# Functions of Representative Terpenoids and Their Biosynthesis Mechanisms in Medicinal Plants
## 6 Terpenoid Biosynthesis Pathway and Regulation
Abbreviations: AACT, acetoacetyl-CoA synthase; HMGR, HMG-CoA reductase;  
HMGS, hydroxymethylglutaryl (HMG)-CoA synthase; MVK, mevalonate kinase; PMK, phospho-mevalonate kinase; MVD, mevalonate diphosphate decarboxylase; FPPS, famesyl diphosphate synthase; DXR, 1-deoxy-D-xylulose-5-phosphate reductase; DXS, 1-deoxy-D-xylulose-5-phosphate synthase; HDR, 1-hydroxy-2-methyl-2-(E)-butenyl-4-diphosphate reductase; HDS, 1-hydroxy-2-methyl-2-(E)-butenyl-4-diphosphate synthase; CMK, 4-diphosphocytidyl-2-C-methyl-D-erythrito kinase; MCT, 4-diphosphocytidyl-2-C-methyl-D-erythrito synthase; MDS, 2-C-methyl-D-erythritol-2, 4-cyclodiphosphate synthase; GGPPS, geranylgeranyl diphosphate synthase; ADS, amorpha-4, 11-diene synthase; ALDH1, aldehyde dehydrogenase 1; DBR2, artemisinin aldehyde A11(13) reductase; LPS, levopimaradiene synthase gene; CYP450, cytochrome P450; TS, taxadiene synthetase; T5\(\alpha\)H, taxadiene 5-\(\alpha\)-hydroxyase; T2\(\alpha\)H, taxoid 2-\(\alpha\)-hydroxyase; T7\(\beta\)H, taxoid 7-\(\beta\)-hydroxyase; TBT, 2-\(\alpha\)-hydroxytaxane 2-O-benzoyltransferase; BAPT, baccatin III amino phenylpropanoyl-13-O-transferase; DBTINBT, 3-\({}^{*}\)N-debenzoyltaxol-N-benzoyltransferase; T13\(\alpha\)H, taxane 13-\(\alpha\)-hydroxyase; KSL, kaurene synthase-like; CPS, copalydiphosphate synthase; AAE, acyl-activating enzyme; OLS, olivetol synthase; GOT, olivetolate geranyltransferase; THCAS, tetrahydrodcannabinolic acid synthase; CBDAS, cannabidionic acid synthase; CBCA, cannabichromenic acid synthase. Dashed square frames represent terpenoid biosynthesis sites. In _S. milliorrhiza_, tanshinone biosynthesis is initiated by the bicyclization of geranylgeranyl diphosphate (GGPP), a common diterpene precursor, and catalyzed by CPP synthases (CPSs), resulting in copalyl diphosphate (CPP). The subsequent cyclization or rearrangement reaction of the alkene is catalyzed by kaurene synthase-like enzymes (KSLs) to produce an olefin. The final production of diterpenoids requires the insertion of oxygen from cytochrome P450 mono-oxygenases (CYPs) [104; 105]. In the genome of _S. milliorrhiza_, 82 terpene synthase genes, which are responsible for producing hemi-, mono-, sesqui-, or di-terpenes, were identified [106]. Physical clustering of terpene synthase genes and CYPs is often linked to consecutive enzymatic actions in the biosynthesis of terpenoids. Four terpenoid synthase genes/CYP pairs have been found in the _S. milliorrhiza_ genome. Among these, _SmCPS1_ and _SmCPS2_ are, respectively, responsible for the tanshinone biosynthesis in roots and leaves, and both are accompanied by genes from the _CYP76 AH_ subfamily. It is worth highlighting that one of these members, _CYP76 AH1_ and _CYP76 AH3_, was previously identified as playing an important role in tanshinone biosynthesis, confirming the functional role of these biosynthetic gene clusters in the biosynthesis of tanshinone [107]. In addition, biochemical and genetic experiments have also demonstrated the crucial role of several _CYP450_ genes implicated in tanshinone biosynthesis, including _SmCYP76 AH1_, _SmCYP76 AH3_, and _SmCYP76 AK1_[108; 109; 110]. In _Taxus_ species, the biosynthesis pathway of toxoids is very complex and involves approximately 19 enzymatic steps. The process begins with the precursor GGPP and includes several key enzymes such as a taxadiene synthase (TS), five P450s (T2\(\alpha\)OH, T5\(\alpha\)OH, T13\(\alpha\)OH, T70\(\beta\)H and T10\(\beta\)OH), five acyltransferases (BAPT, DBTINBT, TAT and TBT), and two extra enzymes (PAM and T2\({}^{\prime}\)\(\alpha\)OH) [111; 112]. The TS catalyst facilitates the cyclization of GGPP to form taxadiene. The expression of the TS gene through heterologous transformation in tobacco can significantly increase taxadiene production [113]. The enzymes taxoid 2\(\alpha\)-hydroxylase and taxoid 7\(\beta\)-hydroxylase utilize taxusin as a substrate to, respectively, produce 2\(\alpha\)-hydroxytaxusin and 7\(\beta\)-hydroxytaxusin [114].