Epicuticular lipids

Several deciduous broadleaved trees have leaf waxes containing triterpenoids in addition to the wax lipids. Epicuticular leaf waxes of T.fomentosa and T. x europaea showed triterpenoids in amounts of 56% and 4H%. Identified could be only one triterpenol B-amyrin. in these leaf waxes B-amyrin was found free as well as esterified with long chain fatty acids and also with acetic acid. B-Amyrin acetate was the dominating wax component with about 3 +% of the wax. In Tilia waxes triterpenones could not be detected. In both Tilia species the same triterpenoids were analysed with one significant dominating main component B-amyrin acetate (U). 

Burke, B. A., Goldsby, G. and, Mudd, J. B. 1987. Polar epicuticular lipids of Lycopersicon pennellii. Phytochemistry 26 2567-2571. 

Chen, X., and B. R. T. Simoneit, Epicuticular Waxes from Vascular Plants and Particles in the Lower Troposphere Analysis of Lipid Classes by Iatroscan Thin-Layer Chromatography with Flame Ionization Detection, . /. Atmos. Chem., 18, 17-31 (1994). 

Schulten, H.-R., Simmleit, N, and Rump, H. H. (1986). Soft ionization mass spectrometry of epicuticular waxes isolated from coniferous needles. Chem. Phys. Lipids 41, 209-224. 

The eggs of B. germanica contain the same types of hydrocarbons as the hemolymph, HDLp, and cuticle of the adult female. Only 150 pg of hydrocarbons accumulate on the epicuticular surface whereas up to 450 pg accumulate within the female during the period of egg maturation (Fan et al., 2002). The internal hydrocarbons are divided primarily between the ovaries, fat body, and 150 pg of HDLp-bound hydrocarbons in the hemolymph. During oocyte maturation ovarian hydrocarbons increase by more than 65-fold - from 3.5 pg on day-1 to 232 pg on day-8 (Fan et al., 2002). However, after oviposition on day-9, ovarian hydrocarbons decline to only 8.2 pg, demonstrating that hydrocarbons were associated with the ovulated oocytes. Radiotracing results indicate that they serve as components of the cuticular lipids of the embryos and first instars (Fan and Schal, unpublished results). 

Hadley, N.F. and Jackson, L.L. (1977). Chemical composition of the epicuticular lipids of the scorpion, Paruroctonus mesaensis. Insect Biochem., 7, 85-89. 

Baker, J.E., Woo, S.M., Nelson, D. R and Fatland, C.L. (1984). Olefins as major components of epicuticular lipids of three Sitophilus weevils. Comp. Biochem. Physiol., 77B, 877-884. 

As stated by Blomquist et al. (1998) in their chapter, the line of demarcation between glandular or cuticular release of semiochemical signals is not always clear . This statement echoes an earlier one by Blum (1985), who reported that insect exocrine glands consisting of modified epidermal cells located throughout the body could perform de novo biosynthesis and secretion of behavioral chemicals. Later, Blum (1987) put forth a unified chemoso-ciality concept proposing that epicuticular lipids carried numerous exocrine compounds and that the cuticle could be compared to a thin layer phase. Nevertheless, it is known that in various non-social insects epicuticular hydrocarbons are synthesized by modified cells often associated with the epidermis, the oenocytes (see above), and that these oenocytes can be located in several sites within insects. 

Transpiration through the cuticle involves more than just the single step of diffusion through the epicuticular lipid layer. Molecules of water must leave the tissues adjacent to the cuticle, diffuse through the cuticle itself, enter the lipid layer, diffuse across the lipids, and enter the gas phase outside the animal. Each step is likely to be affected by temperature to a different extent. Lipid composition and physical properties can also differ from one region of the cuticle to the next, so that the biophysical details of cuticular transpiration may not be homogeneous across the entire animal. Thus, transpiration at the organismal level involves multiple steps, and parallel routes for water flux. 

Eigenbrode, S.D. and Espelie, K.E. (1995). Effects of plant epicuticular lipids on insect herbivores. Annu. Rev. Entomol., 40,171-194. 

Hadley, N.F. (1978). Cuticular permeability of desert tenebrionid beetles correlations with epicuticular lipid composition. Insect Biochem., 8, 17-22. 

Toolson, E.C. and Hadley, N.F. (1979). Seasonal effects on cuticular permeability and epicuticular lipid composition in Centruroides sculpturatus Ewing 1928 (Scorpiones Buthidae)../. Comp. Physiol. B, 129, 319-325. 

Caputo, B Dani, F. R Home, G.L., N Fale, S Diabate, A., Turillazzi, S., Coluzzi, M Costantini, C Priestman, A. A., Petrarca, V. and della Torre, A. (2007). Comparative analysis of epicuticular lipid profiles of sympatric and allopatric field populations of Anopheles gambiae s.s. molecular forms and An. arabiensis from Burkina Faso (West Africa). Insect Biochem. Mol. Biol., 37, 389-398. 

Turillazzi, S Sledge, M.F., Dapporto, L., Landi, M Fanelli, D Fondelli, L., Zanetti, R and Dani, F.R. (2004). Epicuticular lipids and fertility in primitively social wasps (Hymenoptera Stenogastrinae). Physiol. EntomoL, 29,464-471. 

Important solvent properties are volatility, viscosity, surface tension, and lipid solubility. The first three determine the area over which a given volume of solvent spreads the larger the area of contact between insecticide and outer cuticle layers, the larger its total penetration rate will be. Acetone does not spread very far from the site of application, because it is so volatile. Lipid solubility affects the dissolution of the wax components of the epicuticle. By disrupting this layer, e.g., depositing a drop of acetone, the insecticide could bypass the epicuticular barrier. All these effects together may explain why an optimal balance of solvent properties is necessary to obtain maximal penetration rates (Welling and Patterson, 1985). 

In plant tissues Cieo can undergo some other modifications (Figure 3), namely elongation, hydroxylation, oxidation, epoxydation, reduction, oxidative decarboxylation, etc. As a result of these modifications many different lipophilic substances are produced. Among these substanees very long-chain FAs (VLCFAs, C>2o), different unusual FAs (hydroxy-, epoxy-, acetylenic, dicarboxylic), fatty aldehydes and alcohols, hydrocarbons, oxilipins, etc. are formed. Some of them are present in plants in free form (are embedded in the complex cuticular lipid matrix or as a components of epicuticular waxes), the others are used as a substrates for more complex lipids and lipid polymers biosynthesis (see below). 

The waxy material on the surface of the above-ground parts of plants, such as the skins of fruit and leaves of vegetables, seeds and poUen grains, is called surface or epicuticular wax. The next hydrophobic layer of the cuticle is cutin, which is a polymeric material built from hydroxy fatty acids. Wax and cutin together constitute the epicuticular lipids. In most plants, wax is not associated with cutin at all. On the surface of the underground... 

All aerial organs of higher plants are covered primarily with a thin continuous wax layer. These surface or epicuticular waxes consist of a very complex mixture of different components. In most cases these very long chained lipids are found in form of homologous series. The composition of the wax lipids shows species specific and also organ specific patterns. But numerous plants in addition contain triterpenoids,mostly pentacyclic compounds. The composition of triterpenoids from two Euphorbia species and from the leaf waxes of the trees Citrus halimii, Tilia tomentosa and Tilia x europaea will be summarized in this paper (Table 1). 

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