Herbicides that Inhibit Acetohydroxyacid Synthasel

Acetohydroxyacid synthase was discovered as the site of action of imidazolinone and sulfonylurea herbicides over 6 yr ago. In recent years, advances have been made in the understanding of this enzyme as a herbicide target site. Derivatives of both imidazolinones and sulfonylureas have yielded new herbicide chemistry. All of the herbicides display unusual "slow-binding" behavior with the enzyme, and this behavior may help explain efficacy of the herbicides. Resistance to these herbicides has been developed through a number of different procedures, and the mechanism of resistance is through changes in sensitivity of the enzyme to the herbicides. The changes are either selective to only one class of chemistry, or broad to a number of classes of chemistry. These data support the idea that binding sites for the herbicides on the enzyme are only partially overlapping. Progress in purification of AHAS from corn includes discovery of the existence of the enzyme in monomer and oligomer aggregation states. The interac- tion of the enzyme with the herbicides is affected by enzyme aggregation state. Nomenclature: AHAS, aceto- hydroxyacid synthase or acetolactate synthase, E.C. 4.1.3.18; sulfometuron methyl, N-((4,6-dimethyl- pyrimidin-2-yl)aminocarbonyl)-2-methoxycarbonyl-ben- zenesulfonamide; imazapyr, (?)-2-(4,5'-dihydro-4-methyl- 4-(1-methylethyl)-5-oxo-lH-imidazol-2-yl)-3-pyridinecar- boxylic acid; imazethapyr, (?)-2-(4,5-dihydro-4-methyl- 4-(1-methylethyl)-5-oxo-lH-imidazol-2-yl)-5-ethyl-3-pyri- dinecarboxylic acid; 2-(4,5-dihydro-4-methyl-4-(1-methyl- ethyl)-5-oxo-1H-imidazol-imazaquin, 2-(4,5-dihydro-4- methyl-4-(1-methylethyl)-5-oxo-lH-imidazol-imazaquin, 2-(4,5-dihydro-4-methyl-4-(1-methylethyl)-5-oxo-lH-im- idazol-2-yl)-3-quinolinecarboxylic acid. Additional index words. Herbicide, acetohydroxyacid syn- thase, imidazolinone, sulfonylurea.