PLANT PHYSIOLOGY , Vol 103, Issue 4 1221-1226, Copyright © 1993 by American Society of Plant Biologists
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METABOLISM AND ENZYMOLOGY |
Phytotoxicity of Acetohydroxyacid Synthase Inhibitors Is Not Due to Accumulation of 2-Ketobutyrate and/or 2-Aminobutyrate
D. L. Shaner and B. K. Singh
American Cyanamid Company, Princeton, New Jersey 08543-0400
Acetohydroxyacid synthase (AHAS) is the site of action of herbicides of
different chemical classes, such as imidazolinones, sulfonylureas, and
triazolopyrimidines. Inhibition of AHAS causes the accumulation of
2-ketobutyrate (2-KB) and 2-aminobutyrate (2-AB) (the transamination
product of 2-KB), and it has been proposed that the phytotoxicity of these
inhibitors is due to this accumulation. Experiments were done to determine
the relationship between accumulation of 2-KB and 2-AB and the
phytotoxicity of imazaquin to maize (Zea mays). Imazaquin concentrations
that inhibit growth of maize plants also cause the accumulation of 2-KB and
2-AB in the shoots. Supplementation of imazaquin-treated plants with
isoleucine reduced the pools of 2-KB and 2-AB in the plant but did not
protect plants from the growth inhibitory effects of imazaquin. Conversely,
feeding 2-AB to maize plants increased 2-KB and 2-AB pools to much higher
levels than those observed in imazaquin-treated plants, yet such high pools
of 2-KB and 2-AB in the plant had no significant effect on growth. These
results conclusively demonstrate that growth inhibition following imazaquin
treatment is not due to accumulation of 2-KB and/or 2-AB in plants. Changes
in the amino acid profiles after treatment with imazaquin suggest that
starvation for the branched-chain amino acids may be the primary cause of
growth retardation of maize.
