Two mechanisms provide tolerance to cyhalofop-butyl in pond lovegrass [Eragrostis japonica (Thunb.) Trin.]

Pond lovegrass [Eragrostis japonica (Thunb.) Trin.] is an annual grass weed of rice fields worldwide. Cyhalofop-butyl has been widely used for controlling annual grass weeds in rice fields. However, E. japonica is tolerant to cyhalofop-butyl. The effective dose values of cyhalofop-butyl for 29 E. japonica populations causing 50% inhibition of fresh weight (GR50: 130.15 to 187.61 g a.i. ha−1) were much higher than the recommended dose of cyhalofop-butyl (75 g a.i. ha−1) in the field. The mechanisms of tolerance to cyhalofop-butyl in E. japonica were identified. In vitro activity assays revealed that the cyhalofop-butyl concentration required to inhibit 50% of the acetyl-coenzyme A carboxylase (ACCase) activity (IC50) was 6.22-fold higher in E. japonica than that in the cyhalofop-butyl-susceptible Chinese sprangletop [Leptochloa chinensis (L.) Nees]. However, mutations in the ACCase gene, previously found to endow target-site resistance in weeds, were not detected in the sequences obtained. Additionally, the expression level of genes encoding ACCase in E. japonica was found to be as similar to L. chinensis. Tolerance was reduced by two cytochrome P450 monooxygenases (Cyt P450s) inhibitors (1-aminobenzotriazole and piperonyl butoxide) and the activity of NADPH-dependent cytochrome P450 reductase in E. japonica was approximately 4.46-fold higher than that of L. chinensis after cyhalofop-butyl treatment. Taken together, it is concluded that two co-existing mechanisms, an insensitive target ACCase and an enhanced metabolism mediated by Cyt P450s, endow tolerance to cyhalofop-butyl in E. japonica.