Converts the inactive gibberellin (GA) precursors GA9 and GA20 into the bioactives gibberellins GA4 and GA1, respectively. Involved in the production of bioactive GA for vegetative growth and development. 2-oxoglutarate + gibberellin A9 + O2 = CO2 + gibberellin A4 + succinate 2-oxoglutarate + gibberellin A20 + O2 = CO2 + gibberellin A1 + succinate Optimum pH is 7.5. Plant hormone biosynthesis; gibberellin biosynthesis. Expressed in stems, roots, leaves, flowers, and siliques. Highly expressed near the nodes in stems and in the stamen filaments of flowers. Detected in developing cotyledons, vegetative shoot apical meristem and non-meristematic, non-elongation regions of the roots. Found in the cortex and the endodermis of the embryo axis in germinating seeds and in the placenta in developing siliques. Expressed in germinating seeds and in very young seedlings. Expressed in developing siliques 3-13 days after pollination. Inhibited by GA3, indicating the existence of a probable feedback loop. Inhibited by dihydro gibberellins. Regulated by phytochrome. Induced sharply after red light pulse with a peak at 4 hours, and then decreases rapidly as germination occurs. Increases again when etiolated seedling growth begins. Not regulated by long day exposure or auxin. Up-regulated by cold treatment, paclobutrazol and uniconazole P. Semi-dwarf. Ga3ox1 and ga3ox2 double mutant has a severe defect in seed germination and root growth, and a dwarf phenotype. Belongs to the iron/ascorbate-dependent oxidoreductase family. GA3OX subfamily.