New insights on virulence and evolutionary dynamics of the Ralstonia solanacearum species complex -keys and challenges in the search of durable resistance in solanaceae

Bacterial wilt, caused by the soilborne betaproteobacterium Ralstonia solanacearum, is a major diseaseof solanaceous crops, throughout tropical and subtropical areas, and emerging in temperate regions. Ifgenetic resistance is still considered as the most promising control measure, breeding for resistance toRalstonia solanacearum (Rsol}, has been hindered for decades by the scarcity of high level resistancesources, strong genotype x environment interactions, and the huge genomic and phenotypic plasticity ofthe pathogen. Ralstonia solanacearum is a species complex composed of four phylotypes related togeographical origins (1: Africa-Asia, Il: American, III: Africa, IV: Indonesia). Solanaceae-pathogenic strainsbelong to ail four phylotypes, but most epidemiologically active strains belong to phylotype l, liA, IIB,and III, phylotype 1being the most widespread lineage.Extensive screenings of resistant accessions of tomate, eggplant and pepper towards a worldwide R.solcore-collection, lead to formalize grain Solanaceae-Rsol interactions into six main virulence profiles, andallowed to identify highly resistant accessions, mainly in eggplant and pepper. Among the rn, theeggplant MM960 was demonstrated to carry a phylotype I-resistance major dominant gene (Ersl) and two partial resistance QTLs with strain-specific expression. We identified patterns of strain -specificity,but found no evidence of phvlotvpe-specificltv.'phvlotvpe 1containing four of the six pathopprofiles.Searching for durable resistance implies both the identification of bacterial genes responsible foravirulence and virulence, and assessment of the pathogen evolutionary potential. Considering"immune" accessions (five eggplants, two peppers, one tomato) giving true incompatible interactions(no wilt and no colonization), we then searched for genetic factors associated with bacterial virulenceand avirulence using comparative genomics hybridization (CGH) data. We thus identified 61 type IIIeffectors fairly to highly associated to avirulence to eggplants Dingras, SM6, Ceylan, Surya, and MM960,peppers CA8 and Perennial, and tomate HW7996. Interestingly, some of these effectors were elsewhereshown to contribute to bacterial fitness. Focusing on the accessions MM960 and Dingras, less than 10genes were highly associated with avirulence, among which popP2, avirulence gene interacting with theRRS1-Rresistance gene in Arabidopsis thaliana.We inferred evolutionary dynamics of the R.solanacearum species complex (RSSC) at a global scale,performing multi-Iocus sequence analysis (MLSA) and gene genealogies on a worldwide collection.Although recombination was found ubiquitous within the RSSC, we observed phylotype-specificrecombination rates and demographic histories. Phylotype 1 (and, to a lesser extent, phylotype III) ishighly recombinogenic, and carries molecular signatures of a recent and rapid demographic expansion.Phylotype Il is actually composed of two evolutionary lineages, one (liA) being moderatelyrecombinogenic and expanding, and the other (IIB) being clonai and poorly expanding. Taken together,these findings strongly suggest that phylotype 1 is the most prone to adapt rapidly to new hosts andvarieties, thank to its recombination and mutation rates, capacity to disseminate, and large virulencerange.To decipher evolutionary dynamics leading to adaptation at the local, field scale, an experimentalevolution trial is in course to monitor the impact of Ersl-carrying eggplant MM960 successive craps onthe population structure and putative avirulence genes evolution.