RESISTANCE OF WILD OAT (Avena fatua) POPULATIONS TO ACCASE-INHIBITING HERBICIDES AND MOLECULAR BASIS OF RESISTANCE

ACCase-inhibiting herbicides [aryloxyphenoxypropionate (APP) and cyclohexanedione (CHD)] have been used extensively since late 1970s to control wild oat. However, continued reliance on ACCase-inhibiting herbicides has resulted in widespread evolution of resistant wild oat populations. Dose-response studies on four resistant wild oat populations determined the level of resistance to APP herbicides diclofop-methyl and fenoxaprop-p-ethyl, the CHD herbicides clethodim and sethoxydim, and to the phenylpyrazoline (PPZ) herbicide pinoxaden. All four resistant populations exhibited high level diclofop-methyl and fenoxaprop-p-ethyl resistance, but varied in level of resistance to other ACCase-inhibiting herbicides, indicating either different resistance mutations or different resistance mechanisms in these populations. Mutations of ACCase gene are known to endow target-site resistance to ACCase-inhibiting herbicides in other grass species. Therefore, we sequenced the carboxyl-transferase (CT) domain of the plastidic ACCase gene from ACCase-inhibiting herbicides surviving individuals to identify any mutations endowing resistance. In most, but not all individuals, three known amino acid substitutions endowing resistance were identified in resistant populations: the Ile-1781-Leu; Asp-2078-Gly; and Cys-2088-Arg. Polymerase chain reaction (PCR)-based marker analysis further confirmed the mutations are associated with resistance in these populations. Some of the individuals in one population also contained multiple (double and triple) mutations. Evidently, these mutations in ACCase gene endow high level of resistance in diploid grass species such as rigid ryegrass (Lolium rigidum) and blackgrass (Alopecurus myosuroides Huds). Whether the same mutations are to endow similar level of resistance in hexaploid wild oat remains to be investigated.