Technical Advance: a novel technique for the sensitive quantification of acyl CoA esters from plant tissues

Advances in Plant Metabolomics and Its Applications in Stress and Single-Cell Biology

In the past two decades, the post-genomic era envisaged high-throughput technologies, resulting in more species with available genome sequences. In-depth multi-omics approaches have evolved to integrate cellular processes at various levels into a systems biology knowledge base. Metabolomics plays a crucial role in molecular networking to bridge the gaps between genotypes and phenotypes. However, the greater complexity of metabolites with diverse chemical and physical properties has limited the advances in plant metabolomics. For several years, applications of liquid/gas chromatography (LC/GC)-mass spectrometry (MS) and nuclear magnetic resonance (NMR) have been constantly developed. Recently, ion mobility spectrometry (IMS)-MS has shown utility in resolving isomeric and isobaric metabolites. Both MS and NMR combined metabolomics significantly increased the identification and quantification of metabolites in an untargeted and targeted manner. Thus, hyphenated metabolomics tools will narrow the gap between the number of metabolite features and the identified metabolites. Metabolites change in response to environmental conditions, including biotic and abiotic stress factors. The spatial distribution of metabolites across different organs, tissues, cells and cellular compartments is a trending research area in metabolomics. Herein, we review recent technological advancements in metabolomics and their applications in understanding plant stress biology and different levels of spatial organization. In addition, we discuss the opportunities and challenges in multiple stress interactions, multi-omics, and single-cell metabolomics.

Concerted increases of FAE1 expression level and substrate availability improve and singularize the production of very-long-chain fatty acids in Arabidopsis seeds

Our initial goal was to evaluate the contributions of high 18:1 phosphatidylcholine and the expression level of FAE1 to the accumulation of very-long-chain fatty acids (VLCFAs), which have wide applications as industrial feedstocks. Unexpectedly, VLCFAs were not improved by increasing the proportions of 18:1 in fad2-1 mutant, FAD2 artificial miRNA, and FAD2 co-suppression lines. Expressing Arabidopsis FAE1 resulted in co-suppression in 90% of transgenic lines, which was effectively released when it was expressed in the rdr6-11 mutant host. When FAE1 could be highly expressed, apart from its naturally preferred product, 20:1, other saturated and polyunsaturated VLCFAs also accumulated in seeds. We postulated that overabundant FAE1 might cause the diversified VLCFA profile. When FAE1 was highly expressed, knocking down FAD2 increased the content of 20:1, suggesting that the 18:1 availability in the acyl-CoA pool increased from the high 18:1-PC via acyl editing. Concurrent decreases of side products like 22:1 and 20:0 in these lines suggest that increasing availability of the preferred substrate could suppress the side elongation reactions and reverse the effect of VLCFA product diversification due to overabundant FAE1. Re-analysis of FAD2 knockdown lines indicated that increasing 18:1 led to a decrease of 22:1, which also supports the above hypothesis. These results demonstrate that 18:1 substrate could be increased by a downregulation of FAD2 and that a balance between the levels of enzyme and substrate may be crucial for engineering-specific VLCFA products.

Lipid Isolation from Plants

Analysis of plant lipids provides insights into a range of biological processes, from photosynthetic membrane function to oil seed engineering. Many lipid extraction protocols are tailored to fit a specific lipid class. Here we describe a procedure for extraction of glycerolipids from vegetative tissue. This procedure is designed for 1 gram of tissue per sample but maybe scaled for larger samples.

Characterization of an alkylresorcinol synthase that forms phenolics accumulating in the cuticular wax on various organs of rye (Secale cereale)

Alkylresorcinols are bioactive compounds produced in diverse plant species, with chemical structures combining an aliphatic hydrocarbon chain and an aromatic ring with characteristic hydroxyl substituents. Here, we aimed to isolate and characterize the enzyme that forms the alkylresorcinols accumulating in the cuticular wax on the surface of all above-ground organs of rye. Based on sequence homology with other type-III polyketide synthases, a candidate alkylresorcinol synthase was cloned. Yeast heterologous expression showed that the enzyme, ScARS, is highly specific for the formation of the aromatic resorcinol ring structure, through aldol condensation analogous to stilbene synthases. The enzyme accepts long-chain and very-long-chain acyl-CoA starter substrates, preferring saturated over unsaturated chains. It typically carries out three rounds of condensation with malonyl-CoA prior to cyclization, with only very minor activity for a fourth round of malonyl-CoA condensation and cyclization to 5-(2'-oxo)-alkylresorcinols or 5-(2'-hydroxy)-alkylresorcinols. Like other enzymes involved in cuticle formation, ScARS is localized to the endoplasmic reticulum. ScARS expression patterns were found correlated with alkylresorcinol accumulation during leaf development and across different rye organs. Overall, our results thus suggest that ScARS synthesizes the cuticular alkylresorcinols found on diverse rye organ surfaces.

Kinetic improvement of an algal diacylglycerol acyltransferase 1 via fusion with an acyl-CoA binding protein

Microalgal oils in the form of triacylglycerols (TAGs) are broadly used as nutritional supplements and biofuels. Diacylglycerol acyltransferase (DGAT) catalyzes the final step of acyl-CoA-dependent biosynthesis of TAG, and is considered a key target for manipulating oil production. Although a growing number of DGAT1s have been identified and over-expressed in some algal species, the detailed structure-function relationship, as well as the improvement of DGAT1 performance via protein engineering, remain largely untapped. Here, we explored the structure-function features of the hydrophilic N-terminal domain of DGAT1 from the green microalga Chromochloris zofingiensis (CzDGAT1). The results indicated that the N-terminal domain of CzDGAT1 was less disordered than those of the higher eukaryotic enzymes and its partial truncation or complete removal could substantially decrease enzyme activity, suggesting its possible role in maintaining enzyme performance. Although the N-terminal domains of animal and plant DGAT1s were previously found to bind acyl-CoAs, replacement of CzDGAT1 N-terminus by an acyl-CoA binding protein (ACBP) could not restore enzyme activity. Interestingly, the fusion of ACBP to the N-terminus of the full-length CzDGAT1 could enhance the enzyme affinity for acyl-CoAs and augment protein accumulation levels, which ultimately drove oil accumulation in yeast cells and tobacco leaves to higher levels than the full-length CzDGAT1. Overall, our findings unravel the distinct features of the N-terminus of algal DGAT1 and provide a strategy to engineer enhanced performance in DGAT1 via protein fusion, which may open a vista in generating improved membrane-bound acyl-CoA-dependent enzymes and boosting oil biosynthesis in plants and oleaginous microorganisms.

Palmitvaccenic Acid (Δ11-cis-hexadecenoic acid) Is Synthesized by an OLE1-like Desaturase in the Arbuscular Mycorrhiza Fungus Rhizophagus irregularis

Arbuscular mycorrhiza (AM) fungi deliver mineral nutrients to the plant host in exchange for reduced carbon in the form of sugars and lipids. Colonization with AM fungi upregulates a specific host lipid synthesis pathway resulting in the production of fatty acids. Predominantly palmitic acid (16:0) and the unusual palmitvaccenic acid (16:1^Δ11cis) accumulate in the fungus Rhizophagus irregularis. Here, we present the isolation and characterization of RiOLE1-LIKE, the desaturase involved in palmitvaccenic acid synthesis, by heterologous expression in yeast and plants. Results are in line with the scenario in which RiOLE1-LIKE encodes an acyl-CoA desaturase with substrate specificity for C15-C18 acyl groups, in particular C16. Phylogenetic analysis of RiOLE1-LIKE-related sequences revealed that this gene is conserved in AM fungi from the Glomales and Diversisporales but is absent from nonsymbiotic Mortierellaceae and Mucoromycotina fungi, suggesting that 16:1^Δ11cis provides a specific function during AM colonization.

Polyunsaturated linolenoyl-CoA modulates ERF-VII-mediated hypoxia signaling in Arabidopsis

In plants, submergence from flooding causes hypoxia, which impairs energy production and affects plant growth, productivity, and survival. In Arabidopsis, hypoxia induces nuclear localization of the group VII ethylene-responsive transcription factor RELATED TO AP2.12 (RAP2.12), following its dissociation from the plasma membrane-anchored ACYL-COA BINDING PROTEIN1 (ACBP1) and ACBP2. Here, we show that polyunsaturated linolenoyl-CoA (18:3-CoA) regulates RAP2.12 release from the plasma membrane. Submergence caused a significant increase in 18:3-CoA, but a significant decrease in 18:0-, 18:1-, and 18:2-CoA. Application of 18:3-CoA promoted nuclear accumulation of the green fluorescent protein (GFP) fusions RAP2.12-GFP, HYPOXIA-RESPONSIVE ERF1-GFP, and RAP2.3-GFP, and enhanced transcript levels of hypoxia-responsive genes. Plants with decreased ACBP1 and ACBP2 (acbp1 ACBP2-RNAi, produced by ACBP2 RNA interference in the acbp1 mutant) had reduced tolerance to hypoxia and impaired 18:3-CoA-induced expression of hypoxia-related genes. In knockout mutants and overexpression lines of LONG-CHAIN ACYL-COA SYNTHASE2 (LACS2) and FATTY ACID DESATURASE 3 (FAD3), the acyl-CoA pool size and 18:3-CoA levels were closely related to ERF-VII-mediated signaling and hypoxia tolerance. These findings demonstrate that polyunsaturation of long-chain acyl-CoAs functions as important mechanism in the regulation of plant hypoxia signaling, by modulating ACBP-ERF-VII dynamics.

Functional Characterization of Lysophosphatidylcholine: Acyl-CoA Acyltransferase Genes From Sunflower (Helianthus annuus L.)

Lysophosphatidylcholine acyltransferase (LPCAT, EC 2.3.1.23) is an evolutionarily conserved key enzyme in the Lands cycle that catalyzes acylation of lysophosphatidylcholine (LPC) to produce phosphatidylcholine (PC), the main phospholipid in cellular membranes. In this study, three LPCAT genes from sunflower were identified and the corresponding proteins characterized. These HaLPCAT genes encoded functionally active enzymes that were able to complement a deficient yeast mutant. Moreover, enzymatic assays were carried out using microsomal preparations of the yeast cells. When acyl specificities were measured in the forward reaction, these enzymes exhibited a substrate preference for unsaturated acyl-CoAs, especially for linolenoyl-CoA, while in the reverse reaction, linoleoyl or linolenoyl acyl groups were transferred from PC to acyl-CoA to a similar extent. Expression levels of LPCAT genes were studied revealing distinct tissue-specific expression patterns. In summary, this study suggests that the combined forward and reverse reactions catalyzed by sunflower LPCATs facilitate acyl-exchange between the sn-2 position of PC and the acyl-CoA pool. Sunflower LPCATs displayed different characteristics, which could point to different functionalities, favoring the enrichment of seed triacylglycerols (TAGs) with polyunsaturated fatty acid (PUFA).

Intestinal peroxisomal fatty acid β-oxidation regulates neural serotonin signaling through a feedback mechanism

The ability to coordinate behavioral responses with metabolic status is fundamental to the maintenance of energy homeostasis. In numerous species including Caenorhabditis elegans and mammals, neural serotonin signaling regulates a range of food-related behaviors. However, the mechanisms that integrate metabolic information with serotonergic circuits are poorly characterized. Here, we identify metabolic, molecular, and cellular components of a circuit that links peripheral metabolic state to serotonin-regulated behaviors in C. elegans. We find that blocking the entry of fatty acyl coenzyme As (CoAs) into peroxisomal β-oxidation in the intestine blunts the effects of neural serotonin signaling on feeding and egg-laying behaviors. Comparative genomics and metabolomics revealed that interfering with intestinal peroxisomal β-oxidation results in a modest global transcriptional change but significant changes to the metabolome, including a large number of changes in ascaroside and phospholipid species, some of which affect feeding behavior. We also identify body cavity neurons and an ether-a-go-go (EAG)-related potassium channel that functions in these neurons as key cellular components of the circuitry linking peripheral metabolic signals to regulation of neural serotonin signaling. These data raise the possibility that the effects of serotonin on satiety may have their origins in feedback, homeostatic metabolic responses from the periphery.

Agrobacterium-Mediated Transient Gene Expression in Developing Ricinus communis Seeds: A First Step in Making the Castor Oil Plant a Chemical Biofactory

The castor oil plant represents a promising platform to produce oils with industrial applications. However, its use in biotechnology is limited by the absence of a well-established procedure to transform it, and a poor understanding of gene regulation and promoter use in this species. As such, a method has been developed to express proteins or hairpin-RNA in this plant, a method based on the direct injection of Agrobacterium into the developing endosperm of castor oil fruit, enabling different constructs and promoters to be tested. This method produces a high rate of transformation and a good proportion of viable seeds that express reporter genes for up to 20 days after infiltration (DAI). Gene expression under the control of different promoters was tested by quantitative real-time polymerase chain reaction and by directly assaying the activity of the galactouronidase reporter gene, which proved to be strongest when driven by the glycinin promoter. Constructs expressing a fatty acid elongase from Lesquerella fendleri were tested, the expression of which provoked an important increase in the lesquerolic acid in the castor oil endosperm at 5 and 10 DAI, although this fatty acid did not accumulate significantly in the final mature seeds. The nature of this response could reflect the poor availability of substrates for this enzyme. In the light of this data, the potential of this technique to test promoters and different constructs in castor oil plants and other oilseeds is discussed.

Disruptions in valine degradation affect seed development and germination in Arabidopsis

We have functionally characterized the role of two putative mitochondrial enzymes in valine degradation using insertional mutants. Prior to this study, the relationship between branched-chain amino acid degradation (named for leucine, valine and isoleucine) and seed development was limited to leucine catabolism. Using a reverse genetics approach, we show that disruptions in the mitochondrial valine degradation pathway affect seed development and germination in Arabidopsis thaliana. A null mutant of 3-hydroxyisobutyryl-CoA hydrolase (CHY4, At4g31810) resulted in an embryo lethal phenotype, while a null mutant of methylmalonate semialdehyde dehydrogenase (MMSD, At2g14170) resulted in seeds with wrinkled coats, decreased storage reserves, elevated valine and leucine, and reduced germination rates. These data highlight the unique contributions CHY4 and MMSD make to the overall growth and viability of plants. It also increases our knowledge of the role branched-chain amino acid catabolism plays in seed development and amino acid homeostasis.

Identification and characterization of a plastidial phosphatidylglycerophosphate phosphatase in Arabidopsis thaliana

Phosphatidylglycerol (PG) is the only phospholipid in the thylakoid membranes of chloroplasts of plants, and it is also found in extraplastidial membranes including mitochondria and the endoplasmic reticulum. Previous studies showed that lack of PG in the pgp1-2 mutant of Arabidopsis deficient in phosphatidylglycerophosphate (PGP) synthase strongly affects thylakoid biogenesis and photosynthetic activity. In the present study, the gene encoding the enzyme for the second step of PG synthesis, PGP phosphatase, was isolated based on sequence similarity to the yeast GEP4 and Chlamydomonas PGPP1 genes. The Arabidopsis AtPGPP1 protein localizes to chloroplasts and harbors PGP phosphatase activity with alkaline pH optimum and divalent cation requirement. Arabidopsis pgpp1-1 mutant plants contain reduced amounts of chlorophyll, but photosynthetic quantum yield remains unchanged. The absolute content of plastidial PG (34:4; total number of acyl carbons:number of double bonds) is reduced by about 1/3, demonstrating that AtPGPP1 is involved in the synthesis of plastidial PG. PGP 34:3, PGP 34:2 and PGP 34:1 lacking 16:1 accumulate in pgpp1-1, indicating that the desaturation of 16:0 to 16:1 by the FAD4 desaturase in the chloroplasts only occurs after PGP dephosphorylation.

Modulation of Potassium Channel Activity in the Balance of ROS and ATP Production by Durum Wheat Mitochondria-An Amazing Defense Tool Against Hyperosmotic Stress

In plants, the existence of a mitochondrial potassium channel was firstly demonstrated about 15 years ago in durum wheat as an ATP-dependent potassium channel (PmitoKATP). Since then, both properties of the original PmitoKATP and occurrence of different mitochondrial potassium channels in a number of plant species (monocotyledonous and dicotyledonous) and tissues/organs (etiolated and green) have been shown. Here, an overview of the current knowledge is reported; in particular, the issue of PmitoKATP physiological modulation is addressed. Similarities and differences with other potassium channels, as well as possible cross-regulation with other mitochondrial proteins (Plant Uncoupling Protein, Alternative Oxidase, Plant Inner Membrane Anion Channel) are also described. PmitoKATP is inhibited by ATP and activated by superoxide anion, as well as by free fatty acids (FFAs) and acyl-CoAs. Interestingly, channel activation increases electrophoretic potassium uptake across the inner membrane toward the matrix, so collapsing membrane potential (ΔΨ), the main component of the protonmotive force (Δp) in plant mitochondria; moreover, cooperation between PmitoKATP and the K(+)/H(+) antiporter allows a potassium cycle able to dissipate also ΔpH. Interestingly, ΔΨ collapse matches with an active control of mitochondrial reactive oxygen species (ROS) production. Fully open channel is able to lower superoxide anion up to 35-fold compared to a condition of ATP-inhibited channel. On the other hand, ΔΨ collapse by PmitoKATP was unexpectedly found to not affect ATP synthesis via oxidative phosphorylation. This may probably occur by means of a controlled collapse due to ATP inhibition of PmitoKATP; this brake to the channel activity may allow a loss of the bulk phase Δp, but may preserve a non-classically detectable localized driving force for ATP synthesis. This ability may become crucial under environmental/oxidative stress. In particular, under moderate hyperosmotic stress (mannitol or NaCl), PmitoKATP was found to be activated by ROS, so inhibiting further large-scale ROS production according to a feedback mechanism; moreover, a stress-activated phospholipase A2 may generate FFAs, further activating the channel. In conclusion, a main property of PmitoKATP is the ability to keep in balance the control of harmful ROS with the mitochondrial/cellular bioenergetics, thus preserving ATP for energetic needs of cell defense under stress.

An alternative pathway for the effective production of the omega-3 long-chain polyunsaturates EPA and ETA in transgenic oilseeds

The synthesis and accumulation of omega-3 long-chain polyunsaturated fatty acids in transgenic Camelina sativa is demonstrated using the so-called alternative pathway. This aerobic pathway is found in a small number of taxonomically unrelated unicellular organisms and utilizes a C18 Δ9-elongase to generate C20 PUFAs. Here, we evaluated four different combinations of seed-specific transgene-derived activities to systematically determine the potential of this pathway to direct the synthesis of eicosapentaenoic acid (EPA) in transgenic plants. The accumulation of EPA and the related omega-3 LC-PUFA eicosatetraenoic acid (ETA) was observed up to 26.4% of total seed fatty acids, of which ETA was 9.5%. Seed oils such as these not only represent an additional source of EPA, but also an entirely new source of the bona fide fish oil ETA. Detailed lipidomic analysis of the alternative pathway in Camelina revealed that the acyl-substrate preferences of the different activities in the pathway can still generate a substrate-dichotomy bottleneck, largely due to inefficient acyl-exchange from phospholipids into the acyl-CoA pool. However, significant levels of EPA and ETA were detected in the triacylglycerols of transgenic seeds, confirming the channelling of these fatty acids into this storage lipid.

Successful high-level accumulation of fish oil omega-3 long-chain polyunsaturated fatty acids in a transgenic oilseed crop

Omega-3 (also called n-3) long-chain polyunsaturated fatty acids (≥C20; LC-PUFAs) are of considerable interest, based on clear evidence of dietary health benefits and the concurrent decline of global sources (fish oils). Generating alternative transgenic plant sources of omega-3 LC-PUFAs, i.e. eicosapentaenoic acid (20:5 n-3, EPA) and docosahexaenoic acid (22:6 n-3, DHA) has previously proved problematic. Here we describe a set of heterologous genes capable of efficiently directing synthesis of these fatty acids in the seed oil of the crop Camelina sativa, while simultaneously avoiding accumulation of undesirable intermediate fatty acids. We describe two iterations: RRes_EPA in which seeds contain EPA levels of up to 31% (mean 24%), and RRes_DHA, in which seeds accumulate up to 12% EPA and 14% DHA (mean 11% EPA and 8% DHA). These omega-3 LC-PUFA levels are equivalent to those in fish oils, and represent a sustainable, terrestrial source of these fatty acids. We also describe the distribution of these non-native fatty acids within C. sativa seed lipids, and consider these data in the context of our current understanding of acyl exchange during seed oil synthesis.

An inhibitor of oil body mobilization in Arabidopsis

Fatty acid β-oxidation is an essential process in many aspects of plant development, and storage oil in the form of triacylglycerol (TAG) is an important food source for humans and animals, for biofuel and for industrial feedstocks. In this study we characterize the effects of a small molecule, diphenyl methylphosphonate, on oil mobilization in Arabidopsis thaliana. Confocal laser scanning microscopy, transmission electron microscopy and quantitative lipid profiling were used to examine the effects of diphenyl methylphosphonate treatment on seedlings. Diphenyl methylphosphonate causes peroxisome clustering around oil bodies but does not affect morphology of other cellular organelles. We show that this molecule blocks the breakdown of pre-existing oil bodies resulting in retention of TAG and accumulation of acyl CoAs. The biochemical and phenotypic effects are consistent with a block in the early part of the β-oxidation pathway. Diphenyl methylphosphonate appears to be a fairly specific inhibitor of TAG mobilization in plants and whilst further work is required to identify the molecular target of the compound it should prove a useful tool to interrogate and manipulate these pathways in a controlled and reproducible manner.

The Arabidopsis acbp1acbp2 double mutant lacking acyl-CoA-binding proteins ACBP1 and ACBP2 is embryo lethal

*In Arabidopsis thaliana, the amino acid sequences of membrane-associated acyl-CoA-binding proteins ACBP1 and ACBP2 are highly conserved. We have shown previously that, in developing seeds, ACBP1 accumulates in the cotyledonary cells of embryos and ACBP1 is proposed to be involved in lipid transfer. We show here by immunolocalization, using ACBP2-specific antibodies, that ACBP2 is also expressed in the embryos at various stages of seed development in Arabidopsis. *Phenotypic analyses of acbp1 and acbp2 single mutants revealed that knockout of either ACBP1 or ACBP2 alone did not affect their life cycle as both single mutants exhibited normal growth and development similar to the wild-type. However, the acbp1acbp2 double mutant was embryo lethal and was also defective in callus induction. *On lipid and acyl-CoA analyses, the siliques, but not the leaves, of the acbp1 mutant accumulated galactolipid monogalactosyldiacylglycerol and 18:0-CoA, but the levels of most polyunsaturated species of phospholipid, such as phosphatidylcholine, phosphatidylethanolamine, phosphatidylinositol and phosphatidylserine, declined. *As recombinant ACBP1 and ACBP2 bind unsaturated phosphatidylcholine and acyl-CoA esters in vitro, we propose that ACBP1 and ACBP2 are essential in lipid transfer during early embryogenesis in Arabidopsis.

Substrate preferences of a lysophosphatidylcholine acyltransferase highlight its role in phospholipid remodeling

An important enzyme involved in phospholipid turnover is the acyl-CoA: lysophosphatidylcholine acyltransferase (LPCAT). Here, we report characterization of a newly discovered human LPCAT (LPCAT3), which has distinct substrate preferences strikingly consistent with a role in phosphatidylcholine (PtdCho) remodeling and modulating fatty acid composition of PtdCho. LPCAT3 prefers lysophosphatidylcholine (lysoPtdCho) with saturated fatty acid at the sn-1 position and exhibits acyl donor preference towards linoleoyl-CoA and arachidonoyl-CoA. Furthermore, LPCAT3 is active in mediating 1-O-alkyl-sn-glycero-3-phosphocholine acylation with long chain fatty acyl-CoAs to generate 1-O-alkyl-phosphatidylcholine, another very important constitute of mammalian membrane systems. These properties are precisely the known attributes of LPCAT previously ascribed to the isoform involved in Lands' cycle, and thus strongly suggest that LPCAT3 is involved in phospholipids remodeling to achieve appropriate membrane lipid fatty acid composition.

An Arabidopsis mutant impaired in coenzyme A biosynthesis is sugar dependent for seedling establishment

Once the plant coenzyme A (CoA) biosynthetic pathway has been elucidated by comparative genomics, it is feasible to analyze the physiological relevance of CoA biosynthesis in plant life. To this end, we have identified and characterized Arabidopsis (Arabidopsis thaliana) T-DNA knockout mutants of two CoA biosynthetic genes, HAL3A and HAL3B. The HAL3A gene encodes a 4'-phosphopantothenoyl-cysteine decarboxilase that generates 4'-phosphopantetheine. A second gene, HAL3B, whose gene product is 86% identical to that of HAL3A, is present in the Arabidopsis genome. HAL3A appears to have a predominant role over HAL3B according to their respective mRNA expression levels. The hal3a-1, hal3a-2, and hal3b mutants were viable and showed a similar growth rate as that in wild-type plants; in contrast, a hal3a-1 hal3b double mutant was embryo lethal. Unexpectedly, seedlings that were null for HAL3A and heterozygous for HAL3B (aaBb genotype) displayed a sucrose (Suc)-dependent phenotype for seedling establishment, which is in common with mutants defective in beta-oxidation. This phenotype was genetically complemented in aaBB siblings of the progeny and chemically complemented by pantethine. In contrast, seedling establishment of Aabb plants was not Suc dependent, proving a predominant role of HAL3A over HAL3B at this stage. Total fatty acid and acyl-CoA measurements of 5-d-old aaBb seedlings in medium lacking Suc revealed stalled storage lipid catabolism and impaired CoA biosynthesis; in particular, acetyl-CoA levels were reduced by approximately 80%. Taken together, these results provide in vivo evidence for the function of HAL3A and HAL3B, and they point out the critical role of CoA biosynthesis during early postgerminative growth.

Identification of a long-chain polyunsaturated fatty acid acyl-coenzyme A synthetase from the diatom Thalassiosira pseudonana

The draft genome of the diatom Thalassiosira pseudonana was searched for DNA sequences showing homology with long-chain acyl-coenzyme A synthetases (LACSs), since the corresponding enzyme may play a key role in the accumulation of health-beneficial polyunsaturated fatty acids (PUFAs) in triacylglycerol. Among the candidate genes identified, an open reading frame named TplacsA was found to be full length and constitutively expressed during cell cultivation. The predicted amino acid sequence of the corresponding protein, TpLACSA, exhibited typical features of acyl-coenzyme A (acyl-CoA) synthetases involved in the activation of long-chain fatty acids. Feeding experiments carried out in yeast (Saccharomyces cerevisiae) transformed with the algal gene showed that TpLACSA was able to activate a number of PUFAs, including eicosapentaenoic acid and docosahexaenoic acid (DHA). Determination of acyl-CoA synthetase activities by direct measurement of acyl-CoAs produced in the presence of different PUFA substrates showed that TpLACSA was most active toward DHA. Heterologous expression also revealed that TplacsA transformants were able to incorporate more DHA in triacylglycerols than the control yeast.