Gibberellins (GAs) are a large class of tetracyclic diterpenoid plant hormones that regulate numerous growth and developmental processes, such as seed germination, organ elongation, and flowering induction. All known gibberellins share an ent-gibberellane skeleton and follow the same synthesis pathway. Biosynthesis begins in the plasmids via the terpenoid pathway and finishes in the endoplasmic reticulum and cytosol where they undergo modification until a biologically-active form is reached (GA1, GA3, GA4, or GA7). Gibberellin biosynthesis via early C-3 hydroxylation occurs in the cytosol and converts the inactive GA12 to the active GA4. First, a predicted enzyme gibberellin 3beta-hydroxylase (coloured orange in the image) is theorized to hydroxylate the C3 carbon of gibberellin A12, synthesizing gibberellin A14. Second, gibberellin 20-oxidase catalyzes the conversion of gibberellin A14 into gibberellin A37. This is the first of two reactions catalyzed by this enzyme in this pathway. Gibberellin 20-oxidase requires Fe2+ and L-ascorbate as cofactors. Alternatively, gibberellin A37 can be synthesized from gibberellin A15 via gibberellin 3-oxidase. This enzyme also requires Fe2+ and L-ascorbate as cofactors. Third, the conversion of gibberellin A37 into gibberellin A36 is catalyzed by a yet unelucidated enzyme (unofficially termed gibberellin oxidase for reference purposes). Last, gibberellin 20-oxidase catalyzes the conversion of gibberellin A36 into the active gibberellin A4.

Gibberellin Biosynthesis I (Early C-3 Hydroxylation) References