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

Abstract

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 nonmotile 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.

Virulence attenuation in Salmonella enterica rcsC mutants with constitutive activation of the Rcs system

“…Control of flagella and chemotaxis is exercised through the master operon flhDC (Macnab, 1996). Salmonella flhDC mutants are defective in invasion of in vitro cultured epithelial cells due to their lack of motility (Schmitt et al, 2001;Young et al, 2000) and a role in virulence for these genes has been shown in other bacteria (Givaudan & Lanois, 2000;Tans-Kersten et al, 2004). However, a S. enterica serovar Typhimurium flhDC mutant is not attenuated (Fig.…”

Chemotaxis Is Required for Virulence and Competitive Fitness of the Bacterial Wilt Pathogen Ralstonia solanacearum

“…For complementation studies, a 12-kb EcoRI fragment containing the intact cheA and cheW genes was subcloned into pSTBlue-1 from R. solanacearum strain K60 genomic library cosmid 2-89, which contains a large cluster of motility and taxis genes (50,52), to create pSJYcheYAW12.0. The 12-kb insert of pSJYcheYAW12.0 was transferred into pBluescript II SK(Ϫ) with BamHI-HindIII to create pBJYcheYAW12.0.…”

Global regulation of pathogenicity mechanism of Ralstonia solanacearum

“…Active PhcA is regulated in response to cell density by a quorum-sensing mechanism that involves the specific autoinducer molecule 3-hydroxy palmitic acid methyl ester (3-OH PAME) (Flavier et al 1997). At a low cell density, presumably corresponding to saprophytic life and early plant colonization, PhcA is not expressed in culture, leading to expression of early disease virulence factors, including several polygalacturonases and both twitching and swimming motility (Liu et al 2001;Kang et al 2002;Tans-Kersten et al 2004). At a later stage of infection, at a high cell density, the accumulation of 3-OH PAME leads to activation of PhcA and, subsequently, production of EPS and activation of potent CWDEs (cellulases and pectin methylesterase).…”