Effects of 3-Methylgibberellin Analogs on Gibberellin 3β-Hydroxylases and Plant Growth.

Six 3-methylgibberellin analogs were synthesized, and their effects on the GA 3β-hydroxylases from immature seeds of Phaseolus vulgaris and Cucurbita maxima, and/or on the growth of dwarf rice (Oryza sativa L. cv. Tan-ginbozu) and cucumber (Cucumis sativus L. cv. Spacemaster) were investigated. 3-Methyl-GA5 and 2, 3-didehydro-3-methyl-GA9· inhibited the conversion of [2, 3-(3)H2]GA9 to [2-(3)H]GA4 by GA 3β-hydroxylases from both P. vulgaris and C. maxima at 3 μM and higher. Their C/D-ring-rearranged isomers, 2, 3-didehydro-3-methyl-DGC and 16-deoxo-2, 3-didehydro-3-methyl-DGC, inhibited 3β-hydroxylation by the enzyme from P. vulgaris threefold more strongly than the non-C/D-ring-rearranged compounds, but exhibited no effect on 3β-hydroxylation by the enzyme from C. maxima. In a dwarf rice seedling assay, 3-methyl-GA5 and 2, 3-didehydro-3-methyl-GA9 promoted shoot elongation at doses of 300 ng/plant and higher, and 3α-methyl-GA1 and 3α-methyl-GA4 at doses of 30 ng/plant and higher. In contrast 2, 3-didehydro-3-methyl-DGC inhibited shoot growth to half that of the control at a dose of 300 ng/plant, and 16-deoxo-2, 3-didehydro-3-methyl-DGC showed no effect on growth. In a cucumber seedling assay, 3α-methyl-GA4 promoted hypocotyl elongation at doses of 300 ng/plant and higher. The other C-3 methyl compounds showed no effect on the hypocotyl elongation of cucumber seedlings.