Description:
<jats:title>Summary</jats:title><jats:p>The Gram‐negative bacterium <jats:styled-content style="fixed-case"><jats:italic>X</jats:italic></jats:styled-content><jats:italic>anthomonas euvesicatoria</jats:italic> (<jats:styled-content style="fixed-case"><jats:italic>X</jats:italic></jats:styled-content><jats:italic>cv</jats:italic>) is the causal agent of bacterial spot disease in pepper and tomato. <jats:styled-content style="fixed-case"><jats:italic>X</jats:italic></jats:styled-content><jats:italic>cv</jats:italic> pathogenicity depends on a type <jats:styled-content style="fixed-case">III</jats:styled-content> secretion (<jats:styled-content style="fixed-case">T3S</jats:styled-content>) system that delivers effector proteins into host cells to suppress plant immunity and promote disease. The pool of known <jats:styled-content style="fixed-case"><jats:italic>X</jats:italic></jats:styled-content><jats:italic>cv</jats:italic> effectors includes approximately 30 proteins, most identified in the 85‐10 strain by various experimental and computational techniques. To identify additional <jats:styled-content style="fixed-case"><jats:italic>X</jats:italic></jats:styled-content><jats:italic>cv</jats:italic> 85‐10 effectors, we applied a genome‐wide machine‐learning approach, in which all open reading frames (<jats:styled-content style="fixed-case">ORFs</jats:styled-content>) were scored according to their propensity to encode effectors. Scoring was based on a large set of features, including genomic organization, taxonomic dispersion, <jats:italic>hypersensitive response and pathogenicity</jats:italic> (<jats:italic>hrp</jats:italic>)‐dependent expression, 5′ regulatory sequences, amino acid composition bias and <jats:styled-content style="fixed-case">GC</jats:styled-content> content. Thirty‐six predicted effectors were tested for translocation into plant cells using the hypersensitive response (<jats:styled-content style="fixed-case">HR</jats:styled-content>)‐inducing domain of AvrBs2 as a reporter. Seven proteins (<jats:styled-content style="fixed-case">XopAU</jats:styled-content>, <jats:styled-content style="fixed-case">XopAV</jats:styled-content>, <jats:styled-content style="fixed-case">XopAW</jats:styled-content>, <jats:styled-content style="fixed-case">XopAP</jats:styled-content>, <jats:styled-content style="fixed-case">XopAX</jats:styled-content>, <jats:styled-content style="fixed-case">XopAK</jats:styled-content> and <jats:styled-content style="fixed-case">XopAD</jats:styled-content>) harboured a functional translocation signal and their translocation relied on the <jats:styled-content style="fixed-case">HrpF</jats:styled-content> translocon, indicating that they are <jats:italic>bona fide</jats:italic> <jats:styled-content style="fixed-case">T3S</jats:styled-content> effectors. Remarkably, four belong to novel effector families. Inactivation of the <jats:italic>xop</jats:italic><jats:styled-content style="fixed-case"><jats:italic>AP</jats:italic></jats:styled-content> gene reduced the severity of disease symptoms in infected plants. A decrease in cell death and chlorophyll content was observed in pepper leaves inoculated with the <jats:italic>xop</jats:italic><jats:styled-content style="fixed-case"><jats:italic>AP</jats:italic></jats:styled-content> mutant when compared with the wild‐type strain. However, populations of the <jats:italic>xop</jats:italic><jats:styled-content style="fixed-case"><jats:italic>AP</jats:italic></jats:styled-content> mutant in infected leaves were similar in size to those of wild‐type bacteria, suggesting that the reduction in virulence was not caused by impaired bacterial growth.</jats:p>