Key oxidase enzyme in the biosynthesis of gibberellin that catalyzes the conversion of GA12 to GA9, via a three-step oxidation at C-20 of the GA skeleton. GA53 is less effectively oxidized than GA12 and is only oxidized one step to GA44 (PubMed:7630935). Involved in the promotion of the floral transition, fertility and silique elongation, but plays only a minor role in elongation of seedling organs. Acts redundantly with GA20OX2 (PubMed:18069939). 2 2-oxoglutarate + gibberellin A12 + H(+) + 3 O2 = 3 CO2 + gibberellin A9 + 2 H2O + 2 succinate 2-oxoglutarate + gibberellin A12 + O2 = CO2 + gibberellin A15 + succinate 2-oxoglutarate + gibberellin A15 + O2 = CO2 + gibberellin A24 + H2O + succinate 2-oxoglutarate + gibberellin A53 + O2 = CO2 + gibberellin A44 + succinate Binds 1 Fe(2+) ion per subunit. Plant hormone biosynthesis; gibberellin biosynthesis. Highly expressed in stems and inflorescence tissues. Detected in seeds, roots, leaves and siliques. Circadian-regulation. Up-regulated by auxin, paclobutrazol and cold treatment. Negatively controlled by the level of physiologically active gibberellin. Semi-dwarf. Belongs to the iron/ascorbate-dependent oxidoreductase family. GA20OX subfamily.