## 4 Biosynthesis of Phenolic Acids in _S. miltiorrhiza_  
There are more than 20 phenolic acids in _S. miltiorrhiza_, with RA and SAB as the typical water-soluble constituents which contribute to the effects in the treatment of cardiovascular disease. RA was a dominant phenolic acid distributing in Lamiaceae. The biosynthetic pathway of RA in _Coleus blumei_ has been characterized [13]. In Lamiaceae including _S. miltiorrhiza_, two biosynthetic pathways are involved in the biosynthesis of the processor of phenolic acid: the phenylpropanol pathway and the tyrosine-derived pathway. In the phenylpropanol pathway, phenylalanine is transformed into 4-coumaroyl-CoA by phenylalanine ammonia-lyase (PAL), cinnamic acid 4-hydroxylase (C4H) and 4-coumarate: CoA ligase (4 CL). In the tyrosine-derived pathway, tyrosine is metabolized to 4-hydroxyphenyllactate through tyrosine aminotransferase (TAT) and 4-hydroxyphenylpyruvate reductase (HPPR), finally forming 3,4-dihydroxyphenyllactic acid (DHPL, also known as salvianic acid A or danshensu). It was reported that the expression of enzymes involved in the tyrosine-derived pathway were more correlated to rosmarinic and salvianic acid B biosynthesis after elicited with MJ, hyphal extracts or other elicitors, which indicated that the tyrosine-derived pathway might be the rate-limiting step in biosynthesis of phenolic acid [64, 65]. 4-coumaroyl-CoA and DHPL were further coupled by the ester-forming enzyme rosmarinic acid synthase (RAS) to form 4-coumaroyl-3'4'-dihydroxyphenyllactic acid (4C-DHPL). 4C-DHPL is oxidized by _CYP98A14_, which introduces the 3-hydroxyl group to form RA. RA was believed to be the processor of SAB. [PERSON] _et al._[15] employed isotopic labeling experiments _in vivo_ using [ring-13C]-phenylalanine to trace the metabolic origin of phenolic acids in _S. miltiorrhiza_. It was speculated that RA might be transformed into a phenoxyl radical by oxidation, and two phenoxyl radicals unite spontaneously to form SAB [15]. In _S. miltiorrhiza_, 4C-DHPL was considered as the main intermediate for RA biosynthesis, and DHPL was involved in this pathway as a principal intermediate. However, the pathway from RA to LAB remains to be elucidated (Figure 2). The majority of genes involved in phenolic acid biosynthesis have been cloned in _S. miltiorrhiza_, including _SmPAL_, _SmC4H_, _Sm4 CL_, _SmTAT_, _SmHPPR_, _SmRAS_ and _CYP98A14_ (Table 2). Three _SmPALs_ were identified in the working draft of the _S. miltiorrhiza_ genome. Coexpression analyses with the accumulation of phenolic acids revealed that _SmPAL1_ and _SmPAL3_ were more abundant in the root which indicated that they might be more important in the biosynthesis of rosmarinic acids [66]. The function of _SmPAL1_ was further verified by _in vivo_ and _in vitro_ analyses, which revealed that _SmPAL1_ exhibited high catalyzation activity using L-Phe as substrate [67]. Otherwise, when the expression of _SmPAL1_ was suppressed in _S. miltiorrhiza_, the rosmarinic acid and salvianic acid B content was significantly decreased which proved the key role of _SmPAL1_ in biosynthesis of phenolic acids [68]. Several Sm4 CLs have been identified in _S. miltiorrhiza_. The enzyme activity analyses and expression analyses of _Sm4 CL2_ and _Sm4 CL3_ indicated that these two genes are more likely to be involved in the biosynthesis of water-soluble phenolic compounds [69; 70; 71]. In addition to PAL and 4 CL, the function of _SmC4H1_, _SmTAT1_, and _SmHPPR1_ were also confirmed _in vivo_. Overexpression in the hairy roots of _S. multiorrhiza_ could increase the accumulation of RA and SAB [18]. The functional characterization genes involved in the biosynthesis of phenolic acids could be used as targets in metabolic engineering to accelerate the production of these compounds. Figure 2: Biosynthethsis of phenolic acids in _S. multiorrhiza_[15]. Solid arrows indicate the established relationships, and dashed arrows indicate hypothetical relationships. RA is mainly biosynthesized from the pathway showed with black arrows in _S.