# Biosynthetic Pathways of Hormones in Plants
## 10 Gibberellins
Although vegetative rice tissues contain predominantly 13-hydroxylated GAs, the overexpression of the CYP714B gene, which resulted in an increase in the concentration of various 13-hydroxy types of GAs, including GA\({}_{1}\), also caused semidwarfism [173]. However, _A. thaliana_ has two members of the CYP714A subfamily that are highly expressed in developing seeds. CYP714A1 transforms GA\({}_{12}\) into 16\(\alpha\)-carboxy-17-norGA\({}_{12}\), whereas CYP714A2 converts GA\({}_{12}\) into 12\(\alpha\)-hydroxyGA\({}_{12}\) (GA\({}_{111}\)) and, at a lower level of efficiency, GA\({}_{53}\) via the 13-hydroxylation process. This enzyme is also capable of hydrolyzing _ent_-kaureneoic acid in the C-13 position to steviol [174]. Another member of the CYP714 family that is present in rice--CYP714D1--catalyzes the reaction of the epoxidation of the 16,17-double bond of 13-deoxyGAs, including GA\({}_{12}\). This reaction leads to synthesis of inactive compounds [157]. Thus, members of the CYP714 family are responsible for the inactivation of GAs and _ent_-kaureneoids via their oxidation at the C- and D-rings. _A. thaliana_ contains eight tandem CYP72A genes. Among these genes, CYP72A9 was shown to 13-hydroxylate GA\({}_{12}\), as well as GA\({}_{9}\) and GA\({}_{4}\). Therefore, CYP72A9 is the main source of the 13-hydroxy types of Gas [175]. The transformation of GA\({}_{12}\) and GA\({}_{53}\) into GA\({}_{9}\) and GA\({}_{20}\), respectively, are catalyzed in plants by a family of 2-oxoglutate/Fe(II)-dependent dioxygenases, which are also known as GA 20-oxidases (GA20 ox) (Figure 11). These enzymes are responsible for the cleavage of C-20 with 19,10-\(\gamma\)-lactone formation, which is a characteristic of C\({}_{19}\)-GA. Firstly, C-20 methyl is oxidized into alcohol form. Next, it is transformed into the aldehyde. During this oxidation process, C-20 is removed as CO\({}_{2}\), and 20-oic acid is a minor by-product. However, this biochemical mechanism is not well understood [176]. Vascular plants contain a family of _GA20 ox_ genes, which have different developmental, environmental, and tissue expression patterns. In many plants, GA20 ox proteins are involved in the regulation of the GA rate of the synthesis of GA, and expressions of _GA20 ox_ genes are tightly regulated by developmental and environmental signals to maintain GA homeostasis [177]. In the final step in the biosynthesis of bioactive GAs, the C\({}_{19}\)-GAs (i.e., GA\({}_{9}\) and GA\({}_{20}\)) are 3\(\beta\)-hydroxylated to GA\({}_{4}\) and GA\({}_{1}\), respectively, by the enzyme GA 3-oxidase (GA3 ox) (Figure 11) [178]. GA\({}_{4}\) is the first biologically active GA. This GA is desaturated to GA\({}_{7}\), which is then converted into GA\({}_{3}\) via the late 13-hydroxylation reaction. Enzymes GA3 ox are highly active in vegetative tissues and function only as 3\(\beta\)-hydroxylases. They are characterized by high regiospecificity in dicots. On the other hand, GA3 ox in monocots are less specific to the substrates. Therefore, GA\({}_{3}\) is synthesized from GA20 as a minor by-product via the GA\({}_{1}\) production route. The oxidation at the 2\(\beta\) and 3\(\beta\) positions leads to formation of the 2,3-unsaturated intermediate GA\({}_{5}\), which is then converted into GA\({}_{3}\) via oxidation at C-1\(\beta\). This reaction is catalyzed by the same enzyme [157]. Therefore, monocots contain low levels of GA\({}_{3}\), which are usually less than 10% of the GA\({}_{1}\) content, while they are generally undetectable in vegetative tissues of dicots. In plants, the last step in GA synthesis is the 3\(\beta\)-hydroxylation of GA\({}_{9}\) and GA\({}_{20}\) into GA\({}_{4}\) and GA\({}_{1}\), respectively. GA\({}_{4}\) and GA\({}_{1}\) are the main GAs. The _GA3 ox_ genes are present in small families. _A. thaliana_ contains four members, while rice and barley (_Hordeum vulgare_) contain only two members. Two enzymes of GA3 ox--AGA3 ox1 and AGA3 ox2--present in _A.