Understanding the Roles of DSF-regulated Virulence Factors in Xanthomonas campestris Pathogenicity

ADRIÁN A. VOJNOV; DOW JM; KAMAL BOUARAB

Pest Technology

Año: 2007; p. 117 - 126

Xanthomonas campestris pathovar campestris (Xcc) is the causal agent of black rot disease of cruciferous plants. A cell-cell signaling system encoded by genes within the rpf cluster is required for the full virulence of this plant pathogen. This system has been implicated in regulation of production of extracellular enzymes, cyclic glucan and the exopolysaccharide xanthan and in the regulation of biofilm formation in Xcc. Cell-cell communication is mediated by the diffusible signal factor (DSF), an unsaturated fatty acid whose synthesis requires RpfF and RpfB. Here we review current progress on our understanding of DSF signal transduction and of the roles of xanthan, cyclic glucan and biofilm development in the interaction of Xcc with plants. Recent observations have shown that the perception of the DSF signal requires the sensor kinase RpfC and is linked to the degradation of the intracellular second messenger cyclic di-GMP by the HD-GYP domain regulator RpfG. The mechanisms by which cyclic di-GMP exerts its regulatory influence on xanthan, cyclic glucan and biofilm formation remain obscure however. It is now established that DSF signaling has to be finely balanced for the formation of structured biofilms in static cultures in minimal medium and for virulence to plants. New roles for xanthan and cyclic glucan in disease through suppression of plant immune responses have been uncovered. Xanthan induces susceptibility to Xcc in Nicotiana benthamiana and Arabidopsis thaliana by suppressing callose deposition. Unlike xanthan, which acts only locally, the effects of cyclic glucan on plant defense suppression and callose deposition occur in a systemic fashion. These advances contribute to the increased understanding of the molecular basis of bacterial disease, which is a major aim in the post-genomic era in plant-bacterial interactions.