Exoribonuclease R (RNase R) is a 3' hydrolytic exoribonuclease that can degrade structured RNA. Mutation in RNase R affects virulence of certain human pathogenic bacteria. The aim of this study was to determine whether RNase R is necessary for virulence of the phytopathogen that causes bacterial blight in rice, Xanthomonas oryzae pv. oryzae (Xoo). In silico analysis has indicated that RNase R is highly conserved among various xanthomonads. Amino acid sequence alignment of Xoo RNase R with RNase R from various taxa indicated that Xoo RNase R clustered with RNase R of order Xanthomonadales. To study its role in virulence, we generated a gene disruption mutant of Xoo RNase R. The Xoo rnr- mutant is moderately virulence deficient, and the complementing strain (rnr-/pHM1::rnr) rescued the virulence deficiency of the mutant. We investigated swimming and swarming motilities in both nutrient-deficient minimal media and nutrient-optimal media. We observed that RNase R mutation has adversely affected the swimming and swarming motilities of Xoo in optimal media. However, in nutrient-deficient media only swimming motility was noticeably affected. Growth curves in optimal media at suboptimal temperature (15°C cold stress) indicate that the Xoo rnr- mutant grows more slowly than the Xoo wild type and complementing strain (rnr-/pHM1::rnr). Given these findings, we report for the first time that RNase R function is necessary for complete virulence of Xoo in rice. It is also important for motility of Xoo in media and for growth of Xoo at suboptimal temperature.
Keywords: Xanthomonas oryzae pv. oryzae; bacterial pathogens; disease control and pest management; exoribonuclease R (RNase R); growth curve; host–parasite interactions; rice; swarming motility; swimming motility; virulence.