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). Gibberellins are named in the order that they are discovered (GA1 through GAn). Gibberellin biosynthesis via non C-3, non C-13 hydroxylation occurs in the cytosol and converts the inactive GA12 to the active GA4, and inactive GA36 and GA13. The first two reactions are catalyzed by gibberellin 20-oxidase, requiring Fe2+ and L-ascorbate as cofactors. It first converts gibberellin A12 into gibberellin A15 and then into gibberellin A24. Gibberellin A24 has three different fates. The first route involves the conversion of gibberellin A24 into gibberellin A9 by gibberellin 20-oxidase and then the subsequent conversion of gibberellin A9 into the active gibberellin A4 by gibberellin 3-oxidase. It requires Fe2+ and L-ascorbate as cofactors. The second route involves the conversion of gibberellin A24 into gibberellin A36 by gibberellin 3-oxidase. The third route involves the conversion of gibberellin A24 into gibberellin A25 by gibberellin 20-oxidase and then the subsequent conversion of gibberellin A25 into gibberellin A13 by a not yet elucidated gibberellin oxidase (coloured orange in the image).

Gibberellin Biosynthesis III (Non C-3, Non C-13 Hydroxylation) References