Roles of DSF-regulated virulence factors and biofilm formation in Xanthomonas pathogenicity

MALAMUD F.; CONFORTE V.P.; TOUM L.; RIGANO L.; TORES, P.; YARYURA P; CASTAGNARO A. P.; MARANO M. R.; GUDESBLAT G. ; VOJNOV A. A

ANGERS

Conferencia; 4th Xanthomonas Genomics Conference; 2012

INRA

Xanthomonas campestris pv. campestris and Xanthomonas citri subsp. citri are the causative agents of black rot and citrus canker diseases, in cruciferous and citrus plants, respectively. Cell-cell signalling systems encoded by genes within the rpf cluster are required for the full virulence of both plant pathogens. Those systems have been implicated in regulation of production of extracellular enzymes, cyclic glucan and the exopolysaccharide xanthan and in the regulation of biofilm formation. Cell-cell communication is mediated by the diffusible signal factor (DSF), an unsaturated fatty acid. We have made progress in understanding the roles of xanthan, cyclic glucan and biofilm development in the interaction of X. campestris and X. citri with plants and of the mechanistic basis of regulation of these processes by DSF. New roles for xanthan and cyclic glucan in disease through suppression of plant immune responses have been uncovered. We have also revealed the capacity of X. campestris to modulate stomatal aperture as other bacterial strategy of defence suppression by a DSF-regulated factor with unknown structure. Xanthan induces susceptibility to X. campestris in Arabidopsis thaliana and Nicotiana benthamiana by suppressing basal defences such as 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. Crystal violet staining and confocal laser scanning microscopy analysis of the bacteria expressing the green fluorescent protein were used to evaluate attachment and biofilm formation on abiotic and biotic (leaf) surfaces. The extracellular polysaccharide xanthan (EPS) played a key role in the biofilm maturation, survival on leaf surfaces and virulence. By generation of two mutants: X. citri fliC (flagelin gene) and X. citri flgE (hook gene), both involved in the flagellar structure, we demonstrated that biofilm formation is a flagellar-dependent process in X. citri being important in the formation of mushroom-shaped structures and water channels, and in the dispersion of pioneer cells from the mature biofilm. The absence of flagellin produced a slight reduction in X. citri pathogenicity and this reduction was more severe when the complete flagellum structure was absent.