PHYTOTOXICITY OF ACETOHYDROXYACID SYNTHASE INHIBITORS IS NOT DUE TO ACCUMULATION OF 2-KETOBUTYRATE AND OR 2-AMINOBUTYRATE

Acetohydroxyacid synthase (AHAS) is the site of action of herbicides o f different chemical classes, such as imidazolinones, sulfonylureas, a nd triazolopyrimidines. Inhibition of AHAS causes the accumulation of 2-ketobutyrate (2-KB) and 2-aminobutyrate (2-AB) (the transamination p roduct of 2-KB), and it has been proposed that the phytotoxicity of th ese inhibitors is due to this accumulation. Experiments were done to d etermine the relationship between accumulation of 2-KB and 2-AB and th e phytotoxicity of imazaquin to maize (Zea mays). Imazaquin concentrat ions that inhibit growth of maize plants also cause the accumulation o f 2-KB and 2-AB in the shoots. Supplementation of imazaquin-treated pl ants with isoleucine reduced the pools of 2-KB and 2-AB in the plant b ut did not protect plants from the growth inhibitory effects of imazaq uin. Conversely, feeding 2-AB to maize plants increased 2-KB and 2-AB pools to much higher levels than those observed in imazaquin-treated p lants, yet such high pools of 2-KB and 2-AB in the plant had no signif icant effect on growth. These results conclusively demonstrate that gr owth inhibition following imazaquin treatment is not due to accumulati on of 2-KB and/or 2-AB in plants. Changes in the amino acid profiles a fter treatment with imazaquin suggest that starvation for the branched -chain amino acids may be the primary cause of growth retardation of m aize.