The role of rhizosphere-induced traits in pathogenic fitness of Ralstonia solanacearum

Traits that allow adaptation and survival in the rhizosphere environment are important for success of Ralstonia solanacearum, a soilborne pathogen that causes bacterial wilt disease of tomato. A taxis-based expression technology strategy identified seven R. solanacearum race 3 biovar 2 strain UW551 rex (root exudate expressed) genes that were induced in the presence of tomato root exudate. These included genes known to be important in the R. solanacearum-host interaction as well as new genes that were further characterized for their contribution to bacterial wilt disease development.;rex4 encodes a ferredoxin protein belonging to a cluster of genes encoding a Cbb3-type cytochrome c oxidase (Cbb3-cco) that is important in aerobic respiration under microaerobic conditions in other proteobacteria. This gene was significantly induced in microaerobic environments. A mutant lacking ccoN, which encodes a structural protein in the cytochrome c oxidase complex, grew poorly at 0.5% O2. This mutant was also reduced in its ability to adhere to tomato roots and in colonization of tomato stems. The mutant's growth deficiencies in planta and under hypoxia likely account for a significant lag in bacterial wilt disease development of UW551 cco relative to wild-type on tomato whether inoculated by soil soak or through a cut petiole. This suggested that the pathogen encounters microaerobic environments during its life cycle and that Cbb3-cco contributes to pathogen fitness and success.;rex5 encodes Dps, a putative non-specific DNA binding protein from starved cells. This gene was highly expressed after prolonged incubation or nutrient stress. Starved dps mutant cells had increased sensitivity to hydrogen peroxide at both high and low cell densities. This indicated that Dps contributes to oxidative stress tolerance not just in populations experiencing starvation due to stationary phase growth in culture but also in the less dense populations that are more likely to occur in the rhizosphere and tomato xylem during early bacterial wilt disease. Plant assays demonstrated that Dps contributed quantitatively to root adherence, stem colonization, and bacterial wilt disease development on tomato. These results suggest R. solanacearum encounters a nutrient-poor and/or oxidative environment in the rhizosphere and xylem of susceptible tomato plants.