OCCURRENCE OF A HERBICIDE-RESISTANT ACETYL-COENZYME-A CARBOXYLASE MUTANT IN ANNUAL RYEGRASS (LOLIUM-RIGIDUM) SELECTED BY SETHOXYDIM

The spectrum of herbicide resistance was determined in an annual ryegr ass (Lolium rigidum Gaud.) biotype (SLR 3) that had been exposed to th e grass herbicide sethoxydim, an inhibitor of the plastidic enzyme ace tyl-coenzyme A carboxylase (ACCase, EC 6.4.1.2), for three consecutive years. This biotype has an 18-fold resistance to sethoxydim and enhan ced resistance to other cyclohexanedione herbicides compared with a su sceptible biotype (VLR 1). The resistant biotype also has a 47- to > 3 00-fold cross-resistance to the aryloxyphenoxypropanoate herbicides wh ich share ACCase as a target site. No resistance is evident to herbici de with a target site different from ACCase. The absorption of [4-C-14 ]sethoxydim, the rate of metabolic degradation and the nature of the h erbicide metabolites are similar in the resistant and susceptible biot ypes. While the total activity of the herbicide target enzyme ACCase i s similar in extracts from the two biotypes, the kinetics of herbicide inhibition differ. The concentrations of sethoxydim and tralkoxydim r equired to inhibit the activity of ACCase by 50% are 7.8 and > 9.5 tim es higher, respectively, in the resistant biotype. The activity of ACC ase from the resistant biotype was also less sensitive to aryloxypheno xypropanode herbicides than the susceptible biotype. The spectrum of r esistance at the whole-plant level is correlated with resistance at th e ACCase level and confirms that a less sensitive form of the target e nzyme endows resistance in biotype SLR 3.