Phosphorylation of HopQ1, a type III effector from Pseudomonas syringae, creates a binding site for host 14-3-3 proteins

14-3-3 proteins as a major hub for plant immunity

14-3-3 proteins, ubiquitously present in eukaryotic cells, are regulatory proteins involved in a plethora of cellular processes. In plants, they have been studied in the context of metabolism, development, and stress responses. Recent studies have highlighted the pivotal role of 14-3-3 proteins in regulating plant immunity. The ability of 14-3-3 proteins to modulate immune responses is primarily attributed to their function as interaction hubs, mediating protein-protein interactions and thereby regulating the activity and overall function of their binding partners. Here, we shed light on how 14-3-3 proteins contribute to plant defense mechanisms, the implications of their interactions with components of plant immunity cascades, and the potential for leveraging this knowledge for crop improvement strategies.

Formation of HopQ1:14-3-3 complex in the host cytoplasm modulates nuclear import rate of Pseudomonas syringae effector in Nicotiana benthamiana cells

HopQ1, a type three effector from Pseudomonas syringae upon phosphorylation coopts plant 14-3-3 proteins to control its stability and subcellular localization. Mass spectrometry of the cytoplasm-restricted effector revealed that HopQ1 already in this subcellular compartment undergoes phosphorylation at serine 51 within the canonical 14-3-3 binding motif and within the second putative 14-3-3 binding site, 24RTPSES29. Our analyses revealed that the stoichiometry of the HopQ1:14-3-3a complex is 1:2 indicating that both binding sites of HopQ1 are involved in the interaction. Notably, 24RTPSES29 comprises a putative nuclear translocation signal (NTS). Although a peptide containing NTS mediates nuclear import of a Cargo protein suggesting its role in the nuclear trafficking of HopQ1, a deletion of 25TPS27 does not change HopQ1 distribution. In contrast, elimination of 14-3-3 binding site, accelerates nuclear trafficking the effector. Collectively, we show that formation of the HopQ1:14-3-3 complex occurs in the host cytoplasm and slows down the effector translocation into the nucleus. These results provide a mechanism that maintains the proper nucleocytoplasmic partitioning of HopQ1, and at the same time is responsible for the relocation of 14-3-3s from the nucleus to cytoplasm in the presence of the effector.

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.

Multiple host targets of Pseudomonas effector protein HopM1 form a protein complex regulating apoplastic immunity and water homeostasis

Bacterial type III effector proteins injected into the host cell play a critical role in mediating bacterial interactions with plant and animal hosts. Notably, some bacterial effectors are reported to target sequence-unrelated host proteins with unknown functional relationships. The Pseudomonas syringae effector HopM1 is such an example; it interacts with and/or degrades several HopM1-interacting (MIN) Arabidopsis proteins, including HopM1-interacting protein 2 (MIN2/RAD23), HopM1-interacting protein 7 (MIN7/BIG5), HopM1-interacting protein 10 (MIN10/14-3-3ĸ), and HopM1-interacting protein 13 (MIN13/BIG2). In this study, we purified the MIN7 complex formed in planta and found that it contains MIN7, MIN10, MIN13, as well as a tetratricopeptide repeat protein named HLB1. Mutational analysis showed that, like MIN7, HLB1 is required for pathogen-associated molecular pattern (PAMP)-, effector-, and benzothiadiazole (BTH)-triggered immunity. HLB1 is recruited to the trans-Golgi network (TGN)/early endosome (EE) in a MIN7-dependent manner. Both min7 and hlb1 mutant leaves contained elevated water content in the leaf apoplast and artificial water infiltration into the leaf apoplast was sufficient to phenocopy immune-suppressing phenotype of HopM1. These results suggest that multiple HopM1-targeted MIN proteins form a protein complex with a dual role in modulating water level and immunity in the apoplast, which provides an explanation for the dual phenotypes of HopM1 during bacterial pathogenesis.

Comparative sequence analysis of pPATH pathogenicity plasmids in Pantoea agglomerans gall-forming bacteria

Acquisition of the pathogenicity plasmid pPATH that encodes a type III secretion system (T3SS) and effectors (T3Es) has likely led to the transition of a non-pathogenic bacterium into the tumorigenic pathogen Pantoea agglomerans. P. agglomerans pv. gypsophilae (Pag) forms galls on gypsophila (Gypsophila paniculata) and triggers immunity on sugar beet (Beta vulgaris), while P. agglomerans pv. betae (Pab) causes galls on both gypsophila and sugar beet. Draft sequences of the Pag and Pab genomes were previously generated using the MiSeq Illumina technology and used to determine partial T3E inventories of Pab and Pag. Here, we fully assembled the Pab and Pag genomes following sequencing with PacBio technology and carried out a comparative sequence analysis of the Pab and Pag pathogenicity plasmids pPATHpag and pPATHpab. Assembly of Pab and Pag genomes revealed a ~4 Mbp chromosome with a 55% GC content, and three and four plasmids in Pab and Pag, respectively. pPATHpag and pPATHpab share 97% identity within a 74% coverage, and a similar GC content (51%); they are ~156 kb and ~131 kb in size and consist of 198 and 155 coding sequences (CDSs), respectively. In both plasmids, we confirmed the presence of highly similar gene clusters encoding a T3SS, as well as auxin and cytokinins biosynthetic enzymes. Three putative novel T3Es were identified in Pab and one in Pag. Among T3SS-associated proteins encoded by Pag and Pab, we identified two novel chaperons of the ShcV and CesT families that are present in both pathovars with high similarity. We also identified insertion sequences (ISs) and transposons (Tns) that may have contributed to the evolution of the two pathovars. These include seven shared IS elements, and three ISs and two transposons unique to Pab. Finally, comparative sequence analysis revealed plasmid regions and CDSs that are present only in pPATHpab or in pPATHpag. The high similarity and common features of the pPATH plasmids support the hypothesis that the two strains recently evolved into host-specific pathogens.

14-3-3 proteins facilitate the activation of MAP kinase cascades by upstream immunity-related kinases

Activation of mitogen-activated protein kinase (MAP kinase) cascades is essential for plant immunity. Upon activation by surface-localized immune receptors, receptor-like cytoplasmic kinases (RLCKs) in the cytoplasm phosphorylate MAP kinase kinase kinases (MAPKKKs) to initiate MAP kinase activation. Surprisingly, we found that both the phosphorylation of Arabidopsis (Arabidopsis thaliana) MAPKKKs and the subsequent activation of MAP kinase cascades require the λ and κ isoforms of 14-3-3 proteins, which directly interact with multiple RLCKs and MAPKKKs. The N- and C-termini of MAPKKK5 interact intramolecularly to inhibit the access to the C terminus by RLCKs, whereas the 14-3-3 proteins relieve this inhibition and facilitate the interaction of RLCKs with the C-terminus of MAPKKK5. This enables the phosphorylation of MAPKK5 at Ser599 and Ser682, thus promoting MAP kinase activation and enhancing plant disease resistance. Our study reveals a role of 14-3-3 proteins as scaffolds and activators in the regulation of the RLCK-MAPKKK5 module and provides insight into the mechanism of plant immune signaling.

Phytophthora infestans RxLR Effector PITG06478 Hijacks 14-3-3 to Suppress PMA Activity Leading to Necrotrophic Cell Death

Pathogens often induce cell death for their successful proliferation in the host plant. Plasma membrane H⁺-ATPases (PMAs) are targeted by either pathogens or plant immune receptors in immune response regulation. Although PMAs play pivotal roles in host cell death, the molecular mechanism of effector-mediated regulation of PMA activity has not been described. Here, we report that the Phytophthora infestans RxLR effector PITG06478 can induce cell death in Nicotiana benthamiana but the induced cell death is inhibited by fusicoccin (FC), an irreversible PMA activator. PITG06478, which is localized at the plasma membrane, is not directly associated with the PMA but is associated with Nb14-3-3s, a PMA activator. Immunoblot analyses revealed that the interaction between PITG06478 and Nb14-3-3s was disrupted by FC. PMA activity in PITG06478-expressing plants was eventually inhibited, and cell death likely occurred because the 14-3-3 protein was hijacked. Our results further confirm the significance of PMA activity in host cell death and provide new insight into how pathogens utilize essential host components to sustain their life cycle. [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.

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.

What's new in protein kinase/phosphatase signalling in the control of plant immunity?

Plant immunity is crucial to plant health but comes at an expense. For optimal plant growth, tight immune regulation is required to prevent unnecessary rechannelling of valuable resources. Pattern- and effector-triggered immunity (PTI/ETI) represent the two tiers of immunity initiated after sensing microbial patterns at the cell surface or pathogen effectors secreted into plant cells, respectively. Recent evidence of PTI-ETI cross-potentiation suggests a close interplay of signalling pathways and defense responses downstream of perception that is still poorly understood. This review will focus on controls on plant immunity through phosphorylation, a universal and key cellular regulatory mechanism. Rather than a complete overview, we highlight “what’s new in protein kinase/phosphatase signalling” in the immunity field. In addition to phosphoregulation of components in the pattern recognition receptor (PRR) complex, we will cover the actions of the major immunity-relevant intracellular protein kinases/phosphatases in the ‘signal relay’, namely calcium-regulated kinases (e.g. calcium-dependent protein kinases, CDPKs), mitogen-activated protein kinases (MAPKs), and various protein phosphatases. We discuss how these factors define a phosphocode that generates cellular decision-making ‘logic gates’, which contribute to signalling fidelity, amplitude, and duration. To underscore the importance of phosphorylation, we summarize strategies employed by pathogens to subvert plant immune phosphopathways. In view of recent game-changing discoveries of ETI-derived resistosomes organizing into calcium-permeable pores, we speculate on a possible calcium-regulated phosphocode as the mechanistic control of the PTI-ETI continuum.

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.

Interaction of the Xanthomonas effectors XopQ and XopX results in induction of rice immune responses

Xanthomonas oryzae pv. oryzae uses several type III secretion system (T3SS) secreted effectors, namely XopN, XopQ, XopX and XopZ, to suppress rice immune responses that are induced following treatment with cell wall degrading enzymes. Here we show that a T3SS secreted effector XopX interacts with two of the eight rice 14-3-3 proteins. Mutants of XopX that are defective in 14-3-3 binding are also defective in suppression of immune responses, suggesting that interaction with 14-3-3 proteins is required for suppression of host innate immunity. However, Agrobacterium-mediated delivery of both XopQ and XopX into rice cells results in induction of rice immune responses. These immune responses are not observed when either protein is individually delivered into rice cells. XopQ-XopX-induced rice immune responses are not observed with a XopX mutant that is defective in 14-3-3 binding. Yeast two-hybrid, bimolecular fluorescence complementation and co-immunoprecipitation assays indicate that XopQ and XopX interact with each other. A screen for Xanthomonas effectors that can suppress XopQ-XopX-induced rice immune responses led to the identification of five effectors, namely XopU, XopV, XopP, XopG and AvrBs2, that could individually suppress these immune responses. These results suggest a complex interplay of Xanthomonas T3SS effectors in suppression of both pathogen-triggered immunity and effector-triggered immunity to promote virulence on rice.

The Immune Receptor Roq1 Confers Resistance to the Bacterial Pathogens Xanthomonas, Pseudomonas syringae, and Ralstonia in Tomato

Xanthomonas species, Pseudomonas syringae and Ralstonia species are bacterial plant pathogens that cause significant yield loss in many crop species. Generating disease-resistant crop varieties can provide a more sustainable solution to control yield loss compared to chemical methods. Plant immune receptors encoded by nucleotide-binding, leucine-rich repeat (NLR) genes typically confer resistance to pathogens that produce a cognate elicitor, often an effector protein secreted by the pathogen to promote virulence. The diverse sequence and presence/absence variation of pathogen effector proteins within and between pathogen species usually limits the utility of a single NLR gene to protecting a plant from a single pathogen species or particular strains. The NLR protein Recognition of XopQ 1 (Roq1) was recently identified from the plant Nicotiana benthamiana and mediates perception of the effector proteins XopQ and HopQ1 from Xanthomonas and P. syringae respectively. Unlike most recognized effectors, alleles of XopQ/HopQ1 are highly conserved and present in most plant pathogenic strains of Xanthomonas and P. syringae. A homolog of XopQ/HopQ1, named RipB, is present in most Ralstonia strains. We found that Roq1 confers immunity to Xanthomonas, P. syringae, and Ralstonia when expressed in tomato. Strong resistance to Xanthomonas perforans was observed in three seasons of field trials with both natural and artificial inoculation. The Roq1 gene can therefore be used to provide safe, economical, and effective control of these pathogens in tomato and other crop species and reduce or eliminate the need for traditional chemical controls.

Bacterial type III effector protein HopQ inhibits melanoma motility through autophagic degradation of vimentin

Malignant melanoma is a fatal disease that rapidly spreads to the whole body. Treatments have limited efficiency owing to drug resistance and various side effects. Pseudomonas syringae pv. tomato (Pto) is a model bacterial pathogen capable of systemic infection in plants. Pto injects the effector protein HopQ into the plant cytosol via a type III secretion machinery and suppresses the host immunity. Intriguingly, host plant proteins regulated by HopQ are conserved even in humans and conferred in tumor metastasis. Nevertheless, the potential for HopQ to regulate human cancer metastasis was unknown. In this study, we addressed the suitability of HopQ as a possible drug against melanoma metastasis. In melanoma cells, overexpressed HopQ is phosphorylated and bound to 14-3-3 through its N-terminal domain, resulting in stronger interaction between HopQ and vimentin. The binding of HopQ to vimentin allowed for degradation of vimentin via p62-dependent selective autophagy. Attenuation of vimentin expression by HopQ inhibited melanoma motility and in vivo metastasis. These findings demonstrated that HopQ directly degraded vimentin in melanoma cells and could be applied to an inhibitor of melanoma metastasis.

Pathogenic Bacteria Target Plant Plasmodesmata to Colonize and Invade Surrounding Tissues

A hallmark of multicellular organisms is their ability to maintain physiological homeostasis by communicating among cells, tissues, and organs. In plants, intercellular communication is largely dependent on plasmodesmata (PD), which are membrane-lined channels connecting adjacent plant cells. Upon immune stimulation, plants close PD as part of their immune responses. Here, we show that the bacterial pathogen Pseudomonas syringae deploys an effector protein, HopO1-1, that modulates PD function. HopO1-1 is required for P. syringae to spread locally to neighboring tissues during infection. Expression of HopO1-1 in Arabidopsis (Arabidopsis thaliana) increases the distance of PD-dependent molecular flux between neighboring plant cells. Being a putative ribosyltransferase, the catalytic activity of HopO1-1 is required for regulation of PD. HopO1-1 physically interacts with and destabilizes the plant PD-located protein PDLP7 and possibly PDLP5. Both PDLPs are involved in bacterial immunity. Our findings reveal that a pathogenic bacterium utilizes an effector to manipulate PD-mediated host intercellular communication for maximizing the spread of bacterial infection.

Salicylic acid-mediated plasmodesmal closure via Remorin-dependent lipid organization

Plasmodesmata (PD) create cytoplasmic and membrane continuities between adjacent cells to facilitate cell–cell communication and virus movement. Plant cells have evolved diverse mechanisms to regulate PD plasticity against plant pathogens, including the accumulation of the defense hormone, salicylic acid (SA). However, the mechanism of how this occurs is not well understood. Here, we uncover a mechanism by which SA triggers Remorin-dependent membrane lipid nanodomain assembly, leading to enhancement of the liquid-ordered phase. The higher-ordered lipids, which are particularly enriched at PD membrane, decreased PD membrane plasticity, and thus restricted PD opening and impeded virus spreading. Our findings address a knowledge gap in plant defense mechanisms at the membrane level that rely on SA-controlled lipid order and PD closure.

Plasmodesmata (PD) are plant-specific membrane-lined channels that create cytoplasmic and membrane continuities between adjacent cells, thereby facilitating cell–cell communication and virus movement. Plant cells have evolved diverse mechanisms to regulate PD plasticity in response to numerous environmental stimuli. In particular, during defense against plant pathogens, the defense hormone, salicylic acid (SA), plays a crucial role in the regulation of PD permeability in a callose-dependent manner. Here, we uncover a mechanism by which plants restrict the spreading of virus and PD cargoes using SA signaling by increasing lipid order and closure of PD. We showed that exogenous SA application triggered the compartmentalization of lipid raft nanodomains through a modulation of the lipid raft-regulatory protein, Remorin (REM). Genetic studies, superresolution imaging, and transmission electron microscopy observation together demonstrated that Arabidopsis REM1.2 and REM1.3 are crucial for plasma membrane nanodomain assembly to control PD aperture and functionality. In addition, we also found that a 14-3-3 epsilon protein modulates REM clustering and membrane nanodomain compartmentalization through its direct interaction with REM proteins. This study unveils a molecular mechanism by which the key plant defense hormone, SA, triggers membrane lipid nanodomain reorganization, thereby regulating PD closure to impede virus spreading.

A Bacterial Effector Mimics a Host HSP90 Client to Undermine Immunity

The molecular chaperone HSP90 facilitates the folding of several client proteins, including innate immune receptors and protein kinases. HSP90 is an essential component of plant and animal immunity, yet pathogenic strategies that directly target the chaperone have not been described. Here, we identify the HopBF1 family of bacterial effectors as eukaryotic-specific HSP90 protein kinases. HopBF1 adopts a minimal protein kinase fold that is recognized by HSP90 as a host client. As a result, HopBF1 phosphorylates HSP90 to completely inhibit the chaperone's ATPase activity. We demonstrate that phosphorylation of HSP90 prevents activation of immune receptors that trigger the hypersensitive response in plants. Consequently, HopBF1-dependent phosphorylation of HSP90 is sufficient to induce severe disease symptoms in plants infected with the bacterial pathogen, Pseudomonas syringae. Collectively, our results uncover a family of bacterial effector kinases with toxin-like properties and reveal a previously unrecognized betrayal mechanism by which bacterial pathogens modulate host immunity.

Xanthomonas oryzae pv. oryzae XopQ protein suppresses rice immune responses through interaction with two 14-3-3 proteins but its phospho-null mutant induces rice immune responses and interacts with another 14-3-3 protein

Many bacterial phytopathogens employ effectors secreted through the type-III secretion system to suppress plant innate immune responses. The Xanthomonas type-III secreted non-TAL effector protein Xanthomonas outer protein Q (XopQ) exhibits homology to nucleoside hydrolases. Previous work indicated that mutations which affect the biochemical activity of XopQ fail to affect its ability to suppress rice innate immune responses, suggesting that the effector might be acting through some other pathway or mechanism. In this study, we show that XopQ interacts in yeast and in planta with two rice 14-3-3 proteins, Gf14f and Gf14g. A serine to alanine mutation (S65A) of a 14-3-3 interaction motif in XopQ abolishes the ability of XopQ to interact with the two 14-3-3 proteins and to suppress innate immunity. Surprisingly, the S65A mutant gains the ability to interact with a third 14-3-3 protein that is a negative regulator of innate immunity. The XopQS65A mutant is an inducer of rice immune responses and this property is dominant over the wild-type function of XopQ. Taken together, these results suggest that XopQ targets the rice 14-3-3 mediated immune response pathway and that its differential phosphorylation might enable interaction with alternative 14-3-3 proteins.

Tomato 14-3-3 Proteins Are Required for Xv3 Disease Resistance and Interact with a Subset of Xanthomonas euvesicatoria Effectors

The 14-3-3 phospho-binding proteins with scaffolding activity play central roles in the regulation of enzymes and signaling complexes in eukaryotes. In plants, 14-3-3 isoforms are required for disease resistance and key targets of pathogen effectors. Here, we examined the requirement of the tomato (Solanum lycopersicum) 14-3-3 isoform (TFT) protein family for Xv3 disease resistance in response to the bacterial pathogen Xanthomonas euvesicatoria. In addition, we determined whether TFT proteins interact with the repertoire of X. euvesicatoria type III secretion effector proteins, including AvrXv3, the elicitor of Xv3 resistance. We show that multiple TFT contribute to Xv3 resistance. We also show that one or more TFT proteins physically interact with multiple effectors (AvrXv3, XopE1, XopE2, XopN, XopO, XopQ, and XopAU). Genetic analyses indicate that none of the identified effectors interfere with AvrXv3-elicited resistance into Xv3 tomato leaves; however, XopE1, XopE2, and XopO are required to suppress symptom development in susceptible tomato leaves. Phospho-peptide mapping revealed that XopE2 is phosphorylated at multiple residues in planta and residues T66, T131, and S334 are required for maximal binding to TFT10. Together, our data support the hypothesis that multiple TFT proteins are involved in immune signaling during X. euvesicatoria infection.

NRG1 functions downstream of EDS1 to regulate TIR-NLR-mediated plant immunity in Nicotiana benthamiana

Effector-triggered immunity (ETI) in plants involves a large family of nucleotide-binding leucine-rich repeat (NLR) immune receptors, including Toll/IL-1 receptor-NLRs (TNLs) and coiled-coil NLRs (CNLs). Although various NLR immune receptors are known, a mechanistic understanding of NLR function in ETI remains unclear. The TNL Recognition of XopQ 1 (Roq1) recognizes the effectors XopQ and HopQ1 from Xanthomonas and Pseudomonas, respectively, which activates resistance to Xanthomonas euvesicatoria and Xanthomonas gardneri in an Enhanced Disease Susceptibility 1 (EDS1)-dependent way in Nicotiana benthamiana In this study, we found that the N. benthamiana N requirement gene 1 (NRG1), a CNL protein required for the tobacco TNL protein N-mediated resistance to tobacco mosaic virus, is also essential for immune signaling [including hypersensitive response (HR)] triggered by the TNLs Roq1 and Recognition of Peronospora parasitica 1 (RPP1), but not by the CNLs Bs2 and Rps2, suggesting that NRG1 may be a conserved key component in TNL signaling pathways. Besides EDS1, Roq1 and NRG1 are necessary for resistance to Xanthomonas and Pseudomonas in N. benthamiana NRG1 functions downstream of Roq1 and EDS1 and physically associates with EDS1 in mediating XopQ-Roq1-triggered immunity. Moreover, RNA sequencing analysis showed that XopQ-triggered gene-expression profile changes in N. benthamiana were almost entirely mediated by Roq1 and EDS1 and were largely regulated by NRG1. Overall, our study demonstrates that NRG1 is a key component that acts downstream of EDS1 to mediate various TNL signaling pathways, including Roq1 and RPP1-mediated HR, resistance to Xanthomonas and Pseudomonas, and XopQ-regulated transcriptional changes in N. benthamiana.

Pseudomonas syringae evades phagocytosis by animal cells via type III effector-mediated regulation of actin filament plasticity

Certain animal and plant pathogenic bacteria have developed virulence factors including effector proteins that enable them to overcome host immunity. A plant pathogen, Pseudomonas syringae pv. tomato (Pto) secretes a large repertoire of effectors via a type III secretory apparatus, thereby suppressing plant immunity. Here, we show that Pto causes sepsis in mice. Surprisingly, the effector HopQ1 disrupted animal phagocytosis by inhibiting actin rearrangement via direct interaction with the LIM domain of the animal target protein LIM kinase, a key regulator of actin polymerization. The results provide novel insight into animal host-plant pathogen interactions. In addition, the current study firstly demonstrates that certain plant pathogenic bacteria such as Pto evade phagocytosis by animal cells due to cross-kingdom suppression of host immunity.

Nucleus- and plastid-targeted annexin 5 promotes reproductive development in Arabidopsis and is essential for pollen and embryo formation

BACKGROUND

Pollen development is a strictly controlled post-meiotic process during which microspores differentiate into microgametophytes and profound structural and functional changes occur in organelles. Annexin 5 is a calcium- and lipid-binding protein that is highly expressed in pollen grains and regulates pollen development and physiology. To gain further insights into the role of ANN5 in Arabidopsis development, we performed detailed phenotypic characterization of Arabidopsis plants with modified ANN5 levels. In addition, interaction partners and subcellular localization of ANN5 were analyzed to investigate potential functions of ANN5 at cellular level.

RESULTS

Here, we report that RNAi-mediated suppression of ANN5 results in formation of smaller pollen grains, enhanced pollen lethality, and delayed pollen tube growth. ANN5 RNAi knockdown plants also displayed aberrant development during the transition from the vegetative to generative phase and during embryogenesis, reflected by delayed bolting time and reduced embryo size, respectively. At the subcellular level, ANN5 was delivered to the nucleus, nucleolus, and cytoplasm, and was frequently localized in plastid nucleoids, suggesting a likely role in interorganellar communication. Furthermore, ANN5-YFP co-immunoprecipitated with RABE1b, a putative GTPase, and interaction in planta was confirmed in plastidial nucleoids using FLIM-FRET analysis.

CONCLUSIONS

Our findings let us to propose that ANN5 influences basal cell homeostasis via modulation of plastid activity during pollen maturation. We hypothesize that the role of ANN5 is to orchestrate the plastidial and nuclear genome activities via protein-protein interactions however not only in maturing pollen but also during the transition from the vegetative to the generative growth and seed development.

An Engineered Distant Homolog of Pseudomonas syringae TTSS Effector From Physcomitrella patens Can Act as a Bacterial Virulence Factor

Pseudomonas syringae pv. phaseolicola is the causative agent of halo blight in common bean (Phaseolus vulgaris). Similar to other pathogenic gram-negative bacteria, it secrets a set of type III effectors into host cells to subvert defense mechanisms. HopQ1 (for Hrp outer protein Q) is one of these type III effectors contributing to virulence of bacteria. Upon delivery into a plant cell, HopQ1 undergoes phosphorylation, binds host 14-3-3 proteins and suppresses defense-related signaling. Some plants however, evolved systems to recognize HopQ1 and respond to its presence and thus to prevent infection. HopQ1 shows homology to Nucleoside Hydrolases (NHs), but it contains a modified calcium binding motif not found in the canonical enzymes. CLuster ANalysis of Sequences (CLANS) revealed that HopQ1 and alike proteins make a distinct group of putative NHs located distantly from the classical enzymes. The HopQ1 – like protein (HLP) group comprises sequences from plant pathogenic bacteria, fungi, and lower plants. Our data suggest that the evolution of HopQ1 homologs in bacteria, fungi, and algae was independent. The location of moss HopQ1 homologs inside the fungal clade indicates a possibility of horizontal gene transfer (HGT) between those taxa. We identified a HLP in the moss Physcomitrella patens. Our experiments show that this protein (referred to as PpHLP) extended by a TTSS signal of HopQ1 promoted P. syringae growth in bean and was recognized by Nicotiana benthamiana immune system. Thus, despite the low sequence similarity to HopQ1 the engineered PpHLP acted as a bacterial virulence factor and displayed similar to HopQ1 virulence properties.

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.

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.

Two Strategies of Pseudomonas syringae to Avoid Recognition of the HopQ1 Effector in Nicotiana Species

Pseudomonas syringae employs a battery of type three secretion effectors to subvert plant immune responses. In turn, plants have developed receptors that recognize some of the bacterial effectors. Two strain-specific HopQ1 effector variants (for Hrp outer protein Q) from the pathovars phaseolicola 1448A (Pph) and tomato DC3000 (Pto) showed considerable differences in their ability to evoke disease symptoms in Nicotiana benthamiana. Surprisingly, the variants differ by only six amino acids located mostly in the N-terminal disordered region of HopQ1. We found that the presence of serine 87 and leucine 91 renders PtoHopQ1 susceptible to N-terminal processing by plant proteases. Substitutions at these two positions did not strongly affect PtoHopQ1 virulence properties in a susceptible host but they reduced bacterial growth and accelerated onset of cell death in a resistant host, suggesting that N-terminal mutations rendered PtoHopQ1 susceptible to processing in planta and, thus, represent a mechanism of recognition avoidance. Furthermore, we found that co-expression of HopR1, another effector encoded within the same gene cluster masks HopQ1 recognition in a strain-dependent manner. Together, these data suggest that HopQ1 is under high host-pathogen co-evolutionary selection pressure and P. syringae may have evolved differential effector processing or masking as two independent strategies to evade HopQ1 recognition, thus revealing another level of complexity in plant - microbe interactions.

Dissecting virulence function from recognition: cell death suppression in Nicotiana benthamiana by XopQ/HopQ1-family effectors relies on EDS1-dependent immunity

Many Gram-negative plant pathogenic bacteria express effector proteins of the XopQ/HopQ1 family which are translocated into plant cells via the type III secretion system during infection. In Nicotiana benthamiana, recognition of XopQ/HopQ1 proteins induces an effector-triggered immunity (ETI) reaction which is not associated with strong cell death but renders plants immune against Pseudomonas syringae and Xanthomonas campestris pv. vesicatoria strains. Additionally, XopQ suppresses cell death in N. benthamiana when transiently co-expressed with cell death inducers. Here, we show that representative XopQ/HopQ1 proteins are recognized similarly, likely by a single resistance protein of the TIR-NB-LRR class. Extensive analysis of XopQ derivatives indicates the recognition of structural features. We performed Agrobacterium-mediated protein expression experiments in wild-type and EDS1-deficient (eds1) N. benthamiana leaves, not recognizing XopQ/HopQ1. XopQ recognition limits multiplication of Agrobacterium and attenuates levels of transiently expressed proteins. Remarkably, XopQ fails to suppress cell death reactions induced by different effectors in eds1 plants. We conclude that XopQ-mediated cell death suppression in N. benthamiana is due to the attenuation of Agrobacterium-mediated protein expression rather than the cause of the genuine XopQ virulence activity. Thus, our study expands our understanding of XopQ recognition and function, and also challenges the commonly used co-expression assays for elucidation of in planta effector activities, at least under conditions of ETI induction.

CDPKs and 14-3-3 Proteins: Emerging Duo in Signaling

Calcium-dependent protein kinases (CDPKs) are Ca²⁺-sensors that play pivotal roles in plant development and stress responses. They have the unique ability to directly translate intracellular Ca²⁺ signals into reversible phosphorylation events of diverse substrates which can mediate interactions with 14-3-3 proteins to modulate protein functions. Recent studies have revealed roles for the coordinated action of CDPKs and 14-3-3s in regulating diverse aspects of plant biology including metabolism, development, and stress responses. We review here the underlying interaction and cross-regulation of the two signaling proteins, and we discuss how this insight has led to the emerging concept of CDPK/14-3-3 signaling modules that could contribute to response specificity.

The Ralstonia solanacearum Type III Effector RipAY Is Phosphorylated in Plant Cells to Modulate Its Enzymatic Activity

Most bacterial pathogens subvert plant cellular functions using effector proteins delivered inside plant cells. In the plant pathogen Ralstonia solanacearum, several of these effectors contain domains with predicted enzymatic activities, including acetyltransferases, phosphatases, and proteases, among others. How these enzymatic activities get activated inside plant cells, but not in the bacterial cell, remains unknown in most cases. In this work, we found that the R. solanacearum effector RipAY is phosphorylated in plant cells. One phosphorylated serine residue, S131, is required for the reported gamma-glutamyl cyclotransferase activity of RipAY, responsible for the degradation of gamma-glutamyl compounds (such as glutathione) inside host cells. Accordingly, non-phosphorylable mutants in S131 abolish RipAY-mediated degradation of glutathione in plant cells and the subsequent suppression of plant immune responses. In this article, we examine our results in relation to the recent reports on the biochemical activities of RipAY, and discuss the potential implications of phosphorylation in plant cells as a mechanism to modulate the enzymatic activity of RipAY.

Subcellular Localization of Pseudomonas syringae pv. tomato Effector Proteins in Plants

Animal and plant pathogenic bacteria use type III secretion systems to translocate proteinaceous effectors to subvert innate immunity of their host organisms. Type III secretion/effector systems are a crucial pathogenicity factor in many bacterial pathogens of plants and animals. Pseudomonas syringae pv. tomato (Pst) DC3000 injects a total of 36 protein effectors that target a variety of host proteins. Studies of a subset of Pst DC3000 effectors demonstrated that bacterial effectors, once inside the host cell, are localized to different subcellular compartments, including plasma membrane, cytoplasm, mitochondria, chloroplast, and Trans-Golgi network, to carry out their virulence functions. Identifying the subcellular localization of bacterial effector proteins in host cells could provide substantial clues to understanding the molecular and cellular basis of the virulence activities of effector proteins. In this chapter, we present methods for transient or stable expression of bacterial effector proteins in tobacco and/or Arabidopsis thaliana for live cell imaging as well as confirming the subcellular localization in plants using fluorescent organelle markers or chemical treatment.

The type 3 effector NopL of Sinorhizobium sp. strain NGR234 is a mitogen-activated protein kinase substrate

Pathogenic bacteria utilize type 3 secretion systems to inject type 3 effectors (T3Es) into host cells, thereby subverting host defense reactions. Similarly, T3Es of symbiotic nitrogen-fixing rhizobia can affect nodule formation on roots of legumes. Previous work showed that NopL (nodulation outer protein L) of Sinorhizobium(Ensifer) sp. strain NGR234 is multiply phosphorylated in eukaryotic cells and that this T3E suppresses responses mediated by mitogen-activated protein (MAP) kinase signaling in yeast (mating pheromone signaling) and plant cells (expression of pathogenesis-related defense proteins). Here, we show that NopL is a MAP kinase substrate. Microscopic observations of fluorescent fusion proteins and bimolecular fluorescence complementation analysis in onion cells indicated that NopL is targeted to the nucleus and forms a complex with SIPK (salicylic acid-induced protein kinase), a MAP kinase of tobacco. In vitro experiments demonstrated that NopL is phosphorylatyed by SIPK. At least nine distinct spots were observed after two-dimensional gel electrophoresis, indicating that NopL can be hyperphosphorylated by MAP kinases. Senescence symptoms in nodules of beans (Phaseolus vulgaris cv. Tendergreen) were analyzed to determine the symbiotic effector activity of different NopL variants with serine to alanine substitutions at identified and predicted phosphorylation sites (serine-proline motif). NopL variants with six or eight serine to alanine substitutions were partially active, whereas NopL forms with 10 or 12 substituted serine residues were inactive. In conclusion, our findings provide evidence that NopL interacts with MAP kinases and reveals the importance of serine-proline motifs for effector activity during symbiosis.

A 14-3-3 Family Protein from Wild Soybean (Glycine Soja) Regulates ABA Sensitivity in Arabidopsis

It is widely accepted that the 14-3-3 family proteins are key regulators of multiple stress signal transduction cascades. By conducting genome-wide analysis, researchers have identified the soybean 14-3-3 family proteins; however, until now, there is still no direct genetic evidence showing the involvement of soybean 14-3-3s in ABA responses. Hence, in this study, based on the latest Glycine max genome on Phytozome v10.3, we initially analyzed the evolutionary relationship, genome organization, gene structure and duplication, and three-dimensional structure of soybean 14-3-3 family proteins systematically. Our results suggested that soybean 14-3-3 family was highly evolutionary conserved and possessed segmental duplication in evolution. Then, based on our previous functional characterization of a Glycine soja 14-3-3 protein GsGF14o in drought stress responses, we further investigated the expression characteristics of GsGF14o in detail, and demonstrated its positive roles in ABA sensitivity. Quantitative real-time PCR analyses in Glycine soja seedlings and GUS activity assays in PGsGF14O:GUS transgenic Arabidopsis showed that GsGF14o expression was moderately and rapidly induced by ABA treatment. As expected, GsGF14o overexpression in Arabidopsis augmented the ABA inhibition of seed germination and seedling growth, promoted the ABA induced stomata closure, and up-regulated the expression levels of ABA induced genes. Moreover, through yeast two hybrid analyses, we further demonstrated that GsGF14o physically interacted with the AREB/ABF transcription factors in yeast cells. Taken together, results presented in this study strongly suggested that GsGF14o played an important role in regulation of ABA sensitivity in Arabidopsis.

14-3-3 proteins in plant-pathogen interactions

14-3-3 proteins define a eukaryotic-specific protein family with a general role in signal transduction. Primarily, 14-3-3 proteins act as phosphosensors, binding phosphorylated client proteins and modulating their functions. Since phosphorylation regulates a plethora of different physiological responses in plants, 14-3-3 proteins play roles in multiple signaling pathways, including those controlling metabolism, hormone signaling, cell division, and responses to abiotic and biotic stimuli. Increasing evidence supports a prominent role of 14-3-3 proteins in regulating plant immunity against pathogens at various levels. In this review, potential links between 14-3-3 function and the regulation of plant-pathogen interactions are discussed, with a special focus on the regulation of 14-3-3 proteins in response to pathogen perception, interactions between 14-3-3 proteins and defense-related proteins, and 14-3-3 proteins as targets of pathogen effectors.

The cyst nematode effector protein 10A07 targets and recruits host posttranslational machinery to mediate its nuclear trafficking and to promote parasitism in Arabidopsis

Plant-parasitic cyst nematodes synthesize and secrete effector proteins that are essential for parasitism. One such protein is the 10A07 effector from the sugar beet cyst nematode, Heterodera schachtii, which is exclusively expressed in the nematode dorsal gland cell during all nematode parasitic stages. Overexpression of H. schachtii 10A07 in Arabidopsis thaliana produced a hypersusceptible phenotype in response to H. schachtii infection along with developmental changes reminiscent of auxin effects. The 10A07 protein physically associates with a plant kinase and the IAA16 transcription factor in the cytoplasm and nucleus, respectively. The interacting plant kinase (IPK) phosphorylates 10A07 at Ser-144 and Ser-231 and mediates its trafficking from the cytoplasm to the nucleus. Translocation to the nucleus is phosphorylation dependent since substitution of Ser-144 and Ser-231 by alanine resulted in exclusive cytoplasmic accumulation of 10A07. IPK and IAA16 are highly upregulated in the nematode-induced syncytium (feeding cells), and deliberate manipulations of their expression significantly alter plant susceptibility to H. schachtii in an additive fashion. An inactive variant of IPK functioned antagonistically to the wild-type IPK and caused a dominant-negative phenotype of reduced plant susceptibility. Thus, exploitation of host processes to the advantage of the parasites is one mechanism by which cyst nematodes promote parasitism of host plants.

Mutations in the Predicted Active Site of Xanthomonas oryzae pv. oryzae XopQ Differentially Affect Virulence, Suppression of Host Innate Immunity, and Induction of the HR in a Nonhost Plant

Xanthomonas oryzae pv. oryzae, the bacterial blight pathogen of rice, secretes a number of effectors through a type 3 secretion system. One of these effectors, called XopQ, is required for virulence and suppression of rice innate immune responses induced by the plant cell-wall-degrading enzyme lipase/esterase A (LipA). Bioinformatic analysis suggested that XopQ is homologous to inosine-uridine nucleoside hydrolases (NH). A structural model of XopQ with the protozoan Crithidia fasciculata purine NH suggested that D116 and Y279 are potential active site residues. X. oryzae pv. oryzae xopQ mutants (xopQ-/pHM1::xopQD116A and xopQ-/pHM1::xopQY279A) show reduced virulence on rice compared with xopQ-/pHM1::xopQ. The two predicted XopQ active site mutants (xopQ-/pHM1::xopQD116A and xopQ-/pHM1::xopQY279A) exhibit a reduced hypersensitive response (HR) on Nicotiana benthamiana, a nonhost. However, Arabidopsis lines expressing either xopQ or xopQY279A are equally proficient at suppression of LipA-induced callose deposition. Purified XopQ does not show NH activity on standard nucleoside substrates but exhibits ribose hydrolase activity on the nucleoside substrate analogue 4-nitrophenyl β-D-ribofuranoside. The D116A and Y279A mutations cause a reduction in biochemical activity. These results indicate that mutations in the predicted active site of XopQ affect virulence and induction of the HR but do not affect suppression of innate immunity.

How pathogens use linear motifs to perturb host cell networks

Molecular mimicry is one of the powerful stratagems that pathogens employ to colonise their hosts and take advantage of host cell functions to guarantee their replication and dissemination. In particular, several viruses have evolved the ability to interact with host cell components through protein short linear motifs (SLiMs) that mimic host SLiMs, thus facilitating their internalisation and the manipulation of a wide range of cellular networks. Here we present convincing evidence from the literature that motif mimicry also represents an effective, widespread hijacking strategy in prokaryotic and eukaryotic parasites. Further insights into host motif mimicry would be of great help in the elucidation of the molecular mechanisms behind host cell invasion and the development of anti-infective therapeutic strategies.

NudC Nudix hydrolase from Pseudomonas syringae, but not its counterpart from Pseudomonas aeruginosa, is a novel regulator of intracellular redox balance required for growth, motility and biofilm formation

Nudix pyrophosphatases, ubiquitous in all organisms, have not been well studied. Recent implications that some of them may be involved in response to stress and in pathogenesis indicate that they play important biological functions. We have investigated NudC Nudix proteins from the plant pathogen Pseudomonas syringae pv. tomato str. DC3000 and from the human pathogen Pseudomonas aeruginosa PAO1161. We found that these homologous enzymes are homodimeric and in vitro preferentially hydrolyse NADH. The P. syringae mutant strain deficient in NudC accumulated NADH and displayed significant defects in growth, motility and biofilm formation. The wild type copy of the nudC gene with its cognate promoter delivered in trans into the nudC mutant restored its fitness. However, introduction of the P. syringae nudC gene under the control of the strong tacp promoter into either P. syringae or P. aeruginosa cells had a toxic effect on both strains. Opposite to P. syringae NudC, the P. aeruginosa NudC deficiency as well as its overproduction had no visible impact on cells. Moreover, P. aeruginosa NudC does not compensate the lack of its counterpart in the P. syringae mutant. These results indicate that NudC from P. syringae, but not from P. aeruginosa is vital for bacteria.

The crystal structure of type III effector protein XopQ from Xanthomonas oryzae complexed with adenosine diphosphate ribose

Effector proteins are virulence factors that promote pathogenesis by interfering with various cellular events and are delivered directly into host cells by the secretion systems of many Gram-negative bacteria. Type III effector protein XOO4466 from the plant pathogen Xanthomonas oryzae pv. oryzae (XopQ(Xoo)) and XopQ homologs from other phytopathogens have been predicted to be nucleoside hydrolases based on their sequence similarities. However, despite such similarities, recent structural and functional studies have revealed that XopQ(Xoo) does not exhibit the expected activity of a nucleoside hydrolase. On the basis of the conservation of a Ca(2+) coordination shell of a ribose-binding site and the spacious active site in XopQ(Xoo), we hypothesized that a novel compound containing a ribosyl moiety could serve as a substrate for XopQ(Xoo). Here, we report the crystal structure of XopQ(Xoo) in complex with adenosine diphosphate ribose (ADPR), which is involved in regulating cytoplasmic Ca(2+) concentrations in eukaryotic cells. ADPR is bound to the active site of XopQ(Xoo) with its ribosyl end tethered to the Ca(2+) coordination shell. The binding of ADPR is further stabilized by interactions mediated by hydrophobic residues that undergo ligand-induced conformational changes. These data showed that XopQ(Xoo) is capable of binding a novel chemical bearing a ribosyl moiety, thereby providing the first step toward understanding the functional role of XopQ(Xoo).

Emerging role of SGT1 as a regulator of NB-LRR-receptor nucleocytoplasmic partitioning

Plant nucleotide-binding (NB) and leucine-rich repeat (LRR) receptors mediate effector-triggered immunity. Two major classes of NB-LRR proteins are involved in this process, namely, toll-interleukin receptor (TIR)-NB-LRR and coiled coil (CC)-NB-LRR proteins. Recent reports show that some of the TIR-NB-LRRs and CC-NB-LRRs localize to the cytoplasm and nucleus. Equilibrium between these pools is required for full resistance, suggesting tight regulation of nucleocytoplasmic receptor shuttling. We recently showed that SGT1, a protein that controls NB-LRR receptor stability and activity, facilitates nuclear import of N protein, which is a TIR-NB-LRR receptor. In this addendum, we show that the subcellular localization of Rx, a CC-NB-LRR protein, reflects the positions of SGT1 ectopic variants in the cell. This suggests that SGT1 might have a general role in maintaining the nucleocytoplasmic balance of NB-LRR receptors. We discuss these results in light of differences in the N and Rx systems of effector-triggered immunity.

Emerging role of SGT1 as a regulator of NB-LRR-receptor nucleocytoplasmic partitioning

Plant nucleotide-binding (NB) and leucine-rich repeat (LRR) receptors mediate effector-triggered immunity. Two major classes of NB-LRR proteins are involved in this process, namely, toll-interleukin receptor (TIR)-NB-LRR and coiled coil (CC)-NB-LRR proteins. Recent reports show that some of the TIR-NB-LRRs and CC-NB-LRRs localize to the cytoplasm and nucleus. Equilibrium between these pools is required for full resistance, suggesting tight regulation of nucleocytoplasmic receptor shuttling. We recently showed that SGT1, a protein that controls NB-LRR receptor stability and activity, facilitates nuclear import of N protein, which is a TIR-NB-LRR receptor. In this addendum, we show that the subcellular localization of Rx, a CC-NB-LRR protein, reflects the positions of SGT1 ectopic variants in the cell. This suggests that SGT1 might have a general role in maintaining the nucleocytoplasmic balance of NB-LRR receptors. We discuss these results in light of differences in the N and Rx systems of effector-triggered immunity.

The bacterial effector HopM1 suppresses PAMP-triggered oxidative burst and stomatal immunity

Successful pathogens counter immunity at multiple levels, mostly through the action of effectors. Pseudomonas syringae secretes c. 30 effectors, some of which have been shown to inhibit plant immunity triggered upon perception of conserved pathogen-associated molecular patterns (PAMPs). One of these is HopM1, which impairs late immune responses through targeting the vesicle trafficking-related AtMIN7 for degradation. Here, we report that in planta expressed HopM1 suppresses two early PAMP-triggered responses, the oxidative burst and stomatal immunity, both of which seem to require proteasomal function but are independent of AtMIN7. Notably, a 14-3-3 protein, GRF8/AtMIN10, was found previously to be a target of HopM1 in vivo, and expression of HopM1 mimics the effect of chemically and genetically disrupting 14-3-3 function. Our data further show that the function of 14-3-3 proteins is required for PAMP-triggered oxidative burst and stomatal immunity, and chemical-mediated disruption of the 14-3-3 interactions with their client proteins restores virulence of a HopM1-deficient P. syringae mutant, providing a link between HopM1 and the involvement of 14-3-3 proteins in plant immunity. Taken together, these results unveil the impact of HopM1 on the PAMP-triggered oxidative burst and stomatal immunity in an AtMIN7-independent manner, most likely acting at the function of (a) 14-3-3 protein(s).

Xanthomonas euvesicatoria type III effector XopQ interacts with tomato and pepper 14-3-3 isoforms to suppress effector-triggered immunity

Effector-triggered immunity (ETI) to host-adapted pathogens is associated with rapid cell death at the infection site. The plant-pathogenic bacterium Xanthomonas euvesicatoria (Xcv) interferes with plant cellular processes by injecting effector proteins into host cells through the type III secretion system. Here, we show that the Xcv effector XopQ suppresses cell death induced by components of the ETI-associated MAP kinase cascade MAPKKKα MEK2/SIPK and by several R/avr gene pairs. Inactivation of xopQ by insertional mutagenesis revealed that this effector inhibits ETI-associated cell death induced by avirulent Xcv in resistant pepper (Capsicum annuum), and enhances bacterial growth in resistant pepper and tomato (Solanum lycopersicum). Using protein-protein interaction studies in yeast (Saccharomyces cerevisiae) and in planta, we identified the tomato 14-3-3 isoform SlTFT4 and homologs from other plant species as XopQ interactors. A mutation in the putative 14-3-3 binding site of XopQ impaired interaction of the effector with CaTFT4 in yeast and its virulence function in planta. Consistent with a role in ETI, TFT4 mRNA abundance increased during the incompatible interaction of tomato and pepper with Xcv. Silencing of NbTFT4 in Nicotiana benthamiana significantly reduced cell death induced by MAPKKKα. In addition, silencing of CaTFT4 in pepper delayed the appearance of ETI-associated cell death and enhanced growth of virulent and avirulent Xcv, demonstrating the requirement of TFT4 for plant immunity to Xcv. Our results suggest that the XopQ virulence function is to suppress ETI and immunity-associated cell death by interacting with TFT4, which is an important component of ETI and a bona fide target of XopQ.

Mp10 and Mp42 from the aphid species Myzus persicae trigger plant defenses in Nicotiana benthamiana through different activities

Aphids are phloem-feeding insects that, like other plant parasites, deliver effectors inside their host to manipulate host responses. The Myzus persicae (green peach aphid) candidate effectors Mp10 and Mp42 were previously found to reduce aphid fecundity upon intracellular transient overexpression in Nicotiana benthamiana. We performed functional analyses of these proteins to investigate whether they activate defenses through similar activities. We employed a range of functional characterization experiments based on intracellular transient overexpression in N. benthamiana to determine the subcellular localization of Mp10 and Mp42 and investigate their role in activating plant defense signaling. Mp10 and Mp42 showed distinct subcellular localization in planta, suggesting that they target different host compartments. Also, Mp10 reduced the levels of Agrobacterium-mediated overexpression of proteins. This reduction was not due to an effect on Agrobacterium viability. Transient overexpression of Mp10 but not Mp42 activated jasmonic acid and salicylic acid signaling pathways and decreased susceptibility to the hemibiotrophic plant pathogen Phytophthora capsici. We found that two candidate effectors from the broad-host-range aphid M. persicae can trigger aphid defenses through different mechanisms. Importantly, we found that some (candidate) effectors such as Mp10 interfere with Agrobacterium-based overexpression assays, an important tool to study effector activity and function.

The Pseudomonas type III effector HopQ1 activates cytokinin signaling and interferes with plant innate immunity

We characterized the molecular function of the Pseudomonas syringae pv. tomato DC3000 (Pto) effector HopQ1. In silico studies suggest that HopQ1 might possess nucleoside hydrolase activity based on the presence of a characteristic aspartate motif. Transgenic Arabidopsis lines expressing HopQ1 or HopQ1 aspartate mutant variants were characterized with respect to flagellin triggered immunity, phenotype and changes in phytohormone content by high-performance liquid chromatography-MS (HPLC-MS). We found that HopQ1, but not its aspartate mutants, suppressed all tested immunity marker assays. Suppression of immunity was the result of a lack of the flagellin receptor FLS2, whose gene expression was abolished by HopQ1 in a promoter-dependent manner. Furthermore, HopQ1 induced cytokinin signaling in Arabidopsis and the elevation in cytokinin signaling appears to be responsible for the attenuation of FLS2 expression. We conclude that HopQ1 can activate cytokinin signaling and that moderate activation of cytokinin signaling leads to suppression of FLS2 accumulation and thus defense signaling.

Cell wall degrading enzyme induced rice innate immune responses are suppressed by the type 3 secretion system effectors XopN, XopQ, XopX and XopZ of Xanthomonas oryzae pv. oryzae

Innate immune responses are induced in plants and animals through perception of Damage Associated Molecular Patterns. These immune responses are suppressed by pathogens during infection. A number of studies have focussed on identifying functions of plant pathogenic bacteria that are involved in suppression of Pathogen Associated Molecular Pattern induced immune responses. In comparison, there is very little information on functions used by plant pathogens to suppress Damage Associated Molecular Pattern induced immune responses. Xanthomonasoryzae pv. oryzae, a gram negative bacterial pathogen of rice, secretes hydrolytic enzymes such as LipA (Lipase/Esterase) that damage rice cell walls and induce innate immune responses. Here, we show that Agrobacterium mediated transient transfer of the gene for XopN, a X. oryzae pv. oryzae type 3 secretion (T3S) system effector, results in suppression of rice innate immune responses induced by LipA. A xopN^- mutant of X. oryzae pv. oryzae retains the ability to suppress these innate immune responses indicating the presence of other functionally redundant proteins. In transient transfer assays, we have assessed the ability of 15 other X. oryzae pv. oryzae T3S secreted effectors to suppress rice innate immune responses. Amongst these proteins, XopQ, XopX and XopZ are suppressors of LipA induced innate immune responses. A mutation in any one of the xopN, xopQ, xopX or xopZ genes causes partial virulence deficiency while a xopN^- xopX^- double mutant exhibits a greater virulence deficiency. A xopN^- xopQ^- xopX^- xopZ^- quadruple mutant of X. oryzae pv. oryzae induces callose deposition, an innate immune response, similar to a X. oryzae pv. oryzae T3S^- mutant in rice leaves. Overall, these results indicate that multiple T3S secreted proteins of X. oryzae pv. oryzae can suppress cell wall damage induced rice innate immune responses.

Crystal structure of the effector protein XOO4466 from Xanthomonas oryzae

Many Gram-negative bacteria deliver their virulence factors into host cells through a secretion system. Those factors, called effector proteins, are involved in the pathogenicity in host cells by interfering with various cellular events. The phytopathogen Xanthomonas oryzae pv. oryzae uses a type III secretion system to inject its effectors, but the functional roles of these proteins remain largely uncharacterized. Here, we determined a crystal structure of XOO4466, an effector from X. oryzae pv. oryzae, and performed a functional analysis. We determined that XOO4466 is similar in sequence to Xanthomonas outer protein Q, a putative nucleoside hydrolase (NH). The overall structure of XOO4466 is homologous to that of NHs, including a metal-binding site, but differs in its oligomeric state and active site topology. Further analysis indicated that antiparallel β-strands commonly found in NHs adjacent to the active site loop are replaced in XOO4466 with a short loop, causing the active site loop to adopt a conformation distinct from that of NHs. Thus, the catalytic residues emanating from the respective active site loop of NHs are absent in the putative active site of XOO4466. Consistent with these structural features, a functional assay indicated that XOO4466 does not exhibit NH activity and possibly catalyzes yet unknown reactions.