# Biosynthetic Pathways of Hormones in Plants
## 8 Brassinosteroids
Furthermore, 28-homoDL and CS can be produced from isofucosterol using a series of intermediates: 22-hydroxyisofusedocosterol, 6-deoxy-28-homoDS, and 28-homoDS. Castasterone can also be synthesized from \(\beta\)-sitosterol using intermediary compounds such as 22-homositosterol, 6-deoxyhomositosterol, and 28-homoCS. It has been discovered that 28-homoTE, 28-homoTY, and 28-homoCS can be converted into 26-nor-28-homoTE, 26-nor-28-homoTY, and 26-nor-28-homoCS, respectively [116; 124; 125; 126; 127; 128]. In the second pathway, 24-methylelenophenol is converted into campesterol, which acts as a precursor for C\({}_{28}\)-BRs (Figure 9) [122; 130; 131]. In this C\({}_{28}\)-BRs biosynthesis, two parallel pathways, i.e., the late and early C-22 oxidation pathways, occur. Campesterol  
Figure 9: The biosynthetic pathways of different types of brassinosteroids (phytohormones are red, green, and orange). is then converted into (24\(R\))-ergostan-4-\(en\)-3\(\beta\)-one and campestanol (CN). In the parallel early C-22 oxidation pathway, C-22\(\alpha\) hydroxylation of 24-methyleneCR by the enzyme C-22\(\alpha\) hydroxylase leads to the biosynthesis of 22-hydroxy-24-methyleneCR [131]. Subsequent processes resemble the late C-22 oxidation pathway, resulting in the generation of 22-hydroxy variants of the related substances. The distinguishing aspect between the C-22 oxidation sub-pathways is the formation of 6-deoxycathasterone (6-deoxycCT) from (22S,24R)-22-hydroxy-5\(\alpha\)-ergostan-3-one, circumventing the synthesis of CN. This process is referred to as the CN-independent route of BR biosynthesis. Nonetheless, during each phase of the late C-22 oxidation pathway, the compound involved could undergo hydroxylation via C-22\(\alpha\) hydroxylase, leading to the formation of hydroxylated versions of compounds that are part of the early C-22 pathway [132]. Transformation in 22-hydroxymethylyleneCR results in the creation of 6-deoxycolationosterone, which is converted into dolichosterone (DS), dolicholide (DL), castosterone (CS), and brassinolide (BL) [120; 128]. The biosynthesis of C\({}_{27}\)-BR involves the late C6 oxidation pathway. In these reactions, cholesterol is converted into cholestanol by the 5\(\alpha\)-reductase enzyme DET2. Consecutive reactions then produce a sequence of compounds: 6-deoxy-28-norcathosterone, 6-deoxy-28-norcastearone, 6-deoxy-28-norcastearone, 6-deoxy-28-noryphasterol, and 6-deoxyc-28-norcastearone. In addition, the early C6 oxidation pathway starts with the oxidation of cholestanol to create 6-oxocholestanol. This process leads to the synthesis of a series of compounds, including 28-norcathasterone, 28-norcastearone, 28-nor-3-dehydroteasterone, 28-nortyphasterol, 28-norcastearone, and, eventually, 28-nortressinolide [124; 133; 134]. The late C6 pathway begins with the hydroxylation of CN to form 6-deoxycathosterone (6-deoxycCT), which is a process catalyzed by the enzyme 22\(\alpha\)-hydroxylase. Interestingly, 6-deoxycCT can also be directly synthesized from 22-hydroxy-5\(\alpha\)-ergostan-3-one. Following this step, 6-deoxycCT can be hydroxylated by C-23 hydroxylase to form 6-deoxycastearone. This compound then undergoes C-3 oxidation to form 3-dehydro-6-deoxycateosterone (6-deoxy-3-DT) under the action of CYP90D C3-oxidase. Next, 6-deoxy-3-DT is converted into 6-deoxycytphasetrol (6-deoxycTY) by the D11 CYP724B1 enzyme. In the subsequent step, 6-deoxycTY is hydroxylated to form 6-deoxycastosterone (6-deoxycCS) by the enzyme 2\(\alpha\)-hydroxylase. 6-deoxycS is then converted into castasterone (CS) by the enzymes BR-6-oxidase1 and BR-6-oxidase2. Afterward, CS is transformed into brassinolide (BL) via a process called Baeyer-Villiger oxidation, which is facilitated by the enzyme BR-6-oxidase2 (CYP85A2). At the start of the early oxidation pathway, CN is hydrated to form 6\(\alpha\)-hydroxyCN. This compound is then oxidized to create 6-oxo-CN. The enzyme 22\(\alpha\)-hydroxylase is responsible for the conversion of 6-oxo-CN into cathasterone (CT).