## 3 Discussion  
Plants can store and release a wide range of volatile compounds through their specialized organs and tissues, which play a significant role in atmospheric composition [46]. These volatile compounds are also important for the formation of the characteristic floral scent in different plants which improves the aesthetic properties as well as the economic value of the plant [15]. However, the regulatory mechanisms behind the formation of the floral aroma in ornamental plants are poorly characterized. To date, no bHLH transcription factor has been identified and verified by its function in the ornamental and economically significant shrub \"wintersweet\". A few attempts have been made to illustrate the transcriptomic analysis of these genes, but the functional characterization remains underestimated. This might be due to the lack of genetic resources as well as the difficulties in the transformation of the genes in wintersweet [9; 47]. The present study was designed to decipher the molecular mechanism behind flower fragrance production in wintersweet. The motives behind the study were to identify and find out the role and function of the bHLH transcription factors _CpMYC2_ and _CbpHLH13_ in wintersweet. Phylogenetic analysis demonstrates that _CpMYC2_ is closely related to Arabidopsis MYC2 (_AT1G32640_), whereas _CbpHLH13_ is most homologous to _AtbHLH13_ (_AT1G01260_), also known as JASMONATE-ASSOCIATED MYC2-like2 (JAM2). MYC2 was previously reported to promote and activate the sesquiterpene synthase genes TPS21 and TPS11 through interacting with the DELLA proteins (MYC2 interacts with DELLA proteins and regulates the expression of the sesquiterpene synthase gene). It has also been reported that MYC2 is directly involved in the induction of JA and GA in sesquiterpene biosynthesis [48] and induces monoterpene synthase in arabidopsis, which suggests that MYC2 endorses the biosynthesis of a wider range of terpen [49], whereas JAM2 (_AtbHLH13_) is involved in the negative regulation of jasmonate signaling in plants [50]. They have also been reported as novel factors regulating various metabolic pathways in arabidopsis in JA signaling [51]. It has been reported that MYC2 is differentially regulated and induced during the flower opening of wintersweet, especially at the fully-open flower stage [37]. In the present study, high expression of both the genes was found at the fully-open flower stage compared to the other flower developmental stages along with the highest expression in flowers among the other developing tissues of the plant (leaves, stems, and fruits). This indicates the possible role of these genes in volatile production during flowering. MYC2 has been identified to enhance the production of volatile compounds, especially monoterenes and sesquiterpenes, in plants [20; 52]. The expression of the linalool- and \(\beta\)-caryophyllene-producing genes increases in plants transformed by _CpMYC2_ and _CbpHLH13_, respectively. This illustrates the role of the transcription factors _CpMYC2_ and _CpBHLH13_ in the production of monoterenes and sesquiterpenes in wintersweet plants. Our findings also indicate that the overexpression of _CpMYC2_ and _CbpHLH13_ in model plants shows an increase in the production of monoterpene, i.e., linalool as well as the sesquiterpene (E)-\(\beta\)-caryophyllene, respectively. It is possible that the significant increment in compounds could be provoked because it is a MYC2 transcription factor and part of the bHLH family, which is involved in the regulation of many regulatory functions of the plant and also involved in plant growth and the mechanism of flower development in plants [22; 30; 53; 54]. The results also show that _CpMYC2_ possibly increases the production of linalool by inducing the expression of the linalool- and limonene-producing genes _At1g61680_ and _At3g25810_ in arabidopsis.