## 1 Introduction  
_Cinnamomum porrectum_ (Roxb.) Kosterm (_C. porrectum_), an evergreen tree of the family Lauraceae and genus Cinnamomum, that grows in evergreen broad-leaved forests or as shrubs, is mainly found in southern China, Bhutan, Cambodia, India, Indonesia, Laos, Malaysia, Myanmar, Nepal, Pakistan, Thailand and Vietnam [1]. The leaves of _C. porrectum_ are rich in essential oils, and the whole plant serves as an important source of natural essential oils. _C. porrectum_ is widely applied in foods, medicines and the fine chemical industry, including in natural spices and chemical materials [2]. The extracts of the leaves, bark, wood and roots of _C. porrectum_ have antioxidant and antibacterial effects [3; 4; 5], and _C. porrectum_ leaves are traditionally used in carminative tonic, stomachic tonic, and antipyretic preparations [6]. Studies have shown that the essential oil of _C. porrectum_ leaves is rich in monoterpenes, sesquiterpenes and aromatic compounds [2; 7], and the terpenids with high contents in this leaf essential oil are mainly linalool, eucalyptol, camphor, citral, and meridolol. According to the first principal component of its leaf essential oil, _C. porrectum_ can be divided into the linalool, camphor, eucalyptol, citral and meridolol chemotypes [2]. Terpenoids in the essential oil of _C. porrectum_ are widely used and have a high development value. Among these terpenoids, linalool is frequently used as an odorant for perfumes and in common chemical products; in addition, linalool has analgesic [8], anti-inflammatory [9], and antitumor [10; 11] effects. Eucalyptol, with its antimicrobial and insecticidal effects is widely used in medicine, spices and industry. Citral has a strong lemon odor and is widely used in many perfume compositions; in addition, citral has bacteriostatic effects [12]. In recent years, the biosynthesis and metabolic regulation of plant terpenids have been studied [13; 14; 15], showing that two independent pathways in plants are involved in the biosynthesis of the 5-carbon structural unit isopentenyl pyrophosphate (IPP) and dimethylallyl diphosphate (DMAPP) of the universal precursor to isomers of terpenoids. In the 2-C-methyl-D-erythritol 4-phosphate (MEP) pathway, the biosynthesis of IPP/DMAPP in plastids begins with pyruvate and glyceraldehyde-3-phosphate [16; 17], but in the cytoplasm, the mevalonate (MVA) pathway produces IPP [18; 19] from acetyl coenzyme A (CoA). The balance between IPP and DMAPP is controlled by IPP delta-isomerase (IDI), which converts IPP to DMAPP reversibly [20]. Terpenoids are generally formed by catalysis of corresponding precursors by terpene synthases (TPSs), while some terpenids in plants required further modifications, mainly involving methyltransferases, glycosyltransferases, cytochrome P450 monooxygenase, dehydrogenase and reductase. The existence of these modification enzymes greatly enriches the variety of terpenoids and makes the structure and biological activity of terpenids tend to be diversified [21]. Monoterpenes were important components of leaf essential oil in _C. porrectum_, and the study of monoterpenes biosynthesis was of great significance. Monoterpene synthase was the key enzyme in monoterpene biosynthesis, and now many monoterpene synthases have been isolated from plants [22; 23; 24; 25; 26; 27; 28; 29]. Linalool synthase (LIS) gene was isolated from different source of _Cinnamomum osmophloeum_[26]. The recombinant _LIS_ protein can produce s-(+)-linalool from GPP and (E) - Neroli from FPP. Bornyl diphosphate synthase (_LaBPPS_) was cloned from _Larandula angustilgia_[27], which can catalyze the production of Bornyl or camphor. Through bioinformatics analysis, a monoterpene synthetase was identified from _Laurus nobilis_, and mainly catalyzes the formation of 1,8-cineole [28]. A monoterpene synthetase was cloned from _Cinnamom tunuipilum_, which was specifically expressed in the geraniol chemotype.