# Transcriptomic Analysis of the Candidate Genes Related to Aroma Formation in _Osmanthus fragrans_
## Discussion
The biosynthetic enzymes involved and the emission of terpen have been identified and functionally characterized in many plants, such as Arabidopsis [30], Snapdragon [31; 32], _Clarkia breweri_[33; 34; 35; 36], Petunia [37], and Rose [38; 39]. Terpenoids, derived from premyl diphosphate precursors, are synthesized by two independent pathways: The MVA pathway in the cytoplasm and the MEP pathway in plastids [40]. Based on sequence annotations and the analysis of changes in gene expressions during each flowering stage, key floral scent-synthesizing genes, _DXS_, _DXR_, _HDR_, _TPS_, and _GPPS_, were identified. The results indicated that the expressions of _DXS_, _DXR_, _HDR_, and monoterpene synthases were all positively correlated with monoterpene emission. _DXS_ was the first gene identified and is thought to be an important rate-controlling step of the MEP pathway [8]. Two _OfDXSs_ showed differential expression patterns that the transcript level of _OfDXS1_ was down-regulated at the full flowering stage, while _OfDXS2_ increased sharply. It seemed that the _OfDXS2_ transcript was correlated with monoterpene emission. These genes showed similar expression patterns in _Hedychiumoronarium_[41]. _DXR_ may also serve as a significant control factor of the metabolic flux through the MEP pathway, since it catalyzed the first committed step of the pathway towards terpenoid biosynthesis [42; 43]. _HDR_ catalyzed the last step of the MEP pathway and the highest expression was at the full flowering stage. The TPSs were directly responsible for the synthesis of various terpenes, _OfTPS2_ and _OfTPS3_, andhad almost opposite expression patterns at each flowering stage. A total of 1327 TF genes were differentially expressed in the _O. fragrans_ transcriptome. The role of transcriptional regulation in reprogramming metabolic processes during plant development and in response to biotic and abiotic cues has been recognized in several plant systems. Among the regulatory networks governing secondary metabolisms, there have been many studies on flavonoid biosynthesis, but very fewwere known for the regulation of floral volatile production at the molecular level [44; 45; 46; 47]. Only a few MYB and WRKY TF family membershave been characterized and shown to have crucial roles in regulating the biosynthesis of floral volatiles. To date, of the R2R3-MYB TF family, _PhODO1_, _PhEOBI_, _PhEOBII_, and _PhMYB4_ have been characterized as regulators of volatile phenylpropanoids in petunia [48; 49; 50; 51]. The _AtPAP1_ transcription factor enhanced the production of phenylpropanoid and terpenoid scent compounds in rose flowers [20]. MYC2, a bHLH TF, directly bound to the promoters of the sesquiterpene synthase genes, _TPS21_ and _TPS11_, and activated their expression in _Arabidopsis_ flowers [17]. In the WRKY family, _GaWRKY1_, _NaWRKY3_, _NaWRKY6_, and _SIWRKY73_ were found to be related to volatile terpenoid production [15; 16; 21]. The qRT-PCR analysis revealed that the _bHLH_, _MYB_, and _WRKY_ family members were differentially expressed at each flowering stage. The expression patterns of _O/MYB1_, _O/MYB6_, _O/JWRKY1_, and _O/JWRKY6_ were consistent with the release of floral volatiles, which may play critical roles in regulating the formation of floral scent. Further experiments, including yeast one-hybrid and trans-activation assays using promoters of key structural genes, need to be carried out to determine their involvement in floral scent production.