The Pseudomonas syringae type III effector AvrRpt2 promotes virulence independently of RIN4, a predicted virulence target in Arabidopsis thaliana

The IAA17.1/HSFA5a module enhances salt tolerance in Populus tomentosa by regulating flavonol biosynthesis and ROS levels in lateral roots

Auxin signaling provides a promising approach to controlling root system architecture and improving stress tolerance in plants. However, how the auxin signaling is transducted in this process remains unclear. The Aux indole-3-acetic acid (IAA) repressor IAA17.1 is stabilized by salinity, and primarily expressed in the lateral root (LR) primordia and tips in poplar. Overexpression of the auxin-resistant form of IAA17.1 (IAA17.1m) led to growth inhibition of LRs, markedly reduced salt tolerance, increased reactive oxygen species (ROS) levels, and decreased flavonol content. We further identified that IAA17.1 can interact with the heat shock protein HSFA5a, which was highly expressed in roots and induced by salt stress. Overexpression of HSFA5a significantly increased flavonol content, reduced ROS accumulation, enhanced LR growth and salt tolerance in transgenic poplar. Moreover, HSFA5a could rescue the defective phenotypes caused by IAA17.1m. Expression analysis showed that genes associated with flavonol biosynthesis were altered in IAA17.1m- and HAFA5a-overexpressing plants. Furthermore, we identified that HSFA5a directly activated the expression of key enzyme genes in the flavonol biosynthesis pathway, while IAA17.1 suppressed HSFA5a-mediated activation of these genes. Collectively, the IAA17.1/HSFA5a module regulates flavonol biosynthesis, controls ROS accumulation, thereby modulating the root system of poplar to adapt to salt stress.

Polycomb Repressive Complex 2 and KRYPTONITE regulate pathogen-induced programmed cell death in Arabidopsis

The Polycomb Repressive Complex 2 (PRC2) is well-known for its role in controlling developmental transitions by suppressing the premature expression of key developmental regulators. Previous work revealed that PRC2 also controls the onset of senescence, a form of developmental programmed cell death (PCD) in plants. Whether the induction of PCD in response to stress is similarly suppressed by the PRC2 remained largely unknown. In this study, we explored whether PCD triggered in response to immunity- and disease-promoting pathogen effectors is associated with changes in the distribution of the PRC2-mediated histone H3 lysine 27 trimethylation (H3K27me3) modification in Arabidopsis thaliana. We furthermore tested the distribution of the heterochromatic histone mark H3K9me2, which is established, to a large extent, by the H3K9 methyltransferase KRYPTONITE, and occupies chromatin regions generally not targeted by PRC2. We report that effector-induced PCD caused major changes in the distribution of both repressive epigenetic modifications and that both modifications have a regulatory role and impact on the onset of PCD during pathogen infection. Our work highlights that the transition to pathogen-induced PCD is epigenetically controlled, revealing striking similarities to developmental PCD.

Cutting the line: manipulation of plant immunity by bacterial type III effector proteases

Pathogens and their hosts are engaged in an evolutionary arms race. Pathogen-derived effectors promote virulence by targeting components of a host's innate immune system, while hosts have evolved proteins that sense effectors and trigger a pathogen-specific immune response. Many bacterial effectors are translocated into host cells using type III secretion systems. Type III effector proteases irreversibly modify host proteins by cleavage of peptide bonds and are prevalent among both plant and animal bacterial pathogens. In plants, the study of model effector proteases has yielded important insights into the virulence mechanisms employed by pathogens to overcome their host's immune response, as well as into the mechanisms deployed by their hosts to detect these effector proteases and counteract their effects. In recent years, the study of a larger number of effector proteases, across a wider range of pathogens, has yielded novel insights into their functions and recognition. One key limitation that remains is the lack of methods to detect protease cleavage at the proteome-wide level. We review known substrates and mechanisms of plant pathogen type III effector proteases and compare their functions with those of known type III effector proteases of mammalian pathogens. Finally, we discuss approaches to uncover their function on a system-wide level.

The Ptr1 Locus of Solanum lycopersicoides Confers Resistance to Race 1 Strains of Pseudomonas syringae pv. tomato and to Ralstonia pseudosolanacearum by Recognizing the Type III Effectors AvrRpt2 and RipBN

Race 1 strains of Pseudomonas syringae pv. tomato, which cause bacterial speck disease of tomato, are becoming increasingly common and no simply inherited genetic resistance to such strains is known. We discovered that a locus in Solanum lycopersicoides, termed Pseudomonas tomato race 1 (Ptr1), confers resistance to race 1 P. syringae pv. tomato strains by detecting the activity of type III effector AvrRpt2. In Arabidopsis, AvrRpt2 degrades the RIN4 protein, thereby activating RPS2-mediated immunity. Using site-directed mutagenesis of AvrRpt2, we found that, like RPS2, activation of Ptr1 requires AvrRpt2 proteolytic activity. Ptr1 also detected the activity of AvrRpt2 homologs from diverse bacteria, including one in Ralstonia pseudosolanacearum. The genome sequence of S. lycopersicoides revealed no RPS2 homolog in the Ptr1 region. Ptr1 could play an important role in controlling bacterial speck disease and its future cloning may shed light on an example of convergent evolution for recognition of a widespread type III effector.

The structure of Erwinia amylovora AvrRpt2 provides insight into protein maturation and induced resistance to fire blight by Malus × robusta 5

The AvrRpt2 protein of the phytopathogenic bacterium Erwinia amylovora (AvrRpt2_EA) is a secreted type III effector protein, which is recognised by the FB_MR5 resistance protein of Malus × robusta 5, the only identified resistance protein from a Malus species preventing E. amylovora infection. The crystal structure of the immature catalytic domain of AvrRpt2_EA, a C70 family cysteine protease and type III effector, was determined to a resolution of 1.85 Å. The structure provides insights into the cyclophilin-dependent activation of AvrRpt2, and identifies a cryptic leucine of a non-canonical cyclophilin binding motif. The structure also suggests that residue Cys156, responsible for the gene induced resistance, is not involved in substrate determination, and hints that recognition by FB_MR5 is due to direct interaction.

A Single Effector Protein, AvrRpt2EA, from Erwinia amylovora Can Cause Fire Blight Disease Symptoms and Induces a Salicylic Acid-Dependent Defense Response

The AvrRpt2_EA effector protein of Erwinia amylovora is important for pathogen recognition in the fire blight-resistant crabapple Malus × robusta 5; however, little is known about its role in susceptible apples. To study its function in planta, we expressed a plant-optimized version of AvrRpt2_EA driven by a heat shock-inducible promoter in transgenic plants of the fire blight-susceptible cultivar Pinova. After induced expression of AvrRpt2_EA, transgenic lines showed shoot necrosis and browning of older leaves, with symptoms similar to natural fire blight infections. Transgenic expression of this effector protein resulted in an increase in the expression of the salicylic acid (SA)-responsive PR-1 gene but, also, in the levels of SA and its derivatives, with diverse kinetics in leaves of different ages. In contrast, no increase of expression levels of VSP2 paralogs, used as marker genes for the activation of the jasmonic acid (JA)-dependent defense pathway, could be detected, which is in agreement with metabolic profiling of JA and its derivatives. Our work demonstrates that AvrRpt2_EA acts as a virulence factor and induces the formation of SA and SA-dependent systemic acquired resistance.

Behind the lines-actions of bacterial type III effector proteins in plant cells

Pathogenicity of most Gram-negative plant-pathogenic bacteria depends on the type III secretion (T3S) system, which translocates bacterial effector proteins into plant cells. Type III effectors modulate plant cellular pathways to the benefit of the pathogen and promote bacterial multiplication. One major virulence function of type III effectors is the suppression of plant innate immunity, which is triggered upon recognition of pathogen-derived molecular patterns by plant receptor proteins. Type III effectors also interfere with additional plant cellular processes including proteasome-dependent protein degradation, phytohormone signaling, the formation of the cytoskeleton, vesicle transport and gene expression. This review summarizes our current knowledge on the molecular functions of type III effector proteins with known plant target molecules. Furthermore, plant defense strategies for the detection of effector protein activities or effector-triggered alterations in plant targets are discussed.

The Pseudomonas syringae type III effector AvrRpt2 promotes pathogen virulence via stimulating Arabidopsis auxin/indole acetic acid protein turnover

To accomplish successful infection, pathogens deploy complex strategies to interfere with host defense systems and subvert host physiology to favor pathogen survival and multiplication. Modulation of plant auxin physiology and signaling is emerging as a common virulence strategy for phytobacteria to cause diseases. However, the underlying mechanisms remain largely elusive. We have previously shown that the Pseudomonas syringae type III effector AvrRpt2 alters Arabidopsis (Arabidopsis thaliana) auxin physiology. Here, we report that AvrRpt2 promotes auxin response by stimulating the turnover of auxin/indole acetic acid (Aux/IAA) proteins, the key negative regulators in auxin signaling. AvrRpt2 acts additively with auxin to stimulate Aux/IAA turnover, suggesting distinct, yet proteasome-dependent, mechanisms operated by AvrRpt2 and auxin to control Aux/IAA stability. Cysteine protease activity is required for AvrRpt2-stimulated auxin signaling and Aux/IAA degradation. Importantly, transgenic plants expressing the dominant axr2-1 mutation recalcitrant to AvrRpt2-mediated degradation ameliorated the virulence functions of AvrRpt2 but did not alter the avirulent function mediated by the corresponding RPS2 resistance protein. Thus, promoting auxin response via modulating the stability of the key transcription repressors Aux/IAA is a mechanism used by the bacterial type III effector AvrRpt2 to promote pathogenicity.

The role of NOI-domain containing proteins in plant immune signaling

Here we present an overview of our existing knowledge on the function of RIN4 as a regulator of plant defense and as a guardee of multiple plant R-proteins. Domain analysis of RIN4 reveals two NOI domains. The NOI domain was originally identified in a screen for nitrate induced genes. The domain is comprised of approximately 30 amino acids and contains 2 conserved motifs (PXFGXW and Y/FTXXF). The NOI gene family contains members exclusively from the plant lineage as far back as moss. In addition to the conserved NOI domain, members within the family also contain conserved C-terminal cysteine residue(s) which are sites for acylation and membrane tethering. Other than these two characteristic features, the sequence of the family of NOI-containing proteins is diverse and, with the exception of RIN4, their functions are not known. Recently published interactome data showing interactions between RIN4 and components of the exocyst complex prompt us to raise the hypothesis that RIN4 might be involved in defense associated vesicle trafficking.

Gene-for-gene relationship in the host-pathogen system Malus × robusta 5-Erwinia amylovora

Fire blight is a destructive bacterial disease caused by Erwinia amylovora affecting plants in the family Rosaceae, including apple. Host resistance to fire blight is present mainly in accessions of Malus spp. and is thought to be quantitative in this pathosystem. In this study we analyzed the importance of the E. amylovora effector avrRpt2(EA) , a homolog of Pseudomonas syringae avrRpt2, for resistance of Malus × robusta 5 (Mr5). The deletion mutant E. amylovora Ea1189ΔavrRpt2(EA) was able to overcome the fire blight resistance of Mr5. One single nucleotide polymorphism (SNP), resulting in an exchange of cysteine to serine in the encoded protein, was detected in avrRpt2(EA) of several Erwinia strains differing in virulence to Mr5. E. amylovora strains encoding serine (S-allele) were able to overcome resistance of Mr5, whereas strains encoding cysteine (C-allele) were not. Allele specificity was also observed in a coexpression assay with Arabidopsis thaliana RIN4 in Nicotiana benthamiana. A homolog of RIN4 has been detected and isolated in Mr5. These results suggest a system similar to the interaction of RPS2 from A. thaliana and AvrRpt2 from P. syringae with RIN4 as guard. Our data are suggestive of a gene-for-gene relationship for the host-pathogen system Mr5 and E. amylovora.

Pseudomonas syringae pv. tomato DC3000: a model pathogen for probing disease susceptibility and hormone signaling in plants

Since the early 1980s, various strains of the gram-negative bacterial pathogen Pseudomonas syringae have been used as models for understanding plant-bacterial interactions. In 1991, a P. syringae pathovar tomato (Pst) strain, DC3000, was reported to infect not only its natural host tomato but also Arabidopsis in the laboratory, a finding that spurred intensive efforts in the subsequent two decades to characterize the molecular mechanisms by which this strain causes disease in plants. Genomic analysis shows that Pst DC3000 carries a large repertoire of potential virulence factors, including proteinaceous effectors that are secreted through the type III secretion system and a polyketide phytotoxin called coronatine, which structurally mimics the plant hormone jasmonate (JA). Study of Pst DC3000 pathogenesis has not only provided several conceptual advances in understanding how a bacterial pathogen employs type III effectors to suppress plant immune responses and promote disease susceptibility but has also facilitated the discovery of the immune function of stomata and key components of JA signaling in plants. The concepts derived from the study of Pst DC3000 pathogenesis may prove useful in understanding pathogenesis mechanisms of other plant pathogens.

AvrRpm1 missense mutations weakly activate RPS2-mediated immune response in Arabidopsis thaliana

Plants recognize microbes via specific pattern recognition receptors that are activated by microbe-associated molecular patterns (MAMPs), resulting in MAMP-triggered immunity (MTI). Successful pathogens bypass MTI in genetically diverse hosts via deployment of effectors (virulence factors) that inhibit MTI responses, leading to pathogen proliferation. Plant pathogenic bacteria like Pseudomonas syringae utilize a type III secretion system to deliver effectors into cells. These effectors can contribute to pathogen virulence or elicit disease resistance, depending upon the host plant genotype. In disease resistant genotypes, intracellular immune receptors, typically belonging to the nucleotide binding leucine-rich repeat family of proteins, perceive bacterial effector(s) and initiate downstream defense responses (effector triggered immunity) that include the hypersensitive response, and transcriptional re-programming leading to various cellular outputs that collectively halt pathogen growth. Nucleotide binding leucine-rich repeat sensors can be indirectly activated via perturbation of a host protein acting as an effector target. AvrRpm1 is a P. syringae type III effector. Upon secretion into the host cell, AvrRpm1 is acylated by host enzymes and directed to the plasma membrane, where it contributes to virulence. This is correlated with phosphorylation of Arabidopsis RIN4 in vivo. RIN4 is a negative regulator of MAMP-triggered immunity, and its modification in the presence of four diverse type III effectors, including AvrRpm1, likely enhances this RIN4 regulatory function. The RPM1 nucleotide binding leucine-rich repeat sensor perceives RIN4 perturbation in disease resistant plants, leading to a successful immune response. Here, demonstrate that AvrRpm1 has a fold homologous to the catalytic domain of poly(ADP-ribosyl) polymerase. Site-directed mutagenesis of each residue in the putative catalytic triad, His63-Tyr122-Asp185 of AvrRpm1, results in loss of both AvrRpm1-dependent virulence and AvrRpm1-mediated activation of RPM1, but, surprisingly, causes a gain of function: the ability to activate the RPS2 nucleotide binding leucine-rich repeat sensor.

Pseudomonas syringae pv. phaseolicola effector HopF1 inhibits pathogen-associated molecular pattern-triggered immunity in a RIN4-independent manner in common bean (Phaseolus vulgaris)

Plant pathogens usually promote pathogenesis by secreting effector proteins into host plant cells. One of the secreted effectors of Pseudomonas syringae pv. phaseolicola, the causative agent of halo-blight disease in common bean (Phaseolus vulgaris), HopF1, activates effector-triggered immunity (ETI) in a bean cultivar containing R1 resistance gene, but displays virulence function in a bean cultivar without the R1 gene. The virulence mechanism of the effector remained unknown, although it was identified more than a decade ago. Here we demonstrated that HopF1 can inhibit pathogen-associated molecular pattern-triggered immunity (PTI) in a susceptible bean cultivar Tendergreen. HopF1 directly interacted with two RPM1-interacting protein 4 (RIN4) orthologs of bean, PvRIN4a and PvRIN4b. Like RIN4 in Arabidopsis, both PvRIN4 orthologs negatively regulated the PTI responses in bean. However, the virulence function of HopF1 was enhanced in Tendergreen silencing PvRIN4. Furthermore, silencing PvRIN4a compromised the avrβ1-induced hypersensitive response (HR), which previously was reported to be suppressed by HopF1. Together, these results demonstrated that PvRIN4 orthologs were not the virulence target of HopF1 for inhibiting PTI, but probably for interfering with ETI.

Plant immunity: evolutionary insights from PBS1, Pto, and RIN4

Two layers of plant immune systems are used by plants to defend against phytopathogens. The first layer is pathogen-associate molecular patterns (PAMPs)-triggered immunity (PTI), which is activated by plant cell-surface pattern recognition receptors (PRRs) upon perception of microbe general elicitors. The second layer is effector-triggered immunity (ETI), which is initiated by specific recognition of pathogen type III secreted effectors (T3SEs) with plant intracellular resistance (R) proteins. Current opinions agree that ETI was evolved from PTI, and the impetus for the evolution of plant immunity is pathogen T3SEs, which exhibit virulence functions through blocking PTI, but show avirulence functions for triggering ETI. A Decoy Model was put forward and explained that the avirulence targets of pathogen T3SEs were evolved as decoys to compete with the virulence targets for binding with pathogen T3SEs. However, little direct evidence for the evolutionary mode has been offered. Here, we reviewed the recent progresses about Pto, PBS1, and RIN4 to present our viewpoints about the evolution of plant immunity.

RIN4-like proteins mediate resistance protein-derived soybean defense against Pseudomonas syringae

Resistance (R) protein mediated recognition of pathogen avirulence effectors triggers signaling that induces a very robust form of species-specific immunity in plants. The soybean Rpg1-b protein mediates this form of resistance against the bacterial blight pathogen, Pseudomonas syringae expressing AvrB Pgyrace4. Likewise, the Arabidopsis RPM1 protein also mediates species-specific resistance against AvrB expressing bacteria. RPM1 and Rpg1-b are non-orthologous and differ in their requirements for downstream signaling components. We recently showed that the activation of Rpg1-b derived resistance signaling requires two host proteins that directly interact with AvrB. These proteins share high sequence similarity with the Arabidopsis RPM1 interacting protein 4 (RIN4), which is essential for RPM1-derived resistance. The two soybean RIN4-like proteins (GmRIN4a and b) differ in their abilities to interact with Rpg1-b as well as to complement the Arabidopsis rin4 mutation. Because the two GmRIN4 proteins interact with each other, we proposed that they might function as a heteromeric complex in mediating Rpg1-b-derived resistance. Absence of GmRIN4a or b enhanced basal resistance against bacterial and oomycete pathogens in soybean. Lack of GmRIN4a also enhanced the virulence of avrB bacteria in plants lacking Rpg1-b. Our studies suggest that multiple RIN4-like proteins proteins mediate R-mediated signaling, in soybean.

The Pseudomonas syringae effector protein HopZ1a suppresses effector-triggered immunity

*The Pseudomonas syringae pv syringae type III effector HopZ1a is a member of the HopZ effector family of cysteine-proteases that triggers immunity in Arabidopsis. This immunity is dependent on HopZ1a cysteine-protease activity, and independent of known resistance genes. We have previously shown that HopZ1a-triggered immunity is partially additive to that triggered by AvrRpt2. These partially additive effects could be caused by at least two mechanisms: their signalling pathways share a common element(s), or one effector interferes with the response triggered by the other. *Here, we investigate the molecular basis for the partially additive effect displayed by AvrRpt2- and HopZ1a-triggered immunities, by analysing competitive indices, hypersensitive response and symptom induction, PR-1 accumulation, expression of PR genes, and systemic acquired resistance (SAR) induction. *Partially additive effects between these defence responses require HopZ1a cysteine-protease activity, and also take place between HopZ1a and AvrRps4 or AvrRpm1-triggered responses. We establish that HopZ1a-triggered immunity is independent of salicylic acid (SA), EDS1, jasmonic acid (JA) and ethylene (ET)-dependent pathways, and show that HopZ1a suppresses the induction of PR-1 and PR-5 associated with P. syringae pv tomato (Pto)-triggered effector-triggered immunity (ETI)-like defences, AvrRpt2-triggered immunity, and Pto or Pto (avrRpt2) activation of SAR, and that suppression requires HopZ1a cysteine-protease activity. *Our results indicate that HopZ1a triggers an unusual resistance independent of known pathways and suppresses SA and EDS1-dependent resistance.

Advances in experimental methods for the elucidation of Pseudomonas syringae effector function with a focus on AvrPtoB

Pseudomonas syringae infects a wide range of plant species through the use of a type III secretion system. The effector proteins injected into the plant cell through this molecular syringe serve as promoters of disease by subverting the plant immune response to the benefit of the bacteria in the intercellular space. The targets and activities of a subset of effectors have been elucidated recently. In this article, we focus on the experimental approaches that have proved most successful in probing the molecular basis of effectors, ranging from loss-of-function to gain-of-function analyses utilizing several techniques for effector delivery into plants. In particular, we highlight how these diverse approaches have been applied to the study of one effector--AvrPtoB--a multifunctional protein with the ability to suppress both effector-triggered immunity and pathogen (or microbe)-associated molecular pattern-triggered immunity. Taken together, advances in this field illustrate the need for multiple experimental approaches when elucidating the function of a single effector.

From bacterial avirulence genes to effector functions via the hrp delivery system: an overview of 25 years of progress in our understanding of plant innate immunity

Cloning the first avirulence (avr) gene has led not only to a deeper understanding of gene-for-gene interactions in plant disease, but also to fundamental insights into the suppression of basal defences against microbial attack. This article (focusing on Pseudomonas syringae) charts the development of ideas and research progress over the 25 years following the breakthrough achieved by Staskawicz and coworkers. Advances in gene cloning technology underpinned the identification of both avr and hrp genes, the latter being required for the activation of the defensive hypersensitive reaction (HR) and pathogenicity. The delivery of Avr proteins through the type III secretion machinery encoded by hrp gene clusters was demonstrated, and the activity of the proteins inside plant cells as elicitors of the HR was confirmed. Key roles for avr genes in pathogenic fitness have now been established. The rebranding of Avr proteins as effectors, proteins that suppress the HR and cell wall-based defences, has led to the ongoing search for their targets, and is generating new insights into the co-ordination of plant resistance against diverse microbes. Bioinformatics-led analysis of effector gene distribution in genomes has provided a remarkable view of the interchange of effectors and also their functional domains, as the arms race of attack and defence drives the evolution of microbial pathogenicity. The application of our accrued knowledge for the development of disease control strategies is considered.

The Pseudomonas syringae effector protein, AvrRPS4, requires in planta processing and the KRVY domain to function

A Pseudomonas syringae pv. pisi effector protein, AvrRPS4, triggers RPS4-dependent immunity in Arabidopsis. We characterized biochemical and genetic aspects of AvrRPS4 function. Secretion of AvrRPS4 from Pst DC3000 is type III secretion-dependent, and AvrRPS4 is processed into a smaller form in plant cells but not in bacteria or yeast. Agrobacterium-mediated transient expression analysis of N-terminally truncated AvrRPS4 mutants revealed that the C-terminal 88 amino acids are sufficient to trigger the hypersensitive response in turnip. N-terminal sequencing of the processed AvrRPS4 showed that processing occurs between G133 and G134. The processing-deficient mutant, R112L, still triggers RPS4-dependent immunity, suggesting that the processing is not required for the AvrRPS4 avirulence function. AvrRPS4 enhances bacterial growth when delivered by Pta 6606 into Nicotiana benthamiana in which AvrRPS4 is not recognized. Transgenic expression of AvrRPS4 in the Arabidopsis rps4 mutant enhances the growth of Pst DC3000 and suppresses PTI (PAMP-triggered immunity), showing that AvrRPS4 promotes virulence in two distinct host plants. Furthermore, full virulence activity of AvrRPS4 requires both proteolytic processing and the KRVY motif at the N-terminus of processed AvrRPS4. XopO, an Xcv effector, shares the amino acids required for AvrRPS4 processing and the KRVY motif. XopO is also processed into a smaller form in N. benthamiana, similar to AvrRPS4, suggesting that a common mechanism is involved in activation of the virulence activities of both AvrRPS4 and XopO.

Phytopathogen type III effector weaponry and their plant targets

Phytopathogenic bacteria suppress plant innate immunity and promote pathogenesis by injecting proteins called type III effectors into plant cells using a type III protein secretion system. These type III effectors use at least three strategies to alter host responses. One strategy is to alter host protein turnover, either by direct cleavage or by modulating ubiquitination and targeting the 26S proteasome. Another strategy involves alteration of RNA metabolism by transcriptional activation or ADP-ribosylation of RNA-binding proteins. A third major strategy is to inhibit the kinases involved in plant defence signaling, either by the removal of phosphates or by direct inhibition. The wide array of strategies that bacterial pathogens employ to suppress innate immunity suggest that circumvention of innate immunity is crucial for bacterial pathogenicity of plants.

The Cladosporium fulvum virulence protein Avr2 inhibits host proteases required for basal defense

Cladosporium fulvum (syn. Passalora fulva) is a biotrophic fungal pathogen that causes leaf mold of tomato (Solanum lycopersicum). During growth in the apoplast, the fungus establishes disease by secreting effector proteins, 10 of which have been characterized. We have previously shown that the Avr2 effector interacts with the apoplastic tomato Cys protease Rcr3, which is required for Cf-2-mediated immunity. We now show that Avr2 is a genuine virulence factor of C. fulvum. Heterologous expression of Avr2 in Arabidopsis thaliana causes enhanced susceptibility toward extracellular fungal pathogens, including Botrytis cinerea and Verticillium dahliae, and microarray analysis showed that Avr2 expression triggers a global transcriptome reflecting pathogen challenge. Cys protease activity profiling showed that Avr2 inhibits multiple extracellular Arabidopsis Cys proteases. In tomato, Avr2 expression caused enhanced susceptibility toward Avr2-defective C. fulvum strains and also toward B. cinerea and V. dahliae. Cys protease activity profiling in tomato revealed that, in this plant also, Avr2 inhibits multiple extracellular Cys proteases, including Rcr3 and its close relative Pip1. Finally, silencing of Avr2 significantly compromised C. fulvum virulence on tomato. We conclude that Avr2 is a genuine virulence factor of C. fulvum that inhibits several Cys proteases required for plant basal defense.

Pseudomonas syringae type III effector AvrRpt2 alters Arabidopsis thaliana auxin physiology

The Pseudomonas syringae type III effector AvrRpt2 promotes bacterial virulence on Arabidopsis thaliana plants lacking a functional RPS2 gene (rps2 mutant plants). To investigate the mechanisms underlying the virulence activity of AvrRpt2, we examined the phenotypes of transgenic A. thaliana rps2 seedlings constitutively expressing AvrRpt2. These seedlings exhibited phenotypes reminiscent of A. thaliana mutants with altered auxin physiology, including longer primary roots, increased number of lateral roots, and increased sensitivity to exogenous auxin. They also had increased levels of free indole acetic acid (IAA). The presence of AvrRpt2 also was correlated with a further increase in free IAA levels during infection with P. syringae pv. tomato strain DC3000 (PstDC3000). These results indicate that AvrRpt2 alters A. thaliana auxin physiology. Application of the auxin analog 1-naphthaleneacetic acid promoted disease symptom development in PstDC3000-infected plants, suggesting that elevated auxin levels within host tissue promote PstDC3000 virulence. Thus, AvrRpt2 may be among the virulence factors of P. syringae that modulate host auxin physiology to promote disease.

Elicitation and suppression of microbe-associated molecular pattern-triggered immunity in plant-microbe interactions

Recent studies have uncovered fascinating molecular mechanisms underlying plant-microbe interactions that coevolved dynamically. As in animals, the primary plant innate immunity is immediately triggered by the detection of common pathogen- or microbe-associated molecular patterns (PAMPs/MAMPs). Different MAMPs are often perceived by distinct cell-surface pattern-recognition receptors (PRRs) and activate convergent intracellular signalling pathways in plant cells for broad-spectrum immunity. Successful pathogens, however, have evolved multiple virulence factors to suppress MAMP-triggered immunity. Specifically, diverse pathogenic bacteria have employed the type III secretion system to deliver a repertoire of virulence effector proteins to interfere with host immunity and promote pathogenesis. Plants challenged by pathogens have evolved the secondary plant innate immunity. In particular, some plants possess the specific intracellular disease resistance (R) proteins to effectively counteract virulence effectors of pathogens for effector-triggered immunity. This potent but cultivar-specific effector-triggered immunity occurs rapidly with localized programmed cell death/hypersensitive response to limit pathogen proliferation and disease development. Remarkably, bacteria have further acquired virulence effectors to block effector-triggered immunity. This review covers the latest findings in the dynamics of MAMP-triggered immunity and its interception by virulence factors of pathogenic bacteria.

Plant NBS-LRR proteins in pathogen sensing and host defense

Plant proteins belonging to the nucleotide-binding site-leucine-rich repeat (NBS-LRR) family are used for pathogen detection. Like the mammalian Nod-LRR protein 'sensors' that detect intracellular conserved pathogen-associated molecular patterns, plant NBS-LRR proteins detect pathogen-associated proteins, most often the effector molecules of pathogens responsible for virulence. Many virulence proteins are detected indirectly by plant NBS-LRR proteins from modifications the virulence proteins inflict on host target proteins. However, some NBS-LRR proteins directly bind pathogen proteins. Association with either a modified host protein or a pathogen protein leads to conformational changes in the amino-terminal and LRR domains of plant NBS-LRR proteins. Such conformational alterations are thought to promote the exchange of ADP for ATP by the NBS domain, which activates 'downstream' signaling, by an unknown mechanism, leading to pathogen resistance.

The Erwinia amylovora avrRpt2EA gene contributes to virulence on pear and AvrRpt2EA is recognized by Arabidopsis RPS2 when expressed in pseudomonas syringae

The enterobacterium Erwinia amylovora is a devastating plant pathogen causing necrotrophic fire blight disease of apple, pear, and other rosaceous plants. In an attempt to identify genes induced during infection of host plants, we identified and cloned a putative effector gene, avrRpt2EA. The deduced amino-acid sequence of the translated AvrRpt2EA protein is homologous to the effector protein AvrRpt2 previously reported in Pseudomonas syringae pv. tomato. These two proteins share 58% identity (70% similarity) in the functional domain; however, the secretion and translocation signal domain varied. The avrRpt2EA promoter region contains a typical 'hrp box,' which suggests that avrRpt2EA is regulated by the alternative sigma factor, HrpL. avrRpt2EA was detected in all E. amylovora strains tested but not in other closely related Erwinia species. An avrRpt2EA deletion mutant was reduced in its ability to cause systemic infection on immature pear fruits as compared with the wild-type strain, indicating that avrRpt2EA acts as a virulence factor on its native host. Growth of P. syringae pv. tomato DC3000 expressing avrRpt2EA was 10-fold higher than that of P. syringae pv. tomato DC3000 in an Arabidopsis rps2 mutant, indicating that avrRpt2EA promotes virulence of P. syringae pv. tomato DC3000 on Arabidopsis similar to P. syringae pv. tomato avrRpt2. When avrRpt2EA was expressed in P. syringae pv. tomato DC3000 in its native form, a weak hypersensitive response (HR) was induced in Arabidopsis; however, a hybrid protein containing the P. syringae pv. tomato avrRpt2 signal sequence, when expressed from the P syringae pv. tomato avrRpt2 promoter, caused a strong HR. Thus, the signal sequence and promoter of avrRpt2EA may affect its expression, secretion, or translocation, singly or in combination, in P. syringae pv. tomato DC3000. These results indicated that avrRpt2EA is genetically recognized by the RPS2 disease resistance gene in Arabidopsis when expressed in P. syringae pv. tomato DC3000. The results also suggested that although distinct pathogens such as E. amylovora and P. syringae may contain similar effector genes, expression and secretion of these effectors can be under specific regulation by the native pathogen.

AvrB mutants lose both virulence and avirulence activities on soybean and Arabidopsis

The Pseudomonas syringae pv. glycinea effector protein AvrB induces resistance responses in soybean varieties that contain the resistance gene Rpg1-b and Arabidopsis varieties that carry RPM1. In addition to this avirulence activity, AvrB also enhances bacterial virulence on soybean plants that lack Rpg1-b and induces a chlorotic phenotype on Arabidopsis plants that lack RPM1. We screened a library of avrB mutants for loss of avirulence on soybean and Arabidopsis, and assayed selected avirulence mutants for loss of virulence on both plants. All mutants screened were recognized similarly on both plant species. Nine single-site avrB mutations that affected avirulence localized to a solvent-accessible pocket in the protein structure. Seven of these mutated residues are absolutely conserved between AvrB and its nine homologues. Avirulence mutants generally lost virulence enhancement on susceptible soybean varieties and lost the ability to induce a chlorotic response on the rpm1 null Arabidopsis variety Mt-0. Three of four avirulence mutants tested failed to interact with RIN4, an Arabidopsis protein previously shown to be required for RPM1 function. Our results suggest that soybean and Arabidopsis recognize AvrB in the same manner, and that AvrB enzymatic activity is required for its function as an avirulence and virulence effector on two different plant species.

Microbial avirulence determinants: guided missiles or antigenic flak?

SUMMARY Avirulence (avr) determinants are incompatibility factors which elicit host plant defence responses in a gene-for-gene manner. They are produced by fungi, bacteria and viruses, and their recognition by resistance genes has been extensively studied for decades. But why should a microbe keep a molecule that allows it to be recognized? One argument is that avr genes perform some essential function and must be kept despite giving the pathogen away. Many bacterial avr determinants have been shown to be effectors, which contribute to virulence and aggressiveness. If this were always the case, mutants lacking these essential molecules would be at a serious disadvantage. Some disadvantage has been shown for a small number, but for the majority there is no effect on virulence. This has been explained by functional redundancy for bacterial and fungal avr determinants, with other molecules compensating for the deletion of these essential genes. However, this argument is counter-intuitive because by definition these individual genes are no longer essential; so why keep them? With increasing numbers of avr genes being identified, efforts to elucidate their function are increasing. In this review, we take stock of the accumulating literature, and consider what the real function of avr determinants might be.

Suppression of host defense in compatible plant-Pseudomonas syringae interactions

Despite impressive advances in the study of plant resistance to pathogens, little is known about the molecular basis of plant susceptibility to virulent pathogens. Recent progress in susceptible plant-Pseudomonas syringae interactions has provided a glimpse into the battles fought between plants and bacterial pathogens. A key step for pathogenesis appears to be the suppression of host defenses. Suppression of host defenses, including basal defense, gene-for-gene resistance and nonhost resistance, is a key step for pathogenesis. Defense suppression is mediated by bacterial effector proteins, which are secreted through the type III secretion system, and by coronatine, a bacterial toxin that structurally and functionally mimics methyl jasmonate, a plant defense signaling molecule.

The Pseudomonas syringae avrRpt2 gene contributes to virulence on tomato

In order to cause disease on plants, gram-negative phytopathogenic bacteria introduce numerous virulence factors into the host cell in order to render host tissue more hospitable for pathogen proliferation. The mode of action of such bacterial virulence factors and their interaction with host defense pathways remain poorly understood. avrRpt2, a gene from Pseudomonas syringae pv. tomato JL1065, has been shown to promote the virulence of heterologous P. syringae strains on Arabidopsis thaliana. However, the contribution of avrRpt2 to the virulence of JL1065 has not been examined previously. We show that a mutant derivative of JL1065 that carries a disruption in avrRpt2 is impaired in its ability to cause disease on tomato (Lycopersicon esculentum), indicating that avrRpt2 also acts as a virulence gene in its native strain on a natural host. The virulence activity of avrRpt2 was detectable on tomato lines that are defective in either ethylene perception or the accumulation of salicylic acid, but could not be detected on a tomato mutant insensitive to jasmonic acid. The enhanced virulence conferred by the expression of avrRpt2 in JL1065 was not associated with the suppression of several defense-related genes induced during the infection of tomato.

R gene expression induced by a type-III effector triggers disease resistance in rice

Disease resistance (R) genes in plants encode products that specifically recognise incompatible pathogens and trigger a cascade of events leading to disease resistance in the host plant. R-gene specificity is dictated by both host R genes and cognate avirulence (avr) genes in pathogens. However, the basis of gene-for-gene specificity is not well understood. Here, we report the cloning of the R gene Xa27 from rice and the cognate avr gene avrXa27 from Xanthomonas oryzae pv. oryzae. Resistant and susceptible alleles of Xa27 encode identical proteins. However, expression of only the resistant allele occurs when a rice plant is challenged by bacteria harbouring avrXa27, whose product is a nuclear localized type-III effector. Induction of Xa27 occurs only in the immediate vicinity of infected tissue, whereas ectopic expression of Xa27 resulted in resistance to otherwise compatible strains of the pathogen. Thus Xa27 specificity towards incompatible pathogens involves the differential expression of the R gene in the presence of the AvrXa27 effector.

Molecular characterization of proteolytic cleavage sites of the Pseudomonas syringae effector AvrRpt2

During infection of Arabidopsis thaliana, the bacterium Pseudomonas syringae pv tomato delivers the effector protein AvrRpt2 into the plant cell cytosol. Within the plant cell, AvrRpt2 undergoes N-terminal processing and causes elimination of Arabidopsis RIN4. Previous work established that AvrRpt2 is a putative cysteine protease, and AvrRpt2 processing and RIN4 elimination require an intact predicted catalytic triad in that AvrRpt2. In this work, proteolytic events that depend on AvrRpt2 activity were characterized. The amino acid sequence surrounding the processing site of AvrRpt2 and two related sequences from RIN4 triggered Avr-Rpt2-dependent proteolytic cleavage of a synthetic substrate, demonstrating that these sequences are cleavage recognition sites for AvrRpt2 activity. Processing-deficient AvrRpt2 mutants were identified and shown to retain their ability to eliminate wild-type RIN4. Single amino acid substitutions were made in each of the two RIN4 cleavage sites, and mutation of both sites resulted in cleavage-resistant RIN4. Growth of Pseudomonas expressing AvrRpt2 was significantly higher than catalytically inactive mutants on Arabidopsis rin4/rps2 mutant plants, suggesting there are additional protein targets of AvrRpt2 that account for the virulence activity of this effector. Bioinformatics analysis identified putative Arabidopsis proteins containing sequences similar to the proteolytic cleavage sites conserved in AvrRpt2 and RIN4. Several of these proteins were eliminated in an AvrRpt2-dependent manner in a transient in planta expression system. These results identify amino acids important for AvrRpt2 substrate recognition and cleavage as well as demonstrate AvrRpt2 protease activity eliminates multiple Arabidopsis proteins in a transient expression system.

Contrasting mechanisms of defense against biotrophic and necrotrophic pathogens

It has been suggested that effective defense against biotrophic pathogens is largely due to programmed cell death in the host, and to associated activation of defense responses regulated by the salicylic acid-dependent pathway. In contrast, necrotrophic pathogens benefit from host cell death, so they are not limited by cell death and salicylic acid-dependent defenses, but rather by a different set of defense responses activated by jasmonic acid and ethylene signaling. This review summarizes results from Arabidopsis-pathogen systems regarding the contributions of various defense responses to resistance to several biotrophic and necrotrophic pathogens. While the model above seems generally correct, there are exceptions and additional complexities.