# Biosynthetic Pathways of Hormones in Plants
## 8 Brassinosteroids  
Brassinosteroids (BRs) are steroid phytohormones that elicit a wide spectrum of morphological and physiological responses, as well as a tolerance of abiotic and biotic stress [108; 109; 110; 111; 112; 113; 114]. BRs are categorized into three primary groups based on the number of carbon atoms present in each steroid molecule, and BRs are divided into three main groups on the basis of each steroid molecule, i.e., C\({}_{27}\), C\({}_{28}\), and C\({}_{29}\)[115; 116]. The basic structure of C\({}_{27}\)-BRs is the 5\(\alpha\)-cholestane skeleton; 5\(\alpha\)-ergostane serves as the foundational structure of C\({}_{28}\)-BRs, while 5\(\alpha\)-stigmastane forms the basis of C\({}_{29}\)-BRs. The structures of these hormones vary due to the type and orientation of oxygenated functions in the A- and B-rings, as well as the number and position of functional groups in the side chain. These variations arise from oxidation and reduction reactions during their synthesis in plants [117; 118; 119]. Three pathways of BR biosynthesis that lead to the production of BRs of type C\({}_{27}\), C\({}_{28}\), or C\({}_{29}\) were identified in plants. Early steps involved in their synthesis, which are common for all BRs, occur via the MVA or non-MVA (MEP) pathways (Figure 7). Later steps differentiate between the biosynthesis pathways (cycloartenol- and cycloartanol-dependent) of this group of plant hormones (Figure 9) [116; 120; 121]. Isopentenyl pyrophosphate (IPP), which is an intermediary compound involved in the steroid synthesis pathway (for example, cholesterol), can be created via two routes: the MEP pathway (in lower plants) and the MVA pathway (in most higher plants) (Figure 7) [122]. The conversion of IPP into geranyl pyrophosphate and, subsequently, farnesly pyrophosphate leads to the creation of squalene (Figure 7). The squalene is then oxidized to create squalene-2,3-oxide under the influence of the enzyme squalene epoxidase. Following this step, cycloartenol synthase transforms squalene-2,3-oxide into cycloartenol, which is a crucial substrate used in the synthesis of C\({}_{27}\), C\({}_{28}\), and C\({}_{29}\)-BRs. Cycloartenol can then be transformed into cycloartanol and, through a series of reactions, into cholesterol/cholesterol and/or 6-oxochoelastanol, leading to the creation of C\({}_{27}\)-BR (Figure 9). In another process, cycloartenol may serve as a substrate for the C-24 methylation reaction, which is catalyzed by sterol C-24 methyltransferase(SMT1), resulting in the production of 24-methylenediocartanol. Subsequent reactions, which are driven by enzymes such as C-4 sterol methyl oxidase (SMO1), cyclopropylsterol isomerase, obtusifoliol 14\(\alpha\)-demethylase (CYP51), and sterol C-14 reductase, lead to the formation of 4\(\alpha\)-methylergostatrienol. The intermediates of these reactions include cycloucalenol and obtusifoliol. Next, the enzyme sterol C-14 reductase catalyzes the reduction in 4\(\alpha\)-methylergostatrienol to create 4\(\alpha\)-methylergostadienol, which is subsequently transformed into 24-methylelenophenol by the enzyme sterol \(\Delta^{7}\)-isomerase. 24-methylelenophenol can serve as a substrate involved in two distinct sterol biosynthesis pathways. The first pathway leads to the synthesis of isofucosterol/\(\beta\)-sitosterol, which are precursors of C\({}_{29}\)-BRs. 24-methylelenophenol is converted by sterol methyltransferase 2 (SMT2) into 24-ethylidenophenol. This compound is then converted into avenasterol by sterol methyl oxidase 2. Next, avenasterol is transformed into isofucosterol and \(\beta\)-sitosterol, which is hydroxylated into 6-deoxy-28-homoTY and then oxygenated into 28-homoTY by CYP724B2 and CYP90B3 C-22 hydroxylase, respectively [123]. 28-homoTY can also be generated from 28-homoTE, though the intermediates involved in this reaction have not yet been identified [116]. 28-homoTY is transformed into 28-homoCS and 28-homoBL via the actions of enzymes CYP85A1/A2 oxidases.