Molecular epidemiology and virulence typing of Ralstonia solanacearum raise new prospects for sustainable control of Solanaceae bacterial wilt

Intensification of inter-continental trade exchanges have favored the migration of plant pathogens over global scales on quiet short time frames. Implementation of efficient and durable strategies for management of plant bacterial diseases require to deep investigate pathways and circuits of dissemination, as well as evolutionary mechanisms involved in bacterial adaptation to a new host specie s or to a new resistant host. Whereas research has mostly focused on molecular plant-bacteria interactions, there is a huge need to fill the gap regarding bacterial ecology, pathogen evolutionary potential, and population dynamics allowing adaptation to host. To this aspect, Ralstonia soltmacearum, causing bacterial wilts on a huge range of species (more than 54 botanical families) over the tropical and subtropical belt, is a fascinating model. This species complex (RSSC) is composed of four phylotypes of different geographical origins (phylotype I from Asia, phylotype II from America, phylotype III from Africa, phylotype IV from Indonesia), within which have regularly appeared emerging Iineages of virulence variants. Notably, breeding for Solanaceae resistance to R.solanacearum has been hindered for decades by the scarcity of high level resistance sources, strong genotype x environment interactions, and the huge gellomic and phenotypic plasticity of the pathogen. Recently, the R.solanacearum x Solanaceae interactions were formalized into six virulence types, named pathoprofiles. The evolutionary dynamics within the R.solanacearum species complex (RSSC) was investigated using multi-locus sequence analysis (MLSA) on a worldwide collection. Recombination was found ubiquitous within the RSSC, but with contrasting recombination rates and demographic histories across phylotypes. Phylotype I (and, to a lesser extent, phylotype III) is highly recombinogenic, and carries molecular signatures of a recent and rapid demograhic expansion. Taken together, these findings strongly suggest that phylotypes display contrasting evolutionary potentials: highest within phylotype I, lowest within IIB. Given its worldwide prevalence, high virulence variability, and evolutionary potential, phylotype I thus may constitute the priority group to control. The ecological dynamics of phylotype I was thus further investigated using fast-evolving minisatellite (VNTR) markers, developed thank to genomic resources available at GENOSCOPE. Using the 26 VNTR loci identified, four phylotype-specific MLVA schemes were deigned. The phylotype I- specific 13 loci-MLVA scheme identified 11 clonal complexes (CC) within the global collection, among which seven were specific to Africa and five to Cote d'Ivoire. The CC06, prevalent in South-Africa, Kenya and Cote d'Ivoire, gathered highly virulent and aggressive strains, bypassing the main tomato and eggplant resistance sources, and whose putative avirulence type III effector repertoire was specifie. The promising prospects offered by MLV A typing for source tracing and monitoring population evolution to host resistance will be discussed. (Résumé d'auteur