## 4 Identification of Biochemical Pathways in Glandular Trichomes  
Interest in trichome-produced compounds, combined with technical breakthroughs in analytical equipment and the possibility of applying genomic approaches, has greatly increased the understanding of the biochemical pathways that operate in trichomes, as well as the products they generate. Sequencing of Expressed Sequence Tag (EST) libraries generated from mRNA from isolated trichomes has resulted in large databases, which, in combination with metabolite profile analysis of glandular trichomes and proteomics, has led to a much more detailed general insight into the biosynthesis of these specialized metabolites than obtained previously via chemical-analytical methods [16]. Based on analyses of EST databases, it has been suggested that trichomes operate mostly as a self-supporting system [9] and have highly active biochemical pathways for both primary and secondary metabolism [9]. In glandular trichomes of tomato and tobacco, genes encoding enzymes and proteins related to photosynthesis and carbon fixation are significantly expressed [149, 150], indicating that at least some of the carbon necessary for secondary metabolism can be fixed within trichome cells [22]. Earlier studies in cultivated tobacco had also indicated that glandular trichomes possibly can fix carbon and produce sugar and triterpenoids (_i.e._, duvatrienediol) independent from the rest of the plant, although a role for additional carbon imported from the tissues below the trichome cannot be excluded [151, 152]. In mint, however, photosynthesis-related genes were not expressed [153], indicating significant differences between plant species or between trichome types. It has been suggested that the total amount of secondary metabolites produced by glandular cells could be related to their capacity to fix carbon [154] since exudates from photosynthetically active glandular cells can constitute up to 20% of the leaf dry weight biomass in wild Solanaceae [118, 120] while exudates from species with leucoplasts instead of chloroplasts in their trichomes, like the Lamiaceae and Fabaceae, contribute less than 2% to the leaf dry weight [39, 62, 154, 155]. Leucoplasts are non-pigmented plastid-type organelles specialized for _de novo_ biosynthesis of (precursors for) the metabolites that often will be secreted [18]. However, photosynthetically active glandular trichomes are probably supplemented with carbon substrates (e.g., CO\({}_{2}\), sucrose, glucose) as well, because their own primary metabolism is most likely incapable of meeting the huge carbon demands [22, 154]. Besides carbon, other compounds, like nitrogen, phosphate and micronutrients, are required for metabolism as well. According to [PERSON] _et al._[9], import of amino acids into trichomes is minimal, begging the question of how trichomes acquire their nitrogen necessary for the large amount of proteins synthesized in trichomes [37, 139]. Possibly, nitrogen can be recycled by amino transferases in glandular trichomes [9], but it cannot be excluded that nitrogen and other essential substances are imported via the trichome stalk, as well. EST analyses have played an important role in identifying enzymes of trichome secondary metabolism, for instance, in the synthesis of geraniol [156] and eugenol [99] in basil; the synthesis of methyl ketones [41, 110], monoterenes [24] and sesquiterpenes [74, 157, 158] in _Solanum_ species; and, xanthohumol synthesis in hops [159]. In mint, the first species from which trichome-specific ESTswere sequenced, 35% of the sequences were estimated to be involved in secondary metabolism of which 25% in monoterpene biosynthesis [153].