Target site–resistance mechanisms to imazamox in imidazolinone herbicide-resistant weedy rice (<i>Oryza sativa</i> f. <i>spontanea</i>) in China

Abstract Weedy rice ( Oryza sativa f. spontanea Roshev.), a widespread and troublesome weed in rice ( Oryza sativa L.) fields, is typically controlled using imazamox in imidazolinone-tolerant rice fields. However, suspected resistance to imazamox has emerged in weedy rice populations in Jiangsu Province, China. This study aimed to evaluate the degree of resistance and investigate the resistance mechanisms. A whole-plant bioassay was performed on 35 weedy rice populations, demonstrating that 26 populations developed resistance to imazamox. The effective dose values causing 50% inhibition of growth reduction (GR 50 ) in resistant (R) populations ranged from 129.2 to 280.2 g ai ha −1 , exceeding the recommended application rate of imazamox (120 g ai ha −1 ) in imazamox-tolerant rice fields. R populations displayed cross-resistance to other acetolactate synthase (ALS)-inhibiting herbicides, except for certain sulfonylurea herbicides. Sequencing of the ALS gene identified a Ser-653-Asn substitution in resistant populations. A novel derived cleaved amplified polymorphic sequence (dCAPS) method was developed for the rapid and efficient detection of the Ser-653-Asn mutation in O. sativa f. spontanea . In vitro ALS activity assays revealed that the imazamox concentration required to inhibit 50% (IC 50 ) of ALS activity was 80.0- to 88.3-fold higher in R populations compared with a susceptible (S) population. After imazamox treatment, the ALS expression levels in both the S and R populations of weedy rice increased. Resistance was not reversed by cytochrome P450 oxidase system (CYP450) or glutathione S -transferase (GST) inhibitors, suggesting that metabolic resistance mechanisms were not involved. In conclusion, weedy rice developed a different resistance level to imazamox, and the Ser-653-Asn mutation in the target ALS was the main reason. To the best of our knowledge, this study is the first to reveal the mechanism of resistance to imazamox in weedy rice in China.