## 4 Discussion
### Key Genes for Terpenecoid Synthesis in YJ  
In this study, RNA-seq technology was employed for transcriptome sequencing of YJ and NW. To identify the DEGs that contribute to floral fragrance, we conducted a KEGG pathway analysis on terpenoid metabolic pathways. Our results revealed that during the growth and development of both rhododendrons, the most enriched pathways were primarily related to ko00909. Notably, NW and YJ exhibited a higher number of DEGs during the flower bud and decay stages. It can be inferred that a large number of genes regulating the synthesis of volatile terpenoids and their precursors are expressed during the flower bud stage, and subsequently contribute to the synthesis of terpenoid VOCs in rhododend flowers in later developmental stages. The difference in gene expression between YJ and NW during the bud stage is minimal, and this gene expression pattern corresponds to the accumulation and release pattern of metabolites. The biosynthesis of floral fragrance is complex and dynamically changing, often regulated by multiple genes. Subsequently, further analysis was conducted on DEGs to identify the following nine candidate enzyme genes involved in terpenoid biosynthesis pathways, specifically in the MVA pathway: _TPS_, _GGPS_, _FDPS_, _AACT_, _IDI_, _HMGS_, _HMGR_, _MVD_, and _PMK_. The expression levels of these genes are significantly higher in YJ compared to NW, which is consistent with the higher accumulation of terpenoids in YJ. Among these, the _RfCDPS_ gene in YJ is considered a key regulator of terpenoid synthesis in this variety. _TPS_, as a key enzyme gene in terpene synthesis, is responsible for producing a range of secondary metabolites [43; 50]. In our study, _TPS_ was highly expressed in YJ and significantly positively correlated with the expression levels of multiple types of terpene compounds. Research has demonstrated that introducing the linalool synthase gene isolated from _Clarkia pulchella_ Pursh into tomatoes substantially enhances the synthesis and release of linalool and light-based linalool in mature fruits [51]. In _Zephranthes candidia_, a biochemical characterization of ZcTPS02 was conducted, and it was found that ZcTPS02 can catalyze the synthesis of various products from GPP, playing a vital role in the biosynthesisof \(\beta\)-ocimene. This finding has also been validated in studies on _Litsea cubbea_, _Medicago truncatula_, and lilies [52; 53; 54; 55; 56; 57]. As the main allyl transferase, _GGPS_ is significantly upregulated during the decay period and positively correlated with the expression levels of various terpene compounds. [PERSON] et al. [58] found that the synthase enzymes responsible for the conversion of geranyl diphosphate and geranylgeranyl diphosphate are involved in the biosynthesis of monoterpenes and diterpenes in fir oil resin. In other research, the overexpression of IbGGPS in sweet potatoes also upregulates the expression of MEP pathway genes [59]. Similarly, the study also underscores the significant role of the _AACT_ gene in triterpene biosynthesis. [PERSON] et al. [60] also postulated that AACT is the main regulatory factor for the synthesis of terpenes in pconies  
_IDI, HMGS_, and HMGR_ exhibited high expression levels in YJC and YJD, positively correlating with the expression of multiple terpenes. HMGS and HMGR play a crucial role in regulating the biosynthesis of terpenoids, thereby enhancing the production of secondary metabolites such as triterpen glycosides [61; 62]. The heterologous expression of the IDI gene in Streptococcus pneumoniae in polycystic can regulate the balance ratio of DMAPP and IPP, improve the production rate and yield of isoprene, and thereby regulate the production of downstream floral fragrance substances. In 1993, [PERSON] et al. [63] proposed that the increase in ergosterol levels in brewing yeast may be specifically linked to enhanced IDI activity. Studies have shown that IDI is also a key enzyme in the biosynthesis pathway of diterpenoid alkaloids [64].