The imidazolinones, a new chemical class of herbicides, were shown to be uncompetitive inhibitors of acetohydroxyacid synthase from corn. This is the first common enzyme in the biosynthetic pathway for valine, leucine, and isoleucine. The K, for the imidazolinones tested ranged from 2 to 12 micromolar. These results may explain the mechanism of action of these new herbicides.The imidazolinones are a new chemical class of herbicides discovered and being developed by American Cyanamid Company (see structures in Table I). These herbicides kill both monocotyledonous and dicotyledonous species, and selectivity is achieved by differential metabolism of the compounds to nonherbicidal forms (7, 9). Death of the whole plant can require several weeks. The meristematic tissues die first, followed by slow necrosis of the mature tissue.The precise mechanism of action of many herbicides are unknown. The imidazolinones are somewhat unusual in that a biochemical pathway has been identified which is inhibited by these herbicides. The imidazolinones interfere with the biosynthesis of the branched chain amino acids valine, leucine, and isoleucine (10). Corn tissue grown in suspension culture and treated with AC 243,997 (2-[4-isopropyl-4-methyl-5-oxo-2-imidazolin-2-yl]nicotinic acid) showed changes in the free amino acid levels (10). Most of the amino acids either increased or remained constant, but the levels of valine, leucine, and isoleucine decreased. Exogenously supplying these three amino acids to corn tissue cultures and seedlings prevented the phytotoxic effects of AC 243,997 (10). All three of these amino acids were necessary for maximal protection to occur.There are four enzymes that catalyze biosynthetic steps leading to valine, leucine, and isoleucine. Acetohydroxyacid synthase (EC 4.1.3.18) is the first enzyme in the biosynthesis ofvaline and leucine and the second enzyme in the biosynthesis of isoleucine. Acetohydroxyacid synthase is feedback inhibited cooperatively by valine and leucine in higher plants (4). Thus, it is a key controlling point for the levels of the branched chain amino acids and is a prime site for inhibition by a xenobiotic. The imidazolinones were found to be potent inhibitors of acetohydroxyacid synthase and this may explain why the levels ofvaline, leucine, and isoleucine decreased after treatment with the imidazolinones.MATERIALS AND METHODS Chemicals. Analytical grade imidazolinones were synthesized at the American Cyanamid Agricultural Research Center, Princeton, NJ. Acetoin was obtained from Eastman Kodak.Plant Material. Corn (Zea mays L. Pioneer var 3547) seeds were planted in sand and watered with 0.2 mm CaC12. The plants were grown in darkness at 28°C for 3 d. The roots and shoots from the etiolated seedlings were used.Acetohydroxyacid Synthase Assay. The procedure used was a modification ofone used by B. J. Miflin (5). Roots and coleoptiles of the corn seedlings were homogenized in an extraction buffer (50 mm K-phosphate buffer (pH 7.5),5 mM EDTA, 5 mm MgCl2, 1 mM valine, 1 mM leu...

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Three corn (Zea mays L.) lines resistant to imidazolinone herbicides were developed by in vitro selection and plant regeneration. For all three lines, resistance is inherited as a single semidominant allele. The resistance alleles from resistant lines XA17, XI12, and QJ22 have been crossed into the inbred line B73, and in each case homozygotes are tolerant of commercial use rates of imidazolinone herbicides. All resistant selections have herbicide-resistant forms of acetohydroxyacid synthase (AHAS), the known site of action of imidazolinone herbicides. The herbicide-resistant phenotypes displayed at the whole plant level correlate directly with herbicide insensitivity of the AHAS activities of the selections. The AHAS activities from all three selections have normal feedback regulation by valine and leucine, and plants containing the mutations display a normal phenotype.

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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. AH 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 interaction of the enzyme with the herbicides is affected by enzyme aggregation state.