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Science: Tricking the Guard: Exploiting Plant Defense for Disease Susceptibility

Science: Tricking the Guard: Exploiting Plant Defense for Disease Susceptibility | Plant-microbe interaction | Scoop.it

Abstract

Typically, pathogens deploy virulence effectors to disable defense. Plants defeat effectors with resistance proteins that guard effector targets. Here, we show that a pathogen exploits a resistance protein by activating it to confer susceptibility. Interactions of victorin, an effector produced by the necrotrophic fungus Cochliobolus victoriae, TRX-h5, a defense-associated thioredoxin, and LOV1, an Arabidopsis susceptibility protein, recapitulate the guard mechanism of plant defense. In LOV1's absence, victorin inhibits TRX-h5 resulting in compromised defense but not disease by C. victoriae. In LOV1's presence, victorin binding to TRX-h5 activates LOV1 and elicits a resistance-like response that confers disease susceptibility. We propose that victorin is or mimics a conventional pathogen virulence effector that was defeated by LOV1 and confers virulence to C. victoriae solely because it incites defense.

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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 Arabidopsis Malectin-Like Leucine-Rich Repeat Receptor-Like Kinase IOS1 Associates with the Pattern Recognition Receptors FLS2 and EFR and Is Critical for Priming of Pattern-Triggered Immunity

The Arabidopsis Malectin-Like Leucine-Rich Repeat Receptor-Like Kinase IOS1 Associates with the Pattern Recognition Receptors FLS2 and EFR and Is Critical for Priming of Pattern-Triggered Immunity | Plant-microbe interaction | Scoop.it

Plasma membrane-localized pattern recognition receptors such as FLAGELLIN SENSING2 (FLS2) and EF-TU RECEPTOR (EFR) recognize microbe-associated molecular patterns (MAMPs) to activate the first layer of plant immunity termed pattern-triggered immunity (PTI). A reverse genetics approach with genes responsive to the priming agent β-aminobutyric acid (BABA) revealed IMPAIRED OOMYCETE SUSCEPTIBILITY1 (IOS1) as a critical PTI player. Arabidopsis thaliana ios1 mutants were hypersusceptible to Pseudomonas syringae bacteria. Accordingly, ios1 mutants demonstrated defective PTI responses, notably delayed upregulation of PTI marker genes, lower callose deposition, and mitogen-activated protein kinase activities upon bacterial infection or MAMP treatment. Moreover, Arabidopsis lines overexpressing IOS1 were more resistant to P. syringae and demonstrated a primed PTI response. In vitro pull-down, bimolecular fluorescence complementation, coimmunoprecipitation, and mass spectrometry analyses supported the existence of complexes between the membrane-localized IOS1 and FLS2 and EFR. IOS1 also associated with BRASSINOSTEROID INSENSITIVE1-ASSOCIATED KINASE1 (BAK1) in a ligand-independent manner and positively regulated FLS2/BAK1 complex formation upon MAMP treatment. Finally, ios1 mutants were defective in BABA-induced resistance and priming. This work reveals IOS1 as a regulatory protein of FLS2- and EFR-mediated signaling that primes PTI activation upon bacterial elicitation.

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Frontiers | Killing two birds with one stone: trans-kingdom suppression of PAMP/MAMP-induced immunity by T3E from enteropathogenic bacteria | Plant-Microbe Interaction

Frontiers | Killing two birds with one stone: trans-kingdom suppression of PAMP/MAMP-induced immunity by T3E from enteropathogenic bacteria | Plant-Microbe Interaction | Plant-microbe interaction | Scoop.it

Within the past decade, remarkable similarities between the molecular organization of animal and plant systems for non-self discrimination were revealed. Obvious parallels exist between the molecular structures of the receptors mediating the recognition of pathogen- or microbe-associated molecular patterns (PAMPs/MAMPs) with plant pattern recognition receptors strikingly resembling mammalian Toll-like receptors. Mitogen-activated protein kinase cascades, leading to the transcriptional activation of immunity-associated genes, illustrate the conservation of whole molecular building blocks of PAMP/MAMP-induced signaling. Enteropathogenic Salmonella and Escherichia coli use a type three secretion system (T3SS) to inject effector proteins into the mammalian host cell to subvert defense mechanisms and promote gut infection. Lately, disease occurrence was increasingly associated with bacteria-contaminated fruits and vegetables and common themes have emerged with regard to whether and how effectors target innate immune responses in a trans-kingdom manner. We propose that numerous Salmonella or E. coli effectors may be active in planta and tend to target central components (hubs) of immune signaling pathways.


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Agrobacterium tumefaciens Deploys a Superfamily of Type VI Secretion DNase Effectors as Weapons for Interbacterial Competition In Planta: Cell Host & Microbe

Agrobacterium tumefaciens Deploys a Superfamily of Type VI Secretion DNase Effectors as Weapons for Interbacterial Competition In Planta: Cell Host & Microbe | Plant-microbe interaction | Scoop.it

The type VI secretion system (T6SS) is a widespread molecular weapon deployed by many Proteobacteria to target effectors/toxins into both eukaryotic and prokaryotic cells. We report that Agrobacterium tumefaciens, a soil bacterium that triggers tumorigenesis in plants, produces a family of type VI DNase effectors (Tde) that are distinct from previously known polymorphic toxins and nucleases. Tde exhibits an antibacterial DNase activity that relies on a conserved HxxD motif and can be counteracted by a cognate immunity protein, Tdi. In vitro, A. tumefaciens T6SS could kill Escherichia coli but triggered a lethal counterattack by Pseudomonas aeruginosa upon injection of the Tde toxins. However, in an in planta coinfection assay, A. tumefaciens used Tde effectors to attack both siblings cells and P. aeruginosa to ultimately gain a competitive advantage. Such acquired T6SS-dependent fitness in vivo and conservation of Tde-Tdi couples in bacteria highlights a widespread antibacterial weapon beneficial for niche colonization

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Characterization of the Largest Effector Gene Cluster of Ustilago maydis

Characterization of the Largest Effector Gene Cluster of Ustilago maydis | Plant-microbe interaction | Scoop.it

In the genome of the biotrophic plant pathogen Ustilago maydis, many of the genes coding for secreted protein effectors modulating virulence are arranged in gene clusters. The vast majority of these genes encode novel proteins whose expression is coupled to plant colonization. The largest of these gene clusters, cluster 19A, encodes 24 secreted effectors. Deletion of the entire cluster results in severe attenuation of virulence. Here we present the functional analysis of this genomic region. We show that a 19A deletion mutant behaves like an endophyte, i.e. is still able to colonize plants and complete the infection cycle. However, tumors, the most conspicuous symptoms of maize smut disease, are only rarely formed and fungal biomass in infected tissue is significantly reduced. The generation and analysis of strains carrying sub-deletions identified several genes significantly contributing to tumor formation after seedling infection. Another of the effectors could be linked specifically to anthocyanin induction in the infected tissue. As the individual contributions of these genes to tumor formation were small, we studied the response of maize plants to the whole cluster mutant as well as to several individual mutants by array analysis. This revealed distinct plant responses, demonstrating that the respective effectors have discrete plant targets. We propose that the analysis of plant responses to effector mutant strains that lack a strong virulence phenotype may be a general way to visualize differences in effector function.

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The Plant Membrane-Associated REMORIN1.3 Accumulates in Discrete Perihaustorial Domains and Enhances Susceptibility to Phytophthora infestans

The Plant Membrane-Associated REMORIN1.3 Accumulates in Discrete Perihaustorial Domains and Enhances Susceptibility to Phytophthora infestans | Plant-microbe interaction | Scoop.it

Filamentous pathogens such as the oomycete Phytophthora infestans infect plants by developing specialized structures termed haustoria inside the host cells. Haustoria are thought to enable the secretion of effector proteins into the plant cells. Haustorium biogenesis, therefore, is critical for pathogen accommodation in the host tissue. Haustoria are enveloped by a specialized host-derived membrane, the extrahaustorial membrane (EHM), which is distinct from the plant plasma membrane. The mechanisms underlying the biogenesis of the EHM are unknown. Remarkably, several plasma membrane-localized proteins are excluded from the EHM, but the remorin REM1.3 accumulates around P. infestans haustoria. Here, we used overexpression, colocalization with reporter proteins, and superresolution microscopy in cells infected by P. infestans to reveal discrete EHM domains labeled by REM1.3 and the P. infestans effector AVRblb2. Moreover, SYNAPTOTAGMIN1, another previously identified perihaustorial protein, localized to subdomains that are mainly not labeled by REM1.3 and AVRblb2. Functional characterization of REM1.3 revealed that it is a susceptibility factor that promotes infection by P. infestans. This activity, and REM1.3 recruitment to the EHM, require the REM1.3 membrane-binding domain. Our results implicate REM1.3 membrane microdomains in plant susceptibility to an oomycete pathogen.


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HopW1 from Pseudomonas syringae Disrupts the Actin Cytoskeleton to Promote Virulence in Arabidopsis

HopW1 from Pseudomonas syringae Disrupts the Actin Cytoskeleton to Promote Virulence in Arabidopsis | Plant-microbe interaction | Scoop.it

A central mechanism of virulence of extracellular bacterial pathogens is the injection into host cells of effector proteins that modify host cellular functions. HopW1 is an effector injected by the type III secretion system that increases the growth of the plant pathogen Pseudomonas syringae on the Columbia accession of Arabidopsis. When delivered by P. syringae into plant cells, HopW1 causes a reduction in the filamentous actin (F-actin) network and the inhibition of endocytosis, a known actin-dependent process. When directly produced in plants, HopW1 forms complexes with actin, disrupts the actin cytoskeleton and inhibits endocytosis as well as the trafficking of certain proteins to vacuoles. The C-terminal region of HopW1 can reduce the length of actin filaments and therefore solubilize F-actin in vitro. Thus, HopW1 acts by disrupting the actin cytoskeleton and the cell biological processes that depend on actin, which in turn are needed for restricting P. syringae growth in Arabidopsis.

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The S-domain Receptor Kinase AtARK2 and the U-box/ARM-repeat-Containing E3 Ubiquitin Ligase AtPUB9 Module Mediates Lateral Root Development Under Phosphate Starvation in Arabidopsis

The S-domain Receptor Kinase AtARK2 and the U-box/ARM-repeat-Containing E3 Ubiquitin Ligase AtPUB9 Module Mediates Lateral Root Development Under Phosphate Starvation in Arabidopsis | Plant-microbe interaction | Scoop.it

When plants encounter nutrient limiting conditions in the soil, the root architecture is redesigned to generate numerous lateral roots that increase the surface area of roots, promoting efficient uptake of these deficient nutrients. Of the many essential nutrients, reduced availability of inorganic phosphate (Pi) has a major impact on plant growth due to requirement of Pi for synthesis of essential organic molecules. In our screens to identify a potential role for the S-domain receptor kinase (SDK1-6 (ARK2)) and its interacting downstream signalling partner AtPUB9, a U-box/ARM repeat containing E3 ligase, we identified a role for this module in regulating lateral root (LR) development under phosphate starved conditions. Our results show that Arabidopsis double mutant plants lacking AtPUB9 and AtARK2 (ark2-1/pub9-1) display severely reduced lateral roots when grown under phosphate-starved conditions. Under these starvation conditions, these plants accumulated very low to no auxin in their primary root and LR tips as observed through expression of the auxin reporter DR5::uidA transgene. Exogenous auxin was sufficient to rescue the LR developmental defects in the ark2-1/pub9-1 lines indicating a requirement of auxin accumulation for this process. Our subcellular localization studies with tobacco suspension-cultured cells indicate that interaction between ARK2 and AtPUB9 results in accumulation of AtPUB9 in the autophagosomes. Inhibition of autophagy in wild-type plants resulted in reduction of LR development and auxin accumulation under phosphate starved condition suggesting a role for autophagy in regulating LR development. Thus, our study has uncovered a previously unknown signalling module (ARK2-PUB9) that is required for auxin-mediated lateral root development under phosphate-starved conditions.

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Host-induced bacterial cell wall decomposition mediates pattern-triggered immunity in Arabidopsis

Host-induced bacterial cell wall decomposition mediates pattern-triggered immunity in Arabidopsis | Plant-microbe interaction | Scoop.it

Peptidoglycans (PGN) are immunogenic bacterial surface patterns that trigger immune activation in metazoans and plants. It is generally unknown, how complex bacterial structures, such as PGN, are perceived by plant pattern recognition receptors (PRR) and whether host hydrolytic activities facilitate decomposition of bacterial matrices and generation of soluble PRR ligands. Here, we show that Arabidopsis thaliana upon bacterial infection or exposure to microbial patterns produces a metazoan lysozyme-like hydrolase (lysozyme 1, LYS1). LYS1 activity releases soluble PGN fragments from insoluble bacterial cell walls and cleavage products are able to trigger responses typically associated with plant immunity. Importantly, LYS1 mutant genotypes exhibit super-susceptibility to bacterial infections similar to that observed on PGN receptor mutants. We propose that plants employ hydrolytic activities for the decomposition of complex bacterial structures, and that soluble pattern generation might aid PRR-mediated immune activation in cell layers adjacent to infection sites. - See more at: http://elifesciences.org/content/early/2014/06/23/eLife.01990#api_box

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Plant ubiquitin ligases as signaling hubs : Nature Structural & Molecular Biology : Nature Publishing Group

Plant ubiquitin ligases as signaling hubs : Nature Structural & Molecular Biology : Nature Publishing Group | Plant-microbe interaction | Scoop.it
The past decade has witnessed an explosion in the identification of ubiquitin-ligase complexes as the missing receptors for important small-molecule hormones regulating plant growth and development. These breakthroughs were initiated by genetic approaches, with structural analysis providing mechanistic insights into how hormone perception and signaling are coupled to protein ubiquitination. Although there are still many unknowns, plants have imparted valuable lessons about the pharmacology of ubiquitin modification.
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Post-Doc in Plant-Pathogen Interactions/Plant Metabolism available in our lab

Post-Doc in Plant-Pathogen Interactions/Plant Metabolism available in our lab | Plant-microbe interaction | Scoop.it
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EDS1 mediates pathogen resistance and virulence function of a bacterial effector in soybean

EDS1 mediates pathogen resistance and virulence function of a bacterial effector in soybean | Plant-microbe interaction | Scoop.it

Enhanced disease susceptibility 1 (EDS1) and phytoalexin deficient 4 (PAD4) are well known regulators of both basal and resistance (R) protein-mediated plant defense. We identified two EDS1- (GmEDS1a/b) and one PAD4-like (GmPAD4) protein that are required for resistance signaling in soybean. Consistent with their significant structural conservation to Arabidopsis counterparts, constitutive expression of GmEDS1 or GmPAD4 complemented the pathogen resistance defects of Arabidopsis eds1 and pad4 mutants, respectively. Interestingly however, the GmEDS1 and GmPAD4 did not complement pathogen-inducible salicylic acid accumulation in the eds1/pad4 mutants. Furthermore, the GmEDS1a/b proteins were unable to complement the turnip crinkle virus coat protein-mediated activation of the Arabidopsis R protein HRT, even though both interacted with HRT. Silencing GmEDS1a/b or GmPAD4 reduced basal and pathogen-inducible SA accumulation and enhanced soybean susceptibility to virulent pathogens. The GmEDS1a/b and GmPAD4 genes were also required for Rpg2-mediated resistance to Pseudomonas syringae. Notably the GmEDS1a/b proteins interacted with the cognate bacterial effector AvrA1 and were required for its virulence function in rpg2 plants. Together, these results show that despite significant structural similarities, conserved defense signaling components from diverse plants can differ in their functionalities. In addition, we demonstrate a role for GmEDS1 in regulating the virulence function of a bacterial effector.

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MPMI: P. syringae CC1557: a highly virulent strain with an unusually small type III effector repertoire that includes a novel effector (2014)

MPMI: P. syringae CC1557: a highly virulent strain with an unusually small type III effector repertoire that includes a novel effector (2014) | Plant-microbe interaction | Scoop.it

Both type III effector proteins and non-ribosomal peptide toxins play important roles for Pseudomonas syringae pathogenicity in host plants, but whether and how these pathways interact to promote infection remains unclear. Genomic evidence from one clade of P. syringae suggests a tradeoff between the total number of type III effector proteins and presence of syringomycin, syringopeptin, and syringolin A toxins. Here we report the complete genome sequence from P. syringae CC1557, which contains the lowest number of known type III effectors to date and has also acquired genes similar to sequences encoding syringomycin pathways from other strains. We demonstrate that this strain is pathogenic on Nicotiana benthamiana and that both the type III secretion system and a new type III effector family, hopBJ1, contribute to pathogenicity. We further demonstrate that activity of HopBJ1 is dependent on residues structurally similar to the catalytic site of E. coli CNF1 toxin. Taken together, our results provide additional support for a negative correlation between type III effector repertoires and the potential to produce syringomycin-like toxins while also highlighting how genomic synteny and bioinformatics can be used to identify and characterize novel virulence proteins.


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Call for participation - Front. Plant Sci.: Research Topic on Genomics and Effectomics of the crop killer Xanthomonas

Call for participation - Front. Plant Sci.: Research Topic on Genomics and Effectomics of the crop killer Xanthomonas | Plant-microbe interaction | Scoop.it

Phytopathogenic bacteria of the Xanthomonas genus cause severe diseases on hundreds host plants, including economically important crops, such as rice, wheat, cassava, banana, mango, tomato, citrus, cabbage, pepper, bean and cotton. Diseases occurring in nature comprise black rot, leaf/fruit spot, canker, wilt, leaf blight and streak. These bacteria are present worldwide where some phytopathogenic strains are emergent or re-emergent and, consequently, dramatically impact agriculture, economy and food safety.

Xanthomonas bacteria provide excellent models for genomic studies and hundreds of Xanthomonas genome sequences have been obtained since 2002 and many other are underway (www.xanthomonas.org/genomes.html). Comparative genomics between and/or within bacterial species and/or pathovars will be of a great help to decipher commonalities and particularities that underly host range definition.

Most of the Xanthomonas possesses a type III secretion system (T3SS) that is required for injection of various effectors inside plant cells, thus contributing to pathogenicity. Transcription Activator-Like (tal) genes, encode bacterial transcription factors which are injected through the T3SS by many Xanthomonas to promote pathogenicity. Some Ralstonia, Bulkholderia and marine bacteria also express TAL-like proteins which function and mode of action is starting to be deciphered. TALs are addressed to the plant nucleus where they activate plant gene expression by direct binding to the corresponding promoter sequences. Targeted genes essentially act as susceptibility genes. A few years after the cracking of the code allowing the TAL/Host promoter sequence recognition, combined to the ever-growing availability of plant genomes, many efforts have been done to identify TAL targets. These data collected for many Xanthomonas/host pathosystems will assuredly help breeders to breed resistance resistant in important crops.

In this Research Topic we aim to collect manuscripts covering the current knowledge of Xanthomonas genomics and effectomics, with a special focus on TAL effector biology. Specifically, we encourage the submission of manuscripts (Original Research, Hypothesis & Theory, Methods, Reviews, Mini Reviews, Perspective and Opinion) covering the following topics:
1. Manuscripts reporting genome sequencing of Xanthomonas strains.
2. Manuscripts describing functional and comparative genomics in Xanthomonas species/pathovars.
3. Manuscripts describing functional studies on Xanthomonas type III effectors.
3. Manuscripts describing discovery, evolution, bio-informatics and functional genomics of TAL effectors and their targets in plant genomes, as well as for TAL-like in non-Xanthomonas bacteria.
4. Manuscripts describing applications of TAL effector research for resistance breeding in crops.

We anticipate that this Research Topic will be of importance for plant pathologists and breeders.

 

Nicolas Denancé
Guest Associate Editor, Plant-Microbe Interaction
www.frontiersin.org


Via Nicolas Denancé
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Great topic-excited!

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Pto Kinase Binds Two Domains of AvrPtoB and Its Proximity to the Effector E3 Ligase Determines if It Evades Degradation and Activates Plant Immunity

Pto Kinase Binds Two Domains of AvrPtoB and Its Proximity to the Effector E3 Ligase Determines if It Evades Degradation and Activates Plant Immunity | Plant-microbe interaction | Scoop.it

The tomato—Pseudomonas syringae pv. tomato (Pst)—pathosystem is one of the best understood models for plant-pathogen interactions. Certain wild relatives of tomato express two closely related members of the same kinase family, Pto and Fen, which recognize the Pst virulence protein AvrPtoB and activate effector-triggered immunity (ETI). AvrPtoB, however, contains an E3 ubiquitin ligase domain in its carboxyl terminus which causes degradation of Fen and undermines its ability to activate ETI. In contrast, Pto evades AvrPtoB-mediated degradation and triggers ETI in response to the effector. It has been reported recently that Pto has higher kinase activity than Fen and that this difference allows Pto to inactivate the E3 ligase through phosphorylation of threonine-450 (T450) in AvrPtoB. Here we show that, in contrast to Fen which can only interact with a single domain proximal to the E3 ligase of AvrPtoB, Pto binds two distinct domains of the effector, the same site as Fen and another N-terminal domain. In the absence of E3 ligase activity Pto binds to either domain of AvrPtoB to activate ETI. However, the presence of an active E3 ligase domain causes ubiquitination of Pto that interacts with the domain proximal to the E3 ligase, identical to ubiquitination of Fen. Only when Pto binds its unique distal domain can it resist AvrPtoB-mediated degradation and activate ETI. We show that phosphorylation of T450 is not required for Pto-mediated resistance in vivo and that a kinase-inactive version of Pto is still capable of activating ETI in response to AvrPtoB. Our results demonstrate that the ability of Pto to interact with a second site distal to the E3 ligase domain in AvrPtoB, and not a higher kinase activity or T450 phosphorylation, allows Pto to evade ubiquitination and to confer immunity to Pst.

 

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Jim Alfano's curator insight, July 25, 7:51 AM

Important paper showing why the Pto kinase can evade degradation induced by AvrptoB and why the Fen kinase can't. 

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Interaction of the Arabidopsis GTPase RabA4c with Its Effector PMR4 Results in Complete Penetration Resistance to Powdery Mildew

Interaction of the Arabidopsis GTPase RabA4c with Its Effector PMR4 Results in Complete Penetration Resistance to Powdery Mildew | Plant-microbe interaction | Scoop.it

The (1,3)-β-glucan callose is a major component of cell wall thickenings in response to pathogen attack in plants. GTPases have been suggested to regulate pathogen-induced callose biosynthesis. To elucidate the regulation of callose biosynthesis in Arabidopsis thaliana, we screened microarray data and identified transcriptional upregulation of the GTPase RabA4c after biotic stress. We studied the function of RabA4c in its native and dominant negative (dn) isoform in RabA4c overexpression lines. RabA4c overexpression caused complete penetration resistance to the virulent powdery mildew Golovinomyces cichoracearum due to enhanced callose deposition at early time points of infection, which prevented fungal ingress into epidermal cells. By contrast, RabA4c(dn) overexpression did not increase callose deposition or penetration resistance. A cross of the resistant line with the pmr4 disruption mutant lacking the stress-induced callose synthase PMR4 revealed that enhanced callose deposition and penetration resistance were PMR4-dependent. In live-cell imaging, tagged RabA4c was shown to localize at the plasma membrane prior to infection, which was broken in the pmr4 disruption mutant background, with callose deposits at the site of attempted fungal penetration. Together with our interactions studies including yeast two-hybrid, pull-down, and in planta fluorescence resonance energy transfer assays, we concluded that RabA4c directly interacts with PMR4, which can be seen as an effector of this GTPase.

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The Xanthomonas campestris effector protein XopDXcc8004 triggers plant disease tolerance by targeting DELLA proteins - Tan - 2014 - New Phytologist - Wiley Online Library

The Xanthomonas campestris effector protein XopDXcc8004 triggers plant disease tolerance by targeting DELLA proteins - Tan - 2014 - New Phytologist - Wiley Online Library | Plant-microbe interaction | Scoop.it
SummaryPlants protect themselves from the harmful effects of pathogens by resistance and tolerance. Disease resistance, which eliminates pathogens, can be modulated by bacterial type III effectors. Little is known about whether disease tolerance, which sustains host fitness with a given pathogen burden, is regulated by effectors.Here, we examined the effects of the Xanthomonas effector protein XopDXcc8004 on plant disease defenses by constructing knockout and complemented Xanthomonas strains, and performing inoculation studies in radish (Raphanus sativus L. var. radiculus XiaoJinZhong) and Arabidopsis plants.XopDXcc8004 suppresses disease symptoms without changing bacterial titers in infected leaves. In Arabidopsis, XopDXcc8004 delays the hormone gibberellin (GA)-mediated degradation of RGA (repressor of ga1-3), one of five DELLA proteins that repress GA signaling and promote plant tolerance under biotic and abiotic stresses. The ERF-associated amphiphilic repression (EAR) motif-containing region of XopDXcc8004 interacts with the DELLA domain of RGA and might interfere with the GA-induced binding of GID1, a GA receptor, to RGA.The EAR motif was found to be present in a number of plant transcriptional regulators. Thus, our data suggest that bacterial pathogens might have evolved effectors, which probably mimic host components, to initiate disease tolerance and enhance their survival.
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Proteasome-Mediated Processing of Nrf1 Is Essential for Coordinate Induction of All Proteasome Subunits and p97: Current Biology

Proteasome-Mediated Processing of Nrf1 Is Essential for Coordinate Induction of All Proteasome Subunits and p97: Current Biology | Plant-microbe interaction | Scoop.it

Highlights

 

•Nrf1 induces all 26S subunits and p97 upon treatment with proteasome inhibitors•Partially inhibited proteasomes process Nrf1 to allow its nuclear translocation•Complete 26S inhibition blocks Nrf1 processing and transcriptional activity•Nrf1 processing requires its deglycosylation (involving p97), then ubiquitination

 

SummaryBackground

Proteasome inhibitors are widely used in the treatment of multiple myeloma and as research tools. Additionally, diminished proteasome function may contribute to neuronal dysfunction. In response to these inhibitors, cells enhance the expression of proteasome subunits by the transcription factor Nrf1. Here, we investigate the mechanisms by which decreased proteasome function triggers production of new proteasomes via Nrf1.

Results

Exposure of myeloma or neuronal cells to proteasome inhibitors (bortezomib, epoxomicin, and MG132), but not to proteotoxic or ER stress, caused a 2- to 4-fold increase within 4 hr in mRNAs for all 26S subunits. In addition, p97 and its cofactors (Npl4, Ufd1, and p47), PA200, and USP14 were induced, but expression of immunoproteasome-specific subunits was suppressed. Nrf1 mediates this induction of proteasomes and p97, but only upon exposure to low concentrations of inhibitors that partially inhibit proteolysis. Surprisingly, high concentrations of these inhibitors prevent this compensatory response. Nrf1 is normally ER-bound, and its release requires its deglycosylation and ubiquitination. Normally ubiquitinated Nrf1 is rapidly degraded, but when partially inhibited, proteasomes carry out limited proteolysis and release the processed Nrf1 (lacking its N-terminal region) from the ER, which allows it to enter the nucleus and promote gene expression.

Conclusions

When fully active, proteasomes degrade Nrf1, but when partially inhibited, they perform limited proteolysis that generates the active form of Nrf1. This elegant mechanism allows cells to compensate for reduced proteasome function by enhancing production of 26S subunits and p97.

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Salicylic acid signaling controls the maturation and localization of the Arabidopsis defense protein ACCELERATED CELL DEATH6

Salicylic acid signaling controls the maturation and localization of the Arabidopsis defense protein ACCELERATED CELL DEATH6 | Plant-microbe interaction | Scoop.it

ACCELERATED CELL DEATH6 (ACD6) is a multipass membrane protein with an ankyrin domain that acts in a positive feedback loop with the defense signal salicylic acid (SA). This study implemented biochemical approaches to infer changes in ACD6 complexes and localization. In addition to forming endoplasmic reticulum (ER)- and plasma membrane (PM)-localized complexes, ACD6 forms soluble complexes, where it is bound to cytosolic HSP70, ubiquitinated and degraded via the proteasome. Thus, ACD6 constitutively undergoes ER-associated degradation. During SA signaling, the soluble ACD6 pool decreases, whereas the PM pool increases. Similarly, ACD6-1, an activated version of ACD6 that induces SA, is present at low levels in the soluble fraction and high levels in the PM. However, ACD6 variants with amino acid substitutions in the ankyrin domain form aberrant, inactive complexes, are induced by a SA agonist, but show no PM localization. SA signaling also increases the PM pools of FLAGELLIN SENSING2 (FLS2) and BRI1-ASSOCIATED RECEPTOR KINASE 1 (BAK1). FLS2 forms complexes ACD6; both FLS2 and BAK1 require ACD6 for maximal accumulation at the PM in response to SA signaling. A plausible scenario is that SA increases the efficiency of productive folding and/or complex formation in the ER, such that ACD6, together with FLS2 and BAK1, reaches the cell surface to more effectively promote immune responses.

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Frontiers | Phosphoinositide-signaling is one component of a robust plant defense response | Plant-Microbe Interaction

Frontiers | Phosphoinositide-signaling is one component of a robust plant defense response | Plant-Microbe Interaction | Plant-microbe interaction | Scoop.it
The phosphoinositide pathway and inositol-1,4,5-triphosphate (InsP3) have been implicated in plant responses to many abiotic stresses; however, their role in response to biotic stress is not well characterized. In the current study, we show that both basal defense and systemic acquired resistance responses are affected in transgenic plants constitutively expressing the human type I inositol polyphosphate 5-phosphatase (InsP 5-ptase) which have greatly reduced InsP3 levels. Flagellin induced Ca2+-release as well as the expressions of some flg22 responsive genes were attenuated in the InsP 5-ptase plants. Furthermore, the InsP 5-ptase plants were more susceptible to virulent and avirulent strains of Pseudomonas syringae pv. tomato (Pst)DC3000. The InsP 5-ptase plants had lower basal salicylic acid (SA) levels and the induction of SAR in systemic leaves was reduced and delayed. Reciprocal exudate experiments showed that although the InsP 5-ptase plants produced equally effective molecules that could trigger PR-1 gene expression in wild type plants, exudates collected from either wild type or InsP 5-ptase plants triggered less PR-1 gene expression in InsP 5-ptase plants. Additionally, expression profiles indicated that several defense genes including PR-1, PR-2, PR-5 and AIG1 were basally down regulated in the InsP 5-ptase plants compared with wild type. Upon pathogen attack, expression of these genes was either not induced or showed delayed induction in systemic leaves. Our study shows that phosphoinositide signaling is one component of the plant defense network and is involved in both basal and systemic responses. The dampening of InsP3-mediated signaling affects Ca2+ release, modulates defense gene expression and compromises plant defense responses.
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Environmental History Modulates Arabidopsis Pattern-Triggered Immunity in a HISTONE ACETYLTRANSFERASE1–Dependent Manner

Environmental History Modulates Arabidopsis Pattern-Triggered Immunity in a HISTONE ACETYLTRANSFERASE1–Dependent Manner | Plant-microbe interaction | Scoop.it

In nature, plants are exposed to a fluctuating environment, and individuals exposed to contrasting environmental factors develop different environmental histories. Whether different environmental histories alter plant responses to a current stress remains elusive. Here, we show that environmental history modulates the plant response to microbial pathogens. Arabidopsis thaliana plants exposed to repetitive heat, cold, or salt stress were more resistant to virulent bacteria than Arabidopsis grown in a more stable environment. By contrast, long-term exposure to heat, cold, or exposure to high concentrations of NaCl did not provide enhanced protection against bacteria. Enhanced resistance occurred with priming of Arabidopsis pattern-triggered immunity (PTI)–responsive genes and the potentiation of PTI-mediated callose deposition. In repetitively stress-challenged Arabidopsis, PTI-responsive genes showed enrichment for epigenetic marks associated with transcriptional activation. Upon bacterial infection, enrichment of RNA polymerase II at primed PTI marker genes was observed in environmentally challenged Arabidopsis. Finally, repetitively stress-challenged histone acetyltransferase1-1 (hac1-1) mutants failed to demonstrate enhanced resistance to bacteria, priming of PTI, and increased open chromatin states. These findings reveal that environmental history shapes the plant response to bacteria through the development of a HAC1-dependent epigenetic mark characteristic of a primed PTI response, demonstrating a mechanistic link between the primed state in plants and epigenetics.

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The Arabidopsis LecRK-VI.2 associates with the pattern-recognition receptor FLS2 and primes Nicotiana benthamiana pattern-triggered immunity - Huang - 2014 - The Plant Journal - Wiley Online Library

The Arabidopsis LecRK-VI.2 associates with the pattern-recognition receptor FLS2 and primes Nicotiana benthamiana pattern-triggered immunity - Huang - 2014 - The Plant Journal - Wiley Online Library | Plant-microbe interaction | Scoop.it

Pattern-triggered immunity (PTI) is broad spectrum and manipulation of PTI is believed to represent an attractive way to engineer plants with broad-spectrum disease resistance. PTI is activated upon perception of microbe-associated molecular patterns (MAMPs) by pattern-recognition receptors (PRRs). We have recently demonstrated that the L-type lectin receptor kinase-VI.2 (LecRK-VI.2) positively regulates Arabidopsis thaliana PTI. Here we show through in vitro pull-down, bimolecular fluorescence complementation and co-immunoprecipitation analyses that LecRK-VI.2 associates with the PRR FLS2. We also demonstrated that LecRK-VI.2 from the cruciferous plant Arabidopsis remains functional after interfamily transfer to the Solanaceous plant Nicotiana benthamiana. Wild tobacco plants ectopically expressing LecRK-VI.2 were indeed more resistant to virulent hemi-biotrophic and necrotrophic bacteria, but not to the fungal pathogen Botrytis cinerea suggesting that, as with Arabidopsis, the LecRK-VI.2 protective effect in N. benthamiana is bacteria specific. Ectopic expression of LecRK-VI.2 in N. benthamiana primed PTI-mediated reactive oxygen species production, mitogen-activated protein kinase (MAPK) activity, callose deposition and gene expression upon treatment with the MAMP flagellin. Our findings identified LecRK-VI.2 as a member of the FLS2 receptor complex and suggest that heterologous expression of components of PRR complexes can be used as tools to engineer plant disease resistance to bacteria.

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Intervention of Phytohormone Pathways by Pathogen Effectors

Intervention of Phytohormone Pathways by Pathogen Effectors | Plant-microbe interaction | Scoop.it

The constant struggle between plants and microbes has driven the evolution of multiple defense strategies in the host as well as offense strategies in the pathogen. To defend themselves from pathogen attack, plants often rely on elaborate signaling networks regulated by phytohormones. In turn, pathogens have adopted innovative strategies to manipulate phytohormone-regulated defenses. Tactics frequently employed by plant pathogens involve hijacking, evading, or disrupting hormone signaling pathways and/or crosstalk. As reviewed here, this is achieved mechanistically via pathogen-derived molecules known as effectors, which target phytohormone receptors, transcriptional activators and repressors, and other components of phytohormone signaling in the host plant. Herbivores and sap-sucking insects employ obligate pathogens such as viruses, phytoplasma, or symbiotic bacteria to intervene with phytohormone-regulated defenses. Overall, an improved understanding of phytohormone intervention strategies employed by pests and pathogens during their interactions with plants will ultimately lead to the development of new crop protection strategies.

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Opening the Ralstonia solanacearum type III effector tool box: insights into host cell subversion mechanisms

Opening the Ralstonia solanacearum type III effector tool box: insights into host cell subversion mechanisms | Plant-microbe interaction | Scoop.it

Effectors delivered to host cells by the Type III secretion system are essential to Ralstonia solanacearum pathogenicity, as in several other plant pathogenic bacteria. The establishment of exhaustive effector repertoires in multiple R. solanacearum strains drew a first picture of the evolutionary dynamics of the pathogen effector suites. Effector repertoires are diversified, with a core of 20–30 effectors present in most of the strains and the obtention of mutants lacking one or more effector genes revealed the functional overlap among this effector network. Recent functional studies have provided insights into the ability of single effectors to manipulate the host proteasome, elicit cell death, trigger the expression of plant genes, and/or display biochemical activities on plant protein targets.

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Plant Cell: Colletotrichum orbiculare Secretes Virulence Effectors to a Biotrophic Interface at the Primary Hyphal Neck via Exocytosis Coupled with SEC22-Mediated Traffic (2014)

Plant Cell: Colletotrichum orbiculare Secretes Virulence Effectors to a Biotrophic Interface at the Primary Hyphal Neck via Exocytosis Coupled with SEC22-Mediated Traffic (2014) | Plant-microbe interaction | Scoop.it

The hemibiotrophic pathogen Colletotrichum orbiculare develops biotrophic hyphae inside cucumber (Cucumis sativus) cells via appressorial penetration; later, the pathogen switches to necrotrophy. C. orbiculare also expresses specific effectors at different stages. Here, we found that virulence-related effectors of C. orbiculare accumulate in a pathogen–host biotrophic interface. Fluorescence-tagged effectors accumulated in a ring-like region around the neck of the biotrophic primary hyphae. Fluorescence imaging of cellular components and transmission electron microscopy showed that the ring-like signals of the effectors localized at the pathogen–plant interface. Effector accumulation at the interface required induction of its expression during the early biotrophic phase, suggesting that transcriptional regulation may link to effector localization. We also investigated the route of effector secretion to the interface. An exocytosis-related component, the Rab GTPase SEC4, localized to the necks of biotrophic primary hyphae adjacent to the interface, thereby suggesting focal effector secretion. Disruption of SEC4 in C. orbiculare reduced virulence and impaired effector delivery to the ring signal interface. Disruption of the v-SNARE SEC22also reduced effector delivery. These findings suggest that biotrophy-expressed effectors are secreted, via the endoplasmic reticulum-to-Golgi route and subsequent exocytosis, toward the interface generated between C. orbiculare and the host cell.


Via Kamoun Lab @ TSL, Jim Alfano
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Effector MiSSP7 of the mutualistic fungus Laccaria bicolor stabilizes the Populus JAZ6 protein and represses jasmonic acid (JA) responsive genes

Effector MiSSP7 of the mutualistic fungus Laccaria bicolor stabilizes the Populus JAZ6 protein and represses jasmonic acid (JA) responsive genes | Plant-microbe interaction | Scoop.it

Ectomycorrhizal fungi, such as Laccaria bicolor, support forest growth and sustainability by providing growth-limiting nutrients to their plant host through a mutualistic symbiotic relationship with host roots. We have previously shown that the effector protein MiSSP7 (Mycorrhiza-induced Small Secreted Protein 7) encoded by L. bicolor is necessary for the establishment of symbiosis with host trees, although the mechanistic reasoning behind this role was unknown. We demonstrate here that MiSSP7 interacts with the host protein PtJAZ6, a negative regulator of jasmonic acid (JA)-induced gene regulation in Populus. As with other characterized JASMONATE ZIM-DOMAIN (JAZ) proteins, PtJAZ6 interacts with PtCOI1 in the presence of the JA mimic coronatine, and PtJAZ6 is degraded in plant tissues after JA treatment. The association between MiSSP7 and PtJAZ6 is able to protect PtJAZ6 from this JA-induced degradation. Furthermore, MiSSP7 is able to block—or mitigate—the impact of JA on L. bicolor colonization of host roots. We show that the loss of MiSSP7 production by L. bicolor can be complemented by transgenically varying the transcription of PtJAZ6 or through inhibition of JA-induced gene regulation. We conclude that L. bicolor, in contrast to arbuscular mycorrhizal fungi and biotrophic pathogens, promotes mutualism by blocking JA action through the interaction of MiSSP7 with PtJAZ6.

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