Acetolactate Synthase Is the Site of Action of Two Sulfonylurea Herbicides in Higher Plants

Abstract

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.

The development of sulfonylurea herbicide-resistant birdsfoot trefoil (Lotus corniculatus) plants from in vitro selection

“…Detoxification of the herbicide in tolerant crops is by hydroxylation of the phenyl ring, followed by conjugation with a plant carbohydrate (Sweetser et al, 1982). Chaleff and Mauvais (1984) and Ray (1Y84) analyzed the resistance at the molecular level and have shown that growth inhibit10n in tobacco is caused by CS binding to and inactivating the acetolactate synthas~ enzyme (ALS). The same mechanism exists for bacteria (LaRossa and Schloss, 1984), yeast (Falco and Dumas, 1985) and other higher plants such as…”

“…Blair (1988) contends that metabolism is likely the basis of tolerance, as ALS extracted frnm tolerant plants, wheat and oats, is as sensitive to CS inhibilion as ALS from sensi ti ve plants, such as peas and soybeans. However, Chaleff and Mauvais (1984), using tobacco suspension cultures, isolated CS resistant lines and found that the phenotype was inherited as a single dominant or semidominant mutation in the ALS gene, and that thi~ single nuclear gene mutation was responsible for resistance exhibited at the whole plant level.…”

“…However, resistance, such as that described by Chaleff and Mauvais (1984) and Haughn and Somerville (1986), is due to a genetic mutation of the ALS gene. The latter resistance has been characterised as 100 to 1000 fald greater th an the wild type.…”

“…ALS activity was measured using the method described by Chaleff and Mauvais (1984) with modifications by Swanson et al (1988) and J. Granger (Ag. Canada; personal communication).…”

“…This, however, is not certain, sinee Most gene mutations associated with increased SU resistance ci ted in the li terature describe ALS resistance levels of 10 to 1000 fold that of the wild type levels (Chaleff and Mauvais, 1984;Haughn and Somerville, 1986;Swanson et al, 1988 andKing, 1988). A CS resistant tobacco mutant isolated from cell culture had an I~ value that was >8000 nM compared to 14 nM for the wild type (Chaleff and Mauvais, 1984).…”