Quizalofop resistance in weedy rice (Oryza sativa L.) is mainly conferred by an Ile1781Leu mutation

Weedy rice (Oryza sativa L.) is an economically important weed species in rice (Oryza sativa L.) cropping systems. Two weedy rice samples (acc7 and acc8) suspected to be resistant to quizalofop-ethyl (quizalofop) were collected in Arkansas. In this research, susceptibility to quizalofop and resistance mechanisms have been explored. Dose-response assays displayed a resistance index of 42- and 58-fold for the acc7 and acc8, respectively. Experiments with metabolism inhibitors demonstrated that NBD-Cl (4-chloro-7-nitrobenzofurazan) increased quizalofop efficacy slightly in acc8, whereas malathion did not improve effectiveness in resistant samples. Sequencing of the ACCase gene displayed an Ile1781Leu substitution in the resistant samples, like the mutation present in Provisia™ rice. In addition, an allele-specific PCR was developed to genotype the Ile1781Leu mutation. The gene copy number of ACCase showed similar values among samples. In the resistant plants, a KASP (Kompetitive Allele Specific PCR) assay to detect the ALSS653D (acetolactate synthase) and HIS1 (HPPD Inhibitor Sensitive 1) traits revealed that 37.5% of plants carried the ALSS653D trait, whereas 25% showed the HIS1 allele. In summary, a target-site mutation is the main resistance mechanism to quizalofop in weedy rice. Results also suggest the presence of herbicide metabolism (a non-target site resistance mechanism) mediated by glutathione-S-transferases (GSTs) in one resistant sample. •Weedy rice (Oryza sativa L.) has evolved resistance to quizalofop herbicide.•Gene amplification of the ACCase gene is not involved in the resistance mechanism.•A target-site mutation at position 1781 mainly confers resistance to quizalofop in the resistant weedy rice samples.•Herbicide metabolism mediated by GSTs is involved in one resistant weedy rice sample.