Xanthomonas campestris pv. campestris (Xcc) is a rod-shaped, aerobic,yellow-pigmented, gram-negative bacterium which possesses a single polar flagellum and isthe causal agent of black rot in crucifers. Xcc produces a range of extracellular enzymes andan extracellular polysaccharide known as xanthan gum. Extracellular enzymes and xanthanare 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 isimplicated 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, aphenomenon 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 andxanthan, 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 systemmight 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 sensingquenching molecules. The aims of this project are to (1) characterize the role ofquorum-sensing in the expression regulation of virulence factors in free-living Xcc or inbacteria under interaction with their host plants; (2) study the activity of exogenous signalingfactors (i.e. autoinducer or DSF) in different Xanthomonas species; (3) search for putative Xcc DSF-quenching molecules; (4) evaluate the DSF-quenching molecules as a diseaseprevention drug of black rot in crucifers.
