## 1 Introduction  
The floral scent is one of the main features that define the aesthetic value of cut flowers and ornamental plants [1]. It is made up of the specialized metabolites of plants, which enable them to interact with their environment by attracting pollinators and repelling pests, such as herbivores, pathogens, and parasites [2]. The floral scent is the composite of different volatile organic compounds (VOCs), which mainly include terpenoids, phenylpropanoids, benzenoid compounds, and fatty acid derivatives [3]. Moreover, the content and composition of VOCs vary among species and contribute to their distinct fragrance [4]. Overall, terpenoids account for a range of secondary metabolites with numerous volatile constituents, which are derived from two interconvertible five-carbon (C5) precursors (isopentenyl diphosphate (IPP) and its allylic isomer, dimethylallyl diphosphate (DMAPP)) [5]. These C5-isoprene building units are produced from two independent pathways: (1) the mevalonicacid pathway, which leads to the formation of sesquiterpenes; and (2) the methylerythnitol phosphate (MEP) pathway, which contributes to the biosynthesis of monoterenes and diterpenes [6]. In the MEP pathway, DMAPP and IPP molecules are condensed (head to tail) by the geranyl diphosphate synthase (GPPS), resulting in the formation of geranyl diphosphate (GPP) which is a universal precursor of monoterenes. In several plant species, GPPSs have been characterized as heterodimeric and homodimeric forms [7; 8]. Structurally, the heterodimeric GPPS is composed of one large subunit (LSU) and a small subunit (SSU) [9; 10; 11]. The LSU of heterodimeric GPPS shares high sequence similarity (50-75%) with the geranylgeranyl diphosphate synthases (GGPPS) and possess prenyltransferase activity, producing GGPP and often lower levels of GPP and FPP, while the SSU shares only 22-38% sequence similarity with the GGPPS and have no prenyltransferase activity [9]. The interaction of LSU with SSU causes a shift in the composition and content of the product, which normally favors the formation of GPP as compared to the activity displayed by the LSU alone. The homodimeric GPPS is catalytically active and produces GPP as a product [9; 12; 13]. Recent studies have shown the involvement of both homodimeric and heterodimeric GPPSs in monoterenes formation in different plant species, such as _Antirhinum majus, Humulus lupulus, Clarkia breweri, Phlaenopsis bellina, Arabidopsis thaliana_, and _Mangifera indica_[9; 13; 14; 15; 16; 17]. Except for monoterenes, there are also few reports which support the participation of homodimeric GPPS and heterodimeric GPPS in gibberellin and carotenoid biosynthesis in tomato and red pepper, respectively [7; 18]. Wintersweet (_Chimonanthus praecox_ L.) is a diploid (2n = 22) deciduous flowering shrub and ornamental plant, which belongs to the family Calycanthaceae. It is native to China and has been cultivated as a garden and potted plant for the past 1000 years [19]. Moreover, its unique time to flower (November to March), distinct soothing scent, and prominent yellow color of flower enhance its ornamental values and make it a popular landscape, as well as a cut flower plant [19]. The scent of wintersweet flower is mainly dominated by volatile monoterenoid, sesquiterpenoid, and benzenoid compounds [20]. However, less is known about their biosynthesis-related molecular mechanism(s). To date, only _CpFPPS_ (farnesyl diphosphate synthase) and _CpSAMT_ (salicylic acid carboxyl methyltransferase) have been investigated [21; 22]. In addition, [PERSON] et al. [23] used transcriptome and proteome approaches to identify potential genes responsible for differences in terpenids and benzenoid compounds between wintersweet cultivars and suggested that GPPS might play an important role in regulating floral fragrance.