Imidazolinones

Abstract

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...

Efeito residual dos herbicidas imazaquin e imazethapyr, aplicados na soja, sobre o milho safrinha

“…4 Os dois produtos pertencem ao grupo químico das imidazolinonas e estão registrados no Brasil para uso exclusivo na cultura da soja, sendo o primeiro utilizado em pré-plantio e incorporado (PPI) e em pré-emergência da cultura e o segundo, em pós-emergência da mesma (Rodrigues & Almeida, 1995). Segundo Shaner et al (1984), ambos os produtos possuem características de persistência no solo, o que para a cultura da soja é muito favorável, pois proporciona o controle residual das plantas daninhas durante o seu ciclo. Mas esta característica também constitui num risco para as culturas sucessivas à soja.…”

Seed Bank Depletion: The Key to Long-Term Downy Brome ( Bromus tectorum L.) Management

“…Plateau and other imidazolinone herbicides inhibit acetohydroxyacid synthase (AHAS), which catalyzes the biosynthesis of three amino acids, valine, leucine, and isoleucine (Shaner et al 1984). Protein synthesis in susceptible plant species is incomplete and they perish over time.…”

Herbicidal Activity of an Isopropylmalate Dehydrogenase Inhibitor

“…What we have found is an excellent inhibitor of yet another enzyme in the biosynthetic pathway for branched-chain amino acids. Excellent inhibitors have now been identified for four enzymes of the branched-chain amino acid biosynthetic pathway (ALS, KARI, isopropylmalate isomerase, and IPMDH [LaRossa and Schloss, 1984;Shaner et al, 1984;Schulz et al, 1988;Aulabaugh and Schloss, 1990;Wittenbach et al, 1992;Hawkes et al, 1993]), yet only inhibitors of one of these enzymes (ALS) have resulted in commercial, low-use-rate herbicides. Thus, simply inhibiting an enzyme of a vital biosynthetic pathway does not appear to be sufficient to result in a good herbicide, even when there are no apparent limitations in uptake, translocation, and metabolism of the inhibitor.…”

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With the discovery of ALS as the site of action of the sulfonylurea and imidazolinone herbicides (LaRossa and Schloss, 1984;Ray, 1984;Shaner et al, 1984), considerable attention tumed to other enzymes in branched-chain amino acid biosynthesis as potential herbicidal target sites (Schulz et al, 1988; Pirmng, 1989;Aulabaugh and Schloss, 1990;Schloss and Aulabaugh, 1990;Wittenbach et al, 1991 Wittenbach et al, , 1992Hawkes, 1993). In attempting to improve the herbicidal activity of some KARI inhibitors (Wittenbach et al, 1991), we discovered a class of chemicals with increased herbicidal activity that had essentially no in vitro activity on the plant enzyme.

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