## 4 Volatile Aroma Compounds Biosynthetic Pathways and Related Enzymes
### Terpenoids Pathway  
The terpenoids compose the largest class of plant secondary metabolites with many volatile representatives. Hemiterpenes (C\({}_{5}\)), monoterpenes (C\({}_{10}\)), sesquiterpenes (C\({}_{15}\)), homoterpenes (C\({}_{11}\) and C\({}_{16}\)), and some diterpenes (C\({}_{20}\)) have a high vapor pressure allowing their release into the atmosphere. As shown in Scheme 3, terpenids are derived from the universal C\({}_{5}\) precursor isopentenyl diphosphate (IPP) and its allylic isomer dimethylallyl diphosphate (DMAPP), which in higher plants are generated from two independent pathways located in separate intracellular compartments. In cytosol, IPP is derived from the long-known mevalonic acid (MVA) pathway that starts with the condensation of acetyl-CoA [151]. In plastids, IPP is formed from a MVA-independent pathway (or MEP pathway) with pyruvate and glyceraldehyde s-phosphate as direct precursors and methylerythritol phosphate (MEP) as the key intermediate [152]. Initial research indicated that the cytosolic IPP serves as a precursor of farnesyl diphosphate (FPP) for sesquiterpenenes and triterpenenes, whereas the plastidial IPP provides the precursors for geranyl diphosphate (GPP) and GGPP for mono-, di-, and tetra-terpenes. However, cross-talk between these two IPP biosynthetic pathways is prevalent, particularly in the direction from plastids to cytosol [153,154]. **Scheme 2. ** Biosynthetic routes for amino acid degradation to volatiles in plants and microorganisms. In plastids, DMAPP generated from the MEP pathway is used by isoprene synthases for isoprene formation [155,156]. In the cytosol and in plastids, IPP and DMAPP are used by prenyltransferases to produce prenyl diphosphates. In cytosol, the condensation of two molecules of IPP and one molecule of DMAPP catalyzed by the enzyme farnesyl pyrophosphate synthase (FPPS) results in the formation of FPP (C\({}_{15}\)), the natural precursor of sesquiterpenenes [157]. In plastids, a head-to-tail condensation of one molecule of IPP and one molecule of DMAPP catalyzed by geranyl pyrophosphate synthase (GPPS) forms GPP (C\({}_{10}\)), the universal precursor of all the monoterenes [158]. The condensation of one molecule of DMAPP with three molecules of IPP by the action of geranylgeranyl pyrophosphate synthase (GGPPS) yields GGPP, the C\({}_{20}\) diphosphate precursor of diterpenes [158,159]. Following the formation of the acyclic precursors GPP, FPP, and GGPP, a wide range of structurally diverse cyclic and acyclic monoterpenes, sesquiterpenes, and diterpenes is generated through the action of a large family of enzymes known as terpene synthases/cyclases (TPSs) [160,161]. One of the most outstanding properties of these enzymes is their proclivity for making multiple products from a single prenyl diphosphate substrate [162]. **Scheme 3. ** The biosynthesis pathway of isoprenoids in plant cell. DMAPP, dimethylallyl diphosphate; DXP, 1-deoxy-D-xylulose-5-phosphate; DXS, DXP synthase; DXR, DXP reductoisomerase; FPP, farnesyl diphosphate; G3P, glyceraldehyde 3-phosphate; GPP, geranyl diphosphate; GGPP, geranyl geranyl diphosphate; HMGR, HMG-CoA reductase; IPP, isopentenyl diphosphate; MEP, methylerythnitol phosphate; MVA, mevalonic acid. Many of the terpene volatiles are direct products of terpene synthases, while others are formed through alterations of the primary terpene skeletons made by TPSs by hydroxylation, dehydrogenation, acylation, and other reaction types [124]. 3-Hydroxylation of limonene by a P450 enzyme yields _trans_-isopiperitenol, a volatile compound found in mint (_Mentha aquatic_) [163], while 6-hydroxylation of limonene by another P450 enzyme results in the formation of _trans_-carveol, which undergoes further oxidation by nonspecific dehydrogenases to form carvone, a major aroma volatile of caraway fruits (_Carum carvi_ L.) [164,165]. Another monoterpene alcohol, geraniol, was also found to be converted to the corresponding aldehyde by dehydrogenases in the glands of sweet basil [166].