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 Expressed in nodes and the ear of the elongating stem. Highly expressed in the embryo and the surrounding maternal tissues, the pericarp and the integuments. Also found in the germinating grain. Belongs to the iron/ascorbate-dependent oxidoreductase family. GA20OX subfamily.