EFFECT OF EXO-16,17-DIHYDRO-GIBBERELLIN-A(5) ON GIBBERELLIN-A(20) 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-Ga-5 (diHGA(5)) on the metabolism of GA(20) in dwarf rice (Oryza sativa cv. Tan-ginbozu). A mixture of [H-3]- and [H-3]-GA(20) (100 ng per plant) was applied in microdrops to 4 d old seedlings which were harvested 72 h later. Concurrent treatment with diHGA(5) at 100 ng or 333 ng per plant reduced GA(20)-induced elongation of the second leaf sheath by 41-66%. There was a concomitant reduction in the amount of [H-2(2)]GA(1) present at harvest, measured by gas chromatography-mass spectrometry-selected ion monitoring. The [H-2(2)]GA(29) content was also reduced. There was no clear effect of diHGA(5) on the total radioactivity recovered, or on conversion of the [H-3]GA(20) to putative [H-3]GA conjugates, or on the amount of [H-2(2)]GA(20) found. No free [H-2(2)]GA(8) was detected. In other experiments there was little effect of diHGA(5) on elongation induced by treatment with GA(1). We conclude that diHGA(5) inhibited GA(20)-induced elongation in dwarf rice shoots at least partly by reducing the 3 beta-hydroxylation of GA(20) to GA(1).