Trp2027Cys mutation evolves in Digitaria insularis with cross-resistance to ACCase inhibitors

Sourgrass (Digitaria insularis) is one of the most problematic weeds in South America because glyphosate resistance is widespread across most crop production regions. Acetyl coenzyme A carboxylase (ACCase)-inhibiting herbicides have been intensively used to manage D. insularis, which substantially increased selection pressure for this class of herbicides. We confirmed resistance to ACCase herbicides in a D. insularis population from Brazil and characterized its molecular basis. Resistant plants showed high level of resistance to haloxyfop (resistance factor, RF = 613-fold), low level of resistance to pinoxaden (RF = 3.6-fold), and no resistance to clethodim. A target-site mutation, Trp2027Cys, was found in the ACCase sequence from resistant plants. A protein homology model shows that the Trp2027Cys mutation is near the herbicide-binding pocket formed between two ACCase chains, and is predicted to obstruct the access of aryloxyphenoxypropionates (FOP) herbicides to the binding site. A qPCR-based single nucleotide polymorphism genotyping method was validated to discriminate susceptible (wild-type Trp2027) and resistant (mutant Cys2027) alleles. All resistant plants were homozygous for the mutation and the assay could be used for early detection of resistance in D. insularis field samples with suspected resistance to ACCase inhibitors.