Molecular mechanism and resistance risk assessment of laboratory-induced streptomycin-resistant mutants of Xanthomonas oryzae pv.oryzae

Six streptomycin-resistant mutants of Xanthomonas oryzae pv.oryzae were obtained by ultraviolet light induction in the laboratory.The minimal inhibition concentrations(MICs) of streptomycin inhabiting wild-type strain ZJ173 and resistant mutants were 0.10 and 600 μg/mL,respectively.The medium effective concentration(EC50) of wild-type strain ZJ173 was 0.03 μg/mL and the mean EC50 value of six mutants was 11.64 μg/mL. The resistance factor(RF) of laboratory mutants relative to their parent strain ZJ173 was 388.Mutations in the rpsL(encoding S12 protein) and rrs gene(encoding 16S rRNA) and the presence of strA gene were examined in sensitive and resistant strains of X.oryzae pv.oryzae by PCR amplification and sequencing.Four of five tested laboratory mutants had mutation at codon 43(Lys→Arg) and the other one at 88(Lys→Arg) in the rpsL gene.Neither the strA gene nor mutation in the rrs has been found.The resistant mutants maintained their resistance through 10 times of transfers on NA plates.The pathogenicity and growth rate showed no difference between resistant mutants and sensitive strain,while the competitiveness of resistant mutants is lower or slightly lower than that of sensitive strain.Considering the high spontaneous mutation frequency,single-gene mutation pattern and a typical multi-cycle disease,the risk of streptomycin-resistance caused by rpsL mutation in X.oryzae pv.oryzae would be high.