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Identification of putative PAMPs in Ralstonia solanacearum proteins using Tajima’s D test

Noam Eckshtain-Levi: PPWS Department, Virginia Tech

<div>Host perception of pathogen-associated molecular patterns (PAMPs) constitutes the first layer of plant innate immunity with the best known PAMP being the flg22 epitope of flagellin. Plants unable to recognize PAMPs exhibit enhanced disease susceptibility. Therefore, bacterial pathogens have evolved several strategies to evade recognition. For example, <em>Ralstonia solanacearum</em>, the causal agent of bacterial wilt disease, has multiple non-synonymous mutations in the flg22 region that enable it to escape recognition by the pattern recognition receptor FLS2. To date, elf18, an epitope of elongation factor Tu (EF-Tu), is the only known PAMP of <em>R. solanacearum.</em> In order to discover new PAMPs in this pathogen, we applied a population genetics test, Tajima's D, to the genome sequences of 37 <em>R. solanacearum</em> strains to search for genes that are under strong selective pressure. The screen identified 49 genes, including the one encoding EF-Tu, of the 3003 genes annotated in <em>R. solanacearum</em>. A gene encoding a cold shock protein, which contains a sequence corresponding to the <em>Pseudomonas syringae</em> PAMP csp22, was identified by the Tajima’s D test and experimentally confirmed. <em>R. solanacearum</em> genes encoding the <em>P. syringae</em> PAMPs flg22 and xup25 were not identified in our screen and were confirmed not to be PAMPs. We conclude that identification of non-neutrally evolving genes using Tajima's D is a promising approach to identify novel PAMPs in bacterial plant pathogens.</div>