Redirection of metabolic flux for high levels of omega-7 monounsaturated fatty acid accumulation in camelina seeds

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.

De Novo Biosynthesis of Free Vaccenic Acid with a Low Content of Oleic Acid in Saccharomyces cerevisiae

Omega-7 (ω-7) fatty acids have potential application in the fields of nutraceutical, agricultural, and food industry. The natural ω-7 fatty acids are currently from plants or vegetable oils, which are unsustainable and limited by the availability of plant sources. Here, we developed an innovative biosynthetic route to produce vaccenic acid (C18:1 ω-7) while minimizing oleic acid (C18:1 ω-9) content in Saccharomyces cerevisiae. We have engineered S. cerevisiaeto produce C18:1 ω-7 by expressing a fatty acid elongase from Rattus norvegicus. To reduce the content of C18:1 ω-9, the endogenous desaturase Ole1 was replaced by the desaturase, which has specific activity on palmitoyl-coenzyme A (C16:0-CoA). Finally, the production of free C18:1 ω-7 was improved by optimizing the source of cytochrome b5 and overexpressing endoplasmic reticulum chaperones. After combining these strategies, the yield of C18:1 ω-7 was increased from 0 to 9.3 mg/g DCW and C18:1 ω-9 was decreased from 25.2 mg/g DCW to 1.6 mg/g DCW. This work shows a de novo synthetic pathway to produce the highest amount of free C18:1 ω-7 with a low content of C18:1 ω-9 in S. cerevisiae.

Essential Components from Plant Source Oils: A Review on Extraction, Detection, Identification, and Quantification

Oils derived from plant sources, mainly fixed oils from seeds and essential oil from other parts of the plant, are gaining interest as they are the rich source of beneficial compounds that possess potential applications in different industries due to their preventive and therapeutic actions. The essential oils are used in food, medicine, cosmetics, and agriculture industries as they possess antimicrobial, anticarcinogenic, anti-inflammatory and immunomodulatory properties. Plant based oils contain polyphenols, phytochemicals, and bioactive compounds which show high antioxidant activity. The extractions of these oils are a crucial step in terms of the yield and quality attributes of plant oils. This review paper outlines the different modern extraction techniques used for the extraction of different seed oils, including microwave-assisted extraction (MAE), pressurized liquid extraction (PLE), cold-pressed extraction (CPE), ultrasound-assisted extraction (UAE), supercritical-fluid extraction (SFE), enzyme-assisted extraction (EAE), and pulsed electric field-assisted extraction (PEF). For the identification and quantification of essential and bioactive compounds present in seed oils, different modern techniques-such as high-performance liquid chromatography (HPLC), gas chromatography-mass spectrometry (GC-MS), Fourier transform infrared spectroscopy (FTIR), gas chromatography-infrared spectroscopy (GC-IR), atomic fluorescence spectroscopy (AFS), and electron microscopy (EM)-are highlighted in this review along with the beneficial effects of these essential components in different in vivo and in vitro studies and in different applications. The primary goal of this research article is to pique the attention of researchers towards the different sources, potential uses and applications of oils in different industries.

A review on the identification of transgenic oilseeds and oils

Transgenic technology can increase the quantity and quality of vegetable oils worldwide. However, people are skeptical about the safety of transgenic oil-bearing crops and the oils they produce. In order to protect consumers' rights and avoid transgenic oils being adulterated or labeled as nontransgenic oils, the transgenic detection technology of oilseeds and oils needs careful consideration. This paper first summarized the current research status of transgenic technologies implemented at oil-bearing crops. Then, an inspection process was proposed to detect a large number of samples to be the subject rapidly, and various inspection strategies for transgenic oilseeds and oils were summarized according to the process sequence. The detection indicators included oil content, fatty acid, triglyceride, tocopherol, and nucleic acid. The detection technologies involved chromatography, spectroscopy, nuclear magnetic resonance, and polymerase chain reaction. It is hoped that this article can provide crucial technical reference and support for staff engaging in the supervision of transgenic food and for researchers developing fast and efficient monitoring methods in the future.

The coexpression of two desaturases provides an optimized reduction of saturates in camelina oil

Reducing the saturate content of vegetable oils is key to increasing their utility and adoption as a feedstock for the production of biofuels. Expression of either the FAT5 16 : 0-CoA desaturase from Caenorhabditis elegans, or an engineered cyanobacterial 16 : 0/18 : 0-glycerolipid desaturase, DES9*, in seeds of Arabidopsis (Arabidopsis thaliana) substantially lowered oil saturates. However, because pathway fluxes and regulation of oil synthesis are known to differ across species, translating this transgene technology from the model plant to crop species requires additional investigation. In the work reported here, we found that high expression of FAT5 in seeds of camelina (Camelina sativa) provided only a moderate decrease in saturates, from 12.9% of total oil fatty acids in untransformed controls to 8.6%. Expression of DES9* reduced saturates to 4.6%, but compromised seed physiology and oil content. However, the coexpression of the two desaturases together cooperatively reduced saturates to only 4.0%, less than one-third of the level in the parental line, without compromising oil yield or seedling germination and establishment. Our successful lowering of oil saturates in camelina identifies strategies that can now be integrated with genetic engineering approaches that reduce polyunsaturates to provide optimized oil composition for biofuels in camelina and other oil seed crops.

Progress in understanding and improving oil content and quality in seeds

The world's population is projected to increase by two billion by 2050, resulting in food and energy insecurity. Oilseed crops have been identified as key to address these challenges: they produce and store lipids in the seeds as triacylglycerols that can serve as a source of food/feed, renewable fuels, and other industrially-relevant chemicals. Therefore, improving seed oil content and composition has generated immense interest. Research efforts aiming to unravel the regulatory pathways involved in fatty acid synthesis and to identify targets for metabolic engineering have made tremendous progress. This review provides a summary of the current knowledge of oil metabolism and discusses how photochemical activity and unconventional pathways can contribute to high carbon conversion efficiency in seeds. It also highlights the importance of 13C-metabolic flux analysis as a tool to gain insights on the pathways that regulate oil biosynthesis in seeds. Finally, a list of key genes and regulators that have been recently targeted to enhance seed oil production are reviewed and additional possible targets in the metabolic pathways are proposed to achieve desirable oil content and quality.

A toolkit for plant lipid engineering: Surveying the efficacies of lipogenic factors for accumulating specialty lipids

Plants produce energy-dense lipids from carbohydrates using energy acquired via photosynthesis, making plant oils an economically and sustainably attractive feedstock for conversion to biofuels and value-added bioproducts. A growing number of strategies have been developed and optimized in model plants, oilseed crops and high-biomass crops to enhance the accumulation of storage lipids (mostly triacylglycerols, TAGs) for bioenergy applications and to produce specialty lipids with increased uses and value for chemical feedstock and nutritional applications. Most successful metabolic engineering strategies involve heterologous expression of lipogenic factors that outperform those from other sources or exhibit specialized functionality. In this review, we summarize recent progress in engineering the accumulation of triacylglycerols containing - specialized fatty acids in various plant species and tissues. We also provide an inventory of specific lipogenic factors (including accession numbers) derived from a wide variety of organisms, along with their reported efficacy in supporting the accumulation of desired lipids. A review of previously obtained results serves as a foundation to guide future efforts to optimize combinations of factors to achieve further enhancements to the production and accumulation of desired lipids in a variety of plant tissues and species.

Divergent evolution of extreme production of variant plant monounsaturated fatty acids

The dietary and oleochemical value of vegetable oils is determined by their component fatty acids. Double bonds or “unsaturation” in fatty acids are critical for vegetable oil functionality. Seeds containing vegetable oils with extremely high levels of a single fatty acid have provided insights into enzyme-substrate recognition and metabolic plasticity and genes for biotechnological improvement of oilseeds. We report the discovery of species with seed oils containing >90% of an unusual monounsaturated fatty acid. We identified the variant enzyme that produces this fatty acid and elucidated its three-dimensional structure. We used this information to develop enzymes that produce nonnaturally occurring monounsaturated fatty acids and sourced genes from these species to engineer oilseeds and bacteria for modified fatty acid compositions.

Metabolic extremes provide opportunities to understand enzymatic and metabolic plasticity and biotechnological tools for novel biomaterial production. We discovered that seed oils of many Thunbergia species contain up to 92% of the unusual monounsaturated petroselinic acid (18:1Δ6), one of the highest reported levels for a single fatty acid in plants. Supporting the biosynthetic origin of petroselinic acid, we identified a Δ6-stearoyl-acyl carrier protein (18:0-ACP) desaturase from Thunbergia laurifolia, closely related to a previously identified Δ6-palmitoyl-ACP desaturase that produces sapienic acid (16:1Δ6)-rich oils in Thunbergia alata seeds. Guided by a T. laurifolia desaturase crystal structure obtained in this study, enzyme mutagenesis identified key amino acids for functional divergence of Δ6 desaturases from the archetypal Δ9-18:0-ACP desaturase and mutations that result in nonnative enzyme regiospecificity. Furthermore, we demonstrate the utility of the T. laurifolia desaturase for the production of unusual monounsaturated fatty acids in engineered plant and bacterial hosts. Through stepwise metabolic engineering, we provide evidence that divergent evolution of extreme petroselinic acid and sapienic acid production arises from biosynthetic and metabolic functional specialization and enhanced expression of specific enzymes to accommodate metabolism of atypical substrates.

Genome sequence and population genomics provide insights into chromosomal evolution and phytochemical innovation of Hippophae rhamnoides

Plants of the Elaeagnaceae family are widely used to treat various health disorders owing to their natural phytochemicals. Seabuckthorn (Hippophae rhamnoides L.) is an economically and ecologically important species within the family with richness of biologically and pharmacologically active substances. Here, we present a chromosome-level genome assembly of seabuckthorn (http://hipp.shengxin.ren/), the first genome sequence of Elaeagnaceae, which has a total length of 849.04 Mb with scaffold N50 of 69.52 Mb and 30 864 annotated genes. Two sequential tetraploidizations with one occurring ~36-41 million years ago (Mya) and the last ~24-27 Mya were inferred, resulting in expansion of genes related to ascorbate and aldarate metabolism, lipid biosynthesis, and fatty acid elongation. Comparative genomic analysis reconstructed the evolutionary trajectories of the seabuckthorn genome with the predicted ancestral genome of 14 proto-chromosomes. Comparative transcriptomic and metabonomic analyses identified some key genes contributing to high content of polyunsaturated fatty acids and ascorbic acid (AsA). Additionally, we generated and analysed 55 whole-genome sequences of diverse accessions, and identified 9.80 million genetic variants in the seabuckthorn germplasms. Intriguingly, genes in selective sweep regions identified through population genomic analysis appeared to contribute to the richness of AsA and fatty acid in seabuckthorn fruits, among which GalLDH, GMPase and ACC, TER were the potentially major-effect causative genes controlling AsA and fatty acid content of the fruit, respectively. Our research offers novel insights into the molecular basis underlying phytochemical innovation of seabuckthorn, and provides valuable resources for exploring the evolution of the Elaeagnaceae family and molecular breeding.

Regioselectivity mechanism of the Thunbergia alata Δ6-16:0-acyl carrier protein desaturase

Plant plastidial acyl-acyl carrier protein (ACP) desaturases are a soluble class of diiron-containing enzymes that are distinct from the diiron-containing integral membrane desaturases found in plants and other organisms. The archetype of this class is the stearoyl-ACP desaturase which converts stearoyl-ACP into oleoyl (18:1Δ9cis)-ACP. Several variants expressing distinct regioselectivity have been described including a Δ6-16:0-ACP desaturase from black-eyed Susan vine (Thunbergia alata). We solved a crystal structure of the T. alata desaturase at 2.05 Å resolution. Using molecular dynamics (MD) simulations, we identified a low-energy complex between 16:0-ACP and the desaturase that would position C6 and C7 of the acyl chain adjacent to the diiron active site. The model complex was used to identify mutant variants that could convert the T. alata Δ6 desaturase to Δ9 regioselectivity. Additional modeling between ACP and the mutant variants confirmed the predicted regioselectivity. To validate the in-silico predictions, we synthesized two variants of the T. alata desaturase and analyzed their reaction products using gas chromatography-coupled mass spectrometry. Assay results confirmed that mutants designed to convert T. alata Δ6 to Δ9 selectivity exhibited the predicted changes. In complementary experiments, variants of the castor desaturase designed to convert Δ9 to Δ6 selectivity lost some of their Δ9 desaturation ability and gained the ability to desaturate at the Δ6 position. The computational workflow for revealing the mechanistic understanding of regioselectivity presented herein lays a foundation for designing acyl-ACP desaturases with novel selectivities to increase the diversity of monoenes available for bioproduct applications.

Plant monounsaturated fatty acids: Diversity, biosynthesis, functions and uses

Monounsaturated fatty acids are straight-chain aliphatic monocarboxylic acids comprising a unique carbon‑carbon double bond, also termed unsaturation. More than 50 distinct molecular structures have been described in the plant kingdom, and more remain to be discovered. The evolution of land plants has apparently resulted in the convergent evolution of non-homologous enzymes catalyzing the dehydrogenation of saturated acyl chain substrates in a chemo-, regio- and stereoselective manner. Contrasted enzymatic characteristics and different subcellular localizations of these desaturases account for the diversity of existing fatty acid structures. Interestingly, the location and geometrical configuration of the unsaturation confer specific characteristics to these molecules found in a variety of membrane, storage, and surface lipids. An ongoing research effort aimed at exploring the links existing between fatty acid structures and their biological functions has already unraveled the importance of several monounsaturated fatty acids in various physiological and developmental contexts. What is more, the monounsaturated acyl chains found in the oils of seeds and fruits are widely and increasingly used in the food and chemical industries due to the physicochemical properties inherent in their structures. Breeders and plant biotechnologists therefore develop new crops with high monounsaturated contents for various agro-industrial purposes.

Enhancing growth and oil accumulation of a palmitoleic acid-rich Scenedesmus obliquus in mixotrophic cultivation with acetate and its potential for ammonium-containing wastewater purification and biodiesel production

A palmitoleic acid-rich Scenedesmus obliquus strain SXND-02 was isolated from ammonium-containing wastewater. Biomass and lipid production were examined for this microalgal strain in photoautotrophic, heterotrophic, and mixotrophic cultivations, respectively, in order to extend its application in wastewater purification coupled with production of valued bio-products. Among the tested conditions, the microalga had better growth and higher lipid accumulation in mixotrophy. NH₄Cl inhibited the microalgal growth in photoautotrophic cultivation. However, NaAc alleviated this inhibition in both heterotrophy and mixotrophy. Using 7 g L^-1 NaAc and 0.5 g L^-1 NH₄Cl as carbon and nitrogen sources significantly increased the algal biomass and lipid yields under mixotrophic cultivation, with the highest levels up to 1.0 g L^-1 and 59.88%, respectively. Fatty acid profiling indicated that palmitoleic acid was 23% in the S. obliquus SXND-02 under mixotrophic condition, which was about 21-fold higher than that in the control S. obliquus. Furthermore, this microalgal strain was tested in the chicken farm wastewater (CFW) containing high ammonium. Compared with other treatments, the S. obliquus SXND-02 cultivated in the 1/2 CFW + NaAc medium produced larger amounts of biomass (2.18 g L^-1) and lipids (50.22%), and simultaneously higher removal rates of total nitrogen (TN) (80%), total ammonium nitrogen (TAN) (68%), total phosphate (TP) (82%), biological oxygen demand (BOD) (86%) and chemical oxygen demand (COD) (89%) from wastewater. The present data indicate that this excellent microalga can be used in mixotrophic cultivation for wastewater purification coupled with commercial production of valued biomass and high-quality algal oils.

Triacylglycerol remodeling in Physaria fendleri indicates oil accumulation is dynamic and not a metabolic endpoint

Oilseed plants accumulate triacylglycerol (TAG) up to 80% of seed weight with the TAG fatty acid composition determining its nutritional value or use in the biofuel or chemical industries. Two major pathways for production of diacylglycerol (DAG), the immediate precursor to TAG, have been identified in plants: de novo DAG synthesis and conversion of the membrane lipid phosphatidylcholine (PC) to DAG, with each pathway producing distinct TAG compositions. However, neither pathway fits with previous biochemical and transcriptomic results from developing Physaria fendleri seeds for accumulation of TAG containing >60% lesquerolic acid (an unusual 20 carbon hydroxylated fatty acid), which accumulates at only the sn-1 and sn-3 positions of TAG. Isotopic tracing of developing P. fendleri seed lipid metabolism identified that PC-derived DAG is utilized to initially produce TAG with only one lesquerolic acid. Subsequently a nonhydroxylated fatty acid is removed from TAG (transiently reproducing DAG) and a second lesquerolic acid is incorporated. Thus, a dynamic TAG remodeling process involving anabolic and catabolic reactions controls the final TAG fatty acid composition. Reinterpretation of P. fendleri transcriptomic data identified potential genes involved in TAG remodeling that could provide a new approach for oilseed engineering by altering oil fatty acid composition after initial TAG synthesis; and the comparison of current results to that of related Brassicaceae species in the literature suggests the possibility of TAG remodeling involved in incorporation of very long-chain fatty acids into the TAG sn-1 position in various plants.

Multiple functions of the vacuole in plant growth and fruit quality

Vacuoles are organelles in plant cells that play pivotal roles in growth and developmental regulation. The main functions of vacuoles include maintaining cell acidity and turgor pressure, regulating the storage and transport of substances, controlling the transport and localization of key proteins through the endocytic and lysosomal-vacuolar transport pathways, and responding to biotic and abiotic stresses. Further, proteins localized either in the tonoplast (vacuolar membrane) or inside the vacuole lumen are critical for fruit quality. In this review, we summarize and discuss some of the emerging functions and regulatory mechanisms associated with plant vacuoles, including vacuole biogenesis, vacuole functions in plant growth and development, fruit quality, and plant-microbe interaction, as well as some innovative research technology that has driven advances in the field. Together, the functions of plant vacuoles are important for plant growth and fruit quality. The investigation of vacuole functions in plants is of great scientific significance and has potential applications in agriculture.

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.

Production of C6-C14 Medium-Chain Fatty Acids in Seeds and Leaves via Overexpression of Single Hotdog-Fold Acyl-Lipid Thioesterases

ACYL-LIPID THIOESTERASES (ALT) are a type of plant acyl-acyl carrier protein thioesterase that generate a wide range of medium-chain fatty acids and methylketone (MK) precursors when expressed heterologously in Escherichia coli. While this makes ALT-type thioesterases attractive as metabolic engineering targets to increase production of high-value medium-chain fatty acids and MKs in plant systems, the behavior of ALT enzymes in planta was not well understood before this study. To profile the substrate specificities of ALT-type thioesterases in different plant tissue types, AtALT1-4 from Arabidopsis thaliana, which have widely varied chain length and oxidation state preferences in E. coli, were overexpressed in Arabidopsis seeds, Camelina sativa seeds, and Nicotiana benthamiana leaves. Seed-specific overexpression of ALT enzymes led to medium-chain fatty acid accumulation in Arabidopsis and Camelina seed triacylglycerols, and transient overexpression in N. benthamiana demonstrated that the substrate preferences of ALT-type thioesterases in planta generally agree with those previously determined in E. coli. AtALT1 and AtALT4 overexpression in leaves and seeds resulted in the accumulation of 12-14 carbon-length fatty acids and 6-8 carbon-length fatty acids, respectively. While it was difficult to completely profile the products of ALT-type thioesterases that generate MK precursors (i.e. β-keto fatty acids), our results nonetheless demonstrate that ALT enzymes are catalytically diverse in planta. The knowledge gained from this study is a significant step towards being able to use ALT-type thioesterases as metabolic engineering tools to modify the fatty acid profiles of oilseed crops, other plants, and microorganisms.

Expression of a high-activity diacylglycerol acetyltransferase results in enhanced synthesis of acetyl-TAG in camelina seed oil

Acetyl-triacylglycerols (acetyl-TAG) contain an acetate group in the sn-3 position instead of the long-chain fatty acid present in regular triacylglycerol (TAG). The acetate group confers unique physical properties such as reduced viscosity and a lower freezing point to acetyl-TAG, providing advantages for use as emulsifiers, lubricants, and 'drop-in' biofuels. Previously, the synthesis of acetyl-TAG in the seeds of the oilseed crop camelina (Camelina sativa) was achieved through the heterologous expression of the diacylglycerol acetyltransferase gene EaDAcT, isolated from Euonymus alatus seeds that naturally accumulate high levels of acetyl-TAG. Subsequent work identified a similar acetyltransferase, EfDAcT, in the seeds of Euonymus fortunei, that possesses higher in vitro activity compared to EaDAcT. In this study, the seed-specific expression of EfDAcT in camelina led to a 20 mol% increase in acetyl-TAG levels over that of EaDAcT. Coupling EfDAcT expression with suppression of the endogenous competing enzyme DGAT1 further enhanced acetyl-TAG accumulation, up to 90 mol% in the best transgenic lines. Accumulation of high levels of acetyl-TAG was stable over multiple generations, with minimal effect on seed size, weight, and fatty acid content. Slight delays in germination were noted in transgenic seeds compared to the wild type. EfDAcT transcript and protein levels were correlated during seed development with a limited window of EfDAcT protein accumulation. In high acetyl-TAG producing lines, EfDAcT protein expression in developing seeds did not reflect the eventual acetyl-TAG levels in mature seeds, suggesting that other factors limit acetyl-TAG accumulation.

A Review of Nervonic Acid Production in Plants: Prospects for the Genetic Engineering of High Nervonic Acid Cultivars Plants

Nervonic acid (NA) is a very-long-chain monounsaturated fatty acid that plays crucial roles in brain development and has attracted widespread research interest. The markets encouraged the development of a refined, NA-enriched plant oil as feedstocks for the needed further studies of NA biological functions to the end commercial application. Plant seed oils offer a renewable and environmentally friendly source of NA, but their industrial production is presently hindered by various factors. This review focuses on the NA biosynthesis and assembly, NA resources from plants, and the genetic engineering of NA biosynthesis in oil crops, discusses the factors that affect NA production in genetically engineered oil crops, and provides prospects for the application of NA and prospective trends in the engineering of NA. This review emphasizes the progress made toward various NA-related topics and explores the limitations and trends, thereby providing integrated and comprehensive insight into the nature of NA production mechanisms during genetic engineering. Furthermore, this report supports further work involving the manipulation of NA production through transgenic technologies and molecular breeding for the enhancement of crop nutritional quality or creation of plant biochemical factories to produce NA for use in nutraceutical, pharmaceutical, and chemical industries.

Increased expression of fatty acid and ABC transporters enhances seed oil production in camelina

BACKGROUND

Lipid transporters play an essential role in lipid delivery and distribution, but their influence on seed oil production in oilseed crops is not well studied.

RESULTS

Here, we examined the effect of two lipid transporters, FAX1 (fatty acid export1) and ABCA9 (ATP-binding cassette transporter subfamily A9) on oil production and lipid metabolism in the oilseed plant Camelina sativa. Overexpression (OE) of FAX1 and ABCA9 increased seed weight and size, with FAX1-OEs and ABCA9-OEs increasing seed length and width, respectively, whereas FAX1/ABCA9-OEs increasing both. FAX1-OE and ABCA9-OE displayed additive effects on seed oil content and seed yield. Also, OE of FAX1 and ABCA9 affected membrane lipid composition in developing pods, especially on phosphatidylcholine, phosphatidylethanolamine, and phosphatidylglycerol. The expression of some genes involved in seed oil synthesis, such as DGAT2, PDAT1, and LEC1, was increased in developing seeds of FAX1- and/or ABCA9-OEs.

CONCLUSION

These results indicate that increased expression of FAX1 and ABCA9 can potentially be applied to improving camelina oil production.

Molecular Control of Oil Metabolism in the Endosperm of Seeds

In angiosperm seeds, the endosperm develops to varying degrees and accumulates different types of storage compounds remobilized by the seedling during early post-germinative growth. Whereas the molecular mechanisms controlling the metabolism of starch and seed-storage proteins in the endosperm of cereal grains are relatively well characterized, the regulation of oil metabolism in the endosperm of developing and germinating oilseeds has received particular attention only more recently, thanks to the emergence and continuous improvement of analytical techniques allowing the evaluation, within a spatial context, of gene activity on one side, and lipid metabolism on the other side. These studies represent a fundamental step toward the elucidation of the molecular mechanisms governing oil metabolism in this particular tissue. In particular, they highlight the importance of endosperm-specific transcriptional controls for determining original oil compositions usually observed in this tissue. In the light of this research, the biological functions of oils stored in the endosperm of seeds then appear to be more diverse than simply constituting a source of carbon made available for the germinating seedling.

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.

Alterations in allocation and composition of lipid classes in Euonymus fruits and seeds

The spindle tree (Euonymus europaeus L.) is a much-branched deciduous shrub or small tree. Its fruit capsules contain seeds with remarkably high content of oil interesting for industry, especially the 3-acetyl-1,2-diacyl-sn-glycerols (AcDAG) synthesized by a specific acetyl-CoA diacylglycerol acetyltransferase. The distribution and amount of individual triacylglycerols (TAG) and especially acetyl-triacylglycerols (AcDAG) in Euonymus fruit have previously been assigned to specific tissues. Using anatomical and microscopical observations, we studied the fruit morphology, and for the first time, we identified a more detailed allocation of oil bodies in individual tissue structures. Thin layer chromatography separation of extracts from immature and mature fruits confirmed TAG and AcDAG as the most abundant lipid classes in both endosperm and embryo, followed by fatty acids and polar lipids. The abundance of fatty acids was further studied in the TAG and AcDAG fractions using gas chromatography. Data revealed particular FAs in both fractions allocated in tissue-specific manner and/or as indicators of maturation of E. europaeus seeds. While the abundance of cis-vaccenic-, linoleic as well as α-linolenic acids in the AcDAG structures generally drop with maturation in both embryo and endosperm, content of oleic acid increases. Abundance of cis-vaccenic acid in TAG was recorded in immature endosperm. For embryo, the abundance of stearic acid in AcDAG and oleic acid in TAG fraction was distinctive. Deeper understanding of underlying metabolic processes will be essential for targeted metabolic engineering and/or application for oilseed crops.

Characterization of a Novel Acyl-ACP Δ9 Desaturase Gene Responsible for Palmitoleic Acid Accumulation in a Diatom Phaeodactylum tricornutum

Palmitoleic acid (16:1Δ⁹) possesses a double bond at the seventh carbon atom from methyl end of the acyl chain and belongs to unusual ω-7 monounsaturated fatty acids with broad applications in food, pharmaceuticals, cosmetics, biofuel, and other industries. This high-value fatty acid accumulates up to >40% of total lipid in the marine diatom Phaeodactylum tricornutum. The present study was conducted to determine the key gene responsible for 16:1Δ⁹ biosynthesis in this unicellular alga. A new full-length cDNA and genomic DNA encoding acyl-ACP Δ⁹ desaturase (PtAAD) were isolated from P. tricornutum cells. Expression levels of PtAAD gene under normal and stress culture conditions were both positively correlated with 16:1Δ⁹ accumulation, implying its potential role for fatty acid determination. Functional complementation assay of a yeast mutant strain BY4839 evidenced that PtAAD could restore the synthesis of unsaturated fatty acid, especially generating high levels of 16:1Δ⁹. Further transient expression of PtAAD gene in Nicotiana benthamiana leaves was accompanied by the accumulation of 16:1Δ⁹, which was absent from control groups. Three-dimensional structure modeling studies showed that functional domain of PtAAD contained three variant amino acids (F160, A223, and L156), which may narrow the space shape of substrate-binding cavity to ensure the entry of 16:0-ACP. Consistent with this prediction, the mutated version of PtAAD gene (F160L, A223T, and L156M) in N. benthamiana systems failed to accumulate 16:1Δ⁹, but increased levels of 18:1Δ⁹. Taken together, PtAAD exhibits a strong enzymatic activity and substrate preference for 16:0-ACP, acting as the key player for high biosynthesis and accumulation of 16:1Δ⁹ in this alga. These findings provide new insights for better understanding the palmitoleic acid and oil biosynthetic mechanism in P. tricornutum, indicating that PtAAD gene may have practical applications for enriching palmitoleic acid and oil yield in other commercial oleaginous algae and crops.

Enhancing the Nutritional Quality of Major Food Crops Through Conventional and Genomics-Assisted Breeding

Nutritional stress is making over two billion world population malnourished. Either our commercially cultivated varieties of cereals, pulses, and oilseed crops are deficient in essential nutrients or the soils in which these crops grow are becoming devoid of minerals. Unfortunately, our major food crops are poor sources of micronutrients required for normal human growth. To overcome the problem of nutritional deficiency, greater emphasis should be laid on the identification of genes/quantitative trait loci (QTLs) pertaining to essential nutrients and their successful deployment in elite breeding lines through marker-assisted breeding. The manuscript deals with information on identified QTLs for protein content, vitamins, macronutrients, micro-nutrients, minerals, oil content, and essential amino acids in major food crops. These QTLs can be utilized in the development of nutrient-rich crop varieties. Genome editing technologies that can rapidly modify genomes in a precise way and will directly enrich the nutritional status of elite varieties could hold a bright future to address the challenge of malnutrition.

Biotechnology tools and applications for development of oilseed crops with healthy vegetable oils

Vegetable oils, consisting principally of triacylglycerols (TAG), are major sources of calories and essential fatty acids in the human diet. The fatty acid composition of TAG is a primary determinant of the nutritional quality and health-promoting properties of vegetable oils. TAG fatty acid composition also affects the functionality and properties of vegetable oils in food applications and in food processing and preparation. Vegetable oils with improved nutritional and functional properties have been developed for oilseed crops by selection and breeding of fatty acid biosynthetic mutants. These efforts have been effective at generating vegetable oils with altered relative amounts of saturated and unsaturated fatty acids in seed TAG, but are constrained by insufficient genetic diversity for producing oils with "healthy" fatty acids that are not typically found in major oilseeds. The development and application of biotechnological tools have instead enabled the generation of oilseeds that produce novel fatty acid compositions with improved nutritional value by the introduction of genes from alternative sources, including plants, bacteria, and fungi. These tools have also allowed the generation of desired oil compositions that have proven difficult to obtain by breeding without compromised performance in selected oilseed crops. Here, we review biotechnological tools for increasing crop genetic diversity and their application for commercial or proof-of-principal development of oilseeds with expanded utility for food and feed applications and higher value nutritional and nutraceutical markets.

Towards model-driven characterization and manipulation of plant lipid metabolism

Plant lipids have versatile applications and provide essential fatty acids in human diet. Therefore, there has been a growing interest to better characterize the genetic basis, regulatory networks, and metabolic pathways that shape lipid quantity and composition. Addressing these issues is challenging due to context-specificity of lipid metabolism integrating environmental, developmental, and tissue-specific cues. Here we systematically review the known metabolic pathways and regulatory interactions that modulate the levels of storage lipids in oilseeds. We argue that the current understanding of lipid metabolism provides the basis for its study in the context of genome-wide plant metabolic networks with the help of approaches from constraint-based modeling and metabolic flux analysis. The focus is on providing a comprehensive summary of the state-of-the-art of modeling plant lipid metabolic pathways, which we then contrast with the existing modeling efforts in yeast and microalgae. We then point out the gaps in knowledge of lipid metabolism, and enumerate the recent advances of using genome-wide association and quantitative trait loci mapping studies to unravel the genetic regulations of lipid metabolism. Finally, we offer a perspective on how advances in the constraint-based modeling framework can propel further characterization of plant lipid metabolism and its rational manipulation.

Plant unusual fatty acids: learning from the less common

The plant kingdom contains an abundance of structurally diverse fatty acids referred to as unusual fatty acids. Unusual fatty acids on plant surfaces can form polyesters that contribute to the function of cutin as a barrier for water loss and pathogen protection. Unusual fatty acids are also found as abundant components of seed oils of selected species and often confer desirable properties for industrial and nutritional applications. Here, we review recent findings on the biosynthesis and metabolism of unusual fatty acids in cutin and seed oils and use of this information for enzyme structure-function studies and seed oil metabolic engineering. We also highlight the recent discovery of unusual fatty acids that are formed from a previously undescribed variation of fatty acid elongation.

Metabolic Engineering a Model Oilseed Camelina sativa for the Sustainable Production of High-Value Designed Oils

Camelina sativa (L.) Crantz is an important Brassicaceae oil crop with a number of excellent agronomic traits including low water and fertilizer input, strong adaptation and resistance. Furthermore, its short life cycle and easy genetic transformation, combined with available data of genome and other "-omics" have enabled camelina as a model oil plant to study lipid metabolism regulation and genetic improvement. Particularly, camelina is capable of rapid metabolic engineering to synthesize and accumulate high levels of unusual fatty acids and modified oils in seeds, which are more stable and environmentally friendly. Such engineered camelina oils have been increasingly used as the super resource for edible oil, health-promoting food and medicine, biofuel oil and high-valued chemical production. In this review, we mainly highlight the latest advance in metabolic engineering towards the predictive manipulation of metabolism for commercial production of desirable bio-based products using camelina as an ideal platform. Moreover, we deeply analysis camelina seed metabolic engineering strategy and its promising achievements by describing the metabolic assembly of biosynthesis pathways for acetyl glycerides, hydroxylated fatty acids, medium-chain fatty acids, ω-3 long-chain polyunsaturated fatty acids, palmitoleic acid (ω-7) and other high-value oils. Future prospects are discussed, with a focus on the cutting-edge techniques in camelina such as genome editing application, fine directed manipulation of metabolism and future outlook for camelina industry development.

Whole-genome mapping identified novel "QTL hotspots regions" for seed storability in soybean (Glycine max L.)

BACKGROUND

Seed aging in soybean is a serious challenge for agronomic production and germplasm preservation. However, its genetic basis remains largely unclear in soybean. Unraveling the genetic mechanism involved in seed aging, and enhancing seed storability is an imperative goal for soybean breeding. The aim of this study is to identify quantitative trait loci (QTLs) using high-density genetic linkage maps of soybean for seed storability. In this regard, two recombinant inbred line (RIL) populations derived from Zhengyanghuangdou × Meng 8206 (ZM6) and Linhefenqingdou × Meng 8206 (LM6) crosses were evaluated for three seed-germination related traits viz., germination rate (GR), normal seedling length (SL) and normal seedling fresh weight (FW) under natural and artificial aging conditions to map QTLs for seed storability.

RESULTS

A total of 34 QTLs, including 13 QTLs for GR, 11 QTLs for SL and 10 QTLs for FW, were identified on 11 chromosomes with the phenotypic variation ranged from 7.30 to 23.16% under both aging conditions. All these QTLs were novel, and 21 of these QTLs were clustered in five QTL-rich regions on four different chromosomes viz., Chr3, Chr5, Chr17 &Chr18, among them the highest concentration of seven and six QTLs were found in "QTL hotspot A" (Chr17) and "QTL hotspot B" (Chr5), respectively. Furthermore, QTLs within all the five QTL clusters are linked to at least two studied traits, which is also supported by highly significant correlation between the three germination-related traits. QTLs for seed-germination related traits in "QTL hotspot B" were found in both RIL populations and aging conditions, and also QTLs underlying "QTL hotspot A" are identified in both RIL populations under artificial aging condition. These are the stable genomic regions governing the inheritance of seed storability in soybean, and will be the main focus for soybean breeders.

CONCLUSION

This study uncovers the genetic basis of seed storability in soybean. The newly identified QTLs provides valuable information, and will be main targets for fine mapping, candidate gene identification and marker-assisted breeding. Hence, the present study is the first report for the comprehensive and detailed investigation of genetic architecture of seed storability in soybean.

Harnessing Biotechnology for the Development of New Seed Lipid Traits in Brassica

The seed oil quality of Brassica oilseed species has been improved in the last few decades, using conventional breeding approaches. Modern biotechnology has enabled the significant development of new seed lipid traits in many oil crops. Alternation of seed lipid component with gene knockout by RNAi gene silencing, artificial microRNA or gene editing within the crop is relative straightforward. Introducing a new pathway from an exogenous source via biotechnology enables the creation of a new trait, where the biosynthetic pathway for such a new trait is not available in the host crop. This review updates the recent development of new seed lipid traits in six major Brassica species and highlights the capability of biotechnology to improve the composition of important fatty acids for both industrial and nutritional purposes.

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.

Stearoyl-ACP Δ9 Desaturase 6 and 8 (GhA-SAD6 and GhD-SAD8) Are Responsible for Biosynthesis of Palmitoleic Acid Specifically in Developing Endosperm of Upland Cotton Seeds

Palmitoleic acid (16:1Δ⁹) is one kind of ω-7 fatty acids (ω-7 FAs) widely used in food, nutraceutical and industry. However, such high-valued ω-7 FA only has a trace level in mature seeds of cotton and other common oil crops. We found that palmitoleic acid (>10.58 Mol%) was specially enriched in developing cotton endosperm which is disappeared in its mature seed. The present study was conducted to investigate the mechanism underlying high accumulation of palmitoleic acid in developing endosperm but not in embryo of upland cotton (Gossypium hirsutum L.) seed. Of 17 stearoyl-ACP Δ⁹ desaturases (SAD) gene family members identified in upland cotton, six GhSADs may specifically work in the desaturation of palmitic acid (16:0-ACP) to produce palmitoleic acid (16:1Δ⁹-ACP), which were revealed by examining the key amino acids in the catalytic center and their cis-elements. Gene expression analysis showed that spatial patterns of these GhSADs were different in developing ovules, with GhA-SAD6 and GhD-SAD8 preferentially expressed in developing endosperms. Functional analysis by transient expression in Nicotiana benthamiana leaves and genetic complementary assay using yeast mutant BY4389 strain unable to synthesize unsaturated fatty acids demonstrated that GhA-SAD6 and GhD-SAD8 have strong substrate specificity for 16:0-ACP. In contrast, GhA-SAD5 and GhA-SAD7 exhibited high specific activity on 18:0-ACP. Taken together, these data evidence that GhA-SAD6 and GhD-SAD8 are responsible for making palmitoleic acid in developing cotton endosperms, and provide endogenous gene targets for genetic modification to enrich ω-7 FAs in cotton seed oil required for sustainable production of functionality-valued products.

Microbial Lipid Alternatives to Plant Lipids

Lipids are in high demand in food production, nutritional supplements, detergents, lubricants, and biofuels. Different oil seeds produced from plants are conventionally extracted to yield lipids. With increasing population and reduced availability of cultivable land, conventional methods of producing lipids alone will not satisfy increasing demand. Lipids produced using different microbial sources are considered as sustainable alternative to plant derived lipids. Various microorganisms belonging to the genera of algae, bacteria, yeast, fungi, or marine-derived microorganisms such as thraustochytrids possess the ability to accumulate lipids in their cells. A variety of microbial production technologies are being used to cultivate these organisms under specific conditions using agricultural residues as carbon source to be cost competitive with plant derived lipids. Microbial oils, also known as single cell oils, have many advantages when compared with plant derived lipids, such as shorter life cycle, less labor required, season and climate independence, no use of arable land and ease of scale-up. In this chapter we compare the lipids derived from plants and different microorganisms. We also highlight various analytical techniques that are being used to characterize the lipids produced in oleaginous organisms and their applications in various processes.

Analysis of Interdependent Kinetic Controls of Fatty Acid Synthases

Biocatalytic systems (e.g., multienzyme pathways or complexes) enable the conversion of simple sugars into complex products under ambient conditions and, thus, represent promising platforms for the synthesis of renewable fuels and chemicals. Unfortunately, to date, many of these systems have proven difficult to engineer without a detailed understanding of the kinetic relationships that regulate the concerted action of their constituent enzymes. This study develops a mechanistic kinetic model of the fatty acid synthase (FAS) of Escherichia coli and uses that model to determine how different FAS components work together to control the production of free fatty acids-precursors to a wide range of oleochemicals. Perturbational analyses indicate that the modification or overexpression of a single FAS component can depress fatty acid production (a commonly observed phenomenon) by sequestering the proteins with which it interacts and/or by depleting common substrate pools. Compositional studies, in turn, suggest that simple changes in the ratios of FAS components can alter the average length of fatty acids but show that specialized enzymes (i.e., highly specific ketoacyl synthases or thioesterases) are required for narrow product profiles. Intriguingly, a sensitivity analysis indicates that two components primarily influence-and, thus, enable fine control over-total production, but suggests that the enzymes that regulate product profile are more broadly influential. Findings thus reveal the general importance of kinetic considerations in efforts to engineer fatty acid biosynthesis and provide strategies-and a kinetic model-for incorporating those considerations into FAS designs.

Camelina sativa, an oilseed at the nexus between model system and commercial crop

The rapid assessment of metabolic engineering strategies in plants is aided by crops that provide simple, high throughput transformation systems, a sequenced genome, and the ability to evaluate the resulting plants in field trials. Camelina sativa provides all of these attributes in a robust oilseed platform. The ability to perform field evaluation of Camelina is a useful, and in some studies essential benefit that allows researchers to evaluate how traits perform outside the strictly controlled conditions of a greenhouse. In the field the plants are subjected to higher light intensities, seasonal diurnal variations in temperature and light, competition for nutrients, and watering regimes dictated by natural weather patterns, all which may affect trait performance. There are difficulties associated with the use of Camelina. The current genetic resources available for Camelina pale in comparison to those developed for the model plant Arabidopsis thaliana; however, the sequence similarity of the Arabidopsis and Camelina genomes often allows the use of Arabidopsis as a reference when additional information is needed. Camelina's genome, an allohexaploid, is more complex than other model crops, but the diploid inheritance of its three subgenomes is straightforward. The need to navigate three copies of each gene in genome editing or mutagenesis experiments adds some complexity but also provides advantages for gene dosage experiments. The ability to quickly engineer Camelina with novel traits, advance generations, and bulk up homozygous lines for small-scale field tests in less than a year, in our opinion, far outweighs the complexities associated with the crop.

Seeds as oil factories

Studying seed oil metabolism. The seeds of higher plants represent valuable factories capable of converting photosynthetically derived sugars into a variety of storage compounds, including oils. Oils are the most energy-dense plant reserves and fatty acids composing these oils represent an excellent nutritional source. They supply humans with much of the calories and essential fatty acids required in their diet. These oils are then increasingly being utilized as renewable alternatives to petroleum for the chemical industry and for biofuels. Plant oils therefore represent a highly valuable agricultural commodity, the demand for which is increasing rapidly. Knowledge regarding seed oil production is extensively exploited in the frame of breeding programs and approaches of metabolic engineering for oilseed crop improvement. Complementary aspects of this research include (1) the study of carbon metabolism responsible for the conversion of photosynthetically derived sugars into precursors for fatty acid biosynthesis, (2) the identification and characterization of the enzymatic actors allowing the production of the wide set of fatty acid structures found in seed oils, and (3) the investigation of the complex biosynthetic pathways leading to the production of storage lipids (waxes, triacylglycerols). In this review, we outline the most recent developments in our understanding of the underlying biochemical and molecular mechanisms of seed oil production, focusing on fatty acids and oils that can have a significant impact on the emerging bioeconomy.

Towards the synthetic design of camelina oil enriched in tailored acetyl-triacylglycerols with medium-chain fatty acids

The ability to manipulate expression of key biosynthetic enzymes has allowed the development of genetically modified plants that synthesise unusual lipids that are useful for biofuel and industrial applications. By taking advantage of the unique activities of enzymes from different species, tailored lipids with a targeted structure can be conceived. In this study we demonstrate the successful implementation of such an approach by metabolically engineering the oilseed crop Camelina sativa to produce 3-acetyl-1,2-diacyl-sn-glycerols (acetyl-TAGs) with medium-chain fatty acids (MCFAs). Different transgenic camelina lines that had been genetically modified to produce MCFAs through the expression of MCFA-specific thioesterases and acyltransferases were retransformed with the Euonymus alatus gene for diacylglycerol acetyltransferase (EaDAcT) that synthesises acetyl-TAGs. Concomitant RNAi suppression of acyl-CoA:diacylglycerol acyltransferase increased the levels of acetyl-TAG, with up to 77 mole percent in the best lines. However, the total oil content was reduced. Analysis of the composition of the acetyl-TAG molecular species using electrospray ionisation mass spectrometry demonstrated the successful synthesis of acetyl-TAG containing MCFAs. Field growth of high-yielding plants generated enough oil for quantification of viscosity. As part of an ongoing design-test-learn cycle, these results, which include not only the synthesis of 'designer' lipids but also their functional analysis, will lead to the future production of such molecules tailored for specific applications.

Identification of bottlenecks in the accumulation of cyclic fatty acids in camelina seed oil

Modified fatty acids (mFA) have diverse uses; for example, cyclopropane fatty acids (CPA) are feedstocks for producing coatings, lubricants, plastics and cosmetics. The expression of mFA-producing enzymes in crop and model plants generally results in lower levels of mFA accumulation than in their natural-occurring source plants. Thus, to further our understanding of metabolic bottlenecks that limit mFA accumulation, we generated transgenic Camelina sativa lines co-expressing Escherichia coli cyclopropane synthase (EcCPS) and Sterculia foetida lysophosphatidic acid acyltransferase (SfLPAT). In contrast to transgenic CPA-accumulating Arabidopsis, CPA accumulation in camelina caused only minor changes in seed weight, germination rate, oil accumulation and seedling development. CPA accumulated to much higher levels in membrane than storage lipids, comprising more than 60% of total fatty acid in both phosphatidylcholine (PC) and phosphatidylethanolamine (PE) versus 26% in diacylglycerol (DAG) and 12% in triacylglycerol (TAG) indicating bottlenecks in the transfer of CPA from PC to DAG and from DAG to TAG. Upon co-expression of SfLPAT with EcCPS, di-CPA-PC increased by ~50% relative to lines expressing EcCPS alone with the di-CPA-PC primarily observed in the embryonic axis and mono-CPA-PC primarily in cotyledon tissue. EcCPS-SfLPAT lines revealed a redistribution of CPA from the sn-1 to sn-2 positions within PC and PE that was associated with a doubling of CPA accumulation in both DAG and TAG. The identification of metabolic bottlenecks in acyl transfer between site of synthesis (phospholipids) and deposition in storage oils (TAGs) lays the foundation for the optimizing CPA accumulation through directed engineering of oil synthesis in target crops.

High-level accumulation of oleyl oleate in plant seed oil by abundant supply of oleic acid substrates to efficient wax ester synthesis enzymes

BACKGROUND

Biotechnology enables the production of high-valued industrial feedstocks from plant seed oil. The plant-derived wax esters with long-chain monounsaturated acyl moieties, like oleyl oleate, have favorite properties for lubrication. For biosynthesis of wax esters using acyl-CoA substrates, expressions of a fatty acyl reductase (FAR) and a wax synthase (WS) in seeds are sufficient.

RESULTS

For optimization of the enzymatic activity and subcellular localization of wax ester synthesis enzymes, two fusion proteins were created, which showed wax ester-forming activities in Saccharomyces cerevisiae. To promote the formation of oleyl oleate in seed oil, WSs from Acinetobactor baylyi (AbWSD1) and Marinobacter aquaeolei (MaWS2), as well as the two created fusion proteins were tested in Arabidopsis to evaluate their abilities and substrate preference for wax ester production. The tested seven enzyme combinations resulted in different yields and compositions of wax esters. Expression of a FAR of Marinobacter aquaeolei (MaFAR) with AbWSD1 or MaWS2 led to a high incorporation of C₁₈ substrates in wax esters. The MaFAR/TMMmAWAT2-AbWSD1 combination resulted in the incorporation of more C_18:1 alcohol and C_18:0 acyl moieties into wax esters compared with MaFAR/AbWSD1. The fusion protein of a WS from Simmondsia chinensis (ScWS) with MaFAR exhibited higher specificity toward C_20:1 substrates in preference to C_18:1 substrates. Expression of MaFAR/AbWSD1 in the Arabidopsis fad2 fae1 double mutant resulted in the accumulation of oleyl oleate (18:1/18:1) in up to 62 mol% of total wax esters in seed oil, which was much higher than the 15 mol% reached by MaFAR/AbWSD1 in Arabidopsis Col-0 background. In order to increase the level of oleyl oleate in seed oil of Camelina, lines expressing MaFAR/ScWS were crossed with a transgenic high oleate line. The resulting plants accumulated up to >40 mg g seed^-1 of wax esters, containing 27-34 mol% oleyl oleate.

CONCLUSIONS

The overall yields and the compositions of wax esters can be strongly affected by the availability of acyl-CoA substrates and to a lesser extent, by the characteristics of wax ester synthesis enzymes. For synthesis of oleyl oleate in plant seed oil, appropriate wax ester synthesis enzymes with high catalytic efficiency and desired substrate specificity should be expressed in plant cells; meanwhile, high levels of oleic acid-derived substrates need to be supplied to these enzymes by modifying the fatty acid profile of developing seeds.

Cloning and characterization of a monoterpene synthase gene from flowers of Camelina sativa

CsTPS1 encodes for a monoterpene synthase that contributes to the emission of a blend of volatile compounds emitted from flowers of Camelina sativa. The work describes the in vitro characterization of a monoterpene synthase and its regulatory region that we cloned from Camelina sativa (Camelina). Here, we named this gene as C. sativa terpene synthase 1 (CsTPS1). In vitro experiments performed with the CsTPS1 protein after expression and purification from Escherichia coli (E. coli) showed production of a blend of monoterpene volatile organic compounds, of which the emission was also detected in the floral bouquet of wild-type Camelina plants. Quantitative-PCR measurements revealed a high abundance of CsTPS1 transcripts in flowers and experiments performed with the GUS reporter showed high CsTPS1 expression in the pistil, in the cells of the wall of the ovary and in the stigma. Subcellular localization of the CsTPS1 protein was investigated with a GFP reporter construct that showed expression in plastids. The CsTPS1 gene identified in this study belongs to a mid-size family of 60 genes putatively codifying for TPS enzymes. This enlarged family of TPS genes suggests that Camelina has the structural framework for the production of terpenes and other secondary metabolites of relevance for the consumers.

Overexpression of MYB115, AAD2, or AAD3 in Arabidopsis thaliana seeds yields contrasting omega-7 contents

Omega-7 monoenoic fatty acids (ω-7 FAs) are increasingly exploited both for their positive effects on health and for their industrial potential. Some plant species produce fruits or seeds with high amounts of ω-7 FAs. However, the low yields and poor agronomic properties of these plants preclude their commercial use. As an alternative, the metabolic engineering of oilseed crops for sustainable ω-7 FA production has been proposed. Two palmitoyl-ACP desaturases (PADs) catalyzing ω-7 FA biosynthesis were recently identified and characterized in Arabidopsis thaliana, together with MYB115 and MYB118, two transcription factors that positively control the expression of the corresponding PAD genes. In the present research, we examine the biotechnological potential of these new actors of ω-7 metabolism for the metabolic engineering of plant-based production of ω-7 FAs. We placed the PAD and MYB115 coding sequences under the control of a promoter strongly induced in seeds and evaluated these different constructs in A. thaliana. Seeds were obtained that exhibit ω-7 FA contents ranging from 10 to >50% of the total FAs, and these major compositional changes have no detrimental effect on seed germination.

Next-Generation Plant Metabolic Engineering, Inspired by an Ancient Chinese Irrigation System

Specialized secondary metabolites serve not only to protect plants against abiotic and biotic challenges, but have also been used extensively by humans to combat diseases. Due to the great importance of medicinal plants for health, we need to find new and sustainable ways to improve the production of the specialized metabolites. In addition to direct extraction, recent progress in metabolic engineering of plants offers an alternative supply option. We argue that metabolic engineering for producing the secondary metabolites in plants may have distinct advantages over microbial production platforms, and thus propose new approaches of plant metabolic engineering, which are inspired by an ancient Chinese irrigation system. Metabolic engineering strategies work at three levels: introducing biosynthetic genes, using transcription factors, and improving metabolic flux including increasing the supply of precursors, energy, and reducing power. In addition, recent progress in biotechnology contributes markedly to better engineering, such as the use of specific promoters and the deletion of competing branch pathways. We propose that next-generation plant metabolic engineering will improve current engineering strategies, for the purpose of producing valuable metabolites in plants on industrial scales.

Simultaneous Targeting of Multiple Gene Homeologs to Alter Seed Oil Production in Camelina sativa

The ability to transform Camelina sativa easily with biosynthetic enzymes derived from other plants has made this oil seed crop an ideal platform for the production of unusual lipids valuable for different applications. However, in addition to expressing transgenic enzymes, the suppression of endogenous enzyme activity to reduce competition for common substrates or cofactors is also required to enhance the production of target compounds. As camelina possesses a relatively undifferentiated hexaploid genome, up to three gene homeologs can code for any particular enzymatic activity, complicating efforts to alter endogenous biosynthetic pathways. New genome editing technologies, such as that offered by the CRISPR/Cas (clustered regularly interspaced short palindromic repeats/CRISPR-associated protein) system, offer the capability to introduce mutations into specifically targeted genomic sites. Here, by using a carefully designed guide RNA identical to all three homeologs, we demonstrate the ability of the CRISPR/Cas genome editing system to introduce mutations in all three CsDGAT1 or CsPDAT1 homeologous genes important for triacylglycerol (TAG) synthesis in developing seeds. Sequence analysis from transgenic T1 plants revealed that each CsDGAT1 or each CsPDAT1 homeolog was altered by multiple mutations, resulting in a genetic mosaic in the plants. Interestingly, seed harvested from both CsDGAT1- and CsPDAT1-targeted lines was often shrunken and wrinkled. Further, lipid analysis revealed that many lines produced seed with reduced oil content and altered fatty acid composition, consistent with the role of the targeted genes in seed oil biosynthesis. The CRISPR/Cas system therefore represents a useful method to alter endogenous biosynthetic pathways efficiently in polyploid species such as camelina.

The plant lipidome in human and environmental health

Lipids and oils derived from plant and algal photosynthesis constitute much of human daily caloric intake and provide the basis for high-energy bioproducts, chemical feedstocks for countless applications, and even fossil fuels over geological time scales. Sustainable production of high-energy compounds from plants is essential to preserving fossil fuel sources and ensuring the well-being of future generations. As a result of progress in basic research on plant and algal lipid metabolism, in combination with advances in synthetic biology, we can now tailor plant lipids for desirable biological, physical, and chemical properties. We highlight recent advances in plant lipid translational biology and discuss untapped areas of research that might expand the application of plant lipids.

Accumulation of medium-chain, saturated fatty acyl moieties in seed oils of transgenic Camelina sativa

With its high seed oil content, the mustard family plant Camelina sativa has gained attention as a potential biofuel source. As a bioenergy crop, camelina has many advantages. It grows on marginal land with low demand for water and fertilizer, has a relatively short life cycle, and is stress tolerant. As most other crop seed oils, camelina seed triacylglycerols (TAGs) consist of mostly long, unsaturated fatty acyl moieties, which is not desirable for biofuel processing. In our efforts to produce shorter, saturated chain fatty acyl moieties in camelina seed oil for conversion to jet fuel, a 12:0-acyl-carrier thioesterase gene, UcFATB1, from California bay (Umbellularia californica Nutt.) was expressed in camelina seeds. Up to 40% of short chain laurate (C12:0) and myristate (C14:0) were present in TAGs of the seed oil of the transgenics. The total oil content and germination rate of the transgenic seeds were not affected. Analysis of positions of these two fatty acyl moieties in TAGs indicated that they were present at the sn-1 and sn-3 positions, but not sn-2, on the TAGs. Suppression of the camelina KASII genes by RNAi constructs led to higher accumulation of palmitate (C16:0), from 7.5% up to 28.5%, and further reduction of longer, unsaturated fatty acids in seed TAGs. Co-transformation of camelina with both constructs resulted in enhanced accumulation of all three medium-chain, saturated fatty acids in camelina seed oils. Our results show that a California bay gene can be successfully used to modify the oil composition in camelina seed and present a new biological alternative for jet fuel production.

Expression of Camelina WRINKLED1 Isoforms Rescue the Seed Phenotype of the Arabidopsis wri1 Mutant and Increase the Triacylglycerol Content in Tobacco Leaves

Triacylglycerol (TAG) is an energy-rich reserve in plant seeds that is composed of glycerol esters with three fatty acids. Since TAG can be used as a feedstock for the production of biofuels and bio-chemicals, producing TAGs in vegetative tissue is an alternative way of meeting the increasing demand for its usage. The WRINKLED1 (WRI1) gene is a well-established key transcriptional regulator involved in the upregulation of fatty acid biosynthesis in developing seeds. WRI1s from Arabidopsis and several other crops have been previously employed for increasing TAGs in seed and vegetative tissues. In the present study, we first identified three functional CsWRI1 genes (CsWRI1A. B, and C) from the Camelina oil crop and tested their ability to induce TAG synthesis in leaves. The amino acid sequences of CsWRI1s exhibited more than 90% identity with those of Arabidopsis WRI1. The transcript levels of the three CsWRI1 genes showed higher expression levels in developing seeds than in vegetative and floral tissues. When the CsWRI1A. B, or C was introduced into Arabidopsis wri1-3 loss-of-function mutant, the fatty acid content was restored to near wild-type levels and percentages of the wrinkled seeds were remarkably reduced in the transgenic lines relative to wri1-3 mutant line. In addition, the fluorescent signals of the enhanced yellow fluorescent protein (eYFP) fused to the CsWRI1 genes were observed in the nuclei of Nicotiana benthamiana leaf epidermal cells. Nile red staining indicated that the transient expression of CsWRI1A. B, or C caused an enhanced accumulation of oil bodies in N. benthamiana leaves. The levels of TAGs was higher by approximately 2.5- to 4.0-fold in N. benthamiana fresh leaves expressing CsWRI1 genes than in the control leaves. These results suggest that the three Camelina WRI1s can be used as key transcriptional regulators to increase fatty acids in biomass.

The developmental transcriptome atlas of the biofuel crop Camelina sativa

Camelina sativa is currently being embraced as a viable industrial bio-platform crop due to a number of desirable agronomic attributes and the unique fatty acid profile of the seed oil that has applications for food, feed and biofuel. The recent completion of the reference genome sequence of C. sativa identified a young hexaploid genome. To complement this work, we have generated a genome-wide developmental transcriptome map by RNA sequencing of 12 different tissues covering major developmental stages during the life cycle of C. sativa. We have generated a digital atlas of this comprehensive transcriptome resource that enables interactive visualization of expression data through a searchable database of electronic fluorescent pictographs (eFP browser). An analysis of this dataset supported expression of 88% of the annotated genes in C. sativa and provided a global overview of the complex architecture of temporal and spatial gene expression patterns active during development. Conventional differential gene expression analysis combined with weighted gene expression network analysis uncovered similarities as well as differences in gene expression patterns between different tissues and identified tissue-specific genes and network modules. A high-quality census of transcription factors, analysis of alternative splicing and tissue-specific genome dominance provided insight into the transcriptional dynamics and sub-genome interplay among the well-preserved triplicated repertoire of homeologous loci. The comprehensive transcriptome atlas in combination with the reference genome sequence provides a powerful resource for genomics research which can be leveraged to identify functional associations between genes and understand the regulatory networks underlying developmental processes.