# Origin and Function of Structural Diversity in the Plant Specialized Metabolome
## 2 The Four Major Specialized Metabolome Pathways
### Terpenoids  
Terpenoids comprise the largest group of specialized metabolites in the plant kingdom [30; 31; 32]. All terpenoids are derived from the five-carbon building blocks, isopentenyl diphosphate (IPP) and dimethylallyl diphosphate (DMAPP), that are derived via either the mevalonic acid (MVA) or the plastitial 2-methyl-D-erythrid 4-phosphate (MEP) pathway [33] (Figure 2). In the MVA pathway, three acetyl-CoA molecules are condensed to form 3-hydroxy-3-methylglutaryl (HMG)-CoA. Following reduction in HMG-CoA to MVA, phosphorylation yields MVA diphosphate (MVAPP), from which a combined decarboxylation and dehydration results in the formation of IPP. Isomerization of IPP provides DMAPP [33]. However, most plant terpenoids are produced via the MEP pathway [34]. Here, decarboxylation of pyruvate acid is followed by a condensation with glyceraldehyde-3-phosphate, yielding 1-deoxy-D-xylulose-5-phosphate. The latter compound is then converted to MEP. Upon activation with cytidine triphosphate, a phosphoanhydride is formed of which the cleavage leads to 4-hydroxy-3-methyl-but-2-enyl diphosphate, which is then the immediate precursor of both IPP and DMAPP [33]. Dephosphorylation of IPP and DMAPP yields the herimetepenes (C\({}_{5}\)), whereas their concatenation provides geranyl  
Figure 1: Biosynthesis of polyketides and (un)saturated fatty acids. Dashed arrows indicate multiple conversions.  
diphosphate (GPP) from which the monoterpenes (C\({}_{10}\)) are derived. The further addition of IPP to GPP produces farnesyl diphosphate (FPP) that is central in sesquiterpene (C\({}_{15}\)) biosynthesis. Linking another FPP to IPP yields then geranylgeranyl diphosphate (GGPP), the precursor of the diterpenes (C\({}_{20}\)). All these coupling reactions occur via a head-to-tail condensation, yet the triterpenes (C\({}_{30}\)) result from joining, tail-to-tail, two FPP molecules. Dependent on the compound class, tetraterpenes (C\({}_{40}\)) are synthesized via tail-to-tail condensation of two GGPP molecules or via head-to-tail condensations [33].  
In plants, terpenoids play a role in plant growth and development as photosynthetic pigments (e.g., carotenoids, C\({}_{40}\)), electron-carriers (e.g., ubiquinone, C\({}_{40}\)), regulators of growth and development (e.g., gibberellins, C\({}_{20}\)) or as part of membrane structures (e.g., phytosterols, C\({}_{30}\)) [35; 36]. In addition, volatile, low-molecular weight terpenoids (C\({}_{5}\)-C\({}_{10}\)) are involved in the protection of plants against biotic (e.g., microbial pathogens) and abiotic  
Figure 2: Biosynthesis of terpenoids. Dashed arrows indicate multiple conversions.  
(e.g., thermal stress) stress above- and belowground [37; 38]. Furthermore, volatile terpenoids, as constituents of floral scent, play a role in the attraction of plant pollinators [39]. Maize produces a range of non-volatile terpenoids that are elicited in response to biotic attack [40]. For example, phytoplankton, kauralexins and zealexins (C\({}_{15}\)) play a role in maize seedling resistance to _Fusarium verticillioides_[41]. Furthermore, herbivory-damaged maize roots release volatile sesquiterpenes that attract entomopathogenic nematodes which are able to infest the attacking larvae [42]. Herbivore attack of poplar leaves induced the emission of a complex mixture of volatile terpenoids (e.g., (E)-\(\beta\)-ocimene and (E,E)-\(\alpha\)-farnescene) and GLVs aboveground [43]. In addition, cockchafer larvae-damaged roots of _Populus trichocarpa_ and _Populus nigra_ release a mixture of monoterpenes, including \(\beta\)-pinene, camphene and 1,8-cineole [44].