Photochemical degradation of bismerthiazol: structural characterisation of the photoproducts and their inhibitory activities against Xanthomonas oryzae pv. oryzae

Background: Bismerthiazol is a commonly used bactericide against rice bacterial leaf blight in China. Although previous research determined that bismerthiazol is susceptible to photolytic degradation, the photodegradation pathway and degradation products, except for 2-amino-5-mercapto-1,3,4-thiadiazole, have remained unknown.

Results: The photodegradation of bismerthiazol was investigated after 4 and 8 h of irradiation in a solar simulator. Inhibition of Xanthomonas oryzae pv. oryzae (Xoo) was greater with a photolysed solution than with a non-photolysed solution of bismerthiazol. Six photoproducts of bismerthiazol were characterised by liquid chromatography coupled with mass spectrometry, and, based on these products, a photodegradation pathway was inferred. Inhibition of Xoo was significantly greater with bismerthiazol and 2-amino-5-mercapto-1,3,4-thiadiazole than with 5-amino-1,3,4-thiadiazole. In addition, Xoo strain 2-1-1 was bismerthiazol- and 2-amino-5-mercapto-1,3,4-thiadiazole resistant in vivo.

Conclusion: Photodegradation increased the inhibitory activity of bismerthiazol against Xoo. The photodegradation pathway was inferred on the basis of the photoproducts of bismerthiazol. In vitro assays indicated that the sulfhydryl group was crucial for the inhibition of Xoo by bismerthiazol and its photoproducts. Bismerthiazol and 2-amino-5-mercapto-1,3,4-thiadiazole might have a similar mode action in vivo and in vitro.