## 1 Introduction  
Conifers rely on constitutive and induced defense systems, primarily based on terpenoids, against various biotic and abiotic stresses [1; 2; 3]. The coniferous terpenoid defense system consists of monoterpenoids, sesquiterpenoids, diterpenoids, and the corresponding resin ducts and cells [4; 5]. Conifers can actively change the composition of terpenoids to defend against insect herbivores, affecting insect behavior and performance or attracting natural enemies [6; 7; 8]. For example, a methyl jasmonate-stimulated feeding treatment affected the release of terpene volatiles from five different conifers [9]. Methyl jasmonate treatment changes the chemical composition of _Pinus sylvestris_ and influences the meal properties of _Hylobius abietis_[10]. Many herbivorous insects have evolved to use host-produced terpenoids, particularly volatile monoterpenoids and sesquiterpenoids, as critical chemical signals for host recognition and location. For instance, _Dendroclous frontalis_ and _D. terebrans_ have olfactory and behavioral responses to volatiles (\(\alpha\)-pinene, \(\beta\)-pinene, myrrene, limonene, and 4-allylanisole) released from the resins excreted by their hosts [11]. Furthermore, _Xyleborus glabratus_ is attracted to sesquiterpenes, such as \(\alpha\)-copaene, \(\alpha\)-cubebene, \(\alpha\)-humulene, and calamenene, emitted from the cambium of their lauraceous hosts [12]. The components and composition of constitutive terpenoids vary under different biotic and abiotic stresses [13], which is directly related to the distinct gene expression levels of terpene synthases (TPSs) under different conditions [14; 15]. In _Picea abies_ and _Pinus massoniana_ (Pm), terpenoid biosynthesis pathways are activated under feeding stress or methyl jasmonate induction. This activation leads to upregulated gene expression of mono-TPS PaJF67 and Pm TPSs, which finally enhances the biosynthesis of some terpenoids, such as \(\alpha\)-pinene, \(\beta\)-pinene, and (+)-3-Carene [16; 17; 18]. Identification and characterization of TPS genes for genetic improvement can improve the genetic improvement of plants and enhance resistance against invaders. For example, transgenic soybean with overexpression of GmAFS showed significantly improved resistance to soybean cyst nematode [19]; expression of the 7-epizingbierene synthase and Z-Z-farnesyl-diphosphate synthases in the glandular trichomes of cultivated tomato resulted in the resistance of tomatoes to several herbivore pests [20]. In plants, TPSs catalyze the formation of many monoterpenes, sesquiterpenes, and diterpenes with different structures, using precursors [21]. Monoterpenes, sesquiterpenes, and diterpenes are synthesized from geranyl diphosphate (GPP), farnesyl diphosphate (FPP), and geranylgeranyl diphosphate, respectively. They are, in turn, synthesized from different numbers of C5s by isoprenyl transferase. Terpenoids usually exist as enantiomers but often with one enantiomer as the dominant product [22]. Terpene synthases can produce one or more products [23]. The production of sesquiterpenes is usually lower than that of monoterpenes and diterpenes but has the highest structural diversity in coniferous trees. Due to difficulty in the structural identification of sesquiterpenes and the diversity of sesquiterpene synthase products, the physiology and biochemistry of sesquiterpene synthases have barely been studied in coniferous trees. Phylogenetic analysis based on protein sequences has divided gymnosperm and some angiosperm TPSs into seven subfamilies: TPS-a, TPS-b, TPS-c, TPS-e/f, TPS-g, and TPS-h [24; 25; 26; 27; 28]. The TPS-d subfamily is entirely composed of TPSs from gymnosperms that are involved in primary and secondary metabolism [21]. Some TPS genes in _P. abies_, _P.sistchenis_, _P. glauca_, _Abies grandis_, _Pseudotsuga menziesisi_, _P. taeda_, _P. contorta_, and _P. bunksiana_ have been identified, cloned, and functionally verified [29; 30; 31; 32; 33; 34].