The Plant Pathogen Ralstonia solanacearum Needs Aerotaxis for Normal Biofilm Formation and Interactions with Its Tomato Host

Ralstonia solanacearum is a soilborne pathogen that causes bacterial wilt of diverse plant species. To locate and infect host plant rootsR. solanacearum needs taxis, the ability to move toward more favorable conditions. However, the specific signals that attract this pathogen were unknown. One candidate is aerotaxis, or energy taxis, which guides bacteria toward optimal intracellular energy levels. TheR. solanacearum genome encodes two putative aerotaxis transducers. ClonedR. solanacearum aer1 andaer2 genes restored aerotaxis to anEscherichia coli aer mutant, demonstrating that both genes encode heterologously functional aerotaxis transducers. Site-directed mutants lackingaer1 ,aer2 , or bothaer1 andaer2 were significantly less able to move up an oxygen gradient than the wild-type parent strain; in fact, the aerotaxis of theaer mutants was indistinguishable from that of a completely nonmotile strain. Tomato plants inoculated with either theaer2 or theaer1/aer2 mutant had slightly delayed wilt disease development. Furthermore, theaer1/aer2 double mutant was significantly impaired in the ability to rapidly localize on tomato roots compared to its wild-type parent. Unexpectedly, all nonaerotactic mutants formed thicker biofilms on abiotic surfaces than the wild type. These results indicate that energy taxis contributes significantly to the ability ofR. solanacearum to locate and effectively interact with its host plants.