Metabolism of halosulfuron-methyl by corn and wheat

Halosulfuron-methyl is the active ingredient in a new class of herbicide products by Monsanto which are used for the control of broadleaf and sedge weeds in corn, turf, sugarcane, and other crops. The metabolic basis for the corn and wheat selectivity of halosulfuron-methyl was studied using cell suspension cultures and young seedlings. Both seedlings and cell suspensions were found to completely metabolize halosulfuron-methyl within hours of incubation, suggesting that corn and wheat tolerance to halosulfuron-methyl was due to rapid metabolism or detoxification. In marked contrast, in a halosulfuron-methyl sensitive species such as soybean very little metabolism occurred. The key metabolic route by which corn rapidly inactivates halosulfuron-methyl was found to be hydroxylation of the pyrimidine ring followed by rapid conjugation with glucose. The predominant pathway in wheat was O-demethylation of the methoxy group of the pyrimidine ring. Further oxidative biotransformation of the early primary metabolites led to the cleavage of the pyrimidine ring to form pyrazolesulfonamide as a terminal metabolite.