Imidazolinone herbicides are active against the enzyme acetohydroxyacid synthase (AHAS), also known as acetolactate synthase (ALS) or acetolactate pyruvate- lyase. <br>This enzyme catalyzes the first step in the biosynthesis of the essential branched chain amino acids isoleucine, leucine and valine. Herbicide induced AHAS inhibition results in a lethal decrease in protein synthesis. Unmodified wheat is not tolerant to imidazolinones, while soybeans, peas, white beans and kidney beans are naturally tolerant to them. <br>Depending on the species, plants contain single or multiple copies of the AHAS genes. The genome of <i>T. aestivum</i> is believed to contain an AHAS multigene family. <br>Modifications of AHAS genes in various plant species can result in herbicide tolerant phenotypes and typically consist of one amino acid substitution, sufficient to alter the binding site for imidazolinones such that the herbicide no longer inhibits the AHAS enzyme. Several modified AHAS plants have been isolated.<br>AP205CL was selected  from a population of <i>T. aestivum</i> derived by chemical-induced mutagenesis of seed of the wheat variety Gunner.  One mutant, tolerant to imidazoline was selected and designated AP205CL.  Tolerance to imadazoline was demonstrated by comparison of the activity of the AHAS enzyme extracted from AP205CL to that of wild type plants.