Diversity of ACCase mutations in resistant Lolium populations: evaluation using clethodim

The acetyl-CoA carboxylase (ACCase)-inhibiting cyclohexanedione herbicide clethodim is used to control grass weeds infesting dicot crops. In Australia clethodim is widely used to control the weed Lolium rigidum. However, clethodim resistant Lolium populations have appeared over the last five years and now are present in many populations across the Western Australian wheat belt (Owen et al. 2007). An Asp-2078-Gly mutation in the plastidic ACCase enzyme has been identified as the only known mutation endowing clethodim resistance (Délye et al. 2005). Here, with 14 clethodim resistant Lolium populations we revealed diversity and complexity in the molecular basis of resistance to ACCase-inhibiting herbicides (clethodim in particular). Several known ACCase mutations (Ile-1781-Leu, Trp-2027-Cys, Ile-2041-Asn and Asp-2078-Gly) and in particular, a new mutation of Cys to Arg at position 2088, were identified in plants surviving the Australian clethodim field rate (60 g ha-1). Twelve combination patterns of mutant alleles were revealed in relation to clethodim resistance. Through a molecular, biochemical and biological approach, we established that the mutation 2078-Gly or 2088-Arg endows sufficient level of resistance to clethodim at the field rate, and in addition, combinations of two mutant 1781-Leu alleles, or two different mutant alleles (i.e. 1781-Leu/2027-Cys, 1781-Leu/2041-Asn), also confer clethodim resistance. Plants homozygous for the mutant 1781, 2078 or 2088 alleles were found to be clethodim resistant and cross resistant to a number of other ACCase inhibitor herbicides including clodinafop, diclofop, fluazifop, haloxyfop, butroxydim, sethoxydim, tralkoxydim, and pinoxaden. We established that the specific mutation, the homo/heterozygous status of a plant for a specific mutation, and combinations of different resistant alleles plus herbicide rates all are important in contributing to the overall level of herbicide resistance in genetically diverse, cross-pollinated Lolium species.