Role of Metabolism in Sensitivity to Quizalofop-P Ethyl in CoAXium Wheat

The CoAXium Wheat Production System (CoAXium) combines quizalofop-resistant wheat varieties with the herbicide quizalofop-P ethyl for postemergence winter annual grass weed control. Resistance is conferred through a target-site mutation in acetyl-CoA carboxylase (ACCase); however, potential contributions to whole-plant response from non-target-site resistance mechanisms, particularly herbicide metabolism, remain poorly understood. Biological experiments revealed differences in whole-plant herbicide sensitivity among quizalofop-resistant wheat varieties despite similar levels of enzyme-based resistance. Herbicide metabolism studies showed that Fusion AX metabolized quizalofop more rapidly than Incline AX, indicating varietal differences in metabolic capacity. The addition of the safener cloquintocet-mexyl significantly increased herbicide resistance and accelerated the metabolism of quizalofop acid (QZA). In contrast, inhibition of glutathione S-transferase (GST) activity increased herbicide injury, reduced biomass, elevated QZA accumulation, and prolonged herbicide half-life, demonstrating the importance of GST-mediated metabolism in herbicide detoxification. A key glutathione-conjugated metabolite of quizalofop (CQX-GSH) was identified in wheat. While the quizalofop-resistant ACCase mutation is required for plant survival following quizalofop application in wheat, non-target-site metabolic mechanisms increase quizalofop resistance in CoAXium wheat beyond the ACCase mutation alone. Cloquintocet enhanced herbicide detoxification and increased whole-plant resistance level, while GST inhibition reduced metabolic capacity and increased herbicide sensitivity. These findings indicate that GST-mediated metabolism plays a major role in quizalofop resistance in CoAXium wheat. Differential metabolic activity among varieties may provide valuable information for breeding programs aimed at improving herbicide resistance levels and developing regionally adapted CoAXium wheat varieties.