## 3 Biosynthesis and Function of Glandular Trichome-Produced Compounds
### Phenylpropenes  
Like terpenoids, phenylpropanoids exhibit great structural diversity [92] and are emitted in significant amounts by plants, but both the quantity and the composition of the phenylpropanoid blend can markedly differ between species [93] and even cultivars [94]. Despite this structural diversity, three successive, very conserved, enzymatic conversions form the core of the phenylpropanoid biosynthetic pathway (Figure 2) [92]. The first committed step comprises the non-oxidativedeamination of phenylalanineine to trans-cinnamic acid, catalyzed by phenylalanine ammonia lyase (_PAL_). Next, trans-cinnamic acid is hydroxylated to para-coumaric acid by cinnamate 4-hydroxylane (_C4H_). Finally, para-coumaric acid is activated by 4-coumarate CoA ligase (_4 CL_), creating para-coumaroyl CoA, which is the general precursor for a wide range of products, including anthocyanins, flavonoids, lignin and phenylropenes [57, 92]. Together with terpenoids, the phenylropenes are the major constituent of essential oils, which are secreted from glandular trichomes of many Lamiaceae [62]. In basil, for instance, eugenol and methylchavicol were shown to be predominantly synthesized and stored in the glandular trichomes [39].  
Benzenoids, which are derived from trans-cinnamic acid by shortening of the side-chain [95, 96], do not appear to be emitted from foliar glandular trichomes in large amounts and/or by many plant species. For instance, [PERSON] _et al._[54] did not find evidence for production of methyl salicate in tomato glandular trichomes and glandular trichomes of alfalfa (_Medicago sativa_) and hop (_Humulus lupulus_) emit only small amounts of benzenoids [97]. In contrast, methyl cinnamate, which is produced by methylation of trans-cinnamic acid, is synthesized in significant amounts by glandular trichomes [98].  
Compared to the extensive knowledge on terpenoid biosynthesis, relatively little is known about the biosynthesis of eugenol, chavicol and their derivatives. The intermediate steps that follow after coumaric acid has been synthesized remain unclear, although an enzyme was identified in basil glandular trichomes that could catalyze the formation of eugenol by using coniferyl acetate and NADPH as substrates [99]. Furthermore, _O_-methyltransferases responsible for the last step in the formation of methylchavicol and methyleugenol have been characterized and were highly expressed in basil glandular trichomes [100].  
Phenylropenes are well known for their role in the attraction of pollinators. For example, methyleugenol from the orchid _Bulbophyllum cheiri_ was shown to attract several fruit fly species (Bactrocera spp.) for pollination [101]. Furthermore, although the evidence is limited, some studies suggest that eugenol may contribute to plant resistance by negatively affecting plant parasites. For example, application of synthetic eugenol caused mortality and repellency in 4 Coleopteran species [102]. Moreover, also nematodes appeared to be susceptible to eugenol [103], as well as some fungi such as _Cladosporium herbarum_ in which eugenol caused morphological deformations of the hyphae [104]. Taken together, it is clear that phenylropenes fulfill dual roles, both in defense against herbivores, as well as in attraction of pollinators.