## 5 Trichome Engineering to Increase Plant Resistance  
The preparation of trichome-specific EST databases did not only facilitate the discovery and characterization of genes in trichome biosynthetic pathways, but also made it more feasible to engineer the production of specific biocides in trichomes [147]. Most engineering strategies are not designed to obtain expression of a transgene exclusively in a target tissue, such as a trichome, and/or at specific moments, but make use of a construct in which the transgene is fused behind the general 35S promoter and is expressed either via stable transformation [15] or via virus-induced gene silencing [121, 162]. Although this can give rise to pleiotropisms [163], such strategies clearly can suffice to manipulate key metabolic steps in the target biocide's metabolic route. Cultivated tobacco, and its related wild species _Nicotiana sylvestris_ and _Nicotiana tomentosiformis_, produce diterpenes, exist in two forms: the macrocyclic cembranoids, including the cembratrien-diols (CBT-diols) and their precursors the cembratrien-ols (CBT-ols), and the bicyclic labdanoids. Cultivated tobacco and _N. tomentosiformis_ produce both labdanoids and cembranoids, while _N. sylvestris_ only produces the latter group of compounds. These diterpenes are produced in large amounts and, specifically, in the glandular capitate trichomes of the plant, and some of these labdanoids contribute to plant resistance to pests, making tobacco an ideal target for terpenoid metabolic engineering [147]. For example, in cultivated tobacco, downregulation of a trichome-specific CYP450, a CBT-ol hydroxylase, via antisense suppression, led to reduction of CBT-diol levels but promoted the levels of its insecticidal precursor CBT-ol, thereby increasing plant resistance to the red aphid (_Myzus nicotianae_) [164]. Thus, silencing genes can be used to increase levels of compounds with biocidal properties in trichomes, thereby enhancing a plant's resistance. The large amount of data collected on trichome-specific gene expression [16, 157, 158] also made it possible to tailor gene overexpression more specifically by using trichome-specific promoters instead of the 35S promoter. The promoter of the trichome-specific CBT-ol hydroxylase gene [164] was one of the first trichome-specific promoters that has been isolated [165] and in _N. sylvestris_ several _cis_-regulatory elements of a CBT-ol synthase promoter were identified, required for specific expression in the secretory cells of glandular trichomes [166] and the CBT-ol synthase promoters have been used to produce novel diterpenoids [167] and heterologous sesquiterpennes in _N. sylvestris_[74]. Moreover, from squash (_Cucurbita maxima_) [PERSON] _et al._[168] isolated the promoters of a protease inhibitor family and found that one of these was trichome-specific while [PERSON] _et al._[169] cloned the cotton fiber-specific _LTP3_ promoter and fused it to \(\beta\)-glucuronidase (GUS), and demonstrated that expression of this construct in transgenic tobacco plants indeed was specific for its trichomes. However, the possibilities to modify plant-pest interactions by altering trichome chemistry via herbivore- or pathogen-specific promoters have hardly been addressed. [PERSON] _et al._[54] characterized a trichome-specific linalool synthase, called MTS1, induced by wounding the plant defense-hormone JA and by spider mite feeding and, potentially, the promoters of such herbivore-inducible trichome-specific genes can be used to re-engineer trichome based resistance. Finally, [PERSON] _et al._[170] provided proof of this concept by demonstrating that expressing 7-epizingiberene synthase from _S. habrochaites_ fused to the MTS1 promoter [54], together with _Z-Z_-farnesyl-diphosphate synthase fused to the MKS1 promoter [111], specifically in the glandular trichomes of the cultivated tomato can improve resistance against herbivores, including whiteflies and spider mites.