ABSTRACT
The plant cuticle, which covers virtually all the aerial surfaces of vascular plants, is chemically heterogeneous in nature, because it is generally composed of a mixture of insoluble polymers and waxes, which are deposited on the outer surface (epicuticular waxes) and embedded in the matrix (intracuticular waxes). The soluble cuticular waxes consist of monoesters of very long chains of fatty acids (C-18-C-22), alcohols (C-22-C26), and a wide variety of other aliphatic and aromatic molecules [1-4]. The insoluble matrix is constituted by (1) cutin, a three-dimensional polymer network of esterified fatty acids [1,5] and (2) cutan, a nondegradable biopolymer made of aliphatic chains presumably cross-linked by ether bonds [6]. In addition, the cuticular layers also contain nonlipid components such as polysaccharides, phenols, and flavonoids. Large variations have been found in cuticular fine structure, wax load, or cutin composition. Indeed, the structure of the cuticle varies considerably depending on the species and on the age of the plant, but also between leaves, fruits, and stems of a single species and even between the upper and lower surfaces of individual leaves [7]. This complexity may explain the wide diversity of functions fulfilled by the cuticular layer. As an interface between the plant and its environment, the cuticle plays a key role in providing protection from mechanical damage, ultraviolet (UV) radiation [8,9], or penetration by fungal hyphae and insect mouthparts [10]. It also constitutes the main barrier limiting the transport across the plant-atmosphere exchange zone, impeding the foliar uptake of xenobiotics but also reducing the uncon-trolled loss of water and apoplastic solutes from plant tissues [11,12]. It seems that waxes are largely responsible for the low permeability of cuticles, causing a lognormal distribution of transport parameters [13] and therefore contributing for example to the regulation of the cuticular transpiration. Permeability of cuticles differs greatly among plant species and it has been argued that these differences are more likely due to the physical arrangement of waxes, rather than their amounts or composition per se [14]. On the other hand, if the constitutive waxes markedly inhibit sorption of a wide range of organic compounds, they may not always form the main barrier to the penetration of chemicals through plant cuticles [15]. They do favor the penetration of biologically active compounds by acting as a compartment in which lipophilic compounds can accumulate [16]. Moreover, these lipids reduce water retention on the plant surface, which bears important consequences on germination of spores, survival of
microorganisms, or deposition of dust, pollen, and so forth. Additionally, the chemical composition of epicuticular waxes may influence the interaction of herbivorous insects, with their plant hosts [17,18] acting as well in tritrophic interactions, by affecting the settling and oviposition behavior of some predators and parasitoids of these herbivores [18]. It was also reported that waxes found in the tryphine layer of pollen grains are essential for proper pollen-stigma signaling required for fertilization [19].
