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Ralstonia solanacearum requires PopS, an ancient virulence effector, to suppress SA-mediated defenses during tomato wilt
J. M. JACOBS (1), A. Milling (1), R. M. Mitra (2), F. Ailloud (3), P. Prior (3), C. Allen (1). (1) University of Wisconsin, Madison, WI, U.S.A.; (2) Carleton College, Northfield, MN, U.S.A.; (3) CIRAD-INRA, Saint-Pierre, La Réunion, France

<i>Ralstonia solanacearum</i> requires a Type III (T3) secretion system for bacterial wilt pathogenesis, but the biological functions of individual effectors remain unknown. During tomato wilt, <i>R. solanacearum</i> expresses <i>popS</i>, which encodes an AvrE-family T3 effector. <i>popS</i> homologs were present in all 17 sequenced <i>R. solanacearum</i> strains, and the phylogeny of <i>popS</i> parallels that of the <i>R. solanacearum</i> species complex, suggesting that PopS is an ancient effector needed for association with plants. We determined that <i>popS</i> is required for full virulence on multiple <i>Solanum</i> crop hosts (susceptible potato and susceptible and quantitatively resistant tomato), but not for virulence on a related epidemiologically relevant weed, <i>S. dulcamara</i>. The <i>popS</i> mutant was also significantly delayed in tomato stem colonization following direct inoculation through cut petioles. AvrE-type effectors in other plant pathogenic bacteria suppress plant defenses triggered by the plant signaling molecule salicylic acid (SA). The <i>popS</i> mutant induced higher expression of SA-responsive tomato <i>PR</i> genes than its wild-type parent. Further, pretreating plant roots with SA exacerbated the <i>popS</i> virulence defect. Finally, PopS was dispensable for bacterial colonization of SA-deficient NahG transgenic tomato plants. These results indicate that this conserved T3 effector suppresses SA-mediated defenses in tomato roots and stems, which are the natural infection courts of this soilborne vascular pathogen.

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