Plant-Microbe Interactions & MAPKs
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PNAS: Distinct modes of derepression of an Arabidopsis immune receptor complex by two different bacterial effectors (2018)

PNAS: Distinct modes of derepression of an Arabidopsis immune receptor complex by two different bacterial effectors (2018) | Plant-Microbe Interactions & MAPKs | Scoop.it

Plants and animals carry intracellular nucleotide-binding leucine-rich repeat (NLR) immune receptors. How NLR receptors activate defense on perceiving pathogen molecules is poorly understood, especially in plants. Some NLRs function in pairs, with one NLR carrying a domain that mimics a pathogen effector target. Effector action on this domain activates the second “helper” NLR. In the Arabidopsis RPS4 and RRS1 pair, RRS1 carries a WRKY transcription factor domain targeted by bacterial effectors AvrRps4 and PopP2. We monitored conformational changes in RPS4–RRS1 during activation and developed a “molecular padlock” to reversibly restrict such changes. This revealed domains within RRS1 required to keep the RRS1–RPS4 complex inactive prior to effector detection, and specific domain–domain interactions whose disruption or modification contributes to defense activation.

Plant intracellular nucleotide-binding leucine-rich repeat (NLR) immune receptors often function in pairs to detect pathogen effectors and activate defense. The Arabidopsis RRS1-R–RPS4 NLR pair recognizes the bacterial effectors AvrRps4 and PopP2 via an integrated WRKY transcription factor domain in RRS1-R that mimics the effector’s authentic targets. How the complex activates defense upon effector recognition is unknown. Deletion of the WRKY domain results in an RRS1 allele that triggers constitutive RPS4-dependent defense activation, suggesting that in the absence of effector, the WRKY domain contributes to maintaining the complex in an inactive state. We show the WRKY domain interacts with the adjacent domain 4, and that the inactive state of RRS1 is maintained by WRKY–domain 4 interactions before ligand detection. AvrRps4 interaction with the WRKY domain disrupts WRKY–domain 4 association, thus derepressing the complex. PopP2-triggered activation is less easily explained by such disruption and involves the longer C-terminal extension of RRS1-R. Furthermore, some mutations in RPS4 and RRS1 compromise PopP2 but not AvrRps4 recognition, suggesting that AvrRps4 and PopP2 derepress the complex differently. Consistent with this, a “reversibly closed” conformation of RRS1-R, engineered in a method exploiting the high affinity of colicin E9 and Im9 domains, reversibly loses AvrRps4, but not PopP2 responsiveness. Following RRS1 derepression, interactions between domain 4 and the RPS4 C-terminal domain likely contribute to activation. Simultaneous relief of autoinhibition and activation may contribute to defense activation in many immune receptors.


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Eudicot plant-specific sphingolipids determine host selectivity of microbial NLP cytolysins

Eudicot plant-specific sphingolipids determine host selectivity of microbial NLP cytolysins | Plant-Microbe Interactions & MAPKs | Scoop.it
Many microbial pathogens produce proteins that are toxic to the cells that they are targeting. Broad-leaved plants are susceptible to NLP (necrosis and ethylene-inducing peptide 1–like protein) toxins. Lenarčič et al. identified the receptors for NLP toxins to be GIPC (glycosylinositol phosphorylceramide) sphingolipids (see the Perspective by Van den Ackerveken). Their findings reveal why these toxins only attack broad-leaved plants (so-called eudicots): If the sphingolipid carries just two hexoses, as is the case for eudicots, the toxin binds and causes cell lysis. But in monocots with sphingolipids that have three hexoses, the toxin is ineffective.

Science , this issue p. [1431][1]; see also p. [1383][2]

[1]: /lookup/doi/10.1126/science.aan6874
[2]: /lookup/doi/10.1126/science.aar4188
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The Arabidopsis CERK1‐associated kinase PBL27 connects chitin perception to MAPK activation

The Arabidopsis CERK1‐associated kinase PBL27 connects chitin perception to MAPK activation | Plant-Microbe Interactions & MAPKs | Scoop.it
Perception of microbe‐associated molecular patterns by host cell surface pattern recognition receptors (PRRs) triggers the intracellular activation of mitogen‐activated protein kinase (MAPK) cascades. However, it is not known how PRRs transmit immune signals to MAPK cascades in plants. Here, we identify a complete phospho‐signaling transduction pathway from PRR‐mediated pathogen recognition to MAPK activation in plants. We found that the receptor‐like cytoplasmic kinase PBL27 connects the chitin receptor complex CERK1‐LYK5 and a MAPK cascade. PBL27 interacts with both CERK1 and the MAPK kinase kinase MAPKKK5 at the plasma membrane. Knockout mutants of MAPKKK5 compromise chitin‐induced MAPK activation and disease resistance to Alternaria brassicicola . PBL27 phosphorylates MAPKKK5 in vitro , which is enhanced by phosphorylation of PBL27 by CERK1. The chitin perception induces disassociation between PBL27 and MAPKKK5 in vivo . Furthermore, genetic evidence suggests that phosphorylation of MAPKKK5 by PBL27 is essential for chitin‐induced MAPK activation in plants. These data indicate that PBL27 is the MAPKKK kinase that provides the missing link between the cell surface chitin receptor and the intracellular MAPK cascade in plants.

![][1]

Chitin receptor CERK1 transmits immune signals to the intracellular MAPK cascade in plants. This occurs via phosphorylation of MAPKKK5 by the CERK1‐associated kinase PBL27, providing a missing link between pathogen perception and signaling output.

[1]: /embed/graphic-1.gif
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PLoS Pathog: The Arabidopsis Protein Phosphatase PP2C38 Negatively Regulates the Central Immune Kinase BIK1 (2016)

PLoS Pathog: The  Arabidopsis  Protein Phosphatase PP2C38 Negatively Regulates the Central Immune Kinase BIK1 (2016) | Plant-Microbe Interactions & MAPKs | Scoop.it
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The Sainsbury Lab's curator insight, August 8, 2016 4:57 AM
Plants recognize pathogen-associated molecular patterns (PAMPs) via cell surface-localized pattern recognition receptors (PRRs), leading to PRR-triggered immunity (PTI). The Arabidopsis cytoplasmic kinase BIK1 is a downstream substrate of several PRR complexes. How plant PTI is negatively regulated is not fully understood. Here, we identify the protein phosphatase PP2C38 as a negative regulator of BIK1 activity and BIK1-mediated immunity. PP2C38 dynamically associates with BIK1, as well as with the PRRs FLS2 and EFR, but not with the co-receptor BAK1. PP2C38 regulates PAMP-induced BIK1 phosphorylation and impairs the phosphorylation of the NADPH oxidase RBOHD by BIK1, leading to reduced oxidative burst and stomatal immunity. Upon PAMP perception, PP2C38 is phosphorylated on serine 77 and dissociates from the FLS2/EFR-BIK1 complexes, enabling full BIK1 activation. Together with our recent work on the control of BIK1 turnover, this study reveals another important regulatory mechanism of this central immune component.
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Escaping Host Immunity: New Tricks for Plant Pathogens

Introduction

Fungal and oomycete plant pathogens cause destructive diseases in crops and pose real economic and food security threats [1]. These filamentous, eukaryotic organisms can also upset natural ecosystems when they spread invasively [2]. The capability of plant immune systems to detect and respond to pathogen effector proteins is a major determinant of disease susceptibility. Plant pathogen effector proteins that trigger host immunity are often encoded by conditionally detrimental genes that are under strong and contrasting selective pressures [3,4]. Pathogen effectors evolved to play a positive role in virulence by enabling growth and reproduction on host plants [5,6]. Nonetheless, effectors can meet their match with host immune receptors that recognize their presence, a result that ends badly for the pathogen. Such immunity-triggering proteins are known as avirulence (Avr) effectors, encoded by Avr genes.
Escaping Host Immunity

When an Avr effector triggers a host immune receptor, the pathogen’s survival depends upon generating variants that escape, suppress, or alter this recognition event in ways that allow the pathogen to grow and reproduce. This can be accomplished by numerous means. Transposon insertions or mutations to the DNA sequence encoding the Avr gene, or its complete loss, are commonly encountered gain of virulence mechanisms. This is well demonstrated in a study on the tomato leaf mold pathogen Cladosporium fulvum [7]. In fact, pathogen genomes have evolved configurations that position Avr effector genes in repetitive [8], transposon-rich [9], and teleomeric regions [10], or in dispensable segments [11], to aid mutation and recombination that results in gain of virulence. There are also ways to defeat immunity without loss or alteration of the DNA sequence of the offending Avr gene. This can occur through acquisition or evolution of an additional, epistatic effector that supresses the immune-triggering event caused by the Avr effector. Such scenarios are well documented in oomycete and fungal plant pathogens, for example, in the potato late blight pathogen Phytophthora infestans [12], the wheat powdery mildew pathogen Blumeria graminis [13], the tomato wilt pathogen Fusarium oxysporum [14], and the canola blackleg pathogen Leptosphaeria maculans [15]. This arms race can go through repeated iterations and lead to difficulties in untangling the molecular basis of host–pathogen compatibilities.
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WRKY57 regulates JAZ genes transcriptionally to compromise Botrytis cinerea resistance in Arabidopsis thaliana

Although necrotrophic pathogens cause many devastating plant diseases, our understanding of the plant defense response to them is limited. Here, we found that loss of function of WRKY57 enhanced the resistance of Arabidopsis thaliana against Botrytis cinerea infection. Further investigation suggested that the negative regulation of WRKY57 against B. cinerea depends on the JA signaling pathway. Chromatin immunoprecipitation experiments revealed that WRKY57 directly binds to the promoters of JASMONATE ZIM-DOMAIN 1 (JAZ1) and JAZ5, encoding two important repressors of the JA signaling pathway, and activates their transcription. In vivo and in vitro experiments demonstrated that WRKY57 interacts with nuclear-encoded SIGMA FACTOR BINDING PROTEIN1 (SIB1) and SIB2. Further experiments display that the same domain, the VQ motif, of SIB1 and SIB2 interact with WRKY33 and WRKY57. Moreover, transient transcriptional activity assays confirmed that WRKY57 and WRKY33 competitively regulate JAZ1 and JAZ5, SIB1 and SIB2 further enhance these competitions of WRKY57 to WRKY33. Therefore, coordinated regulation of Arabidopsis against B. cinerea by transcription activators and repressors would benefit plants by allowing fine regulation of defense.
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Wheat PR-1 proteins are targeted by necrotrophic pathogen effector proteins - Breen - 2016 - The Plant Journal

Recent studies have identified that proteinaceous effectors secreted by Parastagonospora nodorum are required to cause disease on wheat. These effectors interact in a gene-for-gene manner with host dominant susceptibilty loci, resulting in disease. However, whilst the requirement of these effectors for infection is clear, their mechanisms of action remain poorly understood. A yeast-two-hybrid library approach was used to search for wheat proteins that interacted with the necrotrophic effector SnTox3. Using this strategy we indentified an interaction between SnTox3 and the wheat pathogenicity-related protein TaPR-1-1, and confirmed it by in-planta co-immunprecipitation. PR-1 proteins represent a large family (23 in wheat) of proteins that are up-regulated early in the defence response, however their function remains ellusive. Interestingly, the P. nodorum effector SnToxA has recently been shown to interact specifically with TaPR-1-5. Our analysis of the SnTox3-TaPR-1 interaction demonstrated that SnTox3 can interact with a broader range of TaPR-1 proteins. Based on these data we utilised homology modeling to predict, and validate, regions on TaPR-1 proteins that are likely to be involved in the SnTox3 interaction. Precipitating from this work, we identified that a PR-1 derived defence signalling peptide from the C-terminus of TaPR1-1, known as CAPE1, enhanced the infection of wheat by P. nodorum in an SnTox3-dependent manner, but played no role in ToxA-mediated disease. Collectively, our data suggest that P.nodorum has evolved unique effectors that target a common host-protein involved in host defence, albeit with different mechanisms and potentially outcomes.
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A Plasmodium-like virulence effector of the soybean cyst nematode suppresses plant innate immunity - Noon - 2016 - New Phytologist - Wiley Online Library

A Plasmodium-like virulence effector of the soybean cyst nematode suppresses plant innate immunity - Noon - 2016 - New Phytologist - Wiley Online Library | Plant-Microbe Interactions & MAPKs | Scoop.it
Heterodera glycines, the soybean cyst nematode, delivers effector proteins into soybean roots to initiate and maintain an obligate parasitic relationship. HgGLAND18 encodes a candidate H. glycines effector and is expressed throughout the infection process.
We used a combination of molecular, genetic, bioinformatic and phylogenetic analyses to determine the role of HgGLAND18 during H. glycines infection.
HgGLAND18 is necessary for pathogenicity in compatible interactions with soybean. The encoded effector strongly suppresses both basal and hypersensitive cell death innate immune responses, and immunosuppression requires the presence and coordination between multiple protein domains. The N-terminal domain in HgGLAND18 contains unique sequence similarity to domains of an immunosuppressive effector of Plasmodium spp., the malaria parasites. The Plasmodium effector domains functionally complement the loss of the N-terminal domain from HgGLAND18.
In-depth sequence searches and phylogenetic analyses demonstrate convergent evolution between effectors from divergent parasites of plants and animals as the cause of sequence and functional similarity.
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NbCSPR underlies age-dependent immune responses to bacterial cold shock protein in Nicotiana benthamiana

Plants use receptor kinases (RKs) and receptor-like proteins (RLPs) as pattern recognition receptors (PRRs) to sense pathogen-associated molecular patterns (PAMPs) that are typical of whole classes of microbes. After ligand perception, many leucine-rich repeat (LRR)-containing PRRs interact with the LRR-RK BRI1-ASSOCIATED KINASE 1 (BAK1). BAK1 is thus expected to interact with unknown PRRs. Here, we used BAK1 as molecular bait to identify a previously unknown LRR-RLP required for the recognition of the csp22 peptide derived from bacterial cold shock protein. We established a method to identify proteins that interact with BAK1 only after csp22 treatment. BAK1 was expressed transiently in Nicotiana benthamiana and immunopurified after treatment with csp22. BAK1-associated proteins were identified by mass spectrometry. We identified several proteins including known BAK1 interactors and a previously uncharacterized LRR-RLP that we termed RECEPTOR-LIKE PROTEIN REQUIRED FOR CSP22 RESPONSIVENESS (NbCSPR). This RLP associates with BAK1 upon csp22 treatment, and NbCSPR-silenced plants are impaired in csp22-induced defense responses. NbCSPR confers resistance to bacteria in an age-dependent and flagellin-induced manner. As such, it limits bacterial growth and Agrobacterium-mediated transformation of flowering N. benthamiana plants. Transgenic expression of NbCSPR into Arabidopsis thaliana conferred responsiveness to csp22 and antibacterial resistance. Our method may be used to identify LRR-type RKs and RLPs required for PAMP perception/responsiveness, even when the active purified PAMP has not been defined.
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Integration of decoy domains derived from protein targets of pathogen effectors into plant immune receptors is widespread - Kroj - 2016 - New Phytologist - Wiley Online Library

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Frontiers | Plant Ribosomal Proteins, RPL12 and RPL19, Play a Role in Nonhost Disease Resistance against Bacterial Pathogens | Plant Biotic Interactions

Characterizing the molecular mechanism involved in nonhost disease resistance is important to understand the adaptations of plant-pathogen interactions. In this study, virus-induced gene silencing (VIGS)-based forward genetics screen was utilized to identify genes involved in nonhost resistance in Nicotiana benthamiana. Genes encoding ribosomal proteins, RPL12 and RPL19, were identified in the screening. These genes when silenced in N. benthamiana caused a delay in nonhost bacteria induced hypersensitive response (HR) with concurrent increase in nonhost bacterial multiplication. Arabidopsis mutants of AtRPL12 and AtRPL19 also compromised in nonhost resistance. The studies on NbRPL12 and NbRPL19 double silenced plants suggested that both RPL12 and RPL19 act in the same pathway to confer nonhost resistance. Our work suggests a role for RPL12 and RPL19 in nonhost disease resistance in N. benthamiana and Arabidopsis. In addition, we show that these genes also play a minor role in basal resistance against virulent pathogens.
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Nucleocytoplasmic Trafficking is Essential for BAK1 and BKK1-Mediated Cell-Death Control - Du - 2016 - The Plant Journal - Wiley Online Library

BAK1 was initially identified as a co-receptor of the brassinosteroid (BR) receptor, BRI1. Genetic analyses also revealed that BAK1 and its closest homolog BKK1 regulate a BR-independent cell-death control pathway. A double null mutant bak1 bkk1 displays a salicylic acid (SA)- and light-dependent cell-death phenotype even without pathogen invasion. Molecular mechanisms of the spontaneous cell death mediated by BAK1 and BKK1 remain elusive. Here we report our identification of a suppressor of bak1 bkk1 (sbb1-1). Genetic analyses indicated that the cell-death symptom in a weak double mutant, bak1-3 bkk1-1, was completely suppressed by the loss-of-function mutation of SBB1, which encodes a nucleoporin (NUP) 85-like protein. Genetic analyses also demonstrated that knocking-out three other NUPs individually from the SBB1-located sub-complex was also able to rescue the cell-death phenotype of bak1-3 bkk1-1. In addition, a DEAD box RNA helicase, DRH1, was identified in the same protein complex of SBB1 via a proteomic approach. The drh1 mutant can also rescue the cell-death symptom of bak1-3 bkk1-1. Further analyses indicated that the export of poly (A)+ RNA was greatly blocked, resulting in accumulation of significant amount of mRNAs in the nuclei of the nup and drh1 mutants. Overexpression of a bacterial NahG gene to inactivate SA can rescue the cell-death symptom of bak1-3 bkk1-1. Meanwhile, mutants suppressing the cell death symptoms always showed greatly reduced SA contents. These results suggest that nucleocytoplasmic trafficking, especially for the molecules directly or indirectly involved in endogenous SA accumulation, is critical in BAK1 and BKK1-mediated cell-death control.

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An effector of the Irish potato famine pathogen antagonizes a host autophagy cargo receptor

Plants use autophagy to safeguard against infectious diseases. However, how plant pathogens interfere with autophagy related processes is unknown. Here we show that PexRD54, an effector from the Irish potato famine pathogen Phytophthora infestans, binds host autophagy protein ATG8CL to stimulate autophagosome formation. PexRD54 depletes the autophagy cargo receptor Joka2 out of ATG8CL complexes and interferes with Joka2's positive effect on pathogen defense. Thus a plant pathogen effector has evolved to antagonize a host autophagy cargo receptor in order to counteract host defenses.
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An extracellular network of Arabidopsis leucine-rich repeat receptor kinases

An extracellular network of Arabidopsis leucine-rich repeat receptor             kinases | Plant-Microbe Interactions & MAPKs | Scoop.it
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A non canonical subtilase attenuates the transcriptional activation of defence responses in Arabidopsis thaliana

A non canonical subtilase attenuates the transcriptional activation of defence responses in Arabidopsis thaliana | Proteases play crucial physiological functions in all organisms by controlling the lifetime of proteins. Here, we identified an atypical protease of the subtilase family [SBT5.2(b)] that attenuates the transcriptional activation of plant defence independently of its protease activity. The SBT5.2 gene produces two distinct transcripts encoding a canonical secreted subtilase [SBT5.2(a)] and an intracellular protein [SBT5.2(b)]. Concomitant to SBT5.2(a) downregulation, SBT5.2(b) expression is induced after bacterial inoculation. SBT5.2(b) localizes to endosomes where it interacts with and retains the defence-related TF MYB30. Nuclear exclusion of MYB30 results in its reduced transcriptional activation and, thus, suppressed resistance. sbt5.2 mutants, with abolished SBT5.2(a) and SBT5.2(b) expression, display enhanced defence that is suppressed in a myb30 mutant background. Moreover, overexpression of SBT5.2(b), but not SBT5.2(a), in sbt5.2 plants reverts the phenotypes displayed by sbt5.2 mutants. Overall, we uncover a regulatory mode of the transcriptional activation of defence responses previously undescribed in eukaryotes.
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Science: Detection of the plant parasite Cuscuta reflexa by a tomato cell surface receptor (2016)

Science: Detection of the plant parasite Cuscuta reflexa by a tomato cell surface receptor (2016) | Plant-Microbe Interactions & MAPKs | Scoop.it
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The Sainsbury Lab's curator insight, July 29, 2016 4:05 AM
Parasitic plants are a constraint on agriculture worldwide. Cuscuta reflexa is a stem holoparasite that infests most dicotyledonous plants. One exception is tomato, which is resistant to C. reflexa. We discovered that tomato responds to a small peptide factor occurring in Cuscuta spp. with immune responses typically activated after perception of microbe-associated molecular patterns. We identified the cell surface receptor-like protein CUSCUTA RECEPTOR 1 (CuRe1) as essential for the perception of this parasite-associated molecular pattern. CuRe1 is sufficient to confer responsiveness to the Cuscuta factor and increased resistance to parasitic C. reflexa when heterologously expressed in otherwise susceptible host plants. Our findings reveal that plants recognize parasitic plants in a manner similar to perception of microbial pathogens.
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The Arabidopsis Malectin-Like/LRR-RLK IOS1 Is Critical for BAK1-Dependent and BAK1-Independent Pattern-Triggered Immunity

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Arabidopsis CLAVATA1 and CLAVATA2 receptors contribute to Ralstonia solanacearum pathogenicity through a miR169-dependent pathway

Arabidopsis CLAVATA1 and CLAVATA2 receptors contribute to Ralstonia solanacearum pathogenicity through a miR169-dependent pathway | Plant-Microbe Interactions & MAPKs | Scoop.it
Summary
Bacterial wilt caused by Ralstonia solanacearum is one of the most destructive bacterial plant diseases. Although many molecular determinants involved in R. solanacearum adaptation to hosts and pathogenesis have been described, host components required for disease establishment remain poorly characterized. Phenotypical analysis of Arabidopsis mutants for leucine-rich repeat (LRR)-receptor-like proteins revealed that mutations in the CLAVATA1 (CLV1) and CLAVATA2 (CLV2) genes confer enhanced disease resistance to bacterial wilt. We further investigated the underlying mechanisms using genetic, transcriptomic and molecular approaches. The enhanced resistance of both clv1 and clv2 mutants to the bacteria did not require the well characterized CLV signalling modules involved in shoot meristem homeostasis, and was conditioned by neither salicylic acid nor ethylene defence-related hormones. Gene expression microarray analysis performed on clv1 and clv2 revealed deregulation of genes encoding nuclear transcription factor Y subunit alpha (NF-YA) transcription factors whose post-transcriptional regulation is known to involve microRNAs from the miR169 family. Both clv mutants showed a defect in miR169 accumulation. Conversely, overexpression of miR169 abrogated the resistance phenotype of clv mutants. We propose that CLV1 and CLV2, two receptors involved in CLV3 perception during plant development, contribute to bacterial wilt through a signalling pathway involving the miR169/NF-YA module.Empty description
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An innate immunity pathway in the moss Physcomitrella patens

MAP kinase (MPK) cascades in Arabidopsis thaliana and other vascular plants are activated by developmental cues, abiotic stress, and pathogen infection. Much less is known of MPK functions in nonvascular land plants such as the moss Physcomitrella patens. Here we provide evidence for a signaling pathway in P. patens required for immunity triggered by pathogen associated molecular patterns (PAMPs). This pathway induces rapid growth inhibition, a novel fluorescence burst, cell wall depositions, and accumulation of defense-related transcripts. Two P. patens MPKs (MPK4a and MPK4b) are phosphorylated and activated in response to PAMPs. This activation in response to the fungal PAMP chitin requires a chitin receptor and one or more MAP kinase kinase kinases and MAP kinase kinases. Knockout lines of MPK4a appear wild type but have increased susceptibility to the pathogenic fungi Botrytis cinerea and Alternaria brassisicola. Both PAMPs and osmotic stress activate some of the same MPKs in Arabidopsis. In contrast, abscisic acid treatment or osmotic stress of P. patens does not activate MPK4a or any other MPK, but activates at least one SnRK2 kinase. Signaling via MPK4a may therefore be specific to immunity, and the moss relies on other pathways to respond to osmotic stress.
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Direct and Indirect Targeting of PP2A by Conserved Bacterial Type-III Effector Proteins

Direct and Indirect Targeting of PP2A by Conserved Bacterial Type-III Effector Proteins | Plant-Microbe Interactions & MAPKs | Scoop.it
Author Summary Gram-negative bacterial pathogens employ type-III effector (T3E) proteins to suppress host immunity and promote disease symptoms. AvrE-family T3Es, which are widely distributed among plant-pathogenic bacteria, suppress host defense responses and also contribute to water-soaking, which is perhaps the most common symptom of bacterial diseases and likely results in the release of nutrients from host cells to promote pathogen growth. Despite the central virulence functions of AvrE-family T3Es, their mode of action remains enigmatic largely due to their cell toxicity. We report here that two AvrE-family T3Es, WtsE from the maize pathogen Pantoea stewartii subsp. stewartii and AvrE1 from the tomato and Arabidopsis pathogen Pseudomonas syringae pv. tomato, each target protein phosphatase 2A (PP2A) complexes in susceptible hosts via direct interaction/association with specific B’ regulatory subunits. Chemical inhibitors were used to demonstrate that PP2A activity is required for the virulence functions of WtsE and AvrE1. PP2A isoform specificity was also tested using mutants of Arabidopsis. More generally, PP2A subunits regulate, both positively and negatively, rapid pattern-triggered immune responses in Arabidopsis. Thus, bacterial T3Es target sub-component specific PP2A complexes to manipulate host immunity and cause disease symptoms during infection.
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The stripe rust fungal effector PEC6 suppresses pattern-triggered immunity in a host species-independent manner and interacts with adenosine kinases - Liu - 2016 - New Phytologist - Wiley Online Li...

The stripe rust fungal effector PEC6 suppresses pattern-triggered immunity in a host species-independent manner and interacts with adenosine kinases - Liu - 2016 - New Phytologist - Wiley Online Li... | Plant-Microbe Interactions & MAPKs | Scoop.it
Summary
We identified a wheat stripe rust (Puccinia striiformis) effector candidate (PEC6) with pattern-triggered immunity (PTI) suppression function and its corresponding host target. PEC6 compromised PTI host species-independently. In Nicotiana benthamiana, it hampers reactive oxygen species (ROS) accumulation and callose deposition induced by Pseudomonas fluorescens. In Arabidopsis, plants expressing PEC6 were more susceptible to Pseudomonas syringae pv. tomato (Pto) DC3000 ΔAvrPto/ΔAvrPtoB. In wheat, PEC6-suppression of P. fluorescens-elicited PTI was revealed by the fact that it allowed activation of effector-triggered immunity by Pto DC3000. Knocking down of PEC6 expression by virus-mediated host-induced gene silencing decreased the number of rust pustules, uncovering PEC6 as an important pathogenicity factor. PEC6, overexpressed in plant cells without its signal peptide, was localized to the nucleus and cytoplasm. A yeast two-hybrid assay showed that PEC6 interacts with both wheat and Arabidopsis adenosine kinases (ADKs). Knocking down wheat ADK expression by virus-induced gene silencing reduced leaf growth and enhanced the number of rust pustules, indicating that ADK is important in plant development and defence. ADK plays essential roles in regulating metabolism, cytokinin interconversion and methyl transfer reactions, and our data propose a model where PEC6 may affect one of these processes by targeting ADK to favour fungal growth.
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The type 3 effector NopL of Sinorhizobium sp. strain NGR234 is a mitogen-activated protein kinase substrate

Pathogenic bacteria utilize type 3 secretion systems to inject type 3 effectors (T3Es) into host cells, thereby subverting
host defense reactions. Similarly, T3Es of symbiotic nitrogen-fixing rhizobia can affect nodule formation on roots of
legumes. Previous work showed that NopL (nodulation outer protein L) of Sinorhizobium (Ensifer) sp. strain NGR234
is multiply phosphorylated in eukaryotic cells and that this T3E suppresses responses mediated by mitogen-activated
protein (MAP) kinase signaling in yeast (mating pheromone signaling) and plant cells (expression of pathogenesisrelated
defense proteins). Here, we show that NopL is a MAP kinase substrate. Microscopic observations of fluorescent
fusion proteins and bimolecular fluorescence complementation analysis in onion cells indicated that NopL
is targeted to the nucleus and forms a complex with SIPK (salicylic acid-induced protein kinase), a MAP kinase
of tobacco. In vitro experiments demonstrated that NopL is phosphorylatyed by SIPK. At least nine distinct spots
were observed after two-dimensional gel electrophoresis, indicating that NopL can be hyperphosphorylated by MAP
kinases. Senescence symptoms in nodules of beans (Phaseolus vulgaris cv. Tendergreen) were analyzed to determine
the symbiotic effector activity of different NopL variants with serine to alanine substitutions at identified and
predicted phosphorylation sites (serine–proline motif). NopL variants with six or eight serine to alanine substitutions
were partially active, whereas NopL forms with 10 or 12 substituted serine residues were inactive. In conclusion, our
findings provide evidence that NopL interacts with MAP kinases and reveals the importance of serine–proline motifs
for effector activity during symbiosis.
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Specific control of Arabidopsis BAK1/SERK4-regulated cell death by protein glycosylation

Specific control of Arabidopsis BAK1/SERK4-regulated cell death by protein glycosylation | Plant-Microbe Interactions & MAPKs | Scoop.it
Control of cell death is crucial for plant life. A comprehensive screen for suppressors of BAK1/SERK4-mediated cell death identified a component of protein glycosylation pathways and ERQC, and its cysteine-rich receptor-like kinase (CRK) targets.
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Frontiers | Supramolecular Structure and Functional Analysis of the Type III Secretion System in Pseudomonas fluorescens 2P24 | Plant Biotic Interactions

Frontiers | Supramolecular Structure and Functional Analysis of the Type III Secretion System in Pseudomonas fluorescens 2P24 | Plant Biotic Interactions | Plant-Microbe Interactions & MAPKs | Scoop.it
The type III secretion system (T3SS) of plant and animal bacterial pathogens directs the secretion and injection of proteins into host cells. Some homologous genes of T3SS were found also in nonpathogenic bacteria, but the organization of its machinery and basic function are still unknown. In this study, we identified a T3SS gene cluster from the plant growth-promoting Pseudomonas fluorescens 2P24 and isolated the corresponding T3SS apparatus. The T3SS gene cluster of strain 2P24 is similar organizationally to that of pathogenic P. syringae, except that it lacks the regulator hrpR and the hrpK1 and hrpH genes, which are involved in translocation of proteins. Electron microscopy revealed that the T3SS supramolecular structure of strain 2P24 was comprised of two distinctive substructures: a long extracellular, filamentous pilus and a membrane-embedded base. We show that strain 2P24 deploys a harpin homologue protein, RspZ1, to elicit a hypersensitive response when infiltrated into Nicotiana tabacum cv. xanthi leaves with protein that is partially purified, and by complementing the hrpZ1 mutation of pHIR11. The T3SS of strain 2P24 retained ability to secrete effectors, whereas its effector translocation activity appeared to be excessively lost. Mutation of the rscC gene from 2P24 T3SS abolished the secretion of effectors, but the general biocontrol properties were unaffected. Remarkably, strain 2P24 induced functional MAMP-triggered immunity that included a burst of reactive oxygen species, strong suppression of challenge cell death, and disease expansion, while it was not associated with the secretion functional T3SS.
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Attenuation of pattern recognition receptor signaling is mediated by a MAP kinase kinase kinase

Attenuation of pattern recognition receptor signaling is mediated by a MAP kinase kinase kinase | Plant-Microbe Interactions & MAPKs | Scoop.it

Pattern recognition receptors (PRRs) play a key role in plant and animal innate immunity. PRR binding of their cognate ligand triggers a signaling network and activates an immune response. Activation of PRR signaling must be controlled prior to ligand binding to prevent spurious signaling and immune activation. Flagellin perception in Arabidopsis through FLAGELLIN‐SENSITIVE 2 (FLS2) induces the activation of mitogen‐activated protein kinases (MAPKs) and immunity. However, the precise molecular mechanism that connects activated FLS2 to downstream MAPK cascades remains unknown. Here, we report the identification of a differentially phosphorylated MAP kinase kinase kinase that also interacts with FLS2. Using targeted proteomics and functional analysis, we show that MKKK7 negatively regulates flagellin‐triggered signaling and basal immunity and this requires phosphorylation of MKKK7 on specific serine residues. MKKK7 attenuates MPK6 activity and defense gene expression. Moreover, MKKK7 suppresses the reactive oxygen species burst downstream of FLS2, suggesting that MKKK7‐mediated attenuation of FLS2 signaling occurs through direct modulation of the FLS2 comp

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