in nontransgenic plants. In particular, the basal expression of many defense genes is significantly reduced in Y610F plants, which is consistent with the earlier report in reference 4, that BAK1 controls the expression of a number of genes associated with microbial infection. The present results establish a site-specific role for Tyr phosphorylation of BAK1 in BR signaling and regulation of plant defense mechanisms, which may have implications for enhancing agricultural productivity.

Innate immunity: has poplar made its BED?

The perennial plant model species Populus trichocarpa has received considerable attention in the last 5 yr because of its potential use as a bioenergy crop. The completion of its genome sequence revealed extensive homologies with the herbaceous annual species Arabidopsis thaliana. This review highlights the similarities and differences at the qualitative defence response components level, notably in putative NBS-LRR protein content and downstream defence regulators. With almost a twofold NBS-LRR gene complement compared with A. thaliana, P. trichocarpa also encodes some putative R-proteins with unusual architectures and possible DNA-binding capacity. P. trichocarpa also possesses all the known main components characteristic of TIR-NB-LRR and CC-NB-LRR signalling. However, very little has been done with regard to the components involved in the poplar qualitative response to pathogens. In addition, the relationship between plant-biotroph perception/signalling and the role of salicylic acid, an important defence compound, remains uncertain. This review aims to identify the genomic components present in poplar that could potentially participate in the qualitative response and highlights where efforts should be devoted to obtain a better understanding of the poplar qualitative defence response.

Callose deposition: a multifaceted plant defense response

Callose deposition in Arabidopsis has emerged as a popular model system to quantify activity of plant immunity. However, there has been a noticeable rise in contradicting reports about the regulation of pathogen-induced callose. To address this controversy, we have examined the robustness of callose deposition under different growth conditions and in response to two different pathogen-associated molecular patterns, the flagellin epitope Flg22 and the polysaccharide chitosan. Based on a commonly used hydroponic culture system, we found that variations in growth conditions have a major impact on the plant's overall capacity to deposit callose. This environmental variability correlated with levels of hydrogen peroxide (H₂O₂) production. Depending on the growth conditions, pretreatment with abscissic acid stimulated or repressed callose deposition. Despite a similar effect of growth conditions on Flg22- and chitosan-induced callose, both responses showed differences in timing, tissue responsiveness, and colocalization with H₂O₂. Furthermore, mutant analysis revealed that Flg22- and chitosan-induced callose differ in the requirement for the NADPH oxidase RBOHD, the glucosinolate regulatory enzymes VTC1 and PEN2, and the callose synthase PMR4. Our study demonstrates that callose is a multifaceted defense response that is controlled by distinct signaling pathways, depending on the environmental conditions and the challenging pathogen-associated molecular pattern.

Rice RING protein OsBBI1 with E3 ligase activity confers broad-spectrum resistance against Magnaporthe oryzae by modifying the cell wall defence

Emerging evidence suggests that E3 ligases play critical roles in diverse biological processes, including innate immune responses in plants. However, the mechanism of the E3 ligase involvement in plant innate immunity is unclear. We report that a rice gene, OsBBI1, encoding a RING finger protein with E3 ligase activity, mediates broad-spectrum disease resistance. The expression of OsBBI1 was induced by rice blast fungus Magnaporthe oryzae, as well as chemical inducers, benzothiadiazole and salicylic acid. Biochemical analysis revealed that OsBBI1 protein possesses E3 ubiquitin ligase activity in vitro. Genetic analysis revealed that the loss of OsBBI1 function in a Tos17-insertion line increased susceptibility, while the overexpression of OsBBI1 in transgenic plants conferred enhanced resistance to multiple races of M. oryzae. This indicates that OsBBI1 modulates broad-spectrum resistance against the blast fungus. The OsBBI1-overexpressing plants showed higher levels of H(2)O(2) accumulation in cells and higher levels of phenolic compounds and cross-linking of proteins in cell walls at infection sites by M. oryzae compared with wild-type (WT) plants. The cell walls were thicker in the OsBBI1-overexpressing plants and thinner in the mutant plants than in the WT plants. Our results suggest that OsBBI1 modulates broad-spectrum resistance to blast fungus by modifying cell wall defence responses. The functional characterization of OsBBI1 provides insight into the E3 ligase-mediated innate immunity, and a practical tool for constructing broad-spectrum resistance against the most destructive disease in rice.

Plant targets for Pseudomonas syringae type III effectors: virulence targets or guarded decoys?

The phytopathogenic bacterium Pseudomonas syringae can suppress both pathogen-associated molecular pattern (PAMP)-triggered immunity (PTI) and effector-triggered immunity (ETI) by the injection of type III effector (T3E) proteins into host cells. T3Es achieve immune suppression using a variety of strategies including interference with immune receptor signaling, blocking RNA pathways and vesicle trafficking, and altering organelle function. T3Es can be recognized indirectly by resistance proteins monitoring specific T3E targets resulting in ETI. It is presently unclear whether the monitored targets represent bona fide virulence targets or guarded decoys. Extensive overlap between PTI and ETI signaling suggests that T3Es may suppress both pathways through common targets and by possessing multiple activities.

Activation of plant pattern-recognition receptors by bacteria

The first active layer of plant innate immunity relies on the recognition by surface receptors of molecules indicative of non-self or modified-self. The activation of pattern-recognition receptors (PRRs) by pathogen-associated molecular patterns (PAMPs) is in essence sufficient to stop pathogen invasion through transcriptional reprogramming and production of anti-microbials. The few PRR/PAMP pairs that are characterised provide useful models to study the specificity of ligand-binding and likely activation mechanisms. Both classical and new approaches are still required to identify new bacterial PAMPs. Current genetic screens, functional genomics and biochemical analyses have identified the regulation mechanisms of PRR transcription and biogenesis, provided insights into the composition of PRR complexes at the plasma membrane and highlighted the roles of long-known signalling components in PAMP-triggered immunity (PTI).

Phospholipases in action during plant defense signaling

Eukaryotic organisms rely on intricate signaling networks to connect recognition of microbes with the activation of efficient defense reactions. Accumulating evidence indicates that phospholipids are more than mere structural components of biological membranes. Indeed, phospholipid-based signal transduction is widely used in plant cells to relay perception of extracellular signals. Upon perception of the invading microbe, several phospholipid hydrolyzing enzymes are activated that contribute to the establishment of an appropriate defense response. Activation of phospholipases is at the origin of the production of important defense signaling molecules, such as oxylipins and jasmonates, as well as the potent second messenger phosphatidic acid (PA), which has been shown to modulate the activity of a variety of proteins involved in defense signaling. Here, we provide an overview of recent reports describing the different plant phospholipase pathways that are activated during the establishment of plant defense reactions in response to pathogen attack.

A genome-wide survey for Arabidopsis leucine-rich repeat receptor kinases implicated in plant immunity

Receptor-like kinases (RLK) are among the largest gene families encoded by plant genomes. Common structural features of plant RLKs are an extracellular ligand binding domain, a membrane spanning domain, and an intracellular protein kinase domain. The largest subfamily of plant RLKs is characterized by extracellular leucine-rich repeat (LRR-RLK) structures that are known biochemical modules for mediating ligand binding and protein-protein interactions. In the frame of the Arabidopsis Functional Genomics Network initiative of the German Research Foundation (DFG) we have conducted a comprehensive survey for and functional characterization of LRR-RLKs potentially implicated in Arabidopsis thaliana immunity to microbial infection. Arabidopsis gene expression patterns suggested an important role of this class of proteins in biotic stress adaptation. Detailed biochemical and physiological characterization of the brassinosteroid insensitive 1-associated receptor kinase 1 (BAK1) revealed brassinolide-independent roles of this protein in plant immunity, in addition to its well-established function in plant development. The LRR-RLK BAK1 has further been shown to form heteromeric complexes with various other LRR-RLKs in a ligand-dependent manner, suggesting a role as adapter or co-receptor in plant receptor complexes. Here, we review the current status of BAK1 and BAK1-interacting LRR-RLKs in plant immunity.

The plant cell nucleus: a true arena for the fight between plants and pathogens

Communication between the cytoplasm and the nucleus is a fundamental feature shared by both plant and animal cells. Cellular factors involved in the transport of macromolecules through the nuclear envelope, including nucleoporins, importins and Ran-GTP related components, are conserved among a variety of eukaryotic systems. Interestingly, mutations in these nuclear components compromise resistance signalling, illustrating the importance of nucleocytoplasmic trafficking in plant innate immunity. Indeed, spatial restriction of defence regulators by the nuclear envelope and stimulus-induced nuclear translocation constitute an important level of defence-associated gene regulation in plants. A significant number of effectors from different microbial pathogens are targeted to the plant cell nucleus. In addition, key host factors, including resistance proteins, immunity components, transcription factors and transcriptional regulators shuttle between the cytoplasm and the nucleus, and their level of nuclear accumulation determines the output of the defence response, further confirming the crucial role played by the nucleus during the interaction between plants and pathogens. Here, we discuss recent findings that situate the nucleus at the frontline of the mutual recognition between plants and invading microbes.

Perspectives on remorin proteins, membrane rafts, and their role during plant-microbe interactions

Invasion of host cells by pathogenic or mutualistic microbes requires complex molecular dialogues that often determine host survival. Although several components of the underlying signaling cascades have recently been identified and characterized, our understanding of proteins that facilitate signal transduction or assemble signaling complexes is rather sparse. Our knowledge of plant-specific remorin proteins, annotated as proteins with unknown function, has recently advanced with respect to their involvement in host-microbe interactions. Current data demonstrating that a remorin protein restricts viral movement in tomato leaves and the importance of a symbiosis-specific remorin for bacterial infection of root nodules suggest that these proteins may serve such regulatory functions. Direct interactions of other remorins with a resistance protein in Arabidopsis thaliana, and differential phosphorylation upon perception of microbial-associated molecular patterns and during expression of bacterial effector proteins, strongly underline their roles in plant defense. Furthermore, the specific subcellular localization of remorins in plasma membrane microdomains now provides the opportunity to visualize membrane rafts in living plants cells. There, remorins may oligomerize and act as scaffold proteins during early signaling events. This review summarizes current knowledge of this protein family and the potential roles of remorins in membrane rafts.

The SlMKK2 and SlMPK2 genes play a role in tomato disease resistance to Xanthomonas campestris pv. vesicatoria

Xanthomonas campestris pv. vesicatoria (Xcv) is the causal agent of bacterial spot disease in tomato (Solanum lycopersicum) plants. We recently identified a MAPKKK gene, SlMAPKKKε, which is required for tomato resistance to Xcv strains and encodes a positive regulator of cell death. We also provided evidence that the MEK2 MAPKK, and the WIPK and SIPK MAPKs act downstream to MAPKKKε in Nicotiana benthamiana plants. Here, we used the virus-induced gene silencing technique to assess whether tomato homologs of MEK2 (SlMKK2), SIPK (SlMPK1 and SlMPK2), WIPK (SlMPK3), and other components of MAP kinase cascades (SlNPK1, SlMEK1 and SlNTF6), which were previously implicated in plant immunity, are involved in disease resistance to Xcv. Silencing of none of the tested genes caused the appearance of disease symptoms in tomato leaves challenged with an avirulent Xcv strain. However, bacterial populations were significantly higher in leaves of plants silenced for SlMKK2 and SlMPK2, as compared to control plants, suggesting that these two genes contribute to disease resistance to Xcv. It remains to be established whether SlMKK2 and SlMPK2 are activated by SlMAPKKKε directly or through a distinct MAPKKK.  

A successful bacterial coup d'état: how Rhodococcus fascians redirects plant development

Rhodococcus fascians is a gram-positive phytopathogen that induces differentiated galls, known as leafy galls, on a wide variety of plants, employing virulence genes located on a linear plasmid. The pathogenic strategy consists of the production of a mixture of six synergistically acting cytokinins that overwhelm the plant's homeostatic mechanisms, ensuring the activation of a signaling cascade that targets the plant cell cycle and directs the newly formed cells to differentiate into shoot meristems. The shoots that are formed upon infection remain immature and never convert to source tissues resulting in the establishment of a nutrient sink that is a niche for the epiphytic and endophytic R. fascians subpopulations. Niche formation is accompanied by modifications of the transcriptome, metabolome, physiology, and morphology of both host and pathogen. Here, we review a decade of research and set the outlines of the molecular basis of the leafy gall syndrome.

Transcriptional plant responses critical for resistance towards necrotrophic pathogens

Plant defenses aimed at necrotrophic pathogens appear to be genetically complex. Despite the apparent lack of a specific recognition of such necrotrophs by products of major R genes, biochemical, molecular, and genetic studies, in particular using the model plant Arabidopsis, have uncovered numerous host components critical for the outcome of such interactions. Although the JA signaling pathway plays a central role in plant defense toward necrotrophs additional signaling pathways contribute to the plant response network. Transcriptional reprogramming is a vital part of the host defense machinery and several key regulators have recently been identified. Some of these transcription factors positively affect plant resistance whereas others play a role in enhancing host susceptibility toward these phytopathogens.

Comparative analysis of defence responses induced by the endophytic plant growth-promoting rhizobacterium Burkholderia phytofirmans strain PsJN and the non-host bacterium Pseudomonas syringae pv. pisi in grapevine cell suspensions

Plant growth-promoting rhizobacteria (PGPR) are beneficial microorganisms that colonize the rhizosphere of many plant species and confer beneficial effects, such as an increase in plant growth. PGPR are also well known as inducers of systemic resistance to pathogens in plants. However, the molecular mechanisms involved locally after direct perception of these bacteria by plant cells still remain largely unknown. Burkholderia phytofirmans strain PsJN is an endophytic PGPR that colonizes grapevine and protects the plant against the grey mould disease caused by Botrytis cinerea. This report focuses on local defence events induced by B. phytofirmans PsJN after perception by the grapevine cells. It is demonstrated that, after addition to cell suspension cultures, the bacteria were tightly attaching to plant cells in a way similar to the grapevine non-host bacteria Pseudomonas syringae pv. pisi. B. phytofirmans PsJN perception led to a transient and monophasic extracellular alkalinization but no accumulation of reactive oxygen species or cell death were detected. By contrast, challenge with P. syringae pv. pisi induced a sustained and biphasic extracellular alkalinization, a two phases oxidative burst, and a HR-like response. Perception of the PGPR also led to the production of salicylic acid (SA) and the expression of a battery of defence genes that was, however, weaker in intensity compared with defence gene expression triggered by the non-host bacteria. Some defence genes up-regulated after B. phytofirmans PsJN challenge are specifically induced by exogenous treatment with SA or jasmonic acid, suggesting that both signalling pathways are activated by the PGPR in grapevine.

Identification of differentially expressed genes induced by Bamboo mosaic virus infection in Nicotiana benthamiana by cDNA-amplified fragment length polymorphism

BACKGROUND

The genes of plants can be up- or down-regulated during viral infection to influence the replication of viruses. Identification of these differentially expressed genes could shed light on the defense systems employed by plants and the mechanisms involved in the adaption of viruses to plant cells. Differential gene expression in Nicotiana benthamiana plants in response to infection with Bamboo mosaic virus (BaMV) was revealed using cDNA-amplified fragment length polymorphism (AFLP).

RESULTS

Following inoculation with BaMV, N. benthamiana displayed differential gene expression in response to the infection. Isolation, cloning, and sequencing analysis using cDNA-AFLP furnished 90 cDNA fragments with eight pairs of selective primers. Fifteen randomly selected genes were used for a combined virus-induced gene silencing (VIGS) knockdown experiment, using BaMV infection to investigate the roles played by these genes during viral infection, specifically addressing the means by which these genes influence the accumulation of BaMV protein. Nine of the 15 genes showed either a positive or a negative influence on the accumulation of BaMV protein. Six knockdown plants showed an increase in the accumulation of BaMV, suggesting that they played a role in the resistance to viral infection, while three plants showed a reduction in coat protein, indicating a positive influence on the accumulation of BaMV in plants. An interesting observation was that eight of the nine plants showing an increase in BaMV coat protein were associated with cell rescue, defense, death, aging, signal transduction, and energy production.

CONCLUSIONS

This study reports an efficient and straightforward method for the identification of host genes involved in viral infection. We succeeded in establishing a cDNA-AFLP system to help track changes in gene expression patterns in N. benthamiana plants when infected with BaMV. The combination of both DNA-AFLP and VIGS methodologies made it possible to screen a large number of genes and identify those associated with infections of plant viruses. In this report, 9 of the 15 analyzed genes exhibited either a positive or a negative influence on the accumulation of BaMV in N. benthamiana plants.

Neutrophil responses to staphylococcal pathogens and commensals via the formyl peptide receptor 2 relates to phenol-soluble modulin release and virulence

The mechanisms used by the immune system to discriminate between pathogenic and commensal bacteria have remained largely unclear. Recently, we have shown that virulence of Staphylococcus aureus depends on secretion of phenol-soluble modulin (PSM) peptides that disrupt neutrophils at micromolar concentrations. Moreover, all S. aureus PSMs stimulate and attract neutrophils at nanomolar concentrations via interaction with the formyl-peptide receptor 2 (FPR2). Here, we demonstrate that FPR2 allows neutrophils to adjust their responses in relation to the aggressiveness of staphylococcal species, which differ largely in their capacity to infect or colonize humans and animals. PSM-related peptides were detected in all human and animal pathogenic staphylococci, but were absent from most commensal species. Three PSMβ-like peptides produced by the serious human pathogen Staphylococcus lugdunensis were identified as the previously described S. lugdunensis-synergistic hemolysins (SLUSHs). SLUSHs attracted and stimulated human leukocytes in a FPR2-dependent manner, indicating that FPR2 is a general receptor for all PSM-like peptide toxins. Remarkably, the release of PSMs correlated closely with the apparent capacity of staphylococcal species to cause invasive infections and with their ability to activate FPR2. These findings suggest that the innate immune system may be able to respond in different ways to pathogenic or innocuous staphylococci by monitoring the presence of PSMs via FPR2.

Arabidopsis paves the way: genomic and network analyses in crops

Arabidopsis genomic and network analyses have facilitated crop research towards the understanding of many biological processes of fundamental importance for agriculture. Genes that were identified through genomic analyses in Arabidopsis have been used to manipulate crop traits such as pathogen resistance, yield, water-use efficiency, and drought tolerance, with the effects being tested in field conditions. The integration of diverse Arabidopsis genome-wide datasets in probabilistic functional networks has been demonstrated as a feasible strategy to associate novel genes with traits of interest, and novel genomic methods continue to be developed. The combination of genome-wide location studies, using ChIP-Seq, with gene expression profiling data is affording a genome-wide view of regulatory networks previously delineated through genetic and molecular analyses, leading to the identification of novel components and of new connections within these networks.

PAMP (pathogen-associated molecular pattern)-induced changes in plasma membrane compartmentalization reveal novel components of plant immunity

Plasma membrane compartmentalization spatiotemporally regulates cell-autonomous immune signaling in animal cells. To elucidate immediate early protein dynamics at the plant plasma membrane in response to the bacterial pathogen-associated molecular pattern (PAMP) flagellin (flg22) we employed quantitative mass spectrometric analysis on detergent-resistant membranes (DRMs) of Arabidopsis thaliana suspension cells. This approach revealed rapid and profound changes in DRM protein composition following PAMP treatment, prominently affecting proton ATPases and receptor-like kinases, including the flagellin receptor FLS2. We employed reverse genetics to address a potential contribution of a subset of these proteins in flg22-triggered cellular responses. Mutants of three candidates (DET3, AHA1, FER) exhibited a conspicuous defect in the PAMP-triggered accumulation of reactive oxygen species. In addition, these mutants showed altered mitogen-activated protein kinase (MAPK) activation, a defect in PAMP-triggered stomatal closure as well as altered bacterial infection phenotypes, which revealed three novel players in elicitor-dependent oxidative burst control and innate immunity. Our data provide evidence for dynamic elicitor-induced changes in the membrane compartmentalization of PAMP signaling components.

Rhamnolipid biosurfactants as new players in animal and plant defense against microbes

Rhamnolipids are known as very efficient biosurfactant molecules. They are used in a wide range of industrial applications including food, cosmetics, pharmaceutical formulations and bioremediation of pollutants. The present review provides an overview of the effect of rhamnolipids in animal and plant defense responses. We describe the current knowledge on the stimulation of plant and animal immunity by these molecules, as well as on their direct antimicrobial properties. Given their ecological acceptance owing to their low toxicity and biodegradability, rhamnolipids have the potential to be useful molecules in medicine and to be part of alternative strategies in order to reduce or replace pesticides in agriculture.

Genetic dissection of basal resistance to Pseudomonas syringae pv. phaseolicola in accessions of Arabidopsis

We have examined the genetics of nonhost resistance in Arabidopsis, using the bean pathogen Pseudomonas syringae pv. phaseolicola race 6 1448A to probe accessions for natural variation in basal defense. Symptoms rarely developed in leaves of Niedersenz (Nd), some yellowing and occasional necrosis developed in Columbia (Col), whereas tissue collapse was observed in Wassilewskija (Ws) after inoculation by infiltration. Analysis of F2 progeny and recombinant inbred lines (RIL) from a cross between Col and Nd revealed a pattern of continuous symptom increase, indicating the operation of quantitative determinants of resistance. By mapping quantitative trait loci (QTL), significant linkage was determined for resistance (low symptom score) to markers on chromosome 4. Segregation in the F2 cross from Nd × Ws indicated the operation of two dominant genes for resistance, one of which was FLS2 encoding the flagellin receptor. The requirement for FLS2 to confer resistance was confirmed by transgenic experiments, and we showed that the response to P. syringae pv. phaseolicola was affected by FLS2 gene dosage. Using RIL, the second locus was mapped as a QTL to a large interval on chromosome 1. Both FLS2 and the QTL on chromosome 1 were required for the highest level of resistance to bacterial colonization and symptom development in Nd.

Understanding the plant immune system

Plant immunity is controlled by a complex signaling network. Here, we discuss how the complexity of the network affects our views and approaches in studying the plant immune network. We propose that the mode of plant immunity is mainly determined by how the shared signaling network is used rather than by a signaling machinery specific to each mode, that balancing the robustness of immunity and the negative effect of immunity on plant fitness is a key driver in evolution of the immune network, that comparisons of plant mutant to wild-type phenotypes may not be very effective in elucidating the underlying signaling mechanisms, and that mechanistic understanding of the network can improve our ability to predict the performance of immunity.

Endocytosis in plant-microbe interactions

Plants encounter throughout their life all kinds of microorganisms, such as bacteria, fungi, or oomycetes, with either friendly or unfriendly intentions. During evolution, plants have developed a wide range of defense mechanisms against attackers. In return, adapted microbes have developed strategies to overcome the plant lines of defense, some of these microbes engaging in mutualistic or parasitic endosymbioses. By sensing microbe presence and activating signaling cascades, the plasma membrane through its dynamics plays a crucial role in the ongoing molecular dialogue between plants and microbes. This review describes the contribution of endocytosis to different aspects of plant-microbe interactions, microbe recognition and development of a basal immune response, and colonization of plant cells by endosymbionts. The putative endocytic routes for the entry of microbe molecules or microbes themselves are explored with a special emphasis on clathrin-mediated endocytosis. Finally, we evaluate recent findings that suggest a link between the compartmentalization of plant plasma membrane into microdomains and endocytosis.

Autoacetylation of the Ralstonia solanacearum effector PopP2 targets a lysine residue essential for RRS1-R-mediated immunity in Arabidopsis

Type III effector proteins from bacterial pathogens manipulate components of host immunity to suppress defence responses and promote pathogen development. In plants, host proteins targeted by some effectors called avirulence proteins are surveyed by plant disease resistance proteins referred to as “guards”. The Ralstonia solanacearum effector protein PopP2 triggers immunity in Arabidopsis following its perception by the RRS1-R resistance protein. Here, we show that PopP2 interacts with RRS1-R in the nucleus of living plant cells. PopP2 belongs to the YopJ-like family of cysteine proteases, which share a conserved catalytic triad that includes a highly conserved cysteine residue. The catalytic cysteine mutant PopP2-C321A is impaired in its avirulence activity although it is still able to interact with RRS1-R. In addition, PopP2 prevents proteasomal degradation of RRS1-R, independent of the presence of an integral PopP2 catalytic core. A liquid chromatography/tandem mass spectrometry analysis showed that PopP2 displays acetyl-transferase activity leading to its autoacetylation on a particular lysine residue, which is well conserved among all members of the YopJ family. These data suggest that this lysine residue may correspond to a key binding site for acetyl-coenzyme A required for protein activity. Indeed, mutation of this lysine in PopP2 abolishes RRS1-R-mediated immunity. In agreement with the guard hypothesis, our results favour the idea that activation of the plant immune response by RRS1-R depends not only on the physical interaction between the two proteins but also on its perception of PopP2 enzymatic activity.

Plant and animal bacterial pathogens have evolved to produce virulence factors, called type III effectors, which are injected into host cells to suppress host defences and provide an environment beneficial for pathogen growth. Type III effectors from pathogenic bacteria display enzymatic activities, often mimicking an endogenous eukaryotic activity, to target host signalling pathways. Elucidation of strategies used by pathogens to manipulate host protein activities is a subject of fundamental interest in pathology. PopP2 is a YopJ-like effector from the soil borne root pathogen Ralstonia solanacearum. Here, in addition to demonstrating PopP2 ability to stabilize the expression of its cognate Arabidopsis RRS1-R resistance protein and physically interact with it, we investigated the enzymatic activity of PopP2. Bacterial YopJ-like effectors are predicted to act as acetyl-transferases on host components. However, only two YopJ-like proteins from animal pathogens have been shown to be active acetyl-transferases. We show that PopP2 displays autoacetyl-transferase activity targeting a lysine residue well-conserved among YopJ-like family members. This lysine is a critical residue since its mutation prevents autoacetylation of PopP2 and abolishes its recognition by the host. This study provides new clues on the multiple properties displayed by bacterial type III effectors that may be used to target defense-related host components.

Ca2+ signaling by plant Arabidopsis thaliana Pep peptides depends on AtPepR1, a receptor with guanylyl cyclase activity, and cGMP-activated Ca2+ channels

A family of peptide signaling molecules (AtPeps) and their plasma membrane receptor AtPepR1 are known to act in pathogen-defense signaling cascades in plants. Little is currently known about the molecular mechanisms that link these signaling peptides and their receptor, a leucine-rich repeat receptor-like kinase, to downstream pathogen-defense responses. We identify some cellular activities of these molecules that provide the context for a model for their action in signaling cascades. AtPeps activate plasma membrane inwardly conducting Ca(2+) permeable channels in mesophyll cells, resulting in cytosolic Ca(2+) elevation. This activity is dependent on their receptor as well as a cyclic nucleotide-gated channel (CNGC2). We also show that the leucine-rich repeat receptor-like kinase receptor AtPepR1 has guanylyl cyclase activity, generating cGMP from GTP, and that cGMP can activate CNGC2-dependent cytosolic Ca(2+) elevation. AtPep-dependent expression of pathogen-defense genes (PDF1.2, MPK3, and WRKY33) is mediated by the Ca(2+) signaling pathway associated with AtPep peptides and their receptor. The work presented here indicates that extracellular AtPeps, which can act as danger-associated molecular patterns, signal by interaction with their receptor, AtPepR1, a plasma membrane protein that can generate cGMP. Downstream from AtPep and AtPepR1 in a signaling cascade, the cGMP-activated channel CNGC2 is involved in AtPep- and AtPepR1-dependent inward Ca(2+) conductance and resulting cytosolic Ca(2+) elevation. The signaling cascade initiated by AtPeps leads to expression of pathogen-defense genes in a Ca(2+)-dependent manner.

Ectopic expression of rice Xa21 overcomes developmentally controlled resistance to Xanthomonas oryzae pv. oryzae

Recognition of pathogen-associated molecular patterns (PAMPs) by pattern recognition receptors (PRRs) activates the innate immune response. The rice PRR, XA21, confers robust resistance at adult stages to most strains of the bacterial pathogen Xanthomonas oryzae pv. oryzae (Xoo). Seedlings are still easily infected by Xoo, causing severe yield losses. Here we report that Xa21 is induced by Xoo infection and that ectopic expression of Xa21 confers resistance at three leaf stage (three-week-old), overcoming the developmental limitation of XA21-mediated resistance. Ectopic expression of Xa21 also up-regulates a larger set of defense-related genes as compared to Xa21 driven by the native promoter. These results indicate that altered regulation of Xa21 expression is useful for developing enhanced resistance to Xoo at multiple developmental stages.

RLK7, a leucine-rich repeat receptor-like kinase, is required for proper germination speed and tolerance to oxidative stress in Arabidopsis thaliana

The leucine-rich repeat class of receptor-like kinase (LRR-RLKs) encoding genes represents the largest family of putative receptor genes in the Arabidopsis thaliana genome. However, very little is known about the range of biological process that they control. We present in this paper the functional characterization of RLK7 that has all the structural features of a receptor-like kinase of the plant-specific LRR type. To this end, we identified and characterized three independent T-DNA insertion mutants, constructed lines carrying truncated versions of this putative receptor, one lacking the cytoplasmic kinase domain (RLK7Δkin) and the other one lacking 14 LRR repeats (RLK7ΔLRR) and generated RLK7 overexpressing lines. We thus provide evidences that RLK7 is involved in the control of germination speed and the tolerance to oxidant stress. First, consistent with the expression kinetics of the RLK7 gene in the seeds, we found that all three mutants showed a delay in germination, whereas the overexpressors, RLK7Δkin and RLK7ΔLRR lines displayed a phenotype of more precocious germination. Second, a non-hypothesis driven proteomic approach revealed that in the seedlings of the three T-DNA insertion lines, four enzymes directly or indirectly involved in reactive oxygen species detoxification, were significantly less abundant. Consistent with this finding, the three mutants were less tolerant than the wild type to a hydrogen peroxide treatment, whereas the overexpressors, RLK7Δkin and RLK7ΔLRR lines presented the opposite phenotype.

Lipopolysaccharide mobility in leaf tissue of Arabidopsis thaliana

Bacterial lipopolysaccharides (LPS) are triggers of defence responses in plants, and induce local as well as systemic acquired resistance. Arabidopsis thaliana plants pretreated with LPS show an increased resistance to the virulent bacterial plant pathogen Pseudomonas syringae pv. tomato DC3000. To investigate the mobilization and transport of LPS in Arabidopsis leaves, fluorescently labelled LPS (Alexa Fluor® 488 conjugate) from Salmonella minnesota was used. Leaves were pressure infiltrated with fluorescein-labelled LPS and fluorescence microscopy was used to follow the movement and localization of LPS as a function of time. The observation of leaves 1 h after supplementation with fluorescein-labelled LPS revealed a fluorescent signal in the intercellular space. Capillary zone electrophoresis was used for the detection and analysis of the labelled LPS in directly treated leaves and systemic leaves. In addition, gel electrophoresis was used to confirm LPS mobilization. The results indicated that LPS mobilization/translocation occurs through the xylem from local, treated leaves to systemic, untreated leaves. Consequently, care should be taken when ascribing the observed biochemical responses and induced resistance from LPS perception as being uniquely local or systemic, as these responses might overlap because of the mobility of LPS in the plant vascular system.

Effector-triggered innate immunity contributes Arabidopsis resistance to Xanthomonas campestris

Xanthomonas campestris pv. campestris, the causal agent of black rot disease, depends on its type III secretion system (TTSS) to infect cruciferous plants, including Brassica oleracea, B. napus and Arabidopsis. Previous studies on the Arabidopsis-Pseudomonas syringae model pathosystem have indicated that a major function of TTSS from virulent bacteria is to suppress host defences triggered by pathogen-associated molecular patterns. Similar analyses have not been made for the Arabidopsis-X. campestris pv. campestris pathosystem. In this study, we report that X. campestris pv. campestris strain 8004, which is modestly pathogenic on Arabidopsis, induces strong defence responses in Arabidopsis in a TTSS-dependent manner. Furthermore, the induction of defence responses and disease resistance to X. campestris pv. campestris strain 8004 requires NDR1 (NON-RACE-SPECIFIC DISEASE RESISTANCE1), RAR1 (required for Mla12 resistance) and SGT1b (suppressor of G2 allele of skp1), suggesting that effector-triggered immunity plays a large role in resistance to this strain. Consistent with this notion, AvrXccC, an X. campestris pv. campestris TTSS effector protein, induces PR1 expression and confers resistance in Arabidopsis in a RAR1- and SGT1b-dependent manner. In rar1 and sgt1b mutants, AvrXccC acts as a virulence factor, presumably because of impaired resistance gene function.

Tomato MAPKKKε is a positive regulator of cell-death signaling networks associated with plant immunity

Mitogen-activated protein (MAP) kinase cascades are fundamental components of the signaling pathways associated with plant immunity. Despite the large number of MAP kinase kinase kinases (MAPKKK) encoded in the plant genome, only very few of them have an assigned function. Here, we identified MAPKKK gene of tomato (Solanum lycopersicum), SIMAPKKKε, which is required for hypersensitive response cell death and disease resistance against Gram-negative bacterial pathogens. Silencing of SIMAPKKKε compromised tomato resistance to Xanthomonas campestris and Pseudomonas syringae strains, resulting in the appearance of disease symptoms and enhanced bacterial growth. In addition, silencing of NbMAPKKKε in Nicotiana benthamiana plants significantly inhibited the cell death triggered by expression of different R gene/effector gene pairs. Conversely, overexpression of either the full-length SIMAPKKKε gene or its kinase domain in N. benthamiana leaves caused pathogen-independent activation of cell death that required an intact kinase catalytic domain. Moreover, by suppressing the expression of various MAPKK and MAPK genes and overexpressing the SIMAPKKKε kinase domain, we identified a signaling cascade acting downstream of SIMAPKKKε that includes MEK2, WIPK and SIPK. Additional epistasis experiments revealed that SIPKK functions as a negative regulator of SIMAPKKKε-mediated cell death. Our results provide evidence that SIMAPKKKε is a signaling molecule that positively regulates cell death networks associated with plant immunity.

Role of the pepper cytochrome P450 gene CaCYP450A in defense responses against microbial pathogens

Plant cytochrome P450 enzymes are involved in a wide range of biosynthetic reactions, leading to various fatty acid conjugates, plant hormones, or defensive compounds. Herein, we have identified the pepper cytochrome P450 gene CaCYP450A, which is differentially induced during Xanthomonas campestris pv. vesicatoria (Xcv) infection. CaCYP450A contains a heme-binding motif, PXFXXGXRXCXG, located in the C-terminal region and a hydrophobic membrane anchor region at the N terminal. Knock-down of CaCYP450A by virus-induced gene silencing (VIGS) led to increased susceptibility to Xcv infection in pepper. CaCYP450A-overexpressing Arabidopsis plants exhibited lower pathogen growth and reduced disease symptoms, and they were more resistant to Pseudomonas syringae pv. tomato (Pst) and Hyaloperonospora arabidopsidis than wild-type plants. Overexpression of CaCYP450A also enhanced H(2)O(2) accumulation and cell death. However, CaCYP450A Arabidopsis ortholog CYP94B3 mutants showed enhanced susceptibility to virulent Pst DC3000, but not to avirulent Pst DC3000 avrRpm1 or virulent H. arabidopsidis infection. Taken together, these results suggest that CaCYP450A is required for defense responses to microbial pathogens in plants. The nucleotide sequence data reported here has been deposited in the GenBank database under the accession number HM581974.

All hands on deck—the role of chloroplasts, endoplasmic reticulum, and the nucleus in driving plant innate immunity

Plant innate immunity is mediated by cell membrane and intracellular immune receptors that function in distinct and overlapping cell-signaling pathways to activate defense responses. It is becoming increasingly evident that immune receptors rely on components from multiple organelles for the generation of appropriate defense responses. This review analyzes the defense-related functions of the chloroplast, nucleus, and endoplasmic reticulum (ER) during plant innate immunity. It details the role of the chloroplasts in synthesizing defense-specific second messengers and discusses the retrograde signal transduction pathways that exist between the chloroplast and nucleus. Because the activities of immune modulators are regulated, in part, by their subcellular localization, the review places special emphasis on the dynamics and nuclear–cytoplasmic transport of immune receptors and regulators and highlights the importance of this process in generating orderly events during an innate immune response. The review also covers the recently discovered contributions of the ER quality-control pathways in ensuring the signaling competency of cell surface immune receptors or immune regulators.

Structural and functional analysis of the type III secretion system from Pseudomonas fluorescens Q8r1-96

Pseudomonas fluorescens Q8r1-96 represents a group of rhizosphere strains responsible for the suppressiveness of agricultural soils to take-all disease of wheat. It produces the antibiotic 2,4-diacetylphloroglucinol and aggressively colonizes the roots of cereal crops. In this study, we analyzed the genome of Q8r1-96 and identified a type III protein secretion system (T3SS) gene cluster that has overall organization similar to that of the T3SS gene cluster of the plant pathogen Pseudomonas syringae. We also screened a collection of 30 closely related P. fluorescens strains and detected the T3SS genes in all but one of them. The Q8r1-96 genome contained ropAA and ropM type III effector genes, which are orthologs of the P. syringae effector genes hopAA1-1 and hopM1, as well as a novel type III effector gene designated ropB. These type III effector genes encoded proteins that were secreted in culture and injected into plant cells by both P. syringae and Q8r1-96 T3SSs. The Q8r1-96 T3SS was expressed in the rhizosphere, but mutants lacking a functional T3SS were not altered in their rhizosphere competence. The Q8r1-96 type III effectors RopAA, RopB, and RopM were capable of suppressing the hypersensitive response and production of reactive oxygen species, two plant immune responses.

The long and winding road: virulence effector proteins of plant pathogenic bacteria

Plant pathogenic bacteria inject about 30 virulence effector proteins into the host cell using a specialized secretion apparatus. Bacteria which are unable to do this elicit host immunity and cannot grow inside living plant tissue. Thus, the primary function of the effectors is to suppress host immunity. The identity of individual effectors within each complement varies even between closely related bacterial strains, and effectors themselves act redundantly and are apparently interchangeable. Many effectors are known to target components of plant defense pathways, but it is difficult to study their role in molecular terms. For some of them, there is controversy about their mode of action. We propose that effectors act promiscuously by targeting host molecules with low specificity and affinity.

Pseudomonas syringae virulence factor syringolin A counteracts stomatal immunity by proteasome inhibition

The peptide derivative syringolin A, a product of a mixed nonribosomal peptide and polyketide synthetase, is secreted by certain strains of the phytopathogenic bacterium Pseudomonas syringae pv. syringae. Syringolin A was shown to be a virulence factor for P. syringae pv. syringae B728a because disease symptoms on its host Phaseolus vulgaris (bean) were greatly reduced upon inoculation with syringolin A-negative mutants. Syringolin A's mode of action was recently shown to be irreversible proteasome inhibition. Here, we report that syringolin A-producing bacteria are able to open stomata and, thus, counteract stomatal innate immunity in bean and Arabidopsis. Syringolin A-negative mutants, which induce stomatal closure, can be complemented by exogenous addition of not only syringolin A but also MG132, a well-characterized and structurally unrelated proteasome inhibitor. This demonstrates that proteasome activity is crucial for guard cell function. In Arabidopsis, stomatal immunity was salicylic acid (SA)-dependent and required NPR1, a key regulator of the SA-dependent defense pathway whose proteasome-dependent turnover has been reported to be essential for its function. Thus, elimination of NPR1 turnover through proteasome inhibition by syringolin A is an attractive hypothesis to explain the observed inhibition of stomatal immunity by syringolin A.