Seed-specific over-expression of an Arabidopsis cDNA encoding a diacylglycerol acyltransferase enhances seed oil content and seed weight

Transgenic and Genome Editing Approaches for Modifying Plant Oils

Vegetable oils are important for human and animal nutrition and as renewable resources for chemical feedstocks. We provide an overview of transgenic and genome editing approaches for modifying plant oils, describing useful model and crop systems and different strategies for transgenic modifications. We also describe new genome editing approaches that are beginning to be applied to oilseed plants and crops. These approaches are illustrated with examples for modifying the nutritional quality of vegetable oils by altering fatty acid desaturation, producing non-native fatty acids in oilseeds, and enhancing the overall accumulation of oil in seeds and leaves.

Correlation analysis of the transcriptome and metabolome reveals the regulatory network for lipid synthesis in developing Brassica napus embryos

In this manuscript, we explored the key molecular networks for oil biosynthesis with the transcriptome and metabolome of B. napus embryo at different developmental stages. Brassica napus (B. napus) is an important oil crop worldwide, yet the molecular pathways involved in oil biosynthesis in seeds are not fully understood. In this study, we performed a combined investigation of the gene expression profiles and metabolite content in B. napus seeds at 21, 28 and 35 days after flowering (DAF), when seed oil biosynthesis takes place. The total triacylglycerol (TAG) content in seed embryos increased over the course of seed maturation, and was accompanied by changes in the fatty acid profile, an increase in lipid droplets, and a reduction in starch grains. Metabolome analysis showed that the total amino acid, free fatty acid and organic acid contents in seed embryos decreased during seed maturation. In total, the abundance of 76 metabolites was significantly different between 21 and 28 DAF, and 68 metabolites changed in abundance between 28 and 35 DAF. Transcriptome analysis showed that the set of genes differentially expressed between stages was significantly enriched in those related to lipid metabolism, transport, protein and RNA metabolism, development and signaling, covering most steps of plant lipid biosynthesis and metabolism. Importantly, the metabolite and gene expression profiles were closely correlated during seed development, especially those associated with TAG and fatty acid biosynthesis. Further, the expression of major carbohydrate metabolism-regulating genes was closely correlated with carbohydrate content during seed maturation. Our results provide novel insights into the regulation of oil biosynthesis in B. napus seeds and highlights the coordination of gene expression and metabolism in this process.

Crambe hispanica Subsp. abyssinica Diacylglycerol Acyltransferase Specificities Towards Diacylglycerols and Acyl-CoA Reveal Combinatorial Effects That Greatly Affect Enzymatic Activity and Specificity

Crambe is an oil crop suitable for industrial purposes due to the high content of erucic acid (22:1) in the seed oil. The final acylation of diacylglycerols (DAG) with acyl-CoA in the production of triacylglycerols (oil) is catalyzed by acyl-CoA:diacylglycerol acyltransferase (DGAT) enzymes. We identified eight forms of DGATs in crambe and characterized them in microsomal preparations of yeast expressing the enzymes using various acyl-CoAs and both di-6:0-DAG and long-chain DAG species as acyl acceptors. All DGATs accepted 22:1-CoA when using di-6:0-DAG as acyl acceptor. When di-22:1-DAG was the acyl acceptor, the DGAT1 type of enzyme utilized 22:1-CoA at a much-reduced rate compared to assays with sn-1-22:1-sn-2-18:1(oleoyl)-DAG, the most frequently available DAG precursor in crambe seeds. None of the DGAT2 enzymes was able to acylate di-22:1-DAG. Our results indicate that formation of trierucin by crambe DGATs is a limiting step for further increasing the levels of 22:1 in the previously developed transgenic crambe lines due to their poor abilities to acylate di-22:1-DAG. We also show that the acyl-CoA specificities and the enzymatic activities are highly influenced by the fatty acid composition of the DAG acyl acceptor. This finding implies that the use of artificial acyl acceptors (e.g. di-6:0-DAG) may not always reflect the actual acyl-CoA specificities of DGATs in planta. The relevance of the here reported pronounced specificities for specific DAG species exerted by DGAT enzymes is discussed in the context of the findings of DAG pools of distinct catalytic origin in triacylglycerol biosynthesis in the seed oil.

Identification of the major diacylglycerol acyltransferase mRNA in mouse adipocytes and macrophages

Background

Triacylglycerols (TAGs) are the major form of energy storage in eukaryotes. Diacylglycerol acyltransferases (DGATs) catalyze the final and rate-limiting step of TAG biosynthesis. Mammalian DGATs are classified into DGAT1 and DGAT2 subfamilies. It was unclear which DGAT was the major isoform expressed in animal cells. The objective was to identify the major DGAT mRNA expressed in cultured mouse adipocytes and macrophages and compared it to that expressed in tung tree seeds.

Methods

qPCR evaluated DGAT mRNA levels in mouse 3 T3-L1 adipocytes and RAW264.7 macrophages and tung tree seeds.

Results

TaqMan qPCR showed that DGAT2 mRNA levels were 10–30 fold higher than DGAT1 in adipocytes and macrophages, and DGAT mRNA levels in adipocytes were 50–100-fold higher than those in macrophages. In contrast, the anti-inflammatory tristetraprolin/zinc finger protein 36 (TTP/ZFP36) mRNA levels were 2–4-fold higher in macrophages than those in adipocytes and similar to DGAT1 in adipocytes but 100-fold higher than DGAT1 in macrophages. SYBR Green qPCR analyses confirmed TaqMan qPCR results. DGAT2 mRNA as the major DGAT mRNA in the mouse cells was similar to that in tung tree seeds where DGAT2 mRNA levels were 10–20-fold higher than DGAT1 or DGAT3.

Conclusion

The results demonstrated that DGAT2 mRNA was the major form of DGAT mRNA expressed in mouse adipocytes and macrophages and tung tree seeds.

An integrated omics analysis reveals molecular mechanisms that are associated with differences in seed oil content between Glycine max and Brassica napus

BACKGROUND

Rapeseed (Brassica napus L.) and soybean (Glycine max L.) seeds are rich in both protein and oil, which are major sources of biofuels and nutrition. Although the difference in seed oil content between soybean (~ 20%) and rapeseed (~ 40%) exists, little is known about its underlying molecular mechanism.

RESULTS

An integrated omics analysis was performed in soybean, rapeseed, Arabidopsis (Arabidopsis thaliana L. Heynh), and sesame (Sesamum indicum L.), based on Arabidopsis acyl-lipid metabolism- and carbon metabolism-related genes. As a result, candidate genes and their transcription factors and microRNAs, along with phylogenetic analysis and co-expression network analysis of the PEPC gene family, were found to be largely associated with the difference between the two species. First, three soybean genes (Glyma.13G148600, Glyma.13G207900 and Glyma.12G122900) co-expressed with GmPEPC1 are specifically enriched during seed storage protein accumulation stages, while the expression of BnPEPC1 is putatively inhibited by bna-miR169, and two genes BnSTKA and BnCKII are co-expressed with BnPEPC1 and are specifically associated with plant circadian rhythm, which are related to seed oil biosynthesis. Then, in de novo fatty acid synthesis there are rapeseed-specific genes encoding subunits β-CT (BnaC05g37990D) and BCCP1 (BnaA03g06000D) of heterogeneous ACCase, which could interfere with synthesis rate, and β-CT is positively regulated by four transcription factors (BnaA01g37250D, BnaA02g26190D, BnaC01g01040D and BnaC07g21470D). In triglyceride synthesis, GmLPAAT2 is putatively inhibited by three miRNAs (gma-miR171, gma-miR1516 and gma-miR5775). Finally, in rapeseed there was evidence for the expansion of gene families, CALO, OBO and STERO, related to lipid storage, and the contraction of gene families, LOX, LAH and HSI2, related to oil degradation.

CONCLUSIONS

The molecular mechanisms associated with differences in seed oil content provide the basis for future breeding efforts to improve seed oil content.

Newly Identified Essential Amino Acids Affecting Chlorella ellipsoidea DGAT1 Function Revealed by Site-Directed Mutagenesis

Diacylglycerol acyltransferase (DGAT) is a rate-limiting enzyme in the synthesis of triacylglycerol (TAG), the most important form of energy storage in plants. Some residues have previously been proven to be crucial for DGAT1 activity. In this study, we used site-directed mutagenesis of the CeDGAT1 gene from Chlorella ellipsoidea to alter 16 amino acids to investigate effects on DGAT1 function. Of the 16 residues (L482R, E542R, Y553A, G577R, R579D, Y582R, R596D, H603D, H609D, A624R, F629R, S632A, W650R, A651R, Q658H, and P660R), we newly identified 5 (L482, R579, H603, A651, and P660) as being essential for DGAT1 function and 7 (E542, G577, R596, H609, A624, S632, and Q658) that significantly affect DGAT1 function to different degrees, as revealed by heterologous expression of the mutants in yeast strain INVSc1. Importantly, compared with CeDGAT1, expression of the mutant CeDGAT1Y553A significantly increased the total fatty acid and TAG contents of INVSc1. Comparison among CeDGAT1Y553A, GmDGAT1Y341A, AtDGAT1Y364A, BnDGAT1Y347A, and BoDGAT1Y352A, in which tyrosine at the position corresponding to the 553rd residue in CeDGAT1 is changed into alanine, indicated that the impact of changing Y to A at position 553 is specific for CeDGAT1. Overall, the results provide novel insight into the structure and function of DGAT1, as well as a mutant gene with high potential for lipid improvement in microalgae and plants.

The CCCH-type transcription factor BnZFP1 is a positive regulator to control oleic acid levels through the expression of diacylglycerol O-acyltransferase 1 gene in Brassica napus

In China, the high-oleic acid rapeseed has an oil content of ∼42% and oleic acid (18:1) content of ∼80%. Compared to ordinary rapeseed, high-oleic acid rapeseed has higher levels of monounsaturated fatty acids and lower levels of saturated fatty acid and polyunsaturated fatty acids, and thus is of high nutritional and health value. In addition, high-oleic acid rapeseed oil imparts cardiovascular protective effects. Based on these properties, high-oleic acid oil crops have been extensively investigated and cultivated. We previously identified a CCCH-type transcription factor (BnZFP1, GenBank accession number XM_013796508) that is associated with high oleic acid traits from a Brassica napus subtractive hybridization library. In the present study, we overexpressed and silenced the BnZFP1 gene of B. napus. BnZFP1-overexpressing plants exhibited an 18.8% increase in oleic acid levels and a 3.8% increase in oil content. However, BNZFP1-silenced plants showed a 4.5% decrease in oleic acid levels, whereas no significant change in oil content was observed. Microarray and pull-down assays indicated that BnZFP1 has a total of thirty potential target genes. Further analysis and validation of one of the potential target genes, namely, diacylglycerol O-acyltransferases 1 (DGAT1) gene, indicated that it is positively regulated by BnZFP1. We also observed a correlation between elevated DGAT1 gene expression levels and higher oil content and oleic acid levels in rapeseed.

Diacylglycerol acyltransferase 1 is activated by phosphatidate and inhibited by SnRK1-catalyzed phosphorylation

Diacylglycerol acyltransferase 1 (DGAT1) catalyzes the final and committed step in the Kennedy pathway for triacylglycerol (TAG) biosynthesis and, as such, elucidating its mode of regulation is critical to understand the fundamental aspects of carbon metabolism in oleaginous crops. In this study, purified Brassica napus diacylglycerol acyltransferase 1 (BnaDGAT1) in n-dodecyl-β-d-maltopyranoside micelles was lipidated to form mixed micelles and subjected to detailed biochemical analysis. The degree of mixed micelle fluidity appeared to influence acyltransferase activity. BnaDGAT1 exhibited a sigmoidal response and eventual substrate inhibition with respect to increasing concentrations of oleoyl-CoA. Phosphatidate (PA) was identified as a feed-forward activator of BnaDGAT1, enabling the final enzyme in the Kennedy pathway to adjust to the incoming flow of carbon leading to TAG. In the presence of PA, the oleoyl-CoA saturation plot became more hyperbolic and desensitized to substrate inhibition indicating that PA facilitates the transition of the enzyme into the more active state. PA may also relieve possible autoinhibition of BnaDGAT1 brought about by the N-terminal regulatory domain, which was shown to interact with PA. Indeed, PA is a key effector modulating lipid homeostasis, in addition to its well recognized role in lipid signaling. BnaDGAT1 was also shown to be a substrate of the sucrose non-fermenting-1-related kinase 1 (SnRK1), which catalyzed phosphorylation of the enzyme and converted it to a less active form. Thus, this known regulator of carbon metabolism directly influences TAG biosynthesis.

In silico characterization and expression profiling of the diacylglycerol acyltransferase gene family (DGAT1, DGAT2, DGAT3 and WS/DGAT) from oil palm, Elaeis guineensis

The diacylglycerol acyltransferases (DGAT) (diacylglycerol:acyl-CoA acyltransferase, EC 2.3.1.20) are a key group of enzymes that catalyse the final and usually the most important rate-limiting step of triacylglycerol biosynthesis in plants and other organisms. Genes encoding four distinct functional families of DGAT enzymes have been characterised in the genome of the African oil palm, Elaeis guineensis. The contrasting features of the various isoforms within the four families of DGAT genes, namely DGAT1, DGAT2, DGAT3 and WS/DGAT are presented both in the oil palm itself and, for comparative purposes, in 12 other oil crop or model/related plants, namely Arabidopsis thaliana, Brachypodium distachyon, Brassica napus, Elaeis oleifera, Glycine max, Gossypium hirsutum, Helianthus annuus, Musa acuminata, Oryza sativa, Phoenix dactylifera, Sorghum bicolor, and Zea mays. The oil palm genome contains respectively three, two, two and two distinctly expressed functional copies of the DGAT1, DGAT2, DGAT3 and WS/DGAT genes. Phylogenetic analyses of the four DGAT families showed that the E. guineensis genes tend to cluster with sequences from P. dactylifera and M. acuminata rather than with other members of the Commelinid monocots group, such as the Poales which include the major cereal crops such as rice and maize. Comparison of the predicted DGAT protein sequences with other animal and plant DGATs was consistent with the E. guineensis DGAT1 being ER located with its active site facing the lumen while DGAT2, although also ER located, had a predicted cytosol-facing active site. In contrast, DGAT3 and some (but not all) WS/DGAT in E. guineensis are predicted to be soluble, cytosolic enzymes. Evaluation of E. guineensis DGAT gene expression in different tissues and developmental stages suggests that the four DGAT groups have distinctive physiological roles and are particularly prominent in developmental processes relating to reproduction, such as flowering, and in fruit/seed formation especially in the mesocarp and endosperm tissues.

Properties and Biotechnological Applications of Acyl-CoA:diacylglycerol Acyltransferase and Phospholipid:diacylglycerol Acyltransferase from Terrestrial Plants and Microalgae

Triacylglycerol (TAG) is the major storage lipid in most terrestrial plants and microalgae, and has great nutritional and industrial value. Since the demand for vegetable oil is consistently increasing, numerous studies have been focused on improving the TAG content and modifying the fatty-acid compositions of plant seed oils. In addition, there is a strong research interest in establishing plant vegetative tissues and microalgae as platforms for lipid production. In higher plants and microalgae, TAG biosynthesis occurs via acyl-CoA-dependent or acyl-CoA-independent pathways. Diacylglycerol acyltransferase (DGAT) catalyzes the last and committed step in the acyl-CoA-dependent biosynthesis of TAG, which appears to represent a bottleneck in oil accumulation in some oilseed species. Membrane-bound and soluble forms of DGAT have been identified with very different amino-acid sequences and biochemical properties. Alternatively, TAG can be formed through acyl-CoA-independent pathways via the catalytic action of membrane-bound phospholipid:diacylglycerol acyltransferase (PDAT). As the enzymes catalyzing the terminal steps of TAG formation, DGAT and PDAT play crucial roles in determining the flux of carbon into seed TAG and thus have been considered as the key targets for engineering oil production. Here, we summarize the most recent knowledge on DGAT and PDAT in higher plants and microalgae, with the emphasis on their physiological roles, structural features, and regulation. The development of various metabolic engineering strategies to enhance the TAG content and alter the fatty-acid composition of TAG is also discussed.

Cloning and characterization of EgGDSL, a gene associated with oil content in oil palm

Oil palm (Elaeis guineensis, Jacq.) is a key tropical oil crop, which provides over one third of the global vegetable oil production, but few genes related to oil yield have been characterized. In this study, a GDSL esterase/lipase gene, which was significantly associated with oil content, was isolated from oil palm and designated as EgGDSL. Its functional characterization was carried out through ectopic expression in Arabidopsis ecotype Col-0. It was shown that expression of EgGDSL in Arabidopsis led to the increased total fatty acid content by 9.5% compared with the wild type. Further analysis of the fatty acid composition revealed that stearic acid (18:0) increased in the seeds of the transgenic lines, but the levels of linoleic acid (18:2) plus 11-eicosenoic acid drastically declined. Quantitative real-time PCR (qPCR) revealed that in oil palm, EgGDSL was highly expressed in mesocarp followed by leaf, and the expression level was very low in the root. The expression level of EgGDSL gene began to increase at two months after flowering (MAF) and reached its peak by four MAF, then declined rapidly, and reached its lowest level during the mature period (6 MAF). The EgGDSL gene was more highly expressed in oil palm trees with high oil content than that with low oil content, demonstrating that the transcription level of EgGDSL correlated with the amount of oil accumulation. The gene may be valuable for engineering fatty acid metabolism in crop improvement programmes and for marker-assisted breeding.

The synchronous TAG production with the growth by the expression of chloroplast transit peptide-fused ScPDAT in Chlamydomonas reinhardtii

BACKGROUND

The synchronous triacylglycerol (TAG) production with the growth is a key step to lower the cost of the microalgae-based biofuel production. Phospholipid: diacylglycerol acyltransferase (PDAT) has been identified recently and catalyzes the phospholipid contributing acyl group to diacylglycerol to synthesize TAG, and is considered as the important source of TAG in Chlamydomonas reinhardtii.

RESULTS

Using a chimeric Hsp70A-RbcS2 promoter, exogenous PDAT form Saccharomyces cerevisiae fused with a chloroplast transit peptide was expressed in C. reinhardtii CC-137. Proved by western blot, the expression of ScPDAT showed a synchronous trend to the growth in the exponential phase. Compared to the wild type, the strain of Scpdat achieved 22% increase in the content of total fatty acids and 32% increase in TAG content. In addition, the fluctuation of C16 series fatty acid in monogalactosyldiacylglycerol, diacylglyceryltrimethylhomoserine and TAG indicated an enhancement in the TAG accumulation pathway.

CONCLUSION

The TAG production was enhanced in the regular cultivation without the nutrient stress by strengthening the conversion of polar lipid to TAG in C. reinhardtii and the findings provide a candidate strategy for rational engineered strain to overcome the decline in the growth during the TAG accumulation triggered by nitrogen starvation.

Design of a Seed-Specific Chimeric Promoter with a Modified Expression Profile to Improve Seed Oil Content

Increasing the yield of plant oil is an important objective to meet the demand for sustainable resources and energy. Some attempts to enhance the expression of genes involved in oil synthesis in seeds have succeeded in increasing oil content. In many cases, the promoters of seed-storage protein genes have been used as seed-specific promoters. However, conventional promoters are developmentally regulated and their expression periods are limited. We constructed a chimeric promoter that starts to express in the early stage of seed development, and high-level expression is retained until the later stage by connecting the promoters of the biotin carboxyl carrier protein 2 (BCCP2) gene encoding the BCCP2 subunit of acetyl-CoA carboxylase and the fatty acid elongase 1 (FAE1) gene from Arabidopsis. The constructed promoter was ligated upstream of the TAG1 gene encoding diacylglycerol acyltransferase 1 and introduced into Arabidopsis. Seeds from transgenic plants carrying AtTAG1 under the control of the chimeric promoter showed increased oil content (up by 18–73%) compared with wild-type seeds. The novel expression profile of the chimeric promoter showed that this could be a promising strategy to manipulate the content of seed-storage oils and other compounds.

Identification of genes regulating traits targeted for domestication of field cress (Lepidium campestre) as a biennial and perennial oilseed crop

BACKGROUND

The changing climate and the desire to use renewable oil sources necessitate the development of new oilseed crops. Field cress (Lepidium campestre) is a species in the Brassicaceae family that has been targeted for domestication not only as an oilseed crop that produces seeds with a desirable industrial oil quality but also as a cover/catch crop that provides valuable ecosystem services. Lepidium is closely related to Arabidopsis and display significant proportions of syntenic regions in their genomes. Arabidopsis genes are among the most characterized genes in the plant kingdom and, hence, comparative genomics of Lepidium-Arabidopsis would facilitate the identification of Lepidium candidate genes regulating various desirable traits.

RESULTS

Homologues of 30 genes known to regulate vernalization, flowering time, pod shattering, oil content and quality in Arabidopsis were identified and partially characterized in Lepidium. Alignments of sequences representing field cress and two of its closely related perennial relatives: L. heterophyllum and L. hirtum revealed 243 polymorphic sites across the partial sequences of the 30 genes, of which 95 were within the predicted coding regions and 40 led to a change in amino acids of the target proteins. Within field cress, 34 polymorphic sites including nine non-synonymous substitutions were identified. The phylogenetic analysis of the data revealed that field cress is more closely related to L. heterophyllum than to L. hirtum.

CONCLUSIONS

There is significant variation within and among Lepidium species within partial sequences of the 30 genes known to regulate traits targeted in the present study. The variation within these genes are potentially useful to speed-up the process of domesticating field cress as future oil crop. The phylogenetic relationship between the Lepidium species revealed in this study does not only shed some light on Lepidium genome evolution but also provides important information to develop efficient schemes for interspecific hybridization between different Lepidium species as part of the domestication efforts.

Engineering Camelina sativa (L.) Crantz for enhanced oil and seed yields by combining diacylglycerol acyltransferase1 and glycerol-3-phosphate dehydrogenase expression

Plant seed oil-based liquid transportation fuels (i.e., biodiesel and green diesel) have tremendous potential as environmentally, economically and technologically feasible alternatives to petroleum-derived fuels. Due to their nutritional and industrial importance, one of the major objectives is to increase the seed yield and oil production of oilseed crops via biotechnological approaches. Camelina sativa, an emerging oilseed crop, has been proposed as an ideal crop for biodiesel and bioproduct applications. Further increase in seed oil yield by increasing the flux of carbon from increased photosynthesis into triacylglycerol (TAG) synthesis will make this crop more profitable. To increase the oil yield, we engineered Camelina by co-expressing the Arabidopsis thaliana (L.) Heynh. diacylglycerol acyltransferase1 (DGAT1) and a yeast cytosolic glycerol-3-phosphate dehydrogenase (GPD1) genes under the control of seed-specific promoters. Plants co-expressing DGAT1 and GPD1 exhibited up to 13% higher seed oil content and up to 52% increase in seed mass compared to wild-type plants. Further, DGAT1- and GDP1-co-expressing lines showed significantly higher seed and oil yields on a dry weight basis than the wild-type controls or plants expressing DGAT1 and GPD1 alone. The oil harvest index (g oil per g total dry matter) for DGTA1- and GPD1-co-expressing lines was almost twofold higher as compared to wild type and the lines expressing DGAT1 and GPD1 alone. Therefore, combining the overexpression of TAG biosynthetic genes, DGAT1 and GPD1, appears to be a positive strategy to achieve a synergistic effect on the flux through the TAG synthesis pathway, and thereby further increase the oil yield.

Seed-specific overexpression of AtFAX1 increases seed oil content in Arabidopsis

Biosynthesis of plant seed oil is accomplished through the coordinate action of multiple enzymes in multiple subcellular compartments. Fatty acid (FA) has to be transported from plastid to endoplasmic reticulum (ER) for TAG synthesis. However, the role of plastid FA transportation during seed oil accumulation has not been evaluated. AtFAX1 (Arabidopsis fatty acid export1) mediated the FA export from plastid. In this study, we overexpressed AtFAX1 under the control of a seed specific promoter in Arabidopsis. The resultant overexpression lines (OEs) produced seeds which contained 21-33% more oil and 24-30% more protein per seed than those of the wild type (WT). The increased oil content was probably because of the enhanced FA and TAG synthetic activity. The seed size and weight were both increased accordingly. In addition, the seed number per silique and silique number per plant had no changes in transgenic plants. Taken together, our results demonstrated that seed specific overexpression of AtFAX1 could promote oil accumulation in Arabidopsis seeds and manipulating FA transportation is a feasible strategy for increasing the seed oil content.

Soybean (Glycine max) WRINKLED1 transcription factor, GmWRI1a, positively regulates seed oil accumulation

Soybean is the world's most important leguminous crop producing high-quality protein and oil. Elevating oil accumulation in soybean seed is always many researchers' goal. WRINKLED1 (WRI1) encodes a transcription factor of the APETALA2/ethylene responsive element-binding protein (AP2/EREBP) family that plays important roles during plant seed oil accumulation. In this study, we isolated and characterized three distinct orthologues of WRI1 in soybean (Glycine max) that display different organ-specific expression patterns, among which GmWRI1a was highly expressed in maturing soybean seed. Electrophoretic mobility shift assays and yeast one-hybrid experiments demonstrated that the GmWRI1a protein was capable of binding to AW-box, a conserved sequence in the proximal upstream regions of many genes involved in various steps of oil biosynthesis. Transgenic soybean seeds overexpressing GmWRI1a under the control of the seed-specific napin promoter showed the increased total oil and fatty acid content and the changed fatty acid composition. Furthermore, basing on the activated expressions in transgenic soybean seeds and existence of AW-box element in the promoter regions, direct downstream genes of GmWRI1a were identified, and their products were responsible for fatty acid production, elongation, desaturation and export from plastid. We conclude that GmWRI1a transcription factor can positively regulate oil accumulation in soybean seed by a complex gene expression network related to fatty acid biosynthesis.

Transcriptomic analysis of Perilla frutescens seed to insight into the biosynthesis and metabolic of unsaturated fatty acids

Background

Perilla frutescens is well known for its high α-linolenic acid (ALA) accumulation in seeds and medicinal values as well as a source of edible and general-purpose oils. However, the regulatory mechanisms of the biosynthesis of fatty acid in its seeds remain poorly understood due to the lacking of sequenced genome. For better understanding the regulation of lipid metabolism and further increase its oil content or modify oil composition, time-course transcriptome and lipid composition analyses were performed.

Results

Analysis of fatty acid content and composition showed that the α-linolenic acid and oleic acid accumulated rapidly from 5 DAF to 15 DAF and then kept relatively stable. However, the amount of palmitic acid and linoleic acid decreased quickly from 5 DAF to 15DAF. No significant variation of stearic acid content was observed from 5 DAF to 25DAF. Our transcriptome data analyses revealed that 110,176 unigenes were generated from six seed libraries at 5, 10, 20 DAF. Of these, 53 (31 up, 22 down) and 653 (259 up, 394 down) genes showed temporal and differentially expression during the seed development in 5 DAF vs 10 DAF, 20 vs 10 DAF, respectively. The differentially expressed genes were annotated and found to be involved in distinct functional categories and metabolic pathways. Deep mining of transcriptome data led to the identification of key genes involved in fatty acid and triacylglycerol biosynthesis and metabolism. Thirty seven members of transcription factor family AP2, B3 and NFYB putatively involved in oil synthesis and deposition were differentially expressed during seed development. The results of qRT-PCR for selected genes showed a strong positive correlation with the expression abundance measured in RNA-seq analysis.

Conclusions

The present study provides valuable genomic resources for characterizing Perilla seed gene expression at the transcriptional level and will extend our understanding of the complex molecular and cellular events of oil biosynthesis and accumulation in oilseed crops.

Electronic supplementary material

The online version of this article (10.1186/s12864-018-4595-z) contains supplementary material, which is available to authorized users.

A transgene design for enhancing oil content in Arabidopsis and Camelina seeds

BACKGROUND

Increasing the oil yield is a major objective for oilseed crop improvement. Oil biosynthesis and accumulation are influenced by multiple genes involved in embryo and seed development. The leafy cotyledon1 (LEC1) is a master regulator of embryo development that also enhances the expression of genes involved in fatty acid biosynthesis. We speculated that seed oil could be increased by targeted overexpression of a master regulating transcription factor for oil biosynthesis, using a downstream promoter for a gene in the oil biosynthesis pathway. To verify the effect of such a combination on seed oil content, we made constructs with maize (Zea mays) ZmLEC1 driven by serine carboxypeptidase-like (SCPL17) and acyl carrier protein (ACP5) promoters, respectively, for expression in transgenic Arabidopsis thaliana and Camelina sativa.

RESULTS

Agrobacterium-mediated transformation successfully generated Arabidopsis and Camelina lines that overexpressed ZmLEC1 under the control of a seed-specific promoter. This overexpression does not appear to be detrimental to seed vigor under laboratory conditions and did not cause observable abnormal growth phenotypes throughout the life cycle of the plants. Overexpression of ZmLEC1 increased the oil content in mature seeds by more than 20% in Arabidopsis and 26% in Camelina.

CONCLUSION

The findings suggested that the maize master regulator, ZmLEC1, driven by a downstream seed-specific promoter, can be used to increase oil production in Arabidopsis and Camelina and might be a promising target for increasing oil yield in oilseed crops.0.

Characterization and engineering of a dual-function diacylglycerol acyltransferase in the oleaginous marine diatom Phaeodactylum tricornutum

BACKGROUND

Photosynthetic oleaginous microalgae are promising feedstocks for biofuels. Acyl-CoA:diacylglycerol acyltransferases (DGATs) represent rich sources for engineering microalgal lipid production. The principal activity of DGATs has been defined as a single-function enzyme catalyzing the esterification of diacylglycerol with acyl-CoA.

RESULTS

A dual-function PtWS/DGAT associated with diatom Phaeodactylum tricornutum is discovered in the current study. Distinctive to documented microalgal DGAT types, PtWS/DGAT exhibits activities of both a wax ester synthase (WS) and a DGAT. WS/DGATs are broadly distributed in microalgae, with different topology and phylogeny from those of DGAT1s, DGAT2s, and DGAT3s. In vitro and in vivo assays revealed that PtWS/DGAT, functioning as either a WS or a DGAT, exhibited a preference on saturated FA substrate. Endogenous overexpression of PtWS/DGAT demonstrated that the DGAT activity was dominant, whereas the WS activity was condition dependent and relatively minor. Compared with the wild type (WT), overexpression of PtWS/DGAT in the diatom resulted in increased levels of total lipids (TL) and triacylglycerol (TAG) regardless of nitrogen availability. The stability and scalability of the introduced traits were further investigated at a 10-L photobioreactor, where the mutant growth resembled WT, with moderately increased productivity of TL and TAG. Furthermore, the production of wax esters increased considerably (from undetectable levels to 2.83%) under nitrogen-deplete conditions.

CONCLUSIONS

PtWS/DGAT is a bifunctional enzyme and may serve as a promising target for the engineering of microalga-based oils and waxes for future industrial use.

Seed-Specific Expression of AtLEC1 Increased Oil Content and Altered Fatty Acid Composition in Seeds of Peanut (Arachis hypogaea L.)

Peanut (Arachis hypogaea L.) is one of the major oil crops and is the fifth largest source of plant oils in the world. Numerous genes participate in regulating the biosynthesis and accumulation of the storage lipids in seeds or other reservoir organs, among which several transcription factors, such as LEAFY COTYLEDON1 (AtLEC1), LEC2, and WRINKLED1 (WRI1), involved in embryo development also control the lipid reservoir in seeds. In this study, the AtLEC1 gene was transferred into the peanut genome and expressed in a seed-specific manner driven by the NapinA full-length promoter or its truncated 230-bp promoter. Four homozygous transgenic lines, two lines with the longer promoter and the other two with the truncated one, were selected for further analysis. The AtLEC1 mRNA level and the corresponding protein accumulation in different transgenic overexpression lines were altered, and the transgenic plants grew and developed normally without any detrimental effects on major agronomic traits. In the developing seeds of transgenic peanuts, the mRNA levels of a series of genes were upregulated. These genes are associated with fatty acid (FA) biosynthesis and lipid accumulation. The former set of genes included the homomeric ACCase A (AhACC II), the BC subunit of heteromeric ACCase (AhBC4), ketoacyl-ACP synthetase (AhKAS II), and stearoyl-ACP desaturase (AhSAD), while the latter ones were the diacylglycerol acyltransferases and oleosins (AhDGAT1, AhDGAT2, AhOle1, AhOle2, and AhOle3). The oil content and seed weight increased by 4.42-15.89% and 11.1-22.2%, respectively, and the levels of major FA components including stearic acid, oleic acid, and linoleic acid changed significantly in all different lines.

Comparative transcriptome and metabolome analysis suggests bottlenecks that limit seed and oil yields in transgenic Camelina sativa expressing diacylglycerol acyltransferase 1 and glycerol-3-phosphate dehydrogenase

BACKGROUND

Camelina sativa has attracted much interest as alternative renewable resources for biodiesel, other oil-based industrial products and a source for edible oils. Its unique oil attributes attract research to engineering new varieties of improved oil quantity and quality. The overexpression of enzymes catalyzing the synthesis of the glycerol backbone and the sequential conjugation of fatty acids into this backbone is a promising approach for increasing the levels of triacylglycerol (TAG). In a previous study, we co-expressed the diacylglycerol acyltransferase (DGAT1) and glycerol-3-phosphate dehydrogenase (GPD1), involved in TAG metabolism, in Camelina seeds. Transgenic plants exhibited a higher-percentage seed oil content, a greater seed mass, and overall improved seed and oil yields relative to wild-type plants. To further increase seed oil content in Camelina, we utilized metabolite profiling, in conjunction with transcriptome profiling during seed development to examine potential rate-limiting step(s) in the production of building blocks for TAG biosynthesis.

RESULTS

Transcriptomic analysis revealed approximately 2518 and 3136 transcripts differentially regulated at significant levels in DGAT1 and GPD1 transgenics, respectively. These transcripts were found to be involved in various functional categories, including alternative metabolic routes in fatty acid synthesis, TAG assembly, and TAG degradation. We quantified the relative contents of over 240 metabolites. Our results indicate major metabolic switches in transgenic seeds associated with significant changes in the levels of glycerolipids, amino acids, sugars, and organic acids, especially the TCA cycle and glycolysis intermediates.

CONCLUSIONS

From the transcriptomic and metabolomic analysis of DGAT1, GPD1 and DGAT1 + GPD1 expressing lines of C. sativa, we conclude that TAG production is limited by (1) utilization of fixed carbon from the source tissues supported by the increase in glycolysis pathway metabolites and decreased transcripts levels of transcription factors controlling fatty acids synthesis; (2) TAG accumulation is limited by the activity of lipases/hydrolases that hydrolyze TAG pool supported by the increase in free fatty acids and monoacylglycerols. This comparative transcriptomics and metabolomics approach is useful in understanding the regulation of TAG biosynthesis, identifying bottlenecks, and the corresponding genes controlling these pathways identified as limitations, for generating Camelina varieties with improved seed and oil yields.

BnDGAT1s Function Similarly in Oil Deposition and Are Expressed with Uniform Patterns in Tissues of Brassica napus

As an allotetraploid oilcrop, Brassica napus contains four duplicated Acyl-CoA:diacylglycerol acyltransferase 1 (DGAT1) genes, which catalyze one of the rate-limiting steps in triacylglycerol (TAG) biosynthesis in plants. While all four BnDGAT1s have been expressed functionally in yeast, their expression patterns in different germplasms and tissues and also consequent contribution to seed oil accumulation in planta remain to be elucidated. In this study, the coding regions of the four BnDGAT1s were expressed in an Arabidopsis dgat1 mutant. All four BnDGAT1s showed similar effects on oil content and fatty acid composition, a result which is different from that observed in previous studies of their expression in yeast. Expression patterns of BnDGAT1s were analyzed in developing seeds of 34 B. napus inbred lines and in different tissues of 14 lines. Different expression patterns were observed for the four BnDGAT1s, which suggests that they express independently or randomly in different germplasm sources. Higher expression of BnDGAT1s was correlated with higher seed oil content lines. Tissue-specific analyses showed that the BnDGAT1s were expressed in a uniform pattern in different tissues. Our results suggest that it is important to maintain expression of the four BnDGAT1s for maximum return on oil content.

Discovery of a new mechanism for regulation of plant triacylglycerol metabolism: The peanut diacylglycerol acyltransferase-1 gene family transcriptome is highly enriched in alternative splicing variants

Triacylglycerols (TAGs) are the most important energy storage form in oilseed crops. Diacylglycerol acyltransferase (DGAT) catalyzes the rate-limiting step of the Kennedy pathway of TAG biosynthesis. To date, little is known about the regulation of DGAT activity in peanut (Arachis hypogaea), an agronomically important oilseed crop that is cultivated in many parts of the world. In this study, seven distinct forms of type 1 DGAT (AhDGAT1.1-AhDGAT1.7) were identified, cloned, and characterized. Comparisons of the nucleotide sequences and gene structures revealed many different splicing variants of AhDGAT1, some of which displayed different organ-specific expression patterns. A representative gene (AhDGAT1.1) was transformed into wild-type tobacco and was shown to increase seed fatty acid (FA) content by 14.7%-20.9%. All seven AhDGAT1s were expressed in TAG-deficient Saccharomyces cerevisiae strain H1246; the five longest AhDGAT1 variants generated high levels of acyltransferase activity and complemented the free fatty acid lethality phenotype in this strain. The alternative splicing that gives rise to AhDGAT1.2 and AhDGAT1.4 creates predicted protein C-terminal truncations. The proteins encoded by these two variants were not active and did not complement the fatty acid sensitivity in H1246. These results were verified by visualization of intracellular lipid droplets using Nile Red staining. Collectively, the results presented here represent the first comprehensive analysis of the peanut DGAT1 gene family, which, unlike in other published plant DGAT1 sequences, shows widespread alternative splicing that may affect the expression patterns and enzyme activities of some members of the gene family.

Synteny analysis of genes and distribution of loci controlling oil content and fatty acid profile based on QTL alignment map in Brassica napus

Background

Deciphering the genetic architecture of a species is a good way to understand its evolutionary history, but also to tailor its profile for breeding elite cultivars with desirable traits. Aligning QTLs from diverse population in one map and utilizing it for comparison, but also as a basis for multiple analyses assure a stronger evidence to understand the genetic system related to a given phenotype.

Results

In this study, 439 genes involved in fatty acid (FA) and triacylglycerol (TAG) biosyntheses were identified in Brassica napus. B. napus genome showed mixed gene loss and insertion compared to B. rapa and B. oleracea, and C genome had more inserted genes. Identified QTLs for oil (OC-QTLs) and fatty acids (FA-QTLs) from nine reported populations were projected on the physical map of the reference genome “Darmor-bzh” to generate a map. Thus, 335 FA-QTLs and OC-QTLs could be highlighted and 82 QTLs were overlapping. Chromosome C3 contained 22 overlapping QTLs with all trait studied except for C18:3. In total, 218 candidate genes which were potentially involved in FA and TAG were identified in 162 QTLs confidence intervals and some of them might affect many traits. Also, 76 among these candidate genes were found inside 57 overlapping QTLs, and candidate genes for oil content were in majority (61/76 genes). Then, sixteen genes were found in overlapping QTLs involving three populations, and the remaining 60 genes were found in overlapping QTLs of two populations. Interaction network and pathway analysis of these candidate genes indicated ten genes that might have strong influence over the other genes that control fatty acids and oil formation.

Conclusion

The present results provided new information for genetic basis of FA and TAG formation in B. napus. A map including QTLs from numerous populations was built, which could serve as reference to study the genome profile of B. napus, and new potential genes emerged which might affect seed oil. New useful tracks were showed for the selection of population or/and selection of interesting genes for breeding improvement purpose.

Electronic supplementary material

The online version of this article (10.1186/s12864-017-4176-6) contains supplementary material, which is available to authorized users.

Oil body biogenesis and biotechnology in legume seeds

The seeds of many legume species including soybean, Pongamia pinnata and the model legume Medicago truncatula store considerable oil, apart from protein, in their cotyledons. However, as a group, legume storage strategies are quite variable and provide opportunities for better understanding of carbon partitioning into different storage products. Legumes with their ability to fix nitrogen can also increase the sustainability of agricultural systems. This review integrates the cell biology, biochemistry and molecular biology of oil body biogenesis before considering biotechnology strategies to enhance oil body biosynthesis. Cellular aspects of packaging triacylglycerol (TAG) into oil bodies are emphasized. Enhancing seed oil content has successfully focused on the up-regulation of the TAG biosynthesis pathways using overexpression of enzymes such as diacylglycerol acyltransferase1 and transcription factors such as WRINKLE1 and LEAFY COTYLEDON1. While these strategies are central, decreasing carbon flow into other storage products and maximizing the packaging of oil bodies into the cytoplasm are other strategies that need further examination. Overall there is much potential for integrating carbon partitioning, up-regulation of fatty acid and TAG synthesis and oil body packaging, for enhancing oil levels. In addition to the potential for integrated strategies to improving oil yields, the capacity to modify fatty acid composition and use of oil bodies as platforms for the production of recombinant proteins in seed of transgenic legumes provide other opportunities for legume biotechnology.

Diacylglycerol Acyltransferase 1 Is Regulated by Its N-Terminal Domain in Response to Allosteric Effectors

Diacylglycerol acyltransferase 1 (DGAT1) is an integral membrane enzyme catalyzing the final and committed step in the acyl-coenzyme A (CoA)-dependent biosynthesis of triacylglycerol (TAG). The biochemical regulation of TAG assembly remains one of the least understood areas of primary metabolism to date. Here, we report that the hydrophilic N-terminal domain of Brassica napus DGAT1 (BnaDGAT1_1-113) regulates activity based on acyl-CoA/CoA levels. The N-terminal domain is not necessary for acyltransferase activity and is composed of an intrinsically disordered region and a folded segment. We show that the disordered region has an autoinhibitory function and a dimerization interface, which appears to mediate positive cooperativity, whereas the folded segment of the cytosolic region was found to have an allosteric site for acyl-CoA/CoA. Under increasing acyl-CoA levels, the binding of acyl-CoA with this noncatalytic site facilitates homotropic allosteric activation. Enzyme activation, on the other hand, is prevented under limiting acyl-CoA conditions (low acyl-CoA-to-CoA ratio), whereby CoA acts as a noncompetitive feedback inhibitor through interaction with the same folded segment. The three-dimensional NMR solution structure of the allosteric site revealed an α-helix with a loop connecting a coil fragment. The conserved amino acid residues in the loop interacting with CoA were identified, revealing details of this important regulatory element for allosteric regulation. Based on these results, a model is proposed illustrating the role of the N-terminal domain of BnaDGAT1 as a positive and negative modulator of TAG biosynthesis.

High-performance variants of plant diacylglycerol acyltransferase 1 generated by directed evolution provide insights into structure function

Diacylglycerol acyltransferase 1 (DGAT1) catalyzes the acyl-CoA-dependent biosynthesis of triacylglycerol, the predominant component of seed oil. In some oil crops, including Brassica napus, the level of DGAT1 activity can have a substantial effect on triacylglycerol production. Structure-function insights into DGAT1, however, remain limited because of the lack of a three-dimensional detailed structure for this membrane-bound enzyme. In this study, the amino acid residues governing B. napus DGAT1 (BnaDGAT1) activity were investigated via directed evolution, targeted mutagenesis, in vitro enzymatic assay, topological analysis, and transient expression of cDNA encoding selected enzyme variants in Nicotiana benthamiana. Directed evolution revealed that numerous amino acid residues were associated with increased BnaDGAT1 activity, and 67% of these residues were conserved among plant DGAT1s. The identified amino acid residue substitution sites occur throughout the BnaDGAT1 polypeptide, with 89% of the substitutions located outside the putative substrate binding or active sites. In addition, cDNAs encoding variants I447F or L441P were transiently overexpressed in N. benthamiana leaves, resulting in 33.2 or 70.5% higher triacylglycerol content, respectively, compared with native BnaDGAT1. Overall, the results provide novel insights into amino acid residues underlying plant DGAT1 function and performance-enhanced BnaDGAT1 variants for increasing vegetable oil production.

Depressed expression of FAE1 and FAD2 genes modifies fatty acid profiles and storage compounds accumulation in Brassica napus seeds

In plants, the enzymes fatty acid dehydrogenase 2 (FAD2) and fatty acid elongase 1 (FAE1) have been shown in previous studies to play important roles in the de novo biosynthesis of fatty acids. However, the effects of depressed expression of FAD2 and FAE1 on seed storage compounds accumulation remains to be elucidated. In this study, we produced RNA interfering transgenic rapeseeds lines, BnFAD2-Ri, BnFAE1-Ri and BnFAD2/BnFAE1-Ri, which exhibited depressed expression of the BnFAD2 and BnFAE1 genes under the control of seed-specific napin A promoter. These transgenic rapeseeds showed normal growth and development as compared with the wild type (CY2). Depressed expression of BnFAD2 and BnFAE1 genes modified fatty acid profiles, leading to increased oleic acid and decreased erucic acid contents in transgenic seeds. Consistent with these results, the ratios of C18:1/C18:2 and C18:1/C18:3 in C18 unsaturated fatty acids were greatly increased due to increased oleic acid content in transgenic seeds. Moreover, depressed expression of BnFAD2 and BnFAE1 genes resulted in slightly decreased oil contents and increased protein contents in transgenic seeds. Our results demonstrated that depressed expression of BnFAD2 and BnFAE1 greatly improves seed nutritional quality by modulating the fatty acid metabolism and storage products accumulation and that BnFAD2 and BnFAE1 are reliable targets for genetic improvement of rapeseed in seed nutritional quality.

Soybean LEC2 Regulates Subsets of Genes Involved in Controlling the Biosynthesis and Catabolism of Seed Storage Substances and Seed Development

Soybean is an important oilseed crop and major dietary protein resource, yet the molecular processes and regulatory mechanisms involved in biosynthesis of seed storage substances are not fully understood. The B3 domain transcription factor (TF) LEC2 essentially regulates embryo development and seed maturation in other plants, but is not functionally characterized in soybean. Here, we characterize the function of a soybean LEC2 homolog, GmLEC2a, in regulating carbohydrate catabolism, triacylglycerol (TAG) biosynthesis, and seed development. The experimental analysis showed that GmLEC2a complemented Arabidopsis atlec2 mutant defects in seedling development and TAG accumulation. Over-expression of GmLEC2a in Arabidopsis seeds increased the TAG contents by 34% and the composition of long chain fatty acids by 4% relative to the control seeds. Transcriptome analysis showed that ectopic expression of GmLEC2a in soybean hairy roots up-regulated several sets of downstream TF genes GmLEC1, GmFUS3, GmABI3, GmDof11 and GmWRI1 that regulate the seed development and production of seed storage substances. GmLEC2a regulated the lipid transporter genes and oil body protein gene OLEOSIN (OLE1). The genes involved in carbohydrate biosynthesis and storage, such as sucrose synthesis, and catabolism of TAG, such as lipases in GmLEC2a hairy roots were down-regulated. GmLEC2a targeted metabolic genes for seed protein in soybean.

Mutations in the promoter, intron and CDS of two FAD2 generate multiple alleles modulating linoleic acid level in yellow mustard

Linoleic acid (C18:2) is an important polyunsaturated fatty acid in the seed oil of many crops. Here, we report that mutations in the promoter, intron and CDS of the FAD2 genes SalFAD2.LIA1 and SalFAD2.LIA2 generate three alleles LIA ^1a , LIA ^1b and lia ¹ and two alleles LIA ² and lia ², respectively, controlling the C18:2 variation (4.4-32.7%) in yellow mustard. The allelic effect on increasing C18:2 content is LIA ^1a  > LIA ^1b  > lia ¹ , LIA ² > lia ², and LIA ^1a  > LIA ². The five FAD 2 alleles each contain two exons, one intron and a promoter adjacent to exon 1. LIA ^1a has a 1152 bp CDS, a 1221 bp intron with promoter function and a 607 bp promoter. Compared with LIA ^1a , the intron of LIA ^1b has reduced promoter activity and that of LIA ² and lia ² has no promoter function due to extensive SNP and indel mutations. lia ¹ differed from LIA ^1b by having an insertion of 1223 bp retrotransposon in its intron. lia ² with mutations in the promoter has reduced promoter activity compared with LIA ² . This study revealed that complex quantitative variation of trait phenotype in plants could be modulated by multiple alleles of oligogenic loci resulting from mutations in the regulatory region and CDS.

Implications of glycerol metabolism for lipid production

Triacylglycerol (TAG) is an important product in oil-producing organisms. Biosynthesis of TAG can be completed through either esterification of fatty acids to glycerol backbone, or through esterification of 2-monoacylglycerol. This review will focus on the former pathway in which two precursors, fatty acid and glycerol-3-phosphate (G3P), are required for TAG formation. Tremendous progress has been made about the enzymes or genes that regulate the biosynthetic pathway of TAG. However, much attention has been paid to the fatty acid provision and the esterification process, while the possible role of G3P is largely neglected. Glycerol is extensively studied on its usage as carbon source for value-added products, but the modification of glycerol metabolism, which is directly associated with G3P synthesis, is seldom recognized in lipid investigations. The relevance among glycerol metabolism, G3P synthesis and lipid production is described, and the role of G3P in glycerol metabolism and lipid production are discussed in detail with an emphasis on how G3P affects lipid production through the modulation of glycerol metabolism. Observations of lipid metabolic changes due to glycerol related disruption in mammals, plants, and microorganisms are introduced. Altering glycerol metabolism results in the changes of final lipid content. Possible regulatory mechanisms concerning the relationship between glycerol metabolism and lipid production are summarized.

Progress in modification of sunflower oil to expand its industrial value

Increasing the sunflower seed oil content as well as improving its quality makes it compatible for industrial demands. This is an important breeding objective of sunflower which increases its market value and ensures high returns for the producers. The present review focuses on determining the progress of improving sunflower seed oil content and modifying its quality by empirical and advanced molecular breeding methods. It is known that the sunflower oil content and quality have been altered through empirical selection methods and mutation breeding programmes in various parts of the world. Further improvement in seed oil content and its components (such as phytosterols, tocopherols and modified fatty acid profile) has been slowed down due to low genetic variation in elite germplasm and complex of hereditary traits. Introgression from wild species can be carried out to modify the fatty acids profile and tocopherol contents with linkage drags. Different transgenes introduced through biotechnological methods may produce novel long-chain fatty acids within sunflower oil. Bio-engineering of sunflower oil could allow it to be used in diverse industrial products such as bio-diesel or bio-plastics. These results showed that past and current trends of modifying sunflower oil quality are essential for its further expansion as an oilseed crop. © 2017 Society of Chemical Industry.

Accelerated triacylglycerol production and altered fatty acid composition in oleaginous microalga Neochloris oleoabundans by overexpression of diacylglycerol acyltransferase 2

Background

Microalgae are promising sources of lipid triacylglycerol (TAG) for biodiesel production. However, to date, microalgal biodiesel is technically feasible, but not yet economically viable. Increasing TAG content and productivity are important to achieve economic viability of microalgal biodiesel. To increase TAG content, oleaginous microalga Neochloris oleoabundans was genetically engineered with an endogenous key enzyme diacylglycerol acyltransferase 2 (NeoDGAT2) responsible for TAG biosynthesis.

Results

The integration of NeoDGAT2 expression cassettes in N. oleoabundans transformant was confirmed by PCR. The neutral lipid accumulation in the transformant detected by Nile red staining was accelerated and 1.9-fold higher than in wild type; the lipid bodies in the transformant visualized under fluorescence microscope were also larger. The NeoDGAT2 transcript was two-fold higher in the transformant than wild type. Remarkably higher lipid accumulation was found in the transformant than wild type: total lipid content increased 1.6-to 2.3-fold up to 74.5 ± 4.0% dry cell weight (DCW) and total lipid productivity increased 1.6- to 3.2-fold up to 14.6 ± 2.0 mg/L/day; while TAG content increased 1.8- to 3.2-fold up to 46.1 ± 1.6% DCW and TAG productivity increased 1.6- to 4.3-fold up to 8.9 ± 1.3 mg/L/day. A significantly altered fatty acid composition was detected in the transformant compared to wild type; the levels of saturated fatty acid C16:0 increased double to 49%, whereas C18:0 was reduced triple to 6%. Long-term stability was observed in the transformant continuously maintained in solid medium over 100 generations in a period of about 4 years.

Conclusions

Our results demonstrate the increased TAG content and productivity in N. oleoabundans by NeoDGAT2 overexpression that may offer the first step towards making microalgae an economically feasible source for biodiesel production. The strategy for genetically improved microalga presented in this study can be applied to other microalgal species possessing desired characteristics for industrial biofuel production.

Two Acyltransferases Contribute Differently to Linolenic Acid Levels in Seed Oil

Acyltransferases are key contributors to triacylglycerol (TAG) synthesis and, thus, are of great importance for seed oil quality. The effects of increased or decreased expression of ACYL-COENZYME A:DIACYLGLYCEROL ACYLTRANSFERASE1 (DGAT1) or PHOSPHOLIPID:DIACYLGLYCEROL ACYLTRANSFERASE (PDAT) on seed lipid composition were assessed in several Camelina sativa lines. Furthermore, in vitro assays of acyltransferases in microsomal fractions prepared from developing seeds of some of these lines were performed. Decreased expression of DGAT1 led to an increased percentage of 18:3n-3 without any change in total lipid content of the seed. The tri-18:3 TAG increase occurred predominantly in the cotyledon, as determined with matrix-assisted laser desorption/ionization-mass spectrometry, whereas species with two 18:3n-3 acyl groups were elevated in both cotyledon and embryonal axis. PDAT overexpression led to a relative increase of 18:2n-6 at the expense of 18:3n-3, also without affecting the total lipid content. Differential distributions of TAG species also were observed in different parts of the seed. The microsomal assays revealed that C.sativa seeds have very high activity of diacylglycerol-phosphatidylcholine interconversion. The combination of analytical and biochemical data suggests that the higher 18:2n-6 content in the seed oil of the PDAT overexpressors is due to the channeling of fatty acids from phosphatidylcholine into TAG before being desaturated to 18:3n-3, caused by the high activity of PDAT in general and by PDAT specificity for 18:2n-6. The higher levels of 18:3n-3 in DGAT1-silencing lines are likely due to the compensatory activity of a TAG-synthesizing enzyme with specificity for this acyl group and more desaturation of acyl groups occurring on phosphatidylcholine.

Identification and characterization of an efficient acyl-CoA: diacylglycerol acyltransferase 1 (DGAT1) gene from the microalga Chlorella ellipsoidea

BACKGROUND

Oil in the form of triacylglycerols (TAGs) is quantitatively the most important storage form of energy for eukaryotic cells. Diacylglycerol acyltransferase (DGAT) is considered the rate-limiting enzyme for TAG accumulation. Chlorella, a unicellular eukaryotic green alga, has attracted much attention as a potential feedstock for renewable energy production. However, the function of DGAT1 in Chlorella has not been reported.

RESULTS

A full-length cDNA encoding a putative diacylglycerol acyltransferase 1 (DGAT1, EC 2.3.1.20) was obtained from Chlorella ellipsoidea. The 2,142 bp open reading frame of this cDNA, designated CeDGAT1, encodes a protein of 713 amino acids showing no more than 40% identity with DGAT1s of higher plants. Transcript analysis showed that the expression level of CeDGAT1 markedly increased under nitrogen starvation, which led to significant triacylglycerol (TAG) accumulation. CeDGAT1 activity was confirmed in the yeast quadruple mutant strain H1246 by restoring its ability to produce TAG. Upon expression of CeDGAT1, the total fatty acid content in wild-type yeast (INVSc1) increased by 142%, significantly higher than that transformed with DGAT1s from higher plants, including even the oil crop soybean. The over-expression of CeDGAT1 under the NOS promoter in wild-type Arabidopsis thaliana and Brassica napus var. Westar significantly increased the oil content by 8-37% and 12-18% and the average 1,000-seed weight by 9-15% and 6-29%, respectively, but did not alter the fatty acid composition of the seed oil. The net increase in the 1,000-seed total lipid content was up to 25-50% in both transgenic Arabidopsis and B. napus.

CONCLUSIONS

We identified a gene encoding DGAT1 in C. ellipsoidea and confirmed that it plays an important role in TAG accumulation. This is the first functional analysis of DGAT1 in Chlorella. This information is important for understanding lipid synthesis and accumulation in Chlorella and for genetic engineering to enhance oil production in microalgae and oil plants.

Effects of Overexpression of WRI1 and Hemoglobin Genes on the Seed Oil Content of Lepidium campestre

The wild species field cress (Lepidium campestre), belonging to the Brassicaceae family, has potential to be developed into a novel oilseed- and catch crop, however, the species needs to be further improved regarding some important agronomic traits. One of them is its low oil content which needs to be increased. As far as we know there is no study aiming at increasing the oil content that has been reported in this species. In order to investigate the possibility to increase the seed oil content in field cress, we have tried to introduce the Arabidopsis WRINKLED1 (AtWRI1) or hemoglobin (Hb) genes from either Arabidopsis thaliana (AtHb2) or Beta vulgaris (BvHb2) into field cress with the seed specific expression. The hypothesis was that the oil content would be increased by overexpressing these target genes. The results showed that the oil content was indeed increased by up to 29.9, 20.2, and 25.9% in the transgenic lines expressing AtWRI1, AtHb2, and BvHb2, respectively. The seed oil composition of the transgenic lines did not significantly deviate from the seed oil composition of the wild type plants. Our results indicate that genetic modification can be used in this wild species for its fast domestication into a future economically viable oilseed and catch crop.

Genetic enhancement of oil content in potato tuber (Solanum tuberosum L.) through an integrated metabolic engineering strategy

Potato tuber is a high yielding food crop known for its high levels of starch accumulation but only negligible levels of triacylglycerol (TAG). In this study, we evaluated the potential for lipid production in potato tubers by simultaneously introducing three transgenes, including WRINKLED 1 (WRI1), DIACYLGLYCEROL ACYLTRANSFERASE 1 (DGAT1) and OLEOSIN under the transcriptional control of tuber-specific (patatin) and constitutive (CaMV-35S) promoters. This coordinated metabolic engineering approach resulted in over a 100-fold increase in TAG accumulation to levels up to 3.3% of tuber dry weight (DW). Phospholipids and galactolipids were also found to be significantly increased in the potato tuber. The increase of lipids in these transgenic tubers was accompanied by a significant reduction in starch content and an increase in soluble sugars. Microscopic examination revealed that starch granules in the transgenic tubers had more irregular shapes and surface indentations when compared with the relatively smooth surfaces of wild-type starch granules. Ultrastructural examination of lipid droplets showed their close proximity to endoplasmic reticulum and mitochondria, which may indicate a dynamic interaction with these organelles during the processes of lipid biosynthesis and turnover. Increases in lipid levels were also observed in the transgenic potato leaves, likely due to the constitutive expression of DGAT1 and incomplete tuber specificity of the patatin promoter. This study represents an important proof-of-concept demonstration of oil increase in tubers and provides a model system to further study carbon reallocation during development of nonphotosynthetic underground storage organs.

Nannochloropsis, a rich source of diacylglycerol acyltransferases for engineering of triacylglycerol content in different hosts

BACKGROUND

Photosynthetic microalgae are considered a viable and sustainable resource for biofuel feedstocks, because they can produce higher biomass per land area than plants and can be grown on non-arable land. Among many microalgae considered for biofuel production, Nannochloropsis oceanica (CCMP1779) is particularly promising, because following nutrient deprivation it produces very high amounts of triacylglycerols (TAG). The committed step in TAG synthesis is catalyzed by acyl-CoA:diacylglycerol acyltransferase (DGAT). Remarkably, a total of 13 putative DGAT-encoding genes have been previously identified in CCMP1779 but most have not yet been studied in detail.

RESULTS

Based on their expression profile, six out of 12 type-2 DGAT-encoding genes (NoDGTT1-NoDGTT6) were chosen for their possible role in TAG biosynthesis and the respective cDNAs were expressed in a TAG synthesis-deficient mutant of yeast. Yeast expressing NoDGTT5 accumulated TAG to the highest level. Over-expression of NoDGTT5 in CCMP1779 grown in N-replete medium resulted in levels of TAG normally observed only after N deprivation. Reduced growth rates accompanied NoDGTT5 over-expression in CCMP1779. Constitutive expression of NoDGTT5 in Arabidopsis thaliana was accompanied by increased TAG content in seeds and leaves. A broad substrate specificity for NoDGTT5 was revealed, with preference for unsaturated acyl groups. Furthermore, NoDGTT5 was able to successfully rescue the Arabidopsis tag1-1 mutant by restoring the TAG content in seeds.

CONCLUSIONS

Taken together, our results identified NoDGTT5 as the most promising gene for the engineering of TAG synthesis in multiple hosts among the 13 DGAT-encoding genes of N. oceanica CCMP1779. Consequently, this study demonstrates the potential of NoDGTT5 as a tool for enhancing the energy density in biomass by increasing TAG content in transgenic crops used for biofuel production.

Variant Amino Acid Residues Alter the Enzyme Activity of Peanut Type 2 Diacylglycerol Acyltransferases

Diacylglycerol acyltransferase (DGAT) catalyzes the final step in triacylglycerol (TAG) biosynthesis via the acyl-CoA-dependent acylation of diacylglycerol. This reaction is a major control point in the Kennedy pathway for biosynthesis of TAG, which is the most important form of stored metabolic energy in most oil-producing plants. In this study, Arachis hypogaea type 2 DGAT (AhDGAT2) genes were cloned from the peanut cultivar 'Luhua 14.' Sequence analysis of 11 different peanut cultivars revealed a gene family of 8 peanut DGAT2 genes (designated AhDGAT2a-h). Sequence alignments revealed 21 nucleotide differences between the eight ORFs, but only six differences result in changes to the predicted amino acid (AA) sequences. A representative full-length cDNA clone (AhDGAT2a) was characterized in detail. The biochemical effects of altering the AhDGAT2a sequence to include single variable AA residues were tested by mutagenesis and functional complementation assays in transgenic yeast systems. All six mutant variants retained enzyme activity and produced lipid droplets in vivo. The N6D and A26P mutants also displayed increased enzyme activity and/or total cellular fatty acid (FA) content. N6D mutant mainly increased the content of palmitoleic acid, and A26P mutant mainly increased the content of palmitic acid. The A26P mutant grew well both in the presence of oleic and C18:2, but the other mutants grew better in the presence of C18:2. AhDGAT2 is expressed in all peanut organs analyzed, with high transcript levels in leaves and flowers. These levels are comparable to that found in immature seeds, where DGAT2 expression is most abundant in other plants. Over-expression of AhDGAT2a in tobacco substantially increased the FA content of transformed tobacco seeds. Expression of AhDGAT2a also altered transcription levels of endogenous tobacco lipid metabolic genes in transgenic tobacco, apparently creating a larger carbon 'sink' that supports increased FA levels.

Seed-Specific Overexpression of the Pyruvate Transporter BASS2 Increases Oil Content in Arabidopsis Seeds

Seed oil is important not only for human and animal nutrition, but also for various industrial applications. Numerous genetic engineering strategies have been attempted to increase the oil content per seed, but few of these strategies have involved manipulating the transporters. Pyruvate is a major source of carbon for de novo fatty acid biosynthesis in plastids, and the embryo's demand for pyruvate is reported to increase during active oil accumulation. In this study, we tested our hypothesis that oil biosynthesis could be boosted by increasing pyruvate flux into plastids. We expressed the known plastid-localized pyruvate transporter BILE ACID:SODIUM SYMPORTER FAMILY PROTEIN 2 (BASS2) under the control of a seed-specific soybean (Glycine max) glycinin-1 promoter in Arabidopsis thaliana. The resultant transgenic Arabidopsis plants (OEs), which expressed high levels of BASS2, produced seeds that were larger and heavier and contained 10-37% more oil than those of the wild type (WT), but were comparable to the WT seeds in terms of protein and carbohydrate contents. The total seed number did not differ significantly between the WT and OEs. Therefore, oil yield per plant was increased by 24-43% in the OE lines compared to WT. Taken together, our results demonstrate that seed-specific overexpression of the pyruvate transporter BASS2 promotes oil production in Arabidopsis seeds. Thus, manipulating the level of specific transporters is a feasible approach for increasing the seed oil content.

Characterization and Functional Analysis of a Type 2 Diacylglycerol Acyltransferase (DGAT2) Gene from Oil Palm (Elaeis guineensis Jacq.) Mesocarp in Saccharomyces cerevisiae and Transgenic Arabidopsis thaliana

Oil palm (Elaeis guineensis Jacq.) is the highest oil-yielding plant in the world, storing 90 and 60% (dry weight) oil in its mesocarp and kernel, respectively. To gain insights into the oil accumulation mechanism, one of the key enzymes involved in triacylglycerol (TAG) biosynthesis, a Type 2 diacylglycerol acyltransferase (DGAT2) from oil palm, was characterized for its in vivo activity. EgDGAT2 is highly expressed in mesocarp during the last two developmental stages while large amounts of oil are accumulated at the highest rate during ripening. Heterologous expression of EgDGAT2 in mutant yeast H1246 restored TAG biosynthesis with substrate preference toward unsaturated fatty acids (FAs) (16:1 and 18:1). Furthermore, seed-specific overexpression of EgDGAT2 in Arabidopsis thaliana enhanced the content of polyunsaturated FAs 18:2 and 18:3 (each by 6 mol%) in seed TAGs, when compared to that from wild-type Arabidopsis. In turn, the proportion of 18:0 and 20:0 FAs in seed TAGs from EgDGAT2 transgenic lines decreased accordingly. These results provide new insights into understanding the in vivo activity of EgDGAT2 from oil palm mesocarp, which will be of importance for metabolic enhancement of unsaturated FAs production.

The dilemma for lipid productivity in green microalgae: importance of substrate provision in improving oil yield without sacrificing growth

Rising oil prices and concerns over climate change have resulted in more emphasis on research into renewable biofuels from microalgae. Unlike plants, green microalgae have higher biomass productivity, will not compete with food and agriculture, and do not require fertile land for cultivation. However, microalgae biofuels currently suffer from high capital and operating costs due to low yields and costly extraction methods. Microalgae grown under optimal conditions produce large amounts of biomass but with low neutral lipid content, while microalgae grown in nutrient starvation accumulate high levels of neutral lipids but are slow growing. Producing lipids while maintaining high growth rates is vital for biofuel production because high biomass productivity increases yield per harvest volume while high lipid content decreases the cost of extraction per unit product. Therefore, there is a need for metabolic engineering of microalgae to constitutively produce high amounts of lipids without sacrificing growth. Substrate availability is a rate-limiting step in balancing growth and fatty acid (FA) production because both biomass and FA synthesis pathways compete for the same substrates, namely acetyl-CoA and NADPH. In this review, we discuss the efforts made for improving biofuel production in plants and microorganisms, the challenges faced in achieving lipid productivity, and the important role of precursor supply for FA synthesis. The main focus is placed on the enzymes which catalyzed the reactions supplying acetyl-CoA and NADPH.

Transcriptome analysis revealed the dynamic oil accumulation in Symplocos paniculata fruit

BACKGROUND

Symplocos paniculata, asiatic sweetleaf or sapphire berry, is a widespread shrub or small tree from Symplocaceae with high oil content and excellent fatty acid composition in fruit. It has been used as feedstocks for biodiesel and cooking oil production in China. Little transcriptome information is available on the regulatory molecular mechanism of oil accumulation at different fruit development stages.

RESULTS

The transcriptome at four different stages of fruit development (10, 80,140, and 170 days after flowering) of S. paniculata were analyzed. Approximately 28 million high quality clean reads were generated. These reads were trimmed and assembled into 182,904 non-redundant putative transcripts with a mean length of 592.91 bp and N50 length of 785 bp, respectively. Based on the functional annotation through Basic Local Alignment Search Tool (BLAST) with public protein database, the key enzymes involved in lipid metabolism were identified, and a schematic diagram of the pathway and temporal expression patterns of lipid metabolism was established. About 13,939 differentially expressed unigenes (DEGs) were screened out using differentially expressed sequencing (DESeq) method. The transcriptional regulatory patterns of the identified enzymes were highly related to the dynamic oil accumulation along with the fruit development of S. paniculata. In addition, quantitative real-time PCR (qRT-PCR) of six vital genes was significantly correlated with DESeq data.

CONCLUSIONS

The transcriptome sequences obtained and deposited in NCBI would enrich the public database and provide an unprecedented resource for the discovery of the genes associated with lipid metabolism pathway in S. paniculata. Results in this study will lay the foundation for exploring transcriptional regulatory profiles, elucidating molecular regulatory mechanisms, and accelerating genetic engineering process to improve the yield and quality of seed oil of S. paniculata.