## 3 Discussion
In _Jasminum sambac_, the expression level of _JsHMGR_ also shows a trend of increasing and then decreasing along with different flowering stages, which is basically consistent with the trend of changes in the floral fragrance volatiles content [58], which is in agreement with _PsHMGR1_. Currently, _HMGR_ genes have been isolated and functionally characterized mainly in medicinal plants, such as _A. annua_[59], _Panax ginseng_[29], _Pseudostellaria heterophylla_[26], _Catharanthus roseus_[60], and _Lithospermium erythrothizon_[24]. HMGR enzyme activity as well as total triterpene content is significantly higher in tobacco plants transgenic for _PhHMGR_ than in control plants [26]. _LerHMGR1_ and _LerHMGR2_ from _L. erythrothizon_ have been cloned and demonstratedin vitro to catalyze the conversion of HMG-CoA to MVA, which is thought to play a key rate-determining role in the biosynthesis of shikonin [24]. Transgenesis of the _hmgr_ gene from _C. roseus_ into _A. annua_ produces 38.9% higher artemisinin than the control and increased HMGR enzyme activity [60]. However, little has been reported on the functional resolution of the _HMGR_ gene in ornamental flowers. A mature genetic transformation system has not yet been established in tree peeny. In this study, _PsHMGR1_ was transformed into tobacco, and linalool release amount was detected to be significantly higher than the control. Our work complements the functional characterization of _HMGR_ genes in ornamental plants. The expression levels of _JsHMGR_ in _J. sambac_ at different stages of flower development and at different times after bud excision are basically in agreement with the trend of linalool release [58], which further corroborates our results. As a key enzyme in the last step of terpene synthesis, TPS has a direct role in determining the type of terpene compounds synthesized. Although its sequence is somewhat conserved, TPS proteins are diverse during evolution and in different plants [4; 10; 61]. PsTPS1 isolated in this study possessed the typical conserved structural domain Terpene_cyclase_plant_C1, which was usually contained in TPS family members; had the highly conserved DDX and NSE/DTE motifs, which were considered to be active sites necessary for binding divalent metal cations to catalyze the biosynthesis of terpen; and was also found to have a typical R(R,P,Q)(X)\({}_{8}\)W motif at the N-terminus responsible for catalyzing the monoterpenes cyclization [8; 18; 61]. Typically, members of the TPS-a subfamily synthesize mainly sesquiterpens [18; 62]. In _F. hybrida_, TPS-a subfamily members FhTPS6, FhTPS7, and FhTPS8 all synthesize the sesquiterpens [18], and five TPS-a subfamily members in _Rosa hybrida_, RhTPS47, RhTPS26, RhTPS30, RhTPS32, and RhTPS32S, are also identified as sesquiterpene syntheses [61]. The results of phylogenetic tree analysis in this study showed that PsTPS1 was clustered into the TPS-a clade, so it was hypothesized that PsTPS1 might be involved in sesquiterpene synthesis. PdTPS5, PdTPS2, and RhTPS32, which are more homologous to PsTPS1 in the tree, have been reported to produce sesquiterpene germacrene D in vitro [52; 61]. PaTPS, VrTPS, and VvTPS, which were clustered on the same branch as PsTPS1, were all annotated as germacrene D synthases in NCBI, further speculating that PsTPS1 had the potential to synthesize the sesquiterpene germacrene D. Subsequently, qRT-PCR revealed that the expression levels of _PsTPS1_ in 'Oukan' at different flower developmental stages were consistent with the trend of floral scen release, and the highest expression levels were found in the petal. Further, germacrene D, which was not present in the WT lines, was detected in _PsTPS1_ transgenic tobacco flowers, and this result validated our previous analyses. Thus, PsTPS1 could be considered a terpene synthase gene that catalyzes germacrene D synthesis exclusively, with potential in the future molecular breeding of tree peeny floral fragrances.