Plant surface lipids

Oxidation of the methyl terminal carbons of fatty acids is important in formation of the polyfunctional fatty acid components of plant surface lipids, cutins and suberins. This is covered in detail by Kolattukudy (this volume. Chapter 18). 

Long chain aliphatic IB-diketones ind alkan-2-ol esters are common constituents of plant surface lipids. The B-dicarbonyl pattern, two carbonyl groups 1,3 one another, has suggested that the biosynthesis of I3-diketones involves condensation of propriate B-ketoesters and esters, via a biological Claisen reaction qihao... 

Jetter, R., Kunst, L., 2008. Plant surface lipid biosynthetic pathways and their utility for metabolic engineering of waxes and hydrocarbon biofuels. Plant J. 54, 670. 

The chemical constituents in the cotton plant have been extensively studied. Volatile or steam distillable compounds present in cotton buds (245-253) and leaves (254, 255, 256) have been extensively cataloged. A number of the surface lipids have been identified (257). Two anthocyanins, cyanldin-3-8-glucoside (231, 258) and pelargonldin (259), and the flavonols and flavonol glycosides, quercetin, kaempferol, isoquercltrln, quercetin-7-... 

Surface lipids of plants. The thick cuticle (Fig. 1-6) that covers the outer surfaces of green plants consists largely of waxes and other lipids but also contains a complex polymeric matrix of cutin (stems and leaves) or suberin (roots and wound surfaces).135/135a Plant waxes commonly have C10 - C30 chains in both acid and alcohol components. Methyl branches are frequently present. A major function of the waxes is to inhibit evaporation of water and to protect the outer cell layer. In addition, the methyl branched components may inhibit enzymatic breakdown by microbes. Free fatty acids, free alcohols, aldehydes, ketones, 13-dike tones, and alkanes are also present in plant surface waxes. Chain lengths are usually C20 - C35.136 Hydrocarbon formation can occur in other parts of a plant as well as in the cuticle. Thus, normal heptane constitutes up to 98% of the volatile portion of the turpentine of Pin us jeffreyi.81... 

The leaves of higher plants contain upto 7% of their dry weight as fats some of which are present as surface lipids, the others as components of leaf cells, especially in the chloroplast membrane. The fatty acid composition of plant membrane lipids is very simple. Six fatty acids- palmitic, palmitoleic, stearic, oleic, linoleic and y-linolenic generally account for over 90% of the total fatty acids. 

Plants produce the majority of the world s lipids, and most animals, including humans, depend on these lipids as a major source of calories and essential fatty acids. Like other eukaryotes, plants require lipids for membrane biogenesis, as signal molecules, and as a form of stored carbon and energy. In addition, leaves and other aerial surfaces, bark, herbaceous shoots, and roots each have distinctive protective lipids that help prevent desiccation and infection. To what extent does the biochemistry of plant lipid metabolism resemble that in other organisms This chapter mentions a number of similarities, but emphasizes aspects unique to plants. Major differences between lipid metabolism in plants and other organisms are summarized in Table 1. 

To link these various scales, it is necessary to recognize both that each level of conifer - bark beetle - fungal interaction is characterized by a discrete threshold, and that the outcome at each level depends on feedback among multiple variables (Table 4.4). For example, a beetle can either enter or not enter a tree. However, that discrete outcome is determined by monoterpene and phenolic concentrations and composition, beetle age, the number of rejections already made by a beetle, beetle lipid content, beetle density on the plant surface, beetle genotype, beetle population phase, and presumably other factors. Similar relationships characterize thresholds at the levels of aggregation, host establishment, and population eruption (Table 4.4). 

Surface lipids in the form of waxes have much to do with limiting drying rates to values below lethality. Plants have a water barrier composed of cutin, embedded waxes and pectin. Insects and other arthropods have embedded wax in their outer chitin layer. Amphibians have skin that is very permeable to water movement in order to absorb water from their environments. When spending time in the sun or in the ground, however, they either spread exuded lipids on their skins, or else form an impermeable skin layer. Reptiles use keratin as their principal water barrier. Mammals have a thin lipid film that covers their outer skin layer, the stratum corneum (Hadley, 1980). 

Surface waxes serve a number of functions usually associated with protection. In plants and insects they prevent desiccation and in birds they serve to waterproof feathers. While a few components present in surface lipids can prevent growth of pathogens, the total surface wax layer certainly functions to prevent microbial entry into the organism. Some surface lipids serve as chemical communicants such as the hydrocarbon sex attractants and kairomone of insects. Although internal waxes are infrequently found in Nature, where they do occur they act as energy storage (e.g. in jojoba seeds or marine organisms). For a fuller review of all aspects of natural waxes refer to Kolattukudy (1976). 

Figure 6.12 Chromatography and mass spectrometry of plant waxes. (1) Thin-layer chromatogram of broccoli leaf wax and gas-liquid chromatogram of the hydrocarbon fraction isolated from this wax. (2) Partial gas-liquid chromatogram of the alkane fraction from the surface lipids of tobacco leaves. (3) Partial gas-liquid chromatograms of the hydrocarbons from N. muscorum illustrating resolution of 8- (a), 7- (b) and 6- (c) monomethyl heptadecanes on a 750ft capillary column. (A)-(C) Partial mass spectra taken at points indicated in (2) illustrating identification of n- and branched alkanes. Reproduced with permission from Kolattukudy (1980).

Our knowledge of lipolytic enzymes in plants is meager. This is surprising, perhaps, when the commercial importance of seed oils and other plant lipids is considered. In addition to the obvious importance of lipid metabolism in oil seed crops, the involvement of storage and membrane lipids in plant biochemistry and its applications should be backed by a much better understanding of lipid catabolism. The surface lipids of all plants have been almost totally neglected in this respect until the recent developments described by Kolattukudy (this volume. Chapter 18). 

Alkanes and alkenes containing about 30 C-atoms are constituents of plant cuti-cular waxes and the surface lipids of microorganisms and animals (D 3.2.4). These waxes play an important role in the hardiness and in the water economy of plants (E 2.2, E 5.4). Undecane, CH3(CH2)9CH3 tridecane, CH3(CH2)nCH3 and pentadecane, CH3(CH2)i3CH3 are alarm pheromones of ants. 7,8-Epoxy-2-methyl-octadecane is a sex pheromone of Lymantria dispar. In brown algae... 

Plant lipids are of two main types structural and storage. The structiu lipids are present as constituents of various membranes and protective surface layers and make up about 7 per cent of the leaves of higher plants. The surface lipids are mainly waxes, with relatively minor contributions from long-chain hydrocarbons, fatty acids and cutin. The membrane lipids, present in mitochondria, the endoplasmic reticulmn and the plasma membranes, are mainly glycolipids (40-50 per cent) and phosphoglyc-erides. Plant storage lipids occur in fruits and seeds and are, predominantly, triacyl-glycerols. Over 300 different fatty acids have been isolated from plant tissues, but only about seven are of common occurrence. The most abundant is a-linolenic acid the most common saturated acid is palmitic acid and the most common monounsaturated acid is oleic acid. 

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