## 4 Metabolic Engineering of Terpenoids in Plants
In _Arabidopsis_, to produce monoterpenes, the strawberry gene meridold synthase 1 was used, and this resulted in the formation of linalool; this gene also encodes the dual function for the production of monoterpenes and sesquiterpenoids [14]. In another study, _Perilla frutescens_ limonene synthase was introduced to tobacco plants, and limonene formation was detected in the plastids and cytosol of transgenic plants [82]. In petunia, the overexpression of \(S\)-linalool synthase resulted in the monoterpene production of newly formed linalool [10]. The overexpression of the \(S\)-linalool synthase gene in tomato (_Lycopersion esculent_) resulted in high levels of monoterpenes [11]. The results of many of the studies reported to date suggest that, in general, the direct precursor for monoterpene biosynthesis (i.e., GPP) is largely available to introduced monoterpene synthases. The metabolic engineering and overexpression of sesquiterpenoids is limited to certain sources; \(\beta\)-caryophyllene synthase from _Artemisia annua_ was introduced to a viral vector and transferred into _Agrobacteriumium_, which was then agroinfiltrated into \(N\). _benthamiana_ leaves and produced 26.5 mg of \(\beta\)-caryophyllene [83]. In an in vitro regeneration study, sesquiterpene cyclase was transformed in the medicinal plant _A. annua_, and resulted in a 30% artemisinin content [84]. The co-expression of the parthenolide pathway candidate genes was reconstituted by transient co-expression in \(N\). _benthamiana_, and up to 1.4 ug of the final product was produced [85]. The key enzyme amorpha-4,11-diene synthase was transformed into _Agrobacteriumium_ and agroinfiltrated into \(N\). _benthamiana_, which resulted in various sesquiterpene products [86]. Co-expression with different plant-specific genes, such as feverfew germacrene A synthase (GAS), chicory germacrene A oxidase (GAO), and chicory costunolide synthase (COS), was reconstituted and agroinfiltrated into \(N\). _benthamiana_, and results showed 60 ng g\({}^{-1}\) of costunolide production [87]. Furthermore, different plant species have been used successfully to produce valuable sesquiterpenoid compounds using plant suspension cell culture technology [88; 89]. Hitherto, five mevalonate and artemisinin pathway genes were expressed in tobacco plant cell cultures using a single vector, yielding 0.48-6.8 ug/g of artemisinin [90]. Similarly, constructing four genes (amorpha-4,11-diene synthase, amorphadiene monooxygenase, aldehyde \(\Delta\) (13) reductase, and aldehyde dehydrogenase) in \(N\). _tabacum_ leaf cells resulted in 0.01 mg/g of artemisinin alcohol [91]. Likewise, five artemisinin biosynthesis genes were transferred into _Physcomitrella patens_, which produced 0.21 mg/g of artemisinin after three days of cultures [92]. Table 3 summarizes the overall documented list of metabolic engineering of mono-terpenoids and sesquiterpenoids targeting genes and upregulated metabolites. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Source** & **Species** & **Targeted Genes** & **Up/Downregulated** & **References** \\ \hline \multicolumn{5}{c}{**Monoterpeenoids (C15)**} \\ \hline Mentha & _Mentha spicata_ & Limonene synthase & Increased in sesquiterpenoid & [73] \\ \hline \multirow{2}{*}{Lour} & \multirow{2}{*}{_Litsea cubeba_} & Geranyl diphosphate synthase small subunit 1 & Increase in monoterpene content & [74] \\ \cline{3-5}  & & 1-deoxy-d-xylulose-5- & & \\  & & phosphate synthase, & & \\ Lilium & _Lilium_ “Siberia” & 1-deoxy-d-xylulose-5- & & \\  & & phosphate & & \\  & reductio isomerase & & & \\ \hline \multirow{2}{*}{Mentha X piperita} & \multirow{2}{*}{_Nicotiana benthamiana_} & Geranyl diphosphate synthase small subunit & (\(-\)) Limonene, (\(-\))-Linalool, (\(-\))-\(\beta\)-pinene, (\(-\))-\(\alpha\)-pinene, & [76] \\  & & & Myrrcene & \\ \hline Sweet osmanthus & _Osmanthus fragrans_ & Terpene synthase & \(\beta\)-linalool, trans-\(\beta\)-ocimene, \(\alpha\)-farnesene & [77] \\ \hline \hline \end{tabular}
\end{table}