Publications

F. Li, X. Wu, P. Lam, D. Bird, H. Zheng, L. Samuels, R. Jetter and L. Kunst (2008) Identification of the wax ester synthase/acyl-coenzyme A:diacylglycerol acyltransferase WSD1 required for stem wax ester biosynthesis in Arabidopsis. Plant Physiol. 148:97-107.
(abstract) | (full journal in .pdf)

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.

R. Jetter and L. Kunst (2008) Plant surface lipid biosynthetic pathways and their utility for metabolic engineering of waxes and hydrocarbon biofuels. Plant J. 54: 670–683.
(abstract) | (full journal in .pdf)

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.

L. Samuels, L. Kunst, and R. Jetter (2008) Sealing plant surfaces: Cuticular wax formation by epidermal cells. Annu. Rev. Plant Biol. 59: 683-707.
(abstract) | (full journal in .pdf)

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.

S. Greer, M. Wen, D. Bird, X. Wu, L. Samuels, L. Kunst, and R. Jetter (2007) The cytochrome P450 enzyme CYP96A15 is the mid-chain alkane hydroxylase responsible for formation of secondary alcohols and ketones in stem cuticular wax of Arabidopsis thaliana. Plant Physiol. 145: 653-667.
(abstract) | (full journal in .pdf)

Most aerial surfaces of plants are covered by cuticular wax that is synthesized in epidermal cells. The wax mixture on the inflorescence stems of Arabidopsis (Arabidopsis thaliana) is dominated by alkanes, secondary alcohols, and ketones, all thought to be formed sequentially in the decarbonylation pathway of wax biosynthesis. Here, we used a reverse-genetic approach to identify a cytochrome P450 enzyme (CYP96A15) involved in wax biosynthesis and characterized it as a midchain alkane hydroxylase (MAH1). Stem wax of T-DNA insertional mutant alleles was found to be devoid of secondary alcohols and ketones (mah1-1) or to contain much lower levels of these components (mah1-2 and mah1-3) than wild type. All mutant lines also had increased alkane amounts, partially or fully compensating for the loss of other compound classes. In spite of the chemical variation between mutant and wild-type waxes, there were no discernible differences in the epicuticular wax crystals on the stem surfaces. Mutant stem wax phenotypes could be partially rescued by expression of wild-type MAH1 under the control of the native promoter as well as the cauliflower mosaic virus 35S promoter. Cauliflower mosaic virus 35S-driven overexpression of MAH1 led to ectopic accumulation of secondary alcohols and ketones in Arabidopsis leaf wax, where only traces of these compounds are found in the wild type. The newly formed leaf alcohols and ketones had midchain functional groups on or next to the central carbon, thus matching those compounds in wild-type stem wax. Taken together, mutant analyses and ectopic expression of MAH1 in leaves suggest that this enzyme can catalyze the hydroxylation reaction leading from alkanes to secondary alcohols and possibly also a second hydroxylation leading to the corresponding ketones. MAH1 expression was largely restricted to the expanding regions of the inflorescence stems, specifically to the epidermal pavement cells, but not in trichomes and guard cells. MAH1-green fluorescent protein fusion proteins localized to the endoplasmic reticulum, providing evidence that both intermediate and final products of the decarbonylation pathway are generated in this subcellular compartment and must subsequently be delivered to the plasma membrane for export toward the cuticle.

O. Rowland, R. Lee, R. Franke, L. Schreiber and L. Kunst (2007) The CER3 gene from Arabidopsis thaliana is allelic to WAX2/YRE/FLP1 and is required for cuticular wax biosynthesis. FEBS Lett. 581: 3538–3544.
(abstract) | (full journal in .pdf)

The cuticle coats the aerial organs of land plants and is composed of a cutin matrix embedded and overlayed with waxes. The Arabidopsis CER3 gene is important for cuticular wax biosynthesis and was reported to correspond to At5g02310 encoding an E3 ubiquitin ligase. Here, we demonstrate that CER3 is not At5g02310 and instead corresponds to WAX2/ YRE/FLP1 (At5g57800), a gene of unknown function required for wax biosynthesis. CER3 protein has also been implicated in cutin production because strong cer3 alleles display organ fusions. Leaf cutin analysis of two cer3 alleles did not reveal significant differences in cutin load or composition, indicating that CER3 has no major role in leaf cutin formation.

D. Bird, F. Beisson, A. Brigham, J. Shin, S. Greer, R. Jetter, L. Kunst, X. Wu, A. Yephremov, and L. Samuels (2007) Characterization of Arabidopsis ABCG11/WBC11, an ATP binding cassette (ABC) transporter that is required for cuticular lipid secretion. Plant J. 52: 485-498.
(abstract) | (full journal in .pdf)

ABCG11/WBC11, an ATP binding cassette (ABC) transporter from Arabidopsis thaliana, is a key component of the export pathway for cuticular lipids. Arabidopsis wbc11 T-DNA insertional knock-out mutants exhibited lipidic inclusions inside epidermal cells similar to the previously characterized wax transporter mutant cer5, with a similar strong reduction in the alkanes of surface waxes. Moreover, the wbc11 knock-out mutants also showed defects not present in cer5, including post-genital organ fusions, stunted growth and a reduction in cutin load on the plant surface. A mutant line previously isolated in a forward genetics screen, called permeable leaves 1 (pel1), was identified as an allele of ABCG11/WBC11. The double knock-out wbc11cer5 exhibited the same morphological and biochemical phenotypes as the wbc11 knock-out. A YFP-WBC11 fusion protein rescued a T-DNA knock-out mutant and was localized to the plasma membrane. These results show that WBC11 functions in secretion of surface waxes, possibly by interacting with CER5. However, unlike ABCG12/ CER5, ABCG11/WBC11 is important to the normal process of cutin formation.

M. Dauk, P. Lam, L. Kunst, and M.A. Smith (2007) A FAD2 homologue from Lesquerella lindheimeri has predominantly fatty acid hydroxylase activity, Plant Science 173: 43–49.
(abstract) | (full journal in .pdf)

A genomic sequence encoding a polypeptide with 91% sequence identity to the Lesquerella fendleri bifunctional oleate 12-hydroxylase:- desaturase was amplified by PCR from Lesquerella lindheimeri. Expression of the gene in the yeast Saccharomyces cerevisiae resulted in the synthesis of ricinoleic acid and very low levels of di-unsaturated fatty acids. Comparison of the amino acid sequences of the L. lindheimeri and castor bean oleate 12-hydroxylase enzymes to those of the L. fendleri bifunctional oleate 12-hydroxylase:desaturase and oleate 12-desaturase enzymes from 32 diverse species identified a single amino acid (M295) that was conserved in the hydroxylases and different but also conserved in the desaturases and the bifunctional enzyme. Site-directed mutagenesis indicated that this residue was most likely not involved in determining the catalytic outcome of the hydroxylation/desaturation reaction.Transformation of an Arabidopsis fad2/fae1 mutant line with the L. lindheimeri hydroxylase gave further evidence that this enzyme, like the castor oleate 12-hydroxylase, is primarily a fatty acid hydroxylase and should not be considered bifunctional. Total hydroxy fatty acid content of up to 18% of seed fatty acids was measured in homozygous transformants. Lines with the highest hydroxy fatty acid content showed significant reduction in total oil content.

C. Lai, L. Kunst and R. Jetter (2007) Composition of alkyl esters in the cuticular wax on inflorescence stems of Arabidopsis thaliana cer mutants. Plant J. 50: 189-196.
(abstract) | (full journal in .pdf)

Wax biosynthetic pathways proceed via the elongation of 16:0 acyl-CoA to very long-chain fatty acids (VLCFA), and by further modifications that include reduction to primary alcohols and formation of alkyl esters. We have analyzed the alkyl esters in the stem wax of ten cer mutants of Arabidopsis thaliana together with the corresponding wild types. Alkyl esters with chain lengths between C38 and C52 were identified, and the levels of esters ranged from 0.15 lg cm)2 in Wassilewskija (WS) to 1.20 lg cm)2 in cer2. Esters with even numbers of carbons prevailed, with C42, C44 and C46 favoured in the wild types, a predominance of C42 in cer2 and cer6 mutants, and a relative shift towards C46 in cer3 and cer23 mutants. The esters of all mutants and wild types were dominated by 16:0 acyl moieties, whereas the chain lengths of esterified alcohols were between C20 and C32. The alkyl chain-length distributions of the wild-type esters had a maximum for C28 alcohol, similar to the free alcohols accompanying them in the wax mixtures. The esterified alcohols of cer2, cer6 and cer9 had largely increased levels of C26 alcohol, closely matching the patterns of the corresponding free alcohols and, therefore, differing drastically from the corresponding wild type. In contrast, cer1, cer3, cer10, cer13 and cer22 showed ester alcohol patterns with increased levels of C30, only partially following the shift in chain lengths of the free alcohols in stem wax. These results provide information on the composition of substrate pools and/or the specificity of the ester synthase involved in wax ester formation. We conclude that alcohol levels at the site of biosynthesis are mainly limiting the ester formation in the Arabidopsis wild-type epidermis.

T. S. Hooker, P. Lam, H. Zheng, and L. Kunst (2007) A core subunit of the RNA-processing/degrading exosome specifically influences cuticular wax biosynthesis in Arabidopsis. Plant Cell 19: 904–913.
(abstract) | (full journal in .pdf)

The cuticle is an extracellular matrix composed of cutin polyester and waxes that covers aerial organs of land plants and protects them from environmental stresses. The Arabidopsis thaliana cer7 mutant exhibits reduced cuticular wax accumulation and contains considerably lower transcript levels of ECERIFERUM3/WAX2/YORE-YORE (CER3/WAX2/YRE), a key wax biosynthetic gene. We show here that CER7 protein is a putative 39-59 exoribonuclease homologous to yeast ribonuclease PH45 (RRP45p), a core subunit of the RNA processing and degrading exosome that controls the expression of CER3/ WAX2/YRE. We propose that CER7 acts by degrading a specific mRNA species encoding a negative regulator of CER3/WAX2/YRE transcription. A second RRP45p homolog found in Arabidopsis, designated At RRP45a, is partially functionally redundant with CER7, and complete loss of RRP45 function in Arabidopsis is lethal. To our knowledge, CER7 is currently the only example of a core exosomal subunit specifically influencing a cellular process.

O. Rowland, H. Zheng, S.R. Hepworth, P. Lam, R. Jetter, and L. Kunst (2006) CER4 encodes an alcohol-forming fatty acyl-coenzyme A reductase involved in cuticular wax production in Arabidopsis. Plant Physiol. 142: 866-877.
(abstract) | (full journal in .pdf)

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 verylong- 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 b-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.

M.C. Suh, A. L. Samuels, R. Jetter, L. Kunst, M. Pollard, J. Ohlrogge, and F. Beisson (2005) Cuticular lipid composition, surface structure, and gene expression in Arabidopsis stem epidermis. Plant Physiol. 139: 1649–1665. .
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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 mg/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 mg/cm2) to the lower (3 mg/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.

H. Zheng, O. Rowland, and L. Kunst (2005) 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. Plant Cell 17: 1467-1481.
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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.

H. Moon, G. Chowrira, O. Rowland, B. J.Blacklock,M. A. Smith, and L. Kunst (2004) A root-specific condensing enzyme from Lesquerella fendleri that elongates very-long-chain saturated fatty acids. Plant Mol. Biol. 56: 917-927.
(abstract) | (full journal in .pdf)

The LfKCS45 gene with a high sequence similarity to known 3-ketoacyl-CoA synthases of the membrane- bound fatty acid elongase was isolated from Lesquerella fendleri. The LfKCS45 gene has a 1464 bp open reading frame without introns, and is predicted to encode a polypeptide of 487 amino acids with an estimated molecular mass of 54.6 kD. High-stringency DNA blot analysis indicated that there were no closely related genes to LfKCS45 in the L. fendleri genome. Analysis of the fatty acid composition of transformed yeast revealed that expression of the LfKCS45 protein results in the synthesis of two novel very-long-chain fatty acids identified as C28:0 and C30:0. LfKCS45 was found to be not active with acyl-CoA substrates C16 to C24 in length. Reverse transcription-PCR experiments showed that the LfKCS45 gene is expressed only in L. fendleri root tips. Histochemical assays for GUS activity in Arabidopsis transformed with the LfKCS45 promoter-GUS fusion construct confirmed this expression pattern and demonstrated that LfKCS45 transcription is restricted to the cells of the lateral root cap.

J. A. Pighin, H. Zheng, L. J. Balakshin, I. P. Goodman, T. L. Western, R. Jetter, L. Kunst, and A. L. Samuels (2004) Plant cuticular lipid export requires an ABC transporter. Science 306: 702-704.**
(abstract) | (full journal in .pdf) | (commentary)

A waxy protective cuticle coats all primary aerial plant tissues. Its synthesis requires extensive export of lipids from epidermal cells to the plant surface. Arabidopsis cer5 mutants had reduced stem cuticular wax loads and accumulated sheetlike inclusions in the cytoplasm of wax-secreting cells. These inclusions represented abnormal deposits of cuticular wax and resembled inclusions found in a human disorder caused by a defective peroxisomal adenosine triphosphate binding cassette (ABC) transporter. We found that the CER5 gene encodes an ABC transporter localized in the plasma membrane of epidermal cells and conclude that it is required for wax export to the cuticle.

A.L. Samuels, and L. Kunst (2003) Wax biosynthesis and secretion in plants. Progress in Lipid Research 42: 51-80.
(abstract) | (full journal in .pdf)

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.

M.A. Smith, H. Moon, G. Chowrira, and L. Kunst (2003) Heterologous expression of fatty acid hydroxylase genes in developing seeds of Arabidopsis thaliana. Planta 217: 507-516.
(abstract) | (full journal in .pdf)

Abstract Expression of a cDNA encoding the castor bean (Ricinus communis L.) oleate D12-hydroxylase in the developing seeds of Arabidopsis thaliana (L.) Heynh. results in the synthesis of four novel hydroxy fatty acids. These have been previously identified as ricinoleic acid (12-hydroxy-octadec-cis-9-enoic acid: 18:1-OH), densipolic acid (12-hydroxy-octadec-cis- 9,15-enoic acid: 18:2-OH), lesquerolic acid (14-hydroxy- eicos-cis-11-enoic acid: 20:1-OH) and auricolic acid (14-hydroxy-eicos-cis-11,17-enoic acid: 20:2-OH). Using mutant lines of Arabidopsis that lack the activity of the FAE1 condensing enzyme or FAD3 ER D-15-desaturase, we have shown that these enzymes are required for the synthesis of C20 hydroxy fatty acids and polyunsaturated hydroxy fatty acids, respectively. Analysis of the seed fatty acid composition of transformed plants demonstrated a dramatic increase in oleic acid (18:1) levels and a decrease in linoleic acid (18:2) content correlating to the levels of hydroxy fatty acid present in the seed. Plants in which FAD2 (ER D12-desaturase) activity was absent showed a decrease in 18:1 content and a slight increase in 18:2 levels corresponding to hydroxy fatty acid content. Expression of the castor hydroxylase protein in yeast indicates that this enzyme has a low level of fatty acid D12-desaturase activity. Lipase catalysed 1,3-specific lipolysis of triacylglycerol from transformed plants demonstrated that ricinoleic acid is not excluded from the sn-2 position of triacylglycerol, but is the only hydroxy fatty acid present at this position.

H. Zheng, L. Kunst, C. Hawes and I. Moore (2003) A GFP-based assay reveals a role for RHD3 in transport between the endoplasmic reticulum and Golgi apparatus. Plant J. 37: 398-414.
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We describe the use of a secreted form of Aequoria victoria green fluorescent protein (secGFP) in a non-invasive live cell assay of membrane traffic in Arabidopsis thaliana. We show that in comparison to GFP-HDEL, which accumulates in the endoplasmic reticulum (ER), secGFP generates a weak fluorescence signal when transported to the apoplast. The fluorescence of secGFP in the apoplast can be increased by growth of seedlings on culture medium buffered at pH 8.1, suggesting that apoplastic pH is responsible, at least in part, for the low fluorescence intensity of seedlings expressing secGFP. Inhibition of secGFP transport between the ER and plasma membrane (PM), either by Brefeldin A (BFA) treatment or by genetic intervention results in increased intracellular secGFP accumulation accompanied by an increase in the secGFP fluroescence intensity. secGFP thus provides a valuabe tool for forward and reverse genetic analysis of membrane traffic and endomembrane organisation in Arabidopsis. Using this assay for quantitative sub-lethal perturbation of secGFP transport, we identify a role for root hair defective 3 (RHD3) in transport of secreted and Golgi markers between the ER and the Golgi apparatus.

T.S. Hooker, A.A. Millar, and L. Kunst (2002) Significance of the expression of the CER6 condensing enzyme for epicuticular wax production and overproduction in Arabidopsis. Plant Physiol. 129: 1568-1580.
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To learn more about the role of the CER6 condensing enzyme in Arabidopsis surface wax production, we determined CER6 transcription domains and the timing of CER6 transcription in vegetative and reproductive structures from juvenile, mature, and senescing tissues. We found that CER6 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, induce CER6 transcription, we examined the effects of these factors on CER6 transcript abundance. Our results demonstrate that light is essential for CER6 transcription, and that osmotic stress and the presence of abscisic acid enhance CER6 transcript accumulation. CER6 promoter-directed expression of the 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, CER6 promoter-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.

H. Moon, M.A. Smith, and L. Kunst (2001) A condensing enzyme from the seeds of Lesquerella fendleri that specifically elongates hydroxy fatty acids. Plant Physiol. 127: 1635-1643.
(abstract) | (full journal in .pdf)

Lesquerella fendleri seed oil contains up to 60% hydroxy fatty acids, nearly all of which is the 20-carbon hydroxy fatty acid lesquerolic acid (d-14-hydroxyeicos-cis-11-enoic acid). Previous work suggested that lesquerolic acid in L. fendleri was formed by the elongation of the 18-carbon hydroxy fatty acid, ricinoleic acid. To identify a gene encoding the enzyme involved in hydroxy fatty acid elongation, an L. fendleri genomic DNA library was screened using the coding region of the Arabidopsis Fatty Acid Elongation 1 gene as a probe. A gene, LfKCS3, with a high sequence similarity to known very long-chain fatty acid condensing enzymes, was isolated. LfKCS3 has a 2,062-bp open reading frame interrupted by two introns, which encodes a polypeptide of 496 amino acids. LfKCS3 transcripts accumulated only in the embryos of L. fendleri and first appeared in the early stages of development. Fusion of the LfKCS3 promoter to the uidA reporter gene and expression in transgenic Arabidopsis resulted in a high level of glucuronidase activity exclusively in developing embryos. Seeds of Arabidopsis plants transformed with LfKCS3 showed no change in their very long-chain fatty acid content. However, when these Arabidopsis plants were crossed with the transgenic plants expressing the castor oleate 12-hydroxylase, significant amounts of 20-carbon hydroxy fatty acids accumulated in the seed, indicating that the LfKCS3 condensing enzyme specifically catalyzes elongation of 18-carbon hydroxy fatty acids.

M. Rossak, M.A. Smith, and L. Kunst (2001) Expression of the FAE1 gene and FAE1 promoter activity in developing seeds of Arabidopsis thaliana. Plant Mol. Biol. 46: 717-725.
(abstract) | (full journal in .pdf)

Plant fatty acid elongase which catalyzes very-long-chain fatty acid (VLCFA) biosynthesis is a membrane-bound multienzyme complex. It is composed of four enzymes, a 3-ketoacyl-CoA synthase (condensing enzyme), a 3- ketoacyl-CoA reductase, a 3-hydroxyacyl-CoA dehydrase, and an enoyl-CoA reductase required for completion of each step of 2-carbon elongation of fatty acids. To improve our understanding of the overall regulation of the fatty acid elongase, we investigated the spatial and temporal expression of its key component, the FAE1-condensing enzyme, and examined the activity of the promoter of the FAE1 gene in Arabidopsis. In situ hybridization results revealed that FAE1 transcripts were found exclusively in the embryo. RNA blot analysis and histochemical analysis of GUS activity in pFAE1::GUS transgenic Arabidopsis lines demonstrated that the FAE1 gene was already transcribed in the early torpedo stage embryos 4–5 days after flowering, with transcription reaching its peak 9–11 days after flowering. VLCFA deposition closely paralleled FAE1 transcript accumulation. FAE1 promoter was highly active and embryo-specific. Because its timing coincides with the period of major storage lipid accumulation, and because its in vivo activity in Arabidopsis is superior to the napin promoter, FAE1 promoter may be ideal for genetic engineering of seed oil composition.

A.A. Millar, M.A. Smith and L. Kunst (2000) All fatty acids are not equal: discrimination in plant membrane lipids. Trends Plant. Sci. 5: 95-101.
(abstract) | (full journal in .pdf)

Plant membrane lipids are primarily composed of 16-carbon and 18-carbon fatty acids containing up to three double bonds. By contrast, the seed oils of many plant species contain fatty acids with significantly different structures. These unusual fatty acids sometimes accumulate to .90% of the total fatty acid content in the seed triacylglycerols, but are generally excluded from the membrane lipids of the plant, including those of the seed. The reasons for their exclusion and the mechanisms by which this is achieved are not completely understood. Here we discuss recent research that has given new insights into how plants prevent the accumulation of unusual fatty acids in membrane lipids, and how strict this censorship of membrane composition is. We also describe a transgenic experiment that resulted in an excessive buildup of unusual fatty acids in cellular membranes, and clearly illustrated that the control of membrane lipid composition is essential for normal plant growth and development.

A.A. Millar and L. Kunst (1999) Natural genetic variation of the fatty-acyl composition of seed oils in different ecotypes of Arabidopsis thaliana. Phytochemistry 52: 1029-1033.
(abstract) | (full journal in .pdf)

The fatty-acyl composition of the seed oil was determined for 100 ecotypes of Arabidopsis thaliana. Despite coming from diverse geographical locations, seed fatty-acyl pro®les of all ecotypes were remarkably similar. They contained identical fatty acids, including the characteristic C20 and C22 very-long-chain fatty acids (VLCFAs). The total proportions of seed VLCFA varied between 22% and 35% w/w of the total seed fatty acid content.

A.A. Millar, S. Clemens, S. Zachgo, E.M. Giblin, D.C. Taylor and L. Kunst (1999) CUT1, an Arabidopsis gene required for cuticular wax biosynthesis and pollen fertility, encodes a very-long-chain fatty acid condensing enzyme. Plant Cell 11: 825-838. (abstract) | (full journal in .pdf)

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.

A.A. Millar, M. Wrischer and L. Kunst (1998) Accumulation of very long chain fatty acids in membrane glycerolipids is associated with dramatic alterations in plant morphology. Plant Cell 11: 1889-1902.
(abstract) | (full journal in .pdf)

Transgenic Arabidopsis plants overexpressing the Arabidopsis FATTY ACID ELONGATION 1 gene under the control of the 35S promoter from cauliflower mosaic virus accumulated very-long-chain fatty acids ( VLCFAs) throughout the plant. In some transformants, C20 and C22 VLCFAs accounted for . 30% of the total fatty acids, accumulating at the expense of C16 and C18 fatty acids. These C20 and C22 fatty acids were incorporated into all of the major membrane glycerolipid classes. Plants with a high VLCFA content displayed a dramatically altered morphology, which included the failure of flowering shoots to elongate, a modified spatial pattern of siliques, an altered floral phenotype, and a large accumulation of anthocyanins. In addition, these plants also exhibited a unique alteration of the chloroplast membrane structure. We discuss a possible role for VLCFAs in establishing the shape/curvature of the membranes, which in turn may affect the shape of the cell and ultimately that of the whole plant.

A.A. Millar and L. Kunst (1997) Very long chain fatty acid biosynthesis is controlled through the expression and specificity of the condensing enzyme. Plant J. 12: 121-131.
(abstract) | (full journal in .pdf)

The Arabidopsis FATTY ACID ELONGATION1 (FAE1) gene encodes a putative seed-specific condensing enzyme. It is the first of four enzyme activities that comprise the microsomal fatty acid elongase (FAE) involved in the biosynthesis of very-long-chain fatty acids (VLCFAs). FAE1 has been expressed in yeast and in tissues of Arabidopsis and tobacco, where significant quantities of VLCFAs are not found. The introduction of FAE1 alone in these systems is sufficient for the production of VLCFAs, for wherever FAE1 was expressed, VLCFAs accumulated. These results indicate that FAE1 is the rate-limiting enzyme for VLCFA biosynthesis in Arabidopsis seed, because introduction of extra copies of FAE1 resulted in higher levels of the VLCFAs. Furthermore, the condensing enzyme is the activity of the elongase that determines the acyl chain length of the VLCFAs produced. In contrast, it appears that the other three enzyme activities of the elongase are found ubiquitously throughout the plant, are not rate-limiting and play no role in the control of VLCFA synthesis. The ability of yeast containing FAE1 to synthesize VLCFAs suggests that the expression and the acyl chain length specificity of the condensing enzyme, along with the apparent broad specificities of the other three FAE activities, may be a universal eukaryotic mechanism for regulating the amounts and acyl chain length of VLCFAs synthesized.

V. Katavic, D.W. Reed, D.C. Taylor, E.M. Giblin, D.L. Barton, J. Zou, S.L. MacKenzie, P.S. Covello and L. Kunst (1995) Alteration of seed fatty acid composition by an EMS-induced mutation in Arabidopsis thaliana affecting diacylglycerol acyltransferase activity. Plant Physiol. 108: 399-409.
(abstract) | (full journal in .pdf)

In characterizing the enzymes involved in the formation of very long-chain fatty acids (VLCFAs) in the Brassicaceae, we have generated a series of mutants of Arabidopsis thaliana that have reduced VLCFA content. Here we report the characterization of a seed lipid mutant, as11, which, in comparison to wild type (WT), has reduced levels of 20:l and 18:l and accumulates 18:3 as the major fatty acid in triacylglycerols. Proportions of 18:2 remain similar to WT. Genetic analyses indicate that the fatty acid phenotype is caused by a semidominant mutation in a single nuclear gene, designated TAG1, located on chromosome 2. Biochemical analyses have shown that the as11 phenotype is not due to a deficiency in the capacity to elongate 18:l or to an increase in the relative delta 15 or delta 12 desaturase activities. Indeed, the ratio of desaturase/elongase activities measured in vitro is virtually identical in developing WT and as11 seed homogenates. Rather, the fatty acid phenotype of as11 is the result of reduced diacylglycerol acyltransferase activity throughout development, such that triacylglycerol biosynthesis is reduced. This leads to a reduction in 20:l biosynthesis during seed development, leaving more 18:l available for desaturation. Thus, we have demonstrated that changes to triacylglycerol biosynthesis can result in dramatic changes in fatty acid composition and, in particular, in the accumulation of VLCFAs in seed storage lipids.

L. Kunst, D.C. Taylor and E.W. Underhill (1992) Fatty acid elongation in developing seeds of Arabidopsis thaliana. Plant Physiol. Biochem. 30: 425-434.
(abstract) | (full journal in .pdf)

Six mutants of Arabidopsis thaliana (L.)Heynh., with reduced levels of very long chain fatty acids (VLCFAs) in their seed lipids, were isolated by direct screening of ethyl methane sulfonate-mutagenized populations using gas chromatography. the mutants fall into three complementation groups, indicating that the 18:1 elongation is controlled by several different loci. One line, AC56, was analyzed in detail. In this mutant, a single semidominant nuclear mutation in the FAE1 gene results in a deficiency in acyl chain elongation from 18:1 to 20:1, 20:1 to 22:1 and 18:0 to 20:0. This suggests that the product of the FAE1 gene is shared by three different elongation systems involved in saturated and monunsaturated VLCFA biosynthesis. the mutation does not cause any obvious changes in seed physiology, but does exert an effect on the fatty acid compostion of all the major seed lipids. In all cases, except for triacylglycerols, the proportion of 18:1 acyl group is increased at the expense of 16:0. the existence of a regulatory mechanism which determins 18:1 levels in seeds is discussed.

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