## 1 Introduction  
Plants produce a range of small molecule volatile and non-volatile organic compounds called secondary metabolites that are not essential to the growth and development of the producing organism. But enable them to adapt to their living environment. Terpenoids constitute the largest group of volatile secondary metabolites [1]. They are made from the basic C5 building blocks, isopentenyl diphosphate (IPP) and dimethylallyl diphosphate (DMAPP) and are synthesized by the mevalonate (MVA) pathway in the cytosol and the methyl-erythitol phosphate (MEP) pathway in the plastid [2]. IPP and DMAPP are condensed by head to tail reactions to form geranyl diphosphate (GPP), farnesyl diphosphate (FPP) and geranylgeranyl diphosphate (GGPP) [3]. Terpene synthases (TPS) function to catalyze the terminal steps in these pathways and utilize substrates GPP, FPP and GGPP to form monoterpene(C10), sesquiterpene (C15), and diterpene (C20), respectively. The diversity of terpene synthase (TPS) gene members and their catalytic mechanisms generate extremely complex products [4]. Monoterpenes and sesquiterpenes are volatile at room temperature. They are important secondary metabolites with biological functions such as promoting stress resistance or acting as plant pheromones [5; 6]. Moreover, monoterpenoids and sesquiterpenoids contribute to the characteristic aroma of fruits, including citrus fruits [7]. TPS families have been identified in many plants, for example Arabidopsis, tomato, grapevine, and apple [8; 9; 10; 11]. [PERSON] et al. comprehensively introduced the structure, function, and evolution of plant TPS [12]. The modern plant TPS family went through several evolutionary events such as gene duplication, functional differentiation, and divergence, and finally developed into seven subfamilies [8; 13]. [PERSON] et al. found 49 TPS genes from multiple citrus varieties by searching the CitEST database [14]. With the publication of the _Citrus sinensis_ genome, [PERSON] et al. described the first functionally characterized TPS family in citrus [15]. They identified seven genes which encode sesquiterpene synthases. Other terpene synthases from Citrus spp. have been functionally characterized in mandarin [16; 17; 18; 19; 20], lemon [21], sweet orange [22], and yuzu (_Citrus junos_) [23]. Recently, we have identified a sesquiterpene synthase associated with formation of bicyclogermacrene in finger citron [24]. Terpene biosynthesis changes during plant development. Progress has been made in identifying transcription factors (TFs) related to regulation of terpenoid synthesis. In citrus, CitAP2.10 [25] and CitERF71 [26] were reported to regulate the synthesis of the sesquiterpene valencene and monoterenoid (\(E\))-geraniol, respectively. MsMYB negatively regulates monoterpene synthesis in spearmint trichomes [27]. [PERSON] et al. found that mutation in the NAC binding region of TPS gene promoters in two kiwifruit species affect their monoterpene contents [28]. In cotton, GaWRKY1 up-regulates the expression of _CAD1_, leading to the accumulation of sesquiterpene phytoalexins, including gossypol [29] while overexpression of _OsbZIP79_ causes a decrease in the accumulation of phytoalexin in rice cells [30]. [PERSON] et al. found that AtMYC2 combines multiple plant hormone signals in the transcriptional regulation of sesquiterpene biosynthesis and activates the production of (\(E\))-\(\beta\)-caryophyllene [31]. AaERF1 and AaERF2 positively regulate the expression level of _ADS_ and _CYP7 AVI_, leading to the accumulation of artemisinin [32; 33]. Citron is one of the three primitive species of Citrus [34]. The carpels of finger citron split, causing a finger-like fruit shape [35]. It is a popular ornamental tree in oriental countries with dietary and pharmaceutical potentials [36; 37]. Finger citron essential oils have antioxidant, antibacterial, antibiofilm activities [38; 39; 40] and they have been widely used in the food and perfume industry [41].