Abstract EN:

Ralstonia solanacearum is a bacterial plant pathogen responsible of the bacterial wilt disease on more than 250 plant species including agronomical crops such as potatoes, tomato and peanuts. It has been reported several times in the literature, the emergence of new pathogenic variants successfully able to infect and induce bacterial wilt disease on previously described resistant plants. To understand the molecular basis of R. solanacearum adaptation to its host, an evolution experiment has been performed in my host team. R. solanacearum has been kept during more than 300 bacterial generations into the stem of several plant species (susceptible, tolerant and resistant plants). At the end of the experiment, the in planta fitness of the derived clones has been tested compared to the ancestral WT strain, and more than 80% of the derived clones had a better fitness. Among them, 50 clones were re-sequenced. Interestingly, 6 independent mutations were found in the same gene of unknown function (renamed efpR), this strongly suggesting an evolutionary parallelism. Reverse genetics approaches confirmed that efpR was involved in the in planta fitness gain. The goals of my thesis were (1) to characterize the function of the efpR gene and (2) to better understand its role in the fitness gain of the bacterium. To do so, transcriptomic and metabolic analysis were performed followed by phenotypic validations. We demonstrated that EfpR is a global regulator of R. solanacearum controlling metabolic and virulence functions. We also demonstrated that all the mutations appeared during the evolution experiment were "loss of function" mutations, these mutations also induced a phenotypic heterogeneity phenomenon. Indeed, 2 types of colonies were observable on plates, one type similar to the colonies formed by the WT strain and another type less mucoid. We also highlighted another gene, the RSc3149 gene which is an efpR homolog involved in the phenotypic heterogeneity. Finaly, most of the efpR related phenotypes were strikingly similar of those associated with a well described regulator of the bacterium, the PhcA major regulator. We performed transcriptomic analysis of both regulons coupled with reporter gene fusions analysis to understand if there was a link between these two regulons and if we could position efpR in the regulatory network of R. solanacearum.[...]