Effect of mutagenesis at serine 653 of Arabidopsis thaliana acetohydroxyacid synthase on the sensitivity to imidazolinone and sulfonylurea herbicides.

Resistance to sulfonylurea and imidazolinone herbicides can occur by mutations in acetohydroxyacid synthase (EC 4.1.3.18). Changing serine 653 to asparagine is known to cause insensitivity to imidazolinones but not to sulfonylureas. Here, S-653 of the Arabidopsis thaliana enzyme was mutated to alanine, threonine and phenylalanine. The purified mutated enzymes resemble wild-type in their enzymatic properties. The threonine and phenylalanine mutants are imidazolinone-resistant and the latter is also slightly sulfonylurea-resistant. The alanine mutant remains sensitive to both herbicides. The results suggest that the beta-hydroxyl group is not required for imidazolinone binding and that the size of the side-chain determines resistance.