Cytochrome P-450 monooxygenase activity in herbicide-resistant and -susceptible late watergrass (Echinochloa phyllopogon)

Late watergrass is a major weed of rice in California, and several populations showed resistance to multiple herbicides of differing modes of action. Bispyribac-sodium, fenoxaprop-ethyl, and thiobencarb were used in this study as inducers and substrates of cytochrome P-450 monooxygenases (P450s). To examine the role of P450s in herbicide metabolism by a herbicide-resistant (R) and a herbicide-susceptible (S) late watergrass (Echinochloa phyllopogon) biotype, seedlings were grown in absence and presence of the above three herbicides as inducers. The efficacy of the inducer treatments was confirmed by monitoring changes in P450 content and activity. Herbicide induction increased P450 content and activity in both R and S biotypes. This increase was higher in the R biotype. The induced P450s exhibited specificity for the different herbicide substrates. Our study suggests that induction of P450s is the mechanism involved in the multiple-herbicide resistance observed in late watergrass.