# Functions of Representative Terpenoids and Their Biosynthesis Mechanisms in Medicinal Plants
## 7 Regulation of Terpenoid Biosynthesis by Transcription Factors  
Transcription factors (TFs) are DNA-binding proteins that control the transcription of particular genes, thereby controlling their expression within the cell. Several previous studies have identified various TFs that are important in regulating terpenoid biosynthesis (e.g., MYB, WRKY, bHLH, AP2/ERF, and bZIP TFs) (Figure 4). Figure 4: The important TFs and structural genes associated with terpenoid biosynthesis in several important medicinal plants. MYB serves as a key TF that affects the metabolism of terpenoid compounds. Several MYB TFs were shown to play an important role in terpenoid biosynthesis in medicinal plants. In _S. milliorrhiza_, the transcripts of _SmDXS_, _SmGGPPS_, and _SmKSL1_ can be significantly upregulated by SmMYB9b, and an overexpression of _SmMYB97_ and _SmMYB98_ significantly increases the tanshinone content, indicating that these MYB may play a positive regulatory role in tanshinone biosynthesis [122; 123; 124]. In _Taxus_ species, TmMYB3, a phloem-specific R2R3-MYB, was identified to activate the transcripts of _TmTBT_ and _TmTS_ to enhance paclitaxel biosynthesis [125]. In _A. annuua_, AaTAR2 (MYB TF) and AaMYB17 serve as positive regulators for the glandular trichone, and, in turn, increase the level of artemisinin [5; 126], while AaMYB15 directly binds to and represses the transcriptional activity of _AaORA_, thus suppressing artemisinin biosynthesis [127]. In _C. Sativa_, CsMYB1 can bind _CsCBCAS_ and _CsCBDAS_ promoters, negatively regulating cannabinoid accumulation [128]. Overexpression of _CsMIXTA_ (R2R3-MYB TF) in _N. tabacum_ promotes trichoid development, suggesting that it may promote cannabinoid production by increasing the formation of cannabis glandular trichoid [129]. WRKY TFs are specific to plants and have significant functions in defending against pathogens, responding to abiotic factors, and regulating secondary metabolism [130]. Recently, researchers have successfully isolated WRKY genes from various medicinal plants. For example, an overexpression of _AaWRKY1_ in _A. annuua_ can significantly activate the expression of artemisinin biosynthetic genes [131]. The overexpression of the _AaWRKY4_ gene in _A. annuua_ has the potential to regulate the metabolism of artemisinin, leading to an enhancement in artemisinin biosynthesis [87]. To categorize their impacts on taxol biosynthesis in _Taxus_ species, six representative _TcWRKYS_ have been selected [74]. Overexpression experiments demonstrated that the six _TcWRKYS_ had diverse effects on taxol biosynthesis. Specifically, _TcWRKYS8_ and _TcWRKY47_ significantly and substantially enhanced the expression levels of taxol biosynthesis-related genes [132]. In addition, experimental evidence has shown that _TcWRKY33_ has the ability to bind to the w-boxes in the _TcDBAT_ promoter, resulting in the promotion of its expression [133]. In _S. milliorrhiza_, SmWRKY1 was found to bind directly to the promoter of _SmDXR_[134], while SmWRKY2 binds to the _SmCPS_ promoter to promote tanshinone biosynthesis [135]. In _C. sativa_, CsWRKY1 can also influence cannabinoid synthesis by modulating the expression of _CsCBCAS_ and _CsCBDAS_[128]. AP2/ERF (APETALA2/ethylene-responsive element binding factors) TFs are a large family in the plant kingdom, and specific AP2/ERF members are essential for terpenoid formation [136; 137]. Through experiments like EMSA and transient expression assay, it has been demonstrated that AaERF1, AaERF2, and TAR1 are responsible for regulating sesquiterpenoid biosynthesis in _A. annuua_ through binding to both _AaADS_ and _AaCYP1 AV1_ promoters [138]. In _S. milliorrhiza_, _SmERF6_, _SmERF8_, _SmERF128_, and _SmERF11_ have also been shown to activate tanshinone biosynthesis [139; 137; 138; 139]. The overexpression of _SmERF11_ can greatly enhance tanshinone accumulation in transgenic _S. milliorrhiza_ hairy roots by extensively upregulating _SmDXR_, which is important for the tanshinone biosynthetic pathway [139].