Ljerka Kunst

Biosynthesis and secretion of plant cuticular wax.

The cuticle covers the aerial portions of land plants. It consists of amorphous intracuticular wax embedded in cutin polymer, and epicuticular wax crystalloids that coat the outer plant surface and impart a whitish appearance. Cuticular wax is mainly composed of long-chain aliphatic compounds derived from very long chain fatty acids. Wax biosynthesis begins with fatty acid synthesis in the plastid. Here we focus on fatty acid elongation (FAE) to very long chains (C24-C34), and the subsequent processing of these elongated products into alkanes, secondary alcohols, ketones, primary alcohols and wax esters. The identity of the gene products involved in these processes is starting to emerge. Other areas of this field remain enigmatic. For example, it is not known how the hydrophobic wax components are moved intracellularly, how they are exported out of the cell, or translocated through the hydrophilic cell wall. Two hypotheses are presented for intracellular wax transport: direct transfer of lipids from the endoplasmic reticulum to the plasma membrane, and Golgi mediated exocytosis. The potential roles of ABC transporters and non-specific lipid transfer proteins in wax export are also discussed. Biochemical-genetic and genomic approaches in Arabidopsis thaliana promise to be particularly useful in identifying and characterizing gene products involved in wax biosynthesis, secretion and function. The current review will, therefore, focus on Arabidopsis as a model for studying these processes.

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.

CUT1, an Arabidopsis Gene Required for Cuticular Wax Biosynthesis and Pollen Fertility, Encodes a Very-Long-Chain Fatty Acid Condensing Enzyme

Land plants secrete a layer of wax onto their aerial surfaces that is essential for survival in a terrestrial environment. This wax is composed of long-chain, aliphatic hydrocarbons derived from very-long-chain fatty acids (VLCFAs). Using the Arabidopsis expressed sequence tag database, we have identified a gene, designated CUT1, that encodes a VLCFA condensing enzyme required for cuticular wax production. Sense suppression of CUT1 in transgenic Arabidopsis plants results in waxless (eceriferum) stems and siliques as well as conditional male sterility. Scanning electron microscopy revealed that this was a severe waxless phenotype, because stems of CUT1-suppressed plants were completely devoid of wax crystals. Furthermore, chemical analyses of waxless plants demonstrated that the stem wax load was reduced to 6 to 7% of wild-type levels. This value is lower than that reported for any of the known eceriferum mutants. The severe waxless phenotype resulted from the downregulation of both the decarbonylation and acyl reduction wax biosynthetic pathways. This result indicates that CUT1 is involved in the production of VLCFA precursors used for the synthesis of all stem wax components in Arabidopsis. In CUT1-suppressed plants, the C24 chain-length wax components predominate, suggesting that CUT1 is required for elongation of C24 VLCFAs. The unique wax composition of CUT1-suppressed plants together with the fact that the location of CUT1 on the genetic map did not coincide with any of the known ECERIFERUM loci suggest that we have identified a novel gene involved in wax biosynthesis. CUT1 is currently the only known gene with a clearly established function in wax production.

Disruptions of the Arabidopsis Enoyl-CoA Reductase Gene Reveal an Essential Role for Very-Long-Chain Fatty Acid Synthesis in Cell Expansion during Plant Morphogenesis

In the absence of cell migration, plant architecture is largely determined by the direction and extent of cell expansion during development. In this report, we show that very-long-chain fatty acid (VLCFA) synthesis plays an essential role in cell expansion. The Arabidopsis thaliana eceriferum10 (cer10) mutants exhibit severe morphological abnormalities and reduced size of aerial organs. These mutants are disrupted in the At3g55360 gene, previously identified as a gene coding for enoyl-CoA reductase (ECR), an enzyme required for VLCFA synthesis. The absence of ECR activity results in a reduction of cuticular wax load and affects VLCFA composition of seed triacylglycerols and sphingolipids, demonstrating in planta that ECR is involved in all VLCFA elongation reactions in Arabidopsis. Epidermal and seed-specific silencing of ECR activity resulted in a reduction of cuticular wax load and the VLCFA content of seed triacylglycerols, respectively, with no effects on plant morphogenesis, suggesting that the developmental phenotypes arise from abnormal sphingolipid composition. Cellular analysis revealed aberrant endocytic membrane traffic and defective cell expansion underlying the morphological defects of cer10 mutants.

Cuticular Lipid Composition, Surface Structure, and Gene Expression in Arabidopsis Stem Epidermis

All vascular plants are protected from the environment by a cuticle, a lipophilic layer synthesized by epidermal cells and composed of a cutin polymer matrix and waxes. The mechanism by which epidermal cells accumulate and assemble cuticle components in rapidly expanding organs is largely unknown. We have begun to address this question by analyzing the lipid compositional variance, the surface micromorphology, and the transcriptome of epidermal cells in elongating Arabidopsis (Arabidopsis thaliana) stems. The rate of cell elongation is maximal near the apical meristem and decreases steeply toward the middle of the stem, where it is 10 times slower. During and after this elongation, the cuticular wax load and composition remain remarkably constant (32 μg/cm2), indicating that the biosynthetic flux into waxes is closely matched to surface area expansion. By contrast, the load of polyester monomers per unit surface area decreases more than 2-fold from the upper (8 μg/cm2) to the lower (3 μg/cm2) portion of the stem, although the compositional variance is minor. To aid identification of proteins involved in the biosynthesis of waxes and cutin, we have isolated epidermal peels from Arabidopsis stems and determined transcript profiles in both rapidly expanding and nonexpanding cells. This transcriptome analysis was validated by the correct classification of known epidermis-specific genes. The 15% transcripts preferentially expressed in the epidermis were enriched in genes encoding proteins predicted to be membrane associated and involved in lipid metabolism. An analysis of the lipid-related subset is presented.

Plant cuticles shine: advances in wax biosynthesis and export

The plant cuticle is an extracellular lipid structure deposited over the aerial surfaces of land plants, which seals the shoot and protects it from biotic and abiotic stresses. It is composed of cutin polymer matrix and waxes, produced and secreted by epidermal cells. The use of forward and reverse genetic approaches in Arabidopsis has led to the identification of enzymes involved in fatty acid elongation and biosynthesis of wax components, as well as transporters required for lipid delivery to the cuticle. However, major questions concerning alkane formation, intracellular and extracellular wax transport, regulation of wax deposition, and assembly of cuticular components into a functional cuticle remain to be resolved.

CER4 encodes an alcohol-forming fatty acyl-coenzyme A reductase involved in cuticular wax production in Arabidopsis

A waxy cuticle that serves as a protective barrier against uncontrolled water loss and environmental damage coats the aerial surfaces of land plants. It is composed of a cutin polymer matrix and waxes. Cuticular waxes are complex mixtures of very-long-chain fatty acids and their derivatives. We report here the molecular cloning and characterization of CER4, a wax biosynthetic gene from Arabidopsis (Arabidopsis thaliana). Arabidopsis cer4 mutants exhibit major decreases in stem primary alcohols and wax esters, and slightly elevated levels of aldehydes, alkanes, secondary alcohols, and ketones. This phenotype suggested that CER4 encoded an alcohol-forming fatty acyl-coenzyme A reductase (FAR). We identified eight FAR-like genes in Arabidopsis that are highly related to an alcohol-forming FAR expressed in seeds of jojoba (Simmondsia chinensis). Molecular characterization of CER4 alleles and genomic complementation revealed that one of these eight genes, At4g33790, encoded the FAR required for cuticular wax production. Expression of CER4 cDNA in yeast (Saccharomyces cerevisiae) resulted in the accumulation of C24:0 and C26:0 primary alcohols. Fully functional green fluorescent protein-tagged CER4 protein was localized to the endoplasmic reticulum in yeast cells by confocal microscopy. Analysis of gene expression by reverse transcription-PCR indicated that CER4 was expressed in leaves, stems, flowers, siliques, and roots. Expression of a β-glucuronidase reporter gene driven by the CER4 promoter in transgenic plants was detected in epidermal cells of leaves and stems, consistent with a dedicated role for CER4 in cuticular wax biosynthesis. CER4 was also expressed in all cell types in the elongation zone of young roots. These data indicate that CER4 is an alcohol-forming FAR that has specificity for very-long-chain fatty acids and is responsible for the synthesis of primary alcohols in the epidermal cells of aerial tissues and in roots.

Arabidopsis LTPG Is a Glycosylphosphatidylinositol-Anchored Lipid Transfer Protein Required for Export of Lipids to the Plant Surface

Plant epidermal cells dedicate more than half of their lipid metabolism to the synthesis of cuticular lipids, which seal and protect the plant shoot. The cuticle is made up of a cutin polymer and waxes, diverse hydrophobic compounds including very-long-chain fatty acids and their derivatives. How such hydrophobic compounds are exported to the cuticle, especially through the hydrophilic plant cell wall, is not known. By performing a reverse genetic screen, we have identified LTPG, a glycosylphosphatidylinositol-anchored lipid transfer protein that is highly expressed in the epidermis during cuticle biosynthesis in Arabidopsis thaliana inflorescence stems. Mutant plant lines with decreased LTPG expression had reduced wax load on the stem surface, showing that LTPG is involved either directly or indirectly in cuticular lipid deposition. In vitro 2-p-toluidinonaphthalene-6-sulfonate assays showed that recombinant LTPG has the capacity to bind to this lipid probe. LTPG was primarily localized to the plasma membrane on all faces of stem epidermal cells in the growing regions of inflorescence stems where wax is actively secreted. These data suggest that LTPG may function as a component of the cuticular lipid export machinery.

Identification of the Wax Ester Synthase/Acyl-Coenzyme A:Diacylglycerol Acyltransferase WSD1 Required for Stem Wax Ester Biosynthesis in Arabidopsis

Wax esters are neutral lipids composed of aliphatic alcohols and acids, with both moieties usually long-chain (C16 and C18) or very-long-chain (C20 and longer) carbon structures. They have diverse biological functions in bacteria, insects, mammals, and terrestrial plants and are also important substrates for a variety of industrial applications. In plants, wax esters are mostly found in the cuticles coating the primary shoot surfaces, but they also accumulate to high concentrations in the seed oils of a few plant species, including jojoba (Simmondsia chinensis), a desert shrub that is the major commercial source of these compounds. Here, we report the identification and characterization of WSD1, a member of the bifunctional wax ester synthase/diacylglycerol acyltransferase gene family, which plays a key role in wax ester synthesis in Arabidopsis (Arabidopsis thaliana) stems, as first evidenced by severely reduced wax ester levels of in the stem wax of wsd1 mutants. In vitro assays using protein extracts from Escherichia coli expressing WSD1 showed that this enzyme has a high level of wax synthase activity and approximately 10-fold lower level of diacylglycerol acyltransferase activity. Expression of the WSD1 gene in Saccharomyces cerevisiae resulted in the accumulation of wax esters, but not triacylglycerol, indicating that WSD1 predominantly functions as a wax synthase. Analyses of WSD1 expression revealed that this gene is transcribed in flowers, top parts of stems, and leaves. Fully functional yellow fluorescent protein-tagged WSD1 protein was localized to the endoplasmic reticulum, demonstrating that biosynthesis of wax esters, the final products of the alcohol-forming pathway, occurs in this subcellular compartment.

Significance of the Expression of the CER6 Condensing Enzyme for Cuticular Wax Production in Arabidopsis

To learn more about the role of the CER6 condensing enzyme in Arabidopsis surface wax production, we determinedCER6 transcription domains and the timing ofCER6 transcription in vegetative and reproductive structures from juvenile, mature, and senescing tissues. We found thatCER6 is highly transcribed throughout development, exclusively in the epidermal cells in all tissues examined. The only exception to the epidermal expression was observed in anthers nearing maturity, in which CER6 mRNA was localized in the tapetum. To determine if environmental factors such as light and water deficit, which are known to stimulate wax accumulation, induceCER6 transcription, we examined the effects of these factors on CER6 transcript abundance. Our results demonstrate that light is essential for CER6transcription, and that osmotic stress and the presence of abscisic acid enhance CER6 transcript accumulation.CER6 promoter-directed expression of the β-glucuronidase reporter gene in transgenic plants demonstrated that the CER6 promoter was highly effective in directing epidermis-specific expression in Arabidopsis and tobacco (Nicotiana tabacum). Furthermore, CER6promoter-driven CER6 overexpression resulted in increased wax deposition in Arabidopsis stems. These experiments indicate that the expression level of CER6 in the epidermis is one of the factors controlling wax accumulation on Arabidopsis stems.