Identification and characacterization of interstrain chemical weapons in the Ralstonia solanacearum species complex.
A. I. HUERTA (1), F. Ailloud (2), C. Allen (3). (1) University of Wisconsin - Madison, Madison, WI, U.S.A.; (2) CIRAD, UMR Peuplements Végétaux et Bioagresseurs en Milieu Tropical, CIRAD-Université de la Réunion, Pôle de Protection des Plantes, Saint Pierre, La Réunion, France; 3ANSES Plant Health Labo
The globally distributed <i>Ralstonia solanacearum</i> species complex includes thousands of genetically distinct strains. Most are soilborne and many can infect tomato plants, but multiple strains have never been isolated from a single plant in the field. Pairwise inoculations of tomato plants demonstrated large differences in competitive fitness among three phylogenetically distinct <i>R. solanacearum</i> strains. North American strain K60 outcompeted Asian tropical lowland strain GMI1000 and the South American Race 3 highland strain UW551. Similar relative competitive fitness patterns were observed when strains were cultured together in broth, suggesting that this pathogen deploys chemical weapons against related strains. Reciprocal overlay plate inhibition assays using cell-free supernatants from 15 diverse <i>R. solanacearum</i> strains revealed that most strains produce multiple proteinaceous interstrain inhibitors. Screening a strain K60 mutant library for loss of inhibitory ability identified a cluster of Rhs domain genes, which often encode cell surface or secreted toxins. These mutants could not inhibit growth of GMI1000 or UW551 on plates and also could no longer outcompete UW551 in tomato stems. Nearly all sequenced <i>R. solanacearum</i> genomes contain multiple Rhs genes, suggesting that Rhs-type bacteriocins may act generally in competitive fitness and exclusion of this major pathogen.
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