Toxicity and selective mechanism of imazamox by weedy rice and imidazolinone-resistant rice in China

Weedy rice (Oryza spp.) is a notorious weed in direct-seeding paddy fields. Because it has anatomical and physiological traits similar to those of cultivated rice, no selective herbicide is effective in controlling weedy rice growing among conventional rice cultivars. Imidazolinone (IMI)-resistant rice lines JD372 and JJ818 have been planted with imazamox to control weedy rice in Jiangsu and Shanghai, China. Whole-plant dose-response analysis showed that imazamox exhibited high efficacy against three populations of weedy rice. The ED90 of weedy rice populations FN-5, GY-8, and HY-3 were 46.87, 61.43, and 52.17 g a.i. ha(-1), respectively, close to the recommended field dose (50 g a.i. ha(-1)) of imazamox. Conversely, the ED10 values of JD372 and JJ818 were slightly lower than 50 g a.i. ha(-1). These findings indicate that imazamox can control weedy rice production in JD372 and JJ818 fields. The acetolactate synthase (ALS) sensitivity of JD372 in vitro was 1714.89-fold lower to imazamox than was that of FN-5. ALS gene sequencing revealed a Ser653Asn point mutation-a common mutation that confers resistance to IMI herbicides in JD372. In addition, higher ALS expression levels in JD372 were found at 24 and 72 h after imazamox treatment. ALS overexpression might partially compensate for the ALS activity of JD372 that was suppressed by imazamox.