## 1 Introduction
In recent years, _TPS_ genes have been gradually isolated from _Arabidopsis thaliana_[37], _Solanum lycopersicum_[38], _Freesia \(\times\) hybrida_[8; 18], _Lilium 'Siberia'_[39], _Lathyrus odoratus_[35], _Cympidium faberi_[40], and _Chrysanthemum morifolium_[41] and demonstrated their roles in terpene volatile biosynthesis. Based on the functions and homology, plant TPSs have been clustered into seven subfamilies (TPS-a to TPS-h) [18]. Among them, members of the TPS-a/b/g subfamilies are responsible for the majority of floral volatile monoterpene and sesquiterpene compounds, whose emissions are found to be significantly correlated with the expression of _TPSs_[8; 18; 35; 41]. For example, the temporal expression pattern of the _FhTPS1_ (TPS-b) gene during flower development in _Freesia_ petals is consistent with linalool emission [18]. The _LoTPS4_ (TPS-a) gene expressed at a high level in petal at the S4 period has been demonstrated to catalyze the production of ocimene, myrcene, and farresene in _L. odoratus_[35]. Therefore, it is of great significance to study the expression pattern and synthesis mechanism of _TPS_ genes to reveal the biosynthesis of floral terpene volatiles. _Paeconia suffruticosa_, a woody flower with high ornamental and applied value, is one of the traditional Chinese famous flowers and enjoys the reputation of 'national beauty and heavenly fragrance' [42; 43; 44]. Tree peony is famous for its large and colorful flowers, rich flower shapes, and strong fragrance [45; 46]. Its floral fragrance, as an important ornamental trait and evaluation index of flower food flavor, greatly influences the consumer market [47]. In addition, floral scent is often neglected in breeding, resulting in a considerable number of tree peony cultivars with an overly strong aroma and a few varieties suitable for flower forcing with an unpleasant scent [43]. However, compared with research on flower shape and color, studies on floral scent, especially the molecular synthesis mechanism, are still lagging behind, which limits the tree peony commercial development [48; 49; 50]. Therefore,it is important to understand the types of substances and the functions of key synthetic genes for tree peeny floral scent and to breed new tree peeny varieties with pleasant aromas using modern breeding techniques [45]. Currently, studies related to the tree peeny floral fragrance mainly focus on the volatile components [42; 51], among which terpenoids account for a relatively high proportion [44; 46; 48; 52], such as ocimene, linalool, citronellol, and germacrene D [44; 52; 53]. So far, though several terpene synthesis-related enzyme genes have been cloned, including _PdDXS_ in _P. delanayi_[54], _PsDXS_[55], _PsDXR_, and _PsMCS_ in cultivars 'Oukan' [56], _PsGPPS_[46], _PsGDS_[53], _PsTPS14_[57], _PsuLIS_[45], and five _PsTPSs_ (_PsTPS1-PsTPS5_) [52] in 'High Noon', fewer genes are functionally characterized. Transient overexpression of _PsTPS14_ in tobacco leaves and the lightly scented tree peeny flower 'Fengdan' increases enzyme activity and releases amounts of linalool [57]. And _PsTPS1_ and _PsTPS4_ are shown to involve in linalool synthesis after in vitro enzyme activity analysis and transient overexpression in tobacco leaves [52]. The isolation and functional characterization of terpene synthesis pathway genes are very important in the study of tree peeny floral fragrance biosynthesis mechanism; however, the relevant research foundation is relatively weak. Therefore, it is particularly important to clone these genes and perform functional analyses in tree peeny. The strongly scented tree peeny cultivar 'Oukan' has outstanding ornamental value and is an excellent material for the study of floral fragrance biosynthesis. However, little is known about the isolation and functional exploration of terpene biosynthetic pathway genes in 'Oukan'. Our previous study has demonstrated that terpenoids are the main components in the floral fragrance of 'Oukan', and _PsHMGR1_ and _PsTPS_ may play an important role in terpenoid synthesis [42].