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

Bioengineering of long-chain polyunsaturated fatty acids in oilseed crops

Long-chain polyunsaturated fatty acids (LC-PUFAs), especially very long-chain polyunsaturated fatty acids (VLC-PUFAs), are highly beneficial to human health including brain development, cardiovascular health and the immune system. Plant-derived edible oils serve as crucial dietary sources of PUFAs for humans. However, oilseed crops such as soybean, peanut, rapeseed, sesame and flax, generally contain insufficient content of LC-PUFAs and do not naturally produce VLC-PUFAs. This review discusses PUFA biosynthesis, current efforts on LC-PUFA bioengineering in oilseed crops, comparing the advantages of different genetic engineering strategies and highlights the bottlenecks encountered in this field. Combination of high-efficient enzymes from various species has enabled the improvement of LC-PUFAs and slight production of VLC-PUFAs, though under risk of generational instability. These and future intelligently designed enzymes with multidisciplinary approaches in molecular biology, biochemistry and plant physiology can be crucial in developing oilseed crops that meet the growing demand for LC-PUFAs.

Transcriptional engineering for value enhancement of oilseed crops: a forward perspective

Plant-derived oils provide 20%-35% of dietary calories and are a primary source of essential omega-6 (linoleic) and omega-3 (α-linolenic) fatty acids. While traditional breeding has significantly increased yields in key oilseed crops like soybean, sunflower, canola, peanut, and cottonseed, overall gains have plateaued over the past few decades. Oilseed crops also experience substantial yield losses in both prime and marginal agricultural areas due to biotic and abiotic stresses and shifting agro-climates. Recent genomic, transcriptomic, and metabolomics research has expanded our understanding of the genetic and physiological control of fatty acid biosynthesis and composition. Many oilseed species have inherent stress-combating mechanisms, including transcription factor regulation. Advances in genome editing tools like CRISPR/Cas9 offer precise genetic modifications, targeting transcription factors and binding sites to enhance desirable traits, such as the nutritional profile and chemical composition of fatty acids. This review explores the application of genome editing in oilseed improvement, covering recent progress, challenges, and future potential to boost yield and oil content. These advancements could play a transformative role in developing resilient, nutritious crop varieties essential for sustainable food security in a changing climate.

Biotechnological advances in the production of unusual fatty acids in transgenic plants and recombinant microorganisms

Certain plants and microorganisms can produce high amounts of unusual fatty acids (UFAs) such as hydroxy, conjugated, cyclic, and very long-chain polyunsaturated fatty acids, which have distinct physicochemical properties and significant applications in the food, feed, and oleochemical industries. Since many natural sources of UFAs are not ideal for large-scale agricultural production or fermentation, it is attractive to produce them through synthetic biology. Although several UFAs have been commercially or pre-commercially produced in transgenic plants and microorganisms, their contents in transgenic hosts are generally much lower than in natural sources. Moreover, reproducing this success for a wider spectrum of UFAs has remained challenging. This review discusses recent advancements in our understanding of the biosynthesis, accumulation, and heterologous production of UFAs, and addresses the challenges and potential strategies for achieving high UFA content in engineered plants and microorganisms.

Pistil-derived lipids influence pollen tube growth and male fertility in Arabidopsis thaliana

Pollen germination and pollen tube elongation require rapid phospholipid production and remodeling in membrane systems that involve both de novo synthesis and turnover. Phosphatidic acid phosphohydrolase (PAH) and lysophosphatidylcholine acyltransferase (LPCAT) are 2 key enzymes in membrane lipid maintenance. PAH generates diacylglycerol (DAG), a necessary precursor for the de novo synthesis of phosphatidylcholine (PC), while LPCAT reacylates lysophosphatidylcholine to PC and plays an essential role in the remodeling of membrane lipids. In this study, we investigated the synthetic defects of pah and lpcat mutations in sexual reproduction of Arabidopsis (Arabidopsis thaliana) and explored the prospect of pistil lipid provision to pollen tube growth. The combined deficiencies of lpcat and pah led to decreased pollen tube growth in the pistil and reduced male transmission. Interestingly, pistils of the lipid mutant dgat1 ameliorated the male transmission deficiencies of pah lpcat pollen. In contrast, pollination with a nonspecific phospholipase C (NPC) mutant exacerbated the fertilization impairment of the pah lpcat pollen. Given the importance of DAG in lipid metabolism and its contrasting changes in the dgat1 and npc mutants, we further investigated whether DAG supplement in synthetic media could influence pollen performance. DAG was incorporated into phospholipids of germinating pollen and stimulated pollen tube growth. Our study provides evidence that pistil-derived lipids contribute to membrane lipid synthesis in pollen tube growth, a hitherto unknown role in synergistic pollen-pistil interactions.

Arabidopsis diacylglycerol acyltransferase1 mutants require fatty acid desaturation for normal seed development

Diacylglycerol acyltransferase1 (DGAT1) is the major enzyme that synthesizes triacylglycerols (TAG) during Arabidopsis seed development. Mutant dgat1 seeds possess low oil content in addition to a high polyunsaturated fatty acid (PUFA) composition. Two genes encoding endoplasmic reticulum localized desaturase enzymes, fatty acid desaturase2 (FAD2) and fatty acid desaturase3 (FAD3), were upregulated in both dgat1-1 and dgat1-2 developing seeds. Crosses between both dgat1 mutant alleles and fad2-1 failed to generate plants homozygous for both dgat1 and fad2. Reciprocal crosses with wild-type plants demonstrated that both male and female dgat1 fad2 gametophytes were viable. Siliques from DGAT1/dgat1-1 fad2-1/fad2-1 and dgat1-1/dgat1-1 FAD2/fad2-1 possessed abnormal looking seeds that were arrested in the torpedo growth stage. Approximately 25% of the seeds exhibited this arrested phenotype, genetically consistent with them possessing the double homozygous dgat1 fad2 genotype. In contrast, double homozygous dgat1-1 fad3-2 mutant plants were viable. Seeds from these plants possessed higher levels of 18:2 while their fatty acid content was lower than dgat1 mutant controls. The results are consistent with a model where in the absence of DGAT1 activity, desaturation of fatty acids by FAD2 becomes essential to provide PUFA substrates for phospholipid:diacylglycerol acyltransferase (PDAT) to synthesize TAG. In a dgat1 fad2 mutant, seed development is aborted because TAG is unable to be synthesized by either DGAT1 or PDAT.

Variety of Plant Oils: Species-Specific Lipid Biosynthesis

Plant oils represent a large group of neutral lipids with important applications in food, feed and oleochemical industries. Most plants accumulate oils in the form of triacylglycerol within seeds and their surrounding tissues, which comprises three fatty acids attached to a glycerol backbone. Different plant species accumulate unique fatty acids in their oils, serving a range of applications in pharmaceuticals and oleochemicals. To enable the production of these distinctive oils, select plant species have adapted specialized oil metabolism pathways, involving differential gene co-expression networks and structurally divergent enzymes/proteins. Here, we summarize some of the recent advances in our understanding of oil biosynthesis in plants. We compare expression patterns of oil metabolism genes from representative species, including Arabidopsis thaliana, Ricinus communis (castor bean), Linum usitatissimum L. (flax) and Elaeis guineensis (oil palm) to showcase the co-expression networks of relevant genes for acyl metabolism. We also review several divergent enzymes/proteins associated with key catalytic steps of unique oil accumulation, including fatty acid desaturases, diacylglycerol acyltransferases and oleosins, highlighting their structural features and preference toward unique lipid substrates. Lastly, we briefly discuss protein interactomes and substrate channeling for oil biosynthesis and the complex regulation of these processes.

Functional Characterization of the Effects of CsDGAT1 and CsDGAT2 on Fatty Acid Composition in Camelina sativa

Triacylglycerols (TAGs) are the storage oils of plant seeds, and these lipids provide energy for seed germination and valuable oils for human consumption. Three diacylglycerol acyltransferases (DGAT1, DGAT2, and DGAT3) and phospholipid:diacylglycerol acyltransferases participate in the biosynthesis of TAGs. DGAT1 and DGAT2 participate in the biosynthesis of TAGs through the endoplasmic reticulum (ER) pathway. In this study, we functionally characterized CsDGAT1 and CsDGAT2 from camelina (Camelina sativa). Green fluorescent protein-fused CsDGAT1 and CsDGAT2 localized to the ER when transiently expressed in Nicotiana benthamiana leaves. To generate Csdgat1 and Csdgat2 mutants using the CRISPR/Cas9 system, camelina was transformed with a binary vector carrying Cas9 and the respective guide RNAs targeting CsDGAT1s and CsDGAT2s via the Agrobacterium-mediated floral dip method. The EDD1 lines had missense and nonsense mutations in the CsDGAT1 homoeologs, suggesting that they retained some CsDGAT1 function, and their seeds showed decreased eicosaenoic acid (C20:1) contents and increased C18:3 contents compared to the wild type (WT). The EDD2 lines had a complete knockout of all CsDGAT2 homoeologs and a slightly decreased C18:3 content compared to the WT. In conclusion, CsDGAT1 and CsDGAT2 have a small influence on the seed oil content and have an acyl preference for C20:1 and C18:3, respectively. This finding can be applied to develop oilseed plants containing high omega-3 fatty acids or high oleic acid.

Overexpression of the Phosphatidylcholine:DiacylglycerolCholinephosphotransferase (PDCT) gene increases carbon flux toward triacylglycerol (TAG) synthesis in Camelinasativa seeds

Camelinasativa has considerable promise as a dedicated industrial oilseed crop. Its oil-based blends have been tested and approved as liquid transportation fuels. Previously, we utilized metabolomic and transcriptomic profiling approaches and identified metabolic bottlenecks that control oil production and accumulation in seeds. Accordingly, we selected candidate genes for the metabolic engineering of Camelina. Here we targeted the overexpression of Camelina PDCT gene, which encodes the phosphatidylcholine: diacylglycerol cholinephosphotransferase enzyme. PDCT is proposed as a gatekeeper responsible for the interconversions of diacylglycerol (DAG) and phosphatidylcholine (PC) pools and has the potential to increase the levels of TAG in seeds. To confirm whether increased CsPDCT activity in developing Camelina seeds would enhance carbon flux toward increased levels of TAG and alter oil composition, we overexpressed the CsPDCT gene under the control of the seed-specific phaseolin promoter. Camelina transgenics exhibited significant increases in seed yield (19-56%), seed oil content (9-13%), oil yields per plant (32-76%), and altered polyunsaturated fatty acid (PUFA) content compared to their parental wild-type (WT) plants. Results from [14C] acetate labeling of Camelina developing embryos expressing CsPDCT in culture indicated increased rates of radiolabeled fatty acid incorporation into glycerolipids (up to 64%, 59%, and 43% higher in TAG, DAG, and PC, respectively), relative to WT embryos. We conclude that overexpression of PDCT appears to be a positive strategy to achieve a synergistic effect on the flux through the TAG synthesis pathway, thereby further increasing oil yields in Camelina.

PDAT1 genome editing reduces hydroxy fatty acid production in transgenic Arabidopsis

The fatty acids content of castor (Ricinus communis L.) seed oil is 80-90% ricinoleic acid, which is a hydroxy fatty acid (HFA). The structures and functional groups of HFAs are different from those of common fatty acids and are useful for various industrial applications. However, castor seeds contain the toxin ricin and an allergenic protein, which limit their cultivation. Accordingly, many researchers are conducting studies to enhance the production of HFAs in Arabidopsis thaliana, a model plant for oil crops. Oleate 12-hydroxylase from castor (RcFAH12), which synthesizes HFA (18:1-OH), was transformed into an Arabidopsis fae1 mutant, resulting in the CL37 line producing a maximum of 17% HFA content. In addition, castor phospholipid:diacylglycerol acyltransferase 1-2 (RcPDAT1-2), which catalyzes the production of triacylglycerol by transferring HFA from phosphatidylcholine to diacylglycerol, was transformed into the CL37 line to develop a P327 line that produces 25% HFA. In this study, we investigated changes in HFA content when endogenous Arabidopsis PDAT1 (AtPDAT1) of the P327 line was edited using the CRISPR/Cas9 technique. The successful mutation resulted in three independent lines with different mutation patterns, which were transmitted until the T4 generation. Fatty acid analysis of the seeds showed that HFA content decreased in all three mutant lines. These findings indicate that AtPDAT1 as well as RcPDAT1-2 in the P327 line are involved in transferring and increasing HFAs to triacylglycerol. [BMB Reports 2024; 57(2): 86-91].

Intron-mediated enhancement of DIACYLGLYCEROL ACYLTRANSFERASE1 expression in energycane promotes a step change for lipid accumulation in vegetative tissues

BACKGROUND

Metabolic engineering for hyperaccumulation of lipids in vegetative tissues is a novel strategy for enhancing energy density and biofuel production from biomass crops. Energycane is a prime feedstock for this approach due to its high biomass production and resilience under marginal conditions. DIACYLGLYCEROL ACYLTRANSFERASE (DGAT) catalyzes the last and only committed step in the biosynthesis of triacylglycerol (TAG) and can be a rate-limiting enzyme for the production of TAG.

RESULTS

In this study, we explored the effect of intron-mediated enhancement (IME) on the expression of DGAT1 and resulting accumulation of TAG and total fatty acid (TFA) in leaf and stem tissues of energycane. To maximize lipid accumulation these evaluations were carried out by co-expressing the lipogenic transcription factor WRINKLED1 (WRI1) and the TAG protect factor oleosin (OLE1). Including an intron in the codon-optimized TmDGAT1 elevated the accumulation of its transcript in leaves by seven times on average based on 5 transgenic lines for each construct. Plants with WRI1 (W), DGAT1 with intron (Di), and OLE1 (O) expression (WDiO) accumulated TAG up to a 3.85% of leaf dry weight (DW), a 192-fold increase compared to non-modified energycane (WT) and a 3.8-fold increase compared to the highest accumulation under the intron-less gene combination (WDO). This corresponded to TFA accumulation of up to 8.4% of leaf dry weight, a 2.8-fold or 6.1-fold increase compared to WDO or WT, respectively. Co-expression of WDiO resulted in stem accumulations of TAG up to 1.14% of DW or TFA up to 2.08% of DW that exceeded WT by 57-fold or 12-fold and WDO more than twofold, respectively. Constitutive expression of these lipogenic "push pull and protect" factors correlated with biomass reduction.

CONCLUSIONS

Intron-mediated enhancement (IME) of the expression of DGAT resulted in a step change in lipid accumulation of energycane and confirmed that under our experimental conditions it is rate limiting for lipid accumulation. IME should be applied to other lipogenic factors and metabolic engineering strategies. The findings from this study may be valuable in developing a high biomass feedstock for commercial production of lipids and advanced biofuels.

Diacylglycerol acyltransferase (DGAT) in Crangon crangon and Pandalus montagui (Decapoda, Caridea) - Implications for lipid storage capacities and life history traits

Lipids play essential roles in cell-structuring, cell-signaling, and as efficient metabolic energy stores. Lipid storage capacities determine life history traits of organisms and, thus, their ecological function. Among storage lipids, triacylglycerols (TAGs) are widespread in marine invertebrates. However, abilities to accumulate TAGs can vary even between closely related species, such as the caridean shrimps Crangon crangon and Pandalus montagui. The first species shows low TAG levels throughout the year in the main storage organ, the midgut gland, while the latter accumulates high TAG-levels, peaking in summer. TAGs synthesis is facilitated by the terminal step of the Kennedy-pathway, where the enzyme diacylglycerol-acyltransferase (DGAT) catalyzes the esterification of diacylglycerols with activated fatty acids. We investigated DGAT activity in the midgut gland using a fluorescent enzyme assay. Sequence information was extracted from whole transcriptome shotgun assembly data, that is publicly available on NCBI, and catalytic properties were deduced from molecular structure analysis. C. crangon showed significantly lower TAG synthesis rates than P. montagui, which explains the native TAG levels. Transcriptome data yielded several isoforms of DGAT enzymes in both species. C. crangon DGAT showed point mutations, which are capable of obstructing the catalytic capacity. The consequences are limited starvation resistance and, thus, presumably restricting C. crangon to a habitat with year-round sufficient food. In contrast, higher TAG synthesis rates presumably enable P. montagui to extend into northern subarctic habitats with limited food availability in winter. Moreover, the limited TAG synthesis and accumulation in the midgut gland may force C. crangon to direct energy into the ovaries, which results in multiple spawnings.

Tung tree stearoyl-acyl carrier protein Δ9 desaturase improves oil content and cold resistance of Arabidopsis and Saccharomyces cerevisiae

The seed oil of tung tree is rich in a-eleostearic acid (ESA), which endows tung oil with the characteristic of an excellently dry oil. The stearoyl-acyl carrier protein δ9 desaturase (SAD) is a rate-limiting enzyme that converts the stearic acid to the oleic acid, the substrate for the production of the α-ESA. However, the function of the two predicted VfSAD1 and VfSAD2 genes in the tung tree has not been determined. In this study, quantitative real-time PCR (qRT-PCR) analysis showed that VfSAD1 and VfSAD2 were expressed in multiple organs of tung tree but were highly expressed in the seed during the oil rapid accumulation period. Heterologous expression of VfSAD1 and VfSAD2 could promote the production of oleic acid and its derivatives in Arabidopsis thaliana and yeast BY4741, indicating that VfSAD1 and VfSAD2 possess the stearoyl-ACP desaturases function. Furthermore, both VfSAD1 and VfSAD2 could significantly improve seed oil accumulation in Arabidopsis. VfSAD1 could also significantly promote the oil accumulation in the yeast BY4741 strain. In addition, overexpression of VfSAD1 and VfSAD2 enhanced the tolerance of yeast and Arabidopsis seedlings to low temperature stress. This study indicates that the two VfSAD genes play a vital role in the process of oil accumulation and fatty acid biosynthesis in the tung tree seed, and both of them could be used for molecular breeding in tung tree and other oil crops.

Assessing the biotechnological potential of cotton type-1 and type-2 diacylglycerol acyltransferases in transgenic systems

The chemical and physical properties of vegetable oils are largely dictated by the ratios of 4-6 common fatty acids contained within each oil. However, examples of plant species that accumulate from trace amounts to >90% of certain unusual fatty acids in seed triacylglycerols have been reported. Many of the general enzymatic reactions that drive both common and unusual fatty acid biosynthesis and accumulation in stored lipids are known, but which isozymes have evolved to specifically fill this role and how they coordinate in vivo is still poorly understood. Cotton (Gossypium sp.) is the very rare example of a commodity oilseed that produces biologically relevant amounts of unusual fatty acids in its seeds and other organs. In this case, unusual cyclopropyl fatty acids (named after the cyclopropane and cyclopropene moieties within the fatty acids) are found in membrane and storage glycerolipids (e.g. seed oils). Such fatty acids are useful in the synthesis of lubricants, coatings, and other types of valuable industrial feedstocks. To characterize the role of cotton acyltransferases in cyclopropyl fatty acid accumulation for bioengineering applications, we cloned and characterized type-1 and type-2 diacylglycerol acyltransferases from cotton and compared their biochemical properties to that of litchi (Litchi chinensis), another cyclopropyl fatty acid-producing plant. The results presented from transgenic microbes and plants indicate both cotton DGAT1 and DGAT2 isozymes efficiently utilize cyclopropyl fatty acid-containing substrates, which helps to alleviate biosynthetic bottlenecks and enhances total cyclopropyl fatty acid accumulation in the seed oil.

Acyl-CoA-dependent and acyl-CoA-independent avocado acyltransferases positively influence oleic acid content in nonseed triacylglycerols

In higher plants, acyl-CoA:diacylglycerol acyltransferase (DGAT) and phospholipid:diacylglycerol acyltransferase (PDAT) catalyze the terminal step of triacylglycerol (TAG) synthesis in acyl-CoA-dependent and -independent pathways, respectively. Avocado (Persea americana) mesocarp, a nonseed tissue, accumulates significant amounts of TAG (~70% by dry weight) that is rich in heart-healthy oleic acid (18:1). The oil accumulation stages of avocado mesocarp development coincide with high expression levels for type-1 DGAT (DGAT1) and PDAT1, although type-2 DGAT (DGAT2) expression remains low. The strong preference for oleic acid demonstrated by the avocado mesocarp TAG biosynthetic machinery represents lucrative biotechnological opportunities, yet functional characterization of these three acyltransferases has not been explored to date. We expressed avocado PaDGAT1, PaDGAT2, and PaPDAT1 in bakers' yeast and leaves of Nicotiana benthamiana. PaDGAT1 complemented the TAG biosynthesis deficiency in the quadruple mutant yeast strain H1246, and substantially elevated total cellular lipid content. In vitro enzyme assays showed that PaDGAT1 prefers oleic acid compared to palmitic acid (16:0). Both PaDGAT1 and PaPDAT1 increased the lipid content and elevated oleic acid levels when expressed independently or together, transiently in N. benthamiana leaves. These results indicate that PaDGAT1 and PaPDAT1 prefer oleate-containing substrates, and their coordinated expression likely contributes to sustained TAG synthesis that is enriched in oleic acid. This study establishes a knowledge base for future metabolic engineering studies focused on exploitation of the biochemical properties of PaDGAT1 and PaPDAT1.

Genome-wide identification and expression analysis of diacylglycerol acyltransferase genes in soybean (Glycine max)

Background

Soybean (Glycine max) is a major protein and vegetable oil source. In plants, diacylglycerol acyltransferase (DGAT) can exert strong flux control, which is rate-limiting for triacylglycerol biosynthesis in seed oil formation.

Methods

Here, we identified soybean DGAT genes via a bioinformatics method, thereby laying a solid foundation for further research on their function. Based on our bioinformatics analyses, including gene structure, protein domain characteristics, and phylogenetic analysis, 26 DGAT putative gene family members unevenly distributed on 12 of the 20 soybean chromosomes were identified and divided into the following four groups: DGAT1, DGAT2, WS/DGAT, and cytoplasmic DGAT.

Results

The Ka/Ks ratio of most of these genes indicated a significant positive selection pressure. DGAT genes exhibited characteristic expression patterns in soybean tissues. The differences in the structure and expression of soybean DGAT genes revealed the diversity of their functions and the complexity of soybean fatty acid metabolism. Our findings provide important information for research on the fatty acid metabolism pathway in soybean. Furthermore, our results will help identify candidate genes for potential fatty acid-profile modifications to improve soybean seed oil content.

Conclusions

This is the first time that in silico studies have been used to report the genomic and proteomic characteristics of DGAT in soybean and the effect of its specific expression on organs, age, and stages.

Diacylglycerol Acyltransferase 3(DGAT3) Is Responsible for the Biosynthesis of Unsaturated Fatty Acids in Vegetative Organs of Paeonia rockii

'Diacylglycerol acyltransferase (DGAT)' acts as a key rate-limiting enzyme that catalyzes the final step of the de novo biosynthesis of triacylglycerol (TAG). The study was to characterize the function of the DGAT3 gene in Paeonia rockii, which is known for its accumulation of high levels of unsaturated fatty acids (UFAs). We identified a DGAT3 gene which encodes a soluble protein that is located within the chloroplasts of P. rockii. Functional complementarity experiments in yeast demonstrated that PrDGAT3 restored TAG synthesis. Linoleic acid (LA, C18:2) and α-linolenic acid (ALA, C18:3) are essential unsaturated fatty acids that cannot be synthesized by the human body. Through the yeast lipotoxicity test, we found that the yeast cell density was largely increased by adding exogenous LA and, especially, ALA to the yeast medium. Further ectopic transient overexpression in Nicotiana benthamiana leaf tissue and stable overexpression in Arabidopsis thaliana indicated that PrDGAT3 significantly enhanced the accumulation of the TAG and UFAs. In contrast, we observed a significant decrease in the total fatty acid content and in several major fatty acids in PrDGAT3-silenced tree peony leaves. Overall, PrDGAT3 is important in catalyzing TAG synthesis, with a substrate preference for UFAs, especially LA and ALA. These results suggest that PrDGAT3 may have practical applications in improving plant lipid nutrition and increasing oil production in plants.

Lipidome analysis and characterization of Buglossoides arvensis acyltransferases that incorporate polyunsaturated fatty acids into triacylglycerols

Buglossoides arvensis is a burgeoning oilseed crop that contains an unique combination of ω-3 and ω-6 polyunsaturated fatty acids (PUFA), constituting ~80-85% of seed triacylglycerols (TAGs). To uncover the critical TAG biosynthetic pathways contributing for high PUFA accumulation, we performed lipidome of developing seeds and characterized acyltransferases involved in the final step of TAG biosynthesis. During seed development, distribution of lipid molecular species in individual lipid classes showed distinct patterns from an early-stage (6 days after flowering (DAF)) to the middle-stage (12 and 18 DAF) of oil biosynthesis. PUFA-containing TAG species drastically increased from 6 to 12 DAF. The expression profiles of key triacylglycerol biosynthesis genes and patterns of phosphatidylcholine, diacylglycerol and triacylglycerol molecular species during seed development were used to predict the contribution of diacylglycerol acyltransferases (DGAT1 and DGAT2) and phospholipid: diacylglycerol acyltransferases (PDAT1 and PDAT2) to PUFA-rich TAG biosynthesis. Our analysis suggests that DGATs play a crucial role in enriching TAGs with PUFA compared to PDATs. This was further confirmed by fatty acid feeding studies in yeast expressing acyltransferases. BaDGAT2 preferentially incorporated high amounts of PUFAs into TAG, compared to BaDGAT1. Our results provide insight into the molecular mechanisms of TAG accumulation in this plant and identify target genes for transgenic production of SDA in traditional oilseed crops.

Overexpression of WRINKLED1 improves the weight and oil content in seeds of flax (Linum usitatissimum L.)

Seeds of flax (Linum usitatissimum L.) are highly rich in both oil and linolenic acid (LIN). It is crucial for flax agricultural production to identify positive regulators of fatty acid biosynthesis. In this study, we find that WRINKLED1 transcription factors play important positive roles during flax seed oil accumulation. Two WRINKLED1 genes, LuWRI1a and LuWRI1b, were cloned from flax, and LuWRI1a was found be expressed predominantly in developing seeds during maturation. Overexpression of LuWRI1a increased seed size, weight, and oil content in Arabidopsis and increased seed storage oil content in transgenic flax without affecting seed production or seed oil quality. The rise in oil content in transgenic flax seeds was primarily attributable to the increase in seed weight, according to a correlational analysis. Furthermore, overexpression or interference of LuWRI1a upregulated the expression of genes in the fatty acid biosynthesis pathway and LAFL genes, and the expression level of WRI1 was highly significantly positively associated between L1L, LEC1, and BCCP2. Our findings give a theoretical scientific foundation for the future application of genetic engineering to enhance the oil content of plant seeds.

Full-length transcriptome revealed the accumulation of polyunsaturated fatty acids in developing seeds of Plukenetia volubilis

Background

Plukenetia volubilis is cultivated as a valuable oilseed crop, and its mature seeds are rich in polyunsaturated fatty acids (FAs), which are widely used in food and pharmaceutical industries. Recently, next-generation sequencing (NGS) transcriptome studies in P. volubilis indicated that some candidate genes were involved in oil biosynthesis. The NGS were inaccuracies in assembly of some candidate genes, leading to unknown errors in date analyses. However, single molecular real-time (SMRT) sequencing can overcome these assembled errors. Unfortunately, this technique has not been reported in P. volubilis.

Methods

The total oil content of P. volubilis seed (PVS) was determined using Soxhlet extraction system. The FA composition were analyzed by gas chromatography. Combining PacBio SMRT and Illumina technologies, the transcriptome analysis of developing PVS was performed. Functional annotation and differential expression were performed by BLAST software (version 2.2.26) and RSEM software (version 1.2.31), respectively. The lncRNA-targeted transcripts were predicted in developing PVS using LncTar tool.

Results

By Soxhlet extraction system, the oil content of superior plant-type (SPT) was 13.47% higher than that of inferior plant-type (IPT) at mature PVS. The most abundant FAs were C18:2 and C18:3, among which C18:3 content of SPT was 1.11-fold higher than that of IPT. Combined with PacBio and Illumina platform, 68,971 non-redundant genes were obtained, among which 7,823 long non-coding RNAs (lncRNAs) and 7,798 lncRNA-targeted genes were predicted. In developing seed, the expressions of 57 TFs showed a significantly positive correlation with oil contents, including WRI1-like1, LEC1-like1, and MYB44-like. Comparative analysis of expression profiles between SPT and IPT implied that orthologs of FAD3, PDCT, PDAT, and DAGT2 were possibly important for the accumulation of polyunsaturated FAs. Together, these results provide a reference for oil biosynthesis of P. volubilis and genetic improvement of oil plants.

An alternative angiosperm DGAT1 topology and potential motifs in the N-terminus

The highly variable cytoplasmic N-terminus of the plant diacylglycerol acyltransferase 1 (DGAT1) has been shown to have roles in oligomerization as well as allostery; however, the biological significance of the variation within this region is not understood. Comparing the coding sequences over the variable N-termini revealed the Poaceae DGAT1s contain relatively high GC compositional gradients as well as numerous direct and inverted repeats in this region. Using a variety of reciprocal chimeric DGAT1s from angiosperms we show that related N-termini had similar effects (positive or negative) on the accumulation of the recombinant protein in Saccharomyces cerevisiae. When expressed in Camelina sativa seeds the recombinant proteins of specific chimeras elevated total lipid content of the seeds as well as increased seed size. In addition, we combine N- and C-terminal as well as internal tags with high pH membrane reformation, protease protection and differential permeabilization. This led us to conclude the C-terminus is in the ER lumen; this contradicts earlier reports of the cytoplasmic location of plant DGAT1 C-termini.

Transcriptional regulation of oil biosynthesis in seed plants: Current understanding, applications, and perspectives

Plants produce and accumulate triacylglycerol (TAG) in their seeds as an energy reservoir to support the processes of seed germination and seedling development. Plant seed oils are vital not only for the human diet but also as renewable feedstocks for industrial use. TAG biosynthesis consists of two major steps: de novo fatty acid biosynthesis in the plastids and TAG assembly in the endoplasmic reticulum. The latest advances in unraveling transcriptional regulation have shed light on the molecular mechanisms of plant oil biosynthesis. We summarize recent progress in understanding the regulatory mechanisms of well-characterized and newly discovered transcription factors and other types of regulators that control plant fatty acid biosynthesis. The emerging picture shows that plant oil biosynthesis responds to developmental and environmental cues that stimulate a network of interacting transcriptional activators and repressors, which in turn fine-tune the spatiotemporal regulation of the pathway genes.

Applications and prospects of genome editing in plant fatty acid and triacylglycerol biosynthesis

Triacylglycerol (TAG), which is a neutral lipid, has a structure in which three molecules of fatty acid (FA) are ester-bonded to one molecule of glycerol. TAG is important energy source for seed germination and seedling development in plants. Depending on the FA composition of the TAG, it is used as an edible oil or industrial material for cosmetics, soap, and lubricant. As the demand for plant oil is rising worldwide, either the type of FA must be changed or the total oil content of various plants must be increased. In this review, we discuss the regulation of FA metabolism by Clustered regularly interspaced short palindromic repeats (CRISPR)/Cas9, a recent genome-editing technology applicable to various plants. The development of plants with higher levels of oleic acid or lower levels of very long-chain fatty acids (VLCFAs) in seeds are discussed. In addition, the current status of research on acyltransferases, phospholipases, TAG lipases, and TAG synthesis in vegetative tissues is described. Finally, strategies for the application of CRISPR/Cas9 in lipid metabolism studies are mentioned.

Acyl-CoA:diacylglycerol acyltransferase: Properties, physiological roles, metabolic engineering and intentional control

Acyl-CoA:diacylglycerol acyltransferase (DGAT, EC 2.3.1.20) catalyzes the last reaction in the acyl-CoA-dependent biosynthesis of triacylglycerol (TAG). DGAT activity resides mainly in membrane-bound DGAT1 and DGAT2 in eukaryotes and bifunctional wax ester synthase-diacylglycerol acyltransferase (WSD) in bacteria, which are all membrane-bound proteins but exhibit no sequence homology to each other. Recent studies also identified other DGAT enzymes such as the soluble DGAT3 and diacylglycerol acetyltransferase (EaDAcT), as well as enzymes with DGAT activities including defective in cuticular ridges (DCR) and steryl and phytyl ester synthases (PESs). This review comprehensively discusses research advances on DGATs in prokaryotes and eukaryotes with a focus on their biochemical properties, physiological roles, and biotechnological and therapeutic applications. The review begins with a discussion of DGAT assay methods, followed by a systematic discussion of TAG biosynthesis and the properties and physiological role of DGATs. Thereafter, the review discusses the three-dimensional structure and insights into mechanism of action of human DGAT1, and the modeled DGAT1 from Brassica napus. The review then examines metabolic engineering strategies involving manipulation of DGAT, followed by a discussion of its therapeutic applications. DGAT in relation to improvement of livestock traits is also discussed along with DGATs in various other eukaryotic organisms.

Characterization of sucrose binding protein as a seed-specific promoter in transgenic tobacco Nicotiana tabacum L

Seed-specific expression using appropriate promoters is a recommended strategy for the efficiently producing valuable metabolites in transgenic plants. In the present study, we investigated the sequence of sucrose binding protein (SBP) as a seed-specific promoter to find the cis-acting elements specific to gene expression in seeds. The 1860 bp SBP sequence was analyzed using Plant Care and PLACE databases to find cis-acting elements, which resulted in a finding of 22 cis-acting elements required for seed expression. In addition, we have discovered cis- acting elements that are indirectly involved in triacylglycerol synthesis (GATABOX, DOFCOREZM, CACGTGMOTIF). The seed specificity of SBP was analyzed by generating a stable transgenic tobacco plant harboring β-glucuronidase (GUS) reporter gene under the control of the SBP promoter. Histochemical analysis of these transgenic tobacco plants indicated decreasing GUS activity in the leaves during the vegetative stage. However, the mature seeds of transgenic plants showed GUS activity. Moreover, the SBP promoter function in the seed oil content was evaluated by the expression of DGAT1. The expression analysis of DGAT1 in SBP-DGAT1 transgenic tobacco seeds using quantitative real-time PCR revealed a 7.8-fold increase in DGAT1 than in non-transgenic plants. Moreover, oil content increased up to 2.19 times more than in non-transgenic plants. And the oil content of the SBP-DGAT1 transgenic tobacco leaves did not change compared to the control plant. Therefore, we suggested that the SBP promoter could be used as a seed-specific promoter for targeted expression of desired genes in the metabolite engineering of oilseed crops.

Different acyl-CoA:diacylglycerol acyltransferases vary widely in function, and a targeted amino acid substitution enhances oil accumulation

Triacylglycerols (TAGs) are the major component of plant storage lipids such as oils. Acyl-CoA:diacylglycerol acyltransferase (DGAT) catalyzes the final step of the Kennedy pathway, and is mainly responsible for plant oil accumulation. We previously found that the activity of Vernonia DGAT1 was distinctively higher than that of Arabidopsis and soybean DGAT1 in a yeast microsome assay. In this study, the DGAT1 cDNAs of Arabidopsis, Vernonia, soybean, and castor bean were introduced into Arabidopsis. All Vernonia DGAT1-expressing lines showed a significantly higher oil content (49% mean increase compared with the wild-type) followed by soybean and castor bean. Most Arabidopsis DGAT1-overexpressing lines did not show a significant increase. In addition to these four DGAT1 genes, sunflower, Jatropha, and sesame DGAT1 genes were introduced into a TAG biosynthesis-defective yeast mutant. In the yeast expression culture, DGAT1s from Arabidopsis, castor bean, and soybean only slightly increased the TAG content; however, DGAT1s from Vernonia, sunflower, Jatropha, and sesame increased TAG content >10-fold more than the former three DGAT1s. Three amino acid residues were characteristically common in the latter four DGAT1s. Using soybean DGAT1, these amino acid substitutions were created by site-directed mutagenesis and substantially increased the TAG content.

Transcriptional regulation of triacylglycerol accumulation in plants under environmental stress conditions

Triacylglycerol (TAG), a major energy reserve in lipid form, accumulates mainly in seeds. Although TAG concentrations are usually low in vegetative tissues because of the repression of seed maturation programs, these programs are derepressed upon the exposure of vegetative tissues to environmental stresses. Metabolic reprogramming of TAG accumulation is driven primarily by transcriptional regulation. A substantial proportion of transcription factors regulating seed TAG biosynthesis also participates in stress-induced TAG accumulation in vegetative tissues. TAG accumulation leads to the formation of lipid droplets and plastoglobules, which play important roles in plant tolerance to environmental stresses. Toxic lipid intermediates generated from environmental-stress-induced lipid membrane degradation are captured by TAG-containing lipid droplets and plastoglobules. This review summarizes recent advances in the transcriptional control of metabolic reprogramming underlying stress-induced TAG accumulation, and provides biological insight into the plant adaptive strategy, linking TAG biosynthesis with plant survival.

Transcriptome expression profiles reveal response mechanisms to drought and drought-stress mitigation mechanisms by exogenous glycine betaine in maize

Drought stress is one of the major abiotic stresses that limit growth, development and yield of maize crops. To better understand the responses of maize inbred lines with different levels of drought resistance and the molecular mechanism of exogenous glycine betaine (GB) in alleviating drought stress, the responses of two maize inbred lines to drought stress and to the stress-mitigating effects of exogenous GB were investigated. Seedling morphology, physiological and biochemical indexes, root cell morphology and root transcriptome expression profiles were compared between a drought-tolerant inbred line Chang 7-2 and drought-sensitive inbred line TS141. Plants of both lines were subjected to treatments of drought stress alone and drought stress with application of exogenous GB. The results showed that with the increase of drought treatment time, the growth and development of TS141 were inhibited, while those of Chang 7-2 were not significantly different from those of the control (no drought stress and GB). Compared with the corresponding data of the drought-stress group, every index measured from the two inbred lines indicated mitigating effects from exogenous GB, and TS141 produced stronger mitigating responses due to the GB. Transcriptome analysis showed that 562 differentially expressed genes (DEGs) were up-regulated and 824 DEGs were down-regulated in both inbred lines under drought stress. Due to the exogenous GB, 1061 DEGs were up-regulated and 424 DEGs were down-regulated in both lines. In addition, quantitative real-time polymerase chain reaction (qRT-PCR) was used to verify 10 DEGs screened from the different treatments. These results showed that the expression of 9 DEGs were basically consistent with their respective transcriptome expression profiles. The results of this study provide models of potential mechanisms of drought tolerance in maize as well as potential mechanisms of how exogenous GB may regulate drought tolerance.

Increase in alpha-linolenic acid content by simultaneous expression of fatty acid metabolism genes in Sesame (Sesamum indicum L.)

Sesame is considered one of India's important sources of edible oil and an excellent dietary source for its nutritional and medicinal value. Sesame DGAT1 and PDAT1 genes were co-expressed with omega 3 FAD genes. Systemic isolation of sesame DGAT1, PDAT1, ER type FAD3, and chloroplast type FAD7/8 genes were performed. Their sequence was analyzed for genomic organization, amino acid characterization, organ specificity, and phylogenetic relationships. The insilico analysis revealed the unique features of DGAT1, PDAT1, and FAD3 gene sequences, whereas FAD7 and FAD8 sequences had the same protein characters and were grouped in phylogeny analysis, only variation was found in their mRNA UTR regions. Functional expression of sesame TAG synthesis genes and omega-3 FAD genes was studied in yeast mutant H1246 deficient for TAG synthesis. Functional analyses in yeast with the presence of ALA confirmed the identity of sesame FAD3, FAD7 and FAD8 genes. Recombinant expression of pESC + DGAT1 + FAD3 vector in yeast mutant resulted in lipid accumulation with 10% higher ALA content. Thus this gene combination can be co-expressed in sesame and other plant systems to increase the lipid accumulation with high omega-3 fatty acid (ALA) content.

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The online version contains supplementary material available at 10.1007/s12298-022-01152-0.

Metabolic flux analysis of the non-transitory starch tradeoff for lipid production in mature tobacco leaves

The metabolic plasticity of tobacco leaves has been demonstrated via the generation of transgenic plants that can accumulate over 30% dry weight as triacylglycerols. In investigating the changes in carbon partitioning in these high lipid-producing (HLP) leaves, foliar lipids accumulated stepwise over development. Interestingly, non-transient starch was observed to accumulate with plant age in WT but not HLP leaves, with a drop in foliar starch concurrent with an increase in lipid content. The metabolic carbon tradeoff between starch and lipid was studied using 13CO2-labeling experiments and isotopically nonstationary metabolic flux analysis, not previously applied to the mature leaves of a crop. Fatty acid synthesis was investigated through assessment of acyl-acyl carrier proteins using a recently derived quantification method that was extended to accommodate isotopic labeling. Analysis of labeling patterns and flux modeling indicated the continued production of unlabeled starch, sucrose cycling, and a significant contribution of NADP-malic enzyme to plastidic pyruvate production for the production of lipids in HLP leaves, with the latter verified by enzyme activity assays. The results suggest an inherent capacity for a developmentally regulated carbon sink in tobacco leaves and may in part explain the uniquely successful leaf lipid engineering efforts in this crop.

Evolution and Characterization of Acetyl Coenzyme A: Diacylglycerol Acyltransferase Genes in Cotton Identify the Roles of GhDGAT3D in Oil Biosynthesis and Fatty Acid Composition

Cottonseed oil is rich in unsaturated fatty acids (UFAs) and serves as an edible oil in human nutrition. Reports suggest that acyl-coenzyme A: diacylglycerol acyltransferases (DGAT) and wax ester synthase/DGAT (WSD1) genes encode a key group of enzymes that catalyze the final step for triacylglycerol biosynthesis and enable an important rate-limiting process. However, their roles in oil biosynthesis and the fatty acid profile of cotton seed are poorly understood. Therefore, the aim of this study was to identify and characterize DGAT and WSD1 genes in cotton plants and examine their roles in oil biosynthesis, the fatty acid profile of cotton seeds, and abiotic stress responses. In this study, 36 GhDGAT and GhWSD1 genes were identified in upland cotton (G. hirsutum) and found to be clustered into four groups: GhDGAT1, GhDGAT2, GhDGAT3, and GhWSD1. Gene structure and domain analyses showed that the GhDGAT and GhWSD1 genes in each group are highly conserved. Gene synteny analysis indicated that segmental and tandem duplication events occurred frequently during cotton evolution. Expression analysis revealed that GhDGAT and GhWSD1 genes function widely in cotton development and stress responses; moreover, several environmental stress and hormone response-related cis-elements were detected in the GhDGAT and GhWSD1 promoter regions. The predicted target transcription factors and miRNAs imply an extensive role of GhDGAT and GhWSD1 genes in stress responses. Increases in GhDGAT3 gene expression with increases in cottonseed oil accumulation were observed. Transformation study results showed that there was an increase in C18:1 content and a decrease in C18:2 and C18:3 contents in seeds of Arabidopsis transgenic plants overexpressing GhDGAT3D compared with that of control plants. Overall, these findings contributed to the understanding of the functions of GhDGAT and GhWSD1 genes in upland cotton, providing basic information for further research.

Mechanisms and functions of membrane lipid remodeling in plants

Lipid remodeling, defined herein as post-synthetic structural modifications of membrane lipids, play crucial roles in regulating the physicochemical properties of cellular membranes and hence their many functions. Processes affected by lipid remodeling include lipid metabolism, membrane repair, cellular homeostasis, fatty acid trafficking, cellular signaling and stress tolerance. Glycerolipids are the major structural components of cellular membranes and their composition can be adjusted by modifying their head groups, their acyl chain lengths and the number and position of double bonds. This review summarizes recent advances in our understanding of mechanisms of membrane lipid remodeling with emphasis on the lipases and acyltransferases involved in the modification of phosphatidylcholine and monogalactosyldiacylglycerol, the major membrane lipids of extraplastidic and photosynthetic membranes, respectively. We also discuss the role of triacylglycerol metabolism in membrane acyl chain remodeling. Finally, we discuss emerging data concerning the functional roles of glycerolipid remodeling in plant stress responses. Illustrating the molecular basis of lipid remodeling may lead to novel strategies for crop improvement and other biotechnological applications such as bioenergy production.

Production of the infant formula ingredient 1,3-olein-2-palmitin in Arabidopsis thaliana seeds

In human milk fat, palmitic acid (16:0) is esterified to the middle (sn-2 or β) position on the glycerol backbone and oleic acid (18:1) predominantly to the outer positions, giving the triacylglycerol (TG) a distinctive stereoisomeric structure that is believed to assist nutrient absorption in the infant gut. However, the fat used in most infant formulas is derived from plants, which preferentially esterify 16:0 to the outer positions. We have previously showed that the metabolism of the model oilseed Arabidopsis thaliana can be engineered to incorporate 16:0 into the middle position of TG. However, the fatty acyl composition of Arabidopsis seed TG does not mimic human milk, which is rich in both 16:0 and 18:1 and is defined by the high abundance of the TG molecular species 1,3-olein-2-palmitin (OPO). Here we have constructed an Arabidopsis fatty acid biosynthesis 1-1 fatty acid desaturase 2 fatty acid elongase 1 mutant with around 20% 16:0 and 70% 18:1 in its seeds and we have engineered it to esterify more than 80% of the 16:0 to the middle position of TG, using heterologous expression of the human lysophosphatidic acid acyltransferase isoform AGPAT1, combined with suppression of LYSOPHOSPHATIDIC ACID ACYLTRANSFERASE 2 and PHOSPHATIDYLCHOLINE:DIACYLGLYCEROL CHOLINEPHOSPHOTRANSFERASE. Our data show that oilseeds can be engineered to produce TG that is rich in OPO, which is a structured fat ingredient used in infant formulas.

The Role of Sugar Signaling in Regulating Plant Fatty Acid Synthesis

Photosynthates such as glucose, sucrose, and some of their derivatives play dual roles as metabolic intermediates and signaling molecules that influence plant cell metabolism. Such sugars provide substrates for de novo fatty acid (FA) biosynthesis. However, compared with the well-defined examples of sugar signaling in starch and anthocyanin synthesis, until recently relatively little was known about the role of signaling in regulating FA and lipid biosynthesis. Recent research progress shows that trehalose 6-phosphate and 2-oxoglutarate (2-OG) play direct signaling roles in the regulation of FA biosynthesis by modulating transcription factor stability and enzymatic activities involved in FA biosynthesis. Specifically, mechanistic links between sucrose non-fermenting-1-related protein kinase 1 (SnRK1)-mediated trehalose 6-phosphate (T6P) sensing and its regulation by phosphorylation of WRI1 stability, diacylglycerol acyltransferase 1 (DGAT1) enzyme activity, and of 2-OG-mediated relief of inhibition of acetyl-CoA carboxylase (ACCase) activity by protein PII are exemplified in detail in this review.

Ectopic Expression of OLEOSIN 1 and Inactivation of GBSS1 Have a Synergistic Effect on Oil Accumulation in Plant Leaves

During the transformation of wild-type (WT) Arabidopsis thaliana, a T-DNA containing OLEOSIN-GFP (OLE1-GFP) was inserted by happenstance within the GBSS1 gene, resulting in significant reduction in amylose and increase in leaf oil content in the transgenic line (OG). The synergistic effect on oil accumulation of combining gbss1 with the expression of OLE1-GFP was confirmed by transforming an independent gbss1 mutant (GABI_914G01) with OLE1-GFP. The resulting OLE1-GFP/gbss1 transgenic lines showed higher leaf oil content than the individual OLE1-GFP/WT or single gbss1 mutant lines. Further stacking of the lipogenic factors WRINKLED1, Diacylglycerol O-Acyltransferase (DGAT1), and Cys-OLEOSIN1 (an engineered sesame OLEOSIN1) in OG significantly elevated its oil content in mature leaves to 2.3% of dry weight, which is 15 times higher than that in WT Arabidopsis. Inducible expression of the same lipogenic factors was shown to be an effective strategy for triacylglycerol (TAG) accumulation without incurring growth, development, and yield penalties.

Analyzing Mass Spectrometry Imaging Data of 13C-Labeled Phospholipids in Camelina sativa and Thlaspi arvense (Pennycress) Embryos

The combination of ¹³C-isotopic labeling and mass spectrometry imaging (MSI) offers an approach to analyze metabolic flux in situ. However, combining isotopic labeling and MSI presents technical challenges ranging from sample preparation, label incorporation, data collection, and analysis. Isotopic labeling and MSI individually create large, complex data sets, and this is compounded when both methods are combined. Therefore, analyzing isotopically labeled MSI data requires streamlined procedures to support biologically meaningful interpretations. Using currently available software and techniques, here we describe a workflow to analyze ¹³C-labeled isotopologues of the membrane lipid and storage oil lipid intermediate―phosphatidylcholine (PC). Our results with embryos of the oilseed crops, Camelina sativa and Thlaspi arvense (pennycress), demonstrated greater ¹³C-isotopic labeling in the cotyledons of developing embryos compared with the embryonic axis. Greater isotopic enrichment in PC molecular species with more saturated and longer chain fatty acids suggest different flux patterns related to fatty acid desaturation and elongation pathways. The ability to evaluate MSI data of isotopically labeled plant embryos will facilitate the potential to investigate spatial aspects of metabolic flux in situ.

Functional characterization of an novel acyl-CoA:diacylglycerol acyltransferase 3-3 (CsDGAT3-3) gene from Camelina sativa

Diacylglycerol acyltransferases (DGAT) catalyze the final committed step of de novo biosynthesis of triacylglycerol (TAG) in plant seeds. This study was to functionally characterize DGAT3 genes in Camelina sativa, an important oil crops accumulating high levels of unsaturated fatty acids (UFAs) in seeds. Three camelina DGAT3 genes (CsDGAT3-1, CsDGAT3-2 and CsDGAT3-3) were identified, and the encoded proteins were predicted to be cytosolic-soluble proteins present as a homodimer containing the 2Fe-2S domain. They had divergent expression patterns in various tissues, suggesting that they may function in tissue-specific manner with CsDGAT3-1 in roots, CsDGAT3-2 in flowers and young seedlings, and CsDGAT3-3 in developing seeds. Functional complementation assay in yeast demonstrated that CsDGAT3-3 restored TAG synthesis. TAG content and UFAs, particularly eicosenoic acid (EA, 20:1n-9) were largely increased by adding exogenous UFAs in the yeast medium. Further heterogeneously transient expression in N. benthamiana leaves and seed-specific expression in tobacco seeds indicated that CsDGAT3-3 significantly enhanced oil and UFA accumulation with much higher level of EA. Overall, CsDGAT3-3 exhibited a strong abilty catalyzing TAG synthesis and high substrate preference for UFAs, especially for 20:1n-9. The present data provide new insights for further understanding oil biosynthesis mechanism in camelina seeds, indicating that CsDGAT3-3 may have practical applications for increasing both oil yield and quality.

Effects of type I Diacylglycerol O-acyltransferase (DGAT1) genes on soybean (Glycine max L.) seed composition

Type I Diacylglycerol acyltransferase (DGAT1) catalyzes the final step of the biosynthesis process of triacylglycerol (TAG), the major storage lipids in plant seeds, through the esterification of diacylglycerol (DAG). To characterize the function of DGAT1 genes on the accumulation of oil and other seed composition traits in soybean, transgenic lines were generated via trans-acting siRNA technology, in which three DGAT1 genes (Glyma.13G106100, Glyma.09G065300, and Glyma.17G053300) were downregulated. The simultaneous downregulation of the three isoforms in transgenic lines was found to be associated with the reduction of seed oil concentrations by up to 18 mg/g (8.3%), which was correlated with increases in seed protein concentration up to 42 mg/g (11%). Additionally, the downregulations also influenced the fatty acid compositions in the seeds of transgenic lines through increasing the level of oleic acid, up to 121 mg/g (47.3%). The results of this study illustrate the importance of DGAT1 genes in determining the seed compositions in soybean through the development of new potential technology for manipulating seed quality in soybean to meet the demands for its various food and industrial applications.

A MADS-box gene, EgMADS21, negatively regulates EgDGAT2 expression and decreases polyunsaturated fatty acid accumulation in oil palm (Elaeis guineensis Jacq.)

EgMADS21 regulates PUFA accumulation in oil palm. Oil palm (Elaeis guineensis Jacq.) is the most productive world oil crop, accounting for 36% of world plant oil production. However, the molecular mechanism of the transcriptional regulation of fatty acid accumulation and lipid synthesis in the mesocarp of oil palm by up- or downregulating the expression of genes involved in related pathways remains largely unknown. Here, an oil palm MADS-box gene, EgMADS21, was screened in a yeast one-hybrid assay using the EgDGAT2 promoter sequence as bait. EgMADS21 is preferentially expressed in early mesocarp developmental stages in oil palm fruit and presents a negative correlation with EgDGAT2 expression. The direct binding of EgMADS21 to the EgDGAT2 promoter was confirmed by electrophoretic mobility shift assay. Subsequently, transient expression of EgMADS21 in oil palm protoplasts revealed that EgMADS21 not only binds to the EgDGAT2 promoter but also negatively regulates the expression of EgDGAT2. Furthermore, EgMADS21 was stably overexpressed in transgenic oil palm embryoids by Agrobacterium-mediated transformation. In three independent transgenic lines, EgDGAT2 expression was significantly suppressed by the expression of EgMADS21. The content of linoleic acid (C18:2) in the three transgenic embryoids was significantly decreased, while that of oleic acid (C18:1) was significantly increased. Combined with the substrate preference of EgDGAT2 identified in previous research, the results demonstrate the molecular mechanism by which EgMADS21 regulates EgDGAT2 expression and ultimately affects fatty acid accumulation in the mesocarp of oil palm.

Identification and functional characterization of two acyl CoA:diacylglycerol acyltransferase 1 (DGAT1) genes from forage sorghum (Sorghum bicolor) embryo

Elevating the lipid content in high-biomass forage crops has emerged as a new research platform for increasing energy density and improving livestock production efficiency associated with improved human health beneficial meat and milk quality. To gain insights of triacylglycerol (TAG) biosynthesis in forage sorghum, two type-1 diacylglycerol acyltransferase (designated as SbDGAT1-1 and SbDGAT1-2) were characterized for its in vivo function. SbDGAT1-2 is more abundantly expressed in embryo and bran during the early stage of the grain development in comparison to SbDGAT1-1. Heterologous expression of SbDGAT1 genes in TAG deficient H1246 strain restored the TAG accumulation capability with high substrate predilection towards 16:0, 16:1 and 18:1 fatty acids (FA). In parallel, we have identified N-terminal intrinsically disordered region (IDR) in SbDGAT1 proteins. To test the efficacy of the N-terminal region, truncated variants of SbDGAT1-1 (designated as SbDGAT1-1_(39-515) and SbDGAT1-1_(89-515)) were generated and expressed in yeast H1246 strain. Deletion in the N-terminal region resulted in decreased accumulation of TAG and FA (16:0 and 18:0) when compared to the SbDGAT1-1 variant expressed in yeast H1246 strain. The present study provides significant insight in forage sorghum DGAT1 gene function, useful for enhancing the green-forage TAG content through metabolic engineering.

The biochemistry of headgroup exchange during triacylglycerol synthesis in canola

Many pathways of primary metabolism are substantially conserved within and across plant families. However, significant differences in organization and fluxes through a reaction network may occur, even between plants in closely related genera. Assessing and understanding these differences is key to appreciating metabolic diversity, and to attempts to engineer plant metabolism for higher crop yields and desired product profiles. To better understand lipid metabolism and seed oil synthesis in canola (Brassica napus), we have characterized four canola homologues of the Arabidopsis (Arabidopsis thaliana) ROD1 gene. AtROD1 encodes phosphatidylcholine:diacylglycerol cholinephosphotransferase (PDCT), the enzyme that catalyzes a major flux of polyunsaturated fatty acids (PUFAs) in oil synthesis. Assays in yeast indicated that only two of the canola genes, BnROD1.A3 and BnROD1.C3, encode active isozymes of PDCT, and these genes are strongly expressed during the period of seed oil synthesis. Loss of expression of BnROD1.A3 and BnROD1.C3 in a double mutant, or by RNA interference, reduced the PUFA content of the oil to 26.6% compared with 32.5% in the wild type. These results indicate that ROD1 isozymes in canola are responsible for less than 20% of the PUFAs that accumulate in the seed oil compared with 40% in Arabidopsis. Our results demonstrate the care needed when translating results from a model species to crop plants.

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

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

Triacylglycerol and phytyl ester synthesis in Synechocystis sp. PCC6803

Cyanobacteria are unicellular prokaryotic algae that perform oxygenic photosynthesis, similar to plants. The cells harbor thylakoid membranes composed of lipids related to those of chloroplasts in plants to accommodate the complexes of photosynthesis. The occurrence of storage lipids, including triacylglycerol or wax esters, which are found in plants, animals, and some bacteria, nevertheless remained unclear in cyanobacteria. We show here that the cyanobacterium Synechocystis sp. PCC6803 accumulates both triacylglycerol and wax esters (fatty acid phytyl esters). Phytyl esters accumulate in higher levels under abiotic stress conditions. The analysis of an insertional mutant revealed that the acyltransferase slr2103, with sequence similarity to plant esterase/lipase/thioesterase (ELT) proteins, is essential for triacylglycerol and phytyl ester synthesis in Synechocystis The recombinant slr2103 enzyme showed acyltransferase activity with phytol and diacylglycerol, thus producing phytyl esters and triacylglycerol. Acyl-CoA thioesters were the preferred acyl donors, while acyl-ACP (acyl carrier protein), free fatty acids, or galactolipid-bound fatty acids were poor substrates. The slr2103 protein sequence is unrelated to acyltransferases from bacteria (AtfA) or plants (DGAT1, DGAT2, PDAT), and therefore establishes an independent group of bacterial acyltransferases involved in triacylglycerol and wax ester synthesis. The identification of the gene slr2103 responsible for triacylglycerol synthesis in cyanobacteria opens the possibility of using prokaryotic photosynthetic cells in biotechnological applications.

On the Role of DGAT1 in Seed Glycerolipid Metabolic Network and Critical Stages of Plant Development in Arabidopsis

Studies on the model plant Arabidopsis thaliana have uncovered the identities of most enzymatic components involved in seed storage lipid biosynthesis. However, much remains to be learned on how pathway interactions operate in the seed metabolic network. In this study, we dissected seed glycerolipid molecular compositional changes in the Arabidopsis mutant deficient in diacylglycerol acyltransferase 1 (DGAT1). Our results indicate that metabolic adjustments occurred in both phosphatidylcholine synthesis and deacylation in developing seeds. Ultrastructural changes of perturbed oil and protein bodies were also evident in cotyledon parenchyma cells. To unmask the physiological and developmental role associated with DGAT1-mediated neutral lipid biosynthesis, we attempted to combine dgat1 mutation with lpcat2 that harbors a defect in lysophosphatidylcholine acyltransferase 2 (LPCAT2). Disruption in both DGAT1 and LPCAT2 led to an apparent defect in pollen development that manifested as pollen sterility. Collectively, our results highlight a role of DGAT1 in both storage lipid synthesis and plant development.

Molecular Basis of Plant Oil Biosynthesis: Insights Gained From Studying the WRINKLED1 Transcription Factor

Most plant species generate and store triacylglycerol (TAG) in their seeds, serving as a core supply of carbon and energy to support seedling development. Plant seed oils have a wide variety of applications, from being essential for human diets to serving as industrial renewable feedstock. WRINKLED1 (WRI1) transcription factor plays a central role in the transcriptional regulation of plant fatty acid biosynthesis. Since the discovery of Arabidopsis WRI1 gene (AtWRI1) in 2004, the function of WRI1 in plant oil biosynthesis has been studied intensively. In recent years, the identification of WRI1 co-regulators and deeper investigations of the structural features and molecular functions of WRI1 have advanced our understanding of the mechanism of the transcriptional regulation of plant oil biosynthesis. These advances also help pave the way for novel approaches that will better utilize WRI1 for bioengineering oil production in crops.

Characterization and heterologous expression of three DGATs from oil palm (Elaeis guineensis) mesocarp in Saccharomyces cerevisiae

Oil palm (Elaeis guineensis) can accumulate up to 88% oil in fruit mesocarp. A previous transcriptome study of oil palm fruits indicated that genes coding for three diacylglycerol acyltransferases (DGATs), designated as EgDGAT1_3, EgDGAT2_2 and EgWS/DGAT_1 (according to Rosli et al., 2018) were highly expressed in mesocarp during oil accumulation. In the present study, the corresponding open reading frames were isolated, and characterized by heterologous expression in the mutant yeast H1246, which is devoid of neutral lipid synthesis. Expression of EgDGAT1_3 or EgDGAT2_2 could restore TAG synthesis, confirming that both proteins are true DGAT. In contrast, expression of EgWS/DGAT_1 resulted in the synthesis of fatty acid isoamyl esters (FAIEs) with saturated long-chain and very-long-chain fatty acids. In the presence of exogenously supplied fatty alcohols, EgWS/DGAT_1 was able to produce wax esters, indicating that EgWS/DGAT_1 codes for an acyltransferase with wax ester synthase but no DGAT activity. Finally, the complete wax ester biosynthetic pathway was reconstituted in yeast by coexpressing EgWS/DGAT_1 with a fatty acyl reductase from Tetrahymena thermophila. Altogether, our results characterized two novel DGATs from oil palm as well as a putative wax ester synthase that preferentially using medium chain fatty alcohols and saturated very-long chain fatty acids as substrates.

Acyltransferases Regulate Oil Quality in Camelina sativa Through Both Acyl Donor and Acyl Acceptor Specificities

Camelina sativa is an emerging biotechnology oil crop. However, more information is needed regarding its innate lipid enzyme specificities. We have therefore characterized several triacylglycerol (TAG) producing enzymes by measuring in vitro substrate specificities using different combinations of acyl-acceptors (diacylglycerol, DAG) and donors. Specifically, C. sativa acyl-CoA:diacylglycerol acyltransferase (DGAT) 1 and 2 (which both use acyl-CoA as acyl donor) and phospholipid:diacylglycerol acyltransferase (PDAT, with phosphatidylcoline as acyl donor) were studied. The results show that the DGAT1 and DGAT2 specificities are complementary, with DGAT2 exhibiting a high specificity for acyl acceptors containing only polyunsaturated fatty acids (FAs), whereas DGAT1 prefers acyl donors with saturated and monounsaturated FAs. Furthermore, the combination of substrates that resulted in the highest activity for DGAT2, but very low activity for DGAT1, corresponds to TAG species previously shown to increase in C. sativa seeds with downregulated DGAT1. Similarly, the combinations of substrates that gave the highest PDAT1 activity were also those that produce the two TAG species (54:7 and 54:8 TAG) with the highest increase in PDAT overexpressing C. sativa seeds. Thus, the in vitro data correlate well with the changes in the overall fatty acid profile and TAG species in C. sativa seeds with altered DGAT1 and PDAT activity. Additionally, in vitro studies of C. sativa phosphatidycholine:diacylglycerol cholinephosphotransferase (PDCT), another activity involved in TAG biosynthesis, revealed that PDCT accepts substrates with different desaturation levels. Furthermore, PDCT was unable to use DAG with ricineoleyl groups, and the presence of this substrate also inhibited PDCT from using other DAG-moieties. This gives insights relating to previous in vivo studies regarding this enzyme.

Genome-wide analysis and functional characterization of Acyl-CoA:diacylglycerol acyltransferase from soybean identify GmDGAT1A and 1B roles in oil synthesis in Arabidopsis seeds

Acyl-CoA:diacylglycerol acyltransferase (DGAT) is a key enzyme in the Kennedy pathway of triacylglycerol (TAG) synthesis. It catalyzes the acyl-CoA-dependent acylation of sn-1, 2-diacylglycerol to form TAG. DGATs in soybean (Glycine max) have been reported, but their functions are largely unclear. Here we cloned three members of DGAT1 and four members of DGAT2 family from soybean, named GmDGAT1A to GmDGAT1C, and GmDGAT2A to GmDGAT2D, respectively. GmDGAT1A and GmDGAT1C were expressed at a high level in immature seeds, GmDGAT2B in mature seeds, and GmDGAT2C in older leaves. The seven genes were transformed into the H1246 quadruple mutant yeast strain, in which GmDGAT1A, GmDGAT1B, GmDGAT1C, GmDGAT2A, and GmDGAT2B had the ability to produce TAG. Six genes were transformed into Arabidopsis respectively, and constitutive expression of GmDGAT1A and GmDGAT1B resulted in an increase in oil content at the cost of reduced protein content in seeds. Overexpression of GmDGAT1A produced heavier weight of individual seed, but did not affect the weight of total seeds from a plant. Our results reveal the functions of soybean DGATs in seed oil synthesis using transgenic Arabidopsis. The implications for the biotechnological modification of the oil contents in soybeans by altering DGAT expression are discussed.

LYSOPHOSPHATIDIC ACID ACYLTRANSFERASES 4 and 5 are involved in glycerolipid metabolism and nitrogen starvation response in Arabidopsis

Nitrogen (N) deficiency triggers an accumulation of a storage lipid triacylglycerol (TAG) in seed plants and algae. Whereas the metabolic pathway and regulatory mechanism to synthesize TAG from diacylglycerol are well known, enzymes involved in the supply of diacylglycerol remain elusive under N starvation. Lysophosphatidic acid acyltransferase (LPAT) catalyzes an important step of the de novo phospholipid biosynthesis pathway and thus has a strong flux control in the biosynthesis of phospholipids and TAG. Five LPAT isoforms are known in Arabidopsis; however, the functions of LPAT4 and LPAT5 remain elusive. Here, we show that LPAT4 and LPAT5 are functional endoplasmic-reticulum-localized LPATs. Seedlings of the double knockout mutant lpat4-1 lpat5-1 showed reduced content of phospholipids and TAG under normal growth condition. Under N starvation, lpat4-1 lpat5-1 seedlings showed severer growth defect than the wild-type in shoot. The phenotype was similar to dgat1-4, which affects a major TAG biosynthesis pathway and showed similarly reduced TAG content as the lpat4-1 lpat5-1. We suggest that LPAT4 and LPAT5 may redundantly function in endoplasmic-reticulum-localized de novo glycerolipid biosynthesis for phospholipids and TAG, which is important for the N starvation response in Arabidopsis.

The Arabidopsis MYB96 Transcription Factor Mediates ABA-Dependent Triacylglycerol Accumulation in Vegetative Tissues under Drought Stress Conditions

Triacylglycerols (TAGs), a major lipid form of energy storage, are involved in a variety of plant developmental processes. While carbon reserves mainly accumulate in seeds, significant amounts of TAG have also been observed in vegetative tissues. Notably, the accumulation of leaf TAGs is influenced by environmental stresses such as drought stress, although underlying molecular networks remain to be fully elucidated. In this study, we demonstrate that the R2R3-type MYB96 transcription factor promotes TAG biosynthesis in Arabidopsis thaliana seedlings. Core TAG biosynthetic genes were up-regulated in myb96-ox seedlings, but down-regulated in myb96-deficient seedlings. In particular, ABA stimulates TAG accumulation in the vegetative tissues, and MYB96 plays a fundamental role in this process. Considering that TAG accumulation contributes to plant tolerance to drought stress, MYB96-dependent TAG biosynthesis not only triggers plant adaptive responses but also optimizes energy metabolism to ensure plant fitness under unfavorable environmental conditions.