## 3 Discussion  
Floral fragrance is an important trait of tree peony, which not only mediates its pollination and environmental adaptation but also serves as a key ornamental evaluation index, determining its market value and potential for development and application. Compared with flower color and shape, the study of floral fragrance has started later. At present, the reports on tree peony floral scent mainly focus on the detection and analysis of volatile components, and only in the past three years molecular investigations gradually emerge [45; 52; 53; 56; 57]. However, the cloning and preliminary functional analyses of terpenoid synthesis genes for floral scent are focused on the variety of tree peony cultivar 'High Noon' [45; 46; 52; 53; 57]. As a leading cultivar from the Japanese cultivar-group, the tree peony cultivar 'Oukan', with its strong floral scent, unique flower shape, golden color, high adaptability, and ease of cultivation and management, is not only highly attractive but also has great potential in breeding, fragrance products, and food industry applications. Tree peony cultivar 'Oukan' has been reported to have a strong floral fragrance dominated by terpenoids, reaching more than 80% [42]. While there is a lack of investigation on the molecular synthesis mechanism of terpenoids in its floral fragrance. Therefore, in this study, we used 'Oukan' as the material and screened two terpene synthesis pathway genes, _PsHMGR1_ and _PsTPS1_, to carry out further work on the molecular level research. HMGR is widely distributed in various types of organisms. The characterization and amino acid sequence analysis of a large number of HMGR genes reveals that HMGR is divided into two major classes: class I and class II. Class I is found in eukaryotes with highly conserved catalytic domains and a transmembrane region at the N-terminal associated with the degradation of HMGR molecules, while class II is first discovered in prokaryotes and usually have no transmembrane region [25; 26]. In this study, analysis of the amino acid sequence encoded by the _PsHMGR1_ gene of 'Oukan' showed that the putative PsHMGR1 protein contained structural features common to most plant HMGRs: it was attributed to the HMG-CoA_reductase superfamily member and contained a typical HMG-CoA_reductase_class I structural domain; highly conserved at the C-terminus with large sequence variation at the N-terminus; two transmembrane structural domains; and four highly conserved motifs, i.e., two HMG-CoA-binding motifs and two NADP(H)-binding motifs [24; 25; 26]. We found that the amino acids of the second HMG-CoA-binding site and the two NADP(H)-binding sites of PsHMGR1 were identical to those of other plants, whereas the first HMG-CoA-binding site varied somewhat among different plant HMGRs, and the differences in this site might affect the recognition and binding ability to substrates [25]. Phylogenetic tree results showed that PsHMGR1 had high homology with DUHMGR and PtHMGR proteins, and _D. turbinata_ also possessed volatile floral fragrance, as well as producing significantly higher terpene content than the control in the _PtHMGR_ transgenic poplar lines [32]. So it was hypothesized that _PsHMGR1_ was involved in the biosynthesis of floral fragrance volatiles. Further examination of the expression level of _PsHMGR1_ in different organs of 'Oukan' revealed that its expression level in flower was significantly higher than that in other organs. The changes in the expression of _PsHMGR1_ during the four flower developmental stages showed a consistent trend with the changes in floral fragrance release. The study reported that the expression levels of plant _HMGR_ genes in different tissues were closely correlated with the accumulation of metabolites, which further illustrated the potential of _PsHMGR1_ in floral scent biosynthesis [24].