Key oxidase enzyme in the biosynthesis of gibberellin that catalyzes the conversion of GA12 and GA53 to GA9 and GA20 respectively, 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. 2 2-oxoglutarate + gibberellin A12 + H(+) + 3 O2 = 3 CO2 + gibberellin A9 + 2 H2O + 2 succinate 3 2-oxoglutarate + gibberellin A12 + 3 O2 = 3 CO2 + gibberellin A25 + H(+) + H2O + 3 succinate 2 2-oxoglutarate + gibberellin A53 + H(+) + 3 O2 = 3 CO2 + gibberellin A20 + 2 H2O + 2 succinate Binds 1 Fe(2+) ion per subunit. Plant hormone biosynthesis; gibberellin biosynthesis. Expressed at high level in developing siliques. Detected in seeds, roots, leaves and inflorescences. In seeds, specifically detected at the outer layer of the outer integument. Expressed in developing siliques 3-13 days after pollination. Negatively controlled by the level of physiologically active gibberellin. Not regulated by auxin. Up-regulated by paclobutrazol. Belongs to the iron/ascorbate-dependent oxidoreductase family. GA20OX subfamily.