## 3 Biosynthesis and Function of Glandular Trichome-Produced Compounds
### Methyl Ketones  
Methyl ketones constitute a class of fatty-acid derived volatile compounds that are very effective in protecting plants against pests [30]. Methyl ketones that are commonly found in plants have 7 to 15 carbons and include 2-heptanone, 2-nonanone, 2-undecanone, 2-tridecanone and 2-pentadecanone [41]. In _S. habrochaites_, methyl ketone biosynthesis was shown to proceed in two steps. The first step comprises the hydrolysis of 3-ketoacyl-acyl carrier protein intermediates, produced during fatty acid biosynthesis in chloroplasts (Figure 2). This step is catalyzed by an enzyme identified as methyl ketone synthase 2 (MKS2) [110, 111]. The resulting 3-ketoacids are then decarboxylated in a reaction that is catalyzed by MKS1 [41, 111].  
In the 1980s, 2-tridecanone was identified as the major constituent of type VI trichomes of the wild tomato _S. habrochaites_ f. _glabratum_[112]. Methyl ketones in this species were found in concentrations between 2700 and 5500 \(\mu\)g per g fresh weight, whereas the cultivated tomato also contains 2-tridecanone, but in much smaller amounts, of up to 80 \(\mu\)g per g fresh weight [113]. [PERSON] _et al._[112] demonstrated that 2-tridecanone was lethal to several herbivorous arthropods, including the tobacco hornworm (_Manduca sexta_) and the cotton aphid (_Aphis gossypii_). Tomato fruitworm (_Helicoverpa zea_) larvae were shown to be killed by the fume of _S. habrochaites_ f. _glabratum_ and by pure 2-tridecanone [114]. [PERSON] _et al._[115] showed that methyl ketones are toxic to the two-spotted spider mite upon contact. Trichome exudates and 2-tridecanone applied on artificial membranes inhibited feeding and caused mortality of the potato aphid (_Macrosiphum euphorbiae_) [116]. A second methyl ketone from tomato, identified as 2-undecanone [117], appeared to be less toxic since it did not negatively affect the potato aphid [116] nor did it cause larval mortality in the tobacco hornworm [117]. 2-undecanone did, however, cause increased mortality in the two-spotted spider mite [115] and it also increased mortality of pupae of the tomato fruitworm, and even more so when larvae of this species were reared on an artificial diet containing both 2-tridecanone and 2-undecanone [117].  
Figure 2: Simplified schematic overview of the biosynthesis of the main secondary metabolites stored and/or secreted by tomato glandular trichome cells. Major pathway names are shown in red, key enzymes or enzyme complexes in purple, and stored and/or secreted compounds in blue. Metabolic routes are projected onto their subcellular location, however final modification reactions (e.g., glycosylations, acylations, methylations, hydroxylations), which can take place at various organelles, are not shown for clarity. Abbreviations used: 4 CL, 4-coumarate CoA ligase; ACP, acyl carrier protein; BCKD, branched-chain keto acid dehydrogenase (multi-enzyme complex); C4H, cinnamate 4-hydroxylase; CoA, coenzyme A; DMAPP, dimethylallyl diphosphate; DTS, diterpene synthase; E4P, erythrose 4-phosphate; ER, endoplasmic reticulum; FAS, fatty acid synthesis, FPP, farnesyl diphosphate; GA3P, glyceraldehyde 3-phosphate; GGPP, geranylgeranyldiphosphate; GPP, geranyldiphosphate; IPP, isopentenyl diphosphate; Leu, leucine; the non-mevalonate pathway, also known as the 2-C-methyl-D-erythritol 4-phosphate (MEP) or 1-deoxy-D-xylulose 5-phosphate (DOXP) pathway; MTS, monoterpene synthase; MVA pathway, mevalonate pathway; NPP, neryldiphosphate; PAL, phenylalanine ammonia lyase; PEP, phosphoenolpyruvate; Phe, phenylalanine; STS, sesquiterpene synthase; Val, valine. Solid black arrows indicate established biochemical reactions. Dashed black arrows indicate hypothetical reactions. A single arrow does not necessarily represent a single enzymatic conversion.