Bell and R. Juniper and R. Southwood, eds. Stowe, M. Spencer, ed. Taylor, S. Olsen, J. Vavra, W. Roberts, G. Aubertin, G. Gorsline, J. Law, Jr. Rhoades, Can fatty alcohols reduce water losses?
Wisniak, J. Yermanos, D. McClure and E. Lipinsky, eds. Zaborsky, ed. Moreover, the cer mutant exhibited a tryphine indistinguishable from that of the wild type, which indicates that conditional male sterility is not related to the structure of the tryphine. The cer6 mutant was the first to be characterized as being defective in pollen hydration, and it was originally named pop1 for pollen — pistil interaction1 ; Preuss et al. To date, it is also the only mutant that has been characterized with respect to the lipid composition of the pollen tryphine.
Some of the characteristics of this mutant have been discussed above in connection with cer1. In this article, the authors unequivocally demonstrate that the CER6 gene is located on chromosome 1 at centimorgans cM , and they show by sequence comparison that it is identical to a previously cloned gene, CUT1 , that was mapped erroneously to CUT1 was identified among a group of expressed sequence tags corresponding to a family of FAE1 related genes, and when silenced in transformed Arabidopsis plants by cosuppression, it produced a phenotype identical to that of the cer1 , cer2 , and cer6 mutants.
Despite the wrong map position, the characterization of this gene strongly supported the idea that CUT1 encodes a condensing enzyme. Fiebig et al. Sequence analysis of the cer and cer mutant alleles also highlighted an interesting point. Moreover, the phenotype of the cer mutant plants was stronger than that of other cer6 mutants, suggesting the cer may be a null allele Preuss et al.
The most surprising result described in this article derives from the complementation analysis. The authors observed that the glossy phenotype and male sterility were not rescued equally after the introduction of the CER6 gene in cer mutants. Some of the transformants with restored fertility still had defective stem wax. This is the first report that the two phenotypic effects of the cer mutations can be separated. Even more interesting is the finding of a dominant intragenic suppressor mutation that converts the Pro residue in the cer mutated allele to a Ser residue.
Again, this mutation is in a nonconserved region among condensing enzymes, thus diminishing the possibility that the active site of the enzyme is affected. It would be interesting to visualize a folding model for these three peptides.
Pollen of suppressor plants regains an abundant tryphine with lipid droplets that contain the C29 and C30 VLCFAs, although not at the wild-type level. This brings us back to the question of whether and how much tryphine and lipids are necessary to recover pollen fertility, because some alleles of four independent CER loci have tryphine and some lipids, yet are male sterile.
Epidermal cell walls and cuticles in these mutants are more permeable and show an altered composition of lipids in crude cell wall fractions Lolle et al. Based on these observations, the authors formulated a model in which localized changes in the permeability of the cell wall and the cuticle modify the interaction between epidermal cells. This model is supported by a recent report Sieber et al. Interestingly, though, the chemical wax composition of transgenic plants was identical to that of wild-type plants.
Pruitt et al. For example, they may act as a solvent to release particular compounds of the tryphine or the papillar cells. We have demonstrated that a class of triacylglycerols which are absent in the tryphine of Brassica spp and Arabidopsis are sufficient to establish pollen interaction with pistils and leaves Wolters-Arts et al.
Nevertheless, in our experiments, we always used untreated wild-type pollen, which carries its own components for the interaction. One of these components could be a protein such as oleosin, which has been shown to be necessary for the hydration of Arabidopsis pollen Mayfield and Preuss, The latter hypothesis modification of permeability is closer to the model of Pruitt et al.
Aarts M. Keijzer C. Stiekema W. Pereira A. Molecular characterization of the CER1 gene of Arabidopsis involved in epicuticular wax biosynthesis and pollen fertility. Plant Cell 7 , — Google Scholar. Hodge R. Kalantidis K. Florack D. Wilson Z. Mulligan B. Plant J. Fiebig A. Mayfield J. Miley N. Chau S. Fischer R. Preuss D. Alterations in CER6 , a gene identical to CUT1 , differentially affect long-chain lipid content on the surface of pollen and stems.
Plant Cell 12 , — Guilford W. Schneider D. Labovitz J. Opella S. High resolution solid state 13 C NMR spectroscopy of sporopollenins from different plant taxa. Key Points Natural waxes are typically esters of fatty acids and long chain alcohols. Animal wax esters are derived from a variety of carboxylic acids and fatty alcohols. Plant waxes are derived from mixtures of long-chain hydrocarbons containing functional groups.
Because of their hydrophobic nature, waxes prevent water from sticking on plants and animals. Synthetic waxes are derived from petroleum or polyethylene and consist of long-chain hydrocarbons that lack functional groups. Synthetic and waxes are used in adhesives, cosmetics, food, and many other commercial products. Waxes of mineral origin, such as montan wax, are not discussed. For practical convenience, alkylresorcinols are discussed below, although their connection to main-stream waxes is tenuous.
Biochemists often link waxes with the thin layer of fatty constituents that cover the leaves of plants or provide a surface coating for insects or the skin of animals for which primary requirements are spreadability and chemical and metabolic stability. Such surface waxes are produced by specialized cells or glands, and all tend to contain wax esters as major and perhaps defining components, i. Also, the chain-length and degree of unsaturation and branching of the fatty acids and the other aliphatic constituents, varies with the origin of the wax, but other than in some waxes of marine origin or from some higher animals, the aliphatic moieties tend to be saturated or monoenoic.
In the epidermal cells of plant leaves, the plasma membrane is covered by the cell wall structure, then a cutin layer and finally a coating with a thin layer of hydrophobic waxy material that is microcrystalline in structure and forms the outermost boundary of the cuticular membrane, i.
This wax layer serves many purposes, for example to limit the diffusion of water and solutes and control gas exchange, while permitting a controlled release of volatiles that may deter pests or attract pollinating insects. The wax provides protection from ultraviolet light, disease and insects, and helps the plants resist drought and other environmental stresses.
Most analytical studies have dealt with the epicuticular layer, i. As plants cover much of the earth's surface, it seems likely that plant waxes are among the most abundant of all natural lipids. The range of lipid types in plant waxes is highly variable, both in nature and in composition, and Table 1 illustrates some of this diversity in the main components. Polar complex lipids are rarely encountered. The amount of each lipid class and the nature and proportions of the various molecular species within each class vary greatly according to the plant species and the site of wax deposition leaf, flower, fruit, etc.
It should be noted that these compositions can vary with abiotic and biotic stresses, such as drought or insect predation. Carnauba - The leaves of the carnauba palm, Copernicia cerifera that grows in Brazil, have a thick coating of wax, which can be harvested from the dried leaves.
This is the only leaf wax available in sufficient quantities to be of commercial value. Jojoba - The jojoba plant Simmondsia chinensis , which grows in the semi-arid regions of Mexico and the U. Therefore, it contains C 38 to C 44 esters with one double bond in each alkyl moiety. As methylene-interrupted double bonds are absent, the wax is relatively resistant to oxidation.
It is believed that this may attract birds as an aid to seed dispersal. Biosynthesis and secretion differ from conventional plant waxes and are more closely related to cutin production with distinct pools of acyl donors and the final assembly of triacylglycerols outside of cells.
The Japanese sumac tree Rhus verniciflua produces a similar wax 'Japan wax'. Other - Phytol released by the catabolism of chlorophyll in plants is stored in leaves and fruit of some plant species and in mosses and algae in the form of an ester with fatty acids.
In this form, phytol cannot disrupt membranes, but when required it can be released and utilized as a biosynthetic precursor for tocopherol and perhaps fresh chlorophyll. Because of their biochemical importance and relative ease of study, the waxes of the plant cuticle have received most study. All the aliphatic components of plant waxes are synthesised in the epidermal cells from saturated very-long-chain fatty acids commonly C 20 to C The acyl-ACP products are hydrolysed by thioesterases to free fatty acids, which can then be esterified to coenzyme A before translocation to the endoplasmic reticulum.
In the second stage, multiple elongation steps are catalysed by membrane-associated multi-enzyme complexes, known as fatty acid elongases, see our web page on the biosynthesis of saturated fatty acids , requiring CER2-LIKE proteins related to acyltransferases to change the chain-length specificity of the elongation machinery. These can be released as the free acids for export directly as cuticular waxes, or they can be further processed.
There are then two main pathways for biosynthesis of wax components in the endoplasmic reticulum of plastids of the epidermal cells: an acyl reduction pathway, which yields primary alcohols and wax esters, and a decarbonylation pathway that results in synthesis of aldehydes, alkanes, secondary alcohols and ketones. In the reductive pathway, acyl-CoA esters produced by chain elongation are reduced in a two-step process via a transient aldehyde intermediate, catalysed by a single enzyme, an acyl-CoA reductase though it was once thought that two distinct enzymes were involved.
Primary alcohols can be exported directly to the surface of the plant or are esterified by acyl-CoAs to from wax esters. An enzyme with some sequence similarity to this has been characterized from microsomal fractions of seeds of the jojoba plant and is responsible for production of the storage wax. Similar mechanisms have been observed in studies with insects, algae and birds uropygial glands. In the decarbonylation pathway for the synthesis of wax constituents, the first step is again believed to be the reduction of acyl-CoA ester to an aldehyde by means of an acyl-CoA reductase.
Removal of the carbonyl group by an aldehyde decarbonylase yields an alkane, with one fewer carbon atom than the fatty acid precursor. However, the enzymes involved have not been fully characterized. Further metabolism of the hydrocarbon is then possible, for example by two consecutive reactions catalysed by the cytochrome P enzyme CYP96A15, referred to as mid-chain alkane hydroxylase MAH1, to insert a hydroxyl group to form a secondary alcohol, and thence a ketone, with the position of the substitution depending on the species and the specificities of the enzymes involved.
Secondary alkanols can in turn be esterified to form wax esters. Again, these processes have been studied most in plants, but similar biochemical reactions appear to occur in insects and birds.
Once synthesised, the wax components must be exported from the sites of lipid synthesis in the plastid and the endoplasmic reticulum to the plasma membrane and through the cell wall. They must then pass into the cutin layer that provides a matrix, within and upon which the waxes are deposited. Less is known of how wax is exported, but two groups of transport molecules have been identified that are known to facilitate this process, i. There is also evidence for vesicular transport.
In mammals, the main wax production is associated with the sebaceous glands of the skin, most of which are associated with hair follicles, although there are also related structures on the eyelids termed Meibomian glands.
Sebaceous glands secrete mainly non-polar lipids in the form of sebum onto the skin surface, where they are easily recovered for analysis.
In humans, sebaceous glands are distributed throughout the body with the exception of the palm and sole of the foot. They consist of three type of cells, peripheral undifferentiated cells, cells that produce lipid bodies, and lysing cells loaded lipid that empty their contents into the lumen. Although relatively few species have been studied in real detail, it is evident that a wide range of lipid classes are present and that these vary greatly in amount and nature between species there may also be variation with age.
The composition of human sebum differs appreciably from that of other species, especially in the high content of triacylglycerols and in the nature of the fatty acids.
Some typical data are listed in Table 3. The triterpene hydrocarbon squalene 2,6,10,15,19,hexamethyltetracosa-2,6,10,14,18,hexaene is produced ubiquitously in nature but is found in most tissues as a minor lipid component only, other than in some marine waxes and especially those of sharks Squalus sp.
0コメント