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. Plant Cell Physiol

a new plant growth retardant, was invatigated to determine the inhibitory sites in gibberellin biosynthesis and the comparative effects of its stereoisomcrs in a cell-free system from Cucurbita maxima. After treatment with S-3307, the incorporation of P^CTJmevalonic acid into GAii-aldehyde and GAu 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 uomers and two geometrical isomers. The activities of these isomers as a growth retardant and gibberellin biosynthesis inhibitor were exam-ined 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 th