Identification of Pseudomonas syringae type III effectors that can suppress programmed cell death in plants and yeast

Virulent Effector Hasp155 of Puccinia striiformis f. sp. tritici Suppresses Plant Immunity and Promotes Fungus Infection

As a kind of obligate biotrophic fungus, Puccinia striiformis f. sp. tritici (Pst) secretes vast effectors via haustoria to host cells during the infection to inhibit host defense responses and promote fungal invasion. In this study, based on the completion of genome sequencing and haustorial transcriptome sequencing of Pst, we identified a Pst effector (Hasp155) that is significantly induced in the early stage of Pst infection to wheat. The 18 N-terminal amino acids of Hasp155 encoded a signal peptide with a secretory function. Transient expression of Hasp155 in Nicotiana benthamiana inhibited Bax-induced cell death as well as chitin-triggered callose deposition and defense-related gene expression. Moreover, delivery of the Hasp155 protein into wheat cells via type three secretion systems (TTSS) led to reduced plant immunity to nonpathogenic bacteria and to the avirulent Pst race with decreased H2O2 accumulation and promoted Pst development. Furthermore, transgenic overexpression of Hasp155 significantly renders wheat resistance susceptible, resulting in a decreased defense response and increased Pst pathogenicity. Overall, these results indicate that Hasp155 is an important effector of Pst pathogenicity by suppressing plant immunity.

Assessing the efficacy of phyllospheric growth-promoting and antagonistic bacteria for management of black rot disease of cauliflower incited by Xanthomonas campestris pv. campestris

The study aimed to assess the potential of phyllospheric bacterial strains isolated from cauliflower plants as biocontrol agents against black rot disease caused by Xanthomonas campestris pv. campestris, through both in vitro and in vivo evaluations. A total of 46 bacterial strains were isolated from healthy and infected cauliflower leaves of both resistant and susceptible plants, and evaluated them for various traits, including plant growth-promoting activities and in vitro antagonistic activity against Xanthomonas campestris pv. campestris. Further, a pot experiment was conducted with the susceptible cauliflower genotype (Pusa Sharad) and 10 selected phyllospheric bacterial isolates to assess their biocontrol efficacy against the disease. The results showed that 82.60% of phyllospheric bacterial isolates were positive for phosphate solubilization, 63.04% for ammonia production, 58.69% for HCN production, 36.95% for siderophore production, and 78.26% had the capacity to produce IAA. Out of the 46 isolates, 23 exhibited in vitro antagonistic activity against X. campestris pv. campestris and 10 isolates were selected for a pot experiment under glasshouse conditions based on their good plant growth-promoting activities and antagonistic assay. The results revealed that bacterial isolate CFLB-27 exhibited the highest biocontrol efficiency (65.41%), followed by CFLB-24 (58.30%), CFLB-31 (47.11%), and CFLB-26 (46.03%). These four isolates were identified as Pseudomonas fluorescens CFLB-27, Bacillus velezensis CFLB-24, Bacillus amyloliquefaciens CFLB-31, and Stenotrophomonas rhizophila CFLB-26. This study provides valuable insights into the potential of phyllospheric bacteria as an effective tool for disease management in sustainable agriculture.

Systematic identification and functional characterization of the CFEM proteins in fishscale bamboo rhombic-spot pathogen Neostagonosporella sichuanensis

Fungal effectors play a crucial role in the interaction between pathogenic fungi and their hosts. These interactions directly influence the invasion and spread of pathogens, and the development of diseases. Common in fungal extracellular membrane (CFEM) effectors are closely associated with the pathogenicity, cell wall stability, and pathogenic processes of pathogenic fungi. The aim of this study was to investigate the role of CFEM proteins in Neostagonosporella sichuanensis in pathogen-host interactions. We retrieved 19 proteins containing CFEM structural domains from the genome of N. sichuanensis. By systematic analysis, five NsCFEM proteins had signal peptides but lacked transmembrane structural domains, and thus were considered as potential effectors. Among them, NsCFEM1 and NsCFEM2 were successfully cloned and their functions were further investigated. The validation results show that NsCFEM1 was localized in the cell membrane and nucleus, whereas NsCFEM2 was exclusively observed in the cell membrane. Both were identified as secreted proteins. Additionally, NsCFEM1 inhibited Bax-induced programmed cell death in Nicotiana benthamiana, whereas NsCFEM2 did not induce or inhibit this response. NsCFEM1 was implicated as a virulence factor that contributes to fungal growth, development, stress response, and pathogenicity. NsCFEM2 was implicated in maintenance of cell wall stability. This study lays a foundation for elucidating the role of CFEM proteins in the pathogen of fishscale bamboo rhombic-spot caused by N. sichuanensis. In particular, the functional studies of NsCFEM1 and NsCFEM2 revealed their potential roles in the interaction between N. sichuanensis and the host Phyllostachys heteroclada.

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.

Stripe Rust Effector Pst_9302 Inhibits Wheat Immunity to Promote Susceptibility

Puccinia striiformis f. sp. tritici is an obligate biotrophic fungus that causes destructive stripe rust disease in wheat. During infection, Pst secretes virulence effectors via a specific infection structure-the haustorium-inside host cells to disturb host immunity and promote fungal colonization and expansion. Hence, the identification and functional analyses of Pst effectors are of great significance in deciphering the Pst pathogenicity mechanism. Here, we identified one candidate Pst effector Pst_9302 that could suppress Bax-triggered cell death in Nicotiana benthamiana. qRT-PCR analyses showed that the transcript levels of Pst_9302 were highly increased during the early infection stages of Pst. The transient expression of Pst_9302 in wheat via the type-three secretion system (T3SS) significantly inhibited the callose deposition induced by Pseudomonas syringae EtHAn. During wheat-Pst interaction, Pst_9302 overexpression suppressed reactive oxygen species (ROS) accumulation and cell death caused by the avirulent Pst race CYR23. The host-induced gene silencing (HIGS) of Pst_9302 resulted in decreased Pst pathogenicity with reduced infection area. The results suggest that Pst_9302 plays a virulence role in suppressing plant immunity and promoting Pst pathogenicity. Moreover, wheat voltage-dependent anion channel 1 protein (TaVDAC1) was identified as candidate Pst_9302-interacting proteins by yeast two-hybrid (Y2H) screening. Pull-down assays using the His-Pst_9302 and GST-TaVDAC1 protein verified their interactions. These results suggest that Pst_9302 may modulate wheat TaVDAC1 to regulate plant immunity.

Suppression of NLR-mediated plant immune detection by bacterial pathogens

The plant immune system is constituted of two functionally interdependent branches that provide the plant with an effective defense against microbial pathogens. They can be considered separate since one detects extracellular pathogen-associated molecular patterns by means of receptors on the plant surface, while the other detects pathogen-secreted virulence effectors via intracellular receptors. Plant defense depending on both branches can be effectively suppressed by host-adapted microbial pathogens. In this review we focus on bacterially driven suppression of the latter, known as effector-triggered immunity (ETI) and dependent on diverse NOD-like receptors (NLRs). We examine how some effectors secreted by pathogenic bacteria carrying type III secretion systems can be subject to specific NLR-mediated detection, which can be evaded by the action of additional co-secreted effectors (suppressors), implying that virulence depends on the coordinated action of the whole repertoire of effectors of any given bacterium and their complex epistatic interactions within the plant. We consider how ETI activation can be avoided by using suppressors to directly alter compromised co-secreted effectors, modify plant defense-associated proteins, or occasionally both. We also comment on the potential assembly within the plant cell of multi-protein complexes comprising both bacterial effectors and defense protein targets.

Engineering Agrobacterium tumefaciens with a Type III Secretion System to Express Type III Effectors

Plants elicit defense responses when exposed to pathogens, which partly contribute to the resistance of plants to Agrobacterium tumefaciens-mediated transformation. Some pathogenic bacteria have sophisticated mechanisms to counteract these defense responses by injecting Type III effectors (T3Es) through the Type III secretion system (T3SS). By engineering A. tumefaciens to express T3SS to deliver T3Es, we suppressed plant defense and enhanced plant genetic transformation. Here, we describe the optimized protocols for mobilization of T3SS-expressing plasmid to engineer A. tumefaciens to deliver proteins through T3SS and fractionation of cultures to study proteins from pellet and supernatants to determine protein secretion from engineered A. tumefaciens.

Contrasting effector profiles between bacterial colonisers of kiwifruit reveal redundant roles converging on PTI-suppression and RIN4

Testing effector knockout strains of the Pseudomonas syringae pv. actinidiae biovar 3 (Psa3) for reduced in planta growth in their native kiwifruit host revealed a number of nonredundant effectors that contribute to Psa3 virulence. Conversely, complementation in the weak kiwifruit pathogen P. syringae pv. actinidifoliorum (Pfm) for increased growth identified redundant Psa3 effectors. Psa3 effectors hopAZ1a and HopS2b and the entire exchangeable effector locus (ΔEEL; 10 effectors) were significant contributors to bacterial colonisation of the host and were additive in their effects on virulence. Four of the EEL effectors (HopD1a, AvrB2b, HopAW1a and HopD2a) redundantly contribute to virulence through suppression of pattern-triggered immunity (PTI). Important Psa3 effectors include several redundantly required effectors early in the infection process (HopZ5a, HopH1a, AvrPto1b, AvrRpm1a and HopF1e). These largely target the plant immunity hub, RIN4. This comprehensive effector profiling revealed that Psa3 carries robust effector redundancy for a large portion of its effectors, covering a few functions critical to disease.

Manipulation of the Host Endomembrane System by Bacterial Effectors

The endomembrane system, extending from the nuclear envelope to the plasma membrane, is critical to the plant response to pathogen infection. Synthesis and transport of immunity-related proteins and antimicrobial compounds to and from the plasma membrane are supported by conventional and unconventional processes of secretion and internalization of vesicles, guided by the cytoskeleton networks. Although plant bacterial pathogens reside mostly in the apoplast, major structural and functional modifications of the endomembrane system in the host cell occur during bacterial infection. Here, we review the dynamics of these cellular compartments, briefly, for their essential contributions to the plant defense responses and, in parallel, for their emerging roles in bacterial pathogenicity. We further focus on Pseudomonas syringae, Xanthomonas spp., and Ralstonia solanacearum type III secreted effectors that one or both localize to and associate with components of the host endomembrane system or the cytoskeleton network to highlight the diversity of virulence strategies deployed by bacterial pathogens beyond the inhibition of the secretory pathway. [Formula: see text] Copyright © 2023 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Cooperative virulence via the collective action of secreted pathogen effectors

Although virulence is typically attributed to single pathogenic strains, here we investigated whether effectors secreted by a population of non-virulent strains could function as public goods to enable the emergence of collective virulence. We disaggregated the 36 type III effectors of the phytopathogenic bacterium Pseudomonas syringae strain PtoDC3000 into a 'metaclone' of 36 coisogenic strains, each carrying a single effector in an effectorless background. Each coisogenic strain was individually unfit, but the metaclone was collectively as virulent as the wild-type strain on Arabidopsis thaliana, suggesting that effectors can drive the emergence of cooperation-based virulence through their public action. We show that independently evolved effector suits can equally drive this cooperative behaviour by transferring the effector alleles native to the strain PmaES4326 into the conspecific but divergent strain PtoDC3000. Finally, we transferred the disaggregated PtoDC3000 effector arsenal into Pseudomonas fluorescens and show that their cooperative action was sufficient to convert this rhizosphere-inhabiting beneficial bacterium into a phyllosphere pathogen. These results emphasize the importance of microbial community interactions and expand the ecological scale at which disease may be attributed.

Two Candidate Meloidogyne javanica Effector Genes, MjShKT and MjPUT3: A Functional Investigation of Their Roles in Regulating Nematode Parasitism

During parasitism, root-knot nematode Meloidogyne spp. inject molecules termed effectors that have multifunctional roles in construction and maintenance of nematode feeding sites. As an outcome of transcriptomic analysis of Meloidogyne javanica, we identified and characterized two differentially expressed genes encoding the predicted proteins MjShKT, carrying a Stichodactyla toxin (ShKT) domain, and MjPUT3, carrying a ground-like domain, both expressed during nematode parasitism of the tomato plant. Fluorescence in-situ hybridization revealed expression of MjShKT and MjPUT3 in the dorsal esophageal glands, suggesting their injection into host cells. MjShKT expression was upregulated during the parasitic life stages, to a maximum at the mature female stage, whereas MjPUT3 expression increased in third- to fourth-stage juveniles. Subcellular in-planta localization of MjShKT and MjPUT3 using a fused fluorescence marker indicated MjShKT co-occurrence with the endoplasmic reticulum, the perinuclear endoplasmatic reticulum, and the Golgi organelle markers, while MjPUT3 localized, to some extent, within the endoplasmatic reticulum and was clearly observed within the nucleoplasm. MjShKT inhibited programmed cell death induced by overexpression of MAPKKKα and Gpa2/RBP-1. Overexpression of MjShKT in tomato hairy roots allowed an increase in nematode reproduction, as indicated by the high number of eggs produced on roots overexpressing MjShKT. Roots overexpressing MjPUT3 were characterized by enhanced root growth, with no effect on nematode development on those roots. Investigation of the two candidate effectors suggested that MjShKT is mainly involved in manipulating the plant effector-triggered immune response toward establishment and maintenance of active feeding sites, whereas MjPUT3 might modulate roots morphology in favor of nematode fitness in the host roots. [Formula: see text] Copyright © 2023 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

The Puccinia striiformis effector Hasp98 facilitates pathogenicity by blocking the kinase activity of wheat TaMAPK4

The obligate biotrophic fungus Puccinia striiformis f. sp. tritici (Pst) employs virulence effectors to disturb host immunity and causes devastating stripe rust disease. However, our understanding of how Pst effectors regulate host defense responses remains limited. In this study, we determined that the Pst effector Hasp98, which is highly expressed in Pst haustoria, inhibits plant immune responses triggered by flg22 or nonpathogenic bacteria. Overexpression of Hasp98 in wheat (Triticum aestivum) suppressed avirulent Pst-triggered immunity, leading to decreased H₂ O₂ accumulation and promoting P. striiformis infection, whereas stable silencing of Hasp98 impaired P. striiformis pathogenicity. Hasp98 interacts with the wheat mitogen-activated protein kinase TaMAPK4, a positive regulator of plant resistance to stripe rust. The conserved TEY motif of TaMAPK4 is important for its kinase activity, which is required for the resistance function. We demonstrate that Hasp98 inhibits the kinase activity of TaMAPK4 and that the stable silencing of TaMAPK4 compromises wheat resistance against P. striiformis. These results suggest that Hasp98 acts as a virulence effector to interfere with the MAPK signaling pathway in wheat, thereby promoting P. striiformis infection.

Secreted Glycosyltransferase RsIA_GT of Rhizoctonia solani AG-1 IA Inhibits Defense Responses in Nicotiana benthamiana

Anastomosis group AG-1 IA of Rhizoctonia solani Khün has a wide host range and threatens crop production. Various glycosyltransferases secreted by phytopathogenic fungi play an essential role in pathogenicity. Previously, we identified a glycosyltransferase RsIA_GT (AG11A_09161) as a secreted protein-encoding gene of R. solani AG-1 IA, whose expression levels increased during infection in rice. In this study, we further characterized the virulence function of RsIA_GT. It is conserved not only in Basidiomycota, including multiple anastomosis groups of R. solani, but also in other primary fungal taxonomic categories. RsIA_GT possesses a signal peptide (SP) for protein secretion, and its functionality was proven using yeast and Nicotiana benthamiana. The SP-truncated form of RsIA_GT (RsIA_GT(ΔS)) expressed in Escherichia coli-induced lesion-like phenotype in rice leaves when applied to punched leaves. However, Agrobacterium-mediated transient expressions of both the full-length RsIA_GT and RsIA_GT(ΔS) did not induce cell death in N. benthamiana leaves. Instead, only RsIA_GT(ΔS) suppressed the cell death induced by two reference cell death factors BAX and INF1 in N.benthamiana. RsIA_GT(ΔS)^{R154A D168A D170A}, a mutant RsIA_GT(ΔS) for the glycosyltransferase catalytic domain, still suppressed the BAX- or INF1-induced cell death, suggesting that the cell death suppression activity of RsIA_GT(ΔS) would be independent from its enzymatic activity. RsIA_GT(ΔS) also suppressed the H₂O₂ production and callose deposition and showed an effect on the induction of defense genes associated with the expression of BAX and INF1. The transient expression of RsIA_GT(ΔS) in N. benthamiana enhanced the lesion area caused by R. solani AG-1 IA. The secreted glycosyltransferase, RsIA_GT, of R. solani AG-1 IA is likely to have a dual role in virulence inside and outside of host cells.

A serine-rich effector from the stripe rust pathogen targets a Raf-like kinase to suppress host immunity

Puccinia striiformis f. sp. tritici (Pst) is an important obligate pathogen in wheat (Triticum aestivum L.) and secretes effectors into plant cells to promote infection. Identifying host targets of effector proteins and clarifying their roles in pathogen infection is essential for understanding pathogen virulence. In this study, we identified a serine-rich effector, Pst27791, from Pst that suppresses cell death in Nicotiana benthamiana. Stable overexpression of Pst27791 in wheat suppressed reactive oxygen species accumulation and the salicylic acid-dependent defense response. Transgenic wheat expressing the RNA interference construct of Pst27791 exhibited high resistance to Pst virulent isolate CYR31, indicating its importance in pathogenesis. Pst27791 interacting with wheat rapidly accelerated fibrosarcoma (Raf)-like kinase TaRaf46 in yeast and in planta. Knocking down TaRaf46 expression in wheat attenuated Pst infection and increased wheat immunity. The overexpression of TaRaf46 decreased wheat resistance to Pst and repressed MAPK activation in wheat. Pst27791 may stabilize TaRaf46 through the inhibition of proteasome-mediated degradation in N. benthamiana. The ability of Pst27791 to enhance Pst colonization was compromised when TaRaf46 was silenced, suggesting that the virulence of Pst27791 is mediated by TaRaf46. Overall, these results indicate that Raf-like kinase TaRaf46 is exploited by the Pst effector as a negative regulator of plant immunity to promote infection in wheat.

Diversity, Evolution, and Function of Pseudomonas syringae Effectoromes

Pseudomonas syringae is an evolutionarily diverse bacterial species complex and a preeminent model for the study of plant-pathogen interactions due in part to its remarkably broad host range. A critical feature of P. syringae virulence is the employment of suites of type III secreted effector (T3SE) proteins, which vary widely in composition and function. These effectors act on a variety of plant intracellular targets to promote pathogenesis but can also be avirulence factors when detected by host immune complexes. In this review, we survey the phylogenetic diversity (PD) of the P. syringae effectorome, comprising 70 distinct T3SE families identified to date, and highlight how avoidance of host immune detection has shaped effectorome diversity through functional redundancy, diversification, and horizontal transfer. We present emerging avenues for research and novel insights that can be gained via future investigations of plant-pathogen interactions through the fusion of large-scale interaction screens and phylogenomic approaches.

The effector MJ-10A08 of Meloidogyne javanica is required for parasitism that suppressed programmed cell death in Nicotiana benthamiana

Effectors synthesised in the pharyngeal glands are important in the successful invasion of root-knot nematodes. Meloidogyne javanica is among the nematodes that cause the most damage to various crops. In this study, an effector named MJ-10A08 of M. javanica was identified and investigated. Mj-10A08 was exclusively expressed in the dorsal pharyngeal gland cell and highly expressed in the parasitic second-juvenile stage of M. javanica. Transgenic tobaccos that over-expressed Mj-10A08 were more susceptible to M. javanica; however, host delivered RNAi of Mj-10A08 in tobacco significantly decreased the expression level of Mj-10A08 and the infection efficiency of M. javanica. Transient expression in tobacco leaves demonstrated that MJ-10A08 suppressed programmed cell death caused by BAX and Gpa2/RBP-1. Our results indicated that MJ-10A08 is implicated in the suppression of plant defence response during nematode infection and plays an important role in the parasitism of M. javanica.

Data-driven dynamical modelling of a pathogen-infected plant gene regulatory network: A comparative analysis

Recent advances in synthetic biology have enabled the design of genetic feedback control circuits that could be implemented to build resilient plants against pathogen attacks. To facilitate the proper design of these genetic feedback control circuits, an accurate model that is able to capture the vital dynamical behaviour of the pathogen-infected plant is required. In this study, using a data-driven modelling approach, we develop and compare four dynamical models (i.e. linear, Michaelis-Menten with Hill coefficient (Hill Function), standard S-System and extended S-System) of a pathogen-infected plant gene regulatory network (GRN). These models are then assessed across several criteria, i.e. ease of identifying the type of gene regulation, the predictive capability, Akaike Information Criterion (AIC) and the robustness to parameter uncertainty to determine its viability of balancing between biological complexity and accuracy when modelling the pathogen-infected plant GRN. Using our defined ranking score, we obtain the following insights to the modelling of GRN. Our analyses show that despite commonly used and provide biological relevance, the Hill Function model ranks the lowest while the extended S-System model ranks highest in the overall comparison. Interestingly, the performance of the linear model is more consistent throughout the comparison, making it the preferred model for this pathogen-infected plant GRN when considering data-driven modelling approach.

Metaeffector interactions modulate the type III effector-triggered immunity load of Pseudomonas syringae

The bacterial plant pathogen Pseudomonas syringae requires type III secreted effectors (T3SEs) for pathogenesis. However, a major facet of plant immunity entails the recognition of a subset of P. syringae’s T3SEs by intracellular host receptors in a process called Effector-Triggered Immunity (ETI). Prior work has shown that ETI-eliciting T3SEs are pervasive in the P. syringae species complex raising the question of how P. syringae mitigates its ETI load to become a successful pathogen. While pathogens can evade ETI by T3SE mutation, recombination, or loss, there is increasing evidence that effector-effector (a.k.a., metaeffector) interactions can suppress ETI. To study the ETI-suppression potential of P. syringae T3SE repertoires, we compared the ETI-elicitation profiles of two genetically divergent strains: P. syringae pv. tomato DC3000 (PtoDC3000) and P. syringae pv. maculicola ES4326 (PmaES4326), which are both virulent on Arabidopsis thaliana but harbour largely distinct effector repertoires. Of the 529 T3SE alleles screened on A. thaliana Col-0 from the P. syringae T3SE compendium (PsyTEC), 69 alleles from 21 T3SE families elicited ETI in at least one of the two strain backgrounds, while 50 elicited ETI in both backgrounds, resulting in 19 differential ETI responses including two novel ETI-eliciting families: AvrPto1 and HopT1. Although most of these differences were quantitative, three ETI responses were completely absent in one of the pathogenic backgrounds. We performed ETI suppression screens to test if metaeffector interactions contributed to these ETI differences, and found that HopQ1a suppressed AvrPto1m-mediated ETI, while HopG1c and HopF1g suppressed HopT1b-mediated ETI. Overall, these results show that P. syringae strains leverage metaeffector interactions and ETI suppression to overcome the ETI load associated with their native T3SE repertoires.

Agrobacterium expressing a type III secretion system delivers Pseudomonas effectors into plant cells to enhance transformation

Agrobacterium-mediated plant transformation (AMT) is the basis of modern-day plant biotechnology. One major drawback of this technology is the recalcitrance of many plant species/varieties to Agrobacterium infection, most likely caused by elicitation of plant defense responses. Here, we develop a strategy to increase AMT by engineering Agrobacterium tumefaciens to express a type III secretion system (T3SS) from Pseudomonas syringae and individually deliver the P. syringae effectors AvrPto, AvrPtoB, or HopAO1 to suppress host defense responses. Using the engineered Agrobacterium, we demonstrate increase in AMT of wheat, alfalfa and switchgrass by ~250%-400%. We also show that engineered A. tumefaciens expressing a T3SS can deliver a plant protein, histone H2A-1, to enhance AMT. This strategy is of great significance to both basic research and agricultural biotechnology for transient and stable transformation of recalcitrant plant species/varieties and to deliver proteins into plant cells in a non-transgenic manner.

The Pseudomonas syringae type III effector HopG1 triggers necrotic cell death that is attenuated by AtNHR2B

The plant pathogenic bacterium Pseudomonas syringae pv. tomato DC3000 (Pst DC3000) has become a paradigm to investigate plant-bacteria interactions due to its ability to cause disease in the model plant Arabidopsis thaliana. Pst DC3000 uses the type III secretion system to deliver type III secreted effectors (T3SEs) directly into the plant cytoplasm. Pst DC3000 T3SEs contribute to pathogenicity by suppressing plant defense responses and targeting plant’s physiological processes. Although the complete repertoire of effectors encoded in the Pst DC3000 genome have been identified, the specific function for most of them remains to be elucidated. Among those effectors, the mitochondrial-localized T3E HopG1, suppresses plant defense responses and promotes the development of disease symptoms. Here, we show that HopG1 triggers necrotic cell death that enables the growth of adapted and non-adapted pathogens. We further showed that HopG1 interacts with the plant immunity-related protein AtNHR2B and that AtNHR2B attenuates HopG1- virulence functions. These results highlight the importance of HopG1 as a multi-faceted protein and uncover its interplay with AtNHR2B.

Phosphorylated viral protein evades plant immunity through interfering the function of RNA-binding protein

Successful pathogen infection in plant depends on a proper interaction between the invading pathogen and its host. Post-translational modification (PTM) plays critical role(s) in plant-pathogen interaction. However, how PTM of viral protein regulates plant immunity remains poorly understood. Here, we found that S162 and S165 of Chinese wheat mosaic virus (CWMV) cysteine-rich protein (CRP) are phosphorylated by SAPK7 and play key roles in CWMV infection. Furthermore, the phosphorylation-mimic mutant of CRP (CRP^S162/165D) but not the non-phosphorylatable mutant of CRP (CRP^S162/165A) interacts with RNA-binding protein UBP1-associated protein 2C (TaUBA2C). Silencing of TaUBA2C expression in wheat plants enhanced CWMV infection. In contrast, overexpression of TaUBA2C in wheat plants inhibited CWMV infection. TaUBA2C inhibits CWMV infection through recruiting the pre-mRNA of TaNPR1, TaPR1 and TaRBOHD to induce cell death and H₂O₂ production. This effect can be supressed by CRP^S162/165D through changing TaUBA2C chromatin-bound status and attenuating it’s the RNA- or DNA-binding activities. Taken together, our findings provide new knowledge on how CRP phosphorylation affects CWMV infection as well as the arms race between virus and wheat plants.

Chinese wheat mosaic virus (CWMV) causes a damaging disease in cereal plants. However, CWMV interacts with host factors to facilitate virus infection is not clear yet. Here, we found that S162 and S165 of CWMV cysteine-rich protein (CRP) are phosphorylated by SAPK7 in vivo and in vitro. Mutational analyses have indicated that these two phosphorylation sites of CRP (CRP^S162/165D) promoting CWMV infection in plants, due to the supressed cell death and H₂O₂ production. Further investigations found the CRP^S162/165D can interact with TaUBA2C, while the non-phosphorylatable mutant of CRP (CRP^S162/165A) does not. Futhermore, we have determined that CRP^S162/165D and TaUBA2C interaction inhibited the formation of TaUBA2C speckles in nucleus to attenuate its RNA- and DNA-binding activity. We also showed that TaUBA2C recruit the pre-mRNA of TaNPR1, TaPR1 and TaRBOHD to up-regulated these genes expressions and then induce cell death and H₂O₂ production in plant. This effect can be supressed by the expression of CRP^S162/165D, in a dose-dependent manner. Taken together, our discovery may provide a new sight for the arms race between virus and its host plants.

A phytobacterial TIR domain effector manipulates NAD+ to promote virulence

The Pseudomonas syringae DC3000 type III effector HopAM1 suppresses plant immunity and contains a Toll/interleukin-1 receptor (TIR) domain homologous to immunity-related TIR domains of plant nucleotide-binding leucine-rich repeat receptors that hydrolyze nicotinamide adenine dinucleotide (NAD⁺ ) and activate immunity. In vitro and in vivo assays were conducted to determine if HopAM1 hydrolyzes NAD⁺ and if the activity is essential for HopAM1's suppression of plant immunity and contribution to virulence. HPLC and LC-MS were utilized to analyze metabolites produced from NAD⁺ by HopAM1 in vitro and in both yeast and plants. Agrobacterium-mediated transient expression and in planta inoculation assays were performed to determine HopAM1's intrinsic enzymatic activity and virulence contribution. HopAM1 is catalytically active and hydrolyzes NAD⁺ to produce nicotinamide and a novel cADPR variant (v2-cADPR). Expression of HopAM1 triggers cell death in yeast and plants dependent on the putative catalytic residue glutamic acid 191 (E191) within the TIR domain. Furthermore, HopAM1's E191 residue is required to suppress both pattern-triggered immunity and effector-triggered immunity and promote P. syringae virulence. HopAM1 manipulates endogenous NAD⁺ to produce v2-cADPR and promote pathogenesis. This work suggests that HopAM1's TIR domain possesses different catalytic specificity than other TIR domain-containing NAD⁺ hydrolases and that pathogens exploit this activity to sabotage NAD⁺ metabolism for immune suppression and virulence.

Transcriptome analysis of sesame-Macrophomina phaseolina interactions revealing the distinct genetic components for early defense responses

Sesame (Sesamum indicum L.) is an oilseed crop challenged by many biotic stresses. Charcoal rot caused by Macrophomina phaseolina (MP) is one of the most devastating diseases of sesame. Till date, molecular mechanisms of resistance to charcoal rot in sesame is not yet reported. In this study, two sesame variety GT-10 (resistant) and RT-373 (susceptible) were identified with contrasting disease incidence when infected with MP. To get the molecular insight, root samples were collected at 0, 24, 48- and 72-h post inoculation (hpi) with the pathogen and generated RNAseq data was analyzed. A total of 1153 and 1226 differentially expressed genes (DEGS) were identified in GT-10 and RT-373, respectively. During the inoculation with MP, resistant genotype showed high number DEGs at early time point of 24 hpi and when compared to late expression in susceptible genotype at 48 hpi. Distinct clusters were represented for each time period represented by cytochrome P450 83B1-like, single anchor, hypothetical protein C4D60, kirola like and heat shock proteins in the resistant genotype contributing for resistance. Analysis of differentially expressed genes, catalogued the genes involved in synthesis of pathogenesis-related (PR) proteins, MYB, WRKY, leucine zipper protein, bHLH, bZIP and NAC transcription factors, ABC transporters (B, C and G subfamily), glutathione metabolism, secondary metabolites, fatty acid biosynthesis and phytohormones like auxin, abscisic acid, ethylene and gibberellic acid. Additionally, in the resistant response we have found three unique GO terms including ATP binding, ribonucleotide binding and nucleic acid binding in molecular function category. The molecular clues generated through this work will provide an important resource of genes contributing for disease resistance and could prioritize genes for functional validation in the important oil crop.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s12298-021-01039-6.

The bacterial effector HopZ1a acetylates MKK7 to suppress plant immunity

The Pseudomonas syringae type III secretion system translocates effector proteins into the host cell cytosol to suppress plant basal immunity. Effector HopZ1a suppresses local and systemic immunity triggered by pathogen-associated molecular patterns (PAMPs) and effectors, through target acetylation. HopZ1a has been shown to target several plant proteins, but none fully substantiates HopZ1a-associated immune suppression. Here, we investigate Arabidopsis thaliana mitogen-activated protein kinase kinases (MKKs) as potential targets, focusing on AtMKK7, a positive regulator of local and systemic immunity. We analyse HopZ1a interference with AtMKK7 by translocation of HopZ1a from bacteria inoculated into Arabidopsis expressing MKK7 from an inducible promoter. Reciprocal phenotypes are analysed on plants expressing a construct quenching MKK7 native expression. We analyse HopZ1a-MKK7 interaction by three independent methods, and the relevance of acetylation by in vitro kinase and in planta functional assays. We demonstrate the AtMKK7 contribution to immune signalling showing MKK7-dependent flg22-induced reactive oxygen species (ROS) burst, MAP kinas (MAPK) activation and callose deposition, plus AvrRpt2-triggered MKK7-dependent signalling. Furthermore, we demonstrate HopZ1a suppression of all MKK7-dependent responses, HopZ1a-MKK7 interaction in planta and HopZ1a acetylation of MKK7 with a lysine required for full kinase activity. We demonstrate that HopZ1a targets AtMKK7 to suppress local and systemic plant immunity.

Unrelated Fungal Rust Candidate Effectors Act on Overlapping Plant Functions

Rust fungi cause epidemics that threaten the production of important plant species, such as wheat and soy. Melampsora larici-populina (Mlp) causes the poplar rust and encodes at least 1184 candidate effectors (CEs) whose functions are poorly known. In this study, we sequenced the transcriptome and used mass spectrometry to analyze the metabolome of Arabidopsis plants constitutively expressing 14 Mlp CEs and of a control line to discover alterations leading to plant susceptibility. We found 2299 deregulated genes across the experiment. Genes involved in pattern-triggered immunity, such as FRK1, PR1, RBOHD, and WRKY33, as well as AUX/IAA genes were down-regulated. We further observed that 680 metabolites were deregulated in at least one CE-expressing transgenic line, with “highly unsaturated and phenolic compounds” and “peptides” enriched among down- and up-regulated metabolites. Interestingly, transgenic lines expressing unrelated CEs had correlated patterns of gene and metabolite deregulation, while expression of CEs belonging to the same family deregulated different genes and metabolites. Thus, our results uncouple effector sequence similarity and function. This supports that effector functional investigation in the context of their virulence activity and effect on plant susceptibility requires the investigation of the individual effector and precludes generalization based on sequence similarity.

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.

"Candidatus Liberibacter asiaticus" Secretes Nonclassically Secreted Proteins That Suppress Host Hypersensitive Cell Death and Induce Expression of Plant Pathogenesis-Related Proteins

Although emerging evidence indicates that bacteria extracellularly export many cytoplasmic proteins referred to as non-classically secreted proteins (ncSecPs) for their own benefit, the mechanisms and functional significance of the ncSecPs in extracellular milieu remain elusive. "Candidatus Liberibacter asiaticus" (CLas) is a fastidious Gram-negative bacterium that causes Huanglongbing (HLB), the most globally devastating citrus disease. In this study, using the SecretomeP program coupled with an Escherichia coli alkaline phosphatase assay, we identified 27 ncSecPs from the CLas genome. Further, we demonstrated that 10 of these exhibited significantly higher levels of gene expression in citrus than in psyllid hosts, and particularly suppressed hypersensitive response (HR)-based cell death and H₂O₂ overaccumulation in Nicotiana benthamiana, indicating their opposing effects on early plant defenses. However, these proteins also dramatically enhanced the gene expression of pathogenesis-related 1 protein (PR-1), PR-2, and PR-5, essential components of plant defense mechanisms. Additional experiments disclosed that the increased expression of these PR genes, in particular PR-1 and PR-5, could negatively regulate HR-based cell death development and H₂O₂ accumulation. Remarkably, CLas infection clearly induced gene expression of PR-1, PR-2, and PR-5 in both HLB-tolerant and HLB-susceptible species of citrus plants. Taken together, we hypothesized that CLas has evolved an arsenal of ncSecPs that function cooperatively to overwhelm the early plant defenses by inducing host PR genes.IMPORTANCE In this study, we present a combined computational and experimental methodology that allows a rapid and efficient identification of the ncSecPs from bacteria, in particular the unculturable bacteria like CLas. Meanwhile, the study determined that a number of CLas ncSecPs suppressed HR-based cell death, and thus indicated a novel role for the bacterial ncSecPs in extracellular milieu. More importantly, these ncSecPs were found to suppress cell death presumably by utilizing host PR proteins. The data overall provide a novel clue to understand the CLas pathogenesis and also suggest a new way by which phytopathogens manipulate host cellular machinery to establish infection.

Ralstonia solanacearum Type III Effector RipAC Targets SGT1 to Suppress Effector-Triggered Immunity

Ralstonia solanacearum injects type III effectors into host cells to cause bacterial wilt in Solanaceae plants. To identify R. solanacearum effectors that suppress effector-triggered immunity (ETI) in plants, we evaluated R. solanacearum RS1000 effectors for their ability to suppress a hypersensitive response (HR) induced by the avirulence (Avr) effector RipAA in Nicotiana benthamiana. Out of the 11 effectors tested, 4 suppressed RipAA-triggered HR cell death. Among them, RipAC contains tandem repeats of the leucine-rich repeat (LRR) motif, which serves as the structural scaffold for a protein-protein interaction. We found that the LRR domain of RipAC was indispensable for the suppression of HR cell death during the recognition of RipAA and another Avr effector RipP1. By yeast two-hybrid screening, we identified N. benthamiana SGT1, an adaptor protein that forms a molecular chaperone complex with RAR1, as a host factor of the RipAC target. RipAC interacted with NbSGT1 in yeast and plant cells. Upon the formation of the molecular chaperone complex, the presence of RipAC markedly inhibits the interaction between NbSGT1 and NbRAR1. The RipAA- and RipP1-triggered HR cell deaths were not observed in NbSGT1-silenced plants. The introduction of RipAC was complementary to the reduced growth of the R. solanacearum mutant strain in N. benthamiana. These findings indicate that R. solanacearum uses RipAC to subvert the NbSGT1-mediated formation of the molecular chaperone complex and suppress ETI responses during the recognition of Avr effectors.

The Coxiella burnetii effector protein CaeB modulates endoplasmatic reticulum (ER) stress signalling and is required for efficient replication in Galleria mellonella

The obligate intracellular pathogen Coxiella burnetii is the causative agent of the zoonosis Q fever. C. burnetii infection can have severe outcomes due to the development of chronic infection. To establish and maintain an infection, C. burnetii depends on a functional type IVB secretion system (T4BSS) and, thus, on the translocation of effector proteins into the host cell. Here, we showed that the C. burnetii T4BSS effector protein CaeB targets the conserved endoplasmatic reticulum (ER) stress sensor IRE1 during ER stress in mammalian and plant cells. CaeB-induced upregulation of IRE1 RNase activity was essential for CaeB-mediated inhibition of ER stress-induced cell death. Our data reveal a novel role for CaeB in ER stress signalling modulation and demonstrate that CaeB is involved in pathogenicity in vivo. Furthermore, we provide evidence that C. burnetii infection leads to modulation of the ER stress sensors IRE1 and PERK, but not ATF6 during ER stress. While the upregulation of the RNase activity of IRE1 during ER stress depends on CaeB, modulation of PERK is CaeB independent, suggesting that C. burnetii encodes several factors influencing ER stress during infection.

HaCRT1 of Heterodera avenae Is Required for the Pathogenicity of the Cereal Cyst Nematode

Cereal cyst nematodes are sedentary biotrophic endoparasites that secrete effector proteins into plant tissues to transit normal cells into specialized feeding sites and suppress plant defenses. To understand the function of nematode effectors in Heterodera avenae, here, we identified a calreticulin protein HaCRT1, which could suppress the cell death induced by Bax when expressed in Nicotiana benthamiana. HaCRT1 is synthetized in the subventral gland cells of pre-parasitic second-stage nematodes. Real-time PCR assays indicated that the expression of HaCRT1 was highest in parasitic second-stage juveniles. The expression of an HaCRT1-RFP fusion in N. benthamiana revealed that it was localized in the endoplasmic reticulum of the plant cell. The ability of H. avenae infecting plants was significantly reduced when HaCRT1 was knocked down by RNA interference in vitro. Arabidopsis thaliana plants expressing HaCRT1 were more susceptible than wild-type plants to Pseudomonas syringae. The induction of defense-related genes, PAD4, WRKY33, FRK1, and WRKY29, after treatment with flg22 was suppressed in HaCRT1-transgenic plants. Also, the ROS accumulation induced by flg22 was reduced in the HaCRT1-transgenic plants compared to wild-type plants. HaCRT1 overexpression increased the cytosolic Ca²⁺ concentration in A. thaliana. These data suggested that HaCRT1 may contribute to the pathogenicity of H. avenae by suppressing host basal defense.

Effector prediction and characterization in the oomycete pathogen Bremia lactucae reveal host-recognized WY domain proteins that lack the canonical RXLR motif

Pathogens that infect plants and animals use a diverse arsenal of effector proteins to suppress the host immune system and promote infection. Identification of effectors in pathogen genomes is foundational to understanding mechanisms of pathogenesis, for monitoring field pathogen populations, and for breeding disease resistance. We identified candidate effectors from the lettuce downy mildew pathogen Bremia lactucae by searching the predicted proteome for the WY domain, a structural fold found in effectors that has been implicated in immune suppression as well as effector recognition by host resistance proteins. We predicted 55 WY domain containing proteins in the genome of B. lactucae and found substantial variation in both sequence and domain architecture. These candidate effectors exhibit several characteristics of pathogen effectors, including an N-terminal signal peptide, lineage specificity, and expression during infection. Unexpectedly, only a minority of B. lactucae WY effectors contain the canonical N-terminal RXLR motif, which is a conserved feature in the majority of cytoplasmic effectors reported in Phytophthora spp. Functional analysis of 21 effectors containing WY domains revealed 11 that elicited cell death on wild accessions and domesticated lettuce lines containing resistance genes, indicative of recognition of these effectors by the host immune system. Only two of the 11 recognized effectors contained the canonical RXLR motif, suggesting that there has been an evolutionary divergence in sequence motifs between genera; this has major consequences for robust effector prediction in oomycete pathogens.

Targeted suppression of soybean BAG6-induced cell death in yeast by soybean cyst nematode effectors

While numerous effectors that suppress plant immunity have been identified from bacteria, fungi, and oomycete pathogens, relatively little is known for nematode effectors. Several dozen effectors have been reported from the soybean cyst nematode (SCN). Previous studies suggest that a hypersensitive response-like programmed cell death is triggered at nematode feeding sites in soybean during an incompatible interaction. However, virulent SCN populations overcome this incompatibility using unknown mechanisms. A soybean BAG6 (Bcl-2 associated anthanogene 6) gene previously reported by us to be highly up-regulated in degenerating feeding sites induced by SCN in a resistant soybean line was attenuated in response to a virulent SCN population. We show that GmBAG6-1 induces cell death in yeast like its Arabidopsis homolog AtBAG6 and also in soybean. This led us to hypothesize that virulent SCN may target GmBAG6-1 as part of their strategy to overcome soybean defence responses during infection. Thus, we used a yeast viability assay to screen SCN effector candidates for their ability to specifically suppress GmBAG6-1-induced cell death. We identified several effectors that strongly suppressed cell death mediated by GmBAG6-1. Two effectors identified as suppressors showed direct interaction with GmBAG6-1 in yeast, suggesting that one mechanism of cell death suppression may occur through an interaction with this host protein.

James Robert Alfano, A Giant in Phytopathogenic Bacteria Effector Biology

The worldwide molecular plant-microbe interactions research community was significantly diminished in November 2019 by the death of James "Jim" Robert Alfano at age 56. Jim was a giant in our field, who gained key insights into plant pathogenesis using the model bacterial pathogen Pseudomonas syringae. As a mentor, collaborator, and, above all, a friend, I know Jim's many dimensions and accomplishments and, sadly, the depth of loss being felt by the many people around the world who were touched by him. In tracing the path of Jim's career, I will emphasize the historical context and impact of his advances and, finally, the essence of the person we will so miss.

Lso-HPE1, an Effector of 'Candidatus Liberibacter solanacearum', Can Repress Plant Immune Response

'Candidatus Liberibacter solanacearum' is a plant pathogen affecting the families Solanaceae and Apiaceae in different parts of the world. 'Ca. L. solanacearum' is a Gram-negative, fastidious α-proteobacterium that is vectored by different psyllid species. Plant-pathogenic bacteria are known for interfering with the host physiology or defense mechanisms, often by secreting bacterial effectors. Effector proteins are critical for virulence; therefore, the identification of effectors could help with disease management. In this study, we characterized the Sec-translocon-dependent 'Ca. L. solanacearum'-hypothetical protein effector 1 (Lso-HPE1). We compared this protein sequence in the different 'Ca. L. solanacearum' haplotypes. We predicted the signal peptide and validated its function using Escherichia coli's alkaline phosphatase fusion assay. Agrobacterium tumefaciens-mediated transient expression in Nicotiana benthamiana demonstrated that Lso-HPE1 from 'Ca. L. solanacearum' haplotypes A and B were able to inhibit the induction of cell death in plants. We also compared gene expression of the Lso-HPE1- transcripts in 'Ca. L. solanacearum' haplotypes A and B in tomato and in the vector Bactericera cockerelli. This work validates the identification of a Sec-translocon-dependent 'Ca. L. solanacearum' protein possibly involved in suppression of plant cell death.

A stripe rust effector Pst18363 targets and stabilises TaNUDX23 that promotes stripe rust disease

Wheat stripe rust, caused by Puccinia striiformis f. sp. tritici (Pst), poses a tremendous threat to the production of wheat worldwide. The molecular mechanisms of Pst effectors that regulate wheat immunity are poorly understood. In this study, we identified an effector Pst18363 from Pst that suppresses plant cell death in Nicotiana benthamiana and in wheat. Knocking down Pst18363 expression by virus-mediated host-induced gene silencing significantly decreased the number of rust pustules, indicating that Pst18363 functions as an important pathogenicity factor in Pst. Pst18363 was proven to interact with wheat Nudix hydrolase 23 TaNUDX23. In wheat, silencing of TaNUDX23 by virus-induced gene silencing increased reactive oxygen species (ROS) accumulation induced by the avirulent Pst race CYR23, whereas overexpression of TaNUDX23 suppressed ROS accumulation induced by flg22 in Arabidopsis. In addition, TaNUDX23 suppressed Pst candidate effector Pst322-trigged cell death by decreasing ROS accumulation in N. benthamiana. Knocking down of TaNUDX23 expression attenuated Pst infection, indicating that TaNUDX23 is a negative regulator of defence. In N. benthamiana, Pst18363 stabilises TaNUDX23. Overall, our data suggest that Pst18363 stabilises TaNUDX23, which suppresses ROS accumulation to facilitate Pst infection.

Small RNA ArcZ Regulates Oxidative Stress Response Genes and Regulons in Erwinia amylovora

Erwinia amylovora, causative agent of fire blight disease of apple and pear trees, has evolved to use small RNAs for post-transcriptional regulation of virulence traits important for disease development. The sRNA ArcZ regulates several virulence traits, and to better understand its roles, we conducted a transcriptomic comparison of wild-type and ΔarcZ mutant E. amylovora. We found that ArcZ regulates multiple cellular processes including genes encoding enzymes involved in mitigating the threat of reactive oxygen species (katA, tpx, osmC), and that the ΔarcZ mutant has reduced catalase activity and is more susceptible to exogenous hydrogen peroxide. We quantified hydrogen peroxide production by apple leaves inoculated with E. amylovora and found that the while wild-type E. amylovora cells produce enough catalase to cope with defense peroxide, the ΔarcZ mutant is likely limited in virulence because of inability to cope with peroxide levels in host leaves. We further found that the ArcZ regulon overlaps significantly with the regulons of transcription factors involved in oxidative sensing including Fnr and ArcA. In addition, we show that ArcZ regulates arcA at the post-transcriptional level suggesting a role for this system in mediating adaptations to oxidative state, especially during disease development.

A Novel 'Candidatus Liberibacter asiaticus'-Encoded Sec-Dependent Secretory Protein Suppresses Programmed Cell Death in Nicotiana benthamiana

'Candidatus Liberibacter asiaticus' (CLas) is one of the causal agents of citrus Huanglongbing (HLB), a bacterial disease of citrus trees that greatly reduces fruit yield and quality. CLas strains produce an array of currently uncharacterized Sec-dependent secretory proteins. In this study, the conserved chromosomally encoded protein CLIBASIA_03875 was identified as a novel Sec-dependent secreted protein. We show that CLIBASIA_03875 contains a putative Sec- secretion signal peptide (SP), a 29 amino acid residue located at the N-terminus, with a mature protein (m3875) of 22 amino acids found to localize in multiple subcellular components of the leaf epidermal cells of Nicotiana benthamiana. When overexpressed via a Potato virus X (PVX)-based expression vector in N. benthamiana, m3875 suppressed programmed cell death (PCD) and the H2O2 accumulation triggered by the pro-apoptotic mouse protein BAX and the Phytophthora infestans elicitin INF1. Overexpression also resulted in a phenotype of dwarfing, leaf deformation and mosaics, suggesting that m3875 has roles in plant immune response, growth, and development. Substitution mutagenesis of the charged amino acid (D7, R9, R11, and K22) with alanine within m3875 did not recover the phenotypes for PCD and normal growth. In addition, the transiently overexpressed m3875 regulated the transcriptional levels of N. benthamiana orthologs of CNGCs (cyclic nucleotide-gated channels), BI-1 (Bax-inhibitor 1), and WRKY33 that are involved in plant defense mechanisms. To our knowledge, m3875 is the first PCD suppressor identified from CLas. Studying the function of this protein provides insight as to how CLas attenuates the host immune responses to proliferate and cause Huanglongbing disease in citrus plants.

A host-pathogen interactome uncovers phytopathogenic strategies to manipulate plant ABA responses

The phytopathogen Pseudomonas syringae delivers into host cells type III secreted effectors (T3SEs) that promote virulence. One virulence mechanism employed by T3SEs is to target hormone signaling pathways to perturb hormone homeostasis. The phytohormone abscisic acid (ABA) influences interactions between various phytopathogens and their plant hosts, and has been shown to be a target of P. syringae T3SEs. In order to provide insight into how T3SEs manipulate ABA responses, we generated an ABA-T3SE interactome network (ATIN) between P. syringae T3SEs and Arabidopsis proteins encoded by ABA-regulated genes. ATIN consists of 476 yeast-two-hybrid interactions between 97 Arabidopsis ABA-regulated proteins and 56 T3SEs from four pathovars of P. syringae. We demonstrate that T3SE interacting proteins are significantly enriched for proteins associated with transcription. In particular, the ETHYLENE RESPONSIVE FACTOR (ERF) family of transcription factors is highly represented. We show that ERF105 and ERF8 displayed a role in defense against P. syringae, supporting our overall observation that T3SEs of ATIN converge on proteins that influence plant immunity. In addition, we demonstrate that T3SEs that interact with a large number of ABA-regulated proteins can influence ABA responses. One of these T3SEs, HopF3_Pph6 , inhibits the function of ERF8, which influences both ABA-responses and plant immunity. These results provide a potential mechanism for how HopF3_Pph6 manipulates ABA-responses to promote P. syringae virulence, and also demonstrate the utility of ATIN as a resource to study the ABA-T3SE interface.

Fusarium graminearum arabinanase (Arb93B) Enhances Wheat Head Blight Susceptibility by Suppressing Plant Immunity

Fusarium head blight (FHB) of wheat and barley caused by the fungus Fusarium graminearum reduces crop yield and contaminates grain with mycotoxins. In this study, we investigated two exo-1,5-α-L-arabinanases (Arb93A and Arb93B) secreted by F. graminearum and their effect on wheat head blight development. Arabinan is an important component of plant cell walls but it was not known whether these arabinanases play a role in FHB. Both ARB93A and ARB93B were induced during the early stages of infection. arb93A mutants did not exhibit a detectable change in ability to cause FHB, whereas arb93B mutants caused lower levels of FHB symptoms and deoxynivalenol contamination compared with the wild type. Furthermore, virulence and deoxynivalenol contamination were restored to wild-type levels in ARB93B complemented mutants. Fusion proteins of green fluorescent protein (GFP) with the predicted chloroplast peptide or the mature protein of Arb93B were not observed in the chloroplast. Reactive oxygen species (ROS) production was reduced in the infiltrated zones of Nicotiana benthamiana leaves expressing ARB93B-GFP. Coexpression of ARB93B-GFP and Bax in N. benthamiana leaves significantly suppressed Bax-programmed cell death. Our results indicate that Arb93B enhances plant disease susceptibility by suppressing ROS-associated plant defense responses.

Identifying Pseudomonas syringae Type III Secreted Effector Function via a Yeast Genomic Screen

Gram-negative bacterial pathogens inject type III secreted effectors (T3SEs) directly into host cells to promote pathogen fitness by manipulating host cellular processes. Despite their crucial role in promoting virulence, relatively few T3SEs have well-characterized enzymatic activities or host targets. This is in part due to functional redundancy within pathogen T3SE repertoires as well as the promiscuity of individual T3SEs that can have multiple host targets. To overcome these challenges, we generated and characterized a collection of yeast strains stably expressing 75 T3SE constructs from the plant pathogen Pseudomonas syringae. This collection is devised to facilitate heterologous genetic screens in yeast, a non-host organism, to identify T3SEs that target conserved eukaryotic processes. Among 75 T3SEs tested, we identified 16 that inhibited yeast growth on rich media and eight that inhibited growth on stress-inducing media. We utilized Pathogenic Genetic Array (PGA) screens to identify potential host targets of P. syringae T3SEs. We focused on the acetyltransferase, HopZ1a, which interacts with plant tubulin and alters microtubule networks. To uncover putative HopZ1a host targets, we identified yeast genes with genetic interaction profiles most similar (i.e., congruent) to the PGA profile of HopZ1a and performed a functional enrichment analysis of these HopZ1a-congruent genes. We compared the congruence analyses above to previously described HopZ physical interaction datasets and identified kinesins as potential HopZ1a targets. Finally, we demonstrated that HopZ1a can target kinesins by acetylating the plant kinesins HINKEL and MKRP1, illustrating the utility of our T3SE-expressing yeast library to characterize T3SE functions.

Joining the Crowd: Integrating Plant Virus Proteins into the Larger World of Pathogen Effectors

The first bacterial and viral avirulence ( avr) genes were cloned in 1984. Although virus and bacterial avr genes were physically isolated in the same year, the questions associated with their characterization after discovery were very different, and these differences had a profound influence on the narrative of host-pathogen interactions for the past 30 years. Bacterial avr proteins were subsequently shown to suppress host defenses, leading to their reclassification as effectors, whereas research on viral avr proteins centered on their role in the viral infection cycle rather than their effect on host defenses. Recent studies that focus on the multifunctional nature of plant virus proteins have shown that some virus proteins are capable of suppression of the same host defenses as bacterial effectors. This is exemplified by the P6 protein of Cauliflower mosaic virus (CaMV), a multifunctional plant virus protein that facilitates several steps in the infection, including modulation of host defenses. This review highlights the modular structure and multifunctional nature of CaMV P6 and illustrates its similarities to other, well-established pathogen effectors.

Defining essential processes in plant pathogenesis with Pseudomonas syringae pv. tomato DC3000 disarmed polymutants and a subset of key type III effectors

Pseudomonas syringae pv. tomato DC3000 and its derivatives cause disease in tomato, Arabidopsis and Nicotiana benthamiana. The primary virulence factors include a repertoire of 29 effector proteins injected into plant cells by the type III secretion system and the phytotoxin coronatine. The complete repertoire of effector genes and key coronatine biosynthesis genes have been progressively deleted and minimally reassembled to reconstitute basic pathogenic ability in N. benthamiana, and in Arabidopsis plants that have mutations in target genes that mimic effector actions. This approach and molecular studies of effector activities and plant immune system targets have highlighted a small subset of effectors that contribute to essential processes in pathogenesis. Most notably, HopM1 and AvrE1 redundantly promote an aqueous apoplastic environment, and AvrPtoB and AvrPto redundantly block early immune responses, two conditions that are sufficient for substantial bacterial growth in planta. In addition, disarmed DC3000 polymutants have been used to identify the individual effectors responsible for specific activities of the complete repertoire and to more effectively study effector domains, effector interplay and effector actions on host targets. Such work has revealed that AvrPtoB suppresses cell death elicitation in N. benthamiana that is triggered by another effector in the DC3000 repertoire, highlighting an important aspect of effector interplay in native repertoires. Disarmed DC3000 polymutants support the natural delivery of test effectors and infection readouts that more accurately reveal effector functions in key pathogenesis processes, and enable the identification of effectors with similar activities from a broad range of other pathogens that also defeat plants with cytoplasmic effectors.

A Framework for Engineering Stress Resilient Plants Using Genetic Feedback Control and Regulatory Network Rewiring

Crop disease leads to significant waste worldwide, both pre- and postharvest, with subsequent economic and sustainability consequences. Disease outcome is determined both by the plants' response to the pathogen and by the ability of the pathogen to suppress defense responses and manipulate the plant to enhance colonization. The defense response of a plant is characterized by significant transcriptional reprogramming mediated by underlying gene regulatory networks, and components of these networks are often targeted by attacking pathogens. Here, using gene expression data from Botrytis cinerea-infected Arabidopsis plants, we develop a systematic approach for mitigating the effects of pathogen-induced network perturbations, using the tools of synthetic biology. We employ network inference and system identification techniques to build an accurate model of an Arabidopsis defense subnetwork that contains key genes determining susceptibility of the plant to the pathogen attack. Once validated against time-series data, we use this model to design and test perturbation mitigation strategies based on the use of genetic feedback control. We show how a synthetic feedback controller can be designed to attenuate the effect of external perturbations on the transcription factor CHE in our subnetwork. We investigate and compare two approaches for implementing such a controller biologically-direct implementation of the genetic feedback controller, and rewiring the regulatory regions of multiple genes-to achieve the network motif required to implement the controller. Our results highlight the potential of combining feedback control theory with synthetic biology for engineering plants with enhanced resilience to environmental stress.

Barley stripe mosaic virus infection requires PKA-mediated phosphorylation of γb for suppression of both RNA silencing and the host cell death response

The Barley stripe mosaic virus (BSMV) γb protein is a viral suppressor of RNA silencing (VSR) and symptom determinant. However, it is unclear how post-translational modification affects the different functions of γb. Here, we demonstrate that γb is phosphorylated at Ser-96 by a PKA-like kinase in vivo and in vitro. Mutant viruses containing a nonphosphorylatable substitution (BSMVS96A or BSMVS96R ) exhibited reduced viral accumulation in Nicotiana benthamiana due to transient induction of the cell death response that constrained the virus to necrotic areas. By contrast, a BSMVS96D mutant virus that mimics γb phosphorylation spread similarly to the wild-type virus. Furthermore, the S96A mutant had reduced local and systemic γb VSR activity due to having compromised its binding activity to 21-bp dsRNA. However, overexpression of other VSRs in trans or in cis failed to rescue the necrosis induced by BSMVS96A , demonstrating that suppression of cell death by γb phosphorylation is functionally distinct from its RNA silencing suppressor activities. These results provide new insights into the function of γb phosphorylation in regulating RNA silencing and the BSMV-induced host cell death response, and contribute to our understanding of how the virus optimizes the balance between viral replication and virus survival in the host plants during virus infection.

Differential Suppression of Nicotiana benthamiana Innate Immune Responses by Transiently Expressed Pseudomonas syringae Type III Effectors

The plant pathogen Pseudomonas syringae injects about 30 different virulence proteins, so-called effectors, via a type III secretion system into plant cells to promote disease. Although some of these effectors are known to suppress either pattern-triggered immunity (PTI) or effector-triggered immunity (ETI), the mode of action of most of them remains unknown. Here, we used transient expression in Nicotiana benthamiana, to test the abilities of type III effectors of Pseudomonas syringae pv. tomato (Pto) DC3000 and Pseudomonas syringae pv. tabaci (Pta) 11528 to interfere with plant immunity. We monitored the sequential and rapid bursts of cytoplasmic Ca2+ and reactive oxygen species (ROS), the subsequent induction of defense gene expression, and promotion of cell death. We found that several effector proteins caused cell death, but independently of the known plant immune regulator NbSGT1, a gene essential for ETI. Furthermore, many effectors delayed or blocked the cell death-promoting activity of other effectors, thereby potentially contributing to pathogenesis. Secondly, a large number of effectors were able to suppress PAMP-induced defense responses. In the majority of cases, this resulted in suppression of all studied PAMP responses, suggesting that these effectors target common elements of PTI. However, effectors also targeted different steps within defense pathways and could be divided into three major groups based on their suppressive activities. Finally, the abilities of effectors of both Pto DC3000 and Pta 11528 to suppress plant immunity was conserved in most but not all cases. Overall, our data present a comprehensive picture of the mode of action of these effectors and indicate that most of them suppress plant defenses in various ways.

The role of type III effectors from Xanthomonas axonopodis pv. manihotis in virulence and suppression of plant immunity

Xanthomonas axonopodis pv. manihotis (Xam) causes cassava bacterial blight, the most important bacterial disease of cassava. Xam, like other Xanthomonas species, requires type III effectors (T3Es) for maximal virulence. Xam strain CIO151 possesses 17 predicted T3Es belonging to the Xanthomonas outer protein (Xop) class. This work aimed to characterize nine Xop effectors present in Xam CIO151 for their role in virulence and modulation of plant immunity. Our findings demonstrate the importance of XopZ, XopX, XopAO1 and AvrBs2 for full virulence, as well as a redundant function in virulence between XopN and XopQ in susceptible cassava plants. We tested their role in pathogen-associated molecular pattern (PAMP)-triggered immunity (PTI) and effector-triggered immunity (ETI) using heterologous systems. AvrBs2, XopR and XopAO1 are capable of suppressing PTI. ETI suppression activity was only detected for XopE4 and XopAO1. These results demonstrate the overall importance and diversity in functions of major virulence effectors AvrBs2 and XopAO1 in Xam during cassava infection.

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.

Candidate Effector Pst_8713 Impairs the Plant Immunity and Contributes to Virulence of Puccinia striiformis f. sp. tritici

Puccinia striiformis f. sp. tritici (Pst), the causal agent of stripe rust, is an obligate biotrophic pathogen responsible for severe wheat disease epidemics worldwide. Pst and other rust fungi are acknowledged to deliver many effector proteins to the host, but little is known about the effectors' functions. Here, we report a candidate effector Pst_8713 isolated based on the genome data of CY32 and the expression of Pst_8713 is highly induced during the early infection stage. The Pst_8713 gene shows a low level of intra-species polymorphism. It has a functional N-terminal signal peptide and its product was found in the host cytoplasm and nucleus. Co-infiltrations in Nicotiana benthamiana demonsrated that Pst_8713 was capable of suppressing cell death triggered by mouse pro-apoptotic protein-BAX or Phytophthora infestans PAMP-INF1. Overexpression of Pst_8713 in plants suppressed pattern-triggered immunity (PTI) -associated callose deposition and expression of PTI-associated marker genes and promoted bacterial growth in planta. Effector-triggered immunity (ETI) induced by an avirulent Pst isolate was weakened when we overexpressed Pst_8713 in wheat leaves which accompanied by reduction of reactive oxygen species (ROS) accumulation and hypersensitive response (HR). In addition, the host induced gene silencing (HIGS) experiment showed that knockdown of Pst_8713 weakened the virulence of Pst by producing fewer uredinia. These results indicated that candidate effector Pst_8713 is involved in plant defense suppression and contributes to enhancing the Pst virulence.

CaHDZ27, a Homeodomain-Leucine Zipper I Protein, Positively Regulates the Resistance to Ralstonia solanacearum Infection in Pepper

Homeodomain-leucine zipper class I (HD-Zip I) transcription factors have been functionally characterized in plant responses to abiotic stresses, but their roles in plant immunity are poorly understood. Here, a HD-Zip I gene, CaHZ27, was isolated from pepper (Capsicum annum) and characterized for its role in pepper immunity. Quantitative real-time polymerase chain reaction showed that CaHDZ27 was transcriptionally induced by Ralstonia solanacearum inoculation and exogenous application of methyl jasmonate, salicylic acid, or ethephon. The CaHDZ27-green fluorescent protein fused protein was targeted exclusively to the nucleus. Chromatin immunoprecipitation demonstrated that CaHDZ27 bound to the 9-bp pseudopalindromic element (CAATAATTG) and triggered β-glucuronidase expression in a CAATAATTG-dependent manner. Virus-induced gene silencing of CaHDZ27 significantly attenuated the resistance of pepper plants against R. solanacearum and downregulated defense-related marker genes, including CaHIR1, CaACO1, CaPR1, CaPR4, CaPO2, and CaBPR1. By contrast, transient overexpression of CaHDZ27 triggered strong cell death mediated by the hypersensitive response and upregulated the tested immunity-associated marker genes. Ectopic CaHDZ27 expression in tobacco enhances its resistance against R. solanacearum. These results collectively suggest that CaHDZ27 functions as a positive regulator in pepper resistance against R. solanacearum. Bimolecular fluorescence complementation and coimmunoprecipitation assays indicate that CaHDZ27 monomers bind with each other, and this binding is enhanced significantly by R. solanacearum inoculation. We speculate that homodimerization of CaHZ27 might play a role in pepper response to R. solanacearum, further direct evidence is required to confirm it.

Genome-wide association mapping of resistance to a Brazilian isolate of Sclerotinia sclerotiorum in soybean genotypes mostly from Brazil

BACKGROUND

Sclerotinia Stem Rot (SSR), caused by the fungal pathogen Sclerotinia sclerotiorum, is ubiquitous in cooler climates where soybean crops are grown. Breeding for resistance to SSR remains challenging in crops like soybean, where no single gene provides strong resistance, but instead, multiple genes work together to provide partial resistance. In this study, a genome-wide association study (GWAS) was performed to dissect the complex genetic architecture of soybean quantitative resistance to SSR and to provide effective molecular markers that could be used in breeding programs. A collection of 420 soybean genotypes were selected based on either reports of resistance, or from one of three different breeding programs in Brazil, two commercial, one public. Plant genotype sensitivity to SSR was evaluated by the cut stem inoculation method, and lesion lengths were measured at 4 days post inoculation.

RESULTS

Genotyping-by-sequencing was conducted to genotype the 420 soybean lines. The TASSEL 5 GBSv2 pipeline was used to call SNPs under optimized parameters, and with the extra step of trimming adapter sequences. After filtering missing data, heterozygosity, and minor allele frequency, a total of 11,811 SNPs and 275 soybean genotypes were obtained for association analyses. Using a threshold of FDR-adjusted p-values <0.1, the Compressed Mixed Linear Model (CMLM) with Genome Association and Prediction Integrated Tool (GAPIT), and the Fixed and Random Model Circulating Probability Unification (FarmCPU) methods, both approaches identified SNPs with significant association to disease response on chromosomes 1, 11, and 18. The CMLM also found significance on chromosome 19, whereas FarmCPU also identified significance on chromosomes 4, 9, and 16.

CONCLUSIONS

These similar and yet different results show that the computational methods used can impact SNP associations in soybean, a plant with a high degree of linkage disequilibrium, and in SSR resistance, a trait that has a complex genetic basis. A total of 125 genes were located within linkage disequilibrium of the three loci shared between the two models. Their annotations and gene expressions in previous studies of soybean infected with S. sclerotiorum were examined to narrow down the candidates.