The Rice Pathogen, Xanthomonas oryzae pv. oryzae Exhibits Reversible Phase Variation in LPS O-Antigen Biosynthetic Gene Cluster During Long-Term Stationary Phase

The exopolysaccharide (EPS), xanthan or xanthan gum secreted by the Xanthomonas genus of bacterial pathogens gives them the characteristic mucoid colony phenotype. Lack of xanthan production leads to reduced or attenuated virulence of the pathogen. We have previously described spontaneous non-mucoid variants of Xanthomonas oryzae pv. oryzae (Xoo) which accumulate in long-term stationary phase cultures, and this phenomenon was termed stationary phase variation and variant colonies as stationary phase variants (SPVs). In this study, we demonstrate that many of the SPVs harbour variations in the lipopolysaccharide (LPS) outer antigen (O-antigen) biosynthetic gene cluster. Genotyping of SPVs revealed insertion of endogenous IS elements or slipped strand mispairing (SSM) the LPS O-antigen locus as the major types of variation. Several of these SPVs exhibited reversion to wild type mucoid phenotype via restoration of the wild type genotype. Additionally, the reverting type SPVs generated virulence lesions upon in-planta inoculations. The results indicate that phase variation occurs in the LPS O-antigen biosynthetic gene cluster of Xoo during long term stationary phase cultures and we discuss its possible adaptive significance.