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Oral: Bacterial Virulence Regulation


The in planta regulon of the major Ralstonia solanacearum virulence regulator PhcA
D. KHOKHANI (1), T. Tran Minh (1), C. Allen (1) (1) UNIVERSITY OF WISCONSIN-MADISON, U.S.A.

PhcA, an important quorum sensing responsive virulence regulator of the bacterial wilt pathogen Ralstonia solanacearum. PhcA affects expression of virulence factors like extracellular polysaccharide, endoglucanase, and motility. However, the full extent of the regulon is unknown. We used RNAseq to test the hypothesis that the PhcA system enables Rs to transition from saprophytic to parasitic mode. An impressive 634 genes were differentially regulated at early stage of tomato wilt disease development in ΔphcA compared to the wild-type strain GMI1000, with false discovery rate of less than 0.005. Both, RNA-seq data and validation experiments indicated that multiple anion and sugar transporters are expressed at higher levels in the ΔphcA, suggesting that the pathogen uses this regulator to improve its survival at low cell densities. Increased expression of hemagglutinins and other adhesins in ΔphcA suggest that at low population densities R.solanacearum cells may adhere more to plant surfaces and to each other. Scanning electron microscopy revealed that ΔphcA cells form thick mats in tomato xylem vessels. These were strikingly different from the smaller, looser in planta aggregates of wild-type cells. Further, ΔphcA was largely restricted near the point of infection whereas wild-type colonized tissues beyond the infection site. Taken together, the study identified new targets of PhcA regulation and shed more light on their biological roles in the disease cycle.