## 1 Introduction  
Releasing floral fragrance is an important feature of flowering plants. Floral fragrance, a secondary metabolite released from flowers, is a typical component of plant volatile organic compounds (VOCs), which is crucial in physiological and ecological processes [1, 2]. For the plants, floral fragrance not only helps them to reproduce and improve resistance but also acts as communication signal [3]. Meanwhile, as an important indicator of flower quality, the improvement of floral fragrance traits is significant for enhancing the flowers ornamental and economic value [4]. Compared to flower color and shape, floral fragrance is more likely to influence consumer choice; however, the development of scent in many flower varieties has often been neglected in traditional breeding programs [5, 6, 7, 8]. In addition, floral volatile compounds have been used in a wide range of applications, such as food, pharmaceuticals, additives, and flavoring products [3, 9]. Based on the biosynthetic origin in plants, floral volatile compounds are generally classified into three groups: terpenoids, phenylpropanoids/benzenoids, and fatty acid derivatives [10, 11]. Terpenoids, as the largest group of them, are one of the main components that make up the floral fragrance of ornamental plants [12; 13]. Studies have confirmed the predominance of terpenoids in the floral fragrance in strongly scented _Lilium_ species [10; 14], _Chrysanthhemum morifolium_[15], _Aster_ species [16], _Phalanopsis_ species [17], _Fressia hybrida_[18], _Rosa damescena_[7], and _Osmanthus fragrans_[19; 20]. Terpenoids in plants have been reported to be synthesized through two compartmentsally separated metabolic pathways: the mevalonate (MVA) pathway in the cytosol and the methylerythnitol phosphate (MEP) pathway in plastids [8; 21]. And these two pathways are not completely independent, and there is cross-talk between them [11; 22]. 3-hydroxy-3-methyl-glutaryl CoA (HMG-CoA) reductase (HMGR), the first key rate-limiting enzyme in the MVA pathway, plays a very important role in controlling the flow of carbon sources in the metabolic pathway [23]. Moreover, catalyzing the synthesis of HMG-CoA by HMGR is a critical regulatory step in the biosynthesis of terpenoids [24; 25; 26; 27]. In recent years, more and more _HMGR_ genes from different plants have been isolated and cloned, and their expression patterns have been analyzed and found to be highly expressed in metabolite-enriched organs [23; 25]. The _HbHMGR1_ gene is expressed at significantly higher levels in latex than in leaves, phloem, and xylem and may play an important role in natural rubber biosynthesis in _Hreve brasiliensis_[28]. Both _PgHMGR1_ and _PgHMGR2_ genes are expressed at higher levels in 3- and 6-year-old ginseng roots than in other organs, suggesting that _PgHMGRs_ are involved in the biosynthesis of ginsengoides [29]. The functions of _HMGRs_ in terpene biosynthesis have been investigated in model plants and medicinal plants [26; 27; 30; 31; 32]. In _Artemisia annua_, the artemisinin content of _hmgr_ transgenic lines is higher than in non-transgenic lines [33]. Transient expression of the _AmHMGR_ gene from _Antitrhinum majus_ in tomato results in a 5.7-fold and 1.8-fold increase in monoterere nerolidol and linalool, respectively [34]. However, the characterization of the _HMGR_ gene of ornamental plants needs to be further explored. Terpene synthase (TPS), a key enzyme downstream of the terpene biosynthesis pathway, is directly involved in the synthesis of terpene products. TPS largely determines the structural and functional diversity of terpenoids and plays a crucial role in the overall terpene synthesis process [35; 36; 37; 4]. Therefore, the functional characterization of _TPS_ in plants has received increasing attention.