## 3 Discussion
### Terpene Biosynthesis-Related Genes Contribute to the Fragrance of Herbaceous Peony  
The research of herbaceous peony flower fragrance mainly focused on the recognition of volatile components, but the molecular mechanism of its formation is still unclear. Monoterpenes and 2-PE play leading roles in the fragrance of herbaceous peony. Their different emission patterns may make the most significant contribution to the flavor difference between different cultivars. Thus, exploring the expression level of different genes in the monoterpene and 2-PE biosynthetic pathways is crucial to illuminate the key genes of flower scent metabolism. The MEP pathway is the major monoterpene synthesis pathway in plants, and the genes that play a pivotal role in MEP pathway regulate the synthesis of monoterpenoids [9]. Therefore, studying these key genes is of great importance to reveal the molecular mechanism of monoterpene biosynthesis. Genes related to the MEP pathway were found from the transcriptome database. By comparing the gene expression of strong scented herbaceous peony cultivars with that of non-scented cultivar, _PIDXS2_, _PIDXR1_, _PIHDS1_, _PIHDR1_, _PIGPPS3_, and _PIGPPS4_ in strong scented cultivars with monoterpene release were up-regulated compared with the control (Figure 7A). DXS is a rate-limiting enzyme in the MEP pathway, which catalyzes the first step and controls the synthesis of terpen. [PERSON] et al. [15] found that _FhDXS2A_ played an important role in terpene biosynthesis. _HcDXS2A_ of _Hedychium coronarium_ was consistent with the release of monoterpenoids [59]. The expression level of _PIDXS2_ was higher in the cultivars with large release of monoterpenes. Its expression increased, thus producing sufficient substrates for the downstream DXR to drive MEP pathway. DXR genes of _Mentha piperta_ contributed to the biosynthesis of monoterpene compounds [60]. After the overexpression of DXR gene into _Tripterygium wilfordini_, begonin accumulated rapidly [61]. HDS is an enzyme protein that catalyzes the sixth step of the MEP pathway and plays an important role in the synthesis of monoterpenes. The HDS gene is related to the synthesis of aromatic substances in _Narcissus tazetta_[62]. HDS in _Artemisia annuu_ regulated the synthesis of artemisinin [63]. The _LiHDS_ gene participated in the monoterpene synthesis of _Lilium_ 'Siberia' [64]. HDR catalyzes the synthesis of IPP and DMAPP, provides precursors in the MEP pathway, and plays a role in regulation and speed limitation. The HDR gene of calendula maintained a constant ratio of IPP and DMAPP [65]. _GbHDR_ of _Ginkgo biloba_ played a key role in the synthesis and accumulation of terpen [66]. In this study, the expression patterns of _PIDXS2_, _PIDXR1_, _PIHDS1_, and _PIHDR1_ genes in the upstream of the MEP pathway were consistent with the monoterpene release trend (Figure 7C). GPPS mainly provides precursors for the synthesis of monoterepenes [67]. GPPS is involved in monoterepene production in various plant species, including _Arabidopsis_[68], _Clarkia breweri_[69], _Humulus luplus_[70], and _Phalaenopsis bellina_[71]. In the downstream MEP pathway, two GPPS gene expression levels were consistent with the monoterepene release trend in 16 cultivars. The expression of _PIDXS2_, _PIDXR1_, _PIHDS1_, _PIHDR1_, _PIGPPS3_, and _PIGPPS4_ in herbaceous peony cultivars with high monoterepene release was higher than those with low monoterepene release, indicating that the activation of the MEP pathway of high monoterepene released cultivars was high, thus accelerating the accumulation of GPP, the precursor of monoterepne synthesis (Figure 7A). As the key enzyme of terpene synthesis, TPS can catalyze different substrates to produce different products [20]. GPP can form monoterepenes under the catalysis of TPS [11]. TPS genes have been identified in many plant species. _FluTPS1_ is a mono-product enzyme that catalyzes the formation of linalool [16].