Effect of exo-16,17-dihydro-gibberellin A5 on gibberellin A20 metabolism in seedlings of dwarf rice (Oryza sativa L. cv. Tan-ginbozu)

Several of the 16,17-dihydro gibberellins (GAs) inhibit elongation in a variety of species. In a study of their mechanism of action we have investigated the effect of exo-16,17-dihydro-Ga₅ (diHGA₅) on the metabolism of GA₂₀ in dwarf rice (Oryza sativa cv. Tan-ginbozu). A mixture of ³H]- and ³H]-GA₂₀ (100 ng per plant) was applied in microdrops to 4 d old seedlings which were harvested 72 h later. Concurrent treatment with diHGA₅ at 100 ng or 333 ng per plant reduced GA₂₀-induced elongation of the second leaf sheath by 41–66%. There was a concomitant reduction in the amount of ²H₂]GA₁ present at harvest, measured by gas chromatography-mass spectrometry-selected ion monitoring. The ²H₂]GA₂₉ content was also reduced. There was no clear effect of diHGA₅ on the total radioactivity recovered, or on conversion of the ³H]GA₂₀ to putative ³H]GA conjugates, or on the amount of ²H₂]GA₂₀ found. No free ²H₂]GA₈ was detected. In other experiments there was little effect of diHGA₅ on elongation induced by treatment with GA₁. We conclude that diHGA₅ inhibited GA₂₀-induced elongation in dwarf rice shoots at least partly by reducing the 3-hydroxylation of GA₂₀ to GA₁.Abbreviations diHGA₅ = exo- 16, 17-dihydro-gibberellin A₅ - GA = gibberellin - GC-MS-SIM = gas chromatography-mass spectrometry-selected ion monitoring