An isoleucine to leucine substitution in the ACCase of Alopecurus myosuroides (black-grass) is associated with resistance to the herbicide sethoxydim

The development of herbicide resistance in weeds poses a severe threat to weed control and crop production. For the aryloxyphenoxypropionates and cyclohexanediones which act on the plastidic ACCase of grass weeds, resistance usually occurs by alteration of the target site protein. In three species, Setaria viridis, Lolium rigidum, and Avena fatua, this has been shown to be associated with a mutation in the ACCase gene, resulting in an isoleucine to leucine substitution in the enzyme. Here we report the same amino acid substitution in the ACCase of sethoxydim-resistant Alopecurus myosuroides. The presence of the leucine correlates with a resistant phenotype and the presence of an insensitive ACCase. In one population, plants heterozygous for the mutation showed different levels of sensitivity and in some cases it was demonstrated that both alleles were expressed in the same plant. The fact that the leucine present in the insensitive plastidic ACCases of several graminaceous species with evolved resistance is also present in the naturally occurring insensitive plastidic ACCase of dicotyledons suggests that the selectivity of these herbicides also results from the same amino acid differences.