An enzymic site of inhibition of gibberellin biosynthesis by Amo 1618 and other plant growth retardants.

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Abstract
2''-Isopropyl-4''-(trimethylammonium chloride)-5''-methylphenyl peperidine-1- carboxylate (Amo 1618), a plant growth retardant, inhibited the formation of (-)-kaurene and (-)-kauren-19-ol but not trans- geranylgeraniol from mevalonate in homogenates of endosperm and nucellus from seed of Echinocystis macrocarpa (wild cucumber). The enzymic cyclization of trans-geranylgeranyl pyro-phospahte to (-)-kaurene was directly shown to be inhibited by relatively low levels of this retardant. Several other plant retardants, among them tributyl-2,4-dichlorobenzylammonium chloride (Phosfon S) and tributyl-2,4-dichlorobenzylphosphonium chloride (Phosfon) as well as several substances closely related structurally to Amo 1618, also were found to inhibit (-)-kaurene formation in this system. It is proposed that inhibition of the cyclization reaction with the resulting decrease of gibberellin biosynthesis may account for the growth retarding properties of these substances.