## 3 Biosynthesis and Function of Glandular Trichome-Produced Compounds
### Terpenes  
With over 30,000 known structures, the terpenoids (or isoprenoids) represent the largest and structurally most diverse class of plant metabolites [59]. Terpenoids play important roles in primary plant metabolism, and provide the building blocks for pigments in photosynthesis (chlorophyll), for electron carriers in respiration (quinone) and for the phytohormones abscisic acid, cytokins, gibberellins, strigolactones and the brassinosteroids [60, 61]. The majority of terpenoids, however, are secondary metabolites and have functions related to plant defense [57]. Despite the immense variety of terpenoids, they are basically all assemblies of C5 isoprene units and produced in three consecutive steps, with a concomitant increase of their complexity and diversity. Since the biosynthesis of terpenoids has been reviewed extensively, we will only highlight the major biosynthetic steps here, for excellent reviews on this topic see e.g., [61, 62, 63]. In the cultivated tomato, terpenoids are produced in significant amounts by glandular type VI trichomes [24, 25]. The first committed step of terpenoid biosynthesis comprises the formation of the universal C5 \"building blocks\" isopentenyl diphosphate (IPP) and its isomer dimethylallyl diphosphate (DMAPP). Both IPP and DMAPP are produced via the plasidial 2-C-methyl-D-erythritol 4-phosphate (MEP) pathway from pyruvate and glyceraldehyde-3-phosphate (Figure 2) [64, 65]. Alternatively, IPP can be formed via the mevalonate (MVA) pathway from acetyl-CoA [66]. It has been suggested that the MVA pathway may partly occur in the peroxisomes, instead of the cytosol, but for tomato, this has not been shown [67]. Subsequent steps of terpenoid biosynthesis may take place at various subcellular locations, for instance, in the plastids, the (smooth) endoplasmic reticulum, mitochondria and/or the cytoplasm and, in line with this, different isoforms of the enzyme isopentenyl diphosphate isomerase (IDI), which catalyzes the isomerisation of IPP to DMAPP, can be found in the plastids, mitochondria and/or cytosol [68, 69, 70]. Furthermore, IPP and other terpenoid intermediates can also be shuttled between organelles [61, 69]. Evidence for transport of DMAPP to other cellular compartments is lacking, or perhaps DMAPP is not transported at all [69]. In tobacco, the presence of chloroplasts in trichomes was shown to be necessary for production of diterpenes [71], thereby confirming the importance of these organelles in terpenoid biosynthesis. In the second step of terpenoid biosynthesis, a single (C5) DMAPP serves as the substrate for successive head-to-tail condensations of one or more C5 IPP units. These linear chain elongation reactions are catalyzed by homo and/or heteromeric complexes of prenyltransferases [72]. Any of the intermediate products can be used as starting material for the synthesis of short (up to C20) isoprenyl diphosphates [61, 73]. Interestingly, while most isoprenyl diphosphates are generated only in the cis (\(Z\)or trans (_E_) conformation, some are produced in both isoforms [24, 74]. The head-to-tail condensation reactions lead to the formation of C10 (_E_)-geranyl diphosphate (GPP) and (_Z_)-neryl diphosphate (NPP), the C15 (_E_,_E_)-farnesyl diphosphate (FPP) and (_Z_,_Z_)-farnesyl diphosphate (_Z_,_Z_-FPP), the C20 (_E_,_E_)-geranylgeranyl diphosphate (GGPP) (Figure 2), and the longer oligoprenyl diphosphate (OPP; C25-45) and polyprenyl (C50-130) terpenoid precursor molecules. In the final step, the (_Z_)- or (_E_)-isopropyl diphosphates are converted into cyclic and acyclic terpenoids, catalyzed by a large enzyme family of terpene synthases (TPSs) [75, 76].