Studies of sites of action of a new plant growth retardant (E)-1-(4-chlorophenyl)-4,4-dimethyl-2-(1,2,4-triazol-1-yl)-1-penten-3-ol (S-3307) and comparative effects of its stereoisomers in a cell-free system from Cucurbita maxima

(E)-1-(4-Chlorophenyl)-4,4-dimethyl-2-(1,2,4-triazol-1-yl)-1-penten-3-ol (S-3307), a new plant growth retardant, was investigated to determine the inhibitory sites in gibberellin biosynthesis and the comparative effects of its stereoisomers in a cell-free system from Cucurbita maxima. After treatment with S-3307, the incorporation of [14C]mevalonic acid into GA12-aldehyde and GA12 clearly was inhibited, and kaurene accumulated. Feeding experiments showed that S-3307 inhibited the oxidation of kaurene, kaurenol and kaurenal, but did not affect the oxidation of kaurenoic acid. Thus the reaction sites of S-3307 in gibberellin biosynthesis were shown to be the three oxidation steps from kaurene to kaurenoic acid. Because S-3307 has an asymmetric center and a tri-substituted double bond, there are four stereoisomers; two optical isomers and two geometrical isomers. The activities of these isomers as a growth retardant and gibberellin biosynthesis inhibitor were examined with a rice seedling assay and the cell-free system. The relative activity of these isomers was basically the same in the two assay methods. The (S)-(E) form was the most active, being 7-fold more active in the cell-free system and 70-fold more active in the bioassay than the (R)-(E) form. The (RS)-(Z) form showed no activity in the cell-free system, but weak dwarfing effects in the rice seedling assay.