Inhibition of acetohydroxyacid synthase (AHAS) [EC 4.1.3.18], also known as acetolactate synthase (ALS), with a number of compounds which target this enzyme (imidazolinones, sulfonylureas, triazolopyrimidine sulfonamides and pyrimidyl thio-benzoates), leads to depletion of valine, leucine and isoleucine, and accumulation of 2-oxobutyrate (2-oxobutanoate) and its transamination product, alpha-amino-n-butyrate (Rhodes et al, 1987). Antisense-AHAS potato plants exhibit similar phenotypes to plants treated with AHAS herbicides (Hofgen et al, 1995).

A number of single gene mutants in various plant species have been obtained that are resistant to imidazolinone and sulfonylurea herbicides. These result from herbicide-resistant AHAS activity, arising from a mutation in the coding region of the AHAS gene. Transformation of the mutant AHAS genes confers high levels of herbicide resistance (Singh and Shaner, 1995).

Hofgen R, Laber B, Schuttke I, Klonus A-K, Streber W, Pohlenz H-D 1995 Repression of acelolactate synthase activity through antisense inhibition: Molecular and biochemical analysis of transgenic potato (Solanum tuberosum L. cv. Desiree) plants. Plant Physiol. 107: 469-477.

Rhodes D, Hogan AL, Deal L, Jamison GC, Haworth P 1987 Amino acid metabolism of Lemna minor L. III. Responses to chlorsulfuron. Plant Physiol. 84: 775-780.

Singh BK, Shaner DL 1995 Biosynthesis of branched chain amino acids: From test tube to field. Plant Cell 7: 935-944.