Showing papers on "Effector-triggered immunity published in 2017"

Convergent and Divergent Signaling in PAMP-Triggered Immunity and Effector-Triggered Immunity

Abstract: Plants use diverse immune receptors to sense pathogen attacks. Recognition of pathogen-associated molecular patterns (PAMPs) by pattern recognition receptors localized on the plasma membrane leads to PAMP-triggered immunity (PTI). Detection of pathogen effectors by intracellular or plasma membrane-localized immune receptors results in effector-triggered immunity (ETI). Despite the large variations in the magnitude and duration of immune responses triggered by different PAMPs or pathogen effectors during PTI and ETI, plasma membrane-localized immune receptors activate similar downstream molecular events such as mitogen-activated protein kinase activation, oxidative burst, ion influx, and increased biosynthesis of plant defense hormones, indicating that defense signals initiated at the plasma membrane converge at later points. On the other hand, activation of ETI by immune receptors localized to the nucleus appears to be more directly associated with transcriptional regulation of defense gene expression. Here, we review recent progress in signal transductions downstream of different groups of plant immune receptors, highlighting the converging and diverging molecular events.

Convergent Evolution of Pathogen Effectors toward Reactive Oxygen Species Signaling Networks in Plants.

Abstract: Microbial pathogens have evolved protein effectors to promote virulence and cause disease in host plants. Pathogen effectors delivered into plant cells suppress plant immune responses and modulate host metabolism to support the infection processes of pathogens. Reactive oxygen species (ROS) act as cellular signaling molecules to trigger plant immune responses, such as pathogen-associated molecular pattern (PAMP)-triggered immunity (PTI) and effector-triggered immunity (ETI). In this review, we discuss recent insights into the molecular functions of pathogen effectors that target multiple steps in the ROS signaling pathway in plants. The perception of PAMPs by pattern recognition receptors (PRRs) leads to the rapid and strong production of ROS through activation of NADPH oxidase Respiratory Burst Oxidase Homologs (RBOHs) as well as peroxidases. Specific pathogen effectors directly or indirectly interact with plant nucleotide-binding leucine-rich repeat (NLR) receptors to induce ROS production and the hypersensitive response (HR) in plant cells. By contrast, virulent pathogens possess effectors capable of suppressing plant ROS bursts in different ways during infection. PAMP-triggered ROS bursts are suppressed by pathogen effectors that target mitogen-activated protein kinase (MAPK) cascades. Moreover, pathogen effectors target vesicle trafficking or metabolic priming, leading to the suppression of ROS production. Secreted pathogen effectors block the metabolic coenzyme MADP-malic enzyme (ME), inhibiting the transfer of electrons to the NADPH oxidases (RBOHs) responsible for ROS generation. Collectively, pathogen effectors may have evolved to converge on a common host protein network to suppress the common plant immune system, including the ROS burst and cell death response in plants.

Immune Receptors and Co-receptors in Antiviral Innate Immunity in Plants.

Abstract: Plants respond to pathogens using an innate immune system that is broadly divided into PTI (pathogen-associated molecular pattern- or PAMP-triggered immunity) and ETI (effector-triggered immunity). PTI is activated upon perception of PAMPs, conserved motifs derived from pathogens, by surface membrane-anchored pattern recognition receptors (PRRs). To overcome this first line of defense, pathogens release into plant cells effectors that inhibit PTI and activate effector-triggered susceptibility (ETS). Counteracting this virulence strategy, plant cells synthesize intracellular resistance (R) proteins, which specifically recognize pathogen effectors or avirulence factors and activate ETI. These coevolving pathogen virulence strategies and plant resistance mechanisms illustrate evolutionary arms race between pathogen and host, which is integrated into the zigzag model of plant innate immunity. Although antiviral immune concepts have been initially excluded from the zigzag model, recent studies have provided several lines of evidence substantiating the notion that plants deploy the innate immune system to fight viruses in a manner similar to that used for non-viral pathogens. First, most resistance (R) proteins against viruses so far characterized share structural similarity with antibacterial and antifungal R gene products and elicit typical ETI-based immune responses. Second, virus-derived PAMPs may activate PTI-like responses through immune co-receptors of plant PTI. Finally, and even more compelling, a viral avirulence factor that triggers ETI in resistant genotypes has recently been shown to act as a suppressor of PTI, integrating plant viruses into the coevolutionary model of host-pathogen interactions, the zigzag model. In this review, we summarize these important progresses, focusing on the potential significance of antiviral immune receptors and co-receptors in plant antiviral innate immunity. In light of the innate immune system, we also discuss a newly uncovered layer of antiviral defense that is specific to plant DNA viruses and relies on transmembrane receptor-mediated translational suppression for defense.

Signaling from the plasma-membrane localized plant immune receptor RPM1 requires self-association of the full-length protein

Abstract: Plants evolved intracellular immune receptors that belong to the NOD-like receptor (NLR) family to recognize the presence of pathogen-derived effector proteins. NLRs possess an N-terminal Toll-like/IL-1 receptor (TIR) or a non-TIR domain [some of which contain coiled coils (CCs)], a central nucleotide-binding (NB-ARC) domain, and a C-terminal leucine-rich repeat (LRR). Activation of NLR proteins results in a rapid and high-amplitude immune response, eventually leading to host cell death at the infection site, the so-called hypersensitive response. Despite their important contribution to immunity, the exact mechanisms of NLR activation and signaling remain unknown and are likely heterogenous. We undertook a detailed structure-function analysis of the plasma membrane (PM)-localized CC NLR Resistance to Pseudomonas syringae pv. maculicola 1 (RPM1) using both stable transgenic Arabidopsis and transient expression in Nicotiana benthamiana. We report that immune signaling is induced only by activated full-length PM-localized RPM1. Our interaction analyses demonstrate the importance of a functional P-loop for in planta interaction of RPM1 with the small host protein RPM1-interacting protein 4 (RIN4), for constitutive preactivation and postactivation self-association of RPM1 and for proper PM localization. Our results reveal an additive effect of hydrophobic conserved residues in the CC domain for RPM1 function and RPM1 self-association and their necessity for RPM1–RIN4 interaction. Thus, our findings considerably extend our understanding of the mechanisms regulating NLR activation at, and signaling from, the PM.

Pathogen exploitation of an abscisic acid- and jasmonate-inducible MAPK phosphatase and its interception by Arabidopsis immunity.

Abstract: Phytopathogens promote virulence by, for example, exploiting signaling pathways mediated by phytohormones such as abscisic acid (ABA) and jasmonate (JA). Some plants can counteract pathogen virulence by invoking a potent form of immunity called effector-triggered immunity (ETI). Here, we report that ABA and JA mediate inactivation of the immune-associated MAP kinases (MAPKs), MPK3 and MPK6, in Arabidopsis thaliana ABA induced expression of genes encoding the protein phosphatases 2C (PP2Cs), HAI1, HAI2, and HAI3 through ABF/AREB transcription factors. These three HAI PP2Cs interacted with MPK3 and MPK6 and were required for ABA-mediated MPK3/MPK6 inactivation and immune suppression. The bacterial pathogen Pseudomonas syringae pv. tomato (Pto) DC3000 activates ABA signaling and produces a JA-mimicking phytotoxin, coronatine (COR), that promotes virulence. We found that Pto DC3000 induces HAI1 through COR-mediated activation of MYC2, a master transcription factor in JA signaling. HAI1 dephosphorylated MPK3 and MPK6 in vitro and was necessary for COR-mediated suppression of MPK3/MPK6 activation and immunity. Intriguingly, upon ETI activation, A. thaliana plants overcame the HAI1-dependent virulence of COR by blocking JA signaling. Finally, we showed conservation of induction of HAI PP2Cs by ABA and JA in other Brassicaceae species. Taken together, these results suggest that ABA and JA signaling pathways, which are hijacked by the bacterial pathogen, converge on the HAI PP2Cs that suppress activation of the immune-associated MAPKs. Also, our data unveil interception of JA-signaling activation as a host counterstrategy against the bacterial suppression of MAPKs during ETI.

A plant effector-triggered immunity signaling sector is inhibited by pattern-triggered immunity.

Abstract: Since signaling machineries for two modes of plant‐induced immunity, pattern‐triggered immunity (PTI) and effector‐triggered immunity (ETI), extensively overlap, PTI and ETI signaling likely interact. In an Arabidopsis quadruple mutant, in which four major sectors of the signaling network, jasmonate, ethylene, PAD4, and salicylate, are disabled, the hypersensitive response (HR) typical of ETI is abolished when the Pseudomonas syringae effector AvrRpt2 is bacterially delivered but is intact when AvrRpt2 is directly expressed in planta . These observations led us to discovery of a network‐buffered signaling mechanism that mediates HR signaling and is strongly inhibited by PTI signaling. We named this mechanism the ETI‐Mediating and PTI‐Inhibited Sector (EMPIS). The signaling kinetics of EMPIS explain apparently different plant genetic requirements for ETI triggered by different effectors without postulating different signaling machineries. The properties of EMPIS suggest that information about efficacy of the early immune response is fed back to the immune signaling network, modulating its activity and limiting the fitness cost of unnecessary immune responses.

Conserved RxLR effectors from oomycetes Hyaloperonospora arabidopsidis and Phytophthora sojae suppress PAMP- and effector-triggered immunity in diverse plants

Abstract: Effector proteins are exported to the interior of host cells by diverse plant pathogens. Many oomycete pathogens maintain large families of candidate effector genes, encoding proteins with a secret...

Arabidopsis thaliana methionine sulfoxide reductase B8 influences stress-induced cell death and effector-triggered immunity.

Abstract: Reactive oxygen species (ROS) oxidize methionine to methionine sulfoxide (MetSO) and thereby inactivate proteins. Methionine sulfoxide reductase (MSR) enzyme converts MetSO back to the reduced form and thereby detoxifies the effect of ROS. Our results show that Arabidopsis thaliana MSR enzyme coding gene MSRB8 is required for effector-triggered immunity and containment of stress-induced cell death in Arabidopsis. Plants activate pattern-triggered immunity (PTI), a basal defense, upon recognition of evolutionary conserved molecular patterns present in the pathogens. Pathogens release effector molecules to suppress PTI. Recognition of certain effector molecules activates a strong defense, known as effector-triggered immunity (ETI). ETI induces high-level accumulation of reactive oxygen species (ROS) and hypersensitive response (HR), a rapid programmed death of infected cells. ROS oxidize methionine to methionine sulfoxide (MetSO), rendering several proteins nonfunctional. The methionine sulfoxide reductase (MSR) enzyme converts MetSO back to the reduced form and thereby detoxifies the effect of ROS. Though a few plant MSR genes are known to provide tolerance against oxidative stress, their role in plant–pathogen interaction is not known. We report here that activation of cell death by avirulent pathogen or UV treatment induces expression of MSRB7 and MSRB8 genes. The T-DNA insertion mutant of MSRB8 exaggerates HR-associated and UV-induced cell death and accumulates a higher level of ROS than wild-type plants. The negative regulatory role of MSRB8 in HR is further supported by amiRNA and overexpression lines. Mutants and overexpression lines of MSRB8 are susceptible and resistant respectively, compared to the wild-type plants, against avirulent strains of Pseudomonas syringae pv. tomato DC3000 (Pst) carrying AvrRpt2, AvrB, or AvrPphB genes. However, the MSRB8 gene does not influence resistance against virulent Pst or P. syringae pv. maculicola (Psm) pathogens. Our results altogether suggest that MSRB8 function is required for ETI and containment of stress-induced cell death in Arabidopsis.

Unusual evolutionary mechanisms to escape effector-triggered immunity in the fungal phytopathogen Leptosphaeria maculans.

Abstract: Summary Leptosphaeria maculans is the fungus responsible for the stem canker disease of oilseed rape (Brassica napus). AvrLm3 and AvrLm4-7, two avirulence effector genes of L. maculans, are involved in an unusual relationship: AvrLm4-7 suppresses the Rlm3-mediated resistance. Here, we assessed AvrLm3 polymorphism in a collection of 235 L. maculans isolates. No field isolates exhibited deletion or inactivating mutations in AvrLm3, as observed for other L. maculans avirulence genes. Eleven isoforms of the AvrLm3 protein were found. In isolates virulent towards both Rlm3 and Rlm7 (a3a7), the loss of the Rlm3-mediated resistance response was due to two distinct mechanisms. First, when AvrLm4-7 was inactivated (deletion or inactivating mutations), amino acid substitutions in AvrLm3 generated virulent isoforms of the protein. Secondly, when only point mutations were observed in AvrLm4-7, a3a7 isolates still contained an avirulent allele of AvrLm3. Directed mutagenesis confirmed that some point mutations in AvrLm4-7 were sufficient for the fungus to escape Rlm7-mediated resistance while maintaining the suppression of the AvrLm3 phenotype. Signatures of positive selection were also identified in AvrLm3. The complex evolutionary mechanisms enabling L. maculans to escape Rlm3-mediated resistance while preserving AvrLm3 integrity, along with observed reduced aggressiveness of isolates silenced for AvrLm3, serves to emphasize the importance of this effector in pathogenicity towards B. napus. While the common response to resistance gene pressure is local selection of isolates depleted in the cognate avirulence gene, this example contributes to complexify the gene-for-gene concept of plant-pathogen evolution with a “camouflaged” model allowing retention of non-dispensable avirulence effectors. This article is protected by copyright. All rights reserved.

Harnessing Effector-Triggered Immunity for Durable Disease Resistance.

Abstract: Genetic control of plant diseases has traditionally included the deployment of single immune receptors with nucleotide-binding leucine-rich repeat (NLR) domain architecture. These NLRs recognize corresponding pathogen effector proteins inside plant cells, resulting in effector-triggered immunity (ETI). Although ETI triggers robust resistance, deployment of single NLRs can be rapidly overcome by pathogen populations within a single or a few growing seasons. In order to generate more durable disease resistance against devastating plant pathogens, a multitiered strategy that incorporates stacked NLRs combined with other sources of disease resistance is necessary. New genetic and genomic technologies have enabled advancements in identifying conserved pathogen effectors, isolating NLR repertoires from diverse plants, and editing plant genomes to enhance resistance. Significant advancements have also been made in understanding plant immune perception at the receptor level, which has promise for engineering new sources of resistance. Here, we discuss how to utilize recent scientific advancements in a multilayered strategy for developing more durable disease resistance.

Arabidopsis thaliana serpins AtSRP4 and AtSRP5 negatively regulate stress-induced cell death and effector-triggered immunity induced by bacterial effector AvrRpt2.

Abstract: Protease inhibitors and their cognate proteases regulate growth, development and defense. Serine protease inhibitors (serpins) constitute a large family of genes in most metazoans and plants. Drosophila NECROTIC (NEC) gene and its homologues in the mammalian system are well-characterized serpins, which play a role in regulating proteases that participate in cell death pathways. Though the Arabidopsis genome contains several serpin homologs, biological function is not known for most of them. Here we show that two Arabidopsis serpins, AtSRP4 and AtSRP5, are closest sequence homologue of Drosophila NEC protein, and are involved in stress-induced cell death and defense. Expression of both AtSRP4 and AtSRP5 genes induced upon UV-treatment and inoculation with avirulent pathogens. The knockout mutants and amiRNA lines of AtSRP4 and AtSRP5 exaggerated UV- and hypersensitive response (HR)- induced cell death. Over-expression of AtSRP4 reduced UV- and HR -induced cell death. Mutants of AtSRP4 and AtSRP5 suppressed whereas over-expression of AtSRP4 supported the growth of bacterial pathogen Pseudomonas syringae pv tomato DC3000 carrying the AvrRpt2 effector, but not other avirulent or virulent pathogens. Results altogether identified AtSRP4 and AtSRP5 as negative regulators of stress-induced cell death and AvrRpt2-triggered immunity, however, the influence of AtSRP4 was more prominent than AtSRP5.

8 – Salicylic acid

Abstract: The phytohormone salicylic acid (SA) plays important roles in the regulation of responses to biotic and abiotic stress, as well as developmental processes, but the most extensive research has been focused on characterization of SA signaling in plant–microbial pathogen interactions. In response to pathogen infection, plants activate SA biosynthesis, which is essential for defense responses, such as PAMP (pathogen-associated molecular pattern)-triggered immunity (also known as basal resistance), effector-triggered immunity, and establishment of systemic acquired resistance. Studies revealed that Nonexpresser of Pathogenesis-Related protein 1 (NPR1), 3, and 4 are SA receptors, which are responsible for SA-mediated defense gene expression as well as regulation of cell fate in local infected tissues and systemic noninfected tissues. Defense gene expression is activated by SA mainly through NPR1, the activity of which is tightly regulated by cytoplasm-to-nucleus translocation and posttranslational modifications, such as phosphorylation, ubiquitination, and sumoylation. This chapter mainly summarizes advances concerning SA biosynthesis and signaling pathways.

A bacterial GW-effector targets Arabidopsis AGO1 to promote pathogenicity and induces Effector-triggered immunity by disrupting AGO1 homeostasis

Abstract: SUMMARY Pseudomonas syringae type-III effectors were previously found to suppress the Arabidopsis miRNA pathway through elusive mechanisms. Here, we first show that HopT1-1 effector promotes pathogenicity by suppressing the Arabidopsis AGO1-dependent microRNA (miRNA) pathway. We further demonstrate that HopT1-1 interacts with, and suppresses the activity of, AGO1 through conserved glycine/tryptophan-(GW) motifs. HopT1-1 dampens PAMP-Triggered-Immunity (PTI) in a GW-dependent manner and its presence mimics the impaired PTI responses, which were also observed in ago1 mutants. In addition, the silencing suppression activity of HopT1-1 induces Effector-Triggered-Immunity (ETI), which is correlated with an over-accumulation of silencing factors that are controlled by miRNAs, including AGO1. Remarkably, alleviating miR168-directed silencing of AGO1 was sufficient to trigger an ETI-like response, orchestrated by typical disease resistance-immune signaling factors, suggesting that HopT1-1-induced perturbation of AGO1 homeostasis is a trigger of ETI activation. In summary, this study reports for the first time a strategy used by a bacterial effector to directly target an AGO protein and on how plants perceive its silencing suppression activity to trigger a host counter-counter defense.

Plant signaling pathways activating defence response and interfering mechanisms by pathogen effectors, protein decoys and bodyguards

Abstract: Plants activate an immune response in defense against microbial pathogens. The first layer of immunity consists in the recognition of microbial fingerprints, called Pathogen Associated Molecular Pattern (PAMP), by a set of Pattern Recognition Receptors (PRR). In addition, the degradation products from fungi, bacteria and plant cells are recognised as Damage Associated Molecular Pattern (DAMP). The first layer of plant defence is based on Pattern Recognition Receptors (PRR) on the membrane. These receptors, either receptor kinases or receptor-like proteins (RLPs), associating with cytoplasmic kinases, recognize the presence of PAMPs, thus activating a local response named PAMP-triggered immunity (PTI), that is not strong but effective towards many pathogen species. Here we discuss and focus on Elongation Factor Tu Receptors (EFR) and flagellin sensing (FLS) receptors. In leucine-rich repeat (LRR) receptor proteins, the hydrophobic LLR domains are exposed on external membranes, providing the protein-protein interaction modules. Plants evolved this protein-protein interaction domain several times during the development of mechanisms to defend themselves from viruses, virulence factors, enzymes and effectors of bacterial and fungal pathogens. Pathogens in addition evolved proteins and enzymes that are injected in the plant cell to counterfight plant immune signaling pathways. These effectors are recognised by plant receptors sensing their presence of their cognate avirulence genes. These receptors originated from recombination during evolution and only occur in some specific tomato genotypes, instead of the widely occurring PPRs. Effector Triggered Immunity (ETI) allows a plant response to effector proteins that is more strong, but is race specific. It leads to local necrosis and apoptosis, and to the establishment of the hypersensitive response (HR). For biotrophic or hemibiotrophic pathogens, necrosis is an effective way to limit their spread, while for necrotrophic pathogens this is not efficient and sufficient way to limit their spread, since depends on the timing of infection and on the plant development phase. Pathogenic fungi strategy relies on the formation of specialised structures, or haustoria, that facilitate the nutrient uptake form plant cells. In this review, we summarize the most recent knowledge on plant pathogens and the mechanisms they evolved to circumvent plant defences among which pathogen effectors, protein decoys inactivating plant defence signals. Effectors are recognised through their binding to plant proteins by means of plant receptors, that activate the Effector Triggered Immunity (ETI). In particular, we focus on the Solanaceae, discussing general mechanisms and specific pathways that confer resistance to various pathogens. There is an arm race between plants and fungal and bacterial pathogens that has led to new protein variants and protein decoys (pseudokinases, inhibitors and sponges blocking glucanases, and Transcription Activator Like Effectors). Advances in understanding the function of pathogen effectors will provide new ways to improve plant immunity and mechanisms of defence against their pests. Finally, we present possible combinations of interventions, from gene engineering to chemical priming, acting on signaling pathways regulated by jasmonate and salicylate hormones, to increase plant resistance and activate plant defences without affecting crop yields.

Quantum pathogen evolution by integron-mediated effector capture

Abstract: Plant pathogenic Pseudomonads are responsible for the loss of millions of pounds in crop revenue each year. They export effector molecules via the type three secretion system into the plants’ cells in order to elicit disease. If the plant has the corresponding resistance genes to detect the type three effector molecule then the plant will mount an immune response called the plant hypersensitive response (HR). Type three effector molecules can also supress the plants’ immune response including pathogen associated molecular pattern triggered immunity and effector triggered immunity. Pseudomonads can evade HR by potentially gaining different effector molecules using mobile DNA elements. Integrons are one such type of element. Integrons are elements that allow bacteria to acquire and store genes from the environment particularly during times of stress. They also allow differential expression of the captured genes dependent on the environmental conditions. Integron-like elements (ILEs) within Pseudomonas syringae pathovars and other Pseudomonads can be identified by using conserved genes such as the xerC integrase and the UV damage repair gene rulB. RulB encodes a DNA polymerase V which appears to be a hotspot for ILE insertion. Using the rulAB operon, the xerC gene and the ILE insertion junction, rulB-xerC, it was possible to identify a number of ILEs. The screening of 164 plant pathogenic Pseudomonas strains revealed new ILEs from 21 strains all containing at least one type three effector molecule. The screening also revealed that the xerC integrase was conserved across multiple ILEs within plant pathogens. Expression studies of the ILE integrase genes, type three effector genes and the disrupted rulB gene showed that the genes on both ILEs present in P. syringae pv. pisi 203 and pv. syringae 3023 are upregulated in times of cellular stress and DNA damage. This led to the conclusion that ILEs may be more active when the bacteria was in need of exogenous genes to overcome the cellular stress. The ILE may also be excised following DNA damage to restore full rulB functionality. It was identified that rulB was a hotspot for ILE insertion but it was not known why the ILEs choose this site or if any other genes were required for ILE insertion. Cloned versions of the rulAB operon from the pWW0 plasmid found in Pseudomonas putida PaW340 showed that only rulAB was required for P. fluorescens ILE insertion but rulAB must be intact. P. syringae ILEs were also tested but did not show any insertion. Due to ILEs inserting into and disrupting rulB their effect on UV tolerance was tested. A range of strains containing an intact rulB gene were tested alongside the ILE containing strains with increasing amounts of UVB irradiation applied. The results showed very minor differences in growth rates between the two groups with only one UVB irradiation amount of 60 seconds causing a significant difference in growth rate at the 95% confidence interval between the two groups of strains. This research has contributed to the understanding of ILEs in phytopathogenic bacteria. It has also increased our understanding of the mechanisms of ILE gene expression, the mechanism surrounding ILE excision and insertion and the effect of ILEs on bacterial growth in high UV environments.