## 4 Metabolic Engineering of Terpenoids in Plants  
Changing the availability of precursors is one method used to control terpenoid levels in plants. It is possible that altering the terpenoid precursor pool alone is not enough to elevate levels of target terpenoids, and that simultaneously engineering the downstream genes would also be needed. In metabolic engineering, the availability of precursors is a key concern. The level of an isoprenoid precursor that is limiting for the synthesis of terpenoids is likely to vary depending on the plant tissue, species, and physiological condition [16]. A previous study of the metabolic engineering of monoterpenoids focused on producing heterologous monoterpenes in spearmint, and the knocking down of limonene synthase resulted in a huge reduction of limonene and carvone synthesis, while RNAi plants showed an increased sesquiterpenoid level [73]. The overexpression of geranyl diphosphate synthase small subunit 1(GPS SSU1) in _Listea cubea_ (Lour) plants showed a significant increase in monoterpene levels [74]. Two potential genes in the MEP pathway, namely, DXS and DXR, were cloned and developed in transgenic tobacco plants, which resulted in an increased content of monoterpenoid and sesquiterpenoid linalool and carvophyllene, respectively [75]. The co-expression of _Solanum lycoperscum_ DXP, _Arabidopsis thaliana_ GPS1, and Mentha X piperta GPS SSU through transient expression in _Nicotiana benthamiana_ plants enhanced the production of monoterpenes such as limonene, linalool, alpha/beta-pinene, and myrcene [76]. The monoterpene key enzyme terpene synthase (TPS) subfamily was expressed in _Osmanthus fragrans_ and the transient expression of leaves exhibited a high level of linalool and trans-\(\beta\)-ocimene, hence TPS played an important role in monoterpene production [77]. The development of transgenic _Mentha spicata_ by _Agrobacteriumum tunefaciens_ mediated transformation with isopentyldiphosphaithe (IPP) isomerase, and limonene synthase gene resulted in high levels of terpenoid production [78]. The overexpression of HMG CoA reductase in _Lavandula angustifolia_ resulted in high levels of linalool production [79]. In another study, snapdragon (_Antirinhun majus_) GPPS-SSU was overexpressed in tomato fruits, resulting in the production of monoterpenes including geraniol, geranial, neral, citronella, and citronellal [80]. [PERSON] et al. found that the production level corresponding to three different monoterpene synthases was high in transgenic tobacco plant flowers exhibiting three separate monoterpene synthases, but that \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Source** & **Instrument * & **Method** & **Identifified Terpenes** & **References** \\ \hline Cannabis & GC-MS & ASE & 23 & [65] \\ \hline Cannabis & GC-MS & Headspace, SPME and Liquid injection & 49 & [52] \\ \hline Cannabis varieties & GC-MS & HS-SPME & 30 & [66] \\ \hline Cannabis & GC-MS & Headspace & 30 & [53] \\ \hline Muscat grape & GC-MS & HS-SPME & 28 & [67] \\ \hline Finger Cintron & GC-MS & SPME & 62 & [68] \\ \hline Fourteen Compositae plants & GC-MS & n-hexane & 213 & [54] \\ \hline Goose berry, crabapple, cherry silver berry, scarlet hawthorq & GC \(\times\) GC-TOF-MS & SPME & 79 & [69] \\ \hline _Exocarpium citri_ Grandis & GC-MS & SP, HS-SPME \& solvent extraction & 81 & [70] \\ \hline Basil \& Tobacco & GC-MS & SP, Ultrasound-Assisted & 18 & [71] \\ \hline Zinger & GC-MS/LC-ESI-MS & MeOH & 102 & [72] \\ \hline \hline \end{tabular}  
* GC-MS—Gas chromatography-mass spectrometry; GC-TOF-MS—Gas chromatography time of flight-mass spectrometry; LC-ESI-MS—Liquid chromatography electrospray ionization mass spectrometry; ASE—Accelerated solvent extraction; HS-SPME—Headspace solid-phase microextraction; SPME—Solid-phase microextraction; SP—Solid phase.  
\end{table} 
Table 2: Identification of terpenoid metabolite compounds and analytical methods used. the expression of endogenous linalool production was not affected [81].