Cuticular wax layer

The surface of the green coffee contains a cuticular wax layer (0.2—0.3% db) for both varieties. The wax contains insoluble hydroxytryptamides derived from 5-hydroxytryptamine [61 7-2] and saturated C18—C22 fatty acids. 

Polycyclic aromatic hydrocarbons (PAHs) are hydrophobic compounds which are absorbed by the cuticular wax layer, which acts as a trap for these pollutants. Plants experiencing PAEI exposure (for example, on the verges of roads carrying heavy traffic) often show an increased level of wax... 

Most plants have trichomes on their aerial surfaces. The trichomes may be simple hairs or more specialized glandular trichomes, whose main function may be the production and accumulation of chemicals such as essential oils. The vast majority of these consists of monoterpenoids, sesquiterpenoids and diterpenoids with a high vapour pressure. They may be absorbed on the cuticular wax layer. The trichome secretions are closely related to plant-insect or plant-microbe interactions. Terpenoids can attract, rep>el or initiate defence reactions in insects. Apart from their ecological roles, plant terpenoids are widely used in the pharmaceutical and fragrance industries. The properties of essential oils are correlated with their qualitative and quantitative compositions. 

Powdery mildew infection of wheat leaves was accompanied by increased production of cuticular waxes but did not result in consistent changes in the proportions of the component fractions. The amount of wax present per unit area of leaf surface of healthy plants was greater in resistant than in susceptible cultivars, while the partially resistant cultivar investigated was intermediate in this respect. Although exudates from Erysiphe graminis f sp. hordei have been shown to have esterase and cutinase activity which is likely to be of importance in the initiation of infection (Kunoh et a/., 1990 Nicholson et al, 1993), the present results support other evidence for the importance of the thickness of the cuticular wax layer in resistance to powdery mildew. 

Terrestrial BMOs have also been widely used for monitoring environmental contaminants. In particular, the lipid-like waxy cuticle layer of various types of plant leaves has been used to monitor residues of HOCs in the atmosphere. However, some of the problems associated with aquatic BMOs apply to terrestrial BMOs as well. For example, Bohme et al. (1999) found that the concentrations of HOCs with log KoaS < 9 (i.e., those compounds that should have attained equilibrium) varied by as much as 37-fold in plant species, after normalization of residue concentrations to levels in ryegrass (Lolium spp.). These authors suggested that differences in cuticular wax composition (quality) were responsible for this deviation from equilibrium partition theory. Other characteristics of plant leaves may affect the amount of kinetically-limited and particle-bound HOCs sampled by plant leaves but to a lesser extent (i.e., <4-fold), these include age, surface area, topography of the surface, and leaf orientation. 

The ability of insects to withstand desiccation was recognized in the 1930s to be due to the epicuticular layer of the cuticle. Wigglesworth (1933) described a complex fatty or waxy substance in the upper layers of the cuticle which he called cuticulin . The presence of hydrocarbons in this wax of insects was suggested by Chibnall et al. (1934) and Blount et al. (1937), and over the next few decades the importance of hydrocarbons in the cuticular wax of insects was established (Baker et al., 1963 and references therein). The first relatively complete chemical analyses of the hydrocarbons from any insect, the American cockroach, Periplaneta americana (Baker et al., 1963), occurred after the development of gas-liquid chromatography (GLC). The three major components of the hydrocarbons of this insect, //-pen taco sane, 3-methylpentacosane and (Z,Z)-6,9-heptacosadiene, represent the three major classes of hydrocarbons on insects, n-alkanes, methyl-branched alkanes and alkenes. Baker and co-workers (1963) were able to identify n-pentacosane by its elution time on GLC to a standard and its inclusion in a 5-angstrom molecular sieve. 3-Methylpentacosane... 

Beneath the epicuticular wax layer is the cuticle which bounds the epidermal cells and lines the sub-stomatal cavities. Structurally, it is noncellular and often multi-layered, comprising an inner region which merges with cellulose fibrils of the epidermal cell wall (cuticular layer, fibrillar in organization) (2). The chemical component of the cuticle proper is an... 

The surfaces of all higher plants are covered by a layer of cuticular waxes. These are composed mainly of long-chain aliphatic components but also of cyclic compounds. The primary role of the waxes is to prevent uncontrolled water loss. The chemical composition of plant cuticular waxes can affect the resistance of plants to herbivores and herbivore behaviour. Cuticular waxes and their separate components enhance or deter insect oviposition, movement or feeding. 

The surfaces of insects are also covered by a layer of wax. Insect cuticular waxes are also involved in various types of chemical communication between individuals of a species and reduce the penetration of chemicals and toxins as well as infectious microorganisms. Analyses and identification of insect waxes is the first step towards developing methods of insect control. 

Jetter, R. Schaffer, S. Riederer, M. (2000). Leaf cuticular waxes are arranged in chemically and mechanically distinct layers evidence from Prunus laurocerasus L. Plant, Cell and Environment, Vol.23, No.6, (June 2000), p>p. 619-628, ISSN 1365-3040... 

Cuticular waxes are removed from surface layers of intact tissues by washing with organic solvents such as hexane or chloroform. It is very important to use redistilled solvents particularly since compounds such as hydrocarbons are natural components of most waxes. Internal waxes are extracted from tissues following homogenization by the usual methods (Section 6.3.1). Waxes have also been extracted from suberin-rich barks (Martin and Juniper, 1970). 

Waxes are usually isolated by extracting the tissue with a nonpolar solvent such as chloroform or hexane. Cuticular waxes can be extracted by a quick dip into the solvent at room temperature, but suberin-associated waxes are more difficult to remove because they are embedded in the suberized cell wall (232). Wax from the suberized cells of bark has to be isolated by Soxhlet extraction of dried and powdered tissue to assure its complete removal. Isolated wax can be subjected to GC/MS analysis either after separation into various classes by thin-layer or column chromatography or directly after derivitization of the functional groups (232, 253, 459, 460). 

Free fatty alcohols are perhaps the most common components of cuticular waxes and often are also a major constituent of suberin-associated waxes (232). ThQr comprised between 10% and 45% of the waxes from the periderm of underground storage organs (116). The dominant chain lengths of free fatty alcohols of cuticular waxes are C26 and C28 (232, 253, 292, 460), but the free fatty alcohols of suberin-associated waxes usually have a shorter chain length. For example, the dominant alcohols in the leaf cuticular wax of Agave americana were (36% of the alcohols) and C28 (62%), whereas the dominant component in the fatty alcohols of the suberin-associated wax from the periderm layer surrounding the crystal idioblasts within the same leaf was C22 (88%) (117). The most common fatty alcohols reported as components of bark wax are C24 and C22. 

The examples presented in Figure 3 are just a few of the plants that have been identified as possessing superhydrophobic surfaces. Neinhuis and Barthlott reported the static WCA of 200 water-repellent plant species (Neinhuis and Barthlott, 1997). Most of these plants were classified as having superhydrophobic surfaces, as they exhibited a WCA greater than 150°. The common feature shared by these surfaces is that each of them possesses a very dense layer of 3D cuticular wax crystals arranged randomly or uniformly on their corresponding microscale surface features (papillae). 

Plants were probably the first to have polyester outerwear, as the aerial parts of higher plants are covered with a cuticle whose structural component is a polyester called cutin. Even plants that live under water in the oceans, such as Zoestra marina, are covered with cutin. This lipid-derived polyester covering is unique to plants, as animals use carbohydrate or protein polymers as their outer covering. Cutin, the insoluble cuticular polymer of plants, is composed of inter-esterified hydroxy and hydroxy epoxy fatty acids derived from the common cellular fatty acids and is attached to the outer epidermal layer of cells by a pectinaceous layer (Fig. 1). The insoluble polymer is embedded in a complex mixture of soluble lipids collectively called waxes [1], Electron microscopic examination of the cuticle usually shows an amorphous appearance but in some plants the cuticle has a lamellar appearance (Fig. 2). 

The cuticle, being attached to the epidermal cells via a pectinaceous layer, can be released by disruption of this layer by chemicals such as ammonium oxalate/oxalic acid or by pectin-degrading enzymes. After treatment of the recovered cuticular layer with carbohydrate-hydrolyzing enzymes to remove the remaining attached carbohydrates, the soluble waxes can be removed by ex-... 

The external cuticle of insects is covered by a waxy layer composed of mixtures of hydro-phobic lipids that include long-chain alkanes, alkenes, wax esters, fatty acids, alcohols, aldehydes, and sterols. The primary purpose of this layer is to maintain water balance and prevent desiccation, as described in Chapter 6, but many of the cuticular lipid components have important secondary roles as intraspecific contact chemical signals (pheromones). These roles include species and sex recognition during reproductive interactions, and nestmate recognition and other colony organization functions in social insects. Thus, these compounds are essential mediators of insect behaviors. Cuticular compounds are also exploited by parasitoids and predators as interspecific contact cues (kairomones) to aid in host location. 

Insect cuticular lipids consist of aliphatic material which is present on the outer layer of the integument. In most species, they consist of complex mixtures of hydrophobic compounds including straight-chain saturated, unsaturated, and methyl-branched hydrocarbons, wax esters, sterol esters, ketones, alcohols, aldehydes, and fatty acids (lj 4). In many insects, including the American cockroach (8) and the housefly (9), hydrocarbons are the predominant cuticular lipid component. 

Suberin impregnates the walls of cork cells and in layers covers bark, tubers, roots, wound peridorm and bundle sheaths of monocotyledons to provide a protective mantle impervious to liquids and gases. The associated waxes are not as well studied as those from cutin but do exhibit certain periodicities. The hydrocarbons have a broader distribution of chain lengths than the cuticular material, a predominance of shorter carbon chains and a higher proportion of even-length chains. In addition, no single alkane predominates usually several are present in similar proportions " . 

Suberin and the related polymer, cutin, function primarily as the structural components of barrier layers, which always have waxes associated with them (231, 232). Studies with isolated cuticular layers showed that the wax provides the major barrier to moisture diffusion Removal of the wax resulted in a 300- to 2000-fold increase in permeability (292, 398, 401). Similar studies with cuticular layers from the leaves of Citrus aurantium demonstrated that wax provides the... 

Cuticle constitutes the boundary between higher plants and their environment. Therefore, this layer might be expected to play an important role in the interaction of the plant with environmental factors. The plant cuticle is composed almost entirely of lipids and the role of some of these lipids in the interaction between plants and microbes has become clear in the recent years. In this brief review, we shall confine our discussion to two specific examples of such interactions a detrimental one with pathogenic fungi and a beneficial one with phyllospheric bacteria which might provide fixed nitrogen in return for the use of some of the cuticular components as the carbon source. In this context, we will deal only with the role of the insoluble lipid-derived polymer, cutin, but not the role of soluble waxes that are always constituents of the cuticle. 

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