## 4 Discussion  
Terpenoids are one of the most important secondary metabolites in plants [8], which strongly contribute to the unique flavor of \(Z\). _bungeanum_. In this study, \(\alpha\)-Pinene was detected as having the highest content out of all the VOCs, which suggests that it might be an important internal factor determining the flavor of \(Z\). _bungeanum_. Related research shows that \(\alpha\)-Pinene has good therapeutic effects on some diseases [32]. There needs to be further research on the use of aroma as pharmacology. According to previous studies and the results of this experiment [14], we found that \(DXS\) plays an important role in the synthesis of \(Z\). _bungeanum_ aroma. The \(DXS\) family is known to be very small with three subfamilies [33]. The \(DXS1\) family mainly plays the role of housekeeping genes while \(DXS2\) and \(DXS3\) are mainly involved in plant secondary metabolism [31]. Phylogenetic analysis found that the genetic distance between \(ZbDXS\) and the \(DXS\) gene (_CitDXS_) of _Citrus sinensis_ was the smallest. \(ZbDXS\) and _AtDXS3_ (_Arabidopsis thaliana_) were divided into one clade, and _AtDXS3_ belongs to the \(DXS3\) type gene. Thus, it is speculated that \(ZbDXS\) is also likely to be of the \(DXS3\) type. It is currently \begin{table}
\begin{tabular}{c c c} \hline \hline
**Protein** & **Protein ID in NCBI** & **NOTE** \\ \hline CitDXS & XP\_00648309.1 & _Citrus sinensis_ \\ AtDXS1 & AEE76517.1 & _Arabidopsis thaliana_ \\ RrDXS & AEZ53173.1 & _Rosa rugosa_ \\ GmDXS & NP\_001236070.1 & _Glycine max_ \\ ZmDXS & ACT32136.1 & _Zea mays_ \\ AsDXS & AHI62962.1 & _Aquilaria sinensis_ \\ IrDXS & AMM72794.1 & _Isodorurbescens_ \\ NdDXS & CBA12009.1 & _Nicotiana tabucum_ \\ SmDXS & ACF21004.1 & _Salavia uniformiza_ \\ AbDXS & AGG54706.1 & _Aconitum balfourii_ \\ BnDXS & AHN09416.1 & _Brassica napus_ \\ OsDXS & XP\_015640505.1 & _Oryza sativa_ \\ CrDXS & ABI35993.1 & _Catharanthus roseus_ \\ GbDXS & AAS89341.1 & _Ginkgo biloba_ \\ AaDXS & AAD56390.2 & _Artemisia anntua_ \\ EuDXS & AFU93069.1 & _Eucommita ulmoides_ \\ CaDXS & PHY95029.1 & _Capsicum annuum_ \\ SlDXS & CAZ66648.1 & _Solanum lycopersicum_ \\ SbDXS & XP\_002441088.1 & _Sorghum bicolor_ \\ EsDXS & XP\_006414485.1 & _Eutrenasalsugineum_ \\ AoDXS1 & AEK69518.1 & _Alpiniaofficiinarum_ \\ SmDXS2 & ACO66107.1 & _Salvia milliorrhiza_ \\ AmDXS & AAW28999.1 & _Antrithrium najus_ \\ AtDXS3 & AED1669.1 & _Arabidopsis thaliana_ \\ BrDXS & ABE60813.1 & _Brassica rapa_ \\ HbDXS & ABF18929.1 & _Heevabrasiliensis_ \\ MtDXS1 & CAD22530.1 & _Medicago truncatula_ \\ MtDXS2 & CAN89181.1 & _Medicago truncatula_ \\ PaDXS1 & ABS50518.1 & _Piceaabies_ \\ PaDXS2 & ABS50520.1 & _Piceaabies_ \\ PtDXS1 & ACJ67021.1 & _Pinus taeda_ \\ PtDXS2 & ACJ67020.1 & _Pinus taeda_ \\ RcDXS & XP\_002533688.1 & _Ricinus communuis_ \\ SrDXS & ACJ43010.1 & _Steria rebundiana_ \\ TmDXS & AAS89342.1 & _Taxus\(\times\)media_ \\ DoDXS & AHF22384.1 & _Dendrobium officinal_ \\ CsDXS & BAF75640.1 & _Croton stellatopilosus_ \\ TcDXS & EOY31729.1 & _Thebroma cacao_ \\ BmDXS & ACP30544.2 & _Bacopa nonunieri_ \\ PmDXS & RLN29936.1 & _Panicum miliaccum_ \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Other plant DXS proteins used for phylogenetic tree analysis. known that _DXS3_ family has the fewest member. The expression level of _DXS3_ in maize was low in different tissues, it was speculated that this gene was mainly involved in MEP pathways with lower levels, such as phytohormone gibberellin and abscisic acid [34]. At present, there is no report on the _DXS_ gene family in _Z. bungeanum_, which will be needed to further understand the mechanism related to aroma. The results of RT-qPCR showed that compared to the genes in the MVA pathway, the key genes in the MEP pathway have relatively strong changes in their relative expression. This result presumes that the volatile terpenoids in _Z. bungeanum_ are mainly obtained from the MEP pathway. The results of GC-MS also support this view.