Detection and characterization of bismerthiazol-resistance of Xanthomonas oryzae pv. oryzae

The resistance to bismerthiazol in the Xanthomonas oryzae pv. oryzae population in China has not been clearly determined. Three bismerthiazol-resistant mutants (1-1-1, 2-1-1, 4-1-1) of X. oryzae pv. oryzae were induced on rice plants and the characteristics were compared with their parental strain ZJ173. After 10 subcultures on fresh bismerthiazol-free nutrient agar plates and after being inoculation onto and re-isolation from untreated rice plants two times, the mutants remained pathogenicity on rice plants treated with 300 μg/ml bismerthiazol, indicating that the in vivo-induced bismerthiazol resistance was stable in vivo. The in vitro characteristics of the bismerthiazol-resistant mutants and a wild-type bismerthiazol-sensitive strain (ZJ173) were also compared. The three mutants grew faster than ZJ173 during the logarithmic growth phase but at the same rate during the decline phase. Bismerthiazol had protective and curative activity against strain ZJ173 but not against the three mutants. To evaluate the current degree of bismerthiazol resistance in field populations of X. oryzae pv. oryzae in China, 781 isolates of X. oryzae pv. oryzae were obtained from 1419 leaf samples collected from Yunnan, Sichuan, Jiangsu, Anhui, Hubei, Guangdong, Hainan, and Hunan Provinces in China from 2007 to 2009. The in vivo sensitivity of 505 of the 781 isolates to bismerthiazol was determined by applying them to rice plants treated with 300 μg/ml bismerthiazol and evaluating blast severity. The tested strains were considered resistant, if they showed <70% inhibition relative to strain ZJ173. Of the 505 isolates, 62 were resistant to bismerthiazol. The resistance frequencies among field population ranged from 0 to 21.1%, differed among the provinces, and was 11.2, 20.0 and 10.4% in 2007, 2008 and 2009, respectively. Resistance determined in vivo was not related to resistance determined in vitro. The in vivo-induced bismerthiazol resistance identified in this study will serve as a base line for further studying bismerthiazol resistance in fields, and the current degree of bismerthiazol resistance in field populations of X. oryzae pv. oryzae in China determined would be useful for the rice growers for control of rice bacterial blight.

► The in vivo bismerthiazol-resistant mutants were firstly induced.
► The characters of the mutants and their parental isolate were compared.
► The sensitivity of 505 single-colony isolates to bismerthiazol in vivo was tested.
► The in vivo resistance to bismerthiazol was not related to in vitro.