The branched chain amino acid biosynthetic pathway has received considerable attention in recent years because different chemical classes of highly successful commercial herbicides kill plants by inhibiting this pathway. This discovery has led to identification and design of other inhibitors of this pathway that are also herbicidal. Our recent studies on the mode of action of imidazolinone herbicides have provided clues that may help clarify the role of 2-ketobutyrate and 2-aminobutyrate in the phytotoxic effects of these herbicides. These studies have also provided insight into understanding the regulation of carbon flow through the branched chain amino acid biosynthetic pathway in plants. We have also identified a new form of threonine dehydratase (TD; EC 4.2.1.16) that may have a crucial function in nitrogen metabolism in senescing leaves.

Accumulation of 2-KB/2-AB vs phytotoxidty of AHAS inhibitors

The imidazolinone and sulfonylurea families of highly successful commercial herbicides kill plants by inhibiting acetohydroxyacid synthase (AHAS; EC 4.1.3.18), the first common enzyme in the pathways leading to the biosynthesis of valine, leucine and isoleucine. These extremely potent herbicides kill plants at application rates of grams per hectare. The high potency of AHAS inhibiting herbicides is of great interest because inhibitors of other enzymes in the branched chain amino acid pathway require much higher rates to kill plants (Wittenbach, Aulabaugh & Schloss, 1991; Shaner & Singh, 1992).