# Biosynthetic Pathways of Hormones in Plants
## 10 Gibberellins
This process is part of the mechanism used to decrease GA biosynthesis and its cellular concentration [165]. The second step of the transformation of _ent_-copalyl diphosphate to _ent_-kaurene is catalyzed by a type I cyclase, which is known as _ent_-kaurene synthase (KS) (Figure 11). Cyclization reaction is initiated via metal-dependent heterolytic cleavage of the O-C bond. Figure 11: The biosynthetic pathways of gibberellins (phytohormones are green). A pimeren-8-yl carbocation takes place, which then undergoes rearrangement. The loss of H\({}^{+}\) is important step that leads to tetracycline _ent_-kaurene [166]. Enzyme KS contains RLX(N,D)DX(S,T,G)XX(E,D) and DDXXD motifs, which can bind to ion Mg\({}^{2+}\), which is associated with the diphosphate residue and participates in its ionization [167]. The lycopbyte _Selaginella moellendorfii_, which is one of the first plants to have evolved the ability to synthesize GAs, possesses monofunctional CPS and KS enzymes, which are characteristic of vascular plants. In _S. moellendorfii_, KS is characterized by low substrate specificity, as it converts different stereoisomeric forms of CPP to various organic compounds, while KS enzymes in angiosperms are specific only for _ent_-CPP [168]. In angiosperms, CPS and KS possess the functional diversification required to produce the diterpenoids involved in plants' defense responses [157; 169]. GA\({}_{12}\), which is the first C\({}_{20}\)-GA, is formed from _ent_-kaurene, and the reactions of these synthetic steps of GA productions are catalyzed by the following enzymes: _ent_-kaurene oxidase (KO), two cytochrome P450 mono-oxygenases, and _ent_-kaureneoic acid oxidase (KAO) (Figure 11) [170]. In _A. thaliana_, KO is associated with the outer chloroplast membrane and the endoplasmic reticulum. On the other hand, two KAOs are located in the endoplasmic reticulum. This observation suggests that _ent_-kaurene is oxidized and then transported from plastids to the endoplasmic reticulum [157]. KO catalyzes the oxidation of _ent_-kaurene into _ent_-kaureneoic acid via repeated hydroxylation of C-19. The intermediate aldehyde, i.e., _ent_-kaurenal, is present in this reaction [171]. The first reaction of hydroxylation to _ent_-kaurenel is the rate-limiting step in the GA biosynthesis pathway [172]. The reactions catalyzed by KAO have been studied in cells that were isolated from developing seeds, i.e., endosperm [158]. In plants, _ent_-kaureneoic acid is converted into GA12 in three steps that involve 7/8-hydroxy-_ent_-kaureneoic acid and GA\({}_{12}\)-aldehyde. Enzyme KAO is involved in this conversion (Figure 11). The first step involves stereospecific hydroxylation of C-7/8. In the second reaction (initiated via stereospecific loss of the 6/8-H), ring B contracts between 5 and 6 carbon atoms via migration of the C-7-C-8 bond from C-7 to C-6. The extrusion of C-7 as the aldehyde is observed. In the third step, GA\({}_{12}\)-aldehyde is oxidized to GA\({}_{12}\). Cucurbitaceae plants, such as _Cucurbita maxima_ and _Cucunis sativa_, contain GA\({}_{7}\) oxidases (GA7 ox) that can convert GA\({}_{12}\)-aldehyde into GA\({}_{12}\). These enzymes are present in the development of seeds and vegetative tissues. The pathway branches from GA\({}_{12}\). The reaction of 13-hydroxylation to GA\({}_{53}\) initiates the formation of 13-hydroxylated GAs, such as GA\({}_{1}\). Additionally, a parallel non-13-hydroxylation route from GA\({}_{12}\) results in the synthesis of GA\({}_{4}\)[158]. In higher plants, GA\({}_{12}\) is present at a branch point along the GA pathway. GA\({}_{12}\)-aldehyde can be converted into GA\({}_{12}\) or transformed into GA\({}_{53}\) via the hydroxylation at C-13 (13-hydroxylated GA) (Figure 11). GA\({}_{12}\) and GA\({}_{53}\) are precursors of the non-13-hydroxylation and 13-hydroxylation pathways, respectively. These GAs can be oxidized at C-20, producing GA\({}_{9}\) and GA\({}_{20}\), respectively [159]. In rice, two cytochrome P450s--CYP714B1 and CYP714B2--are responsible for 13-hydroxylation of GA\({}_{12}\).