The selective sulfonylurea herbicide, chlorsulfuron (Fig. 1),
represents a major advancement in weed control technology
combining high herbicidal activity with outstanding tolerance to
cereal crops and very low mammalian toxicity (3). Sensitive
weeds are controlled at 10 to 20 g/ha which is equivalent to 1 to
2 tablespoons of active ingredient per acre. Chlorsulfuron acts to
inhibit plant growth by blocking some process necessary for plant
cell division (7-9). Recent studies have shown that another
sulfonylurea herbicide, sulfometuron methyl, which is the active
ingredient in Oust'( Weed Killer (Fig. 1: Ref. 6), interferes with
the production of branched-chain amino acids in bacteria by
inhibiting the enzyme acetolactate synthase (2). This enzyme
catalyzes the first step in the biosynthesis of Val, Leu, and Ile.
Results reported here demonstrate that in plants chlorsulfuron
and sulfometuron methyl also block the biosynthesis of Val and
Ile by inhibiting the enzyme acetolactate synthase. These herbicides
are extremely potent inhibitors of acetolactate synthase
from plants having Iso values as low as 18.5 nm for chlorsulfuron
and 15.4 nm for sulfometuron methyl.