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, and GA25 is also formed as a minor product. 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 GA20OX1 (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 3 2-oxoglutarate + gibberellin A12 + 3 O2 = 3 CO2 + gibberellin A25 + H(+) + H2O + 3 succinate Binds 1 Fe(2+) ion per subunit. Plant hormone biosynthesis; gibberellin biosynthesis. Expressed in inflorescence and developing siliques. Detected in seeds, roots, cotyledons and leaves. In seeds, specifically detected at the rim of the embryo and the outer integument. Expressed in developing siliques 3-13 days after pollination. Negatively controlled by the level of physiologically active gibberellin. Up-regulated by auxin, paclobutrazol, long day exposure and cold treatment. Slightly smaller than the wild type. Belongs to the iron/ascorbate-dependent oxidoreductase family. GA20OX subfamily.