## 3 Discussion
This intermediate compound is considered an undesirable component under harsh growth conditions. The downregulation of the gene encoding pulegone reductase (PR), and the consistent expression of the gene encoding (\(-\))-menthol dehydrogenase (MR), may explain the reduced proportions of mentone and menthol. [PERSON] and [PERSON] [23] reported that high levels of menthofuran reduced pr transcript levels in immature leaves, consequently increasing pulegone content in peppermint. These results suggest that the flux of (+)-pulegone is controlled at the transcriptional level of pr, and they are consistent with the results of our present research. However, our data are presently insufficient to explain the accumulation of piperitone under low light conditions. The light-dependent transcriptional regulation of MEP pathway genes is conserved in all plant species we investigated. Nevertheless, not much is known about the signal transduction pathways or the transcription factors that convert the environmental information into a transcriptional response [1]. In our transcriptome analysis results, co-expression modules with terpenoid biosynthesis genes from the MEP pathway suggest that the transcription of those genes may be coregulated by the same transcription factor as response to light. In _A. annuau_, HY5, a bZIP transcription factor and a core TF in the light signaling pathway, interacts with the promoter of the monoterpene synthesis gene _QH6_[24]. It has also been reported that AaHY5 acts via trichome-specific and light-induced AaGSW1 to control artemisinin formation [8]. Evidence from _Arabidopsis_ has demonstrated that HY5 and PIFs are direct regulators of the light-modulated expression of the _DXS_ and _DXR_ genes encoding the flux-controlling enzymes of the MEP pathway [25]. In the present transcriptome data, homologous genes of _HY5_ (Unigene0034124), _PIF1_ (Unigene0086430) and _PIF4_ (Unigene0065109) were identified in _M. Canadensis_. The resultant RPKM values indicate that the expression levels of these TF genes were significantly changed under dark conditions. The expression pattern of _HY5_ was the same as that of the menthol synthesis genes, whereas _PIFs_ showed the opposite expression pattern (upregulated under darkness and downregulated after illumination). These results indicate that a similar regulation mechanism to that targeting the monoterpene synthesis gene (operating via the key-master integrators of light signal transduction HY5 and PIFs) may also operate in _M. canadensis_. More evidence should be sought in future experiments. We have established strategies to increase the levels of essential oil in peppermint via the overexpression of unique structural genes in the menthol synthesis pathway, or the overexpression of additional genes that encode enzymes involved in the precursor supply pathway for monoterpene biosynthesis [26]. Transgenic lines were generated, but few of them improved essential oil yields. The overexpression of transcription factors that target and positively control multiple monoterpene biosynthesis, or the suppression of the expression of negatively controlled TFs, could also each represent metabolic engineering strategies for improving essential oil yields. Previously, researchers have elucidated the regulatory roles of several genes encoding the WRKY [27; 28; 29], MYB [30], bHLH [31], AP2/ERF [32; 33], and bZIP [8] classes of TFs in specialized terpenoid biosynthesis. Using RNA-Seq in _Solanum lycopersicum_, SIMYC1 was identified and shown to transiently transactivate the terpene synthase promoters in _Nicotiana benthamiana_ leaves [34]. Heterologous AtWRKY18, AtWRKY40, and AtMYC2 were overexpressed in _S. sclarea_ hairy roots and proven to regulate the expression of several genes encoding enzymes of the MEP-dependent pathway [35]. The overexpression of SmERF128 increased the expression levels of copalyl diphosphate synthase \(1\) (SmCPS1), kaurene synthase-like \(1\) (SmKSL1) and cytochrome P450 monooxygenase [32].