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

The processed C-terminus of AvrRps4 effector suppresses plant immunity via targeting multiple WRKYs

Pathogens generate and secrete effector proteins to the host plant cells during pathogenesis to promote virulence and colonization. If the plant carries resistance (R) proteins that recognize pathogen effectors, effector-triggered immunity (ETI) is activated, resulting in a robust immune response and hypersensitive response (HR). The bipartite effector AvrRps4 from Pseudomonas syringae pv. pisi has been well studied in terms of avirulence function. In planta, AvrRps4 is processed into two parts. The C-terminal fragment of AvrRps4 (AvrRps4C) induces HR in turnip and is recognized by the paired resistance proteins AtRRS1/AtRPS4 in Arabidopsis. Here, we show that AvrRps4C targets a group of Arabidopsis WRKY, including WRKY46, WRKY53, WRKY54, and WRKY70, to induce its virulence function. Indeed, AvrRps4C suppresses the general binding and transcriptional activities of immune-positive regulator WRKY54 and WRKY54-mediated resistance. AvrRps4C interferes with WRKY54's binding activity to target gene SARD1 in vitro, suggesting WRKY54 is sequestered from the SARD1 promoter by AvrRps4C. Through the interaction of AvrRps4C with four WRKYs, AvrRps4 enhances the formation of homo-/heterotypic complexes of four WRKYs and sequesters them in the cytoplasm, thus inhibiting their function in plant immunity. Together, our results provide a detailed virulence mechanism of AvrRps4 through its C-terminus.

Greater than the sum of their parts: an overview of the AvrRps4 effector family

Phytopathogenic microbes use secreted effector proteins to increase their virulence in planta. If these effectors or the results of their activity are detected by the plant cell, the plant will mount an immune response which applies evolutionary pressure by reducing growth and success of the pathogen. Bacterial effector proteins in the AvrRps4 family (AvrRps4, HopK1, and XopO) have commonly been used as tools to investigate plant immune components. At the same time, the in planta functions of this family of effectors have yet to be fully characterized. In this minireview we summarize current knowledge about the AvrRps4 effector family with emphasis on properties of the proteins themselves. We hypothesize that the HopK1 C-terminus and the AvrRps4 C-terminus, though unrelated in sequence and structure, are broadly related in functions that counteract plant defense responses.

Molecular basis for the interference of the Arabidopsis WRKY54-mediated immune response by two sequence-unrelated bacterial effectors

Arabidopsis thaliana WRKY proteins are potential targets of pathogen-secreted effectors. RESISTANT TO RALSTONIA SOLANACEARUM 1 (RRS1; AtWRKY52) is a well-studied Arabidopsis nucleotide-binding and leucine-rich repeat (NLR) immune receptor carrying a C-terminal WRKY domain that functions as an integrated decoy. RRS1-R recognizes the effectors AvrRps4 from Pseudomonas syringae pv. pisi and PopP2 from Ralstonia pseudosolanacearum by direct interaction through its WRKY domain. AvrRps4 and PopP2 were previously shown to interact with several AtWRKYs. However, how these effectors selectively interact with their virulence targets remains unknown. Here, we show that several members of subgroup IIIb of the AtWRKY family are targeted by AvrRps4 and PopP2. We demonstrate that several AtWRKYs induce cell death when transiently expressed in Nicotiana benthamiana, indicating the activation of immune responses. AtWRKY54 was the only cell death-inducing AtWRKY that interacted with both AvrRps4 and PopP2. We found that AvrRps4 and PopP2 specifically suppress AtWRKY54-induced cell death. We also demonstrate that the amino acid residues required for the avirulence function of AvrRps4 and PopP2 are critical for suppressing AtWRKY54-induced cell death. AtWRKY54 residues predicted to form a binding interface with AvrRps4 were predominantly located in the DNA binding domain and necessary for inducing cell death. Notably, one AtWRKY54 residue, E164, contributes to affinity with AvrRps4 and is exclusively present among subgroup IIIb AtWRKYs, yet is located outside of the DNA-binding domain. Surprisingly, AtWRKY54 mutated at E164 evaded AvrRps4-mediated cell death suppression. Taking our observations together, we propose that AvrRp4 and PopP2 specifically target AtWRKY54 to suppress plant immune responses.

Pseudomonas syringae coffee blight is associated with the horizontal transfer of plasmid-encoded type III effectors

The emergence of new pathogens is an ongoing threat to human health and agriculture. While zoonotic spillovers received considerable attention, the emergence of crop diseases is less well studied. Here, we identify genomic factors associated with the emergence of Pseudomonas syringae bacterial blight of coffee. Fifty-three P. syringae strains from diseased Brazilian coffee plants were sequenced. Comparative and evolutionary analyses were used to identify loci associated with coffee blight. Growth and symptomology assays were performed to validate the findings. Coffee isolates clustered in three lineages, including primary phylogroups PG3 and PG4, and secondary phylogroup PG11. Genome-wide association study of the primary PG strains identified 37 loci, including five effectors, most of which were encoded on a plasmid unique to the PG3 and PG4 coffee strains. Evolutionary analyses support the emergence of coffee blight in PG4 when the coffee-associated plasmid and associated effectors derived from a divergent plasmid carried by strains associated with other hosts. This plasmid was only recently transferred into PG3. Natural diversity and CRISPR-Cas9 plasmid curing were used to show that strains with the coffee-associated plasmid grow to higher densities and cause more severe disease symptoms in coffee. This work identifies possible evolutionary mechanisms underlying the emergence of a new lineage of coffee pathogens.

Plant NLRs: Evolving with pathogen effectors and engineerable to improve resistance

Pathogens are important threats to many plants throughout their lifetimes. Plants have developed different strategies to overcome them. In the plant immunity system, nucleotide-binding domain and leucine-rich repeat-containing proteins (NLRs) are the most common components. And recent studies have greatly expanded our understanding of how NLRs function in plants. In this review, we summarize the studies on the mechanism of NLRs in the processes of effector recognition, resistosome formation, and defense activation. Typical NLRs are divided into three groups according to the different domains at their N termini and function in interrelated ways in immunity. Atypical NLRs contain additional integrated domains (IDs), some of which directly interact with pathogen effectors. Plant NLRs evolve with pathogen effectors and exhibit specific recognition. Meanwhile, some NLRs have been successfully engineered to confer resistance to new pathogens based on accumulated studies. In summary, some pioneering processes have been obtained in NLR researches, though more questions arise as a result of the huge number of NLRs. However, with a broadened understanding of the mechanism, NLRs will be important components for engineering in plant resistance improvement.

AvrRps4 effector family processing and recognition in lettuce

During pathogenesis, effector proteins are secreted from the pathogen to the host plant to provide virulence activity for invasion of the host. However, once the host plant recognizes one of the delivered effectors, effector-triggered immunity activates a robust immune and hypersensitive response (HR). In planta, the effector AvrRps4 is processed into the N-terminus (AvrRps4N ) and the C-terminus (AvrRps4C ). AvrRps4C is sufficient to trigger HR in turnip and activate AtRRS1/AtRPS4-mediated immunity in Arabidopsis; on the other hand, AvrRps4N induces HR in lettuce. Furthermore, AvrRps4N -mediated HR requires a conserved arginine at position 112 (R112), which is also important for full-length AvrRps4 (AvrRps4F ) processing. Here, we show that effector processing and effector recognition in lettuce are uncoupled for the AvrRps4 family. In addition, we compared effector recognition by lettuce of AvrRps4 and its homologues, HopK1 and XopO. Interestingly, unlike for AvrRps4 and HopK1, mutation of the conserved R111 in XopO by itself was insufficient to abolish recognition. The combination of amino acid substitutions arginine 111 to leucine with glutamate 114 to lysine abolished the XopO-mediated HR, suggesting that AvrRps4 family members have distinct structural requirements for perception by lettuce. Together, our results provide an insight into the processing and recognition of AvrRps4 and its homologues.

Recognition of Microbe- and Damage-Associated Molecular Patterns by Leucine-Rich Repeat Pattern Recognition Receptor Kinases Confers Salt Tolerance in Plants

In plants, a first layer of inducible immunity is conferred by pattern recognition receptors (PRRs) that bind microbe- and damage-associated molecular patterns to activate pattern-triggered immunity (PTI). PTI is strengthened or followed by another potent form of immunity when intracellular receptors recognize pathogen effectors, termed effector-triggered immunity. Immunity signaling regulators have been reported to influence abiotic stress responses as well, yet the governing principles and mechanisms remain ambiguous. Here, we report that PRRs of a leucine-rich repeat ectodomain also confer salt tolerance in Arabidopsis thaliana, following recognition of cognate ligands such as bacterial flagellin (flg22 epitope) and elongation factor Tu (elf18 epitope), and the endogenous Pep peptides. Pattern-triggered salt tolerance (PTST) requires authentic PTI signaling components; namely, the PRR-associated kinases BAK1 and BIK1 and the NADPH oxidase RBOHD. Exposure to salt stress induces the release of Pep precursors, pointing to the involvement of the endogenous immunogenic peptides in developing plant tolerance to high salinity. Transcriptome profiling reveals an inventory of PTST target genes, which increase or acquire salt responsiveness following a preexposure to immunogenic patterns. In good accordance, plants challenged with nonpathogenic bacteria also acquired salt tolerance in a manner dependent on PRRs. Our findings provide insight into signaling plasticity underlying biotic or abiotic stress cross-tolerance in plants conferred by PRRs.[Formula: see text] Copyright © 2022 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Perception of structurally distinct effectors by the integrated WRKY domain of a plant immune receptor

Plants use intracellular nucleotide-binding domain (NBD) and leucine-rich repeat (LRR)-containing immune receptors (NLRs) to detect pathogen-derived effector proteins. The Arabidopsis NLR pair RRS1-R/RPS4 confers disease resistance to different bacterial pathogens by perceiving the structurally distinct effectors AvrRps4 from Pseudomonas syringae pv. pisi and PopP2 from Ralstonia solanacearum via an integrated WRKY domain in RRS1-R. How the WRKY domain of RRS1 (RRS1WRKY) perceives distinct classes of effector to initiate an immune response is unknown. Here, we report the crystal structure of the in planta processed C-terminal domain of AvrRps4 (AvrRps4C) in complex with RRS1WRKY Perception of AvrRps4C by RRS1WRKY is mediated by the β2-β3 segment of RRS1WRKY that binds an electronegative patch on the surface of AvrRps4C Structure-based mutations that disrupt AvrRps4C-RRS1WRKY interactions in vitro compromise RRS1/RPS4-dependent immune responses. We also show that AvrRps4C can associate with the WRKY domain of the related but distinct RRS1B/RPS4B NLR pair, and the DNA-binding domain of AtWRKY41, with similar binding affinities and how effector binding interferes with WRKY-W-box DNA interactions. This work demonstrates how integrated domains in plant NLRs can directly bind structurally distinct effectors to initiate immunity.

Chromatin accessibility landscapes activated by cell-surface and intracellular immune receptors

Activation of cell-surface and intracellular receptor-mediated immunity results in rapid transcriptional reprogramming that underpins disease resistance. However, the mechanisms by which co-activation of both immune systems lead to transcriptional changes are not clear. Here, we combine RNA-seq and ATAC-seq to define changes in gene expression and chromatin accessibility. Activation of cell-surface or intracellular receptor-mediated immunity, or both, increases chromatin accessibility at induced defence genes. Analysis of ATAC-seq and RNA-seq data combined with publicly available information on transcription factor DNA-binding motifs enabled comparison of individual gene regulatory networks activated by cell-surface or intracellular receptor-mediated immunity, or by both. These results and analyses reveal overlapping and conserved transcriptional regulatory mechanisms between the two immune systems.

Bacterial effector targeting of a plant iron sensor facilitates iron acquisition and pathogen colonization

Acquisition of nutrients from different species is necessary for pathogen colonization. Iron is an essential mineral nutrient for nearly all organisms, but little is known about how pathogens manipulate plant hosts to acquire iron. Here, we report that AvrRps4, an effector protein delivered by Pseudomonas syringae bacteria to plants, interacts with and targets the plant iron sensor protein BRUTUS (BTS) to facilitate iron uptake and pathogen proliferation in Arabidopsis thaliana. Infection of rps4 and eds1 by P. syringae pv. tomato (Pst) DC3000 expressing AvrRps4 resulted in iron accumulation, especially in the plant apoplast. AvrRps4 alleviates BTS-mediated degradation of bHLH115 and ILR3(IAA-Leucine resistant 3), two iron regulatory proteins. In addition, BTS is important for accumulating immune proteins Enhanced Disease Susceptibility1 (EDS1) at both the transcriptional and protein levels upon Pst (avrRps4) infections. Our findings suggest that AvrRps4 targets BTS to facilitate iron accumulation and BTS contributes to RPS4/EDS1-mediated immune responses.

What the Wild Things Do: Mechanisms of Plant Host Manipulation by Bacterial Type III-Secreted Effector Proteins

Phytopathogenic bacteria possess an arsenal of effector proteins that enable them to subvert host recognition and manipulate the host to promote pathogen fitness. The type III secretion system (T3SS) delivers type III-secreted effector proteins (T3SEs) from bacterial pathogens such as Pseudomonas syringae, Ralstonia solanacearum, and various Xanthomonas species. These T3SEs interact with and modify a range of intracellular host targets to alter their activity and thereby attenuate host immune signaling. Pathogens have evolved T3SEs with diverse biochemical activities, which can be difficult to predict in the absence of structural data. Interestingly, several T3SEs are activated following injection into the host cell. Here, we review T3SEs with documented enzymatic activities, as well as T3SEs that facilitate virulence-promoting processes either indirectly or through non-enzymatic mechanisms. We discuss the mechanisms by which T3SEs are activated in the cell, as well as how T3SEs modify host targets to promote virulence or trigger immunity. These mechanisms may suggest common enzymatic activities and convergent targets that could be manipulated to protect crop plants from infection.

Nuclear Localization of HopA1Pss61 Is Required for Effector-Triggered Immunity

Plant resistance proteins recognize cognate pathogen avirulence proteins (also named effectors) to implement the innate immune responses called effector-triggered immunity. Previously, we reported that hopA1 from Pseudomonas syringae pv. syringae strain 61 was identified as an avr gene for Arabidopsis thaliana. Using a forward genetic screen approach, we cloned a hopA1-specific TIR-NBS-LRR class disease resistance gene, RESISTANCE TO PSEUDOMONAS SYRINGAE6 (RPS6). Many resistance proteins indirectly recognize effectors, and RPS6 is thought to interact with HopA1_Pss61 indirectly by surveillance of an effector target. However, the involved target protein is currently unknown. Here, we show RPS6 is the only R protein that recognizes HopA1_Pss61 in Arabidopsis wild-type Col-0 accession. Both RPS6 and HopA1_Pss61 are co-localized to the nucleus and cytoplasm. HopA1_Pss61 is also distributed in plasma membrane and plasmodesmata. Interestingly, nuclear localization of HopA1_Pss61 is required to induce cell death as NES-HopA1_Pss61 suppresses the level of cell death in Nicotiana benthamiana. In addition, in planta expression of hopA1_Pss61 led to defense responses, such as a dwarf morphology, a cell death response, inhibition of bacterial growth, and increased accumulation of defense marker proteins in transgenic Arabidopsis. Functional characterization of HopA1_Pss61 and RPS6 will provide an important piece of the ETI puzzle.

Mutual potentiation of plant immunity by cell-surface and intracellular receptors

The plant immune system involves cell-surface receptors that detect intercellular pathogen-derived molecules, and intracellular receptors that activate immunity upon detection of pathogen-secreted effector proteins that act inside the plant cell. Immunity mediated by surface receptors has been extensively studied¹, but that mediated by intracellular receptors has rarely been investigated in the absence of surface-receptor-mediated immunity. Furthermore, interactions between these two immune pathways are poorly understood. Here, by activating intracellular receptors without inducing surface-receptor-mediated immunity, we analyse interactions between these two distinct immune systems in Arabidopsis. Pathogen recognition by surface receptors activates multiple protein kinases and NADPH oxidases, and we find that intracellular receptors primarily potentiate the activation of these proteins by increasing their abundance through several mechanisms. Likewise, the hypersensitive response that depends on intracellular receptors is strongly enhanced by the activation of surface receptors. Activation of either immune system alone is insufficient to provide effective resistance against the bacterial pathogen Pseudomonas syringae. Thus, immune pathways activated by cell-surface and intracellular receptors in plants mutually potentiate to activate strong defences against pathogens. These findings reshape our understanding of plant immunity and have broad implications for crop improvement.

Mutual potentiation of plant immunity by cell-surface and intracellular receptors

The plant immune system involves cell-surface receptors that detect intercellular pathogen-derived molecules, and intracellular receptors that activate immunity upon detection of pathogen-secreted effector proteins that act inside the plant cell. Immunity mediated by surface receptors has been extensively studied¹, but that mediated by intracellular receptors has rarely been investigated in the absence of surface-receptor-mediated immunity. Furthermore, interactions between these two immune pathways are poorly understood. Here, by activating intracellular receptors without inducing surface-receptor-mediated immunity, we analyse interactions between these two distinct immune systems in Arabidopsis. Pathogen recognition by surface receptors activates multiple protein kinases and NADPH oxidases, and we find that intracellular receptors primarily potentiate the activation of these proteins by increasing their abundance through several mechanisms. Likewise, the hypersensitive response that depends on intracellular receptors is strongly enhanced by the activation of surface receptors. Activation of either immune system alone is insufficient to provide effective resistance against the bacterial pathogen Pseudomonas syringae. Thus, immune pathways activated by cell-surface and intracellular receptors in plants mutually potentiate to activate strong defences against pathogens. These findings reshape our understanding of plant immunity and have broad implications for crop improvement.

The Conserved Arginine Required for AvrRps4 Processing Is Also Required for Recognition of Its N-Terminal Fragment in Lettuce

Pathogens utilize a repertoire of effectors to facilitate pathogenesis, but when the host recognizes one of them, it causes effector-triggered immunity. The Pseudomonas type III effector AvrRps4 is a bipartite effector that is processed in planta into a functional 133-amino acid N-terminus (AvrRps4-N) and 88-amino acid C-terminus (AvrRps4-C). Previous studies found AvrRps4-C to be sufficient to trigger the hypersensitive response (HR) in turnip. In contrast, our recent work found that AvrRps4-N but not AvrRps4-C triggered HR in lettuce, whereas both were required for resistance induction in Arabidopsis. Here, we initially compared AvrRps4 recognition by turnip and lettuce using transient expression. By serial truncation, we identified the central conserved region consisting of 37 amino acids as essential for AvrRps4-N recognition, whereas the putative type III secretion signal peptide or the C-terminal 13 amino acids were dispensable. Surprisingly, the conserved arginine at position 112 (R112) that is required for full-length AvrRps4 processing is also required for the recognition of AvrRps4-N by lettuce. Mutating R112 to hydrophobic leucine or negatively charged glutamate abolished the HR-inducing capacity of AvrRps4-N, while a positively charged lysine at this position resulted in a slow and weak HR. Together, our results suggest an AvrRps4-N recognition-specific role of R112 in lettuce.[Formula: see text] Copyright © 2021 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Phytotoxin synthesis genes and type III effector genes of Pseudomonas syringae pv. actinidiae biovar 6 are regulated by culture conditions

The kiwifruit bacterial canker (Pseudomonas syringae pv. actinidiae; Psa) causes severe damage to kiwifruit production worldwide. Psa biovar 6 (Psa6), which was isolated in Japan in 2015, produces two types of phytotoxins: coronatine and phaseolotoxin. To elucidate the unique virulence of Psa6, we performed transcriptomic analysis of phytotoxin synthesis genes and type III effector genes in in vitro cultivation using various media. The genes related to phytotoxin synthesis and effectors of Psa6 were strictly regulated in the coronatine-inducing mediums (HS and HSC); 14 of 23 effector genes and a hrpL sigma factor gene were induced at 3 h after transferring to the media (early-inducible genes), and phytotoxin synthesis genes such as argD of phaseolotoxin and cfl of coronatine were induced at 6 and 12 h after transferring to the media (late-inducible genes). In contrast, induction of these genes was not observed in the hrp-inducing medium. Next, to examine whether the changes in gene expression in different media is specific to Psa6, we investigated gene expression in other related bacteria. For Psa biovar 1 (Psa1), biovar 3 (Psa3), and P. s. pv. glycinea (Psg), no clear trends were observed in expression behavior across various culture media and incubation times. Therefore, Psa6 seems to exert its virulence efficiently by using two phytotoxins and effectors according to environmental changes. This is not seen in other biovars and pathovars, so it is thought that Psa6 has acquired its own balance of virulence.

Estradiol-inducible AvrRps4 expression reveals distinct properties of TIR-NLR-mediated effector-triggered immunity

Plant nucleotide-binding domain, leucine-rich repeat receptor (NLR) proteins play important roles in recognition of pathogen-derived effectors. However, the mechanism by which plant NLRs activate immunity is still largely unknown. The paired Arabidopsis NLRs RRS1-R and RPS4, that confer recognition of bacterial effectors AvrRps4 and PopP2, are well studied, but how the RRS1/RPS4 complex activates early immediate downstream responses upon effector detection is still poorly understood. To study RRS1/RPS4 responses without the influence of cell surface receptor immune pathways, we generated an Arabidopsis line with inducible expression of the effector AvrRps4. Induction does not lead to hypersensitive cell death response (HR) but can induce electrolyte leakage, which often correlates with plant cell death. Activation of RRS1 and RPS4 without pathogens cannot activate mitogen-associated protein kinase cascades, but still activates up-regulation of defence genes, and therefore resistance against bacteria.

Chloroplasts and Plant Immunity: Where Are the Fungal Effectors?

Chloroplasts play a central role in plant immunity through the synthesis of secondary metabolites and defense compounds, as well as phytohormones, such as jasmonic acid and salicylic acid. Additionally, chloroplast metabolism results in the production of reactive oxygen species and nitric oxide as defense molecules. The impact of viral and bacterial infections on plastids and chloroplasts has been well documented. In particular, bacterial pathogens are known to introduce effectors specifically into chloroplasts, and many viral proteins interact with chloroplast proteins to influence viral replication and movement, and plant defense. By contrast, clear examples are just now emerging for chloroplast-targeted effectors from fungal and oomycete pathogens. In this review, we first present a brief overview of chloroplast contributions to plant defense and then discuss examples of connections between fungal interactions with plants and chloroplast function. We then briefly consider well-characterized bacterial effectors that target chloroplasts as a prelude to discussing the evidence for fungal effectors that impact chloroplast activities.

Genome analysis of Pseudomonas syringae pv. actinidiae biovar 6, which produces the phytotoxins, phaseolotoxin and coronatine

The kiwifruit bacterial canker pathogen, Pseudomonas syringae pv. actinidiae (Psa), causes enormous economic damages in many kiwifruit producing countries. In 2015, biovar 6, the novel biovar of Psa, was found in Nagano Prefecture, Japan. The genomes of two representative strains of biovar 6 (MAFF 212134 and MAFF 212141) were sequenced and analysed, indicating that their genomes are the most similar to that of biovar 3 among the known Psa biovars, based on average nucleotide identity analysis. Biovar 3 has neither the phaseolotoxin synthesis gene cluster nor the coronatine synthesis gene cluster, whereas biovar 6 has both clusters and produces both phytotoxins. We found that biovar 6 possesses 29 type III secreted effector (T3SE) genes, among which avrRps4 and hopBI1 are unique to biovar 6. The expression of T3SE genes and two phytotoxin synthesis gene clusters of biovar 6 during the early stages of host infection was investigated using RNA-Seq analysis, showing that these genes could be grouped into three categories: constantly expressed genes, constantly suppressed genes, and temporarily induced genes. A PCR assay was established to differentiate biovar 6 strains from the other Psa biovars and the closely related pathovar, pv. actinidifoliorum, by using avrRps4 as a biovar 6-specific marker gene.

Generating Transgenic Arabidopsis Plants for Functional Analysis of Pathogen Effectors and Corresponding R Proteins

Inducible expression of a pathogen effector has been proven to be a powerful strategy for dissecting its virulence and avirulence functions. However, leaky expression of some effector proteins can cause drastic physiological changes, such as growth retardation, accelerated senescence, and sterility. Unfortunately, leaky expression from current inducible vectors is unavoidable. To overcome these problems, a highly efficient Arabidopsis transformation protocol is described here, which allows the generation of hundreds to over a thousand T1 plants for selecting appropriate lines. In addition, since transgenic silencing is frequently observed, a principle for screening stable transgenic plants is also introduced.

Chloroplasts at the Crossroad of Photosynthesis, Pathogen Infection and Plant Defense

Photosynthesis, pathogen infection, and plant defense are three important biological processes that have been investigated separately for decades. Photosynthesis generates ATP, NADPH, and carbohydrates. These resources are utilized for the synthesis of many important compounds, such as primary metabolites, defense-related hormones abscisic acid, ethylene, jasmonic acid, and salicylic acid, and antimicrobial compounds. In plants and algae, photosynthesis and key steps in the synthesis of defense-related hormones occur in chloroplasts. In addition, chloroplasts are major generators of reactive oxygen species and nitric oxide, and a site for calcium signaling. These signaling molecules are essential to plant defense as well. All plants grown naturally are attacked by pathogens. Bacterial pathogens enter host tissues through natural openings or wounds. Upon invasion, bacterial pathogens utilize a combination of different virulence factors to suppress host defense and promote pathogenicity. On the other hand, plants have developed elaborate defense mechanisms to protect themselves from pathogen infections. This review summarizes recent discoveries on defensive roles of signaling molecules made by plants (primarily in their chloroplasts), counteracting roles of chloroplast-targeted effectors and phytotoxins elicited by bacterial pathogens, and how all these molecules crosstalk and regulate photosynthesis, pathogen infection, and plant defense, using chloroplasts as a major battlefield.

Comparative genomics of Pseudomonas syringae reveals convergent gene gain and loss associated with specialization onto cherry (Prunus avium)

Genome-wide analyses of the effector- and toxin-encoding genes were used to examine the phylogenetics and evolution of pathogenicity amongst diverse strains of Pseudomonas syringae causing bacterial canker of cherry (Prunus avium), including pathovars P. syringae pv morsprunorum (Psm) races 1 and 2, P. syringae pv syringae (Pss) and P. syringae pv avii. Phylogenetic analyses revealed Psm races and P. syringae pv avii clades were distinct and were each monophyletic, whereas cherry-pathogenic strains of Pss were interspersed amongst strains from other host species. A maximum likelihood approach was used to predict effectors associated with pathogenicity on cherry. Pss possesses a smaller repertoire of type III effectors but has more toxin biosynthesis clusters than Psm and P. syringae pv avii. Evolution of cherry pathogenicity was correlated with gain of genes such as hopAR1 and hopBB1 through putative phage transfer and horizontal transfer respectively. By contrast, loss of the avrPto/hopAB redundant effector group was observed in cherry-pathogenic clades. Ectopic expression of hopAB and hopC1 triggered the hypersensitive reaction in cherry leaves, confirming computational predictions. Cherry canker provides a fascinating example of convergent evolution of pathogenicity that is explained by the mix of effector and toxin repertoires acting on a common host.

Peripheral infrastructure vectors and an extended set of plant parts for the Modular Cloning system

Standardized DNA assembly strategies facilitate the generation of multigene constructs from collections of building blocks in plant synthetic biology. A common syntax for hierarchical DNA assembly following the Golden Gate principle employing Type IIs restriction endonucleases was recently developed, and underlies the Modular Cloning and GoldenBraid systems. In these systems, transcriptional units and/or multigene constructs are assembled from libraries of standardized building blocks, also referred to as phytobricks, in several hierarchical levels and by iterative Golden Gate reactions. Here, a toolkit containing further modules for the novel DNA assembly standards was developed. Intended for use with Modular Cloning, most modules are also compatible with GoldenBraid. Firstly, a collection of approximately 80 additional phytobricks is provided, comprising e.g. modules for inducible expression systems, promoters or epitope tags. Furthermore, DNA modules were developed for connecting Modular Cloning and Gateway cloning, either for toggling between systems or for standardized Gateway destination vector assembly. Finally, first instances of a "peripheral infrastructure" around Modular Cloning are presented: While available toolkits are designed for the assembly of plant transformation constructs, vectors were created to also use coding sequence-containing phytobricks directly in yeast two hybrid interaction or bacterial infection assays. The presented material will further enhance versatility of hierarchical DNA assembly strategies.

The bacterial type III-secreted protein AvrRps4 is a bipartite effector

Bacterial effector proteins secreted into host plant cells manipulate those cells to the benefit of the pathogen, but effector-triggered immunity (ETI) occurs when effectors are recognized by host resistance proteins. The RPS4/RRS1 pair recognizes the Pseudomonas syringae pv. pisi effector AvrRps4. AvrRps4 is processed in planta into AvrRps4^N (133 amino acids), homologous to the N-termini of other effectors including the native P. syringae pv. tomato strain DC3000 effector HopK1, and AvrRps4^C (88 amino acids). Previous data suggested that AvrRps4^C alone is necessary and sufficient for resistance when overexpressed in heterologous systems. We show that delivering AvrRps4^C from DC3000, but not from a DC3000 hopK1^- strain, triggers resistance in the Arabidopsis accession Col-0. Delivering AvrRps4^C in tandem with AvrRps4^N, or as a chimera with HopK1^N, fully complements AvrRps4-triggered immunity. AvrRps4^N in the absence of AvrRps4^C enhances virulence in Col-0. In addition, AvrRps4^N triggers a hypersensitive response in lettuce that is attenuated by coexpression of AvrRps4^C, further supporting the role of AvrRps4^N as a bona fide effector domain. Based on these results we propose that evolutionarily, fusion of AvrRps4^C to AvrRps4^N may have counteracted recognition of AvrRps4^N, and that the plant RPS4/RRS1 resistance gene pair was selected as a countermeasure. We conclude that AvrRps4 represents an unusual chimeric effector, with recognition in Arabidopsis by RPS4/RRS1 requiring the presence of both processed effector moieties.

An important component of the plant immune system relies on the detection of pathogen-derived effectors by immune receptors called resistance proteins. Bacterial pathogens inject effectors into the host cell via a dedicated secretion system to suppress defenses and to manipulate the physiology of the host cell to the pathogen's advantage. Usually, a single resistance protein recognizes a single effector, but an increasing number of exceptions and elaborations on this one-to-one relationship are known. The plant Arabidopsis uses a pair of resistance proteins, RRS1 and RPS4, to detect the effector AvrRps4. After injection into the cell, AvrRps4 is cleaved into two protein parts, and it had been assumed that only the C-terminal part needs to be present to trigger RPS4/RRS1. We show here that both AvrRps4 parts are required for triggering resistance in Arabidopsis, and that the N-terminal part, which previously had been assumed to only function in effector secretion into the host cell, in fact on its own has some functions of an effector. This conclusion is supported by the observation that the N-terminal part of AvrRps4 is sufficient to trigger resistance in lettuce. The fusion of the two AvrRps4 parts may have arisen to counteract plant defenses.

Plant-microbe interactions: organelles and the cytoskeleton in action

Contents Summary 1012 I. Introduction 1012 II. The endomembrane system in plant-microbe interactions 1013 III. The cytoskeleton in plant-microbe interactions 1017 IV. Organelles in plant-microbe interactions 1019 V. Inter-organellar communication in plant-microbe interactions 1022 VI. Conclusions and prospects 1023 Acknowledgements 1024 References 1024 SUMMARY: Plants have evolved a multilayered immune system with well-orchestrated defense strategies against pathogen attack. Multiple immune signaling pathways, coordinated by several subcellular compartments and interactions between these compartments, play important roles in a successful immune response. Pathogens use various strategies to either directly attack the plant's immune system or to indirectly manipulate the physiological status of the plant to inhibit an immune response. Microscopy-based approaches have allowed the direct visualization of membrane trafficking events, cytoskeleton reorganization, subcellular dynamics and inter-organellar communication during the immune response. Here, we discuss the contributions of organelles and the cytoskeleton to the plant's defense response against microbial pathogens, as well as the mechanisms used by pathogens to target these compartments to overcome the plant's defense barrier.

Constant vigilance: plant functions guarded by resistance proteins

Unlike animals, plants do not have an adaptive immune system and have instead evolved sophisticated and multi-layered innate immune mechanisms. To overcome plant immunity, pathogens secrete a diverse array of effectors into the apoplast and virtually all cellular compartments to dampen immune signaling and interfere with plant functions. Here we describe the scope of the arms race throughout the cell and summarize various strategies used by both plants and pathogens. Through studying the ongoing evolutionary battle between plants and key pathogens, we may yet uncover potential ways to achieve the ultimate goal of engineering broad-spectrum resistant crops without affecting food quality or productivity.

Non-host Resistance Induced by the Xanthomonas Effector XopQ Is Widespread within the Genus Nicotiana and Functionally Depends on EDS1

Most Gram-negative plant pathogenic bacteria translocate effector proteins (T3Es) directly into plant cells via a conserved type III secretion system, which is essential for pathogenicity in susceptible plants. In resistant plants, recognition of some T3Es is mediated by corresponding resistance (R) genes or R proteins and induces effector triggered immunity (ETI) that often results in programmed cell death reactions. The identification of R genes and understanding their evolution/distribution bears great potential for the generation of resistant crop plants. We focus on T3Es from Xanthomonas campestris pv. vesicatoria (Xcv), the causal agent of bacterial spot disease on pepper and tomato plants. Here, 86 Solanaceae lines mainly of the genus Nicotiana were screened for phenotypical reactions after Agrobacterium tumefaciens-mediated transient expression of 21 different Xcv effectors to (i) identify new plant lines for T3E characterization, (ii) analyze conservation/evolution of putative R genes and (iii) identify promising plant lines as repertoire for R gene isolation. The effectors provoked different reactions on closely related plant lines indicative of a high variability and evolution rate of potential R genes. In some cases, putative R genes were conserved within a plant species but not within superordinate phylogenetical units. Interestingly, the effector XopQ was recognized by several Nicotiana spp. lines, and Xcv infection assays revealed that XopQ is a host range determinant in many Nicotiana species. Non-host resistance against Xcv and XopQ recognition in N. benthamiana required EDS1, strongly suggesting the presence of a TIR domain-containing XopQ-specific R protein in these plant lines. XopQ is a conserved effector among most xanthomonads, pointing out the XopQ-recognizing R_xopQ as candidate for targeted crop improvement.

Differential response of tomato genotypes to Xanthomonas-specific pathogen-associated molecular patterns and correlation with bacterial spot (Xanthomonas perforans) resistance

Plants depend on innate immune responses to retard the initial spread of pathogens entering through stomata, hydathodes or injuries. These responses are triggered by conserved patterns in pathogen-encoded molecules known as pathogen-associated molecular patterns (PAMPs). Production of reactive oxygen species (ROS) is one of the first responses, and the resulting 'oxidative burst' is considered to be a first line of defense. In this study, we conducted association analyses between ROS production and bacterial spot (BS; Xanthomonas spp.) resistance in 63 genotypes of tomato (Solanum lycopersicum L.). A luminol-based assay was performed on leaf tissues that had been treated with a flagellin 22 (flg22), flagellin 28 and a Xanthomonas-specific flg22 (flg22-Xac) peptide, to measure PAMP-induced ROS production in each genotype. These genotypes were also assessed for BS disease response by inoculation with Xanthomonas perforans, race T4. Although there was no consistent relationship between peptides used and host response to the BS, there was a significant negative correlation (r=-0.25, P<0.05) between foliar disease severity and ROS production, when flg22-Xac was used. This response could potentially be used to identify the Xanthomonas-specific PRR allele in tomato, and eventually PAMP-triggered immunity loci could be mapped in a segregating population. This has potential significance in tomato improvement.

Go in for the kill: How plants deploy effector-triggered immunity to combat pathogens. [Corrected]

Plant resistance (R) proteins perceive specific pathogen effectors from diverse plant pathogens to initiate defense responses, designated effector-triggered immunity (ETI). Plant R proteins are mostly nucleotide binding-leucine rich repeat (NB-LRR) proteins, which recognize pathogen effectors directly or indirectly through sophisticated mechanisms. Upon activation by effector proteins, R proteins elicit robust defense responses, including a rapid burst of reactive oxygen species (ROS), induced biosynthesis and accumulation of salicylic acid (SA), a rapid programmed cell death (PCD) called hypersensitive response (HR) at the infection sites, and increased expression of pathogenesis-related (PR) genes. Initiation of ETI is correlated with a complex network of defense signaling pathways, resulting in defensive cellular responses and large-scale transcriptional reprogramming events. In this review, we highlight important recent advances on the recognition of effectors, regulation and activation of plant R proteins, dynamic intracellular trafficking of R proteins, induction of cell death, and transcriptional reprogramming associated with ETI. Current knowledge gaps and future research directions are also discussed in this review.

A Plant Immune Receptor Detects Pathogen Effectors that Target WRKY Transcription Factors

Defense against pathogens in multicellular eukaryotes depends on intracellular immune receptors, yet surveillance by these receptors is poorly understood. Several plant nucleotide-binding, leucine-rich repeat (NB-LRR) immune receptors carry fusions with other protein domains. The Arabidopsis RRS1-R NB-LRR protein carries a C-terminal WRKY DNA binding domain and forms a receptor complex with RPS4, another NB-LRR protein. This complex detects the bacterial effectors AvrRps4 or PopP2 and then activates defense. Both bacterial proteins interact with the RRS1 WRKY domain, and PopP2 acetylates lysines to block DNA binding. PopP2 and AvrRps4 interact with other WRKY domain-containing proteins, suggesting these effectors interfere with WRKY transcription factor-dependent defense, and RPS4/RRS1 has integrated a "decoy" domain that enables detection of effectors that target WRKY proteins. We propose that NB-LRR receptor pairs, one member of which carries an additional protein domain, enable perception of pathogen effectors whose function is to target that domain.

Two linked pairs of Arabidopsis TNL resistance genes independently confer recognition of bacterial effector AvrRps4

Plant immunity requires recognition of pathogen effectors by intracellular NB-LRR immune receptors encoded by Resistance (R) genes. Most R proteins recognize a specific effector, but some function in pairs that recognize multiple effectors. Arabidopsis thaliana TIR-NB-LRR proteins RRS1-R and RPS4 together recognize two bacterial effectors, AvrRps4 from Pseudomonas syringae and PopP2 from Ralstonia solanacearum. However, AvrRps4, but not PopP2, is recognized in rrs1/rps4 mutants. We reveal an R gene pair that resembles and is linked to RRS1/RPS4, designated as RRS1B/RPS4B, which confers recognition of AvrRps4 but not PopP2. Like RRS1/RPS4, RRS1B/RPS4B proteins associate and activate defence genes upon AvrRps4 recognition. Inappropriate combinations (RRS1/RPS4B or RRS1B/RPS4) are non-functional and this specificity is not TIR domain dependent. Distinct putative orthologues of both pairs are maintained in the genomes of Arabidopsis thaliana relatives and are likely derived from a common ancestor pair. Our results provide novel insights into paired R gene function and evolution.

The nuclear immune receptor RPS4 is required for RRS1SLH1-dependent constitutive defense activation in Arabidopsis thaliana

Plant nucleotide-binding leucine-rich repeat (NB-LRR) disease resistance (R) proteins recognize specific “avirulent” pathogen effectors and activate immune responses. NB-LRR proteins structurally and functionally resemble mammalian Nod-like receptors (NLRs). How NB-LRR and NLR proteins activate defense is poorly understood. The divergently transcribed Arabidopsis R genes, RPS4 (resistance to Pseudomonas syringae 4) and RRS1 (resistance to Ralstonia solanacearum 1), function together to confer recognition of Pseudomonas AvrRps4 and Ralstonia PopP2. RRS1 is the only known recessive NB-LRR R gene and encodes a WRKY DNA binding domain, prompting suggestions that it acts downstream of RPS4 for transcriptional activation of defense genes. We define here the early RRS1-dependent transcriptional changes upon delivery of PopP2 via Pseudomonas type III secretion. The Arabidopsis slh1 (sensitive to low humidity 1) mutant encodes an RRS1 allele (RRS1^SLH1) with a single amino acid (leucine) insertion in the WRKY DNA-binding domain. Its poor growth due to constitutive defense activation is rescued at higher temperature. Transcription profiling data indicate that RRS1^SLH1-mediated defense activation overlaps substantially with AvrRps4- and PopP2-regulated responses. To better understand the genetic basis of RPS4/RRS1-dependent immunity, we performed a genetic screen to identify suppressor ofslh1 immunity (sushi) mutants. We show that many sushi mutants carry mutations in RPS4, suggesting that RPS4 acts downstream or in a complex with RRS1. Interestingly, several mutations were identified in a domain C-terminal to the RPS4 LRR domain. Using an Agrobacterium-mediated transient assay system, we demonstrate that the P-loop motif of RPS4 but not of RRS1^SLH1 is required for RRS1^SLH1 function. We also recapitulate the dominant suppression of RRS1^SLH1 defense activation by wild type RRS1 and show this suppression requires an intact RRS1 P-loop. These analyses of RRS1^SLH1 shed new light on mechanisms by which NB-LRR protein pairs activate defense signaling, or are held inactive in the absence of a pathogen effector.

How plant NB-LRR resistance proteins and the related mammalian Nod-like receptors (NLRs) activate defense is poorly understood. Plant and animal immune receptors can function in pairs. Two Arabidopsis nuclear immune receptors, RPS4 and RRS1, confer recognition of the unrelated bacterial effectors, AvrRps4 and PopP2, and activate defense. Using delivery of PopP2 into Arabidopsis leaf cells via Pseudomonas type III secretion, we define early transcriptional changes upon RPS4/RRS1-dependent PopP2 recognition. We show an auto-active allele of RRS1, RRS1^SLH1, triggers transcriptional reprogramming of defense genes that are also reprogrammed by AvrRps4 or PopP2 in an RPS4/RRS1-dependent manner. To discover genetic requirements for RRS1^SLH1 auto-activation, we conducted a suppressor screen. Many suppressor of slh1 immunity (sushi) mutants that are impaired in RRS1^SLH1-mediated auto-activation carry loss-of-function mutations in RPS4. This suggests that RPS4 functions as a signaling component together with or downstream of RRS1-activated immunity, in contrast to earlier hypotheses, significantly advancing our understanding of how immune receptors activate defense in plants.

Structural basis for assembly and function of a heterodimeric plant immune receptor

Cytoplasmic plant immune receptors recognize specific pathogen effector proteins and initiate effector-triggered immunity. In Arabidopsis, the immune receptors RPS4 and RRS1 are both required to activate defense to three different pathogens. We show that RPS4 and RRS1 physically associate. Crystal structures of the N-terminal Toll-interleukin-1 receptor/resistance (TIR) domains of RPS4 and RRS1, individually and as a heterodimeric complex (respectively at 2.05, 1.75, and 2.65 angstrom resolution), reveal a conserved TIR/TIR interaction interface. We show that TIR domain heterodimerization is required to form a functional RRS1/RPS4 effector recognition complex. The RPS4 TIR domain activates effector-independent defense, which is inhibited by the RRS1 TIR domain through the heterodimerization interface. Thus, RPS4 and RRS1 function as a receptor complex in which the two components play distinct roles in recognition and signaling.

Distinct Pseudomonas type-III effectors use a cleavable transit peptide to target chloroplasts

The pathogen Pseudomonas syringae requires a type-III protein secretion system and the effector proteins it injects into plant cells for pathogenesis. The primary role for P. syringae type-III effectors is the suppression of plant immunity. The P. syringae pv. tomato DC3000 HopK1 type-III effector was known to suppress the hypersensitive response (HR), a programmed cell death response associated with effector-triggered immunity. Here we show that DC3000 hopK1 mutants are reduced in their ability to grow in Arabidopsis, and produce reduced disease symptoms. Arabidopsis transgenically expressing HopK1 are reduced in PAMP-triggered immune responses compared with wild-type plants. An N-terminal region of HopK1 shares similarity with the corresponding region in the well-studied type-III effector AvrRps4; however, their C-terminal regions are dissimilar, indicating that they have different effector activities. HopK1 is processed in planta at the same processing site found in AvrRps4. The processed forms of HopK1 and AvrRps4 are chloroplast localized, indicating that the shared N-terminal regions of these type-III effectors represent a chloroplast transit peptide. The HopK1 contribution to virulence and the ability of HopK1 and AvrRps4 to suppress immunity required their respective transit peptides, but the AvrRps4-induced HR did not. Our results suggest that a primary virulence target of these type-III effectors resides in chloroplasts, and that the recognition of AvrRps4 by the plant immune system occurs elsewhere. Moreover, our results reveal that distinct type-III effectors use a cleavable transit peptide to localize to chloroplasts, and that targets within this organelle are important for immunity.

On the front line: structural insights into plant-pathogen interactions

Over the past decade, considerable advances have been made in understanding the molecular mechanisms that underpin the arms race between plant pathogens and their hosts. Alongside genomic, bioinformatic, proteomic, biochemical and cell biological analyses of plant-pathogen interactions, three-dimensional structural studies of virulence proteins deployed by pathogens to promote infection, in some cases complexed with their plant cell targets, have uncovered key insights into the functions of these molecules. Structural information on plant immune receptors, which regulate the response to pathogen attack, is also starting to emerge. Structural studies of bacterial plant pathogen-host systems have been leading the way, but studies of filamentous plant pathogens are gathering pace. In this Review, we summarize the key developments in the structural biology of plant pathogen-host interactions.

New clues in the nucleus: transcriptional reprogramming in effector-triggered immunity

The robustness of plant effector-triggered immunity is correlated with massive alterations of the host transcriptome. Yet the molecular mechanisms that cause and underlie this reprogramming remain obscure. Here we will review recent advances in deciphering nuclear functions of plant immune receptors and of associated proteins. Important open questions remain, such as the identities of the primary transcription factors involved in control of effector-triggered immune responses, and indeed whether this can be generalized or whether particular effector-resistance protein interactions impinge on distinct sectors in the transcriptional response web. Multiple lines of evidence have implicated WRKY transcription factors at the core of responses to microbe-associated molecular patterns and in intersections with effector-triggered immunity. Recent findings from yeast two-hybrid studies suggest that members of the TCP transcription factor family are targets of several effectors from diverse pathogens. Additional transcription factor families that are directly or indirectly involved in effector-triggered immunity are likely to be identified.

In planta effector competition assays detect Hyaloperonospora arabidopsidis effectors that contribute to virulence and localize to different plant subcellular compartments

The genome of the pathogenic oomycete Hyaloperonospora arabidopsidis is predicted to encode at least 134 high-confidence effectors (HaRxL) carrying the RxLR motif implicated in their translocation into plant cells. However, only four avirulence genes (ATR1, ATR13, ATR5, and ATR39) have been isolated. This indicates that identification of HaRxL effectors based on avirulence is low throughput. We aimed at rapidly identifying H. arabidopsidis effectors that contribute to virulence by developing methods to detect and quantify multiple candidates in bacterial mixed infections using either Illumina sequencing or capillary electrophoresis. In these assays, referred to here as in planta effector competition assays, we estimate the contribution to virulence of individual effectors by calculating the abundance of each HaRxL in the bacterial population recovered from leaves 3 days after inoculation relative to abundance in the initial mixed inoculum. We identified HaRxL that enhance Pseudomonas syringae pv. tomato DC3000 growth in some but not all Arabidopsis accessions. Further analysis showed that HaRxLL464, HaRxL75, HaRxL22, HaRxLL441, and HaRxL89 suppress pathogen-associated molecular pattern-triggered immunity (PTI) and localize to different subcellular compartments in Nicotiana benthamiana, providing evidence for a multilayered suppression of PTI by pathogenic oomycetes and molecular probes for the dissection of PTI.

Arabidopsis TNL-WRKY domain receptor RRS1 contributes to temperature-conditioned RPS4 auto-immunity

In plant effector-triggered immunity (ETI), intracellular nucleotide binding-leucine rich repeat (NLR) receptors are activated by specific pathogen effectors. The Arabidopsis TIR (Toll-Interleukin-1 receptor domain)-NLR (denoted TNL) gene pair, RPS4 and RRS1, confers resistance to Pseudomonas syringae pv tomato (Pst) strain DC3000 expressing the Type III-secreted effector, AvrRps4. Nuclear accumulation of AvrRps4, RPS4, and the TNL resistance regulator EDS1 is necessary for ETI. RRS1 possesses a C-terminal "WRKY" transcription factor DNA binding domain suggesting that important RPS4/RRS1 recognition and/or resistance signaling events occur at the nuclear chromatin. In Arabidopsis accession Ws-0, the RPS4(Ws) /RRS1(Ws) allelic pair governs resistance to Pst/AvrRps4 accompanied by host programed cell death (pcd). In accession Col-0, RPS4(Col) /RRS1(Col) effectively limits Pst/AvrRps4 growth without pcd. Constitutive expression of HA-StrepII tagged RPS4(Col) (in a 35S:RPS4-HS line) confers temperature-conditioned EDS1-dependent auto-immunity. Here we show that a high (28°C, non-permissive) to moderate (19°C, permissive) temperature shift of 35S:RPS4-HS plants can be used to follow defense-related transcriptional dynamics without a pathogen effector trigger. By comparing responses of 35S:RPS4-HS with 35S:RPS4-HS rrs1-11 and 35S:RPS4-HS eds1-2 mutants, we establish that RPS4(Col) auto-immunity depends entirely on EDS1 and partially on RRS1(Col) . Examination of gene expression microarray data over 24 h after temperature shift reveals a mainly quantitative RRS1(Col) contribution to up- or down-regulation of a small subset of RPS4(Col) -reprogramed, EDS1-dependent genes. We find significant over-representation of WRKY transcription factor binding W-box cis-elements within the promoters of these genes. Our data show that RRS1(Col) contributes to temperature-conditioned RPS4(Col) auto-immunity and are consistent with activated RPS4(Col) engaging RRS1(Col) for resistance signaling.

Distinct regions of the Pseudomonas syringae coiled-coil effector AvrRps4 are required for activation of immunity

Gram-negative phytopathogenic bacteria translocate effector proteins into plant cells to subvert host defenses. These effectors can be recognized by plant nucleotide-binding-leucine-rich repeat immune receptors, triggering defense responses that restrict pathogen growth. AvrRps4, an effector protein from Pseudomonas syringae pv. pisi, triggers RPS4-dependent immunity in resistant accessions of Arabidopsis. To better understand the molecular basis of AvrRps4-triggered immunity, we determined the crystal structure of processed AvrRps4 (AvrRps4(C), residues 134-221), revealing that it forms an antiparallel α-helical coiled coil. Structure-informed mutagenesis reveals an electronegative surface patch in AvrRps4(C) required for recognition by RPS4; mutations in this region can also uncouple triggering of the hypersensitive response from disease resistance. This uncoupling may result from a lower level of defense activation, sufficient for avirulence but not for triggering a hypersensitive response. Natural variation in AvrRps4 reveals distinct recognition specificities that involve a surface-exposed residue. Recently, a direct interaction between AvrRps4 and Enhanced Disease Susceptibility 1 has been implicated in activation of immunity. However, we were unable to detect direct interaction between AvrRps4 and Enhanced Disease Susceptibility 1 after coexpression in Nicotiana benthamiana or in yeast cells. How intracellular plant immune receptors activate defense upon effector perception remains an unsolved problem. The structure of AvrRps4(C), and identification of functionally important residues for its activation of plant immunity, advances our understanding of these processes in a well-defined model pathosystem.

HopAS1 recognition significantly contributes to Arabidopsis nonhost resistance to Pseudomonas syringae pathogens

• Plant immunity is activated by sensing either conserved microbial signatures, called pathogen/microbe-associated molecular patterns (P/MAMPs), or specific effectors secreted by pathogens. However, it is not known why most microbes are nonpathogenic in most plant species. • Nonhost resistance (NHR) consists of multiple layers of innate immunity and protects plants from the vast majority of potentially pathogenic microbes. Effector-triggered immunity (ETI) has been implicated in race-specific disease resistance. However, the role of ETI in NHR is unclear. • Pseudomonas syringae pv. tomato (Pto) T1 is pathogenic in tomato (Solanum lycopersicum) yet nonpathogenic in Arabidopsis. Here, we show that, in addition to the type III secretion system (T3SS)-dependent effector (T3SE) avrRpt2, a second T3SE of Pto T1, hopAS1, triggers ETI in nonhost Arabidopsis. • hopAS1 is broadly present in P. syringae strains, contributes to virulence in tomato, and is quantitatively required for Arabidopsis NHR to Pto T1. Strikingly, all tested P. syringae strains that are pathogenic in Arabidopsis carry truncated hopAS1 variants of forms, demonstrating that HopAS1-triggered immunity plays an important role in Arabidopsis NHR to a broad-range of P. syringae strains.

Arabidopsis EDS1 connects pathogen effector recognition to cell compartment-specific immune responses

Pathogen effectors are intercepted by plant intracellular nucleotide binding-leucine-rich repeat (NB-LRR) receptors. However, processes linking receptor activation to downstream defenses remain obscure. Nucleo-cytoplasmic basal resistance regulator EDS1 (ENHANCED DISEASE SUSCEPTIBILITY1) is indispensible for immunity mediated by TIR (Toll-interleukin-1 receptor)-NB-LRR receptors. We show that Arabidopsis EDS1 molecularly connects TIR-NB-LRR disease resistance protein RPS4 recognition of bacterial effector AvrRps4 to defense pathways. RPS4-EDS1 and AvrRps4-EDS1 complexes are detected inside nuclei of living tobacco cells after transient coexpression and in Arabidopsis soluble leaf extracts after resistance activation. Forced AvrRps4 localization to the host cytoplasm or nucleus reveals cell compartment-specific RPS4-EDS1 defense branches. Although nuclear processes restrict bacterial growth, programmed cell death and transcriptional resistance reinforcement require nucleo-cytoplasmic coordination. Thus, EDS1 behaves as an effector target and activated TIR-NB-LRR signal transducer for defenses across cell compartments.

Pathogen effectors target Arabidopsis EDS1 and alter its interactions with immune regulators

Plant resistance proteins detect the presence of specific pathogen effectors and initiate effector-triggered immunity. Few immune regulators downstream of resistance proteins have been identified, none of which are known virulence targets of effectors. We show that Arabidopsis ENHANCED DISEASE SUSCEPTIBILITY1 (EDS1), a positive regulator of basal resistance and of effector-triggered immunity specifically mediated by Toll-interleukin-1 receptor-nucleotide binding-leucine-rich repeat (TIR-NB-LRR) resistance proteins, forms protein complexes with the TIR-NB-LRR disease resistance proteins RPS4 and RPS6 and with the negative immune regulator SRFR1 at a cytoplasmic membrane. Further, the cognate bacterial effectors AvrRps4 and HopA1 disrupt these EDS1 complexes. Tight association of EDS1 with TIR-NB-LRR-mediated immunity may therefore derive mainly from being guarded by TIR-NB-LRR proteins, and activation of this branch of effector-triggered immunity may directly connect to the basal resistance signaling pathway via EDS1.

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.

Dual roles for the variable domain in protein trafficking and host-specific recognition of Heterodera glycines CLE effector proteins

*Soybean cyst nematodes (Heterodera glycines) produce secreted effector proteins that function as peptide mimics of plant CLAVATA3/ESR (CLE)-like peptides probably involved in the developmental reprogramming of root cells to form specialized feeding cells called syncytia. *The site of action and mechanism of delivery of CLE effectors to host plant cells by the nematode, however, have not been established. In this study, immunologic, genetic and biochemical approaches were used to reveal the localization and site of action of H. glycines-secreted CLE proteins in planta. *We present evidence indicating that the nematode CLE propeptides are delivered to the cytoplasm of syncytial cells, but ultimately function in the apoplast, consistent with their proposed role as ligand mimics of plant CLE peptides. We determined that the nematode 12-amino-acid CLE motif peptide is not sufficient for biological activity in vivo, pointing to an important role for sequences upstream of the CLE motif in function. *Genetic and biochemical analysis confirmed the requirement of the variable domain in planta for host-specific recognition and revealed a novel role in trafficking cytoplasmically delivered CLEs to the apoplast in order to function as ligand mimics.

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.