Basis of Selectivity of the Herbicide Flupyrsulfuron-methyl in Wheat

Abstract

Flupyrsulfuron-methyl, methyl 2-[[[[(4,6-dimethoxy-2-pyrimidinyl) amino]carbonyl]amino]sulfonyl]-6-(trifluoromethyl)-3-pyridinecarboxylate monosodium salt, formerly DPX-KE459, is a new postemergence sulfonylurea herbicide for the control of grass and broadleaf weeds in wheat. Similar to other sulfonylureas, the site of action of flupyrsulfuron-methyl is acetolactate synthase (ALS), an enzyme in branched-chain amino acid biosynthesis. Studies ofin vitroinhibition of ALS across sensitive and tolerant species discount differential active site sensitivity as the basis for wheat tolerance to this herbicide. However, wheat and the tolerant weedsAvena fatuaandPhalaris minormetabolized flupyrsulfuron-methyl very rapidly with half-lives of ≤2 h. Metabolism in the moderately tolerant speciesSetaria viridiswas intermediate (half-life of 10 h), and slow inAlopecurus myosuroides(half-life of 20 h), a sensitive species. The initial metabolic pathway of flupyrsulfuron-methyl in wheat primarily involves glutathione conjugation; whereas in an equally tolerant species,P. minor, this herbicide is not metabolized by glutathione conjugation but apparently byO-demethylation. Flupyrsulfuron-methyl is metabolized via both pathways (glutathione conjugation andOdemethylation) inA. fatua, also a tolerant species. Differential rates of metabolism in sensitive and tolerant plant species are responsible for the selectivity of flupyrsulfuron-methyl.