|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.