The in planta regulon of the major Ralstonia solanacearum virulence regulator PhcA
C. Allen (1), D. Khokani (1), T. Tran Minh (1) (1) UW-Madison, U.S.A.

PhcA is an important transcriptional regulator of the bacterial wilt pathogen Ralstonia solanacearum that responds to accumulated quorum sensing signal. When bacterial populations surpass 107 CFU/ml in culture or in the xylem vessels of infected plants, PhcA alters expression of virulence factors like extracellular polysaccharide (EPS), endoglucanase, and motility, through layered regulatory cascades. However, the full extent of the regulon is unknown. We hypothesized that the PhcA system enables R. solanacearum to transition from the saprophytic to the parasitic mode and to improve the host environment for bacterial growth. We compared in planta gene expression levels in wild-type R. solanacearum strain GMI1000 and a ΔphcA mutant strain, early in tomato wilt disease development using RNA-seq. An impressive 634 genes (FDR<0.005) were differentially regulated in the ΔphcA mutant. The study revealed that multiple anion and sugar transporters are expressed at higher levels in the ΔphcA mutant, suggesting that the pathogen uses this regulator to improve its survival at low cell densities. Increased levels of several hemagglutinins and other adhesins in the ΔphcA, suggest that at low cell densities these bacteria may adhere more to plant surfaces and to each other. Scanning electron microscopy revealed that in tomato xylem vessels ΔphcA cells form thick mats connected by many fiber-like structures while wild-type cells form smaller and looser aggregates. Further, the ΔphcA mutant was largely restricted to the point of infection whereas wild-type colonized tissues beyond the infection site. Overall, this study defined the direct and indirect targets of PhcA regulation and suggested testable hypotheses about their biological roles in the bacterial wilt disease cycle.

Abstract Number: P9-236
Session Type: Poster