Multiple mechanisms jointly promoted the infestation of pinoxaden-resistant Beckmannia syzigachne: a case study with 236 populations.

Beckmannia syzigachne (Steud.) Fernald is the most troublesome wheat weed species in eastern China. Pinoxaden, an acetyl-CoA carboxylase (ACCase)-inhibiting herbicide, is one of the most important wheat herbicides against grass weeds including B. syzigachne. Here, we tested 236 B. syzigachne populations for resistance to pinoxaden collected from wheat fields in eastern China. Whole-plant bioassays suggested that 97.0% of the populations were pinoxaden-resistant, with 57.6% exhibiting high to very high resistance, while 11.9% showed pinoxaden-induced hormesis. Among the 85 populations for which the ACCase gene was sequenced, seven amino acid substitutions were detected at six sites, including I1781L (57.6% of populations), D2078G (40.0%), I2041N (35.3%), G2096A (5.9%), W2027C (2.4%), W1999C (1.2%), and W2027L (1.2%). In total, 72 of the 84 resistant populations tested (85.7%) were found to carry target-site mutation(s). Of these populations 48.6%, 23.6%, 19.4% and 8.3% carried one, two, three and four mutation types within a single population, respectively. Metabolic resistance associated with cytochrome P450 monooxygenases (P450) and glutathione S-transferases (GST) was detected in 61.9% and 46.4% of the resistant populations, respectively. Genetic differentiation was detected among populations, with an FST value of 0.20 (P < 0.05). Geographical distance was not significantly correlated with genetic differentiation. Together, our results suggest that multiple mechanisms jointly promoted the widespread occurrence of pinoxaden resistance in B. syzigachne.