Basis of Selectivity of Triflusulfuron Methyl in Sugar Beets ( Beta vulgaris)

Abstract

Triflusulfuron methyl (methyl 2-[[[[[4-(dimethylamino)-6-(2,2,2-triftuoroethoxy)-1,3,5-triazin-2-yl]amino]carbonyl]amino]sulfonyl]-3-methylbenzoate), formerly DPX-66037, is a new selective postemergence sulfonylurea herbicide for the control of annual and perennial broadleaf weeds and grasses in sugar beets ( Beta vulgaris). Similar to other sulfonylureas, the site of action of triflusulfuron methyl is acetolactate synthase (ALS), an enzyme in branched-chain amino acid biosynthesis. This herbicide is unique in this class in its safety on sugar beets, a crop that has often been used as an indicator of residual sulfonylureas in soil. We investigated the mechanism of selectivity by comparing the response of sugar beets with that of sensitive weeds ( Brassica napus, Matricaria inodora, and Veronica persica) and a moderately tolerant weed ( Chenopodium album). Studies of foliar uptake and in vitro inhibition of ALS across these species discount differential uptake or active site sensitivity as the basis for sugar beet tolerance to this herbicide. However, a good correlation between metabolism and plant tolerance does exist. Sugar beets metabolize triflusulfuron methyl very rapidly (half-life of <1 hr) while C. album has an intermediate rate (half-life of 7 hr) and the sensitive weeds have slow rates of metabolism (half-lives of >35 hr). The initial metabolism of triflusulfuron methyl in sugar beets involves nucleophilic attack by glutathione at the urea carbonyl group producing the S-carbamoyl glutathione conjugate plus 7-methylsaccharin and its free acid. The 2,6-disubstituted sulfonamide appears essential for this metabolism, but surprisingly even minor changes on the triazine ring have a profound effect on sugar beet tolerance. Although the data strongly suggest enzymatic involvement, attempts to determine if glutathione- S-transferase was involved proved inconclusive.