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The nuclear ubiquitin proteasome degradation affects WRKY45 function in the rice defense program - Matsushita - The Plant Journal - Wiley Online Library

The nuclear ubiquitin proteasome degradation affects WRKY45 function in the rice defense program - Matsushita - The Plant Journal - Wiley Online Library | Plant-microbe interaction | Scoop.it

Summary
The transcriptional activator WRKY45 plays a major role in the salicylic acid (SA)/benzothiadiazole-induced defense program in rice. Here, we show that the nuclear ubiquitin proteasome system (UPS) plays a role in regulating the function of WRKY45. Proteasome inhibitors induced accumulation of polyubiquitinated WRKY45 and transient upregulation of WRKY45 target genes in rice cells, suggesting that WRKY45 is constantly degraded by the UPS to suppress defense responses in the absence of defense signals. Mutational analysis of the nuclear localization signal (NLS) indicated that UPS-dependent WRKY45 degradation occurs in the nuclei. Interestingly, transcriptional activity of WRKY45 after SA treatment was impaired by proteasome inhibition. The same C-terminal region in WRKY45 was essential for both transcriptional activity and UPS-dependent degradation. These results suggest that UPS regulation also plays a role in transcriptional activity of WRKY45. It has been reported that AtNPR1, the central regulator of the SA pathway in Arabidopsis, is regulated by the UPS. We found that OsNPR1/NH1, the rice counterpart of NPR1, was not stabilized by proteasome inhibition under an uninfected condition. We discuss the differences in the post-translational regulation of the SA-pathway components between rice and Arabidopsis.

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Plant-microbe interaction
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PLOS Pathogens: The Xanthomonas campestris Type III Effector XopJ Targets the Host Cell Proteasome to Suppress Salicylic-Acid Mediated Plant Defence

PLOS Pathogens: The Xanthomonas campestris Type III Effector XopJ Targets the Host Cell Proteasome to Suppress Salicylic-Acid Mediated Plant Defence | Plant-microbe interaction | Scoop.it

The phytopathogenic bacterium Xanthomonas campestris pv. vesicatoria (Xcv) requires type III effector proteins (T3Es) for virulence. After translocation into the host cell, T3Es are thought to interact with components of host immunity to suppress defence responses. XopJ is a T3E protein from Xcv that interferes with plant immune responses; however, its host cellular target is unknown. Here we show that XopJ interacts with the proteasomal subunit RPT6 in yeast andin planta to inhibit proteasome activity. A C235A mutation within the catalytic triad of XopJ as well as a G2A exchange within the N-terminal myristoylation motif abolishes the ability of XopJ to inhibit the proteasome. Xcv ΔxopJ mutants are impaired in growth and display accelerated symptom development including tissue necrosis on susceptible pepper leaves. Application of the proteasome inhibitor MG132 restored the ability of the Xcv ΔxopJ to attenuate the development of leaf necrosis. The XopJ dependent delay of tissue degeneration correlates with reduced levels of salicylic acid (SA) and changes in defence- and senescence-associated gene expression. Necrosis upon infection with Xcv ΔxopJ was greatly reduced in pepper plants with reduced expression of NPR1, a central regulator of SA responses, demonstrating the involvement of SA-signalling in the development of XopJ dependent phenotypes. Our results suggest that XopJ-mediated inhibition of the proteasome interferes with SA-dependent defence response to attenuate onset of necrosis and to alter host transcription. A central role of the proteasome in plant defence is discussed.

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CP's comment, June 14, 2013 9:14 AM
Congrats again!!
Freddy Monteiro's comment, February 20, 2:32 PM
Jim, I agree: another great publication to go side by side with this one: <br> Gimenez-Ibanez et al. The Bacterial Effector HopX1 Targets JAZ Transcriptional Repressors to Activate Jasmonate Signaling and Promote Infection in Arabidopsis. PLOS Biology 2014.<br>http://dx.plos.org/10.1371/journal.pbio.1001792
Freddy Monteiro's curator insight, February 20, 2:33 PM

Another great publication to go side by side with this one:

Gimenez-Ibanez et al. The Bacterial Effector HopX1 Targets JAZ Transcriptional Repressors to Activate Jasmonate Signaling and Promote Infection in Arabidopsis. PLOS Biology 2014.

http://dx.plos.org/10.1371/journal.pbio.1001792

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The Plasmodesmal Protein PDLP1 Localises to Haustoria-Associated Membranes during Downy Mildew Infection and Regulates Callose Deposition

The Plasmodesmal Protein PDLP1 Localises to Haustoria-Associated Membranes during Downy Mildew Infection and Regulates Callose Deposition | Plant-microbe interaction | Scoop.it
The downy mildew pathogen Hyaloperonospora arabidopsidis (Hpa) is a filamentous oomycete that invades plant cells via sophisticated but poorly understood structures called haustoria. Haustoria are separated from the host cell cytoplasm and surrounded by an extrahaustorial membrane (EHM) of unknown origin. In some interactions, including Hpa-Arabidopsis, haustoria are progressively encased by host-derived, callose-rich materials but the molecular mechanisms by which callose accumulates around haustoria remain unclear. Here, we report that PLASMODESMATA-LOCATED PROTEIN 1 (PDLP1) is expressed at high levels in Hpa infected cells. Unlike other plasma membrane proteins, which are often excluded from the EHM, PDLP1 is located at the EHM in Hpa-infected cells prior to encasement. The transmembrane domain and cytoplasmic tail of PDLP1 are sufficient to convey this localization. PDLP1 also associates with the developing encasement but this association is lost when encasements are fully mature. We found that the pdlp1,2,3 triple mutant is more susceptible to Hpa while overexpression of PDLP1 enhances plant resistance, suggesting that PDLPs enhance basal immunity against Hpa. Haustorial encasements are depleted in callose in pdlp1,2,3 mutant plants whereas PDLP1 over-expression elevates callose deposition around haustoria and across the cell surface. These data indicate that PDLPs contribute to callose encasement of Hpa haustoria and suggests that the deposition of callose at haustoria may involve similar mechanisms to callose deposition at plasmodesmata.
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Bacterial effector modulation of host E3 ligase activity suppresses PAMP-triggered immunity in rice : Nature Communications : Nature Publishing Group

Pathogen effector proteins are delivered to host cells to suppress plant immunity. However, the mechanisms by which effector proteins function are largely unknown. Here we show that expression of XopPXoo, an effector of rice pathogen Xanthomonas oryzae pv. oryzae, in rice strongly suppresses peptidoglycan (PGN)- and chitin-triggered immunity and resistance to X. oryzae. XopPXoo targets OsPUB44, a rice ubiquitin E3 ligase with a unique U-box domain. We find that XopPXoo directly interacts with the OsPUB44 U-box domain and inhibits ligase activity. Two amino-acid residues specific for the OsPUB44 U-box domain are identified, which are responsible for the interaction with XopPXoo. Silencing of OsPUB44 suppresses PGN- and chitin-triggered immunity and X. oryzae resistance, indicating that OsPUB44 positively regulates immune responses. Thus, it is likely that XopPXoo suppresses immune responses by directly interacting with and inhibiting a positive regulator of plant immunity.
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Trade-off between growth and immunity: role of brassinosteroids

Trade-off between growth and immunity: role of brassinosteroids | Plant-microbe interaction | Scoop.it

Highlights•

Brassinosteroids (BR) regulate the growth-immunity trade-off in Arabidopsis.

The co-receptor BAK1 is not rate-limiting between the BR and immune pathways.

The BR-activated transcription factor BZR1 is a master regulator of this trade-off.

BZR1 and HBI1 form a core transcriptional cascade and regulatory hub.

The effects of BR on plant–pathogen interactions require further investigation.

A balance between growth and immunity exists in plants. Recently, the growth-promoting hormones brassinosteroids (BR) have emerged as crucial regulators of the growth-immunity trade-off, although the molecular mechanisms underlying this role remained unclear. New evidence obtained from the model plant Arabidopsis thaliana points at an indirect crosstalk between BR signaling and immunity, mediated by the transcription factors BZR1 and HBI1, which suppress immunity upon BR perception. The core transcriptional cascade formed by BZR1 and HBI1 seems to act as a regulatory hub on which multiple signaling inputs impinge, ensuring effective fine-tuning of the trade-off between growth and immunity in a timely and cost-efficient manner.


Via Christophe Jacquet
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Light intensity and temperature affect systemic spread of silencing signal in transient agroinfiltration studies

Light intensity and temperature affect systemic spread of silencing signal in transient agroinfiltration studies | Plant-microbe interaction | Scoop.it

RNA silencing is a sequence-specific post-transcriptional gene inactivation mechanism that operates in diverse organisms and that can extend beyond its site of initiation, owing to the movement of the silencing signal, called non-autonomous gene silencing. Previous studies have shown that several factors manifest the movement of the silencing signal, such as the size (21 or 24 nucleotides) of the secondary small interfering RNA (siRNA) produced, the steady-state concentration of siRNAs and their cognate messenger RNA (mRNA) or a change in the sink–source status of plant parts affecting phloem translocation. Our study shows that both light intensity and temperature have a significant impact on the systemic movement of the silencing signal in transient agroinfiltration studies in Nicotiana benthamiana. At higher light intensities (≥450 μE/m2/s) and higher temperatures (≥30 °C), gene silencing was localized to leaf tissue that was infiltrated, without any systemic spread. Interestingly, in these light and temperature conditions (≥450 μE/m2/s and ≥30 °C), the N. benthamiana plants showed recovery from the viral symptoms. However, the reduced systemic silencing and reduced viral symptom severity at higher light intensities were caused by a change in the sink–source status of the plant, ultimately affecting the phloem translocation of small RNAs or the viral genome. In contrast, at lower light intensities (<300 μE/m2/s) with a constant temperature of 25 °C, there was strong systemic movement of the silencing signal in the N. benthamiana plants and reduced recovery from virus infections. The accumulation of gene-specific siRNAs was reduced at higher temperature as a result of a reduction in the accumulation of transcript on transient agroinfiltration of RNA interference (RNAi) constructs, mostly because of poor T-DNA transfer activity of Agrobacterium, possibly also accompanied by reduced phloem translocation.

 

 


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PLOS Genetics: The Nuclear Immune Receptor RPS4 Is Required for RRS1SLH1-Dependent Constitutive Defense Activation in Arabidopsis thaliana (2014)

PLOS Genetics: The Nuclear Immune Receptor RPS4 Is Required for RRS1SLH1-Dependent Constitutive Defense Activation in Arabidopsis thaliana (2014) | Plant-microbe interaction | Scoop.it

Plant nucleotide-binding leucine-rich repeat (NB-LRR) disease resistance (R) proteins recognize specific “avirulent” pathogen effectors and activate immune responses. NB-LRR proteins structurally and functionally resemble mammalian Nod-like receptors (NLRs). How NB-LRR and NLR proteins activate defense is poorly understood. The divergently transcribed Arabidopsis R genes, RPS4 (resistance to Pseudomonas syringae 4) and RRS1 (resistance to Ralstonia solanacearum 1), function together to confer recognition of PseudomonasAvrRps4 and Ralstonia PopP2. RRS1 is the only known recessive NB-LRR R gene and encodes a WRKY DNA binding domain, prompting suggestions that it acts downstream of RPS4 for transcriptional activation of defense genes. We define here the early RRS1-dependent transcriptional changes upon delivery of PopP2 via Pseudomonas type III secretion. The Arabidopsis slh1 (sensitive to low humidity 1) mutant encodes an RRS1 allele (RRS1SLH1) with a single amino acid (leucine) insertion in the WRKY DNA-binding domain. Its poor growth due to constitutive defense activation is rescued at higher temperature. Transcription profiling data indicate that RRS1SLH1-mediated defense activation overlaps substantially with AvrRps4- and PopP2-regulated responses. To better understand the genetic basis of RPS4/RRS1-dependent immunity, we performed a genetic screen to identify suppressor of slh1 immunity (sushi) mutants. We show that many sushi mutants carry mutations in RPS4, suggesting that RPS4 acts downstream or in a complex with RRS1. Interestingly, several mutations were identified in a domain C-terminal to the RPS4 LRR domain. Using an Agrobacterium-mediated transient assay system, we demonstrate that the P-loop motif of RPS4 but not of RRS1SLH1 is required for RRS1SLH1 function. We also recapitulate the dominant suppression of RRS1SLH1 defense activation by wild type RRS1 and show this suppression requires an intact RRS1 P-loop. These analyses of RRS1SLH1shed new light on mechanisms by which NB-LRR protein pairs activate defense signaling, or are held inactive in the absence of a pathogen effector.


Via Kamoun Lab @ TSL, CP
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Frontiers | Sustained mitogen-activated protein kinase activation reprograms defense metabolism and phosphoprotein profile in Arabidopsis thaliana | Plant Proteomics

Mitogen-activated protein kinases (MAPKs) target a variety of protein substrates to regulate cellular signaling processes in eukaryotes. In plants, the number of identified MAPK substrates that control plant defense responses is still limited. Here, we generated transgenic Arabidopsis thaliana plants with an inducible system to simulate in vivo activation of two stress-activated MAPKs, MPK3 and MPK6. Metabolome analysis revealed that this artificial MPK3/6 activation (without any exposure to pathogens or other stresses) is sufficient to drive the production of major defense-related metabolites, including various camalexin, indole glucosinolate and agmatine derivatives. An accompanying (phospho)proteome analysis led to detection of hundreds of potential phosphoproteins downstream of MPK3/6 activation. Besides known MAPK substrates, many candidates on this list possess typical MAPK-targeted phosphosites and in many cases, the corresponding phosphopeptides were detected by mass spectrometry. Notably, several of these putative phosphoproteins have been reported to be associated with the biosynthesis of antimicrobial defense substances (e.g. WRKY transcription factors and proteins encoded by the genes from the “PEN” pathway required for penetration resistance to filamentous pathogens). Thus, this work provides an inventory of candidate phosphoproteins, including putative direct MAPK substrates, for future analysis of MAPK-mediated defense control. (Proteomics data are available with the identifier PXD001252 via ProteomeXchange, http://proteomecentral.proteomexchange.org).
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A Unique Plant ESCRT Component, FREE1, Regulates Multivesicular Body Protein Sorting and Plant Growth: Current Biology

A Unique Plant ESCRT Component, FREE1, Regulates Multivesicular Body Protein Sorting and Plant Growth: Current Biology | Plant-microbe interaction | Scoop.it
Highlights•FREE1 is a plant-specific and PVC-localized FYVE domain protein binding to PI3P•FREE1 is in a complex with ESCRT-I via a direct interaction with Vps23•FREE1 directly binds to ubiquitin and regulates vacuolar membrane protein sorting•FREE1 is essential for PVC/MVB biogenesis and plant growthSummary

Tight control of membrane protein homeostasis by selective degradation is crucial for proper cell signaling and multicellular organismal development. Membrane proteins destined for degradation, such as misfolded proteins or activated receptors, are usually ubiquitinated and sorted into the intraluminal vesicles (ILVs) of prevacuolar compartments/multivesicular bodies (PVCs/MVBs), which then fuse with vacuoles/lysosomes to deliver their contents to the lumen for degradation by luminal proteases [ 1 ]. The formation of ILVs and the sorting of ubiquitinated membrane cargoes into them are facilitated by the endosomal sorting complex required for transport (ESCRT) machinery [ 2–4 ]. Plants possess most evolutionarily conserved members of the ESCRT machinery but apparently lack orthologs of ESCRT-0 subunits and the ESCRT-I component Mvb12 [ 5–8 ]. Here, we identified a unique plant ESCRT component called FYVE domain protein required for endosomal sorting 1 (FREE1). FREE1 binds to phosphatidylinositol-3-phosphate (PI3P) and ubiquitin and specifically interacts with Vps23 via PTAP-like tetrapeptide motifs to be incorporated into the ESCRT-I complex. Arabidopsis free1 mutant is seedling lethal and defective in the formation of ILVs in MVBs. Consequently, endocytosed plasma membrane (PM) proteins destined for degradation, such as the auxin efflux carrier PIN2 [ 9, 10 ], cannot reach the lumen of the vacuole and mislocalize to the tonoplast. Collectively, our findings provide the first functional characterization of a plant FYVE domain protein, which is essential for plant growth via its role as a unique evolutionary ESCRT component for MVB biogenesis and vacuolar sorting of membrane proteins.

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Expression Profiling during Arabidopsis/Downy Mildew Interaction Reveals a Highly-Expressed Effector That Attenuates Responses to Salicylic Acid

Expression Profiling during Arabidopsis/Downy Mildew Interaction Reveals a Highly-Expressed Effector That Attenuates Responses to Salicylic Acid | Plant-microbe interaction | Scoop.it
Plants have evolved strong innate immunity mechanisms, but successful pathogens evade or suppress plant immunity via effectors delivered into the plant cell. Hyaloperonospora arabidopsidis (Hpa) causes downy mildew on Arabidopsis thaliana, and a genome sequence is available for isolate Emoy2. Here, we exploit the availability of genome sequences for Hpa and Arabidopsis to measure gene-expression changes in both Hpa and Arabidopsis simultaneously during infection. Using a high-throughput cDNA tag sequencing method, we reveal expression patterns of Hpa predicted effectors and Arabidopsis genes in compatible and incompatible interactions, and promoter elements associated with Hpa genes expressed during infection. By resequencing Hpa isolate Waco9, we found it evades Arabidopsis resistance gene RPP1 through deletion of the cognate recognized effector ATR1. Arabidopsis salicylic acid (SA)-responsive genes including PR1 were activated not only at early time points in the incompatible interaction but also at late time points in the compatible interaction. By histochemical analysis, we found that Hpa suppresses SA-inducible PR1 expression, specifically in the haustoriated cells into which host-translocated effectors are delivered, but not in non-haustoriated adjacent cells. Finally, we found a highly-expressed Hpa effector candidate that suppresses responsiveness to SA. As this approach can be easily applied to host-pathogen interactions for which both host and pathogen genome sequences are available, this work opens the door towards transcriptome studies in infection biology that should help unravel pathogen infection strategies and the mechanisms by which host defense responses are overcome.
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Dss1 Is a 26S Proteasome Ubiquitin Receptor

Dss1 Is a 26S Proteasome Ubiquitin Receptor | Plant-microbe interaction | Scoop.it
Summary

The ubiquitin-proteasome system is the major pathway for protein degradation in eukaryotic cells. Proteins to be degraded are conjugated to ubiquitin chains that act as recognition signals for the 26S proteasome. The proteasome subunits Rpn10 and Rpn13 are known to bind ubiquitin, but genetic and biochemical data suggest the existence of at least one other substrate receptor. Here, we show that the phylogenetically conserved proteasome subunit Dss1 (Sem1) binds ubiquitin chains linked by K63 and K48. Atomic resolution data show that Dss1 is disordered and binds ubiquitin by binding sites characterized by acidic and hydrophobic residues. The complementary binding region in ubiquitin is composed of a hydrophobic patch formed by I13, I44, and L69 flanked by two basic regions. Mutations in the ubiquitin-binding site of Dss1 cause growth defects and accumulation of ubiquitylated proteins.

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Cell Host & Microbe: Proline Isomerization of the Immune Receptor-Interacting Protein RIN4 by a Cyclophilin Inhibits Effector-Triggered Immunity in Arabidopsis (2014)

Cell Host & Microbe: Proline Isomerization of the Immune Receptor-Interacting Protein RIN4 by a Cyclophilin Inhibits Effector-Triggered Immunity in Arabidopsis (2014) | Plant-microbe interaction | Scoop.it

In the absence of pathogen infection, plant effector-triggered immune (ETI) receptors are maintained in a preactivation state by intermolecular interactions with other host proteins. Pathogen effector-induced alterations activate the receptor. In Arabidopsis, the ETI receptor RPM1 is activated via bacterial effector AvrB-induced phosphorylation of the RPM1-interacting protein RIN4 at Threonine 166. We find that RIN4 also interacts with the prolyl-peptidyl isomerase (PPIase) ROC1, which is reduced upon RIN4 Thr166 phosphorylation. ROC1 suppresses RPM1 immunity in a PPIase-dependent manner. Consistent with this, RIN4 Pro149 undergoes cis/transisomerization in the presence of ROC1. While the RIN4P149V mutation abolishes RPM1 resistance, the deletion of Pro149 leads to RPM1 activation in the absence of RIN4 phosphorylation. These results support a model in which RPM1 directly senses conformational changes in RIN4 surrounding Pro149 that is controlled by ROC1. RIN4 Thr166 phosphorylation indirectly regulates RPM1 resistance by modulating the ROC1-mediated RIN4 isomerization.


Via Kamoun Lab @ TSL
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Structural basis of the regulatory mechanism of the plant CIPK family of protein kinases controlling ion homeostasis and abiotic stress

Structural basis of the regulatory mechanism of the plant CIPK family of protein kinases controlling ion homeostasis and abiotic stress | Plant-microbe interaction | Scoop.it
Plant cells have developed specific protective molecular machinery against environmental stresses. The family of CBL-interacting protein kinases (CIPK) and their interacting activators, the calcium sensors calcineurin B-like (CBLs), work together to decode calcium signals elicited by stress situations. The molecular basis of biological activation of CIPKs relies on the calcium-dependent interaction of a self-inhibitory NAF motif with a particular CBL, the phosphorylation of the activation loop by upstream kinases, and the subsequent phosphorylation of the CBL by the CIPK. We present the crystal structures of the NAF-truncated and pseudophosphorylated kinase domains of CIPK23 and CIPK24/SOS2. In addition, we provide biochemical data showing that although CIPK23 is intrinsically inactive and requires an external stimulation, CIPK24/SOS2 displays basal activity. This data correlates well with the observed conformation of the respective activation loops: Although the loop of CIPK23 is folded into a well-ordered structure that blocks the active site access to substrates, the loop of CIPK24/SOS2 protrudes out of the active site and allows catalysis. These structures together with biochemical and biophysical data show that CIPK kinase activity necessarily requires the coordinated releases of the activation loop from the active site and of the NAF motif from the nucleotide-binding site. Taken all together, we postulate the basis for a conserved calcium-dependent NAF-mediated regulation of CIPKs and a variable regulation by upstream kinases.
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Molecular and cellular control of cell death and defense signaling in pepper

Molecular and cellular control of cell death and defense signaling in pepper | Plant-microbe interaction | Scoop.it

Pepper (Capsicum annuum L.) provides a good experimental system for studying the molecular and functional genomics underlying the ability of plants to defend themselves against microbial pathogens. Cell death is a genetically programmed response that requires specific host cellular factors. Hypersensitive response (HR) is defined as rapid cell death in response to a pathogen attack. Pepper plants respond to pathogen attacks by activating genetically controlled HR- or disease-associated cell death. HR cell death, specifically in incompatible interactions between pepper and Xanthomonas campestris pv. vesicatoria, is mediated by the molecular genetics and biochemical machinery that underlie pathogen-induced cell death in plants. Gene expression profiles during the HR-like cell death response, virus-induced gene silencing and transient and transgenic overexpression approaches are used to isolate and identify HR- or disease-associated cell death genes in pepper plants. Reactive oxygen species, nitric oxide, cytosolic calcium ion and defense-related hormones such as salicylic acid, jasmonic acid, ethylene and abscisic acid are involved in the execution of pathogen-induced cell death in plants. In this review, we summarize recent molecular and cellular studies of the pepper cell death-mediated defense response, highlighting the signaling events of cell death in disease-resistant pepper plants. Comprehensive knowledge and understanding of the cellular functions of pepper cell death response genes will aid the development of novel practical approaches to enhance disease resistance in pepper, thereby helping to secure the future supply of safe and nutritious pepper plants worldwide.

 

 


Via Christophe Jacquet
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GRIM REAPER peptide binds to receptor kinase PRK5 to trigger cell death in Arabidopsis

GRIM REAPER peptide binds to receptor kinase PRK5 to trigger cell death in Arabidopsis | Plant-microbe interaction | Scoop.it
Recognition of extracellular peptides by plasma membrane‐localized receptor proteins is commonly used in signal transduction. In plants, very little is known about how extracellular peptides are processed and activated in order to allow recognition by receptors. Here, we show that induction of cell death in planta by a secreted plant protein GRIM REAPER (GRI) is dependent on the activity of the type II metacaspase METACASPASE‐9. GRI is cleaved by METACASPASE‐9 in vitro resulting in the release of an 11 amino acid peptide. This peptide bound in vivo to the extracellular domain of the plasma membrane‐localized, atypical leucine‐rich repeat receptor‐like kinase POLLEN‐SPECIFIC RECEPTOR‐LIKE KINASE 5 (PRK5) and was sufficient to induce oxidative stress/ROS‐dependent cell death. This shows a signaling pathway in plants from processing and activation of an extracellular protein to recognition by its receptor.
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Curr Opin Microbiology: Targeting of plant pattern recognition receptor-triggered immunity by bacterial type-III secretion system effectors (2014)

Curr Opin Microbiology: Targeting of plant pattern recognition receptor-triggered immunity by bacterial type-III secretion system effectors (2014) | Plant-microbe interaction | Scoop.it

• Type-III effectors (T3Es) suppress plant immunity using multiple strategies.
• PRR-triggered immunity is redundantly targeted by multiple T3Es from a single bacterial strain.
• A single T3E can have multiple plant targets.
• A given immune component can be targeted by multiple T3Es.
• The study of T3Es reveals important immune components and unique biochemical processes.

 

During infection, microbes are detected by surface-localized pattern recognition receptors (PRRs), leading to an innate immune response that prevents microbial ingress. Therefore, successful pathogens must evade or inhibit PRR-triggered immunity to cause disease. In the past decade, a number of type-III secretion system effector (T3Es) proteins from plant pathogenic bacteria have been shown to suppress this layer of innate immunity. More recently, the detailed mechanisms of action have been defined for several of these effectors. Interestingly, effectors display a wide array of virulence targets, being able to prevent activation of immune receptors and to hijack immune signaling pathways. Besides being a fascinating example of pathogen-host co-evolution, effectors have also emerged as valuable tools to dissect important biological processes in host cells.


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The Calcium-Dependent Protein Kinase CPK28 Buffers Plant Immunity and Regulates BIK1 Turnover: Cell Host & Microbe

The Calcium-Dependent Protein Kinase CPK28 Buffers Plant Immunity and Regulates BIK1 Turnover: Cell Host & Microbe | Plant-microbe interaction | Scoop.it
Plant perception of pathogen-associated molecular patterns (PAMPs) triggers a phosphorylation relay leading to PAMP-triggered immunity (PTI). Despite increasing knowledge of PTI signaling, how immune homeostasis is maintained remains largely unknown. Here we describe a forward-genetic screen to identify loci involved in PTI and characterize the Arabidopsis calcium-dependent protein kinase CPK28 as a negative regulator of immune signaling. Genetic analyses demonstrate that CPK28 attenuates PAMP-triggered immune responses and antibacterial immunity. CPK28 interacts with and phosphorylates the plasma-membrane-associated cytoplasmic kinase BIK1, an important convergent substrate of multiple pattern recognition receptor (PRR) complexes. We find that BIK1 is rate limiting in PTI signaling and that it is continuously turned over to maintain cellular homeostasis. We further show that CPK28 contributes to BIK1 turnover. Our results suggest a negative regulatory mechanism that continually buffers immune signaling by controlling the turnover of this key signaling kinase.
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Functional analysis of plant defense suppression and activation by the Xanthomonas core type III effector XopX

Functional analysis of plant defense suppression and activation by the Xanthomonas core type III effector XopX | Plant-microbe interaction | Scoop.it
Many phytopathogenic type III secretion effectors (T3Es) have been shown to target and suppress plant immune signaling, but perturbation of the plant immune system by T3Es can also elicit a plant response. XopX is a “core” Xanthomonas T3E that contributes to growth and symptom development during Xanthomonas euvesicatoria (Xe) infection of tomato, but its functional role is undefined. We tested the effect of XopX on several aspects of plant immune signaling. XopX promoted ethylene production and plant cell death (PCD) during Xe infection of susceptible tomato and in transient expression assays in Nicotiana benthamiana, which is consistent with its requirement for the development of Xe-induced disease symptoms. Additionally, although XopX suppressed flagellin-induced reactive oxygen species, it promoted the accumulation of pattern-triggered immunity (PTI) gene transcripts. Surprisingly, XopX co-expression with other PCD elicitors resulted in delayed PCD, suggesting antagonism between XopX-dependent PCD and other PCD pathways. However, we found no evidence that XopX contributed to the suppression of effector-triggered immunity during Xe-tomato interactions, suggesting that XopX’s primary virulence role is to modulate PTI. These results highlight the dual role of a core Xanthomonas T3E in simultaneously suppressing and activating plant defense responses.
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Frontiers | The Salmonella effector protein SpvC, a phosphothreonine lyase is functional in plant cells | Plant-Microbe Interaction

Frontiers | The Salmonella effector protein SpvC, a phosphothreonine lyase is functional in plant cells | Plant-Microbe Interaction | Plant-microbe interaction | Scoop.it
Salmonella is one of the most prominent causes of food poisoning and growing evidence indicates that contaminated fruits and vegetables are an increasing concern for human health. Successful infection demands the suppression of the host immune system, which is often achieved via injection of bacterial effector proteins into host cells. In this report we present the function of Salmonella effector protein in plant cell, supporting the new concept of trans-kingdom competence of this bacterium. We screened a range of Salmonella Typhimurium effector proteins for interference with plant immunity. Among these, the phosphothreonine lyase SpvC attenuated the induction of immunity-related genes when present in plant cells. Using in vitro and in vivo systems we show that this effector protein interacts with and dephosphorylates activated Arabidopsis Mitogen-activated Protein Kinase 6 (MPK6), thereby inhibiting defense signaling. Moreover, the requirement of Salmonella SpvC was shown by the decreased proliferation of the ΔspvC mutant in Arabidopsis plants. These results suggest that some Salmonella effector proteins could have a conserved function during proliferation in different hosts. The fact that Salmonella and other Enterobacteriaceae use plants as hosts strongly suggests that plants represent a much larger reservoir for animal pathogens than so far estimated.
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Perturbation of host ubiquitin systems by plant pathogen/pest effector proteins - Banfield - Cellular Microbiology - Wiley Online Library

Perturbation of host ubiquitin systems by plant pathogen/pest effector proteins - Banfield - Cellular Microbiology - Wiley Online Library | Plant-microbe interaction | Scoop.it
Microbial pathogens and pests of animals and plants secrete effector proteins into host cells, altering cellular physiology to the benefit of the invading parasite. Research in the past decade has delivered significant new insights into the molecular mechanisms of how these effector proteins function, with a particular focus on modulation of host immunity-related pathways. One host system that has emerged as a common target of effectors is the ubiquitination system in which substrate proteins are post-translationally modified by covalent conjugation with the small protein ubiquitin. This modification, typically via isopeptide bond formation through a lysine side-chain of ubiquitin, can result in target degradation, re-localisation, altered activity or affect protein-protein interactions. In this review I focus primarily on how effector proteins from bacterial and filamentous pathogens of plants and pests perturb host ubiquitination pathways that ultimately include the 26S proteasome. The activities of these effectors, in how they affect ubiquitin pathways in plants, reveals how pathogens have evolved to identify and exploit weaknesses in this system that deliver increased pathogen fitness.
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Genome Biology | Abstract | Transcriptomic analysis reveals tomato genes whose expression is induced specifically during effector-triggered immunity and identifies the Epk1 protein kinase which is ...

Genome Biology | Abstract | Transcriptomic analysis reveals tomato genes whose expression is induced specifically during effector-triggered immunity and identifies the Epk1 protein kinase which is ... | Plant-microbe interaction | Scoop.it

Background: Plants have two related immune systems to defend themselves against pathogen attack. Initially, pattern-triggered immunity is activated upon recognition of microbe-associated molecular patterns by pattern recognition receptors. Pathogenic bacteria deliver effector proteins into the plant cell that interfere with this immune response and promote disease. However, some plants express resistance proteins that detect the presence of specific effectors leading to a robust defense response referred to as effector-triggered immunity. The interaction of tomato with Pseudomonas syringae pv. tomato is an established model system for understanding the molecular basis of these plant immune responses.ResultsWe apply high-throughput RNA sequencing to this pathosystem to identify genes whose expression changes specifically during pattern-triggered or effector-triggered immunity. We then develop reporter genes for each of these responses that will enable characterization of the host response to the large collection of P. s. pv. tomato strains that express different combinations of effectors. Virus-induced gene silencing of 30 of the effector-triggered immunity-specific genes identifies Epk1 which encodes a predicted protein kinase from a family previously unknown to be involved in immunity. Knocked-down expression of Epk1 compromises effector-triggered immunity triggered by three bacterial effectors but not by effectors from non-bacterial pathogens. Epistasis experiments indicate that Epk1 acts upstream of effector-triggered immunity-associated MAP kinase signaling.ConclusionsUsing RNA-seq technology we identify genes involved in specific immune responses. A functional genomics screen led to the discovery of Epk1, a novel predicted protein kinase required for plant defense activation upon recognition of three different bacterial effectors.

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Myosins VIII and XI Play Distinct Roles in Reproduction and Transport of Tobacco Mosaic Virus

Myosins VIII and XI Play Distinct Roles in Reproduction and Transport of Tobacco Mosaic Virus | Plant-microbe interaction | Scoop.it
Viruses are obligatory parasites that depend on host cellular factors for their replication as well as for their local and systemic movement to establish infection. Although myosin motors are thought to contribute to plant virus infection, their exact roles in the specific infection steps have not been addressed. Here we investigated the replication, cell-to-cell and systemic spread of Tobacco mosaic virus (TMV) using dominant negative inhibition of myosin activity. We found that interference with the functions of three class VIII myosins and two class XI myosins significantly reduced the local and long-distance transport of the virus. We further determined that the inactivation of myosins XI-2 and XI-K affected the structure and dynamic behavior of the ER leading to aggregation of the viral movement protein (MP) and to a delay in the MP accumulation in plasmodesmata (PD). The inactivation of myosin XI-2 but not of myosin XI-K affected the localization pattern of the 126k replicase subunit and the level of TMV accumulation. The inhibition of myosins VIII-1, VIII-2 and VIII-B abolished MP localization to PD and caused its retention at the plasma membrane. These results suggest that class XI myosins contribute to the viral propagation and intracellular trafficking, whereas myosins VIII are specifically required for the MP targeting to and virus movement through the PD. Thus, TMV appears to recruit distinct myosins for different steps in the cell-to-cell spread of the infection.
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Nucleotide-binding oligomerization domain-like receptor cooperativity in effector-triggered immunity

Nucleotide-binding oligomerization domain-like receptor cooperativity in effector-triggered immunity | Plant-microbe interaction | Scoop.it

Intracellular nucleotide-binding oligomerization domain (NOD)-like receptors (NLRs) are basic elements of innate immunity in plants and animals. Whereas animal NLRs react to conserved microbe- or damage-associated molecular patterns, plant NLRs intercept the actions of diverse pathogen virulence factors (effectors). In this review, we discuss recent genetic and molecular evidence for functional NLR pairs, and discuss the significance of NLR self-association and heteromeric NLR assemblies in the triggering of downstream signaling pathways. We highlight the versatility and impact of cooperating NLR pairs that combine pathogen sensing with the initiation of defense signaling in both plant and animal immunity. We propose that different NLR receptor molecular configurations provide opportunities for fine-tuning resistance pathways and enhancing the host's pathogen recognition spectrum to keep pace with rapidly evolving microbial populations.

  
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Salicylic Acid Regulates Arabidopsis Microbial Pattern Receptor Kinase Levels and Signaling

Salicylic Acid Regulates Arabidopsis Microbial Pattern Receptor Kinase Levels and Signaling | Plant-microbe interaction | Scoop.it

In Arabidopsis thaliana, responses to pathogen-associated molecular patterns (PAMPs) are mediated by cell surface pattern recognition receptors (PRRs) and include the accumulation of reactive oxygen species, callose deposition in the cell wall, and the generation of the signal molecule salicylic acid (SA). SA acts in a positive feedback loop with ACCELERATED CELL DEATH6 (ACD6), a membrane protein that contributes to immunity. This work shows that PRRs associate with and are part of the ACD6/SA feedback loop. ACD6 positively regulates the abundance of several PRRs and affects the responsiveness of plants to two PAMPs. SA accumulation also causes increased levels of PRRs and potentiates the responsiveness of plants to PAMPs. Finally, SA induces PRR- and ACD6-dependent signaling to induce callose deposition independent of the presence of PAMPs. This PAMP-independent effect of SA causes a transient reduction of PRRs and ACD6-dependent reduced responsiveness to PAMPs. Thus, SA has a dynamic effect on the regulation and function of PRRs. Within a few hours, SA signaling promotes defenses and downregulates PRRs, whereas later (within 24 to 48 h) SA signaling upregulates PRRs, and plants are rendered more responsive to PAMPs. These results implicate multiple modes of signaling for PRRs in response to PAMPs and SA.

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Cell Host & Microbe: A Plant Phosphoswitch Platform Repeatedly Targeted by Type III Effector Proteins Regulates the Output of Both Tiers of Plant Immune Receptors (2014)

Cell Host & Microbe: A Plant Phosphoswitch Platform Repeatedly Targeted by Type III Effector Proteins Regulates the Output of Both Tiers of Plant Immune Receptors (2014) | Plant-microbe interaction | Scoop.it

Plants detect microbes via two functionally interconnected tiers of immune receptors. Immune detection is suppressed by equally complex pathogen mechanisms. The small plasma-membrane-tethered protein RIN4 negatively regulates microbe-associated molecular pattern (MAMP)-triggered responses, which are derepressed upon bacterial flagellin perception. We demonstrate that recognition of the flagellin peptide MAMP flg22 triggers accumulation of RIN4 phosphorylated at serine 141 (pS141) that mediates derepression of several immune outputs. RIN4 is targeted by four bacterial type III effector proteins, delivered temporally after flagellin perception. Of these, AvrB acts with a host kinase to increase levels of RIN4 phosphorylated at threonine 166 (pT166). RIN4 pT166 is epistatic to RIN4 pS141. Thus, AvrB contributes to virulence by enhancing “rerepression” of immune system outputs. Our results explain the evolution of independent effectors that antagonize accumulation of RIN4 pS141 and of a specific plant intracellular NLR protein, RPM1, which is activated by AvrB-mediated accumulation of RIN4 pT166.


Via Kamoun Lab @ TSL
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Probing formation of cargo/importin-α transport complexes in plant cells using a pathogen effector - Wirthmueller - The Plant Journal - Wiley Online Library

Probing formation of cargo/importin-α transport complexes in plant cells using a pathogen effector - Wirthmueller - The Plant Journal - Wiley Online Library | Plant-microbe interaction | Scoop.it
Importin-αs are essential adapter proteins that recruit cytoplasmic proteins destined for active nuclear import to the nuclear transport machinery. Cargo proteins interact with the importin-α armadillo repeat domain via nuclear localization sequences (NLSs), short amino acids motifs enriched in Lys and Arg residues. Plant genomes typically encode several importin-α paralogs that can have both specific and partially redundant functions. Although some cargos are preferentially imported by a distinct importin-α, it remains unknown how this specificity is generated and to what extent cargos compete for binding to nuclear transport receptors. Here we report that the effector protein HaRxL106 from the oomycete pathogen Hyaloperonospora arabidopsidis co-opts the host cell's nuclear import machinery. We use HaRxL106 as a probe to determine redundant and specific functions of importin-α paralogs from Arabidopsis thaliana. A crystal structure of the importin-α3/MOS6 armadillo repeat domain suggests that five of the six Arabidopsis importin-αs expressed in rosette leaves have an almost identical NLS binding site. Comparison of the importin-α binding affinities of HaRxL106 and other cargos in vitro and in plant cells suggests that relatively small affinity differences in vitro affect the rate of transport complex formation in vivo. Our results suggest that cargo affinity for importin-α, sequence variation at the importin-α NLS binding sites and tissue-specific expression levels of importin-αs determine formation of cargo/importin-α transport complexes in plant cells.
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Ca2+ signalling in plant immune response: from pattern recognition receptors to Ca2+ decoding mechanisms. - Seybold - 2014 - New Phytologist - Wiley Online Library

Ca2+ signalling in plant immune response: from pattern recognition receptors to Ca2+ decoding mechanisms. - Seybold - 2014 - New Phytologist - Wiley Online Library | Plant-microbe interaction | Scoop.it
Summary

Ca2+ is a ubiquitous second messenger for cellular signalling in various stresses and developmental processes. Here, we summarize current developments in the roles of Ca2+ during plant immunity responses. We discuss the early perception events preceding and necessary for triggering cellular Ca2+ fluxes, the potential Ca2+-permeable channels, the decoding of Ca2+ signals predominantly via Ca2+-dependent phosphorylation events and transcriptional reprogramming. To highlight the complexity of the cellular signal network, we briefly touch on the interplay between Ca2+-dependent signalling and selected major signalling mechanisms – with special emphasis on reactive oxygen species at local and systemic levels.

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