## 3 Transcriptional Regulation of TIA Biosynthesis  
Plenty of evidence indicates that the synthesis of TIAs is strictly regulated by transcription factors that target on the key structural genes. Elicitors such as yeast, jasmonate (JA) and related oxylipins, hormones such as auxins as well as environmental cues can stimulate TIA biosynthesis in _C. roseus_[23; 24]. It has been revealed that these stimuli-promoted TIA biosynthesis happen via transcriptional regulation of TIA synthetic genes, such as _STR_, _TDC_, _G10H_. The mechanisms for regulating the biosynthesis of different TIA end products are complicated and diversified; many transcription factors have been characterized for their regulatory functions and some of them have been successfully applied for TIA production [24]. There are many examples that applied different transcription factors to improve the content of plant secondary metabolites. Overexpression of _TSAR1_ or _TSAR2_ in basic helix-loop-helix (bHLH) gene family in _Medicago truncatula_ hairy roots caused the up-regulation of triterpene saponin biosynthetic genes and increased accumulation of triterpene saponins [24]. A novel APETALA2/Ethylene Response Factors (AP2/ERF) family transcription factor PsAP2 from _Papaver somniferum_ was overexpressed in tobacco, which showed improved resistance to both abiotic and biotic stresses [25]. After expressing seven MYB transcription factors (Dof1.1, IQD1-1, MYB28, MYB29, MYB34, MYB51 and MYB122) involved in aliphatic and indolic glucosinolate (GSL) biosynthesis in Chinese cabbage, the aliphatic and indolic GSL contents are changed [26]. Overexpression of AaERF1 and AaERF2 from _Artemisia annua_ could increase the accumulation of artemisinin and artemisinin acid in transgenic _A. annua_ plants [27]. Thus, it is an effective way to apply effective transcription factors for plant secondary metabolic engineering. Two AP2/ERF family transcription factors ORCA2 and ORCA3 were characterized as the critical regulators for TIA biosynthesis. They could specifically bind to the JEE (jasmonate and elicitor-responsive element) in _STR_ promoter and can be up-regulated by JA [28]. In _C. roseus_ hairy roots, the up-regulated expression of _ORCA2_ significantly changed the transcripts of many structural genes in TIA biosynthesis, such as _AS_, _TDC_, _G10H_, _LAMT_, _STR_, _T16H_, _PRX1_, _D4H_, _SGD_ and _DAT_; moreover, the induced _ORCA2_ also caused the changes of the expressions of several TF encoding genes, such as _ORCA3_, _ZCT1_, _ZCT2_, _ZCT3_ and _CrMYC2_ (Figure 2). Accordingly, the accumulation of catharanthine, ajmalicine, serpentine and tabersonine were also changed dramatically after _ORCA2_ induction [29]. Another study found that vindoline in the _ORCA2_ transgenic hairy roots could be significantly increased compared to the control lines [30]. Therefore, ORCA2 plays an important role in the regulation of TIA metabolism. Another AP2/ERF factor ORCA3 was isolated by T-DNA activation tagging approach, which could also regulate the expressions of many TIA synthetic genes [31]. In _C. roseus_ cell suspension cultures, overexpressing _ORCA3_ caused the up-regulation of _TDC_, _STR_, _SLS_, _CPR_, _D4H_, _AS_ and _DXS_, variable expression of _SGD_, as well as unaffected expression of _G10H_ and _DAT_. Therefore, overexpression of _ORCA3_ significantly induced tryptamine in shikimate pathway, while it did not cause secologanin in seco-iridoid pathway. Also, feeding loganin to _ORCA3_-overexpressed lines significantly increase TIA production [31]. Partly different from the results of _C. roseus_ cell suspension cultures, overexpression of _ORCA3_ and, along with JA elicitation in hairy root, induced the expression levels of _AS_, _DXS_, _SLS_ and _STR_, decreased the expression of _SGD_, but not affected _TDC_, _G10H_, _CPR_, _GBFs_ and _ORCA2_. The differences may result from different regulatory mechanisms between two types of tissue culture systems [32].