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-13 hydroxylation occurs in the cytosol and converts the inactive GA12 to the active GA1. First, the predicted enzyme gibberellin 13-hydroxylase (coloured orange in the image) is theorized to catalyze the conversion of gibberellin A12 into gibberellin A53. Second, gibberellin 20-oxidase catalyzes the conversion of gibberellin A53 into gibberellin A44 open lactone, requiring Fe2+ and L-ascorbate as cofactors. The main fate of gibberellin A44 open lactone is conversion into gibberellin A19 which is catalyzed by the not yet elucidated enzyme gibberellin-44 dioxygenase, requiring Fe3+ as a cofactor. The secondary fate of gibberellin A44 open lactone is conversion into gibberellin A38 which is catalyzed by gibberellin 3-oxidase, requiring Fe2+ and L-ascorbate as cofactors. The main fate of gibberellin A19 is conversion into gibberellin A20 and its secondary fate is conversion into gibberellin A17. These reactions' enzymes have not yet been elucidated and are referred to as simply gibberellin oxidase for reference purposes. Last, gibberellin 3-oxidase converts gibberellin A20 into the active gibberellin A1. It requires Fe2+ and L-ascorbate as cofactors.

Gibberellin Biosynthesis II (Early C-13 Hydroxylation) References