## 3 Biosynthesis of Tanshinones in _S. miltiorrhiza_
\begin{table}
\begin{tabular}{c c c c} \hline
**Gene (Genbank No.)** & **Highest Expression Sites** & **Elicitor** & **Reference** \\ \hline _SmIPI2_ (N831106) & stem & MJ & [31] \\ _SmGPPS_ (JN831107) & & & [31] \\ _SmGPPS_.LSU (N831111) & stem and flower & MJ & [31] \\ _SmGPPS_.SSUI (N831108) & leaf & MJ & [31] \\ _SmGPPS_.SSUI.1 (JN831109) & leaf and root cortex & & [31] \\ _SmGPPS_.SSUI.2 (JN831110) & stem & & [31] \\ _SmFPPS_ (EF635968 H0687768) & stem & MJ & [22, 31, 47] \\ _SmGGPPS_ (F1)643617 F1178784) & leaf & NaCl, wounding, high temperature, darkness, pathogen, MJ, ABA, SA and GA, Act12, Ag\({}^{*}\) & [17, 35, 38, 48, 49, 50] \\ _SmGGPPS2_ (JN831112) & root stele & & [31] \\ _SmGGPPS3_ (JN831113) & ubiquitous & & [31] \\ _SmCPS1_ (EU003997) & root cortex & Ag\({}^{*}\), MJ, VE & [35, 51, 52] \\ _SmCPS2_ (JN831114) & leaf & & [31, 53] \\ _SmCPS3_ (JN831115) & stem & & [31, 53] \\ _SmCPS4_ (JN831120) & sepal & & [31, 53] \\ _SmCPS5_ (JN831121) & stem & MJ & [31, 53] \\ _SmKSL1_ (EF635966) & root & Ag\({}^{*}\), MJ & [35, 51, 53] \\ _SmKSL2_ (JN831119) & root xylem & & [31, 53] \\ _CYP64 HI_ (JX422213) & root & Ag\({}^{*}\) & [27, 35] \\ _CYP76s_ (KR140168) (KR140169) & root & Ag\({}^{*}\) & [54] \\ _SmCPRI_(F8693803) & & & [27, 55] \\ _SmCPR2_(JX848592) & & & [27] \\ \hline \end{tabular}
\end{table} 
Table 1: _Cont._Isopentenyl diphosphate (IPP) and its isomer dimethylallyl diphosphate (DMAPP) are the precursors of all terpenoids. In plants, two biosynthetic pathways are involved in the production of the universal isoprene precursors IPP and DMAPP: the mevalonate (MVA) pathway in the cytosol, and the methylerythritol phosphate (MEP) pathway in the plastids. The MEP pathway was identified through labeling experiments in bacteria and plants [56,57]. In plants, the MEP pathway is mainly responsible for production of monoterrepoids, triterpenoids, carotenoids, and the side chains of chlorophylls and plastoquinone [58]. Glyceraldehyde 3-phosphate (G3P) and pyruvate have been identified as the direct precursors of IPP and DMAPP. Most genes in the MEP pathway have been identified in _S. miltiorrhiza_. 1-Deoxy-d-xylulose-5-phosphate synthase (DXS) is the key enzymes of the MEP pathway and is involved in the important rate-limiting reaction. There are five DXSs in the draft genome of _S. miltiorrhiza_, and the expression level of _SmDXS2_ in _S. miltiorrhiza_ roots was highest compared with the other _SmDXSs_[31]. It was also reported that the overexpression of _SmDXS2_ could increase the accumulation of tanshinones in _S. miltiorrhiza_ hairy root [17], indicating that _SmDXS2_ might be closely related to the accumulation of tanshinones. The MVA pathway, which has been studied for decades, is mainly involved in the biosynthesis of sesquiterpenoids, polyisoprenoids and sterols [59]. In _S. miltiorrhiza_, it was considered that tanshinones are mainly synthesized by the MEP pathway, while the MVA pathway could be better explained as being involved in cell growth. However, both cell growth and tanshinone production could partially depend on the crosstalk between the two pathways [60,61]. In the MVA pathway, IPP is synthesized from two acetyl-CoA molecules. The 3-hydroxy-3-methylglutaryl-CoA reductase (HMGR) converts 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) to MVA with two NADPH as the rate-limiting step of the MVA pathway [36]. There are four HMGRs in the draft genome of _S. miltiorrhiza_[31]. _SmHMGR1_, _SmHMGR2_, _SmHMGR3_ are considered to play more important roles in tanshinone biosynthesis [31]. Actually, overexpression of _SmHMGR1_[17,62] and _SmHMGR2_[36] in _S. miltiorrhiza_ hairy root cultures both could significantly increase the tanshinone content, indicating that _SmHMGR1_ and _SmHMGR2_ from MVA pathway might also contribute to the biosynthesis of tanshinones. The conversion of IPP and DMAPP could be catalyzed by IPP isomerase (IPI). DMAPP and IPP are further condensed by geranyl diphosphate synthase (GPPS), farnesyl diphosphate synthase (FPPS), geranylgeranyl diphosphate synthase (GGPPS) to form GGPP [6].