Engineering Trienoic Fatty Acids into Cottonseed Oil Improves Low-Temperature Seed Germination, Plant Photosynthesis and Cotton Fiber Quality

Alpha-linolenic acid (ALA, 18:3Δ9,12,15) and γ-linolenic acid \ (GLA, 18:3Δ6,9,12) are important trienoic fatty acids, which are beneficial for human health in their own right, or as precursors for the biosynthesis of long-chain polyunsaturated fatty acids. ALA and GLA in seed oil are synthesized from linoleic acid (LA, 18:2Δ9,12) by the microsomal ω-3 fatty acid desaturase (FAD3) and Δ6 desaturase (D6D), respectively. Cotton (Gossypium hirsutum L.) seed oil composition was modified by transforming with an FAD3 gene from Brassica napus and a D6D gene from Echium plantagineum, resulting in approximately 30% ALA and 20% GLA, respectively. The total oil content in transgenic seeds remained unaltered relative to parental seeds. Despite the use of a seed-specific promoter for transgene expression, low levels of GLA and increased levels of ALA were found in non-seed cotton tissues. At low temperature, the germinating cottonseeds containing the linolenic acid isomers elongated faster than the untransformed controls. ALA-producing lines also showed higher photosynthetic rates at cooler temperature and better fiber quality compared to both untransformed controls and GLA-producing lines. The oxidative stability of the novel cottonseed oils was assessed, providing guidance for potential food, pharmaceutical and industrial applications of these oils.

Biological Role of Unsaturated Fatty Acid Desaturases in Health and Disease

Polyunsaturated fatty acids (PUFAs) are considered one of the most important components of cells that influence normal development and function of many organisms, both eukaryotes and prokaryotes. Unsaturated fatty acid desaturases play a crucial role in the synthesis of PUFAs, inserting additional unsaturated bonds into the acyl chain. The level of expression and activity of different types of desaturases determines profiles of PUFAs. It is well recognized that qualitative and quantitative changes in the PUFA profile, resulting from alterations in the expression and activity of fatty acid desaturases, are associated with many pathological conditions. Understanding of underlying mechanisms of fatty acid desaturase activity and their functional modification will facilitate the development of novel therapeutic strategies in diseases associated with qualitative and quantitative disorders of PUFA.

Endoplasmic reticulum retention signaling and transmembrane channel proteins predicted for oilseed ω3 fatty acid desaturase 3 (FAD3) genes

Oilseed crop oils contain a variety of unsaturated fatty acids that are synthesized and regulated by fatty acid desaturases (FADs). In this study, 14 FAD3 (ω3 desaturase) protein sequences from oilseeds are analyzed and presented through the application of several computational tools. The results indicated a close relationship between Brassica napus and Camelina sativa, as well as between Salvia hispanica and Perilla frutescens FAD3s, due to a high similarity in codon preferences in codon usage clusters and the phylogenetic tree. The cis-acting element results reveal that the seed-specific promoter region of BnFAD3 contains the critical conserved boxes such as HSE and ABRE, which are involved in responsiveness to heat stress and abscisic acid. The presence of the aforementioned conserved boxes may increase cold acclimation as well as tolerance to drought and high salinity. Omega(ω)3 desaturases contain a Skn-1 motif which is a cis-acting regulatory element required involved in endosperm development. In oilseed FAD3s, leucine is the most repeated amino acid in FAD3 proteins. The study conveyed that B. napus, Camelina sativa, Linum usitatissimum, Vernicia fordii, Gossypium hirsutum, S. hispanica, Cannabis sativa, and P. frutescens have retention signal KXKXX/XKXX at their c-terminus sites, which is one of the most important characteristics of FADs. Additionally, it was found that BnFAD3 is a transmembrane protein that can convert ω6 to ω3 fatty acids and may simultaneously act as a potassium ion channel in the ER.

Ubiquitous Promoters Direct the Expression of Fatty Acid Delta-6 Desaturase from Nile Tilapia (Oreochromis niloticus) in Saccharomyces cerevisiae

In general, promoters have significant influence on recombinant protein production. Herein, we compared the performance of actin (pACT), phosphoglycerate kinase (pPGK), and translational elongation factor (pTEF) promoters for driving the expression of fatty acid delta-6 (Δ6) desaturase from Nile tilapia (Oreochromis niloticus; Oni-fads2) in Saccharomyces cerevisiae. Our results showed that by applying real-time RT-PCR, the highest level of Oni-fads2 mRNA was observed in S. cerevisiae carrying the expression vector driven by pTEF promoters. Exogenous substrate C18:2n-6 was used to determine Δ6 activity by quantitatively determining the C18:3n-6 product. The results showed that highest Δ6 desaturation was observed when using pTEF as a promoter. Recombinant S. cerevisiae cells expressing Oni-fads2 driven by pTEF were tested with the substrate C18:3n-3, and Δ6 desaturation efficiently converted C18:3n-3 to C18:4n-3. Furthermore, crude extract of recombinant yeast also exhibited Δ6 activity. Thus, recombinant S. cerevisiae cells expressing Oni-fads2 driven by the pTEF promoter have potential as a yeast factory for the sustainable production of long-chain polyunsaturated fatty acids.

Strong co-suppression impedes an increase in polyunsaturated fatty acids in seeds overexpressing FAD2

Fatty acid desaturase2 (FAD2) catalyses the conversion of oleic acid to linoleic acid and is the main determinant of the levels of essential poly-unsaturated fatty acids (PUFAs) in seed oils. The very limited number of successful examples of overexpression of FAD2 over the last two decades and a shortage of reports on co-suppression make it uncertain whether FAD2 can increase PUFAs effectively across a broad range of oil crops. In this study, strong co-suppression was observed in about 80% of over 100 transgenic lines when FAD2 was overexpressed in three oilseed crops, namely flax (Linum usitatissimum), carinata (Brassica carinata), and camelina (Camelina sativa), as well as in the model plant Arabidopsis. Further analyses of Arabidopsis transgenic lines revealed both endogenous and transgenic FAD2 gene-silencing. Thus, the commonality and potency of FAD2 co-suppression seemingly imposes an obstacle to engineering oilseed PUFA enhancement by direct FAD2 overexpression. AtFAD2, driven by the 35S promoter, also caused co-suppression in Arabidopsis roots. The FAD2 co-suppression was unstable and PUFA phenotypes of T4 lines were similar to the wild-type, further indicating that high PUFA content cannot be achieved by screening advanced generations. However, we demonstrate that the obstacle of FAD2 co-suppression can be overcome in the Arabidopsis rdr6 mutant, which is impaired in post-transcriptional gene-silencing, and that lines with high PUFA content are stable through four generations.

Dietary fish oil replacement by linseed oil: Effect on growth, nutrient utilization, tissue fatty acid composition and desaturase gene expression in silver barb (Puntius gonionotus) fingerlings

Silver barb (Puntius gonionotus) is considered a promising medium carp species for freshwater aquaculture in Asia. This study in silver barb was carried out to evaluate the effects of total or partial substitution of dietary fish oil (FO) with linseed oil (LO) on growth, nutrient utilization, whole-body composition, muscle and liver fatty acid composition. Fish (12.1±0.4g of initial body weight) were fed for 60days with five experimental iso-proteinous, iso-lipidic and iso-caloric diets in which FO (control diet) was replaced by 33.3%, 50%, 66.7% and 100% LO. Final weight, weight gain, percent weight gain, SGR decreased linearly (p<0.001) with increasing LO levels in the diets. Dietary LO substitution levels did not significantly (p>0.05) affect the feed conversion ratio (FCR), protein efficiency ratio (PER) and whole body proximate composition. Furthermore, enhanced level of LO increased α-linolenic acid (ALA; 18:3n3) and linoleic acid (LA; 18:2n6) and decreased eicosapentaenoic acid (EPA; 20:5n3) and docosahexaenoic acid (DHA; 22:6n3) in muscle and liver. To understand the molecular mechanism of long chain-polyunsaturated fatty acid (LC-PUFA) biosynthesis, we cloned and characterized the fatty acyl Δ6 desaturase (Δ6 fad) cDNA and investigated its expression in various organs/tissues following replacement of FO with LO in the diet. The full-length Δ6 fad cDNA was 2056bp encoding 444 amino acids and was widely expressed in various organs/tissues. Replacement of FO with LO increased the expression of Δ6 fad mRNA in liver, muscle and intestine but no significant difference was found in the brain.

Long-chain polyunsaturated fatty acid sources and evaluation of their nutritional and functional properties

Recent studies have clearly shown the importance of polyunsaturated fatty acids (as essential fatty acids) and their nutritional value for human health. In this review, various sources, nutritional properties, and metabolism routes of long-chain polyunsaturated fatty acids (LC-PUFA) are introduced. Since the conversion efficiency of linoleic acid (LA) to arachidonic acid (AA) and also α-linolenic acid (ALA) to docosahexaenoic acid (DHA) and eicosatetraenoic acid (EPA) is low in humans, looking for the numerous sources of AA, EPA and EPA fatty acids. The sources include aquatic (fish, crustaceans, and mollusks), animal sources (meat, egg, and milk), plant sources including 20 plants, most of which were weeds having a good amount of LC-PUFA, fruits, herbs, and seeds; cyanobacteria; and microorganisms (bacteria, fungi, microalgae, and diatoms).

Transgenic expression of delta-6 and delta-15 fatty acid desaturases enhances omega-3 polyunsaturated fatty acid accumulation in Synechocystis sp. PCC6803

BACKGROUND

Polyunsaturated fatty acids (PUFAs), which contain two or more double bonds in their backbone, are the focus of intensive global research, because of their nutritional value, medicinal applications, and potential use as biofuel. However, the ability to produce these economically important compounds is limited, because it is both expensive and technically challenging to separate omega-3 polyunsaturated fatty acids (ω-3 PUFAs) from natural oils. Although the biosynthetic pathways of some plant and microalgal ω-3 PUFAs have been deciphered, current understanding of the correlation between fatty acid desaturase content and fatty acid synthesis in Synechocystis sp. PCC6803 is incomplete.

RESULTS

We constructed a series of homologous vectors for the endogenous and exogenous expression of Δ6 and Δ15 fatty acid desaturases under the control of the photosynthesis psbA2 promoter in transgenic Synechocystis sp. PCC6803. We generated six homologous recombinants, harboring various fatty acid desaturase genes from Synechocystis sp. PCC6803, Gibberella fujikuroi and Mortierella alpina. These lines produced up to 8.9 mg/l of α-linolenic acid (ALA) and 4.1 mg/l of stearidonic acid (SDA), which are more than six times the corresponding wild-type levels, at 20°C and 30°C. Thus, transgenic expression of Δ6 and Δ15 fatty acid desaturases enhances the accumulation of specific ω-3 PUFAs in Synechocystis sp. PCC6803.

CONCLUSIONS

In the blue-green alga Synechocystis sp. PCC6803, overexpression of endogenous and exogenous genes encoding PUFA desaturases markedly increased accumulation of ALA and SDA and decreased accumulation of linoleic acid and γ-linolenic acid. This study lays the foundation for increasing the fatty acid content of cyanobacteria and, ultimately, for producing nutritional and medicinal products with high levels of essential ω-3 PUFAs.

ω3 fatty acid desaturases from microorganisms: structure, function, evolution, and biotechnological use

The biosynthesis of very-long-chain polyunsaturated fatty acids involves an alternating process of fatty acid desaturation and elongation catalyzed by complex series of enzymes. ω3 desaturase plays an important role in converting ω6 fatty acids into ω3 fatty acids. Genes for this desaturase have been identified and characterized in a wide range of microorganisms, including cyanobacteria, yeasts, molds, and microalgae. Like all fatty acid desaturases, ω3 desaturase is structurally characterized by the presence of three highly conserved histidine-rich motifs; however, unlike some desaturases, it lacks a cytochrome b5-like domain. Understanding the structure, function, and evolution of ω3 desaturases, particularly their substrate specificities in the biosynthesis of very-long-chain polyunsaturated fatty acids, lays the foundation for potential production of various ω3 fatty acids in transgenic microorganisms.

Characterization of fatty acid delta-6 desaturase gene in Nile tilapia and heterogenous expression in Saccharomyces cerevisiae

Fatty acid delta-6 desaturase (fads2)-like gene from Nile tilapia (Oreochromis niloticus) was characterized and designated as oni-fads2. The Oni-FADS2 showed the typical structure of microsomal FADS2. The presence of oni-fads2 transcripts in unfertilized eggs demonstrated the maternal role of Nile tilapia in providing the oni-fads2 transcript in their eggs. In addition, the expression of oni-fads2 was detectable in embryos throughout the hatching stage. Real-time reverse transcription-PCR revealed that oni-fads2 was expressed at a high level in all the brain regions, liver, and testis. Recombinant yeast (RY) was generated by transformation of Saccharomyces cerevisiae with the plasmid containing oni-fads2 driven by the Gal1 promoter (pYoni-fads2). The conspicuous expression of RY was detectable by RT-PCR after induction with galactose for 24h. When RY was induced with galactose, it exhibited 39% and 7% of delta-6 desaturase (∆6) activity toward C18:2n6 and C18:3n3, respectively. Additionally, it displayed 4% of delta-5 desaturase (∆5) activity toward C20:3n6, indicating that Oni-FADS2 had ∆5 and ∆6 bifunction.

Metabolic engineering of the omega-3 long chain polyunsaturated fatty acid biosynthetic pathway into transgenic plants

Omega-3 (ω-3) very long chain polyunsaturated fatty acids (VLC-PUFAs) such as eicosapentaenoic acid (EPA; 20:5 Δ5,8,11,14,17) and docosahexaenoic acid (DHA; 22:6 Δ4,7,10,13,16,19) have been shown to have significant roles in human health. Currently the primary dietary source of these fatty acids are marine fish; however, the increasing demand for fish and fish oil (in particular the expansion of the aquaculture industry) is placing enormous pressure on diminishing marine stocks. Such overfishing and concerns related to pollution in the marine environment have directed research towards the development of a viable alternative sustainable source of VLC-PUFAs. As a result, the last decade has seen many genes encoding the primary VLC-PUFA biosynthetic activities identified and characterized. This has allowed the reconstitution of the VLC-PUFA biosynthetic pathway in oilseed crops, producing transgenic plants engineered to accumulate ω-3 VLC-PUFAs at levels approaching those found in native marine organisms. Moreover, as a result of these engineering activities, knowledge of the fundamental processes surrounding acyl exchange and lipid remodelling has progressed. The application of new technologies, for example lipidomics and next-generation sequencing, is providing a better understanding of seed oil biosynthesis and opportunities for increasing the production of unusual fatty acids. Certainly, it is now possible to modify the composition of plant oils successfully, and, in this review, the most recent developments in this field and the challenges of producing VLC-PUFAs in the seed oil of higher plants will be described.

Transgenic oilseed crops as an alternative to fish oils

Growing evidence suggests that omega-3 long chain polyunsaturated fatty acids (VLC-PUFAs), especially eicosapentaenoic acid (EPA; 20:5Δ5,8,11,14,17) and docosahexaenoic acid (DHA; 22:6Δ4,7,10,13,16,19) play critical roles in human health and development. VLC-PUFAs are mainly found in fish, some fungi, marine bacteria and microalgae. Currently, the predominant dietary sources of VLC-PUFAs are marine fish and seafood. However, the increasing demand for fish and fish oils is putting enormous pressure on marine ecosystems leading to a depletion of fish stocks while commercial cultivation of marine microorganisms and aquaculture are not sustainable and cannot compensate for the shortage in fish supply. Therefore, there is an obvious requirement for an alternative and sustainable source for VLC-PUFAs. Over the last decade, many genes encoding the primary VLC-PUFAs biosynthetic activities became available providing a toolkit for the "reverse-engineering" of transgenic plants to produce fish oils. In this review, we will describe the recent advances in this field and the insights they give us into the complexities of metabolic engineering of oil-seed crops producing VLC-PUFAs.

The front-end desaturase: structure, function, evolution and biotechnological use

Very long chain polyunsaturated fatty acids such as arachidonic acid (ARA, 20:4n-6), eicosapentaenoic acid (EPA, 20:5n-3), docosapentaenoic acid (DPA, 22:5n-3) and docosahexaenoic acid (DHA, 22:6n-3) are essential components of cell membranes, and are precursors for a group of hormone-like bioactive compounds (eicosanoids and docosanoids) involved in regulation of various physiological activities in animals and humans. The biosynthesis of these fatty acids involves an alternating process of fatty acid desaturation and elongation. The desaturation is catalyzed by a unique class of oxygenases called front-end desaturases that introduce double bonds between the pre-existing double bond and the carboxyl end of polyunsaturated fatty acids. The first gene encoding a front-end desaturase was cloned in 1993 from cyanobacteria. Since then, front-end desaturases have been identified and characterized from a wide range of eukaryotic species including algae, protozoa, fungi, plants and animals including humans. Unlike front-end desaturases from bacteria, those from eukaryotes are structurally characterized by the presence of an N-terminal cytochrome b₅-like domain fused to the main desaturation domain. Understanding the structure, function and evolution of front-end desaturases, as well as their roles in the biosynthesis of very long chain polyunsaturated fatty acids offers the opportunity to engineer production of these fatty acids in transgenic oilseed plants for nutraceutical markets.

The production of unusual fatty acids in transgenic plants

The ability to genetically engineer plants has facilitated the generation of oilseeds synthesizing non-native fatty acids. Two particular classes of fatty acids are considered in this review. First, so-called industrial fatty acids, which usually contain functional groups such as hydroxyl, epoxy, or acetylenic bonds, and second, very long chain polyunsaturated fatty acids normally found in fish oils and marine microorganisms. For industrial fatty acids, there has been limited progress toward obtaining high-level accumulation of these products in transgenic plants. For very long chain polyunsaturated fatty acids, although they have a much more complex biosynthesis, accumulation of some target fatty acids has been remarkably successful. In this review, we consider the probable factors responsible for these different outcomes, as well as the potential for further optimization of the transgenic production of unusual fatty acids in transgenic plants.

Enzymes for transgenic biosynthesis of long-chain polyunsaturated fatty acids

Polyunsaturated fatty acids (PUFAs) are important for the normal development and function of all organisms, and are essential in maintaining human health. Impaired PUFA metabolism is thought to be associated with pathogenesis of many chronic diseases. Dietary supplementation of PUFAs, such as gamma-linolenic acid, arachidonic acid, eicosapentaenoic acid, and docosahexaenoic acid, which bypass the defective or dysfunctional steps of the biosynthetic pathway has been found to significantly alleviate the symptoms of the disease. These findings have drawn a great deal of interest from general public and food manufacturers. As the demand of these beneficial PUFAs has drastically increased in recent years, there are also increasing efforts in finding the alternate sources of PUFAs that are more economical and sustainable. One option is to modify the oil-seed crops to produce PUFAs through genetic engineering technique. This review examines the isolation, identification and expression of genes encoding the enzymes required for the biosynthesis of the above mentioned PUFAs in plants.

Isolation and characterization of a delta5 FA desaturase from Pythium irregulare by heterologous expression in Saccharomyces cerevisiae and oilseed crops

By using the polymerase chain reaction approach with two degenerate primers targeting the heme-binding and the third histidine-rich motifs in microsomal carboxyl-directed desaturases, we identified a cDNA PiD5 from Pythium irregulare encoding a delta5 desaturase. The substrate specificity of the enzyme was studied in detail by expressing PiD5 in a yeast (Saccharomyces cerevisiae) mutant strain, AMY-2alpha, where ole1, a delta9 desaturase gene, is disrupted. The result revealed that the encoded enzyme could desaturate unsaturated FA from 16 to 20 carbons beginning with delta9 and delta11 as well as delta8 ethylenic double bonds. Introduction of PiD5 into Brassica juncea under the control of a CaMV 35S constitutive promoter resulted in accumulation of several delta5-unsaturated polymethylene-interrupted FA (delta5-UPIFA) including 18:2-5,9, 18:2-5,11, 18:3-5,9,12, and 18:4-5,9,12,15 in vegetative tissues. The transgenic enzyme could also desaturate the exogenously supplied homo-gamma-linolenic acid (20:3-8,11,14) to arachidonic acid (20:4-5,8,11,14). Introduction of PiD5 into B. juncea and flax under the control of seed-specific promoters resulted in production of delta5-UPIFA, representing more than 10% of the total FA in the seeds.

High-level production of gamma-linolenic acid in Brassica juncea using a delta6 desaturase from Pythium irregulare

gamma-Linolenic acid (GLA), a nutritionally important fatty acid in mammals, is synthesized by a delta6 desaturase. Here, we report identification of PiD6, a new cDNA from the oleaginous fungus, Pythium irregulare, encoding a 459-amino acid protein that shares sequence similarity to carboxyl-directed desaturases from various species. Expression of PiD6 in yeast (Saccharomyces cerevisiae) revealed that it converts exogenously supplied linoleic acid into GLA, indicating that it encodes a delta6 fatty acid desaturase. Expression of the desaturase in Brassica juncea under the control of the Brassica napus napin promoter resulted in production of three delta6 unsaturated fatty acids (18:2-6, 9; 18:3-6, 9, 12; and 18:4-6, 9, 12, 15) in seeds. Among them, GLA (18:3-6, 9, 12) is the most abundant and accounts for up to 40% of the total seed fatty acids. Lipid class and positional analysis indicated that GLA is almost exclusively incorporated into triacylglycerol (98.5%) with only trace amounts found in the other lipids. Within triacylglycerols, GLA is more abundant at the sn-2 position.