Natural transformation in the Ralstonia solanacearum species complex: number and size of DNA that can be transferred

Ralstonia solanacearum is a widely distributed phytopathogenic bacterium that is known to invade more than 200 host species, mainly in tropical areas. Reference strain GMI1000 is naturally transformable at in vitro and also in planta conditions and thus has the ability to acquire free exogenous DNA. We tested the ubiquity and variability of natural transformation in the four phylotypes of this species complex using 55 strains isolated from different hosts and geographical regions. Eighty per cent of strains distributed in all the phylotypes were naturally transformable by plasmids and/or genomic DNA. Transformability can be considered as a ubiquitous physiological trait in the R. solanacearum species complex. Transformation performed with two independent DNA donors showed that multiple integration events occurred simultaneously in two distant genomic regions. We also engineered a fourfold-resistant R. solanacearum GMI1000 mutant RS28 to evaluate the size of DNA exchanged during natural transformation. The results demonstrated that this bacterium was able to exchange large DNA fragments ranging from 30 to 90 kb by DNA replacement. The combination of these findings indicated that the natural transformation mechanism could be the main driving force of genetic diversification of the R. solanacearum species complex.