Deciphering the dual resistance pathways to mesosulfuron-methyl in Lolium multiflorum: target-site and non-target-site insights.

Key message: Lolium multiflorum exhibits resistance to mesosulfuron-methyl through ALS mutations; HZ2 population also shows metabolic resistance through P450 pathways. Lolium multiflorum L., is a weed that frequently appears in wheat fields and is recognized for its strong competitive nature, where it can cause significant damage to grain production. The weeds of L. multiflorum in the wheat fields may have developed resistance to mesosulfuron-methyl. This study explored the response of L. multiflorum populations in certain areas of Henan Province, China, to mesosulfuron-methyl. The study found that, compared to the HX1 sensitive population, the HZ1 and HZ2 populations showed resistance to mesosulfuron-methyl in the full dose-response test, with resistance ratios of 12.38- and 24.19-fold, respectively. Genetic sequencing revealed novel mutations at the Pro-197-Thr and Asp-376-Glu residues of the ALS gene in both resistant populations. A critical finding was the divergent resistance mechanisms between the geographically close populations, with HZ1 resistance solely conferred by target-site mutations and HZ2 exhibiting multiple resistance driven by both target-site mutations and enhanced metabolism mediated by cytochrome P450 monooxygenases. This was conclusively demonstrated by applying the P450 inhibitors malathion and PBO, which reversed resistance in HZ2 by 66.77% and 70.53%, respectively. Furthermore, both resistant populations showed heightened sensitivity to isoproturon, suggesting a potential management strategy. Molecular docking simulations corroborated that the identified mutations reduce herbicide binding affinity. Our findings provide the first evidence of concurrent target-site and non-target-site resistance to mesosulfuron-methyl in Chinese L. multiflorum, offering crucial insights for diagnosing and managing herbicide resistance.