## 4 Discussion  
Terpenes are a component of plant essential oils [29]. Terpenes are closely related to human life and are widely used in pesticides, perfumes, nutraceuticalils, and medicines [30]. Recently, it was reported that terpenes have great potential as biofuels [31]. Due to the great potential of terpenes, Rutaceae plants with developed oil glands and abundant terpenes are worthy of study. Here, we focus on two key genes (_ZbDXS, and ZbFPS_) in the terpenoid metabolic pathway of _Z. bungeanum_. The current research testifies that _ZbDXS_ and _ZbFPS_ are related to terpene biosynthesis, which is based on evidence from an in vitro assay of a recombinant protein and transient expression in tobacco. According to the phylogenetic tree, we found that the _FPS_ gene is mainly distributed in gymnosperms, monocotyledons, and dicotyledons. The evolution of genes leads to functional differentiation, which in turn affects the diversity of metabolism. Genetic relationships between different species can be determined by phylogenetic tree analysis of the terrestrial plants of the _FPS_ gene [32]. According to the phylogenetic tree lineage, it was speculated that the metabolites of different species differ due to their metabolic pathways. The terrestrial plant's _FPS_ gene is located at a branch point in the terpene pathway, directing carbon out of the central part of the isoprene pathway [33]. Subcellular localization prediction showed that _ZbDXS_ was located in the chloroplast, which was consistent with the MEP pathway occurring in the chloroplast. Notably, _ZbFPS_ were localized in mitochondria, consistent with previous reports [34; 35; 36; 37; 38]. Current studies have found that _FPS_ genes are located in chloroplasts, cytosol, and peroxisomes [39; 40; 41]. We deduce that _ZbFPS_ may be transported from the cytoplasm to mitochondria, through which energy is provided for downstream product synthesis. When plants are under stress, the terpenes pathway responds to environmental factors to stress resistance, thereby protecting plants [16]. The interaction of upstream and downstream genes in the terpene pathway forms a feedback regulation mechanism, the core of coping with stress. The interactions of _ZbDXS_ and _ZbFPS_ with other regulatory proteins were predicted by STRING ([[http://string-db.org/](http://string-db.org/)]([http://string-db.org/](http://string-db.org/))) (accessed on 29 May 2020). Input _ZbDXS_ and _ZbFPS_ into the STRING website to predict protein interactions. The specie selection was the _Arabidopsis thaliana_ for analysis. Among them, _ZbDXS_ matches _DXPS1_ (73%), and _ZbFPS_ matches _FPS_2 (80%). We can find from the protein functional connection network that _ZbDXS_ and _ZbFPS_ have complex interactions with various proteins (Figure 6). These proteins are involved in the terpene pathway, where _ISPF_ and _DXR_ are at the center of the functional connection network and interact with various proteins. The results showed that genes in the terpene pathway interact with each other, antagonizing each other and producing terpen in response to stress response. Tissue expression could be used to regulate the supply to biosynthetic pathways localized in these plant parts [14]. The _DXS_ family is involved in the biosynthesis of different isoprene compounds due to its different members [42]. _DXS 1_ is expressed in many plant tissues except the roots, such as corn, tomato, and so on. _DXS 2_ is strongly expressed  
Figure 6: The _ZbDXS_ and _ZbFPS_ protein functional connection network. in mycorrhizal-induced roots and accumulates carotenoids and monoterpenes [12]. _FPS_ is also expressed differently in different tissues of the plant. For _Matricaria recuitia_ L., _MrFPS_ is most highly expressed in flowers and stems [43]. In _H. brasiliensis_, _HbFPS_ is not expressed in roots, stems, leaves, and other tissues but is highly expressed in latex [44]. These studies have shown that the spatiotemporal expression patterns of _ZbDXS_ and _ZbFPS_ genes in different plants are quite different.