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

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.

A 14-3-3 Protein Ca16R Acts Positively in Pepper Immunity against Ralstonia solanacearum by Interacting with CaASR1

Although 14-3-3 proteins have been implicated in plant growth, development, and stress response, their roles in pepper immunity against R. solanacearum remain poorly understood. In this study, a 14-3-3-encoding gene in pepper, Ca16R, was found to be upregulated by R. solanacearum inoculation (RSI), its silencing significantly reduced the resistance of pepper plants to RSI, and its overexpression significantly enhanced the resistance of Nicotiana benthamiana to RSI. Consistently, its transient overexpression in pepper leaves triggered HR cell death, indicating that it acts positively in pepper immunity against RSI, and it was further found to act positively in pepper immunity against RSI by promoting SA but repressing JA signaling. Ca16R was also found to interact with CaASR1, originally using pull-down combined with a spectrum assay, and then confirmed using bimolecular fluorescence complementation (BiFC) and a pull-down assay. Furthermore, we found that CaASR1 transient overexpression induced HR cell death and SA-dependent immunity while repressing JA signaling, although this induction and repression was blocked by Ca16R silencing. All these data indicate that Ca16R acts positively in pepper immunity against RSI by interacting with CaASR1, thereby promoting SA-mediated immunity while repressing JA signaling. These results provide new insight into mechanisms underlying pepper immunity against RSI.

Draft Genome Sequence of Seven Pigmented Strains of Xanthomonas citri pv. anacardii, the Causal Agent of Cashew Angular Spot

Cashew (Anacardium occidentale) angular leaf spot is caused by pigmented and non-pigmented strains of Xanthomonas citri pv. anacardii, which have been isolated from infected plants in Brazil. The disease symptoms can be observed in leaves, stems, and fruits. Given that infection in young fruits results in fruits unsuitable for commercialization, angular leaf spot represents a serious threat to the cashew crop in Brazil. Here, we report the genomic sequencing of seven pigmented strains of X. citri pv. anacardii, obtained from the leaves of cashew trees from São Paulo state, Brazil, in 2009. The construction of the libraries was carried out according to the manufacturer, and whole-genome sequencing was performed using the Illumina HiSeq 2500 platform. Genome size, number of coding sequences, largest contig length, and N50 ranged from 4,996,984 to 5,003,485 bp, 4,621 to 4,643 bp, 212,513 to 362,232 bp, and 113,582 to 141,003 bp, respectively. GC content and RNA numbers were 64.68% and 54, respectively, for all strains. ANIm and dDDH analyses showed values above 99.5 and 92.1% among these strains and the non-pigmented pathotype strain of X. citri pv. anacardii (IBSBF2579PT). A maximum likelihood tree built with 2,708 core genes grouped all X. citri pv. anacardii strains in the same clade, with a 100% bootstrap. These resources will contribute in a relevant way to help understand the ecological, taxonomic, evolutionary, pathogenicity, and virulence aspects of X. citri pv. anacardii, which will be useful for the study and development of techniques for managing cashew angular leaf spot.

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.

Genome-Wide Identification and Expression Analysis of the 14-3-3 (TFT) Gene Family in Tomato, and the Role of SlTFT4 in Salt Stress

The 14-3-3 proteins, which are ubiquitous and highly conserved in eukaryotic cells, play an essential role in various areas of plant growth, development, and physiological processes. The tomato is one of the most valuable vegetable crops on the planet. The main objective of the present study was to perform genome-wide identification and analysis of the tomato 14-3-3 (SlTFT) family to investigate its response to different abiotic stresses and phytohormone treatments in order to provide valuable information for variety improvement. Here, 13 SlTFTs were identified using bioinformatics methods. Characterization showed that they were categorized into ε and non-ε groups with five and eight members, accounting for 38.5% and 61.5%, respectively. All the SlTFTs were hydrophilic, and most of them did not contain transmembrane structural domains. Meanwhile, the phylogeny of the SlTFTs had a strong correlation with the gene structure, conserved domains, and motifs. The SlTFTs showed non-random chromosomal distribution, and the promoter region contained more cis-acting elements related to abiotic stress tolerance and phytohormone responses. The results of the evolutionary analysis showed that the SlTFTs underwent negative purifying selection during evolution. Transcriptional profiling and gene expression pattern analysis showed that the expression levels of the SlTFTs varied considerably in different tissues and periods, and they played a specific role under various abiotic stresses and phytohormone treatments. Meanwhile, the constructed protein-based interaction network systematically broadens our understanding of SlTFTs. Finally, the virus-induced gene silencing of SlTFT4 affected the antioxidant and reactive oxygen species defense systems, increased the degree of cellular damage, and reduced salt resistance in tomatoes.

The 14-3-3 protein GF14c positively regulates immunity by modulating the protein homoeostasis of the GRAS protein OsSCL7 in rice

Various types of transcription factors have been reported to be involved in plant-pathogen interactions by regulating defence-related genes. GRAS proteins, plant- specific transcription factors, have been shown to play essential roles in plant growth, development and stress responses. By performing a transcriptome study on rice early defence responses to Magnaporthe oryzae, we identified a GRAS protein, OsSCL7, which was induced by M. oryzae infection. We characterized the function of OsSCL7 in rice disease resistance. OsSCL7 was upregulated upon exposure to M. oryzae and pathogen-associated molecular pattern treatments, and knocking out OsSCL7 resulted in decreased disease resistance of rice to M. oryzae. In contrast, overexpression of OsSCL7 could improve rice disease resistance to M. oryzae. OsSCL7 was mainly localized in the nucleus and showed transcriptional activity. OsSCL7 can interact with GF14c, a 14-3-3 protein, and loss-of-function GF14c leads to enhanced susceptibility to M. oryzae. Additionally, OsSCL7 protein levels were reduced in the gf14c mutant and knocking out OsSCL7 affected the expression of a series of defence-related genes. Taken together, these findings uncover the important roles of OsSCL7 and GF14c in plant immunity and a potential mechanism by which plants fine-tune immunity by regulating the protein stability of a GRAS protein via a 14-3-3 protein.

Coat proteins of necroviruses target 14-3-3a to subvert MAPKKKα-mediated antiviral immunity in plants

Mitogen-activated protein kinase (MAPK) cascades play an important role in innate immunity against various pathogens in plants and animals. However, we know very little about the importance of MAPK cascades in plant defense against viral pathogens. Here, we used a positive-strand RNA necrovirus, beet black scorch virus (BBSV), as a model to investigate the relationship between MAPK signaling and virus infection. Our findings showed that BBSV infection activates MAPK signaling, whereas viral coat protein (CP) counteracts MAPKKKα-mediated antiviral defense. CP does not directly target MAPKKKα, instead it competitively interferes with the binding of 14-3-3a to MAPKKKα in a dose-dependent manner. This results in the instability of MAPKKKα and subversion of MAPKKKα-mediated antiviral defense. Considering the conservation of 14-3-3-binding sites in the CPs of diverse plant viruses, we provide evidence that 14-3-3-MAPKKKα defense signaling module is a target of viral effectors in the ongoing arms race of defense and viral counter-defense.

QTL dissection and mining of candidate genes for Ascochyta fabae and Orobanche crenata resistance in faba bean (Vicia faba L.)

BACKGROUND

Ascochyta blight caused by Ascochyta fabae Speg. and broomrape (Orobanche crenata) are among the economically most significant pathogens of faba bean. Several QTLs conferring resistance against the two pathogens have been identified and validated in different genetic backgrounds. The aim of this study was to saturate the most stable QTLs for ascochyta and broomrape resistance in two Recombinant Inbred Line (RIL) populations, 29H x Vf136 and Vf6 x Vf136, to identify candidate genes conferring resistance against these two pathogens.

RESULTS

We exploited the synteny between faba bean and the model species Medicago truncatula by selecting a set of 219 genes encoding putative WRKY transcription factors and defense related proteins falling within the target QTL intervals, for genotyping and marker saturation in the two RIL populations. Seventy and 50 of the candidate genes could be mapped in 29H x Vf136 and Vf6 x Vf136, respectively. Besides the strong reduction of the QTL intervals, the mapping process allowed replacing previous dominant and pedigree-specific RAPD flanking markers with robust and transferrable SNP markers, revealing promising candidates for resistance against the two pathogens.

CONCLUSIONS

Although further efforts in association mapping and expression studies will be required to corroborate the candidate genes for resistance, the fine-mapping approach proposed here increases the genetic resolution of relevant QTL regions and paves the way for an efficient deployment of useful alleles for faba bean ascochyta and broomrape resistance through marker-assisted breeding.

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.

N-glycosylation shields Phytophthora sojae apoplastic effector PsXEG1 from a specific host aspartic protease

Hosts and pathogens are engaged in a continuous evolutionary struggle for physiological dominance. A major site of this struggle is the apoplast. In Phytophthora sojae-soybean interactions, PsXEG1, a pathogen-secreted apoplastic endoglucanase, is a key focal point of this struggle, and the subject of two layers of host defense and pathogen counterdefense. Here, we show that N-glycosylation of PsXEG1 represents an additional layer of this coevolutionary struggle, protecting PsXEG1 against a host apoplastic aspartic protease, GmAP5, that specifically targets PsXEG1. This posttranslational modification also attenuated binding by the previously described host inhibitor, GmGIP1. N-glycosylation of PsXEG1 at N174 and N190 inhibited binding and degradation by GmAP5 and was essential for PsXEG1's full virulence contribution, except in GmAP5-silenced soybeans. Silencing of GmAP5 reduced soybean resistance against WT P. sojae but not against PsXEG1 deletion strains of P. sojae. The crucial role of N-glycosylation within the three layers of defense and counterdefense centered on PsXEG1 highlight the critical importance of this conserved apoplastic effector and its posttranslational modification in Phytophthora-host coevolutionary conflict.

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 immunity of Meiwa kumquat against Xanthomonas citri is associated with a known susceptibility gene induced by a transcription activator-like effector

Citrus canker caused by Xanthomonas citri subsp. citri (Xcc) is one of the most devastating diseases in citrus. Meiwa kumquat (Fortunella crassifolia) has shown a durable resistance against Xcc. Here, we aimed to characterize the mechanisms responsible for such a durable resistance by characterizing the transcriptional and physiological responses of Meiwa kumquat to Xcc. Inoculation of Meiwa kumquat with Xcc promoted immune responses such as upregulation of PR genes, accumulation of salicylic acid, hypersensitive response (HR)-like cell death and early leaf abscission. Hypertrophy and hyperplasia symptoms, which are known to be caused by Xcc-induction of the canker susceptibility gene LOB1 through the transcription activator-like effector (TALE) PthA4, always appear prior to the development of cell death. Mutation of pthA4 in Xcc abolished the induction of LOB1, canker symptoms, cell death, and leaf abscission and reduced the expression of PR genes in inoculated kumquat leaves without reducing Xcc titers in planta. Transcriptome analysis demonstrated that PthA4 promotes plant biotic and abiotic stress responses and the biosynthesis of abscisic acid. Transcriptional induction of LOB1 homologs in Meiwa kumquat by Xcc pthA4 mutant strains carrying a repertoire of designer TALEs promoted the elicitation of HR-like phenotype and leaf abscission, suggesting that kumquat response to Xcc is associated with upregulation of LOB1. Our study suggests a novel mechanism of plant resistance to Xanthomonas via elicitation of immune responses by upregulation of a host susceptibility gene.

Tomato bHLH132 Transcription Factor Controls Growth and Defense and Is Activated by Xanthomonas euvesicatoria Effector XopD During Pathogenesis

Effector-dependent manipulation of host transcription is a key virulence mechanism used by Xanthomonas species causing bacterial spot disease in tomato and pepper. Transcription activator-like (TAL) effectors employ novel DNA-binding domains to directly activate host transcription, whereas the non-TAL effector XopD uses a small ubiquitin-like modifier (SUMO) protease activity to represses host transcription. The targets of TAL and non-TAL effectors provide insight to the genes governing susceptibility and resistance during Xanthomonas infection. In this study, we investigated the extent to which the X. euvesicatoria non-TAL effector strain Xe85-10 activates tomato transcription to gain new insight to the transcriptional circuits and virulence mechanisms associated with Xanthomonas euvesicatoria pathogenesis. Using transcriptional profiling, we identified a putative basic helix-loop-helix (bHLH) transcription factor, bHLH132, as a pathogen-responsive gene that is moderately induced by microbe-associated molecular patterns and defense hormones and is highly induced by XopD during X. euvesicatoria infection. We also found that activation of bHLH132 transcription requires the XopD SUMO protease activity. Silencing bHLH132 mRNA expression results in stunted tomato plants with enhanced susceptibility to X. euvesicatoria infection. Our work suggests that bHLH132 is required for normal vegetative growth and development as well as resistance to X. euvesicatoria. It also suggests new transcription-based models describing XopD virulence and recognition in tomato.

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.

Aphid effector Me10 interacts with tomato TFT7, a 14-3-3 isoform involved in aphid resistance

We demonstrated previously that expression of Macrosiphum euphorbiae salivary protein Me10 enhanced aphid reproduction on its host tomato (Solanum lycopersicum). However, the mechanism of action of Me10 remained elusive. To confirm the secretion of Me10 by the aphid into plant tissues, we produced Me10 polyclonal antibodies. To identify the plant targets of Me10, we developed a tomato immune induced complementary DNA yeast two-hybrid library and screened it with Me10 as bait. Immunoprecipitation and bimolecular fluorescence complementation (BiFC) assays were performed to validate one of the interactions in planta, and virus-induced gene silencing was used for functional characterization in tomato. We demonstrated that Me10 is secreted into the plant tissues and interacts with tomato 14-3-3 isoform 7 (TFT7) in yeast. Immunoprecipitation assays confirmed that Me10 and its homologue in Aphis gossypii, Ag10k, interact with TFT7 in planta. Further, BiFC revealed that Me10 interaction with TFT7 occurs in the plant cell cytoplasm. While silencing of TFT7 in tomato leaves did not affect tomato susceptibility to M. euphorbiae, it enhanced longevity and fecundity of A. gossypii, the non-host aphid. Our results suggest the model whereby TFT7 plays a role in aphid resistance in tomato and effectors of the Me10/Ag10k family interfere with TFT7 function during aphid infestation.