## 1 Introduction  
_Rhododendron_ stands as a distinguished woody ornamental plant within the _Rhododendron_ genus of the _Ericaceae_ family, renowned as the \"king of woody flowers\" and celebrated as one of the world's foremost ornamental blooms, widely used in landscaping and garden beautification. _R._ has abundant germplasm resources, with over 1200 varieties worldwide, and boasts approximately 600 of these varieties in China [1, 2], but only a select few varieties exhibit aromatic qualities. _R. fortunei_ Lindl., a member of the _R._ family and subgenus _Evergreen Rhododendronstrum_, predominantly thrives in southern China at elevations ranging from 629 to 2000 m above sea level. The flowers are large, light pink or light purple, elegantly shaped, and have a charming aroma, endowing them with high ornamental value [3]. In contrast, _R._ \"Nova Zembla\", also belonging to the subgenus _Evergreen Rhododendronstrum_, features wide, funnel-shaped blossoms with purplish red petals adorned with dark brown-red spots; when flowering, there is no fragrance emitted [4], classifying it as an odorless rhododendron variant. Notwithstanding, it remains one of the top-selling alpine rhododendron cultivars presently available. While both _R. fortunei Lindl._ and _R._ \"Nova Zembla\" belong to the alpine rhododendron category, the former stands out as a rare aromatic rhododendron variant. Floral aroma represents a paramount characteristic of ornamental plants [5]. Flower fragrance plays various roles in attracting pollinators and improving pollination efficiency during plant pollination [6; 7]. Moreover, floral scents serve as a defense mechanism against environmental stressors [8]. For instance, (E)-\(\beta\)-caryophyllene, which is emitted during Arabidopsis flowering, acts as a deterrent against pathogen invasion [9], while eugenol exhibits potent antimicrobial properties [10]. Floral aroma, comprising numerous low-molecular weight, low-boiling point, and highly volatile compounds, collectively define a plant's floral characteristics. Volatile organic compounds (VOCs) can be synthesized by various plant organs, including the roots, stems, leaves, seeds, and fruits. Presently, over 1700 VOCs have been identified [11]; the highest amount is typically contained in petals, encompassing terpenoids, benzene/phenylpropanol compounds, fatty acid derivatives, amino acid derivatives, and other specific compounds. Numerous studies have elucidated the aroma profiles of ornamental plants [12], and in-depth research has been conducted on their release patterns and molecular regulatory mechanisms, such as roses [13], peonies [14], lilies [15; 16], _Osmanthus fragrans_[17], dendrobiums [18], and prunus mumes [19]. However, our understanding of the transcriptional regulation underlying the synthesis and release of floral aroma substances remains limited. For rhododendrons, the current research focus is still on the relationship between anthocyanins [20; 21] and their analgesic and anti-inflammatory effects [22; 23; 24]. Hence, investigating the genes related to the synthesis and metabolism of its floral aroma substances is beneficial for the study of the mechanism of rhododendron fragrance formation and the selection of new varieties. Terpenoids stand as the predominant chemical constituents among the volatile compounds produced by plants [25]. Isopentenyl diphosphate (IPP) and dimethylallyl pyrophosphate (DMAPP), as the basic C5 structural units of terpene synthesis, are synthesized via two pathways--the mevalonate (MVA) pathway in the cytoplasm and the 2-C-methyl-D-erythritol 4-phosphate (MEP) pathway in the plastids [26]. Under the action of isopentenyl transferase, DMAPP synthesizes various intermediates of different lengths, including prenyl diphosphates (phenyl-PPs), such as geranyl diphosphate (GPP), famesyl pyrophosphate (FPP), and geranylgeranyl pyrophosphate (GGPP).