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. 2 2-oxoglutarate + gibberellin A12 + H(+) + 3 O2 = 3 CO2 + gibberellin A9 + 2 H2O + 2 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 in 3-day-old seedlings and siliques. Detected in dry seeds, roots, old leaves and inflorescences. Not controlled by the level of physiologically active gibberellin. Belongs to the iron/ascorbate-dependent oxidoreductase family. GA20OX subfamily. Truncated N-terminus. Truncated N-terminus.