## 1 Introduction  
_Andrographis paniculata_ (Burm. f.) Nees, known as 'king of bitters', belongs to the Acanthaceae family. It is widely used as a traditional medicine in southern and southeastern Asian countries and has a long history for the treatment of sore throat, flu, upper respiratory tract infections and other disorders [1, 2]. It is utilized in the treatment of colds accompanied by fever, swollen throat, oral ulcers, etc., and as well as abscesses and ulcers and snake and insect bites. The extract exhibits pharmacological effects including anti-inflammatory, antimicrobial, hepatotoxicity, and anticancer properties. Terpenoids are one of the key active ingredients in the main medicinal activity of _A. paniculata_. Therefore, compared with flavonoids, xanthoquinones and other compounds [3],terpenoids in _A. paniculata_ are the most widely and thoroughly studied. Among them, sulfonated derivatives of diterpenoid Andrographolide have been developed into traditional Chinese medicine injections, and Andrographolide and its structural analogues have also been proven to have significant biological activity, including anticancer [4], antiviral [5] and antimicrobial [6] activities. Its biosynthetic pathway is catalyzed by the diterpene synthase ApCPS2 and a series of P450s such as CYP72A399 and CYP72F1 from the CYP72 subfamily [7]. Many plant-derived terpenoids, such as diterpenoid taxol [8] and tanshinone [9], sesquiterpenoid artemisinin [10] and (-)-\(\beta\)-elemene [11] and monoterenoid paeoniflorin [12] have significant biological activity and good clinical efficacy. In the study of the biosynthetic pathways of these terpenoids, researchers have reported and developed a series of structurally similar terpenoid active derivatives and catalytic tools such as terpenoid synthases and P450s for efficient biosynthesis. Therefore, studying terpenoids and their diversity in medicinal plants is of great significance for the exploration of new drug sources and novel biological catalytic elements. However, except for labdane-related diterpenes, there are few reports on other types of terpenoids in _A. paniculata_, which limits the research and development of its terpenoid diversity. The diversity formation of terpenoid compounds is mediated by terpene synthase (TPSs), and the catalytic function of TPSs is closely related to their domains and motifs. TPSs are classified into two distinct categories (class I TPSs and class II TPSs) based on the presence of the conserved motifs 'DDxD' or 'DxDD'. They can be further subdivided into seven subfamilies, TPS-a, -b, -c, -d, -e, -f and -g [13], according to their amino acid sequence similarities. Class I TPSs contain 'DDxD' and 'DTE/NSE' motifs; members include those in the TPS-a, -b, d, -e/f and -g subfamilies, which utilize a trinuclear metal to trigger the ionization of the isoprenoid substrate (e.g., GPP, FPP or GGPP) prepnl diphosphate group and generate a carbocation intermediate to initiate catalysis [14], and activity of the TPS-e/f subfamily members specific for class I diTPSs that are process-catalyzed by GGPP or CPP, resulting in the production of diterpenoid carbon skeletons [15; 16]. The TPS-c subfamily contains a 'DxDD' motif specific for class II TPSs, which facilitates the generation of different bicyclic diphosphate intermediates through the protonation-dependent cyclization of GGPP [15; 16; 17]. As the functions of TPSs are continually being identified, the functions of each subfamily of TPSs are becoming increasingly diverse. Five diTPSs of _A. paniculata_ have been reported in previous studies, showing that three class II diTPSs can convert GGPP to _entent_-CPP or _normal_-CPP, and two class I diTPSs can convert _ent_- or _normal_-copalyl diphosphate into different diterpenoid skeletons [18; 19; 20]. To date, except for diterpene synthase, key genes involved in the formation of other types of terpenoids such as monoterpenes and sesquiterpenes in _A. paniculata_ have not been identified.