# Functions of Representative Terpenoids and Their Biosynthesis Mechanisms in Medicinal Plants
## 6 Terpenoid Biosynthesis Pathway and Regulation
Overexpression of the gene encoding _DBTINBT_ increased paclitaxel yield by 37% in the transgenic _Taxus_ species cells [115]. To establish a foundational pathway for toxoids, the genome of _T. wallichiana_ was assembled and has been found to have a genome size of 10.6 Gb. In this genome, a number of genes involved in the biosynthesis of paclitaxel (terpene synthase family TPS002, cytochrome P450 family CYP725, and transferase family TRF004) were found to have undergone significant expansion [114]. These findings suggest that tandem duplication could be the primary mechanism driving the complex paclitaxel biosynthetic pathway. Through tandem duplication, an increasing number of enzymes were created to modify toxoids, ultimately leading to the production of paclitaxel. There are several key synthetic genes that have been reported in the ginkgolides pathway, including 1-deoxy-D-xylulose-5-phosphate synthase (_DXS_), 1-hydroxy-2-methyl-2-(E)butenyl-4-diphosphate reductase (_HDR_), geranyl geranyl diphosphate synthase (_GGPPS_), and the levopimaradiene synthase (_LPS_) gene [62; 77]. The _GbLPS_ was cloned, and it was demonstrated that it could catalyze the synthesis of ginkgolides [116]. Overexpression of _GbHDR2_ in _N. tabacum_ also elevated the diterpenoid duvatrienediol content at least sixfold [117]. In addition, _CYPs_ genes are considered to be linked to ginkgolide biosynthesis, as the suppression of the _GbCYPs_ genes can inhibit ginkgolide biosynthesis [118]. In recent research, the first steps of ginkgolide biosynthesis have been revealed through the exploration of gene clusters and co-expression analysis. Five _CYPS_ genes (_GbCYP7005C1_, _GbCYP7005C3_, _GbCYP867E38_, _GbCYP867K1_, and _GbCYP720B31_) were identified in close proximity to the _GbLPS_ gene. These genes encode multifunctional enzymes with atypical catalytic activities, resulting in the formation of the tert-butyl group and one of the lactose rings, which are characteristic of all trilactone terpen [117]. The biosynthetic pathway of cannabinoids differs slightly from that of other terpenoids. Cannabinoid biosynthesis begins with the polyketide pathway's synthesis of olivetolic acid (OLA) and geranyl pyrophosphate (GPP). Subsequently, an enzyme known as olivetolate geranyltransferase (GOT) facilitates the alkylation of OLA and GPP, resulting in the synthesis of cannabigerolic acid (CBGA) [118; 119]. This CBGA can be converted to neutral cannabinoids (THCA, CBDA, and CBCA) through heating, drying, and other non-enzymatic decarboxylation reactions [120]. There is also a variant pathway for cannabinoid production, where divarinic acid (DA) and GPP generate the cannabinoid precursor, cannabigerovaninic acid (CBGVA), which is then catalyzed by a series of enzymes to eventually form the annabigerovanirn (CBGV), A9-tetrahydroocannabivarin (THCV), and cannabivarin (CBV). The genes encoding _CsTHCAS_, _CsBDAS_, and _CsCBCAS_ are regarded as key genes in the synthesis process. Among them, _CsTHCAS_ and _CsCBDAS_ have been shown to promote cannabinoid synthesis in glandular trichosomes [121].