## 2 Biosynthesis and Precursors of Terpenoids  
Plants possess two distinct pathways to produce terpenoids: the plastidial 2-C-methyl-d-erythritid-4-phosphate (MEP) pathway and the acetyl-CoA dependent cytosolic mean-onate (MVA) pathway (Figure 1). The C5 unit's IPP (isopentenyl pyrophosphate) and its allylic isomer dimethylallyl pyrophosphate (DMAPP), the fundamental terpenoid biosynthesis building blocks, are produced through a metabolic assembly of multiples in both terpenoids pathways. Prenyl transferases use DMAPP and IPP in condensation reactions to generate bigger prepyl diphosphates, including the sesquiterpene precursor FPP (farnesyl pyrophosphate), monoterpene precursor GPP (geranyl pyrophosphate), and C40 carotenoid and diterpene precursor GGPP (geranylgeranyl pyrophosphate) in both compartments (Figure 1). Although there is increasing evidence that there is an exchange of intermediates between these compartments [32; 33], the cytoplasmic MVA pathway is generally considered to supply the precursors for the production of sesquiterpenes. In the plastids, the MEP pathway supplies the precursors for the production of monoterpenes (Figure 1). The MVA pathway consists of seven enzymatic processes that convert the precursor acetyl-CoA to IPP and DMAPP (Figure 1). The MEP pathway requires eight enzymatic steps to convert the initial materials pyruvate and glyceraldehyde-3-phosphate to IPP and DMAPP (Figure 1). Even though IPP and DMAPP are separated spatially, they are exchanged between cytosol and plastids during the production of various terpenoids [34]. Using precursors such as GPP, FPP and GGPP are cyclized or rearranged by different terpene synthase enzymes, and are responsible for the synthesis of different classes of terpenoids; they can easily acquire new catalytic properties by minor changes in the structure [35]. Monoterpenoids are C10 compounds derived from GPP through the enzymatic activity of geranyl pyrophosphate synthase (GPS). Through the cyclization process, the range of monoterpenes is rapidly increased, and monocyclic or bicyclic compounds can be synthesized. The dephosphorylation and ionization of geranyl diphosphate to geranyl carbo-cation initiates the production of monoterpenes [36]. Linalyl pyrophosphate and neryl pyrophosphate are isomers of GPP by ionization to the allylic cation. This allows for changes in the attachment of the diphosphate group or changes in stereochemistry at the double bond. Monoterpene synthase (or monoterpene cyclase) is an enzyme that catalyzes the formation of cyclic monoterpenoids. There is an essential enzyme involved in the synthesis of each monoterpenoids, such as linalool synthase for linalool, limonene synthase for limonene, pinene synthase for pinene, and myrcene synthase for myrcene [37; 38] (Figure 1). Figure 1: Terpenoid biosynthesis in plants. There are two distinct pathways in plants for the synthesis of the universal precursors isopentenyl pyrophosphate (IPP) and dimethyl-allyl pyrophosphate (DMAPP): the cytoplasm-localized  
mevalonate (MVA) pathway and the plastid-localized methyl erythroid phosphate (MEP) pathway. The brown color indicates intermediate precursors of plastids (geranyl pyrophosphate) and cytosol (farnesyl pyrophosphate). AACT acetoacetyl-CoA thiolase; HMGCAS 3-hydroxy-3-methylglutaryl-CoA synthase; HMGR 3-hydroxy-3-methylglutaryl-CoA reductase; MK mevalonate-5-kinase; PMK phosphomevalonate kinase; MPD mevalonate-5-phosphate decarboxylase; MPPD mevalonate pyrophosphate decarboxylase; IPK isopentyl pyrophosphate kinase; FPS farnesyl pyrophosphate synthase.