Biochemical and genetic studies of a tobacco mutant resistant to the herbicides chlorsulfuron and sulfometuron methyl have demonstrated that these sulfonylurea herbicides inhibit acetolactate synthase, the first enzyme specific to the branched chain amino acid biosynthetic pathway. Resistance of this mutant is accomplished by production of a form of the enzyme that is insensitive to inhibition by the two herbicides.

Several mutants resistant to the herbicides chlorsulfuron and sulfometuron methyl were isolated form cultured cells of Nicotiana tabacum. Resistance was inherited as a single dominant or semidominant mutation in all cases. Linkage analysis of six mutants identified two unlinked genetic loci. Studies of plants homozygous for one mutation showed the mutant plants to be completely resistant to treatment with a concentration of chlorsulfuron 100 times higher than that which produces symptoms of phytotoxicity on normal plants.

A soybean [Glycine max (L.) Merr. I mutation conferring resistance to a wide range of sulfonylurea (SU) herbicides would greatly enhance the weed control options available to soybean farmers. This report describes the selection, characterization, and potential utility of such mutants. Seed mutagenesis (using N‐nitroso‐N‐methylurea and ethyl methanesulfonate) followed by selection for resistance to chlorsulfuron [2‐chloro‐N‐[(4‐methoxy‐6‐methyl‐l,3,5‐triazin‐2‐yl)aminocarbonyl] benzenesulfonamide] yielded a soybean mutant with a high degree of resistance to both postemergence and pre‐emergence applications of a variety of SU herbicides. Resistance was monogenic, semidominant, and not allelic to any of the previously identified recessive genes hs1, hs2, or hs3 that confer tolerance to SU herbicides. Biochemical tests indicate that the mechanism of resistance is reduced sensitivity of acetolactate synthase to SU inhibition. The SU resistance afforded by this mutation (designated Alsl) can be used to enhance soybean weed control options and can serve as a selectable marker for seed purification.