## 3 Production of Plant Secondary Metabolites Using Natural Hosts
### Comparison of Different In Vitro Plant Culture Systems
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Compound** & **Product** & **Host** & **Strategy or Observations** & **Platform** & **Titer** & **Vessel** & **Ref.** \\ \hline BenzylIsoquinoline & & & _Papaver_ & Removal of hormones & Suspension cells & 2.5 mg/g DW & Flask & [83] \\ alkaloids & & & _Somiferum_ & & & & \\  & Morphine & & & & & \\ \hline  & & _Papaver_ & & & & & \\  & & _Somiferum_ & & & & & \\ \hline  & & _Papaver_ & Elicitation by hormone & Suspension cells & 243.2 mg/g & & \\  & & _bractatum_ & indole-3-acetic acid & & & & \\ \hline  & & _Papaver_ & Overexpression of & & & & \\  & & _bractatum_ & codeinone reductase & & & & \\ \hline  & & _Papaver_ & Elicitation with & & & & \\  & _orientate_ & salicylic acid & & & & \\ \hline  & Total terpenoid & & & & & & \\  & indole & & & & & \\  & alkaloids & & & & & \\  & & & & & & \\  & & & & & & \\  & & & & & & \\  & & & & & \\  & & & & & & \\  & & & & & & \\  & & & & & & \\ \hline  & & & Co-overexpression of & & & \\  & & _ORCA3_ and _SGD_ induced & & & & \\  & & & with 3 \(\upmu\)M & & & \\  & & & dexamethasone & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: _Cont._Plant organs have specialized tissue structures and do not experience the yield instability caused by the redifferentiation of suspension cells [94]. Particularly, roots have been found in recent years to be capable of achieving higher yields compared to their corresponding suspension cell lines. Industrial productions of ginseng (20,000 L) using adventitious roots of _Panax ginseng_ has already been achieved [95]. Moreover, various compounds such as artemisinin in _Artemisia annua_, oleanolic acid in _Calendula officinalis_, quercetin in _Polygonum multiforum_, genistein in _Trifolium pratense_, oleanolic acid in _Calendula officinalis_, elcastrol in _Triptergytium wilfordii_, tropane alkaloids in _Duboisia_[96], pyridine alkaloids in _Nicotiana tabacum_, as well as terpenoid indole alkaloids and quinoline alkaloids, all show higher yields in hairy roots (Table 1). However, roots are not beneficial for the production of all secondary metabolites. Some secondary metabolites show no significant difference in content between hairy and natural roots, such as tanshinone in _Salvia militorhiza_[97], and benzyl isoquinoline alkaloids in _P. somiferum_ (opium poppy) and _Eschscholzia californica_[98]. Compared to suspension cells, the content of the sesquiterpene \(\beta\)-eudesmol in _Attractylodes lancea_ adventitious roots is lower than that in suspension cells (5 and 18 \(\mu\)g/g FW, respectively) [35; 36]. The content of benzyl isoquinoline alkaloids, such as morphine, in _P. bracteatum_ is also higher in suspension cells compared to hairy roots. Berberine, shikonin, and Taxol, as mentioned earlier, also have significantly higher content in suspension cells compared to roots. Adventitious roots and hairy roots exhibit comparable abilities in the synthesis of many secondary metabolites, but the latter often demonstrate a more stable morphology, possibly due to the integration of plant hormone biosynthetic genes from _Agrobacteriumrium rhizosphere_ into the genomes. For instance, in our study, the adventitious roots of _Triptergyium wilfordii_ tend to aggregate and undergo partial dedifferentiation [99], while hairy roots do not [100]. Additionally, unlike adventitious roots, hairy roots appear to have lower capability for secreting secondary metabolites into the culture medium, such as the indole alkaloids of _Catharanthus roseus_[101], tropane alkaloids of _Duboisia leichhardtii_[102], sesquiterpene pyridine alkaloids of _Triptergytium wilfordii_[103], terpenoids of _Salvia militorhiza_[104], and red beet pigments [105]. This difference may also be attributed to the genetic manipulation of hairy roots. Comparing suspension cells and root cultures, it appears that structurally complex compounds often require cell organization with differentiation, as found in root or organ forms, to be effectively synthesized.