Xanthomonas oryzae pv. oryzae exoribonuclease R is required for complete virulence in rice, optimal motility, and growth under stress

Abstract

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 required 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. In order to study its role in virulence, a gene disruption mutant of Xoo RNase R (rnr –) was generated. 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 and nutrient optimal media. We observed that rnr – mutant 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 Xoo rnr – mutant grows slower than Xoo wild type (wt) and complementing strain (rnr –/pHM1::rnr). Taken together, 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.