The metabolism of plant and insect waxes

Plant surface lipid biosynthetic pathways and their utility for metabolic engineering of waxes and hydrocarbon biofuels

Due to their unique physical properties, waxes are high-value materials that are used in a variety of industrial applications. They are generated by chemical synthesis, extracted from fossil sources, or harvested from a small number of plant and animal species. As a result, the diversity of chemical structures in commercial waxes is low and so are their yields. These limitations can be overcome by engineering of wax biosynthetic pathways in the seeds of high-yielding oil crops to produce designer waxes for specific industrial end uses. In this review, we first summarize the current knowledge regarding the genes and enzymes generating the chemical diversity of cuticular waxes that accumulate at the surfaces of primary plant organs. We then consider the potential of cuticle biosynthetic genes for biotechnological wax production, focusing on selected examples of wax ester chain lengths and isomers. Finally, we discuss the genes/enzymes of cuticular alkane biosynthesis and their potential in future metabolic engineering of plants for the production of renewable hydrocarbon fuels.

Sealing plant surfaces: cuticular wax formation by epidermal cells

The vital importance of plant surface wax in protecting tissue from environmental stresses is reflected in the huge commitment of epidermal cells to cuticle formation. During cuticle deposition, a massive flux of lipids occurs from the sites of lipid synthesis in the plastid and the endoplasmic reticulum to the plant surface. Recent genetic studies in Arabidopsis have improved our understanding of fatty acid elongation and of the subsequent modification of the elongated products into primary alcohols, wax esters, secondary alcohols, and ketones, shedding light on the enzymes involved in these pathways. In contrast, the biosynthesis of alkanes is still poorly understood, as are the mechanisms of wax transport from the site of biosynthesis to the cuticle. Currently, nothing is known about wax trafficking from the endoplasmic reticulum to the plasma membrane, or about translocation through the cell wall to the cuticle. However, a first breakthrough toward an understanding of wax export recently came with the discovery of ATP binding cassette (ABC) transporters that are involved in releasing wax from the plasma membrane into the apoplast. An overview of our present knowledge of wax biosynthesis and transport and the regulation of these processes during cuticle assembly is presented, including the evidence for coordination of cutin polyester and wax production.

Effect of triacontanol on plant cell cultures in vitro

Triacontanol [CH(3)(CH(2))(28)CH(2)OH] increased growth in vitro of cell cultures of haploid tobacco (Nicotiana tabacum). The fresh weight of cell cultures of tomato (Lycopersicon esculentum), potato (Solanum tuberosum), bean (Phaseolus vulgaris), and barley (Hordeum vulgare x H. jubatum) was also increased. The increase in growth of tobacco callus seems to have been due to an increase in cell number. Another long chain alcohol, octocosanol [CH(3)(CH(2))(26)CH(2)OH], did not increase the growth of tobacco cell cultures.

Sicklepod Surface Wax Response to Photoperiod and S-(2,3-Dichloroallyl)diisopropylthiocarbamate (Diallate)

Sicklepod (Cassia obtusifolia L.) leaflet epicuticular fatty acid, fatty alcohol, and hydrocarbon contents were measured by gas-liquid chromatography from plants grown under 10-, 12-, 14-, or 16-hour photoperiods and treated with S-(2,3-dichloroallyl)diisopropylthiocarbamate (diallate) (0, 0.14, 0.28, 0.56, 1.12 kilograms per hectare). As diallate concentration increased, epicuticular fatty acid content decreased. Fatty alcohol content was maximal in plants treated with 0.28 kilograms per hectare diallate and was reduced from that level at herbicide concentrations above or below this rate. Hydrocarbon content patterns were similar to those of the fatty alcohols except that the hydrocarbons at 0.28 kilograms per hectare were 61% of that present in the control, whereas the concentration of fatty alcohols increased to 200% of the control in treatments of 0.28 kilograms per hectare diallate.Photoperiodic influence on the various epicuticular lipid classes showed increased fatty acid and hydrocarbon contents as the photoperiod was lengthened. Fatty alcohols were maximal under 16-hour photoperiods and minimal under 12-hour photoperiods.Photoperiod-diallate concentration interactions were demonstrated in the epicuticular fatty acid, fatty alcohol, and hydrocarbon contents of sicklepod leaflets. The amount of each lipid class present per gram leaf fresh weight was influenced by the rate of diallate applied and the photoperiod under which the plants were grown. Each lipid class responded differently to the combination of treatments.

Leaf epicuticular waxes

The external surface of the higher plants comprises a cuticular layer covered by a waxy deposit. This deposit is believed to play a major part in such phenomena as the water balance of plants and the behavior of agricultural sprays. The wax contains a wide range of organic compounds. These complex mixtures are amenable to modern microchromatographic and microspectrometric analytical procedures. The few surveys which have been made of the species distribution of certain classes of constituents indicate that such distribution may be of limited taxonomic value; however, the wax composition of a species may differ for different parts of the same plant and may vary with season, locale, and the age of the plant. This fascinating subject, in which the disciplines of botany, biochemistry, chemistry, and physics overlap and interact, is still in a very active state. Much remains to be learned about the composition and fine structure of the wax deposits, and, for this, experimental study of wax crystallization and permeation through artificial membranes will be required. Enzymic studies, radiolabeling, and electron microscopy will be needed to reveal the mode of biogenesis of the wax constituents and their site of formation and subsequent pathway through the cuticle to the leaf surface.

Chemical composition of the wax secreted by a scale insect (Ceroplastes pseudoceriferus Green)

The wax material in the secretion of a scale insect,Ceroplastes pseudoceriferus Green was analyzed chemically with special interest to the composition of higher fatty acids and higher alcohols. The wax consists of 34.2% fatty acids, 27.1% unsaponifiable matter and 29.5% resin acids. The fatty acids were found to be a complex mixture of 15 normal acids ranging from C(8) to C(32). Of these, octacosanoic, triacontanoic and dotriacontanoic acids comprise over 30% of the wax. Presence of relatively large amount of unsaturated fatty acids of the C(18) series (2.8% of the wax) is of particular interest.From the unsaponifiable fraction, only one saturated straight chain aleohol, bexacosanol, was detected (2.7% of the original wax). The other unsaponifiable matter was considered to be cyclic or branched carbon chain, and consisted of at least 12 to 20 compounds. The resin acid fraction was also found to be a complex mixture of at least 13 to 14 components.

Paleobiology of a Precambrian Shale: Geology, organic geochemistry, and paleontology are applied to the problem of detection of ancient life

Investigations have been made of crude oil, pristane, phytane, steranetype and optically active alkanes, porphyrins, microfossils, and the stable isotopes of carbon and of sulfur found in the Nonesuch shale of Precambrian age from Northern Michigan. These sediments are approximately 1 billion years old. Geologic evidence indicates that they were deposited in a nearshore deltaic environment. Porphyrins are found in the siltstones but not in the crude oils of the Nonesuch formation-evidence that these chemical fossils are adsorbed or absorbed and immobile. This immobility makes it highly unlikely that these porphyrins could have moved from younger formations into the Nonesuch sediments, and the widely disseminated particulate organic matters and fossils in this Precambrian shale are certainly indigenous.