## 3 Discussion
This finding aligns well with our hypothesis regarding the function of the FPS protein family, suggesting their important role in the synthesis of isoprenoids and lipid compounds. Previous studies have demonstrated that _FPS_ genes play a pivotal role in isoprenoid metabolism in plants, particularly in the biosynthesis of terpenoids and carotenoids [18; 19; 20; 32]. Moreover, comparative analysis showed that the functional domains of _EhFPS_ genes are highly conserved, including the enzyme activity-regulating motifs DDXXD and GKLNR, further supporting their hypothesized functions as terpene synthases. The type and number of cis-acting elements in the promoter region play important regulatory roles in gene expression [33]. The promoter of the _FPS_ gene of _E. hirta_ contains a large number of cis-acting elements related to light response, plant hormone induction, and stress response, indicating that members of this family are involved in a variety of biological processes. Studies have reported that spectral quality mainly changes the distribution of terpenoids, whereas the intensity and period affect their abundance [34]. For example, illumination duration and temperature have an important impact on the growth of _Ganoderma lucidum_ (a Chinese medicine) and synthesis of triterpenes. The combination of 36 \({}^{\circ}\)C and 16 h of illumination significantly increased the yield of _Ganoderma lucidum_ triterpenes, which was 98.32% higher than that of the control. Light treatment can also induce an increase in the mycelial biomass of _Ganoderma lucidum_ and promote the synthesis of triterpenoids by upregulating the expression of key enzyme genes such as HMGR, SQS, and LAS [35]. Another study on _Eleutherooccus senticosus_ revealed that saponin content significantly increased under varying light conditions. Four _EsbZIP_ genes associated with saponin biosynthesis were identified through correlation analysis: _EsbZIP1, EsbZIP2, EsbZIP4_, and _EsbZIP5_. The promoter region of _EsbZIP_ contains numerous photoresponsive elements, and its activity varies with light intensity [36]. We observed that the promoters of the eight _EthFPS_ genes contained different numbers of light-responsive cis-elements (Box4, G-box, and GT1-motif). Therefore, it can be speculated that the transcription level of _EthFPS_ may be regulated by transcription factors involved in terpene synthesis, such as AP2/ERF, BHLH, MYB, NAC, WRKY, and bZIP, which ultimately affect the synthesis and accumulation of triterpenoids [37]. Plant hormones have been reported to affect terpenoid synthesis through various mechanisms. For example, plant hormones such as JA, MeJA, and SA can regulate the differential expression of key enzyme activities or genes (HMRG, DXR, and FPS) in the terpenoid biosynthesis pathway by acting as signaling molecules or inducers, thereby affecting the accumulation of the final product [38]. Moreover, plant hormones, such as GA, BR, and ABA, are also involved in regulating the synthesis of terpenoids [39; 40]. Seven genes linked to ginseng Rg3 synthesis (FPS, UGT, CYP339, etc.) were screened using trait correlation analysis. Exogenous MeJA treatment markedly increased the expression levels. Six hours after induction, the expression level of the _FPS_ gene peaked and was 7.1 times higher than that in the control group [41]. It has been discovered that treating birch trees with exogenous MeJA at a concentration of 0.5 mmol/L and SA at a concentration of 50 mmol/L can greatly raise the _BrFPS_ gene's expression level and encourage the buildup of total triterpenoids in the leaves and outer bark of the stem [42]. In addition, promoter analysis and RT-qPCR quantitative experiments revealed that the promoters of birch _BrMYB21_ and _BrMYB61_ contain plant hormone response elements, including ABRE, GGTCA-motif, TCA-element, and GARE motifs. These elements can be induced by ABA, JA, and SA and subsequently control the production of triterpenoids by regulating the expression of important enzyme genes (SS and SE) [43].