# Functional Analysis of Two Terpene Synthase Genes Isolated from the Flowers of _Hosta_ 'So Sweet'
## 4 Discussion  
In the present work, we isolated and functionally analyzed two terpene synthase genes from the _Hosta_ flower for the first time, and we used qRT-PCR to investigate the expression of these genes in flowers at four different developmental stages. There was a significant correlation between the release of linalool in _Hosta_ and the expression of Hs_TPS_ genes. To date, several terpene synthases have been found to be bifunctional in vitro but monofunctional in planta [46]. Some terpene synthases synthesize linalool and nerolol in vitro but exhibit monoterpene or sesquiterpene biosynthesis activity in planta [47]. These studies suggest that some sesquiterpene synthases may have evolved from a subset of monoterpene synthases adapted to use farnesyl diphosphate as a substrate [48]. In a recent study, Pam_TPS1_ (_Plectrantthus amboinicus_) was identified as a linalool/nerolidol synthase with the ability to exclusively produce linalool and nerolidol. Thus, some enzymes possess both monoterpene synthase and sesquiterpene synthase activities [49]. Recombinant Sa_NES_/_LIS_ (_Santalum album_ L.) is a bifunctional enzyme that showed the biosynthesis of (_E_)-nerolidol from farnesyl diphosphate and of linalool from geranyl diphosphate [50]. In a study of _Clematis florida_, recombinant proteins Cf_TPS1_ and Cf_TPS2_ were demonstrated to catalyze the conversion of geranyl diphosphate to linalool. In addition, Cf_TPS1_ and Cf_TPS2_ produced nerolidol from farnesyl diphosphate [51]. Although recombinant proteins of Hs_TPS1_ and Hs_TPS2_ showed enzyme activity when provided with GPP and FPP as substrates in vitro, respectively, their role in plants should be inferred in conjunction with subcellular localization. Different localization results were obtained in the subcellular localization analysis of tobacco. The exact role of Hs_TPS_ in _Hosta_ was not clarified in this study; hence, the possibility of Hs_TPS_ acting as both a monoterpene synthase and a sesquiterpene synthase in plants cannot be ruled out. Further experiments are needed to provide evidence for more reliable conclusions.  
Overall, the results showed that there was a certain correlation between the high expression of Hs_TPS_ genes and the accumulation of the flower terpene content at S2 and S3. The release of linalool and its rich monoterpene compounds in _Hosta_ plants was directly related to the level of TPS. It is speculated that Hs_TPS1_ and Hs_TPS2_ are involved in the mass production of linalool in _Hosta_, while there may also be many multifunctional enzymes similar to Hs_TPS1_ and Hs_TPS2_ in _Hosta_.  
These studies lay a foundation for the study of terpene release in _Hosta_ flowers, fill the gap of floral scent research in _Liliaceae_ plants, and provide ideas for further study of the Liliaceae gene and cell engineering. The production of large amounts of linalool in _Hosta_ flowers may be facilitated by various enzymes such as Hs_TPS1_ and Hs_TPS2_, a hypothesis that needs to be further verified. In the future, the identification and expression of key TPS genes can provide information for the study of the correlation between the TPS family and terpene synthesis in _Hosta_.