Sites of Inhibition by a Plant-Growth Regulator, 4'-Chloro-2'-({alpha}-hydroxybenzyl)-isonicotinanilide (Inabenfide), and its Related Compounds in the Biosynthesis of Gibberellins

Inhibition of the biosynthesis of gibberellins by 4'-chloro-2'-(a-hydroxybenzyl)-isonicotin-anilide(inabenfide) was investigated with cell-free systems preparedfrom immature seeds of Cucurbita maxima, Phaseolus vulgarisand Pisum sativum. Inabenfide specifically inhibited the oxidationof kaurene, kaurenol and kaurenal but it did not inhibit theoxidation steps that lead to kaurenoic acid. The (S)- and (R)-formsof inabenfide and 2'-benzoyl-4'-chloro-isonicotinanilide (compound[1]) also inhibited the same oxidation steps. The (S)-form wasthe most active in inhibition of the oxidation of kaurene; theracemic form (inabenfide) was one half as effective; and the(R)-form and compound [1] had little effect on the biosynthesisof gibberellins. Growth-retardant activities of these compoundswere also examined using seedlings of C. maxima. Since the growth-retardantactivity of these compounds was correlated with their abilityto inhibit the oxidation of kaurene in cell-free systems, wesuggest that the growth-retardant activity of inabenfide isdue to inhibition of the three oxidation steps from kaureneto kaurenoic acid in the biosynthesis of gibberellins. (Received August 17, 1989; Accepted December 4, 1989)