# Functions of Representative Terpenoids and Their Biosynthesis Mechanisms in Medicinal Plants
## 1 Introduction  
Medical plants are defined by the World Health Organization as plants that contain a substance in one or more organs that can be used for therapeutic purposes or as a precursor for the synthesis of a useful drug [1]. The biologically active constituents in medicinal plants are mainly terpenoids, alkaloids, and phenylprogene compounds [2]. terpenoids, in particular, constitute a significant and varied group of natural products that includes both non-volatile and volatile compounds. Currently, over 80,000 terpenoid compounds have been identified and are widely used in the pharmaceutical and cosmetic industries [3, 4].  
All terpenoids comprise five-carbon isoprene units (C\({}_{5}\)H\({}_{8}\)). Based on the number of these units, they can be classified as monoterpenes, sesquiterpenenes, diterpenenes, esterterpenes, triterpenenes, tetraterpenes, and polyterpenes [5, 6]. Some specific terpenids with unique medicinal properties have been identified in certain medicinal plants (Figure 1). These include cannabinoids (a constituent of _Cannabis sativa_), tanshinone (a constituent of _Salvia milliorrhiza_), paclitaxel (a diterpenoid compound isolated from _Taxus media_), artemisinin (a bioactive compound of _Artemisia annua_), ginseng (a triterpene compound from _Panax ginseng_), and ginkgolide (a component of _Ginkgo biloba_) [7, 8, 9, 10, 11]. These terpenoids not only play a significant role in the management of cardiovascular and cerebrovascular diseases and malaria but can also improve the resistance of plants to biotic and abiotic stress [12]. In turn, certain stressors and their associated hormones can promote the accumulation of terpenoids, demonstrating a feedback mechanism in terpene biosynthesis. In addition, advances in high-throughput sequencing technology have led to the successful assembly of more than 195 medicinal plant genomes and thediscovery of the synthetic metabolic pathways of essential terpenoids [13; 14]. The terpenoids found in medicinal plants are numerous and serve a wide variety of medicinal functions. In this regard, we have identified several major terpenoids that are well-known for their benefits in medicine. These include pain-relieving cannabinoids, anti-malarial artemisinin, anti-cardiovascular tanshinones and ginkgolides, and anti-tumor paclitaxel. Extensive research has been devoted to studying these terpenoids due to their significant contributions to human health and their potential in drug discovery. These terpenoids are among the most potent and promising compounds in the field of medical applications. Our paper provides a summary of the types and functions of terpenoids in the main medicinal plants. We specifically focus on the mechanisms of structural genes, transcription factors, and transporters participating in the regulation of terpenoid biosynthesis. Additionally, we discuss and compare the terpenoid biosynthesis pathway in these medicinal plants. Given that medicinal plant terpenoids are numerous and specific, we concentrate on the terpenoids of several representative medicinal plants, as defined in Figure 1.