## 1 Introduction  
Plant secondary metabolites including terpen, phenolic compounds, and nitrogen-containing compounds, which are served as signaling or defense molecules, are not only important to plant development and adaptation, but also have significant economic value as resources in human medicine and chemical industries. Many secondary metabolites from plants have been used in the clinical treatment of diseases, such as taxol in the treatment of refractory ovarian cancer and metastatic breast cancer [1], and ginkgolides in the treatment of cardiovascular diseases and diabetic vascular complications [2]. Secondary metabolites are always accumulated at low levels in plants with long growth periods. Moreover, chemical synthesis is challenging and has little practical value for those compounds with complicated structures. Scientists have turned to biotechnology and genetic engineering to produce or improve these compounds in plants or microorganisms. Elucidation of the biosynthetic pathways of secondary metabolites in plants enabled the exploration of metabolic engineering or synthetic biology as potentially effective approaches for producing specific metabolites, which provide alternative for the production of the valuable compounds. Metabolic engineering can be optimized by enhancing the rate-limiting steps or blocking competitive pathways to improve production. Overexpression of genes that encode the rate-limiting enzymes involved in scopolamine biosynthesis has resulted in greater accumulation of scopolamine in hairy roots of transgenic _Hyoscyamus niger_ compared to the wild-type [3]. Contents of oleanane-type ginsenoside (ginsenoside Ro) in _Panax ginseng_ plant were highly increased by overexpression of _CYP716A52v2_, and the amount of ginsenoside Ro declined in an RNA interference transgenic line targeting this gene [4]. Alternatively, synthetic biology can be used to construct engineered strains for the high-yield production of bioactive secondary metabolites. [PERSON] _et al._ used synthetic biology to develop strains of _Saccharomyces cerevisiae_ for the high-yield biological production of artemisinin acid, which led to the heterologous production of 25 g/L of this compound by baker's yeast after multiple steps of optimization [5]. The root and rhizome of _S. miltiorrhiza_ with diterpenoid tanshinones and phenolic acids as its active compounds is a traditional Chinese medicine (usually called _danshen_ in China) has been widely used for the treatment of cardiovascular and cerebrovascular diseases. The Fufang Danshen Dripping Pill with _danshen_ as its main material has been extensively used for treatment of cardiovascular diseases in Chinaand some other countries, and has been approved for phase III clinical trials in the USA (ClinicalTrials.gov Identifier: NCT01659580). The two typical secondary metabolites made _S. miliorrhiza_ as a model for analyzing plant secondary metabolism pathways. Tanshinones including tanshinone I, tanshinone IIA, cryptotamshinone, and dihydrotanshinone I are lipophilic compounds, which are widely accumulated in _Salvia_ (Labiaceae) [6,7]. More than 40 tanshinones have been isolated from _danshen_. It possesses significant antioxidant, anti-inflammatory, and antineoplastic activities and has been shown to have potent anticancer activities both _in vitro_ and _in vivo_[8,9]. The anti-inflammatory activity of tanshinone IIA is especially outstanding, as this compound blocks pro-inflammatory signals _in vivo_ and these effects are conserved in human neutrophils [10]. In addition, the sulfotanshinone sodium injection constituted with sodium tanshinone IIA sulfate, which significantly improves the intestinal absorption of it, has been widely used for treatment of coronary disease, angina pectoris, and myocardial infarction in China [11]. Tanshinones also have significant anti-aging activity. For instance, cryptotamshinone can greatly extend the chronological lifespan of budding yeast _S. cerevisiae_[12].