Converts the inactive gibberellin (GA) precursors GA9 and GA20 in the bioactives gibberellins GA4 and GA1. Involved in the production of bioactive GA for reproductive development. 2-oxoglutarate + gibberellin A20 + O2 = CO2 + gibberellin A1 + succinate Plant hormone biosynthesis; gibberellin biosynthesis. Expressed in siliques and in seeds, specifically at the rim of the embryo and the outer integument. Also expressed in flowers. Not detected in roots, stems and leaves. Expressed in developing siliques 3-13 days after pollination. No visible phenotype, but abnormal formation of the seed coat. Belongs to the iron/ascorbate-dependent oxidoreductase family. GA3OX subfamily.