Site of action of chlorsulfuron: inhibition of valine and isoleucine biosynthesis in plants.

The sulfonylurea herbicide chlorsulfuron blocks the biosynthesis of the amino acids valine and isoleucine in plants. Addition of these two amino acids to excised pea root (Pisum sativum L. var Alaska) cultures incubated in the presence of chlorsulfuron completely alleviates herbicide-induced growth inhibition. The site of action of chlorsulfuron is the enzyme acetolactate synthase which catalyzes the first step in the biosynthesis of valine and isoleucine. This enzyme is extremely sensitive to inhibition by chlorsulfuron having I(50) values ranging from 18 to 36 nanomolar. In addition, acetolactate synthase from a wide variety of tolerant and sensitive plants species is highly sensitive to inhibition by chlorsulfuron.