Swimming Motility, a Virulence Trait of Ralstonia solanacearum, Is Regulated by FlhDC and the Plant Host Environment

Swimming motility allows the bacterial wilt pathogen Ralstonia solanacearum to efficiently invade and colonize host plants. However, the bacteria are essentially nonmotile once inside plant xylem vessels. To determine how and when motility genes are expressed, we cloned and mutated flhDC, which encodes a major regulator of flagellar biosynthesis and bacterial motility. An flhDC mutant was non-motile and less virulent than its wild-type parent on both tomato and Arabidopsis; on Arabidopsis, the flhDC mutant also was less virulent than a nonmotile fliC flagellin mutant. Genes in the R. solanacearum motility regulon had strikingly different expression patterns in culture and in the plant. In culture, as expected, flhDC expression depended on PehSR, a regulator of early virulence factors; and, in turn, FlhDC was required for fliC (flagellin) expression. However, when bacteria grew in tomato plants, flhDC was expressed in both wild-type and pehR mutant backgrounds, although PehSR is necessary for motility both in culture and in planta. Both flhDC and pehSR were significantly induced in planta relative to expression levels in culture. Unexpectedly, the fliC gene was expressed in planta at cell densities where motile bacteria were not observed, as well as in a nonmotile flhDC mutant. Thus, expression of flhDC and flagellin itself are uncoupled from bacterial motility in the host environment, indicating that additional signals and regulatory circuits repress motility during plant pathogenesis.