Deciphering the functions of the outer membrane porin OprBXo involved in virulence, motility, exopolysaccharide production, biofilm formation and stress tolerance in Xanthomonas oryzae pv. oryzae

Xanthomonas oryzae pv. oryzae (Xoo) is a Gram-negative bacterium causing bacterial leaf blight disease in rice. Previously, proteomic analysis has shown that the outer membrane protein B in Xoo (OprBXo) is more abundant in the wildtype strain than is the outer membrane protein 1 in the Xoo (Omp1X) knockout mutant. OprBXo shows high homology with OprB, which has been well characterized as a carbohydrate-selective porin in X. citri ssp. citri and Pseudomonas species. However, the functions of OprBXo in Xoo have not yet been documented. To elucidate the functions of OprBXo, we generated the OprBXo-overexpressing mutant, Xoo(OprBXo), and the knockout mutant, Xoo Delta oprBXo(EV). We found that the virulence and migration of Xoo(OprBXo), but not Xoo Delta oprBXo(EV), were markedly reduced in rice. To postulate the mechanisms affected by OprBXo, comparative proteomic analysis was performed. Based on the results of proteomics, we employed diverse phenotypic assays to characterize the functions of OprBXo. Abnormal twitching motility and reduction in swarming motility were observed in Xoo(OprBXo). Moreover, Xoo(OprBXo) decreased, but Xoo Delta oprBXo(EV) enhanced, exopolysaccharide production and biofilm formation. The chemotactic ability of Xoo Delta oprBXo(EV) was dramatically lower than that of Xoo(EV) in the presence of glucose and xylose. Xoo(OprBXo) was resistant to sodium dodecylsulphate and hydrogen peroxide, but Xoo Delta oprBXo(EV) was highly sensitive compared with Xoo(EV). Thus, OprBXo is not only essential for chemotaxis and stress tolerance, but also for motility, biofilm formation and exopolysaccharide production, which may contribute to the virulence of Xoo. These results will lead to new insights into the functions of a sugar-selective porin in Xoo.