The Arabidopsis thaliana TAG1 mutant has a mutation in a diacylglycerol acyltransferase gene

Mining the bitter melon (momordica charantia l.) seed transcriptome by 454 analysis of non-normalized and normalized cDNA populations for conjugated fatty acid metabolism-related genes

BACKGROUND

Seeds of Momordica charantia (bitter melon) produce high levels of eleostearic acid, an unusual conjugated fatty acid with industrial value. Deep sequencing of non-normalized and normalized cDNAs from developing bitter melon seeds was conducted to uncover key genes required for biotechnological transfer of conjugated fatty acid production to existing oilseed crops. It is expected that these studies will also provide basic information regarding the metabolism of other high-value novel fatty acids.

RESULTS

Deep sequencing using 454 technology with non-normalized and normalized cDNA libraries prepared from bitter melon seeds at 18 DAP resulted in the identification of transcripts for the vast majority of known genes involved in fatty acid and triacylglycerol biosynthesis. The non-normalized library provided a transcriptome profile of the early stage in seed development that highlighted the abundance of transcripts for genes encoding seed storage proteins as well as for a number of genes for lipid metabolism-associated polypeptides, including Δ12 oleic acid desaturases and fatty acid conjugases, class 3 lipases, acyl-carrier protein, and acyl-CoA binding protein. Normalization of cDNA by use of a duplex-specific nuclease method not only increased the overall discovery of genes from developing bitter melon seeds, but also resulted in the identification of 345 contigs with homology to 189 known lipid genes in Arabidopsis. These included candidate genes for eleostearic acid metabolism such as diacylglycerol acyltransferase 1 and 2, and a phospholipid:diacylglycerol acyltransferase 1-related enzyme. Transcripts were also identified for a novel FAD2 gene encoding a functional Δ12 oleic acid desaturase with potential implications for eleostearic acid biosynthesis.

CONCLUSIONS

454 deep sequencing, particularly with normalized cDNA populations, was an effective method for mining of genes associated with eleostearic acid metabolism in developing bitter melon seeds. The transcriptomic data presented provide a resource for the study of novel fatty acid metabolism and for the biotechnological production of conjugated fatty acids and possibly other novel fatty acids in established oilseed crops.

Identification and expression analysis of castor bean (Ricinus communis) genes encoding enzymes from the triacylglycerol biosynthesis pathway

Castor bean (Ricinus communis) oil contains ricinoleic acid-rich triacylglycerols (TAGs). As a result of its physical and chemical properties, castor oil and its derivatives are used for numerous bio-based products. In this study, we survey the Castor Bean Genome Database to report the identification of TAG biosynthesis genes. A set of 26 genes encoding six distinct classes of enzymes involved in TAGs biosynthesis were identified. In silico characterization and sequence analysis allowed the identification of plastidic isoforms of glycerol-3-phosphate acyltransferase and lysophosphatidate acyltransferase enzyme families, involved in the prokaryotic lipid biosynthesis pathway, that form a cluster apart from the cytoplasmic isoforms, involved in the eukaryotic pathway. In addition, two distinct membrane bound diacylglycerol acyltransferase enzymes were identified. Quantitative expression pattern analyses demonstrated variations in gene expressions during castor seed development. A tendency of maximum expression level at the middle of seed development was observed. Our results represent snapshots of global transcriptional activities of genes encompassing six enzyme families involved in castor bean TAG biosynthesis that are present during seed development. These genes represent potential targets for biotechnological approaches to produce nutritionally and industrially desirable oils.

Acyl-lipid metabolism

Acyl lipids in Arabidopsis and all other plants have a myriad of diverse functions. These include providing the core diffusion barrier of the membranes that separates cells and subcellular organelles. This function alone involves more than 10 membrane lipid classes, including the phospholipids, galactolipids, and sphingolipids, and within each class the variations in acyl chain composition expand the number of structures to several hundred possible molecular species. Acyl lipids in the form of triacylglycerol account for 35% of the weight of Arabidopsis seeds and represent their major form of carbon and energy storage. A layer of cutin and cuticular waxes that restricts the loss of water and provides protection from invasions by pathogens and other stresses covers the entire aerial surface of Arabidopsis. Similar functions are provided by suberin and its associated waxes that are localized in roots, seed coats, and abscission zones and are produced in response to wounding. This chapter focuses on the metabolic pathways that are associated with the biosynthesis and degradation of the acyl lipids mentioned above. These pathways, enzymes, and genes are also presented in detail in an associated website (ARALIP: http://aralip.plantbiology.msu.edu/). Protocols and methods used for analysis of Arabidopsis lipids are provided. Finally, a detailed summary of the composition of Arabidopsis lipids is provided in three figures and 15 tables.

Identification and characterization of an acyl-CoA:diacylglycerol acyltransferase 2 (DGAT2) gene from the microalga O. tauri

In order to identify novel genes encoding enzymes involved in the terminal step of triacylglycerol (TAG) formation, a database search was carried out in the genome of the unicellular photoautotrophic green alga Ostreococcus tauri. The search led to the identification of three putative type 2 acyl-CoA:diacylglycerol acyltransferase-like sequences (DGAT; EC 2.3.1.20), and revealed the absence of any homolog to type 1 or type 3 DGAT sequence in the genome of O. tauri. For two of the cDNA sequences (OtDGAT2A and B) enzyme activity was detected by heterologous expression in Saccharomyces cerevisiae mutant strains with impaired TAG metabolism. However, activity of OtDGAT2A was too low for further analysis. Analysis of their amino acid sequences showed that they share limited identity with other DGAT2 from different plant species, such as Ricinus communis and Vernicia fordii with approximately 25 to 30% identity. Lipid analysis of the mutant yeast cells revealed that OtDGAT2B showed broad substrate specificity accepting saturated as well as mono- and poly-unsaturated acyl-CoAs as substrates.

A distinct DGAT with sn-3 acetyltransferase activity that synthesizes unusual, reduced-viscosity oils in Euonymus and transgenic seeds

Endosperm and embryo tissues from the seeds of Euonymus alatus (Burning Bush) accumulate high levels of 3-acetyl-1,2-diacyl-sn-glycerols (acTAGs) as their major storage lipids. In contrast, the aril tissue surrounding the seed produces long-chain triacylglycerols (lcTAGs) typical of most other organisms. The presence of the sn-3 acetyl group imparts acTAGs with different physical and chemical properties, such as a 30% reduction in viscosity, compared to lcTAGs. Comparative transcriptome analysis of developing endosperm and aril tissues using pyrosequencing technology was performed to isolate the enzyme necessary for the synthesis of acTAGs. An uncharacterized membrane-bound O-acyltransferase (MBOAT) family member was the most abundant acyltransferase in the endosperm but was absent from the aril. Expression of this MBOAT in yeast resulted in the accumulation of acTAGs but not lcTAG; hence, the enzyme was named EaDAcT (Euonymus alatus diacylglycerol acetyltransferase). Yeast microsomes expressing EaDAcT possessed acetyl-CoA diacylglycerol acetyltransferase activity but lacked long-chain acyl-CoA diacylglycerol acyltransferase activity. Expression of EaDAcT under the control of a strong, seed-specific promoter in Arabidopsis resulted in the accumulation of acTAGs, up to 40 mol % of total TAG in the seed oil. These results demonstrate the utility of deep transcriptional profiling with multiple tissues as a gene discovery strategy for low-abundance proteins. They also show that EaDAcT is the acetyltransferase necessary and sufficient for the production of acTAGs in Euonymus seeds, and that this activity can be introduced into the seeds of other plants, allowing the evaluation of these unusual TAGs for biofuel and other applications.

Lipid biochemists salute the genome

The biochemistry of plant metabolic pathways has been studied for many generations; nevertheless, numerous new enzymes and metabolic products have been discovered in the last 5-10 years. More importantly, many intriguing questions remain in all areas of metabolism. In this review, we consider these issues with respect to several pathways of lipid metabolism and the contributions made by the Arabidopsis genome sequence and the tools that it has spawned. These tools have allowed identification of enzymes and transporters required for the mobilization of seed storage lipids, as well as transporters that facilitate movement of lipids from the endoplasmic reticulum to the chloroplast in green leaf cells. Genomic tools were important in recognition of novel components of the cutin and suberin polymers that form water-impermeable barriers in plants. The waxes that also contribute to these barriers are exported from cells of the epidermis by transporters that are now being identified. Biochemical and genetic knowledge from yeast and animals has permitted successful homology-based searches of the Arabidopsis genome for genes encoding enzymes involved in the elongation of fatty acids and the synthesis of sphingolipids. Knowledge of the genome has identified novel enzymes for the biosynthesis of the seed storage lipid, triacylglycerol, and provided a refined understanding of how the pathways of fatty acid and triacylglycerol synthesis are integrated into overall carbon metabolism in developing seeds.

DGAT1, DGAT2 and PDAT expression in seeds and other tissues of epoxy and hydroxy fatty acid accumulating plants

Triacylglycerol (TAG) is the main storage lipid in plants. Acyl-CoA: diacylglycerol acyltransferase (DGAT1 and DGAT2) and phospholipid: diacylglycerol acyltransferase (PDAT) can catalyze TAG synthesis. It is unclear how these three independent genes are regulated in developing seeds, and particularly if they have specific functions in the high accumulation of unusual fatty acids in seed oil. The expression patterns of DGAT1, DGAT2 and a PDAT in relation to the accumulation of oil and epoxy and hydroxy fatty acids in developing seeds of the plant species Vernonia galamensis, Euphorbia lagascae, Stokesia laevis and castor that accumulate high levels of these fatty acids in comparison with soybean and Arabidopsis were investigated. The expression patterns of DGAT1, DGAT2 and the PDAT are consistent with all three enzymes playing a role in the high epoxy or hydroxy fatty acid accumulation in developing seeds of these plants. PDAT and DGAT2 transcript levels are present at much higher levels in developing seeds of epoxy and hydroxy fatty acid accumulating plants than in soybeans or Arabidopsis. Moreover, PDAT, DGAT1 and DGAT2 are found to be expressed in many different plant tissues, suggesting that these enzymes may have other roles in addition to seed oil accumulation. DGAT1 appears to be a major enzyme for seed oil accumulation at least in Arabidopsis and soybeans. For the epoxy and hydroxy fatty acid accumulating plants, DGAT2 and PDAT also show expression patterns consistent with a role in the selective accumulation of these unusual fatty acids in seed oil.

Involvement of the phospholipid sterol acyltransferase1 in plant sterol homeostasis and leaf senescence

Genes encoding sterol ester-forming enzymes were recently identified in the Arabidopsis (Arabidopsis thaliana) genome. One belongs to a family of six members presenting homologies with the mammalian Lecithin Cholesterol Acyltransferases. The other one belongs to the superfamily of Membrane-Bound O-Acyltransferases. The physiological functions of these genes, Phospholipid Sterol Acyltransferase1 (PSAT1) and Acyl-CoA Sterol Acyltransferase1 (ASAT1), respectively, were investigated using Arabidopsis mutants. Sterol ester content decreased in leaves of all mutants and was strongly reduced in seeds from plants carrying a PSAT1-deficient mutation. The amount of sterol esters in flowers was very close to that of the wild type for all lines studied. This indicated further functional redundancy of sterol acylation in Arabidopsis. We performed feeding experiments in which we supplied sterol precursors to psat1-1, psat1-2, and asat1-1 mutants. This triggered the accumulation of sterol esters (stored in cytosolic lipid droplets) in the wild type and the asat1-1 lines but not in the psat1-1 and psat1-2 lines, indicating a major contribution of the PSAT1 in maintaining free sterol homeostasis in plant cell membranes. A clear biological effect associated with the lack of sterol ester formation in the psat1-1 and psat1-2 mutants was an early leaf senescence phenotype. Double mutants lacking PSAT1 and ASAT1 had identical phenotypes to psat1 mutants. The results presented here suggest that PSAT1 plays a role in lipid catabolism as part of the intracellular processes at play in the maintenance of leaf viability during developmental aging.

DGAT1 and PDAT1 acyltransferases have overlapping functions in Arabidopsis triacylglycerol biosynthesis and are essential for normal pollen and seed development

Triacylglycerol (TAG) biosynthesis is a principal metabolic pathway in most organisms, and TAG is the major form of carbon storage in many plant seeds. Acyl-CoA:diacylglycerol acyltransferase 1 (DGAT1) is the only acyltransferase enzyme that has been confirmed to contribute to TAG biosynthesis in Arabidopsis thaliana seeds. However, dgat1 null mutants display only a 20 to 40% decrease in seed oil content. To determine whether other enzymes contribute to TAG synthesis, candidate genes were expressed in TAG-deficient yeast, candidate mutants were crossed with the dgat1-1 mutant, and target genes were suppressed by RNA interference (RNAi). An in vivo role for phospholipid:diacylglycerol acyltransferase 1 (PDAT1; At5g13640) in TAG synthesis was revealed in this study. After failing to obtain double homozygous plants from crossing dgat1-1 and pdat1-2, further investigation showed that the dgat1-1 pdat1-2 double mutation resulted in sterile pollen that lacked visible oil bodies. RNAi silencing of PDAT1 in a dgat1-1 background or DGAT1 in pdat1-1 background resulted in 70 to 80% decreases in oil content per seed and in disruptions of embryo development. These results establish in vivo involvement of PDAT1 in TAG biosynthesis, rule out major contributions by other candidate enzymes, and indicate that PDAT1 and DGAT1 have overlapping functions that are essential for normal pollen and seed development of Arabidopsis.

An AP2-type transcription factor, WRINKLED1, of Arabidopsis thaliana binds to the AW-box sequence conserved among proximal upstream regions of genes involved in fatty acid synthesis

Although an APETALA2 (AP2)-type transcription factor, WRINKLED1 (WRI1), has been shown to be required for accumulation of triacylglycerols (TAGs) in Arabidopsis seeds, its direct target genes have not been established. Overexpression of WRI1 up-regulated a set of genes involved in fatty acid (FA) synthesis in plastids, including genes for a subunit of pyruvate kinase (Pl-PKbeta1), acetyl-CoA carboxylase (BCCP2), acyl carrier protein (ACP1), and ketoacyl-acyl carrier protein synthase (KAS1), while expression of these genes is reduced in mutants with reduced WRI1 expression. Transient expression of LUC reporter genes with the proximal sequences upstream from the ATG codon of Pl-PKbeta1, BCCP2, and KAS1 in protoplasts was activated by co-expression of WRI1, and recombinant WRI1 bound to these upstream sequences in vitro. The seven WRI1 binding sites shared a sequence [CnTnG](n)(7)[CG], where n is any nucleotide designated as the AW-box, and mutations in AW-boxes near the transcription start site and in the 5'-untranslated region of Pl-PKbeta1 abolished activation by WRI1 in protoplasts and expression during seed maturation. Although expression of genes for the synthesis of TAGs and packaging into oil bodies in the endoplasmic reticulum in developing seeds required WRI1, their expression was not up-regulated by WRI1 overexpression. Thus, WRI1 promotes the flow of carbon to oil during seed maturation by directly activating genes involved in FA synthesis and controlling genes for assembly and storage of TAG.

Antisense suppression of type 1 diacylglycerol acyltransferase adversely affects plant development in Brassica napus

Diacylglycerol acyltransferase (DGAT) catalyzes the acyl-coenzyme A (CoA) dependent acylation of sn-1,2-diacylglycerol to form triacylglycerol in the terminal step of seed oil formation. Previous work has suggested that the level of DGAT activity may have a substantial effect on the flow of carbon into triacylglycerol, implying that the enzyme may represent a promising target for seed oil modification through biotechnological approaches. In the current study, Brassica napus DH12075 was transformed with an antisense type 1 DGAT construct, resulting in a reduction in DGAT1 gene expression, total DGAT activity and seed oil content. In addition, reduced seed yield and germination rates were observed along with severe developmental abnormalities. These data suggest that in addition to its critical role in seed oil formation, DGAT1 enzyme may also be important for normal seed development in B. napus, although the underlying mechanism(s) remain to be determined.

Oil accumulation in leaves directed by modification of fatty acid breakdown and lipid synthesis pathways

Plant oils in the form of triacylglycerol (TAG) are used for food, industrial feedstock and biofuel manufacture. Although TAG is typically harvested from the fruit or seeds of oil crop species, plants can also accumulate small amounts of TAG in the leaves and other vegetative tissues. Here we show that leaf TAG levels can be increased significantly (10-20 fold) by blocking fatty acid breakdown, particularly during extended dark treatments or leaf senescence in the model plant Arabidopsis. Generation of a double mutant in fatty acid breakdown and diacylglycerol acyltransferase 1 (DGAT1) resulted in a severe vegetative growth phenotype suggesting that partitioning of fatty acids to TAG in leaves is carried out predominantly by this acyltransferase. LEC2, a seed development transcription factor involved in storage product accumulation, was ectopically expressed during senescence in the fatty acid breakdown mutant COMATOSE (cts2). This resulted in accumulation of seed oil type species of TAG in senescing tissue. Our data suggests that recycled membrane fatty acids can be re-directed to TAG by expressing the seed-programme in senescing tissue or by a block in fatty acid breakdown. This work raises the possibility of producing significant amounts of oil in vegetative tissues of biomass crops such as Miscanthus.

Increasing the flow of carbon into seed oil

The demand for vegetable oils for food, fuel (bio-diesel) and bio-product applications is increasing rapidly. In Canada alone, it is estimated that a 50 to 75% increase in canola oil production will be required to meet the demand for seed oil in the next 7-10years. Plant breeding and genetics have demonstrated that seed oil content is a quantitative trait based on a number of contributing factors including embryo genetic effects, cytoplasmic effects, maternal genetic effects, and genotype-environment interactions. Despite the involvement of numerous quantitative trait loci in determining seed oil content, genetic engineering to over-express/repress specific genes encoding enzymes and other proteins involved in the flow of carbon into seed oil has led to the development of transgenic lines with significant increases in seed oil content. Proteins encoded by these genes include enzymes catalyzing the production of building blocks for oil assembly, enzymes involved in oil assembly, enzymes regulating metabolic carbon partitioning between oil, carbohydrate and secondary metabolite fractions, and transcription factors which orchestrate metabolism at a more general level.

Regulation of de novo fatty acid synthesis in maturing oilseeds of Arabidopsis

As a Brassicaceae, Arabidopsis thaliana constitutes an excellent model system to investigate oil biosynthesis in seeds. Extensive tools for the genetic and molecular dissection of this model species are now available. Together with analytical procedures adapted to its tiny seeds, these tools have allowed major advances in isolating and characterising the factors that participate in the metabolic and developmental control of seed filling. Once the biochemical pathways producing storage lipids, namely triacylglycerols, were elucidated, the question of the regulation of this metabolic network has arisen. The coordinated up regulation of genes encoding enzymes of the fatty acid biosynthetic pathway observed at the onset of seed maturation suggests that the pathway may be subjected to a system of global transcriptional regulation. This has been further established by the study of master regulators of the maturation program like LEAFY COTYLEDON2 and the characterisation of the WRINKLED1 transcription factor. These factors have been shown to participate in a regulatory cascade controlling the induction of the genes involved in fatty acid biosynthesis at the onset of the maturation phase. Although much remains to be elucidated, the framework of the regulatory system controlling fatty acid biosynthesis in Arabidopsis seeds is coming into focus.

Directed evolution of acyl-CoA:diacylglycerol acyltransferase: development and characterization of Brassica napus DGAT1 mutagenized libraries

Metabolic flux to triacylglycerol (TAG) may be limited by the level of acyl-CoA:diacylglycerol acyltransferase (DGAT, EC 2.3.1.20) activity. In some species, this enzyme also appears to play a role in the channeling of specific fatty acyl moieties into TAG. The objective of this work is to implement a directed evolution approach to enhance the catalytic efficiency of type-1 DGAT from Brassica napus (BnDGAT1). We generated randomly mutagenized libraries of BnDGAT1 in a yeast expression vector using error-prone PCR. The mutagenized libraries were used to transform a Saccharomyces cerevisiae strain devoid of neutral lipid biosynthesis and analyzed using a high-throughput screening (HTS) system. The HTS, recently developed for this purpose, consisted of a positive selection of clones expressing active DGAT mutants followed by quantification of DGAT activity by fluorescence detection of TAG in yeast cells. The initial results indicated that the positive selection system efficiently eliminated DGAT mutants lacking enzyme activity. Screening of 1528 selected mutants revealed that some DGAT clones had enhanced ability to synthesize TAG in yeast. This was confirmed by analysis of individual clones that could carry mutations resulting in an increased catalytic efficiency. The directed evolution approach could lead to the development of an improved plant DGAT1 for increasing seed oil content in oleaginous crops.

Analysis of acyl fluxes through multiple pathways of triacylglycerol synthesis in developing soybean embryos

The reactions leading to triacylglycerol (TAG) synthesis in oilseeds have been well characterized. However, quantitative analyses of acyl group and glycerol backbone fluxes that comprise extraplastidic phospholipid and TAG synthesis, including acyl editing and phosphatidylcholine-diacylglycerol interconversion, are lacking. To investigate these fluxes, we rapidly labeled developing soybean (Glycine max) embryos with [(14)C]acetate and [(14)C]glycerol. Cultured intact embryos that mimic in planta growth were used. The initial kinetics of newly synthesized acyl chain and glycerol backbone incorporation into phosphatidylcholine (PC), 1,2-sn-diacylglycerol (DAG), and TAG were analyzed along with their initial labeled molecular species and positional distributions. Almost 60% of the newly synthesized fatty acids first enter glycerolipids through PC acyl editing, largely at the sn-2 position. This flux, mostly of oleate, was over three times the flux of nascent [(14)C]fatty acids incorporated into the sn-1 and sn-2 positions of DAG through glycerol-3-phosphate acylation. Furthermore, the total flux for PC acyl editing, which includes both nascent and preexisting fatty acids, was estimated to be 1.5 to 5 times the flux of fatty acid synthesis. Thus, recycled acyl groups (16:0, 18:1, 18:2, and 18:3) in the acyl-coenzyme A pool provide most of the acyl chains for de novo glycerol-3-phosphate acylation. Our results also show kinetically distinct DAG pools. DAG used for TAG synthesis is mostly derived from PC, whereas de novo synthesized DAG is mostly used for PC synthesis. In addition, two kinetically distinct sn-3 acylations of DAG were observed, providing TAG molecular species enriched in saturated or polyunsaturated fatty acids.

System analysis of an Arabidopsis mutant altered in de novo fatty acid synthesis reveals diverse changes in seed composition and metabolism

Embryo-specific overexpression of biotin carboxyl carrier protein 2 (BCCP2) inhibited plastid acetyl-coenzyme A carboxylase (ACCase), resulting in altered oil, protein, and carbohydrate composition in mature Arabidopsis (Arabidopsis thaliana) seed. To characterize gene and protein regulatory consequences of this mutation, global microarray, two-dimensional difference gel electrophoresis, iTRAQ, and quantitative immunoblotting were performed in parallel. These analyses revealed that (1) transgenic overexpression of BCCP2 did not affect the expression of three other ACCase subunits; (2) four subunits to plastid pyruvate dehydrogenase complex were 25% to 70% down-regulated at protein but not transcript levels; (3) key glycolysis and de novo fatty acid/lipid synthesis enzymes were induced; (4) multiple storage proteins, but not cognate transcripts, were up-regulated; and (5) the biotin synthesis pathway was up-regulated at both transcript and protein levels. Biotin production appears closely matched to endogenous BCCP levels, since overexpression of BCCP2 produced mostly apo-BCCP2 and the resulting ACCase-compromised, low-oil phenotype. Differential expression of glycolysis, plastid pyruvate dehydrogenase complex, fatty acid, and lipid synthesis activities indicate multiple, complex regulatory responses including feedback as well as futile "feed-forward" elicitation in the case of fatty acid and lipid biosynthetic enzymes. Induction of storage proteins reveals that oil and protein synthesis share carbon intermediate(s) and that reducing malonyl-coenzyme A flow into fatty acids diverts carbon into amino acid and protein synthesis.

Transgenic increases in seed oil content are associated with the differential expression of novel Brassica-specific transcripts

Background

Seed oil accumulates primarily as triacylglycerol (TAG). While the biochemical pathway for TAG biosynthesis is known, its regulation remains unclear. Previous research identified microsomal diacylglycerol acyltransferase 1 (DGAT1, EC 2.3.1.20) as controlling a rate-limiting step in the TAG biosynthesis pathway. Of note, overexpression of DGAT1 results in substantial increases in oil content and seed size. To further analyze the global consequences of manipulating DGAT1 levels during seed development, a concerted transcriptome and metabolome analysis of transgenic B. napus prototypes was performed.

Results

Using a targeted Brassica cDNA microarray, about 200 genes were differentially expressed in two independent transgenic lines analyzed. Interestingly, 24–33% of the targets showing significant changes have no matching gene in Arabidopsis although these represent only 5% of the targets on the microarray. Further analysis of some of these novel transcripts indicated that several are inducible by ABA in microspore-derived embryos. Of the 200 Arabidopsis genes implicated in lipid biology present on the microarray, 36 were found to be differentially regulated in DGAT transgenic lines. Furthermore, kinetic reverse transcriptase Polymerase Chain Reaction (k-PCR) analysis revealed up-regulation of genes encoding enzymes of the Kennedy pathway involved in assembly of TAGs. Hormone profiling indicated that levels of auxins and cytokinins varied between transgenic lines and untransformed controls, while differences in the pool sizes of ABA and catabolites were only observed at later stages of development.

Conclusion

Our results indicate that the increased TAG accumulation observed in transgenic DGAT1 plants is associated with modest transcriptional and hormonal changes during seed development that are not limited to the TAG biosynthesis pathway. These might be associated with feedback or feed-forward effects due to altered levels of DGAT1 activity. The fact that a large fraction of significant amplicons have no matching genes in Arabidopsis compromised our ability to draw concrete inferences from the data at this stage, but has led to the identification of novel genes of potential interest.

Hormonal regulation of oil accumulation in Brassica seeds: metabolism and biological activity of ABA, 7'-, 8'- and 9'-hydroxy ABA in microspore derived embryos of B. napus

Developing seeds of Brassica napus contain significant levels of ABA and products of oxidation at the 7'- and 9'-methyl groups of ABA, 7'- and 9'-hydroxy ABA, as well stable products of oxidation of the 8'-methyl group, phaseic acid and dihydrophaseic acid. To probe the biological roles of the initially formed hydroxylated compounds, we have compared the effects of supplied ABA and the hydroxylated metabolites in regulating oil synthesis in microspore-derived embryos of B. napus, cv Hero that accumulate long chain fatty acids. Uptake into the embryos and metabolism of each of the hormone metabolites was studied by using deuterium labeled analogs. Supplied ABA, which was rapidly metabolized, induced expression of oleosin and fatty acid elongase genes and increased the accumulation of triacylglycerols and very long chain fatty acids. The metabolites 7'- and 9'-hydroxy ABA had similar effects, with the 9'-hydroxy ABA having even greater activity than ABA. The principal catabolite of ABA, 8'-hydroxy ABA, also had hormonal activity and led to increased oil synthesis but induced the genes weakly. These results indicate that all compounds tested could be involved in lipid synthesis in B. napus, and may have hormonal roles in other ABA-regulated processes.

Thematic review series: glycerolipids. DGAT enzymes and triacylglycerol biosynthesis

Triacylglycerols (triglycerides) (TGs) are the major storage molecules of metabolic energy and FAs in most living organisms. Excessive accumulation of TGs, however, is associated with human diseases, such as obesity, diabetes mellitus, and steatohepatitis. The final and the only committed step in the biosynthesis of TGs is catalyzed by acyl-CoA:diacylglycerol acyltransferase (DGAT) enzymes. The genes encoding two DGAT enzymes, DGAT1 and DGAT2, were identified in the past decade, and the use of molecular tools, including mice deficient in either enzyme, has shed light on their functions. Although DGAT enzymes are involved in TG synthesis, they have distinct protein sequences and differ in their biochemical, cellular, and physiological functions. Both enzymes may be useful as therapeutic targets for diseases. Here we review the current knowledge of DGAT enzymes, focusing on new advances since the cloning of their genes, including possible roles in human health and diseases.

Metabolic engineering of hydroxy fatty acid production in plants: RcDGAT2 drives dramatic increases in ricinoleate levels in seed oil

SUMMARY

A central goal of green chemistry is to produce industrially useful fatty acids in oilseed crops. Although genes encoding suitable fatty acid-modifying enzymes are available from many wild species, progress has been limited because the expression of these genes in transgenic plants produces low yields of the desired products. For example, Ricinus communis fatty acid hydroxylase 12 (FAH12) produces a maximum of only 17% hydroxy fatty acids (HFAs) when expressed in Arabidopsis. cDNA clones encoding R. communis enzymes for additional steps in the seed oil biosynthetic pathway were identified. Expression of these cDNAs in FAH12 transgenic plants revealed that the R. communis type-2 acyl-coenzyme A:diacylglycerol acyltransferase (RcDGAT2) could increase HFAs from 17% to nearly 30%. Detailed comparisons of seed neutral lipids from the single- and double-transgenic lines indicated that RcDGAT2 substantially modified the triacylglycerol (TAG) pool, with significant increases in most of the major TAG species observed in native castor bean oil. These data suggest that RcDGAT2 prefers acyl-coenzyme A and diacylglycerol substrates containing HFAs, and biochemical analyses of RcDGAT2 expressed in yeast cells confirmed a strong preference for HFA-containing diacylglycerol substrates. Our results demonstrate that pathway engineering approaches can be used successfully to increase the yields of industrial feedstocks in plants, and that members of the DGAT2 gene family probably play a key role in this process.

Cloning and characterization of an acyl-CoA-dependent diacylglycerol acyltransferase 1 (DGAT1) gene from Tropaeolum majus, and a study of the functional motifs of the DGAT protein using site-directed mutagenesis to modify enzyme activity and oil content

A full-length cDNA encoding a putative diacylglycerol acyltransferase 1 (DGAT1, EC 2.3.1.20) was obtained from Tropaeolum majus (garden nasturtium). The 1557-bp open reading frame of this cDNA, designated TmDGAT1, encodes a protein of 518 amino acids showing high homology to other plant DGAT1s. The TmDGAT1 gene was expressed exclusively in developing seeds. Expression of recombinant TmDGAT1 in the yeast H1246MATalpha quadruple mutant (DGA1, LRO1, ARE1, ARE2) restored the capability of the mutant host to produce triacylglycerols (TAGs). The recombinant TmDGAT1 protein was capable of utilizing a range of (14)C-labelled fatty acyl-CoA donors and diacylglycerol acceptors, and could synthesize (14)C-trierucin. Collectively, these findings confirm that the TmDGAT1 gene encodes an acyl-CoA-dependent DGAT1. In plant transformation studies, seed-specific expression of TmDGAT1 was able to complement the low TAG/unusual fatty acid phenotype of the Arabidopsis AS11 (DGAT1) mutant. Over-expression of TmDGAT1 in wild-type Arabidopsis and high-erucic-acid rapeseed (HEAR) and canola Brassica napus resulted in an increase in oil content (3.5%-10% on a dry weight basis, or a net increase of 11%-30%). Site-directed mutagenesis was conducted on six putative functional regions/motifs of the TmDGAT1 enzyme. Mutagenesis of a serine residue in a putative SnRK1 target site resulted in a 38%-80% increase in DGAT1 activity, and over-expression of the mutated TmDGAT1 in Arabidopsis resulted in a 20%-50% increase in oil content on a per seed basis. Thus, alteration of this putative serine/threonine protein kinase site can be exploited to enhance DGAT1 activity, and expression of mutated DGAT1 can be used to enhance oil content.

Expression of Umbelopsis ramanniana DGAT2A in seed increases oil in soybean

Oilseeds are the main source of lipids used in both food and biofuels. The growing demand for vegetable oil has focused research toward increasing the amount of this valuable component in oilseed crops. Globally, soybean (Glycine max) is one of the most important oilseed crops grown, contributing about 30% of the vegetable oil used for food, feed, and industrial applications. Breeding efforts in soy have shown that multiple loci contribute to the final content of oil and protein stored in seeds. Genetically, the levels of these two storage products appear to be inversely correlated with an increase in oil coming at the expense of protein and vice versa. One way to overcome the linkage between oil and protein is to introduce a transgene that can specifically modulate one pathway without disrupting the other. We describe the first, to our knowledge, transgenic soy crop with increased oil that shows no major impact on protein content or yield. This was achieved by expressing a codon-optimized version of a diacylglycerol acyltransferase 2A from the soil fungus Umbelopsis (formerly Mortierella) ramanniana in soybean seed during development, resulting in an absolute increase in oil of 1.5% (by weight) in the mature seed.

Storage reserve accumulation in Arabidopsis: metabolic and developmental control of seed filling

In the life cycle of higher plants, seed development is a key process connecting two distinct sporophytic generations. Seed development can be divided into embryo morphogenesis and seed maturation. An essential metabolic function of maturing seeds is the deposition of storage compounds that are mobilised to fuel post-germinative seedling growth. Given the importance of seeds for food and animal feed and considering the tremendous interest in using seed storage products as sustainable industrial feedstocks to replace diminishing fossil reserves, understanding the metabolic and developmental control of seed filling constitutes a major focus of plant research. Arabidopsis thaliana is an oilseed species closely related to the agronomically important Brassica oilseed crops. The main storage compounds accumulated in seeds of A. thaliana consist of oil stored as triacylglycerols (TAGs) and seed storage proteins (SSPs). Extensive tools developed for the molecular dissection of A. thaliana development and metabolism together with analytical and cytological procedures adapted for very small seeds have led to a good description of the biochemical pathways producing storage compounds. In recent years, studies using these tools have shed new light on the intricate regulatory network controlling the seed maturation process. This network involves sugar and hormone signalling together with a set of developmentally regulated transcription factors. Although much remains to be elucidated, the framework of the regulatory system controlling seed filling is coming into focus.

High-value oils from plants

The seed oils of domesticated oilseed crops are major agricultural commodities that are used primarily for nutritional applications, but in recent years there has been increasing use of these oils for production of biofuels and chemical feedstocks. This is being driven in part by the rapidly rising costs of petroleum, increased concern about the environmental impact of using fossil oil, and the need to develop renewable domestic sources of fuel and industrial raw materials. There is also a need to develop sustainable sources of nutritionally important fatty acids such as those that are typically derived from fish oil. Plant oils can provide renewable sources of high-value fatty acids for both the chemical and health-related industries. The value and application of an oil are determined largely by its fatty acid composition, and while most vegetable oils contain just five basic fatty acid structures, there is a rich diversity of fatty acids present in nature, many of which have potential usage in industry. In this review, we describe several areas where plant oils can have a significant impact on the emerging bioeconomy and the types of fatty acids that are required in these various applications. We also outline the current understanding of the underlying biochemical and molecular mechanisms of seed oil production, and the challenges and potential in translating this knowledge into the rational design and engineering of crop plants to produce high-value oils in plant seeds.

Plant triacylglycerols as feedstocks for the production of biofuels

Triacylglycerols produced by plants are one of the most energy-rich and abundant forms of reduced carbon available from nature. Given their chemical similarities, plant oils represent a logical substitute for conventional diesel, a non-renewable energy source. However, as plant oils are too viscous for use in modern diesel engines, they are converted to fatty acid esters. The resulting fuel is commonly referred to as biodiesel, and offers many advantages over conventional diesel. Chief among these is that biodiesel is derived from renewable sources. In addition, the production and subsequent consumption of biodiesel results in less greenhouse gas emission compared to conventional diesel. However, the widespread adoption of biodiesel faces a number of challenges. The biggest of these is a limited supply of biodiesel feedstocks. Thus, plant oil production needs to be greatly increased for biodiesel to replace a major proportion of the current and future fuel needs of the world. An increased understanding of how plants synthesize fatty acids and triacylglycerols will ultimately allow the development of novel energy crops. For example, knowledge of the regulation of oil synthesis has suggested ways to produce triacylglycerols in abundant non-seed tissues. Additionally, biodiesel has poor cold-temperature performance and low oxidative stability. Improving the fuel characteristics of biodiesel can be achieved by altering the fatty acid composition. In this regard, the generation of transgenic soybean lines with high oleic acid content represents one way in which plant biotechnology has already contributed to the improvement of biodiesel.

Cloning and functional analysis of two type 1 diacylglycerol acyltransferases from Vernonia galamensis

Vernonia galamensis accumulates vernolic acid (cis-12-epoxyoctadeca-cis-9-enoic acid) as the major fatty acid in its seed oil. Such epoxy fatty acids are useful in a number of industrial applications. Successful genetic engineering of commercial oilseed crops to produce high levels of vernolic acid depends on a better understanding of the source plant enzymes for vernolic acid accumulation. Developing V. galamensis seed microsome assays demonstrate that diacylglycerol acyltransferase (DGAT), an enzyme for the final step of triacylglycerol synthesis, has a strong substrate preference for vernolic acid bearing substrates including acyl-CoA and diacylglycerol. There are two classes of DGATs known as DGAT1 and DGAT2. Here we report on the isolation, characterization, and functional analysis of two DGAT1 cDNAs from V. galamensis (VgDGAT1a and VgDGAT1b). VgDGAT1a and VgDGAT1b are expressed in all plant tissues examined with highest expression in developing seeds. Enzymatic assays using isolated microsomes from transformed yeast show that VgDGAT1a and VgDGAT1b have the same DGAT activity levels and substrate specificities. Oleoyl-CoA and sn-1,2-dioleoylglycerol are preferred substrates over vernoloyl-CoA and sn-1,2-divernoloylglycerol. This data indicates that the two VgDGAT1s are functional, but not likely to be responsible for the selective accumulation of vernolic acid in V. galamensis seed oil.

Metabolic control analysis is helpful for informed genetic manipulation of oilseed rape (Brassica napus) to increase seed oil content

Top-down control analysis (TDCA) is a useful tool for quantifying constraints on metabolic pathways that might be overcome by biotechnological approaches. Previous studies on lipid accumulation in oilseed rape have suggested that diacylglycerol acyltransferase (DGAT), which catalyses the final step in seed oil biosynthesis, might be an effective target for enhancing seed oil content. Here, increased seed oil content, increased DGAT activity, and reduced substrate:product ratio are demonstrated, as well as reduced flux control by complex lipid assembly, as determined by TDCA in Brassica napus (canola) lines which overexpress the gene encoding type-1 DGAT. Lines overexpressing DGAT1 also exhibited considerably enhanced seed oil content under drought conditions. These results support the use of TDCA in guiding the rational selection of molecular targets for oilseed modification. The most effective lines had a seed oil increase of 14%. Moreover, overexpression of DGAT1 under drought conditions reduced this environmental penalty on seed oil content.

Engineering oilseeds for sustainable production of industrial and nutritional feedstocks: solving bottlenecks in fatty acid flux

Oilseeds provide a unique platform for the production of high-value fatty acids that can replace non-sustainable petroleum and oceanic sources of specialty chemicals and aquaculture feed. However, recent efforts to engineer the seeds of crop and model plant species to produce new types of fatty acids, including hydroxy and conjugated fatty acids for industrial uses and long-chain omega-3 polyunsaturated fatty acids for farmed fish feed, have met with only modest success. The collective results from these studies point to metabolic 'bottlenecks' in the engineered plant seeds that substantially limit the efficient or selective flux of unusual fatty acids between different substrate pools and ultimately into storage triacylglycerol. Evidence is emerging that diacylglycerol acyltransferase 2, which catalyzes the final step in triacylglycerol assembly, is an important contributor to the synthesis of unusual fatty acid-containing oils, and is likely to be a key target for future oilseed metabolic engineering efforts.

The acyltransferase GPAT5 is required for the synthesis of suberin in seed coat and root of Arabidopsis

Suberin and cutin are fatty acid- and glycerol-based plant polymers that act as pathogen barriers and function in the control of water and solute transport. However, despite important physiological roles, their biosynthetic pathways, including the acyl transfer reactions, remain hypothetical. We report the characterization of two suberin mutants (gpat5-1 and gpat5-2) of Arabidopsis thaliana GPAT5, encoding a protein with acyl-CoA:glycerol-3-phosphate acyltransferase activity. RT-PCR and beta-glucuronidase-promoter fusion analyses demonstrated GPAT5 expression in seed coat, root, hypocotyl, and anther. The gpat5 plants showed a 50% decrease in aliphatic suberin in young roots and produced seed coats with a severalfold reduction in very long chain dicarboxylic acid and omega-hydroxy fatty acids typical of suberin but no change in the composition or content of membrane or storage glycerolipids or surface waxes. Consistent with their altered suberin, seed coats of gpat5 mutants had a steep increase in permeability to tetrazolium salts compared with wild-type seed coats. Furthermore, the germination rate of gpat5 seeds under high salt was reduced, and gpat5 seedlings had lower tolerance to salt stress. These results provide evidence for a critical role of GPAT5 in polyester biogenesis in seed coats and roots and for the importance of lipid polymer structures in the normal function of these organs.

Acyl-CoA-binding and self-associating properties of a recombinant 13.3 kDa N-terminal fragment of diacylglycerol acyltransferase-1 from oilseed rape

Background

Diacylglycerol acyltransferase (DGAT, EC 2.3.1.20) catalyzes the acyl-CoA-dependent acylation of sn-1, 2-diacylglycerol to generate triacylglycerol and CoA. The deduced amino acid sequence of cDNAs encoding DGAT1 from plants and mammals exhibit a hydrophilic N-terminal region followed by a number of potential membrane-spanning segments, which is consistent with the membrane-bound nature of this enzyme family. In order to gain insight into the structure/function properties of DGAT1 from Brassica napus (BnDGAT1), we produced and partially characterized a recombinant polyHis-tagged N-terminal fragment of the enzyme, BnDGAT1_(1–116)His₆, with calculated molecular mass of 13,278 Da.

Results

BnDGAT1_(1–116)His₆ was highly purified from bacterial lysate and plate-like monoclinic crystals were grown using this preparation. Lipidex-1000 binding assays and gel electrophoresis indicated that BnDGAT1_(1–116)His₆ interacts with long chain acyl-CoA. The enzyme fragment displayed enhanced affinity for erucoyl (22:1cis^Δ13)-CoA over oleoyl (18:1cis^Δ9)-CoA, and the binding process displayed positive cooperativity. Gel filtration chromatography and cross-linking studies indicated that BnDGAT1_(1–116)His₆ self-associated to form a tetramer. Polyclonal antibodies raised against a peptide of 15 amino acid residues representing a segment of BnDGAT1_(1–116)His₆ failed to react with protein in microsomal vesicles following treatment with proteinase K, suggesting that the N-terminal fragment of BnDGAT1 was localized to the cytosolic side of the ER.

Conclusion

Collectively, these results suggest that BnDGAT1 may be allosterically modulated by acyl-CoA through the N-terminal region and that the enzyme self-associates via interactions on the cytosolic side of the ER.

Diacylglycerol acyltransferase: A key mediator of plant triacylglycerol synthesis

Many plants deposit TAG in seeds and fruits as the major form of storage lipid. TAG production is of tremendous socioeconomic value in food, nutraceutical, and industrial applications, and thus numerous conventional and molecular genetic strategies have been explored in attempts to increase TAG content and modify the FA composition of plant seed oils. Much research has focused on the acyl-CoA-dependent reaction catalyzed by diacylglycerol acyltransferase (DGAT), which is an integral endoplasmic reticulum protein and has also been shown to be present in oil bodies and plastids. DGAT enzymes exhibit diverse biochemical properties among different plant species, many of which are summarized here. In addition to catalyzing a critical step in TAG biosynthesis, there is evidence that DGAT has roles in lipid metabolism associated with germination and leaf senescence. TAG can also be formed in plants via two different acyl-CoA-independent pathways, catalyzed by phospholipid: diacylglycerol acyltransferase and diacylglycerol transacylase. The current understanding of the terminal step in TAG formation in plants and the development of molecular genetic approaches aimed at altering TAG yield and FA composition of TAG are discussed.

Identification and functional expression of a type 2 acyl-CoA:diacylglycerol acyltransferase (DGAT2) in developing castor bean seeds which has high homology to the major triglyceride biosynthetic enzyme of fungi and animals

Seed oil from castor bean (Ricinus communis) contains high amounts of hydroxy fatty acid rich triacylglycerols (TAGs) that can serve as raw material for production of bio-based products such as nylon, cosmetics, lubricants, foams, and surfactants. Diacylglycerol acyltransferase (DGAT) catalyses the terminal reaction in the acyl-CoA dependent Kennedy pathway of triglyceride biosynthesis. There is still some debate whether there are three or four enzymes in yeast that have DGAT activity and catalyse the synthesis of TAG but of these the DGAT2 homologue Dga1 contributes in a major way to TAG biosynthesis. Here we report on the cloning of a cDNA for DGAT2 from castor bean and prove its biological activity following expression in yeast and enzymatic assays using diricinolein as the acceptor and ricinoleoyl-CoA as the donor. Previous reports of DGAT in castor have focussed on DGAT1 which has little amino acid sequence homology to DGAT2. Expressional studies demonstrate that DGAT2 is 18-fold more highly expressed in seeds than in leaves and shows temporal specific expression during seed development. In contrast, DGAT1 shows little difference in expression in seeds versus leaves. We conclude that in castor bean DGAT2 is more likely to play a major role in seed TAG biosynthesis than DGAT1.

Tung tree DGAT1 and DGAT2 have nonredundant functions in triacylglycerol biosynthesis and are localized to different subdomains of the endoplasmic reticulum

Seeds of the tung tree (Vernicia fordii) produce large quantities of triacylglycerols (TAGs) containing approximately 80% eleostearic acid, an unusual conjugated fatty acid. We present a comparative analysis of the genetic, functional, and cellular properties of tung type 1 and type 2 diacylglycerol acyltransferases (DGAT1 and DGAT2), two unrelated enzymes that catalyze the committed step in TAG biosynthesis. We show that both enzymes are encoded by single genes and that DGAT1 is expressed at similar levels in various organs, whereas DGAT2 is strongly induced in developing seeds at the onset of oil biosynthesis. Expression of DGAT1 and DGAT2 in yeast produced different types and proportions of TAGs containing eleostearic acid, with DGAT2 possessing an enhanced propensity for the synthesis of trieleostearin, the main component of tung oil. Both DGAT1 and DGAT2 are located in distinct, dynamic regions of the endoplasmic reticulum (ER), and surprisingly, these regions do not overlap. Furthermore, although both DGAT1 and DGAT2 contain a similar C-terminal pentapeptide ER retrieval motif, this motif alone is not sufficient for their localization to specific regions of the ER. These data suggest that DGAT1 and DGAT2 have nonredundant functions in plants and that the production of storage oils, including those containing unusual fatty acids, occurs in distinct ER subdomains.

Identification of differentially expressed genes in seeds of two near-isogenic Brassica napus lines with different oil content

The regulation of seed oil synthesis in rapeseed is largely unknown. In this study, we compared the gene expression during seed development between two lines of Brassica napus with a 10% difference in oil content. We isolated the immature seeds 15 and 25 days after flowering at periods preceding and including the major accumulation of storage oils and proteins. The differentially expressed gene clones between the two rape lines were isolated by subtractive suppression hybridization (SSH). All SSH clones were arrayed and screened by dot blot hybridization, followed by RT-PCR analysis for selected clones. A total of 217 cDNA clones corresponding to 30 genes were found to have a high expression in seeds with high oil content. Six genes were highly expressed in seeds with low oil content. Northern blot and enzyme activity analysis demonstrated a change in expression pattern of several genes. The results provide information on gene-encoding factors responsible for the regulation of oil synthesis. The possible role of these genes in seeds is discussed. The genes in this study may be suitable as novel targets for genetic improvement of seed oil content and may also provide molecular markers for studies of rape breeding.

Key enzymes for biosynthesis of neutral lipid storage compounds in prokaryotes: properties, function and occurrence of wax ester synthases/acyl-CoA: diacylglycerol acyltransferases

Triacylglycerols (TAGs) and wax esters (WEs) are beside polyhydroxyalkanoates (PHAs) important storage lipids in some groups of prokaryotes. Accumulation of these lipids occurs in cells when they are cultivated under conditions of unbalanced growth in the presence of high concentrations of a suitable carbon source, which can be used for fatty acid and storage lipid biosyntheses. The key enzymes, which mediate both WE and TAG formations from long-chain acyl-coenzyme A (CoA) as acyl donor and long-chain fatty alcohols or diacylglycerols as respective acyl acceptors in bacteria, are WE synthases/acyl-CoA:diacylglycerol acyltransferases (WS/DGATs). The WS/DGATs identified so far represent rather unspecific enzymes with broad spectra of possible substrates; this makes them interesting for many biotechnological applications. This review traces the molecular structure and biochemical properties including the probable regions responsible for acyltransferase properties, enzymatic activity and substrate specifities. The phylogenetic relationships based on amino acid sequence similarities of this unique class of enzymes were revealed. Furthermore, recent advances in understanding the physiological functions of WS/DGATs in their natural hosts including pathogenic Mycobacterium tuberculosis were discussed.

Cytosolic triacylglycerol biosynthetic pathway in oilseeds. Molecular cloning and expression of peanut cytosolic diacylglycerol acyltransferase

Triacylglycerols (TAGs) are the most important storage form of energy for eukaryotic cells. TAG biosynthetic activity was identified in the cytosolic fraction of developing peanut (Arachis hypogaea) cotyledons. This activity was NaF insensitive and acyl-coenzyme A (CoA) dependent. Acyl-CoA:diacylglycerol acyltransferase (DGAT) catalyzes the final step in TAG biosynthesis that acylates diacylglycerol to TAG. Soluble DGAT was identified from immature peanuts and purified by conventional column chromatographic procedures. The enzyme has a molecular mass of 41 +/- 1.0 kD. Based on the partial peptide sequence, a degenerate probe was used to obtain the full-length cDNA. The isolated gene shared less than 10% identity with the previously identified DGAT1 and 2 families, but has 13% identity with the bacterial bifunctional wax ester/DGAT. To differentiate the unrelated families, we designate the peanut gene as AhDGAT. Expression of peanut cDNA in Escherichia coli resulted in the formation of labeled TAG and wax ester from [14C]acetate. The recombinant E. coli showed high levels of DGAT activity but no wax ester synthase activity. TAGs were localized in transformed cells with Nile blue A and oil red O staining. The recombinant and native DGAT was specific for 1,2-diacylglycerol and did not utilize hexadecanol, glycerol-3-phosphate, monoacylglycerol, lysophosphatidic acid, and lysophosphatidylcholine. Oleoyl-CoA was the preferred acyl donor as compared to palmitoyl- and stearoyl-CoAs. These data suggest that the cytosol is one of the sites for TAG biosynthesis in oilseeds. The identified pathway may present opportunities of bioengineering oil-yielding plants for increased oil production.

WRI1 is required for seed germination and seedling establishment

Storage compound accumulation during seed development prepares the next generation of plants for survival. Therefore, processes involved in the regulation and synthesis of storage compound accumulation during seed development bear relevance to germination and seedling establishment. The wrinkled1 (wri1) mutant of Arabidopsis (Arabidopsis thaliana) is impaired in seed oil accumulation. The WRI1 gene encodes an APETALA2/ethylene-responsive element-binding protein transcription factor involved in the control of metabolism, particularly glycolysis, in the developing seeds. Here we investigate the role of this regulatory factor in seed germination and seedling establishment by comparing the wri1-1 mutant, transgenic lines expressing the WRI1 wild-type cDNA in the wri1-1 mutant background, and the wild type. Plants altered in the expression of the WRI1 gene showed different germination responses to the growth factor abscisic acid (ABA), sugars, and fatty acids provided in the medium. Germination of the mutant was more sensitive to ABA, sugars, and osmolites, an effect that was alleviated by increased WRI1 expression in transgenic lines. The expression of ABA-responsive genes AtEM6 and ABA-insensitive 3 (ABI3) was increased in the wri1-1 mutant. Double-mutant analysis between abi3-3 and wri1-1 suggested that WRI1 and ABI3, a transcription factor mediating ABA responses in seeds, act in parallel pathways. Addition of 2-deoxyglucose inhibited seed germination, but did so less in lines overexpressing WRI1. Seedling establishment was decreased in the wri1-1 mutant but could be alleviated by sucrose. Apart from a possible signaling role in germination, sugars in the medium were required as building blocks and energy supply during wri1-1 seedling establishment.

Oil content of Arabidopsis seeds: the influence of seed anatomy, light and plant-to-plant variation

Arabidopsis thaliana is frequently used as a model for the study of oilseed biology and metabolism. However, the very small seeds of Arabidopsis can complicate analysis of their oil content and influence the application of results to larger-seeded plants. Here, we describe how seed anatomy, light, and plant-to-plant variation influence the content and measurement of oil in Arabidopsis seeds. The anatomy of Arabidopsis and Brassica napus seeds were compared and the distribution of mass, oil and the fatty acid composition of different seed parts were determined. In Brassica, 90% of the seed oil resides in the cotyledons that contribute 74% of seed mass. By contrast, the values for Arabidopsis are 60% and 45%, respectively, with a higher fraction of the oil deposited in the radicle, hypocotyl, endosperm and seed coat. Growth of Arabidopsis plants with 600 micromol m(-2) s(-1) light resulted in a two-fold higher seed yield, a 40% increase in mass per seed and a 60% increase in oil per seed compared to growth at 100 micromol m(-2) s(-1). Factors that influence the analysis of oil content were evaluated. Intact-seed transmethylation followed by gas chromatography (GC) analysis provided reproducible analysis of Arabidopsis seed oil. However, plant-to-plant variation in oil content is large and we analyzed how this influences the ability to detect statistically valid changes in oil between different genotypes. These observations establish a reference data set on the fatty acid composition and distribution of mass and oil between tissues of Arabidopsis seeds that should help to predict the applicability of results obtained with Arabidopsis to other oilseeds.

Cloning and comparative analysis of the gene encoding diacylglycerol acyltransferase from wild type and cultivated soybean

Diacylglycerol acyltransferase (DGAT), as an important enzyme in triacylglycerol synthesis, catalyzes the final acylation of the Kennedy pathway. In the present study, the GmDGAT gene was cloned from Glycine max by using AtDGAT as a query to search against the soybean EST database and the rapid amplification of cDNA ends (RACE) method. Allelic genes were also isolated from 13 soybean accessions and the divergence of the deduced amino acid sequences were compared. The comparison reveals that although GmDGAT is a highly conserved protein, several differences of insertion/deletion were identified in the N-terminal region of the GmDGATs from various soybean accessions. In the C-terminal regions, a single amino acid mutation specific to both G. max and G. soja was also found. The GmDGAT genomic sequences were further cloned and the number and size of exons in the DGAT genomic sequence were very similar among different plant species, whereas the introns were more diverged. These results may have significance in elucidating the genetic diversity of the GmDGAT among the soybean subgenus.

Diacylglycerol acyltransferase activity and triacylglycerol synthesis in germinating castor seed cotyledons

The central importance of storage lipid breakdown in providing carbon and energy during seed germination has been demonstrated by isolating the genes encoding the enzymes involved in FA beta-oxidation. In contrast, little is known about the ability of germinating seeds to synthesize TAG. We report that castor cotyledons are capable of TAG synthesis. The rate of incorporation of ricinoleic acid into TAG reached a peak at 7 d after imbibition (DAI) (1.14 nmol/h/mg) and decreased rapidly thereafter, but was sustained at 20 DAI in cotyledons and true leaves. The castor DAG acyltransferase (RcDGAT) mRNA and protein were expressed throughout seed germination at levels considerably enhanced from that in the dormant seed, thus indicating new expression. Significant degradation of the RcDGAT protein was observed after 7 DAI. The DGAT activity was found to be predominantly a function of the level of the intact RcDGAT protein, with the rate of TAG synthesis decreasing as degradation of the RcDGAT protein proceeded. A possible mechanism for the degradation of the RcDGAT protein is discussed. The induction of DGAT mRNA and protein, the capacity for TAG synthesis in vitro and in tissue slices, and the differing TAG composition of dormant seed TAG vs. cotyledonary TAG provide strong circumstantial evidence for active TAG synthesis by cotyledons. However, we have not yet determined the physiological significance of this capability.