DEFENSE AND COUNTER-DEFENSE IN THE RICE-XANTHOMONAS PATHOSYSTEM
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Plants have multi-layered immune systems that survey for and respond to pathogens. Successful pathogens subvert these defenses. This defense and counter-defense dynamic leads to a molecular arms race characterized by rapid evolution of plant immunity and pathogen virulence proteins. Pathogens from the bacterial genus Xanthomonas deploy virulence proteins called transcription activator-like (TAL) effectors. TAL effectors comprise DNA binding and gene activation modules and are able to activate specific host genes. Characterized strains of rice-infecting Xanthomonas oryzae encode as many as 26 TAL effectors, with distinct specificities. Hijacking host transcription is a powerful virulence mechanism, however bacterial proteins delivered into the plant cell are at risk of being detected by host defense proteins. Carolina Gold Select rice has an immunity gene, Xo1, that recognizes TAL effectors and prevents disease. Interestingly, this variety is susceptible to some strains of Xanthomonas oryzae, suggesting a second, unidentified counter-defense pathogen protein. Here, I identify and study a class of Xanthomonas counter-defense proteins encoded by genes originally overlooked as TAL effector pseudogenes due to deletions in the N- and C-terminal coding regions. In fact, these truncated TAL effectors (truncTALEs) are non-DNA-binding virulence factors that suppress Xo1-mediated plant immunity. Further, using long-read sequencing a high-quality genome assembly of Carolina Gold Select was generated. Fourteen nucleotide binding and leucine-rich repeat (NLR) defense protein genes were identified at the Xo1 locus. Seven of these Fourteen are expressed in the presence of the pathogen, and one was a strong Xo1 candidate based on structural similarity to the functionally similar Xa1 gene. A broad comparison of NLRs revealed the presence of Xa1-like genes in diverse Oryzae species. Next, by expressing the Xo1 candidate as a transgene in a susceptible variety and observing resistance, we confirmed it to be Xo1. We showed that, like TAL effectors and truncTALEs, the Xo1 protein localizes to the nucleus. Xo1 co-immunoprecipitates with a tagged truncTALE, suggesting that suppression of Xo1-mediated defense is due to interaction of the two proteins.
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Collmer, Alan