## 4 A Hairy Root Platform for the Production of Bioactive Abietane Diterpenes
### Elicitation with Methyl jasmonate and Its Analogue Coronatine  
Elicitors are biotic or abiotic agents which trigger signal cascades inducing tolerance and immune response in plants. They have been widely applied to stimulate the synthesis of secondary metabolites in HRs as well as in plant cell culture of different plants [43; 44]. Jasmonic acid (JA) and its structural analogue coronatine (Cor) have been extensively reported as elicitors of the MEP-pathway. Both JA or Cor, which structurally mimics JA-isoleucine (JA-Ile), the bioactive JA, bind the CORONATINE INSENSITIVE 1 (COI1), to activate JA signaling and the expression of JA-induced genes. Cor, however, is able to induce other defense pathways in an independent fashion from the JA signaling [45]. The COI1 dependent perception and signaling pathway is regulated by the transcription factor MYC2 that acts as activator and repressor of JA-responsive gene expression in _Arabidopsis_[46]. It is known that MYC2 is repressed by the JAZ proteins, which bind the co-repressor TOPLESS (TPL) protein, directly through the EAR (Ethylene Response Factor-Associated Amphiphilic Repression) motif or indirectly through an additional repressor, the NINJA (Novel Interactor of JAZ) protein. The histone acetylases HDA6 and HDA19 are then recruited to form a closed complex inhibiting JA response (Figure 4a). Under a high level of JA-Ile (caused by stress conditions) or elicitation with Cor, JA-Ile or Cor are transported into the nucleus, where they interact with the F-box protein COI1 within the SCF E3 ubiquitin ligase (Skp-Cullin-F-box-type) and form the SCF-COI1 complex. This complex recruits the JAZ repressor proteins, bound to the promoter of MYC2, and sends them to degradation via the 26S proteasome. Released MYC2 binds the subunit 25 of Mediator complex (MED25) and induces the transcription of early JA-responsive genes [46], \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Gene** & **Modification/** & **Total Abietane Diterpenes** & **Fold-** & **Aethiopinone Content** \\  & **Elicitation** & **mg g\({}^{-1}\) Dry Weight** & **Increase** & **mg g\({}^{-1}\) Dry Weight** \\ \hline HRs control & Not transformed/not elicited & \(1.2\pm 0.12\) & – & \(0.5\pm 0.03\) \\ \hline _AIDS_ & overexpression & \(\pm\) 0.22 ** & 2.1 & 1.01 \(\pm\) 0.25 * \\ \hline _ADXR_ & overexpression & 3.38 \(\pm\) 1.02 ** & 2.8 & 1.54 \(\pm\) 0.4 ** \\ \hline _Sent -CPPS_ & RNA interference & 4.5 \(\pm\) 0.50 ** & 3.7 & 1.86 \(\pm\) 0.11 ** \\ \hline _SSGGPPS_ & overexpression & 25.64 \(\pm\) 1.22 ** & 21.3 & 7.8 \(\pm\) 0.71 ** \\ \hline _SscCPPS_ & overexpression & 37 \(\pm\) 1.60 ** & 30.8 & 10.44 \(\pm\) 0.21 ** \\ \hline _SscGPPS_/_SscCPPS_ & overexpression & 10.24 \(\pm\) 0.68 ** & 8.5 & 5.2 \(\pm\) 0.12 ** \\ \hline cyanobacterial _DXS_ & overexpression & 2.9 \(\pm\) 0.13 ** & 2.4 & 1.21 \(\pm\) 0.11 ** \\ \hline cyanobacterial _DXR_ & overexpression & 4.5 \(\pm\) 0.24 ** & 3.75 & 2.27 \(\pm\) 0.22 ** \\ \hline _AtWRRX’40_ & overexpression & 4.8 \(\pm\) 0.43 ** & 4.0 & 2.9 \(\pm\) 0.12 ** \\ \hline _AtWRRXY18_ & overexpression & 2.5 \(\pm\) 0.03 * & 2.1 & 0.77 \(\pm\) 0.01 * \\ \hline _AtMYC2_ & overexpression & 6.5 \(\pm\) 1.62 ** & 5.4 & 3.1 \(\pm\) 0.10 ** \\ \hline \hline \end{tabular} Mean values obtained from three independent adventitious hairy root lines for each elicitation treatment or transformation event (* \(p\leq\) 0.05, ** \(p\leq\) 0.01; *** \(p\leq\) 0.001). Data in the table are derived from the indicated publications of the authors.  
\end{table} 
Table 2: A summary of the effects on the abietane diterpene accumulation in _Salvia sclarea_ HRs obtained by designing different elicitation and metabolic engineering approaches. as well as genes involved in the plant defense mechanisms, including also genes encoding enzymes of different secondary metabolites (Figure 4b) [47]. Elicitation of _S.