## 3 Biosynthesis and Function of Glandular Trichome-Produced Compounds
### Acyl Sugars  
Sugar esters, also called acyl sugars, are nonvolatile metabolites, produced [118] and stored in glandular trichomes of many Solanaceae, including _Solanum, Nicotiana_, _Datura_[42] and _Petunia_ species [119]. These compounds are conjugates of sugars and aromatic or aliphatic fatty acids and a significant fraction of these are exuded onto the surface of aerial organs, in the case of the wild tomato _Solanum pennellii_ up to 20% of the plant's leaf dry weight [120]. Acyl sugar biosynthesis is especially well studied in tomato [118,121] and tobacco [118,122] species. The backbone of acyl sugars consist of a sugar, predominantly sucrose or glucose, or sometimes a sugar-alcohol, predominantly sorbitol of xylitol, to which one or more straight or branched chain fatty acids, which are usually methyl-branched, are esterified. Depending on the number of acyl groups, _i.e._, the free hydroxyl groups in the sugar, most of these sugar esters are mono-, di- or tri-acyl sugars [35] and are formed via _O_-acylation. For example, type IV glandular trichomes of _S. pennellii_ exude a mixture of 2,3,4-_O_-tri-acyl-glucose [123], 3',3,4-_O_-tri-acyl-sucrose and 3',3,4,6-_O_-tetra-acyl-sucrose polyesters, which have both straight and branched chains, ranging in length from 2 to 12 carbons, that are formed prior to acetylation to glucose and sucrose [9,124,125]. The branched- or straight-chained fatty acid acyl moieties of the glucose esters of _S. pennellii_ are derived from branched-chain amino acids (_i.e._, Val, Leu, and Ile) [124]. In _Solanum_ and _Datura_ species, elongation of fatty acids is mediated via fatty acid synthase (FAS), while in tobacco and petunia this elongation occurs via \(\alpha\)-ketoacid elongation [42]. Biosynthesis and elongation of branched fatty acids involves the branched-chain keto acid dehydrogenase (BCKD) protein complex which generates activated acyl-CoA esters from branched-chain keto acid precursors [118] but how these acyl-CoA esters are exactly used for synthesis of acyl sugars is still unclear [121]. In _S. pennellii_, the acylation steps require sequential action of a glucosyl transferase, which forms the first acyl sugar intermediate, and an acyl transferase that catalyzes the further additions of fatty acids to the backbone [126,127]. Finally, also an acyltransferase (_AT2_) has been identified that catalyzes the transfer of the acetyl group found in the tetra-acyl sucoses of the cultivated tomato [121]. Expression of _AT2_ was shown to be specific for the tip cells of type IV glandular trichomes of an _S. lycopersicum x S. penelli_ introgression line [121]. Acyl sugars may be directly toxic to herbivores, but they are also excellent emulsifiers and surfactants and may easily stick to arthropod cuticles thereby immobilizing or suffocating arthropods [1,35]. [PERSON] _et al._[1] reported that aphids upon contact with tobacco trichomes are rapidly \"coated\" by trichome-produced sugar esters, thereby entrapping the insect and preventing it from further moving around. Staining with Rhodamine B revealed that the highest concentrations of sugar esters are present at the joints of the aphid's antennae and legs where entry of toxins into the body is likely to occur most easily [1]. Also, it was shown that acyl sugars can deter or repel herbivores, such as the potato aphid. Structure and activity studies revealed that acyl glucoses and acyl sucroses were equally repellent to the aphid and differences in the length of the fatty acid chain did not influence repellency [128]. However, according to [PERSON] _et al._[35] the toxic properties of synthetic acyl sugars depend both on sugar backbone and fatty acid chain length, and different acyl sugars caused different mortalities in pear psyllids (_Cacopsyllla pyricola_), tobacco aphids (_Myzus nicotianae_), tobacco hornworms and spider mites. Furthermore, in tomato the density of glandular trichomes and the amount of acyl sugars were shown to correlate with resistance to whiteflies and spidermites [129, 130, 131].