Fig. 4.

Metabolic pathways leading to active GA biosynthesis. Modified redrawn from Hedden and Thomas (2012) and Yamaguchi (2008). The active forms of GA are in yellow ovals; the GA oxidase enzymes are abbreviated as 13ox, 2ox, and 3ox. The enzymes ent-copalyl diphosphate synthase (CPS), ent-kaurene synthase (KS), ent-kaurene oxidase (KO), and ent-kaurenoic acid (KAO) are indicated, together with the metabolites geranylgeranyl diphosphate (GGDP) and ent-copalyl diphosphate (ent-CP). The primary breakdown products of the active GA₁ and GA₄ forms are also shown (GA₈ and GA₃₄). The coloured bonds and atoms in the chemical structures indicate the modifications catalysed by the enzyme shown in the same colour (Yamaguchi, 2008). Isoform numbers are not included, as they appear to be distributed spatially rather than segregating between the two pathways (Suzuki et al., 2005). Note that some oxidized C-20 intermediates might not be released by the enzymes during some of the multistep reactions (Hedden and Sponsel, 2015). Recently, Liu et al. (2019) showed that a 2-oxoglutarate-dependent dioxygenase catalysed the formation of a previously unknown, biologically active gibberellin (DHGA₁₂) in seeds of Arabidopsis thaliana, by hydration of GA₁₂.