Basis for soybean tolerance to thifensulfuron methyl

Abstract

Thifensulfuron methyl (methyl 3-[[(4-methoxy-6-methyl-1,3,5-triazin-2-yl)aminocarbonyl]aminosulfonyl]-2-thiophenecarboxylate, formerly DPX-M6316) is a sulfonylurea herbicide for postemergence broadleaf weed control in wheat, barley, and soybeans. Plant response data are presented demonstrating > 12-fold soybean tolerance to thifensulfuron methyl relative to sensitive weeds, including Amaranthus retroflexus L., Chenopodium album L., and Abutilon theophrasti Medik. Studies of foliar uptake and in vitro inhibition of acetolactate synthase (ALS) discount differential uptake or differential active site sensitivity as the basis for soybean tolerance to this herbicide. Instead, excised soybean ( Glycine max, cv “Williams”) seedlings are found to rapidly metabolize a transpired pulse of thifensulfuron methyl with a half-life of 4–6 hr while sensitive weeds metabolize this herbicide much more slowly. The primary metabolite of thifensulfuron methyl in soybean seedlings is its deesterified free acid (thifensulfuron acid), which is herbicidally inactive and inactive against ALS. Metabolism rates and pathways of thifensulfuron methyl, chlorimuron ethyl, and metsulfuron methyl in soybean seedlings are compared and discussed. It is concluded that soybean tolerance to thifensulfuron methyl is based on rapid metabolic deesterification and that this deesterification reaction is specific to certain sulfonylurea herbicides.