Phase variation in rice pathogen Xanthomonas oryzae pv. oryzae

Abstract

Xanthomonas oryzae pv. oryzae (Xoo) is the causal pathogen of bacterial blight of rice. Diverse factors contribute to its virulence, e.g., secreted enzymes, protein involved in adhesion, gene regulation. Two such studied virulence factors are Exopolysaccharide (EPS) and Lipopolysaccharide (LPS). LPS is a structural component of the outer membrane of all gram-negative bacteria. LPS is necessary for the function of many outer membrane proteins and protect the bacteria from antimicrobial plant products. EPS is a complex polymer secreted by Xanthomonas genus and the known functions of EPS include biofilm formation, and suppression of the plant immune responses. In this study, we isolated non-mucoid and virulence compromised Xoo colonies from stationary phase cultures. These spontaneous mutants are called as stationary phase variants (SPV). Our data show that, SPVs arises due to insertion of endogenous Insertion Sequence (IS) elements in EPS or LPS O-antigen biosynthetic clusters or by slipped-strand mispairing (SSM) in wxoA gene of LPS O-antigen biosynthetic cluster. The SPVs reverts to wild-type colony morphology and showed true reversion, i.e. restoration of wild-type genotype. The results suggest that we are observing phase variation in Xoo. We hypothesize that similar phase variation may be a part of Xoo’s life cycle. Which may help the bacteria survive in nutrient limited conditions during late stages of infection and on rice seeds and plant debris.