Rapid dissemination of host metabolism-manipulating genes via integrative and conjugative elements.

Integrative and conjugative elements (ICEs) are self-transmissible mobile elements that transfer functional genetic units across broad phylogenetic distances. Accessory genes shuttled by ICEs can make significant contributions to bacterial fitness. Most ICEs characterized to date encode readily observable phenotypes contributing to symbiosis, pathogenicity, and antimicrobial resistance, yet the majority of ICEs carry genes of unknown function. Recent observations of rapid acquisition of ICEs in a pandemic lineage of Pseudomonas syringae pv. actinidae led to investigation of the structural and functional diversity of these elements. Fifty-three unique ICE types were identified across the P. syringae species complex. Together they form a distinct family of ICEs (PsICEs) that share a distant relationship to ICEs found in Pseudomonas aeruginosa. PsICEs are defined by conserved backbone genes punctuated by an array of accessory cargo genes, are highly recombinogenic, and display distinct evolutionary histories compared to their bacterial hosts. The most common cargo is a recently disseminated 16-kb mobile genetic element designated Tn6212. Deletion of Tn6212 did not alter pathogen growth in planta, but mutants displayed fitness defects when grown on tricarboxylic acid (TCA) cycle intermediates. RNA-seq analysis of a set of nested deletion mutants showed that a Tn6212-encoded LysR regulator has global effects on chromosomal gene expression. We show that Tn6212 responds to preferred carbon sources and manipulates bacterial metabolism to maximize growth.

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

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

The Ralstonia pseudosolanacearum Type III Effector RipL Delays Flowering and Promotes Susceptibility to Pseudomonas syringae in Arabidopsis thaliana.

The plant defense responses to microbial infection are tightly regulated and integrated with the developmental program for optimal resources allocation. Notably, the defense- associated hormone salicylic acid (SA) acts as a promoter of flowering while several plant pathogens actively target the flowering signaling pathway to promote their virulence or dissemination. Ralstonia pseudosolanacearum inject tens of effectors in the host cells that collectively promote bacterial proliferation in plant tissues. Here, we characterized the function of the broadly conserved R. pseudosolanacearum effector RipL, through heterologous expression in Arabidopsis thaliana . RipL-expressing transgenic lines presented a delayed flowering, which correlated with a low expression of flowering regulator genes. Delayed flowering was also observed in Nicotiana benthamiana plants transiently expressing RipL. In parallel, RipL promoted plant susceptibility to virulent strains of Pseudomonas syringae in the effector-expressing lines or when delivered by the type III secretion system. Unexpectedly, SA accumulation and SA-dependent immune signaling were not significantly affected by RipL expression. Rather, the RNA-seq analysis of infected RipL-expressing lines revealed that the overall amplitude of the transcriptional response was dampened, suggesting that RipL could promote plant susceptibility in an SA-independent manner. Further elucidation of the molecular mechanisms underpinning RipL effect on flowering and immunity may reveal novel effector functions in host cells.

Engineering homoeologs provide a fine scale for quantitative traits in polyploid.

Numerous staple crops exhibit polyploidy and are difficult to genetically modify. However, recent advances in genome sequencing and editing have enabled polyploid genome engineering. The hexaploid black nightshade species Solanum nigrum has immense potential as a beneficial food supplement. We assembled its genome at the scaffold level. After functional annotations, we identified homoeologous gene sets, with similar sequence and expression profiles, based on comparative analyses of orthologous genes with close diploid relatives Solanum americanum and S. lycopersicum. Using CRISPR-Cas9-mediated mutagenesis, we generated various mutation combinations in homoeologous genes. Multiple mutants showed quantitative phenotypic changes based on the genotype, resulting in a broad-spectrum effect on the quantitative traits of hexaploid S. nigrum. Furthermore, we successfully improved the fruit productivity of Boranong, an orphan cultivar of S. nigrum suggesting that engineering homoeologous genes could be useful for agricultural improvement of polyploid crops.

Solanum americanum genome-assisted discovery of immune receptors that detect potato late blight pathogen effectors.

Potato (Solanum tuberosum) and tomato (Solanum lycopersicon) crops suffer severe losses to late blight caused by the oomycete pathogen Phytophthora infestans. Solanum americanum, a relative of potato and tomato, is globally distributed and most accessions are highly blight resistant. We generated high-quality reference genomes of four S. americanum accessions, resequenced 52 accessions, and defined a pan-NLRome of S. americanum immune receptor genes. We further screened for variation in recognition of 315P. infestans RXLR effectors in 52 S. americanum accessions. Using these genomic and phenotypic data, we cloned three NLR-encoding genes, Rpi-amr4, R02860 and R04373, that recognize cognate P. infestans RXLR effectors PITG_22825 (AVRamr4), PITG_02860 and PITG_04373. These genomic resources and methodologies will support efforts to engineer potatoes with durable late blight resistance and can be applied to diseases of other crops.

Diversified host target families mediate convergently evolved effector recognition across plant species.

Recognition of pathogen effectors is a crucial step for triggering plant immunity. Resistance (R) genes often encode for nucleotide-binding leucine-rich repeat receptors (NLRs), and NLRs detect effectors from pathogens to trigger effector-triggered immunity (ETI). NLR recognition of effectors is observed in diverse forms where NLRs directly interact with effectors or indirectly detect effectors by monitoring host guardees/decoys (HGDs). HGDs undergo different biochemical modifications by diverse effectors and expand the effector recognition spectrum of NLRs, contributing robustness to plant immunity. Interestingly, in many cases of the indirect recognition of effectors, HGD families targeted by effectors are conserved across the plant species while NLRs are not. Notably, a family of diversified HGDs can activate multiple non-orthologous NLRs across plant species. Further investigation on HGDs would reveal the mechanistic basis of how the diversification of HGDs confers novel effector recognition by NLRs.

The Ralstonia pseudosolanacearum effector RipE1 is recognized at the plasma membrane by NbPtr1, the Nicotiana benthamiana homologue of Pseudomonas tomato race 1.

The bacterial wilt disease caused by soilborne bacteria of the Ralstonia solanacearum species complex (RSSC) threatens important crops worldwide. Only a few immune receptors conferring resistance to this devastating disease are known so far. Individual RSSC strains deliver around 70 different type III secretion system effectors into host cells to manipulate the plant physiology. RipE1 is an effector conserved across the RSSC and triggers immune responses in the model solanaceous plant Nicotiana benthamiana. Here, we used multiplexed virus-induced gene silencing of the nucleotide-binding and leucine-rich repeat receptor family to identify the genetic basis of RipE1 recognition. Specific silencing of the N. benthamiana homologue of Solanum lycopersicoides Ptr1 (confers resistance to Pseudomonas syringae pv. tomato race 1) gene (NbPtr1) completely abolished RipE1-induced hypersensitive response and immunity to Ralstonia pseudosolanacearum. The expression of the native NbPtr1 coding sequence was sufficient to restore RipE1 recognition in Nb-ptr1 knockout plants. Interestingly, RipE1 association with the host cell plasma membrane was necessary for NbPtr1-dependent recognition. Furthermore, NbPtr1-dependent recognition of RipE1 natural variants is polymorphic, providing additional evidence for the indirect mode of activation of NbPtr1. Altogether, this work supports NbPtr1 relevance for resistance to bacterial wilt disease in Solanaceae.

Ptr1 and ZAR1 immune receptors confer overlapping and distinct bacterial pathogen effector specificities.

Some nucleotide-binding and leucine-rich repeat receptors (NLRs) indirectly detect pathogen effectors by monitoring their host targets. In Arabidopsis thaliana, RIN4 is targeted by multiple sequence-unrelated effectors and activates immune responses mediated by RPM1 and RPS2. These effectors trigger cell death in Nicotiana benthamiana, but the corresponding NLRs have yet not been identified. To identify N. benthamiana NLRs (NbNLRs) that recognize Arabidopsis RIN4-targeting effectors, we conducted a rapid reverse genetic screen using an NbNLR VIGS library. We identified that the N. benthamiana homolog of Ptr1 (Pseudomonas tomato race 1) recognizes the Pseudomonas effectors AvrRpt2, AvrRpm1, and AvrB. We demonstrated that recognition of the Xanthomonas effector AvrBsT and the Pseudomonas effector HopZ5 is conferred independently by the N. benthamiana homolog of Ptr1 and ZAR1. Interestingly, the recognition of HopZ5 and AvrBsT is contributed unequally by Ptr1 and ZAR1 in N. benthamiana and Capsicum annuum. In addition, we showed that the RLCK XII family protein JIM2 is required for the NbZAR1-dependent recognition of AvrBsT and HopZ5. The recognition of sequence-unrelated effectors by NbPtr1 and NbZAR1 provides an additional example of convergently evolved effector recognition. Identification of key components involved in Ptr1 and ZAR1-mediated immunity could reveal unique mechanisms of expanded effector recognition.

A plasma membrane nucleotide-binding leucine-rich repeat receptor mediates the recognition of the Ralstonia pseudosolanacearum effector RipY in Nicotiana benthamiana.

Bacterial wilt disease caused by several Ralstonia species is one of the most destructive diseases in Solanaceae crops. Only a few functional resistance genes against bacterial wilt have been cloned to date. Here, we show that the broadly conserved type III secreted effector RipY is recognized by the Nicotiana benthamiana immune system, leading to cell death induction, induction of defense-related gene expression, and restriction of bacterial pathogen growth. Using a multiplexed virus-induced gene-silencing-based N. benthamiana nucleotide-binding and leucine-rich repeat receptor (NbNLR) library, we identified a coiled-coil (CC) nucleotide-binding and leucine-rich repeat receptor (CNL) required for recognition of RipY, which we named RESISTANCE TO RALSTONIA SOLANACEARUM RIPY (RRS-Y). Genetic complementation assays in RRS-Y-silenced plants and stable rrs-y knockout mutants demonstrated that RRS-Y is sufficient to activate RipY-induced cell death and RipY-induced immunity to Ralstonia pseudosolanacearum. RRS-Y function is dependent on the phosphate-binding loop motif of the nucleotide-binding domain but independent of the characterized signaling components ENHANCED DISEASE SUSCEPTIBILITY 1, ACTIVATED DISEASE RESISTANCE 1, and N REQUIREMENT GENE 1 and the NLR helpers NB-LRR REQUIRED FOR HR-ASSOCIATED CELL DEATH-2, -3, and -4 in N. benthamiana. We further show that RRS-Y localization at the plasma membrane is mediated by two cysteine residues in the CC domain and is required for RipY recognition. RRS-Y also broadly recognizes RipY homologs across Ralstonia species. Lastly, we show that the C-terminal region of RipY is indispensable for RRS-Y activation. Together, our findings provide an additional effector/receptor pair system to deepen our understanding of CNL activation in plants.

The broad host range pathogen Sclerotinia sclerotiorum produces multiple effector proteins that induce host cell death intracellularly.

Sclerotinia sclerotiorum is a broad host range necrotrophic fungal pathogen, which causes disease on many economically important crop species. S. sclerotiorum has been shown to secrete small effector proteins to kill host cells and acquire nutrients. We set out to discover novel necrosis-inducing effectors and characterize their activity using transient expression in Nicotiana benthamiana leaves. Five intracellular necrosis-inducing effectors were identified with differing host subcellular localization patterns, which were named intracellular necrosis-inducing effector 1-5 (SsINE1-5). We show for the first time a broad host range pathogen effector, SsINE1, that uses an RxLR-like motif to enter host cells. Furthermore, we provide preliminary evidence that SsINE5 induces necrosis via an NLR protein. All five of the identified effectors are highly conserved in globally sourced S. sclerotiorum isolates. Taken together, these results advance our understanding of the virulence mechanisms employed by S. sclerotiorum and reveal potential avenues for enhancing genetic resistance to this damaging fungal pathogen.

An Apoplastic Effector Pat-1Cm of the Gram-Positive Bacterium Clavibacter michiganensis Acts as Both a Pathogenicity Factor and an Immunity Elicitor in Plants.

Clavibacter michiganensis, a Gram-positive plant-pathogenic bacterium, utilizes apoplastic effectors for disease development in host plants. Here, we determine the roles of Pat-1Cm (a putative serine protease) in pathogenicity and plant immunity. Pat-1Cm was found to be a genuine secreted protein, and the secreted mature form did not carry the first 33 amino acids predicted to be a signal peptide (SP). The pat-1Cm mutant impaired to cause wilting, but still caused canker symptom in tomato. Moreover, this mutant failed to trigger the hypersensitive response (HR) in a nonhost Nicotiana tabacum. Among orthologs and paralogs of pat-1Cm , only chp-7Cs from Clavibacter sepedonicus, a potato pathogen, successfully complemented pat-1Cm function in pathogenicity in tomato, whereas all failed to complement pat-1Cm function in HR induction in N. tabacum. Based on the structural prediction, Pat-1Cm carried a catalytic triad for putative serine protease, and alanine substitution of any amino acids in the triad abolished both pathogenicity and HR-inducing activities of Pat-1Cm in C. michiganensis. Ectopic expression of pat-1Cm with an SP from tobacco secreted protein triggered HR in N. tabacum, but not in tomato, whereas a catalytic triad mutant failed to induce HR. Inoculation of the pat-1Cm mutant mixed with the mutant of another apoplastic effector CelA (cellulase) caused severe wilting in tomato, indicating that these two apoplastic effectors can functionally cooperate in pathogenicity. Overall, these results indicate that Pat-1Cm is a distinct secreted protein carrying a functional catalytic triad for serine protease and this enzymatic activity might be critical for both pathogenicity and HR-eliciting activities of Pat-1Cm in plants.

Investigation of natural RIN4 variants reveals a motif crucial for function and provides an opportunity to broaden NLR regulation specificity.

Multiple bacterial effectors target RPM1-INTERACTING PROTEIN4 (RIN4), the biochemical modifications of which are recognized by several plant nucleotide-binding and leucine-rich repeat immune receptor (NLR) proteins. Recently, a comparative study of Arabidopsis and apple (Malus domestica) RIN4s revealed that the RIN4 specificity motif (RSM) is critical for NLR regulation. Here, we investigated the extent to which the RSM contributes to the functions of natural RIN4 variants. Functional analysis of 33 natural RIN4 variants from 28 plant species showed that the RSM is generally required yet sometimes dispensable for the RIN4-mediated suppression of NLR auto-activity or effector-triggered NLR activation. Association analysis of the sequences and fire blight resistance gene originating from Malus × robusta 5 (FB_MR5) activation functions of the natural RIN4 variants revealed H167 to be an indispensable residue for RIN4 function in the regulation of NLRs. None of the tested natural RIN4 variants could suppress RESISTANCE TO PSEUDOMONAS SYRINGAE PV. MACULICOLA1 (RPM1) auto-activity and activate FB_MR5. To engineer RIN4 to carry broader NLR compatibility, we generated chimeric RIN4 proteins, several of which could regulate RPM1, RESISTANT TO PSEUDOMONAS SYRINGAE2 (RPS2), and FB_MR5. We propose that the intrinsically disordered nature of RIN4 provides a flexible platform to broaden pathogen recognition specificity by establishing compatibility with otherwise incompatible NLRs.

Pan-Genome Analysis of Effectors in Korean Strains of the Soybean Pathogen Xanthomonas citri pv. glycines.

Xanthomonas citri pv. glycines is a major pathogen of soybean in Korea. Here, we analyzed pathogenicity genes based on a comparative genome analysis of five Korean strains and one strain from the United States, 8ra. Whereas all six strains had nearly identical profiles of carbohydrate-active enzymes, they varied in diversity and number of candidate type III secretion system effector (T3SE) genes. The five Korean strains were similar in their effectors, but differed from the 8ra strain. Across the six strains, transcription activator-like effectors (TALEs) showed diverse repeat sizes and at least six forms of the repeat variable di-residue (RVD) sequences, with differences not correlated with the origin of the strains. However, a phylogenetic tree based on the alignment of RVD sequences showed two distinct clusters with 17.5 repeats, suggesting that two distinct 17.5 RVD clusters have evolved, potentially to adapt Xcg to growth on distinct soybean cultivars. The predicted effector binding elements of the TALEs fell into six groups and were strongly overlapping in sequence, suggesting evolving target specificity of the binding domains in soybean cultivars. Our findings reveal the variability and adaptability of T3SEs in the Xcg strains and enhance our understanding of Xcg pathogenicity in soybean.

Direct acetylation of a conserved threonine of RIN4 by the bacterial effector HopZ5 or AvrBsT activates RPM1-dependent immunity in Arabidopsis.

Plant pathogenic bacteria deliver effectors into plant cells to suppress immunity and promote pathogen survival; however, these effectors can be recognized by plant disease resistance proteins to activate innate immunity. The bacterial acetyltransferase effectors HopZ5 and AvrBsT trigger immunity in Arabidopsis thaliana genotypes lacking SUPPRESSOR OF AVRBST-ELICITED RESISTANCE 1 (SOBER1). Using an Arabidopsis accession, Tscha-1, that naturally lacks functional SOBER1 but is unable to recognize HopZ5, we demonstrated that RESISTANCE TO P. SYRINGAE PV MACULICOLA 1 (RPM1) and RPM1-INTERACTING PROTEIN 4 (RIN4) are indispensable for HopZ5- or AvrBsT-triggered immunity. Remarkably, T166 of RIN4, the phosphorylation of which is induced by AvrB and AvrRpm1, is directly acetylated by HopZ5 and AvrBsT. Furthermore, we demonstrated that the acetylation of RIN4 T166 is required and sufficient for HopZ5- or AvrBsT-triggered RPM1-dependent defense activation. Finally, we showed that SOBER1 interferes with HopZ5- or AvrBsT-triggered immunity by deacetylating RIN4 T166. Collectively, our study elucidates detailed molecular mechanisms underlying the activation and suppression of plant innate immunity triggered by two bacterial acetyltransferases, HopZ5 and AvrBsT, from different bacterial pathogens.

Ralstonia solanacearum Type III Effector RipJ Triggers Bacterial Wilt Resistance in Solanum pimpinellifolium.

Ralstonia solanacearum causes bacterial wilt disease in solanaceous crops. Identification of avirulence type III-secreted effectors recognized by specific disease resistance proteins in host plant species is an important step toward developing durable resistance in crops. In the present study, we show that R. solanacearum effector RipJ functions as an avirulence determinant in Solanum pimpinellifolium LA2093. In all, 10 candidate avirulence effectors were shortlisted based on the effector repertoire comparison between avirulent Pe_9 and virulent Pe_1 strains. Infection assays with transgenic strain Pe_1 individually carrying a candidate avirulence effector from Pe_9 revealed that only RipJ elicits strong bacterial wilt resistance in S. pimpinellifolium LA2093. Furthermore, we identified that several RipJ natural variants do not induce bacterial wilt resistance in S. pimpinellifolium LA2093. RipJ belongs to the YopJ family of acetyltransferases. Our sequence analysis indicated the presence of partially conserved putative catalytic residues. Interestingly, the conserved amino acid residues in the acetyltransferase catalytic triad are not required for effector-triggered immunity. In addition, we show that RipJ does not autoacetylate its lysine residues. Our study reports the identification of the first R. solanacearum avirulence protein that triggers bacterial wilt resistance in tomato. We expect that our discovery of RipJ as an avirulence protein will accelerate the development of bacterial wilt-resistant tomato varieties in the future.[Formula: see text] Copyright © 2021 The Author(s). This is an open access article distributed under the CC BY 4.0 International license.

Identification of RipAZ1 as an avirulence determinant of Ralstonia solanacearum in Solanum americanum.

Ralstonia solanacearum causes bacterial wilt disease in many plant species. Type III-secreted effectors (T3Es) play crucial roles in bacterial pathogenesis. However, some T3Es are recognized by corresponding disease resistance proteins and activate plant immunity. In this study, we identified the R. solanacearum T3E protein RipAZ1 (Ralstonia injected protein AZ1) as an avirulence determinant in the black nightshade species Solanum americanum. Based on the S. americanum accession-specific avirulence phenotype of R. solanacearum strain Pe_26, 12 candidate avirulence T3Es were selected for further analysis. Among these candidates, only RipAZ1 induced a cell death response when transiently expressed in a bacterial wilt-resistant S. americanum accession. Furthermore, loss of ripAZ1 in the avirulent R. solanacearum strain Pe_26 resulted in acquired virulence. Our analysis of the natural sequence and functional variation of RipAZ1 demonstrated that the naturally occurring C-terminal truncation results in loss of RipAZ1-triggered cell death. We also show that the 213 amino acid central region of RipAZ1 is sufficient to induce cell death in S. americanum. Finally, we show that RipAZ1 may activate defence in host cell cytoplasm. Taken together, our data indicate that the nucleocytoplasmic T3E RipAZ1 confers R. solanacearum avirulence in S. americanum. Few avirulence genes are known in vascular bacterial phytopathogens and ripAZ1 is the first one in R. solanacearum that is recognized in black nightshades. This work thus opens the way for the identification of disease resistance genes responsible for the specific recognition of RipAZ1, which can be a source of resistance against the devastating bacterial wilt disease.

Phylogenetic analysis of ABCG subfamily proteins in plants: functional clustering and coevolution with ABCGs of pathogens.

ABCG subfamily proteins are highly enriched in terrestrial plants. Many of these proteins secrete secondary metabolites that repel or inhibit pathogens. To establish why the ABCG subfamily proteins proliferated extensively during evolution, we constructed phylogenetic trees from a broad range of eukaryotic organisms. ABCG proteins were massively duplicated in land plants and in oomycetes, a group of agronomically important plant pathogens, which prompted us to hypothesize that plant and pathogen ABCGs coevolved. Supporting this hypothesis, full-size ABCGs in host plants (Arabidopsis thaliana and Glycine max) and their pathogens (Hyaloperonospora arabidopsidis and Phytophthora sojae, respectively) had similar divergence times and patterns. Furthermore, generalist pathogens with broad ranges of host plants have diversified more ABCGs than their specialist counterparts. The hypothesis was further tested using an example pair of ABCGs that first diverged during multiplication in a host plant and its pathogen: AtABCG31 of A. thaliana and HpaP802307 of H. arabidopsidis. AtABCG31 expression was activated following infection with H. arabidopsidis, and disrupting AtABCG31 led to increased susceptibility to H. arabidopsidis. Together, our results suggest that ABCG genes in plants and their oomycete pathogens coevolved in an arms race, to extrude secondary metabolites involved in the plant's defense response against pathogens.

Host adaptation and microbial competition drive Ralstonia solanacearum phylotype I evolution in the Republic of Korea.

Bacterial wilt caused by the Ralstonia solanacearum species complex (RSSC) threatens the cultivation of important crops worldwide. We sequenced 30 RSSC phylotype I (R. pseudosolanacearum) strains isolated from pepper (Capsicum annuum) and tomato (Solanum lycopersicum) across the Republic of Korea. These isolates span the diversity of phylotype I, have extensive effector repertoires and are subject to frequent recombination. Recombination hotspots among South Korean phylotype I isolates include multiple predicted contact-dependent inhibition loci, suggesting that microbial competition plays a significant role in Ralstonia evolution. Rapid diversification of secreted effectors presents challenges for the development of disease-resistant plant varieties. We identified potential targets for disease resistance breeding by testing for allele-specific host recognition of T3Es present among South Korean phyloype I isolates. The integration of pathogen population genomics and molecular plant pathology contributes to the development of location-specific disease control and development of plant cultivars with durable resistance to relevant threats.

What are the Top 10 Unanswered Questions in Molecular Plant-Microbe Interactions?

This article is part of the Top 10 Unanswered Questions in MPMI invited review series.The past few decades have seen major discoveries in the field of molecular plant-microbe interactions. As the result of technological and intellectual advances, we are now able to answer questions at a level of mechanistic detail that we could not have imagined possible 20 years ago. The MPMI Editorial Board felt it was time to take stock and reassess. What big questions remain unanswered? We knew that to identify the fundamental, overarching questions that drive our research, we needed to do this as a community. To reach a diverse audience of people with different backgrounds and perspectives, working in different areas of plant-microbe interactions, we queried the more than 1,400 participants at the 2019 International Congress on Molecular Plant-Microbe Interactions meeting in Glasgow. This group effort resulted in a list of ten, broad-reaching, fundamental questions that influence and inform our research. Here, we introduce these Top 10 unanswered questions, giving context and a brief description of the issues. Each of these questions will be the subject of a detailed review in the coming months. We hope that this process of reflecting on what is known and unknown and identifying the themes that underlie our research will provide a framework to use going forward, giving newcomers a sense of the mystery of the big questions and inspiring new avenues and novel insights.[Formula: see text] Copyright © 2020 The Author(s). This is an open access article distributed under the CC BY 4.0 International license.