DSF-regulated Virulence Factors and biofilm formation in Xanthomonas Pathogenicity

FLORENCIA MALAMUD; VALERIA CONFORTE; LAILA TOUM; LUCIANO RIGANO; PABLO TORRES; PABLO YARYURA; ATILIO CASTAGNARO; MARÍA ROSA MARANO; GUSTAVO GUDESBLAT; ADRIÁN VOJNOV

Mar del Plata

Congreso; VIII Congreso Argentino de Microbiología General; 2012

Sociedad Argentina de Microbiología General

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. A cell-cell signalling system encoded by genes within the rpf cluster is required for the full virulence of both plant pathogens. Cell-cell communication in Xanthomonas spp. is mediated by the diffusible signal factor (DSF), an unsaturated fatty acid. The rpf/DSF system regulates biofilm formation, as well as the production of extracellular hydrolytic enzymes, cyclic glucan and the exopolysaccharide xanthan. 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 their hosts and in 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. Xanthan induces susceptibility to X. campestris in Arabidopsis thaliana and Nicotiana benthamiana by locally suppressing basal defences such as callose deposition. By contrast, cyclic beta (1,3) glucans intereference with the plant immune response occurs both locally and in a systemic fashion. Crystal violet staining and confocal microscopy analysis of bacteria expressing GFP revealed that xanthan plays a key role in biofilm maturation and in survival both on leaf surfaces and inside the host. In addition, through the use of fliC and flgE mutants affected in the flagellar structure, we have demonstrated that biofilm formation is a flagellar-dependent process in X. citri, as these mutants display abnormal formation of mushroom-shaped structures and water channels, and deficient dispersion of pioneer cells from the mature biofilm. The absence of flagellin produced a slight reduction in X. citri pathogenicity, which was more severe when the complete flagellum structure was absent. We have also revealed the capacity of X. campestris to modulate stomatal aperture as another strategy of host defence suppression by a DSF-regulated factor of unknown structure. This factor can revert stomatal closure induced by bacteria, and is required for X. campestris colonization through these pores.