Effect of exo-16,17-dihydro-gibberellin A5 on gibberellin A20 metabolism in seedlings of dwarf rice (Oryza sativa L. cv. Tan-ginbozu)
1994
Takagi, M. | Pearce, D.W. | Janzen, L.M. | Pharis, R.P.
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]GA20 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 3.3-hydroxylation of GA20 to GA1.
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