Mechanism of Echinochloa crus-galli Resistance to the ALS-Inhibiting Herbicide Pyrazosulfuron-ethyl in China

Rice (Oryza sativa L.) is a staple food crop, feeding more than 3.5 billion people. With the increasing demand for food in the 21st century, weed infestation poses the most significant biotic threat to global food security, and herbicides remain the most effective and economic way to manage it in field. However, weeds can rapidly adapt under herbicide selection pressure due to their high competitiveness, rapid growth, and reproductive capacity. Hence, we collected Echinochloa crus-galli populations from Heilongjiang and Hebei provinces in China and investigated their resistance mechanisms to pyrazosulfuron-ethyl (PSE), a sulfonylurea herbicide that inhibits acetolactate synthase (ALS). Dose–response experiments confirm that the resistant (R) population exhibits 52.9-fold resistance to PSE compared with the susceptible (S) population. Inhibitor bioassays with malathion and NBD-Cl, together with ALS activity assays, ALS gene sequencing, and molecular docking, collectively suggest that resistance is strongly associated with the ALS Trp-574-Leu target-site substitution, with a possible additional contribution from enhanced herbicide metabolism. However, because the S and R populations originate from geographically distinct locations, some of the observed physiological and molecular differences may also reflect inherent population variation. Specifically, the ALS W574L substitution is predicted to reduce key interactions between ALS and PSE. This study provides valuable evidence for the risk of PSE resistance evolution in E. crus-galli and elucidates the molecular mechanism conferring resistance to ALS inhibitors.