Acetolactate synthase-Inhibiting herbicide resistance in two barnyardgrass (Echinochloa crus-galli) biotypes of Arkansas.
Barnyardgrass (Echinochloa crus-galli L.) is the most important weed of rice in Arkansas. Recently, two barnyardgrass biotypes with putative resistance to the acetolactate synthase (ALS)-inhibiting herbicides were collected from Arkansas (herein referred to as AR1 and AR2). Experiments were conducted to confirm and characterize cross-resistance to ALS-inhibiting herbicides and determine the resistance mechanism in these barnyardgrass biotypes. Cross-resistance experiments revealed that the AR1 biotype has evolved cross-resistance to the field rate applications of bispyribac (Regiment), imazamox (Beyond), imazethapyr (Newpath), and penoxsulam (Grasp). The AR2 biotype has also evolved cross-resistance to all these herbicides except bispyribac. Dose-response experiments revealed that AR1 and AR2 biotypes were >94 times more resistant to imazamox; >94 and 30 times, respectively, more resistant to penoxsulam; and 15 and 0.89 times, respectively, more resistant to bispyribac compared to susceptible biotypes based on the lethal dose needed to kill 50% of plants (LD50). Addition of malathion (Prentox) at 0.89 lb ai/acre to penoxsulam reduced dry weight of AR1 and AR2 biotypes by 40% and 94%, respectively, at 21 days after treatment (DAT) compared to penoxsulam applied alone. However, addition of malathion to imazethapyr had no effect on dry weight or mortality of resistant biotypes at 21 DAT. Addition of malathion to bispyribac did not reduce dry weight; however, it increased mortality of the AR1 biotype by 30%. Malathion inhibits the activity of cytochrome P450 monooxygenase, an enzyme known to metabolize various herbicides. Reduction in dry weight after addition of malathion confirms that cytochrome P450 monooxygenase is imparting resistance to penoxsulam. Partial ALS gene coding sequence (CDS, 1701 bp) of AR1 and AR2 biotypes was sequenced and analyzed, which revealed that there is alanine122 to valine substitution in the AR1 biotype and alanine122 to threonine substitution in the AR2 biotype. Mutation at this position usually imparts a high level of resistance to imidazolinone herbicides (e.g., imazamox and imazethapyr) as seen in our experiments. These experiments show that different resistance mechanisms are involved in imparting cross-resistance to ALS-inhibiting herbicides in these resistant barnyardgrass biotypes.