Xanthomonas campestris pv. campestris (Xcc) is a rod-shaped, aerobic, yellow-pigmented, gram-negative bacterium which possesses a single polar flagellum and is the causal agent of black rot in crucifers. Xcc produces a range of extracellular enzymes and an extracellular polysaccharide known as xanthan gum. Extracellular enzymes and xanthan are collectively required for bacterial pathogenicity. The production of these factors is regulated by a cluster of genes called rpf (for regulation of pathogenicity factors). rpfF is implicated in the synthesis of a diffusible regulatory molecule, DSF. Cells of the bacterial population can sense the accumulation of DSF and modulate expression of specific genes, a phenomenon called quorum sensing. RpfG and RpfC are involved in the signal transduction of DSF.
Biosynthesis of the major Xcc virulence factors, such as extracellular enzymes and xanthan, are positively regulated by DSF. rpfF mutants can not produce DSF and lose the virulence. These data suggest that inhibition of this bacterial cell-cell communication system might efficiently block the microbial pathogenesis. Based on this strategy, we will find out the mechanism of Xcc quorum-sensing system and to identify effective Xcc quorum sensing quenching molecules. The preliminary result of the project showed that some plant extracts have anti-quorum sensing activity. The following work is to identify the mode of action of these extracts and evaluate the DSF-quenching molecules as a disease prevention drug of black rot in crucifers.
