The role of bacterial taxis in Ralstonia solanacearum-host interactions

Ralstonia solanacearum is a soilborne plant pathogen of considerable economic importance. Qualitative and quantitative taxis assays revealed that this bacterium is specifically attracted to diverse compounds, and especially to root exudates from a host plant. Two site-directed R. solanacearum mutants lacking either CheA or CheW were created. Both mutants were completely non-tactic but retained normal swimming motility. Both non-tactic mutants had significantly reduced virulence that was indistinguishable from that of a nonmotile mutant, demonstrating that directed motility, not simply random motion, is required for full virulence. Competition assays showed that tactic ability also contributes to bacterial fitness during plant invasion and colonization by R. solanacearum.; The role of a specific type of taxis, aerotaxis in the virulence of R. solanacearum was investigated. Two putative aerotaxis transducer (aer) genes from R. solanacearum were cloned. Heterologous complementation studies showed that both R. solanacearum aerotaxis transducers functioned in Escherichia coli. Single and double site-directed aer mutants of R. solanacearum failed to respond to an oxygen gradient in an aerotaxis assay. Mutants lacking aer2 or both aer genes were slightly delayed in disease development on unwounded tomato plants, possibly due to their inability to rapidly locate and aggregate on plant roots. Surprisingly, all aerotaxis mutants over-produced biofilms on polyvinylchloride surfaces, indicating a specific role for aerotaxis in this bacterial behavior.; A two-component regulatory system, VsrA/D, was known to be required for virulence and EPS production. VsrA/D negatively regulates swimming motility and thus taxis through the FlhDC motility regulator, especially under nutrient stress conditions. VsrA/D positively controls many stress response processes including cold tolerance, resistance to reactive oxygen species and biofilm formation. VsrA/D also directly regulates expression of the global regulator PhcA and the SolI/R acyl-homoserine lactone quorum sensing locus. Moreover, vsrAD mutants grow poorly in planta because VsrA/D positively regulates the hrp-encoded Type Three Secretion System. Overall, VsrA/D integrates signals of nutritional and possibly other stresses to control diverse functions required for R. solanacearum virulence and stress tolerance.