## 4 Discussion
### Key Genes for Terpenecoid Synthesis in YJ
On the contrary, _MVD_ is downregulated in YJB and YJC and negatively correlated with various terpenes, indicating its role as the final rate-limiting enzyme for the production of terpene isopentenyl pyrophosphate precursors in the MVA pathway [65]. PMK is highly expressed in YJA and YJB, but significantly negatively correlated with terpene VOC expression, indicating its role in regulating terpene biosynthesis. In Sanghuangporus bhumii SbPMK, an important rate-limiting enzyme in the triterpenoid biosynthesis pathway was cloned and overexpressed. Consequently, this led to a significant increase in the content of pharmacologically active triterpenoids in Sanghuangporus bhumii [66]. Based on the transcriptome database of the camphor tree, [PERSON] et al. [67] and [PERSON] et al. [68] found that PMK plays an important role in the biosynthesis of terpenoids in camphor trees, and previous studies have shown that the overexpression of PMK leads to a threefold increase in terpenoid production. _FDPS_ is a key rate-limiting enzyme in the mevalonate pathway, catalyzing the synthesis of isoprenoid products, including farnesyl pyrophosphate and geranyl geranyl pyrophosphate. The farnesyl pyrophosphate produced by FDPS catalyzes the condensation reaction between dimethylallyl diphosphate and two units of isopentene pyrophosphate, serving as a precursor for the synthesis of many terpenoids. In order to understand and elucidate the role of RfFDPS in terpenoid biosynthesis in YJ, we conducted functional validation of RfFDPS through transient overexpression and transient gene silencing experiments. The discovery that it has a negative correlation with the content of monoterpenes and a positive correlation with the content of sesquiterpenes indicates its importance in the biosynthesis of terpenes. The high activity level of FDPS in wild ginseng is believed to be related to the biosynthesis pathways of sesquiterpenes and triterpenes, which is consistent with the conclusion of this study [69]. [PERSON] et al. [70] cloned PgFPS from Platly-_odon grandiflora_, and after overexpression in tobacco, the content of squalene significantly increased, verifying its function in terpenoid biosynthesis. Through the exogenous addition of inducers to _Imonotus obliquus_ to regulate FPS expression levels, significant effects were also observed on the accumulation of triterpenoids [71]. In addition, the overexpression of FDPS is beneficial for increasing triterpenoid biosynthesis activity and yield in other plant species. [PERSON] et al. [72] used the genetic algorithm for recombination detection (GARD) method for recombination detection and concluded that the diversity of FDPS enhances enzyme activity in the triterpen biosynthesis pathway. In the high-yield 001 system of Artemisia annua, the overexpression of FDPS resulted in a significant increase in artemisininin production compared to wild plants [73]. [PERSON] et al. [48] obtained _PcFPPS_ from patchouli and achieved heterologous overexpression in tobacco plants. They found that FPP enzyme activity was significantly upregulated, and more terpenoid metabolites were detected, including stigmasterol, phytosterol, and neophytodiente, which are consistent with the results of this study. Through functional verification of _RfFDPS_, it was found that it positively regulates the synthesis of YJ terpenoid floral fragrance compounds and is a key gene in the synthesis of volatile terpenoids. However, in this study, there are also some monoterpenes, such as terpinen-4-ol, that positively correlate with the expression of _RfFDPS_. It is speculated that this is due to the differential competition among various terpene synthases for GPP. Monoterpenes are mostly synthesized in plastids and are the main metabolic products of the MVA pathway. On the other hand, sesquiterpenes and triterpenes are often synthesized more in the cytosol, primarily relying on the MEP pathway.