Plant-microbe interaction
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J. Bact. (2012): Pseudomonas syringae pv. tomato DC3000 CmaL (PSPTO4723), a DUF1330 family member, is needed to produce L-allo-isoleucine, a precursor for the phytotoxin coronatine

J. Bact. (2012): Pseudomonas syringae pv. tomato DC3000 CmaL (PSPTO4723), a DUF1330 family member, is needed to produce L-allo-isoleucine, a precursor for the phytotoxin coronatine | Plant-microbe interaction | Scoop.it

Pseudomonas syringae pathovar tomato DC3000 produces the phytotoxin coronatine, a major determinant of the leaf chlorosis associated with DC3000 pathogenesis. The DC3000 PSPTO4723 (cmaL) gene is located in a genomic region encoding type III effectors, however it promotes chlorosis in the model plant Nicotiana benthamiana in a manner independent of type III secretion. Coronatine is produced by ligation of two moieties, coronafacic acid (CFA) and coronamic acid (CMA), which are produced by biosynthetic pathways encoded in separate operons. Cross-feeding experiments, performed in N. benthamiana with cfa, cma, and cmaL mutants, implicate CmaL in CMA production. Furthermore, analysis of bacterial supernatants under coronatine-inducing conditions revealed that mutants lacking either the cma operon or cmaL accumulate CFA rather than coronatine, supporting a role for CmaL in the regulation or biosynthesis of CMA. CmaL does not appear to regulate CMA production, as cmaL mutants are unaltered in the expression of proteins with known roles in CMA production. Rather, CmaL is needed for the first step in CMA synthesis, as wild-type levels of coronatine production are restored to a ΔcmaL mutant when supplemented with 50 μg/ml L-allo-isoleucine, the starting unit for CMA production. cmaL is found in all other sequenced P. syringae strains with coronatine biosynthesis genes. This characterization of CmaL identifies a critical missing factor in coronatine production and provides a foundation for further investigation of a member of the widespread DUF1330 protein family.

 

Jay N. Worley, Alistair B. Russell, Aaron G. Wexler, Philip A. Bronstein, Brian H. Kvitko, Stuart B. Krasnoff, Kathy R. Munkvold, Bryan Swingle, Donna M. Gibson, and Alan Collmer


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Plant-microbe interaction
Current research on plant immunity, effector proteins, the proteasome and autophagy
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The proteasome acts as a hub for plant immunity and is targeted by Pseudomonas type-III effectors

The proteasome acts as a hub for plant immunity and is targeted by Pseudomonas type-III effectors | Plant-microbe interaction | Scoop.it
Recent evidence suggests that the ubiquitin-proteasome system (UPS) is involved in several aspects of plant immunity and a range of plant pathogens subvert the UPS to enhance their virulence. Here we show that proteasome activity is strongly induced during basal defense in Arabidopsis. Mutant lines of the proteasome subunits RPT2a and RPN12a support increased bacterial growth of virulent Pseudomonas syringae pv. tomato DC3000 (Pst) and Pseudomonas syringae pv. maculicola ES4326. Both proteasome subunits are required for Pathogen-associated molecular patterns (PAMP)-triggered immunity (PTI) responses. Analysis of bacterial growth after a secondary infection of systemic leaves revealed that the establishment of systemic-acquired resistance (SAR) is impaired in proteasome mutants, suggesting that the proteasome also plays an important role in defense priming and SAR. In addition, we show that Pst inhibits proteasome activity in a type-III secretion dependent manner. A screen for type-III effector proteins from Pst for their ability to interfere with proteasome activity revealed HopM1, HopAO1, HopA1 and HopG1 as putative proteasome inhibitors. Biochemical characterization of HopM1 by mass-spectrometry indicates that HopM1 interacts with several E3 ubiquitin ligases and proteasome subunits. This supports the hypothesis that HopM1 associates with the proteasome leading to its inhibition. Thus, the proteasome is an essential component of PTI and SAR, which is targeted by multiple bacterial effectors.
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TALE-induced bHLH transcription factors that activate a pectate lyase contribute to water soaking in bacterial spot of tomato

TALE-induced bHLH transcription factors that activate a pectate lyase contribute to water soaking in bacterial spot of tomato | Plant-microbe interaction | Scoop.it
AvrHah1 [avirulence (avr) gene homologous to avrBs3 and hax2, no. 1] is a transcription activator-like (TAL) effector (TALE) in Xanthomonas gardneri that induces water-soaked disease lesions on fruits and leaves during bacterial spot of tomato. We observe that water from outside the leaf is drawn into the apoplast in X. gardneri-infected, but not X. gardneriΔavrHah1 (XgΔavrHah1)-infected, plants, conferring a dark, water-soaked appearance. The pull of water can facilitate entry of additional bacterial cells into the apoplast. Comparing the transcriptomes of tomato infected with X. gardneri vs. XgΔavrHah1 revealed the differential up-regulation of two basic helix–loop–helix (bHLH) transcription factors with predicted effector binding elements (EBEs) for AvrHah1. We mined our RNA-sequencing data for differentially up-regulated genes that could be direct targets of the bHLH transcription factors and therefore indirect targets of AvrHah1. We show that two pectin modification genes, a pectate lyase and pectinesterase, are targets of both bHLH transcription factors. Designer TALEs (dTALEs) for the bHLH transcription factors and the pectate lyase, but not for the pectinesterase, complement water soaking when delivered by XgΔavrHah1. By perturbing transcriptional networks and/or modifying the plant cell wall, AvrHah1 may promote water uptake to enhance tissue damage and eventual bacterial egression from the apoplast to the leaf surface. Understanding how disease symptoms develop may be a useful tool for improving the tolerance of crops from damaging disease lesions.
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ATG9 regulates autophagosome progression from the endoplasmic reticulum in Arabidopsis

ATG9 regulates autophagosome progression from the endoplasmic reticulum in Arabidopsis | Plant-microbe interaction | Scoop.it
Autophagy is a conserved pathway for bulk degradation of cytoplasmic material by a double-membrane structure named the autophagosome. The initiation of autophagosome formation requires the recruitment of autophagy-related protein 9 (ATG9) vesicles to the preautophagosomal structure. However, the functional relationship between ATG9 vesicles and the phagophore is controversial in different systems, and the molecular function of ATG9 remains unknown in plants. Here, we demonstrate that ATG9 is essential for endoplasmic reticulum (ER)-derived autophagosome formation in plants. Through a combination of genetic, in vivo imaging and electron tomography approaches, we show that Arabidopsis ATG9 deficiency leads to a drastic accumulation of autophagosome-related tubular structures in direct membrane continuity with the ER upon autophagic induction. Dynamic analyses demonstrate a transient membrane association between ATG9 vesicles and the autophagosomal membrane during autophagy. Furthermore, trafficking of ATG18a is compromised in atg9 mutants during autophagy by forming extended tubules in a phosphatidylinositol 3-phosphate–dependent manner. Taken together, this study provides evidence for a pivotal role of ATG9 in regulating autophagosome progression from the ER membrane in Arabidopsis.
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ATG8 Expansion: A Driver of Selective Autophagy Diversification?

ATG8 Expansion: A Driver of Selective Autophagy Diversification? | Plant-microbe interaction | Scoop.it
Selective autophagy is a conserved homeostatic pathway that involves engulfment of specific cargo molecules into specialized organelles called autophagosomes. The ubiquitin-like protein ATG8 is a central player of the autophagy network that decorates autophagosomes and binds to numerous cargo receptors. Although highly conserved across eukaryotes, ATG8 diversified from a single protein in algae to multiple isoforms in higher plants. We present a phylogenetic overview of 376 ATG8 proteins across the green plant lineage that revealed family-specific ATG8 clades. Because these clades differ in fixed amino acid polymorphisms, they provide a mechanistic framework to test whether distinct ATG8 clades are functionally specialized. We propose that ATG8 expansion may have contributed to the diversification of selective autophagy pathways in plants.
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Prohibitin 2 Is an Inner Mitochondrial Membrane Mitophagy Receptor

Prohibitin 2 Is an Inner Mitochondrial Membrane Mitophagy Receptor | Plant-microbe interaction | Scoop.it
Highlights

•PHB2 is an inner mitochondrial membrane mitophagy receptor
•The interaction between PHB2 and LC3 requires outer mitochondrial membrane rupture
•PHB2 is required for Parkin-mediated mitophagy
•PHB2 is required for paternal mitochondrial elimination in C. elegans

Summary

The removal of unwanted or damaged mitochondria by autophagy, a process called mitophagy, is essential for key events in development, cellular homeostasis, tumor suppression, and prevention of neurodegeneration and aging. However, the precise mechanisms of mitophagy remain uncertain. Here, we identify the inner mitochondrial membrane protein, prohibitin 2 (PHB2), as a crucial mitophagy receptor involved in targeting mitochondria for autophagic degradation. PHB2 binds the autophagosomal membrane-associated protein LC3 through an LC3-interaction region (LIR) domain upon mitochondrial depolarization and proteasome-dependent outer membrane rupture. PHB2 is required for Parkin-induced mitophagy in mammalian cells and for the clearance of paternal mitochondria after embryonic fertilization in C. elegans. Our findings pinpoint a conserved mechanism of eukaryotic mitophagy and demonstrate a function of prohibitin 2 that may underlie its roles in physiology, aging, and disease.
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Phosphoribosylation of Ubiquitin Promotes Serine Ubiquitination and Impairs Conventional Ubiquitination

Phosphoribosylation of Ubiquitin Promotes Serine Ubiquitination and Impairs Conventional Ubiquitination | Plant-microbe interaction | Scoop.it
Conventional ubiquitination involves the ATP-dependent formation of amide bonds between the ubiquitin C terminus and primary amines in substrate proteins. Recently, SdeA, an effector protein of pathogenic Legionella pneumophila, was shown to mediate NAD-dependent and ATP-independent ubiquitin transfer to host proteins. Here, we identify a phosphodiesterase domain in SdeA that efficiently catalyzes phosphoribosylation of ubiquitin on a specific arginine via an ADP-ribose-ubiquitin intermediate. SdeA also catalyzes a chemically and structurally distinct type of substrate ubiquitination by conjugating phosphoribosylated ubiquitin to serine residues of protein substrates via a phosphodiester bond. Furthermore, phosphoribosylation of ubiquitin prevents activation of E1 and E2 enzymes of the conventional ubiquitination cascade, thereby impairing numerous cellular processes including mitophagy, TNF signaling, and proteasomal degradation. We propose that phosphoribosylation of ubiquitin potently modulates ubiquitin functions in mammalian cells.
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p62- and ubiquitin-dependent stress-induced autophagy of the mammalian 26S proteasome

p62- and ubiquitin-dependent stress-induced autophagy of the mammalian 26S proteasome | Plant-microbe interaction | Scoop.it

The ubiquitin-proteasome system and autophagy are the two main proteolytic systems involved in, among other functions, the maintenance of cell integrity by eliminating misfolded and damaged proteins and organelles. Both systems remove their targets after their conjugation with ubiquitin. An interesting, yet incompletely understood problem relates to the fate of the components of the two systems. Here we provide evidence that amino acid starvation enhances polyubiquitination on specific sites of the proteasome, a modification essential for its targeting to the autophagic machinery. The uptake of the ubiquitinated proteasome is mediated by its interaction with the ubiquitin-associated domain of p62/SQSTM1, a process that also requires interaction with LC3. Importantly, deletion of the PB1 domain of p62, which is important for the targeting of ubiquitinated substrates to the proteasome, has no effect on stress-induced autophagy of this proteolytic machinery, suggesting that the domain of p62 that binds to the proteasome determines the function of p62 in either targeting substrates to the proteasome or targeting the proteasome to autophagy.

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Nature: Bacteria establish an aqueous living space in plants crucial for virulence (2016)

Nature: Bacteria establish an aqueous living space in plants crucial for virulence (2016) | Plant-microbe interaction | Scoop.it

High humidity has a strong influence on the development of numerous diseases affecting the above-ground parts of plants (the phyllosphere) in crop fields and natural ecosystems, but the molecular basis of this humidity effect is not understood. Previous studies have emphasized immune suppression as a key step in bacterial pathogenesis. Here we show that humidity-dependent, pathogen-driven establishment of an aqueous intercellular space (apoplast) is another important step in bacterial infection of the phyllosphere. Bacterial effectors, such as Pseudomonas syringae HopM1, induce establishment of the aqueous apoplast and are sufficient to transform non-pathogenic P. syringae strains into virulent pathogens in immunodeficient Arabidopsis thaliana under high humidity. Arabidopsis quadruple mutants simultaneously defective in a host target (AtMIN7) of HopM1 and in pattern-triggered immunity could not only be used to reconstitute the basic features of bacterial infection, but also exhibited humidity-dependent dyshomeostasis of the endophytic commensal bacterial community in the phyllosphere. These results highlight a new conceptual framework for understanding diverse phyllosphere–bacterial interactions.


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An extracellular cysteine-rich protein kinase associates with a membrane immune complex and is required for cell death

An extracellular cysteine-rich protein kinase associates with a membrane immune complex and is required for cell death | Plant-microbe interaction | Scoop.it
Membrane localized proteins perceive and respond to biotic and abiotic stresses. We performed quantitative proteomics on plasma membrane-enriched samples from Arabidopsis treated with bacterial flagellin. We identified multiple receptor-like protein kinases (RLKs) changing in abundance, including cysteine-rich receptor-like kinases (CRKs) that are upregulated upon perception of flagellin. CRKs possess extracellular cysteine-rich domains and comprise a gene family consisting of 46 members in Arabidopsis. Single T-DNA insertion lines in CRK28 and CRK29, two CRKs induced in response to flagellin perception, did not exhibit robust alterations in immune responses. In contrast, silencing multiple bacterial flagellin-induced CRKs resulted in enhanced susceptibility to pathogenic Pseudomonas syringae, indicating functional redundancy in this large gene family. Enhanced expression of CRK28 in Arabidopsis increased disease resistance to P. syringae. Expression of CRK28 in N. benthamiana induced cell death, which required intact extracellular cysteine residues and a conserved kinase active site. CRK28-mediated cell death required the common RLK co-receptor BAK1. CRK28 associated with BAK1 as well as the activated FLS2 immune receptor complex. CRK28 self-associated as well as associated with the closely related CRK29. These data support a model where Arabidopsis CRKs are synthesized upon pathogen perception, associate with the FLS2 complex, and coordinately act to enhance plant immune responses
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The hijacking of a receptor kinase–driven pathway by a wheat fungal pathogen leads to disease

The hijacking of a receptor kinase–driven pathway by a wheat fungal pathogen leads to disease | Plant-microbe interaction | Scoop.it
Necrotrophic pathogens live and feed on dying tissue, but their interactions with plants are not well understood compared to biotrophic pathogens. The wheat Snn1 gene confers susceptibility to strains of the necrotrophic pathogen Parastagonospora nodorum that produce the SnTox1 protein. We report the positional cloning of Snn1 , a member of the wall-associated kinase class of receptors, which are known to drive pathways for biotrophic pathogen resistance. Recognition of SnTox1 by Snn1 activates programmed cell death, which allows this necrotroph to gain nutrients and sporulate. These results demonstrate that necrotrophic pathogens such as P. nodorum hijack host molecular pathways that are typically involved in resistance to biotrophic pathogens, revealing the complex nature of susceptibility and resistance in necrotrophic and biotrophic pathogen interactions with plants.

Via Tatsuya Nobori, Jim Alfano
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The ATG conjugation systems are important for degradation of the inner autophagosomal membrane

The ATG conjugation systems are important for degradation of the inner autophagosomal membrane | Plant-microbe interaction | Scoop.it
In macroautophagy, cytoplasmic contents are sequestered into the double-membrane autophagosome, which fuses with the lysosome to become the autolysosome. It has been thought that the autophagy-related (ATG) conjugation systems are required for autophagosome formation. Here, we found that autophagosomal SNARE syntaxin17-positive autophagosome-like structures could be generated even in the absence of the ATG conjugation systems, although at a reduced rate. These syntaxin17-positive structures could further fuse with lysosomes, but degradation of the inner autophagosomal membrane was significantly delayed. Accordingly, autophagic activity in ATG conjugation-deficient cells was strongly suppressed. We suggest that the ATG conjugation systems, which are likely required for the closure (i.e., fission) of the autophagosomal edge, are not absolutely essential for autolysosome formation but important for efficient degradation of the inner autophagosomal membrane.
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Salicylic acid receptors activate jasmonic acid signalling through a non-canonical pathway to promote effector-triggered immunity

Salicylic acid receptors activate jasmonic acid signalling through a non-canonical pathway to promote effector-triggered immunity | Plant-microbe interaction | Scoop.it
It is an apparent conundrum how plants evolved effector-triggered immunity (ETI), involving programmed cell death (PCD), as a major defence mechanism against biotrophic pathogens, because ETI-associated PCD could leave them vulnerable to necrotrophic pathogens that thrive on dead host cells. Interestingly, during ETI, the normally antagonistic defence hormones, salicylic acid (SA) and jasmonic acid (JA) associated with defence against biotrophs and necrotrophs respectively, both accumulate to high levels. In this study, we made the surprising finding that JA is a positive regulator of RPS2-mediated ETI. Early induction of JA-responsive genes and de novo JA synthesis following SA accumulation is activated through the SA receptors NPR3 and NPR4, instead of the JA receptor COI1. We provide evidence that NPR3 and NPR4 may mediate this effect by promoting degradation of the JA transcriptional repressor JAZs. This unique interplay between SA and JA offers a possible explanation of how plants can mount defence against a biotrophic pathogen without becoming vulnerable to necrotrophic pathogens.

Via Francis Martin, Jim Alfano
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Characterization of a pipecolic acid biosynthesis pathway required for systemic acquired resistance

Characterization of a pipecolic acid biosynthesis pathway required for systemic acquired resistance | Plant-microbe interaction | Scoop.it

Characterization of a pipecolic acid biosynthesis pathway required for systemic acquired resistance

Pingtao Ding, Dmitrij Rekhter, Yuli Ding, Kirstin Feussner, Lucas Busta, Sven Haroth, Shaohua Xu, Xin Li, Reinhard Jetter, Ivo Feussner, and Yuelin Zhang

Plant Cell 2016 tpc.16.00486; Advance Publication October 6, 2016; doi:10.1105/tpc.16.00486


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The receptor kinase FER is a RALF-regulated scaffold controlling plant immune signaling

The receptor kinase FER is a RALF-regulated scaffold controlling plant immune signaling | Plant-microbe interaction | Scoop.it
In plants, perception of invading pathogens involves cell-surface immune receptor kinases. Here, we report that the Arabidopsis SITE-1 PROTEASE (S1P) cleaves endogenous RAPID ALKALINIZATION FACTOR (RALF) propeptides to inhibit plant immunity. This inhibition is mediated by the malectin-like receptor kinase FERONIA (FER), which otherwise facilitates the ligand-induced complex formation of the immune receptor kinases EF-TU RECEPTOR (EFR) and FLAGELLIN-SENSING 2 (FLS2) with their co-receptor BRASSINOSTEROID INSENSITIVE 1–ASSOCIATED KINASE 1 (BAK1) to initiate immune signaling. We show that FER acts as a RALF-regulated scaffold that modulates receptor kinase complex assembly. A similar scaffolding mechanism may underlie FER function in other signaling pathways.
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A paralogous decoy protects Phytophthora sojae apoplastic effector PsXEG1 from a host inhibitor

A paralogous decoy protects Phytophthora sojae apoplastic effector PsXEG1 from a host inhibitor | Plant-microbe interaction | Scoop.it
The extracellular space (apoplast) of plant tissue represents a critical battleground between plants and attacking microbes. Here we show that a pathogen-secreted apoplastic Xyloglucan-specific EndoGlucanase PsXEG1 is a focus of this struggle in the Phytophthora sojae-soybean interaction. We show that soybean produces an apoplastic Glucanase Inhibitor Protein, (GmGIP1), that binds to PsXEG1 to block its contribution to virulence. P. sojae however, secretes a paralogous PsXEG1-Like Protein (PsXLP1) that has lost enzyme activity but binds to GmGIP1 more tightly than does PsXEG1, thus freeing PsXEG1 to support P. sojae infection. The PsXEG1 and PsXLP1gene pair is conserved in many Phytophthora species, and the P. parasitica orthologs PpXEG1 and PpXLP1 have similar functions. Thus this apoplastic decoy strategy maybe widely employed in Phytophthora pathosystems.
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Fluorescence‐based ATG8 sensors monitor localization and function of LC3/GABARAP proteins

Fluorescence‐based ATG8 sensors monitor localization and function of LC3/GABARAP proteins | Plant-microbe interaction | Scoop.it
Autophagy is a cellular surveillance pathway that balances metabolic and energy resources and transports specific cargos, including damaged mitochondria, other broken organelles, or pathogens for degradation to the lysosome. Central components of autophagosomal biogenesis are six members of the LC3 and GABARAP family of ubiquitin‐like proteins (mATG8s). We used phage display to isolate peptides that possess bona fide LIR (LC3‐interacting region) properties and are selective for individual mATG8 isoforms. Sensitivity of the developed sensors was optimized by multiplication, charge distribution, and fusion with a membrane recruitment (FYVE) or an oligomerization (PB1) domain. We demonstrate the use of the engineered peptides as intracellular sensors that recognize specifically GABARAP, GABL1, GABL2, and LC3C, as well as a bispecific sensor for LC3A and LC3B. By using an LC3C‐specific sensor, we were able to monitor recruitment of endogenous LC3C to Salmonella during xenophagy, as well as to mitochondria during mitophagy. The sensors are general tools to monitor the fate of mATG8s and will be valuable in decoding the biological functions of the individual LC3/GABARAPs.
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Induced Genome-Wide Binding of Three Arabidopsis WRKY Transcription Factors during Early MAMP-Triggered Immunity

Induced Genome-Wide Binding of Three Arabidopsis WRKY Transcription Factors during Early MAMP-Triggered Immunity | Plant-microbe interaction | Scoop.it

Induced Genome-Wide Binding of Three Arabidopsis WRKY Transcription Factors during Early MAMP-Triggered Immunity

Rainer P Birkenbihl, Barbara Kracher, and Imre E. Somssich

Plant Cell 2016 tpc.16.00681; Advance Publication December 23, 2016; doi:10.1105/tpc.16.00681


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An E3 Ubiquitin Ligase-BAG Protein Module Controls Plant Innate Immunity and Broad-Spectrum Disease Resistance

An E3 Ubiquitin Ligase-BAG Protein Module Controls Plant Innate Immunity and Broad-Spectrum Disease Resistance | Plant-microbe interaction | Scoop.it
Programmed cell death (PCD) and immunity in plants are tightly controlled to promote antimicrobial defense while preventing autoimmunity. However, the mechanisms contributing to this immune homeostasis are poorly understood. Here, we isolated a rice mutant ebr1 (enhanced blight and blast resistance 1) that shows enhanced broad-spectrum bacterial and fungal disease resistance, but displays spontaneous PCD, autoimmunity, and stunted growth. EBR1 encodes an E3 ubiquitin ligase that interacts with OsBAG4, which belongs to the BAG (Bcl-2-associated athanogene) family that functions in cell death, growth arrest, and immune responses in mammals. EBR1 directly targets OsBAG4 for ubiquitination-mediated degradation. Elevated levels of OsBAG4 in rice are necessary and sufficient to trigger PCD and enhanced disease resistance to pathogenic infection, most likely by activating pathogen-associated molecular patterns-triggered immunity (PTI). Together, our study suggests that an E3-BAG module orchestrates innate immune homeostasis and coordinates the trade-off between defense and growth in plants.
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bioRxiv: NLR signaling network mediates immunity to diverse plant pathogens (2016)

bioRxiv: NLR signaling network mediates immunity to diverse plant pathogens (2016) | Plant-microbe interaction | Scoop.it

Plant and animal nucleotide-binding domain and leucine-rich repeat-containing (NLR) proteins often function in pairs to mediate innate immunity to pathogens. However, the degree to which NLR proteins form signaling networks beyond genetically linked pairs is poorly understood. In this study, we discovered that a large NLR immune signaling network with a complex genetic architecture confers immunity to oomycetes, bacteria, viruses, nematodes, and insects. The network emerged over 100 million years ago from a linked NLR pair that diversified into up to one half of the NLR of asterid plants. We propose that this NLR network increases robustness of immune signaling to counteract rapidly evolving plant pathogens.


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Science: Regulation of sugar transporter activity for antibacterial defense in Arabidopsis (2016)

Science: Regulation of sugar transporter activity for antibacterial defense in Arabidopsis (2016) | Plant-microbe interaction | Scoop.it

Microbial pathogens strategically acquire metabolites from their hosts during infection. Here we show that the host can intervene to prevent such metabolite loss to pathogens. Phosphorylation-dependent regulation of sugar transporter 13 (STP13) is required for antibacterial defense in the plant Arabidopsis thaliana. STP13 physically associates with the flagellin receptor flagellin-sensitive 2 (FLS2) and its co-receptor BRASSINOSTEROID INSENSITIVE 1-associated receptor kinase 1 (BAK1). BAK1 phosphorylates STP13 at threonine 485, which enhances its monosaccharide uptake activity to compete with bacteria for extracellular sugars. Limiting the availability of extracellular sugar deprives bacteria of an energy source and restricts virulence factor delivery. Our results reveal that control of sugar uptake, managed by regulation of a host sugar transporter, is a defense strategy deployed against microbial infection. Competition for sugar thus shapes host-pathogen interactions.


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Dying two deaths — programmed cell death regulation in development and disease

Dying two deaths — programmed cell death regulation in development and disease | Plant-microbe interaction | Scoop.it
Programmed cell death (PCD) is a fundamental cellular process that has adopted a plethora of vital functions in multicellular organisms. In plants, PCD processes are elicited as an inherent part of regular development in specific cell types or tissues, but can also be triggered by biotic and abiotic stresses. Although over the last years we have seen progress in our understanding of the molecular regulation of different plant PCD processes, it is still unclear whether a common core machinery exists that controls cell death in development and disease. In this review, we discuss recent advances in the field, comparing some aspects of the molecular regulation controlling developmental and pathogen-triggered PCD in plants.

Via Tatsuya Nobori
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A core function of EDS1 with PAD4 is to protect the salicylic acid defense sector in Arabidopsis immunity - Cui - 2016 - New Phytologist - Wiley Online Library

A core function of EDS1 with PAD4 is to protect the salicylic acid defense sector in Arabidopsis immunity - Cui - 2016 - New Phytologist - Wiley Online Library | Plant-microbe interaction | Scoop.it
Plant defenses induced by salicylic acid (SA) are vital for resistance against biotrophic pathogens. In basal and receptor-triggered immunity, SA accumulation is promoted by Enhanced Disease Susceptibility1 with its co-regulator Phytoalexin Deficient4 (EDS1/PAD4). Current models position EDS1/PAD4 upstream of SA but their functional relationship remains unclear.
In a genetic and transcriptomic analysis of Arabidopsis autoimmunity caused by constitutive or conditional EDS1/PAD4 overexpression, intrinsic EDS1/PAD4 signaling properties and their relation to SA were uncovered.
A core EDS1/PAD4 pathway works in parallel with SA in basal and effector-triggered bacterial immunity. It protects against disabled SA-regulated gene expression and pathogen resistance, and is distinct from a known SA-compensatory route involving MAPK signaling. Results help to explain previously identified EDS1/PAD4 regulated SA-dependent and SA-independent gene expression sectors.
Plants have evolved an alternative route for preserving SA-regulated defenses against pathogen or genetic perturbations. In a proposed signaling framework, EDS1 with PAD4, besides promoting SA biosynthesis, maintains important SA-related resistance programs, thereby increasing robustness of the innate immune system.
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PARylation of the forkhead-associated domain protein DAWDLE regulates plant immunity - Feng - 2016 - EMBO reports - Wiley Online Library

PARylation of the forkhead-associated domain protein DAWDLE regulates plant immunity - Feng - 2016 - EMBO reports - Wiley Online Library | Plant-microbe interaction | Scoop.it
Protein poly(ADP-ribosyl)ation (PARylation) primarily catalyzed by poly(ADP-ribose) polymerases (PARPs) plays a crucial role in controlling various cellular responses. However, PARylation targets and their functions remain largely elusive. Here, we deployed an Arabidopsis protein microarray coupled with in vitro PARylation assays to globally identify PARylation targets in plants. Consistent with the essential role of PARylation in plant immunity, the forkhead-associated (FHA) domain protein DAWDLE (DDL), one of PARP2 targets, positively regulates plant defense to both adapted and non-adapted pathogens. Arabidopsis PARP2 interacts with and PARylates DDL, which was enhanced upon treatment of bacterial flagellin. Mass spectrometry and mutagenesis analysis identified multiple PARylation sites of DDL by PARP2. Genetic complementation assays indicate that DDL PARylation is required for its function in plant immunity. In contrast, DDL PARylation appears to be dispensable for its previously reported function in plant development partially mediated by the regulation of microRNA biogenesis. Our study uncovers many previously unknown PARylation targets and points to the distinct functions of DDL in plant immunity and development mediated by protein PARylation and small RNA biogenesis, respectively.
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Enrichment of Phosphatidylethanolamine in Viral Replication Compartments via Co-opting the Endosomal Rab5 Small GTPase by a Positive-Strand RNA Virus

Enrichment of Phosphatidylethanolamine in Viral Replication Compartments via Co-opting the Endosomal Rab5 Small GTPase by a Positive-Strand RNA Virus | Plant-microbe interaction | Scoop.it
Positive-strand RNA viruses build extensive membranous replication compartments to support replication and protect the virus from antiviral responses by the host. These viruses require host factors and various lipids to form viral replication complexes (VRCs). The VRCs built by Tomato bushy stunt virus (TBSV) are enriched with phosphatidylethanolamine (PE) through a previously unknown pathway. To unravel the mechanism of PE enrichment within the TBSV replication compartment, in this paper, the authors demonstrate that TBSV co-opts the guanosine triphosphate (GTP)-bound active form of the endosomal Rab5 small GTPase via direct interaction with the viral replication protein. Deletion of Rab5 orthologs in a yeast model host or expression of dominant negative mutants of plant Rab5 greatly decreases TBSV replication and prevents the redistribution of PE to the sites of viral replication. We also show that enrichment of PE in the viral replication compartment is assisted by actin filaments. Interestingly, the closely related Carnation Italian ringspot virus, which replicates on the boundary membrane of mitochondria, uses a similar strategy to the peroxisomal TBSV to hijack the Rab5-positive endosomes into the viral replication compartments. Altogether, usurping the GTP-Rab5–positive endosomes allows TBSV to build a PE-enriched viral replication compartment, which is needed to support peak-level replication. Thus, the Rab family of small GTPases includes critical host factors assisting VRC assembly and genesis of the viral replication compartment.
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Activation-Dependent Destruction of a Co-receptor by a Pseudomonas syringae Effector Dampens Plant Immunity

Activation-Dependent Destruction of a Co-receptor by a Pseudomonas syringae Effector Dampens Plant Immunity | Plant-microbe interaction | Scoop.it
The Arabidopsis immune receptor FLS2 and co-receptor BAK1 perceive the bacterial flagellin epitope flg22 to activate plant immunity. To prevent this response, phytopathogenic bacteria deploy a repertoire of effector proteins to perturb immune signaling. However, the effector-induced perturbation is often sensed by the host, triggering another layer of immunity. We report that the Pseudomonas syringae effector HopB1 acts as a protease to cleave immune-activated BAK1. Prior to activation, HopB1 constitutively interacts with FLS2. Upon activation by flg22, BAK1 is recruited to the FLS2-HopB1 complex and is phosphorylated at Thr455. HopB1 then specifically cleaves BAK1 between Arg297 and Gly298 to inhibit FLS2 signaling. Although perturbation of BAK1 is known to trigger increased immune responses in plants, the HopB1-mediated cleavage of BAK1 leads to enhanced virulence, but not disease resistance. This study thus reveals a virulence strategy by which a pathogen effector attacks the plant immune system with minimal host perturbation.
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Structure of ubiquitylated-Rpn10 provides insight into its autoregulation mechanism

Structure of ubiquitylated-Rpn10 provides insight into its autoregulation mechanism | Plant-microbe interaction | Scoop.it
Ubiquitin receptors decode ubiquitin signals into many cellular responses. Ubiquitin receptors also undergo coupled monoubiquitylation, and rapid deubiquitylation has hampered the characterization of the ubiquitylated state. Using bacteria that express a ubiquitylation apparatus, we purified and determined the crystal structure of the proteasomal ubiquitin-receptor Rpn10 in its ubiquitylated state. The structure shows a novel ubiquitin-binding patch that directs K84 ubiquitylation. Superimposition of ubiquitylated-Rpn10 onto electron-microscopy models of proteasomes indicates that the Rpn10-conjugated ubiquitin clashes with Rpn9, suggesting that ubiquitylation might be involved in releasing Rpn10 from the proteasome. Indeed, ubiquitylation on immobilized proteasomes dissociates the modified Rpn10 from the complex, while unmodified Rpn10 mainly remains associated. In vivo experiments indicate that contrary to wild type, Rpn10-K84R is stably associated with the proteasomal subunit Rpn9. Similarly Rpn10, but not ubiquitylated-Rpn10, binds Rpn9 in vitro. Thus we suggest that ubiquitylation functions to dissociate modified ubiquitin receptors from their targets, a function that promotes cyclic activity of ubiquitin receptors.
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