Expression of a borage desaturase cDNA containing an N-terminal cytochrome b5 domain results in the accumulation of high levels of delta6-desaturated fatty acids in transgenic tobacco

Molecular cloning and functional analysis of a plastidial ω3 desaturase from Emiliania huxleyi

Emiliania huxleyi is a marine microalga playing a significant ecological and biogeochemical role in oceans. It can produce several polyunsaturated fatty acids (PUFAs), such as docosahexaenoic acid (DHA, 22:6-4,7,10,13,16,19) and octadecapentaenoic acid (OPA, 18:5-3,6,9,12,15), providing a primary source for nutritionally important ω3 PUFAs in the marine food chain. However, the biosynthesis of these PUFAs in this organism is not well understood. In this study, a full length plastidial ω3 desaturase cDNA (EhN3) was cloned from this alga. Heterologous expression of EhN3 with and without the chloroplast targeting peptide (cTP) in cyanobacterium Synechococcus elongatus showed that it possessed high desaturation activity toward C18-ω6 PUFAs, linoleic acid (LA, 18:2-9,12), γ-linolenic acid (GLA, 18:3-6,9,12), and C20-ω6 PUFAs, dihomo-γ-linolenic acid (DGLA, 20:3-8,11,14) and arachidonic acid (ARA, 20:4-5,8,11,14) that were exogenously supplied. Desaturation efficiency could reach almost 100% in a time course. On the other hand, when expressed in Saccharomyces cerevisiae, EhN3 with and without cTP did not exhibit any activity. Lipid analysis of Synechococcus transformants expressing EhN3 showed that it utilized galactolipids as substrates. Transcriptional expression analysis revealed that the expression of the gene increased while the growth temperature decreased, which was correlated with the increased production of ω3-PUFAs, particularly OPA. This is the first report of a plastidial ω3 desaturase from microalgae that can effectively introduce an ω3 double bond into both C18-ω6 and C20-ω6 PUFAs. EhN3 might also be one of the key enzymes involved in the biosynthesis of OPA in E. huxleyi through the plastidial aerobic pathway.

Acyl-Lipid Δ6-Desaturase May Act as a First FAD in Cyanobacteria

Fatty acid desaturases (FADs) play important roles in various metabolic and adaptive pathways in all living organisms. They represent a superfamily of oxygenases that introduce double bonds into the acyl chains of fatty acids (FAs). These enzymes are highly specific to the length of the carbon chain, position of double bonds formation, etc. The modes by which FADs "count" the position of the double bond formation may differ. In cyanobacteria, the first double bond is formed between 9th and 10th carbons (position Δ⁹), counting from the carboxylic end of an FA. Other FADs that produce polyunsaturated FAs may introduce double bonds counting from the carboxyl (Δ) or methyl (ω) terminus, or from a pre-existing double bond towards carboxyl or methyl terminus of an FA chain. Here, we expressed the desD gene for the Δ6-FAD from Synechocystis sp. PCC 6803 in Synechococcus elongatus PCC 7942 (which is capable of synthesizing only monoenoic FAs desaturated mainly at position Δ⁹) and observed the appearance of unusual monoenoic FAs desaturated at position Δ⁶, as well as Δ^6,9 dienoic FAs. Exogenously added cis-10-heptadecenoic acid (17:1Δ¹⁰) was converted into cis-6,10-heptadecadienoic (17:2Δ^6,10). These data demonstrate the ability of Δ6-FAD to introduce the first double bond into the unsaturated substrates and suggests that it "counts" from the carboxyl end, irrespective of the absence or presence of a previous double bond in an FA chain.

Microbes: A Hidden Treasure of Polyunsaturated Fatty Acids

Microbes have gained a lot of attention for their potential in producing polyunsaturated fatty acids (PUFAs). PUFAs are gaining scientific interest due to their important health-promoting effects on higher organisms including humans. The current sources of PUFAs (animal and plant) have associated limitations that have led to increased interest in microbial PUFAs as most reliable alternative source. The focus is on increasing the product value of existing oleaginous microbes or discovering new microbes by implementing new biotechnological strategies in order to compete with other sources. The multidisciplinary approaches, including metabolic engineering, high-throughput screening, tapping new microbial sources, genome-mining as well as co-culturing and elicitation for the production of PUFAs, have been considered and discussed in this review. The usage of agro-industrial wastes as alternative low-cost substrates in fermentation for high-value single-cell oil production has also been discussed. Multidisciplinary approaches combined with new technologies may help to uncover new microbial PUFA sources that may have nutraceutical and biotechnological importance.

A Cytochrome B5-Like Heme/Steroid Binding Domain Protein, PlCB5L1, Regulates Mycelial Growth, Pathogenicity and Oxidative Stress Tolerance in Peronophythora litchii

As an electron transport component, cytochrome b₅ is an essential component of the Class II cytochrome P450 monooxygenation system and widely present in animals, plants, and fungi. However, the roles of Cyt-b₅ domain proteins in pathogenic oomycetes remain unknown. Peronophythora litchii is an oomycete pathogen that causes litchi downy blight, the most destructive disease of litchi. In this study, we identified a gene, designated PlCB5L1, that encodes a Cyt-b₅ domain protein in P. litchii, and characterized its function. PlCB5L1 is highly expressed in the zoospores, cysts, germinated cysts, and during early stages of infection. PlCB5L1 knockout mutants showed reduced growth rate and β-sitosterol utilization. Importantly, we also found that PlCB5L1 is required for the full pathogenicity of P. litchii. Compared with the wild-type strain, the PlCB5L1 mutants exhibited significantly higher tolerance to SDS and sorbitol, but impaired tolerance to cell wall stress, osmotic stress, and oxidative stress. Further, the expression of genes involved in oxidative stress tolerance, including peroxidase, cytochrome P450, and laccase genes, were down-regulated in PlCB5L1 mutants under oxidative stress. This is the first report that a Cyt-b₅ domain protein contributes to the development, stress response, and pathogenicity in plant pathogenic oomycetes.

Natural ultraviolet radiation exposure alters photosynthetic biology and improves recovery from desiccation in a desert moss

Plants in dryland ecosystems experience extreme daily and seasonal fluctuations in light, temperature, and water availability. We used an in situ field experiment to uncover the effects of natural and reduced levels of ultraviolet radiation (UV) on maximum PSII quantum efficiency (Fv/Fm), relative abundance of photosynthetic pigments and antioxidants, and the transcriptome in the desiccation-tolerant desert moss Syntrichia caninervis. We tested the hypotheses that: (i) S. caninervis plants undergo sustained thermal quenching of light [non-photochemical quenching (NPQ)] while desiccated and after rehydration; (ii) a reduction of UV will result in improved recovery of Fv/Fm; but (iii) 1 year of UV removal will de-harden plants and increase vulnerability to UV damage, indicated by a reduction in Fv/Fm. All field-collected plants had extremely low Fv/Fm after initial rehydration but recovered over 8 d in lab-simulated winter conditions. UV-filtered plants had lower Fv/Fm during recovery, higher concentrations of photoprotective pigments and antioxidants such as zeaxanthin and tocopherols, and lower concentrations of neoxanthin and Chl b than plants exposed to near natural UV levels. Field-grown S. caninervis underwent sustained NPQ that took days to relax and for efficient photosynthesis to resume. Reduction of solar UV radiation adversely affected recovery of Fv/Fm following rehydration.

Multifaceted regulation and functions of fatty acid desaturase 2 in human cancers

As an important hallmark of metabolic reprogramming in cancer, a disruption in fatty acid metabolism contributes to tumor proliferation, cell migration and invasion, and other tumor cell behaviors. In recent years, more and more studies have been conducted on fatty acid desaturase 2 (FADS2), the first rate-limiting enzyme for the biosynthesis of polyunsaturated fatty acids. These studies have found that FADS2 is abnormally expressed in cancers of the breast, lung, liver, and esophagus; melanoma; leukemia; and other malignant tumors. Furthermore, its expression is significantly correlated with tumor proliferation, cell migration and invasion, clonal formation, angiogenesis, ferroptosis, resistance to radiotherapy, histological grade, metastasis to lymph nodes, clinical stage, and prognosis. The abnormal expression of FADS2 results in an imbalance of cell membrane phospholipids, which disrupts the fluidity of the membrane structure and the transmission of signals and promotes the production of proinflammatory factors and arachidonic acid (AA) metabolites, ultimately harming human health. This article aims to systematically review the structural characteristics of FADS2; its function, expression, and mechanism of action; and the factors affecting its activity. This review also provides new ideas and strategies for the development of treatments aimed at the metabolic reprogramming of tumors.

Δ6 fatty acid desaturases in polyunsaturated fatty acid biosynthesis: insights into the evolution, function with substrate specificities and biotechnological use

Δ6 fatty acid desaturases (FADS6) have different substrate specificities that impact the ratio of omega-6/omega-3 polyunsaturated fatty acids, which are involved in regulating multiple signalling pathways associated with various diseases. For decades, FADS6 with different substrate specificities have been characterized and the functions of these crucial enzymes have been investigated, while it remains enigmatic that the substrate specificities of FADS6 from various species have a huge difference. This review summarizes the substrate specificities of FADS6 in different species and reveals the underlying relationship. Further evaluation of biochemical properties has revealed that the FADS6 prefer linoleic acid that is more hydrophilic and stable. Domain-swapping and site-directed mutagenesis have been employed to delineate the regions and sites that affect the substrate specificities of FADS6. These analyses improve our understanding of the functions of FADS6 and offer information for the discovery of novel biological resources. KEY POINTS: • Outline of the excavation and identification of Δ6 fatty acid desaturases. • Overview of methods used to determine the pivotal resides of desaturases. • Application of substrate properties to generate specific fatty acids.

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.

Plastidic Δ6 Fatty-Acid Desaturases with Distinctive Substrate Specificity Regulate the Pool of C18-PUFAs in the Ancestral Picoalga Ostreococcus tauri

Eukaryotic Δ6-desaturases are microsomal enzymes that balance the synthesis of ω-3 and ω-6 C18-polyunsaturated fatty acids (C18-PUFAs) according to their specificity. In several microalgae, including Ostreococcus tauri, plastidic C18-PUFAs are strictly regulated by environmental cues suggesting an autonomous control of Δ6-desaturation of plastidic PUFAs. Here, we identified two putative front-end Δ6/Δ8-desaturases from O tauri that, together with putative homologs, cluster apart from other characterized Δ6-desaturases. Both were plastid-located and unambiguously displayed a Δ6-desaturation activity when overexpressed in the heterologous hosts Nicotiana benthamiana and Synechocystis sp. PCC6803, as in the native host. Detailed lipid analyses of overexpressing lines unveiled distinctive ω-class specificities, and most interestingly pointed to the importance of the lipid head-group and the nonsubstrate acyl-chain for the desaturase efficiency. One desaturase displayed a broad specificity for plastidic lipids and a preference for ω-3 substrates, while the other was more selective for ω-6 substrates and for lipid classes including phosphatidylglycerol as well as the peculiar 16:4-galactolipid species occurring in the native host. Overexpression of both Δ6-desaturases in O tauri prevented the regulation of C18-PUFA under phosphate deprivation and triggered glycerolipid fatty-acid remodeling, without causing any obvious alteration in growth or photosynthesis. Tracking fatty-acid modifications in eukaryotic hosts further suggested the export of plastidic lipids to extraplastidic compartments.

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.

The expression of several pepper fatty acid desaturase genes is robustly activated in an incompatible pepper-tobamovirus interaction, but only weakly in a compatible interaction

The replication of positive strand RNA viruses in plant cells is markedly influenced by the desaturation status of fatty acid chains in lipids of intracellular plant membranes. At present, little is known about the role of lipid desaturation in the replication of tobamoviruses. Therefore, we investigated the expression of fatty acid desaturase (FAD) genes and the fatty acid composition of pepper leaves inoculated with two different tobamoviruses. Obuda pepper virus (ObPV) inoculation induced a hypersensitive reaction (incompatible interaction) while Pepper mild mottle virus (PMMoV) inoculation caused a systemic infection (compatible interaction). Changes in the expression of 16 FADs were monitored in pepper leaves following ObPV and PMMoV inoculations. ObPV inoculation rapidly and markedly upregulated seven Δ12-FADs that encode enzymes putatively located in the endoplasmic reticulum membrane. In contrast, PMMoV inoculation resulted in a weaker but rapid upregulation of two Δ12-FADs and a Δ15-FAD. The expression of genes encoding plastidial FADs was not influenced neither by ObPV nor by PMMoV. In accordance with gene expression results, a significant accumulation of linoleic acid was observed by gas chromatography-mass spectrometry in ObPV-, but not in PMMoV-inoculated leaves. ObPV inoculation led to a marked accumulation of H₂O₂ in the inoculated leaves. Therefore, the effect of H₂O₂ treatments on the expression of six tobamovirus-inducible FADs was also studied. The expression of these FADs was upregulated to different degrees by H₂O₂ that correlated with ObPV-inducibility of these FADs. These results underline the importance of further studies on the role of pepper FADs in pepper-tobamovirus interactions.

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.

Functional Characterization of Lysophosphatidylcholine: Acyl-CoA Acyltransferase Genes From Sunflower (Helianthus annuus L.)

Lysophosphatidylcholine acyltransferase (LPCAT, EC 2.3.1.23) is an evolutionarily conserved key enzyme in the Lands cycle that catalyzes acylation of lysophosphatidylcholine (LPC) to produce phosphatidylcholine (PC), the main phospholipid in cellular membranes. In this study, three LPCAT genes from sunflower were identified and the corresponding proteins characterized. These HaLPCAT genes encoded functionally active enzymes that were able to complement a deficient yeast mutant. Moreover, enzymatic assays were carried out using microsomal preparations of the yeast cells. When acyl specificities were measured in the forward reaction, these enzymes exhibited a substrate preference for unsaturated acyl-CoAs, especially for linolenoyl-CoA, while in the reverse reaction, linoleoyl or linolenoyl acyl groups were transferred from PC to acyl-CoA to a similar extent. Expression levels of LPCAT genes were studied revealing distinct tissue-specific expression patterns. In summary, this study suggests that the combined forward and reverse reactions catalyzed by sunflower LPCATs facilitate acyl-exchange between the sn-2 position of PC and the acyl-CoA pool. Sunflower LPCATs displayed different characteristics, which could point to different functionalities, favoring the enrichment of seed triacylglycerols (TAGs) with polyunsaturated fatty acid (PUFA).

Identification and functional characterization of Buglossoides arvensis microsomal fatty acid desaturation pathway genes involved in polyunsaturated fatty acid synthesis in seeds

Buglossoides arvensis seed oil is the richest natural source of stearidonic acid (SDA), an ω-3 fatty acid with nutraceutical potential superior to α-linolenic acid (ALA). The molecular basis of polyunsaturated fatty acid synthesis in B. arvensis is unknown. Here, we describe the identification of B. arvensis fatty acid desaturase2 (BaFAD2), fatty acid desaturase3 (BaFAD3), and Delta-6-desaturase (BaD6D-1 and BaD6D-2) genes by mining the transcriptome of developing seeds and their functional characterization by heterologous expression in Saccharomyces cerevisiae. In silico analysis of their encoded protein sequences showed conserved histidine-boxes and signature motifs essential for desaturase activity. Expression profiling of these genes showed higher transcript abundance in reproductive tissues than in vegetative tissues, and their expression varied with temperature stress treatments. Yeast expressing BaFAD2 was found to desaturate both oleic acid and palmitoleic acid into linoleic acid (LA) and hexadecadienoic acid, respectively. Fatty acid supplementation studies in yeast expressing BaFAD3 and BaD6D-1 genes revealed that the encoded enzyme activities of BaFAD3 efficiently converted LA to ALA, and BaD6D-1 converted LA to γ-linolenic acid and ALA to SDA, but with an apparent preference to LA. BaD6D-2 did not show the encoded enzyme activity and is not a functional D6D. Our results provide an insight into SDA biosynthesis in B. arvensis and expand the repository of fatty acid desaturase targets available for biotechnological production of SDA in traditional oilseed crops.

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.

High accumulation of γ-linolenic acid and Stearidonic acid in transgenic Perilla (Perilla frutescens var. frutescens) seeds

BACKGROUND

Polyunsaturated fatty acids such as linoleic acid (LA) and α-linolenic acid (ALA) are abundant in vegetable oils and are important for human health. In the body, LA and ALA are respectively converted to the omega-6 fatty acid γ-linolenic acid (GLA) and the omega-3 fatty acid stearidonic acid (SDA) by Δ6 desaturase (D6DES). Currently, dietary GLA and SDA are mainly obtained from marine organisms, but given their benefits to human health, many studies have aimed to enhance their accumulation in transgenic crops. Perilla frutescens (perilla) accumulates more ALA in its seed oil compared to other oilseed crops, making it a good candidate for the production of fatty acids via the fatty acid desaturase D6DES.

RESULTS

In this study, we cloned the D6DES gene from Phytophthora citrophthora and confirmed its function in budding yeast. We then transformed the functional D6DES gene under the control of the seed-specific vicilin promoter into the perilla cultivar Yeobsil. The resulting transgenic perilla seeds accumulated significant levels of GLA and SDA, as well as putative C18:2Δ^6,9 at minor levels. Developing seeds and leaves also accumulated GLA and SDA, although PcD6DES expression and GLA and SDA levels were much lower in leaves compared to developing seeds. GLA and SDA accumulated in both polar lipids and neutral lipids in mature perilla seeds expressing PcD6DES, especially in neutral lipids. Although the seed weight in PcD6DES perilla was 87-96% that of wild type, the total oil content per seed weight was similar between lines. The PcD6DES perilla plants contained very high content (over 45%) of both GLA and SDA in seed oil.

CONCLUSIONS

Thus, PcD6DES perilla plants may represent a feasible alternative to traditional marine sources for the production of omega-3 oil capsules and to evening primrose seed oil for GLA as health food. In addition, these plants can be used to create other transgenic lines harboring additional genes to produce other desirable fish-oil like oils.

Insights into the phylogenetic and molecular evolutionary histories of Fad and Elovl gene families in Actiniaria

The biosynthesis of long-chain polyunsaturated fatty acids (LC-PUFAs, ≥ C20) is reliant on the action of desaturase and elongase enzymes, which are encoded by the fatty acyl desaturase (Fad) and elongation of very long-chain fatty acid (Elovl) gene families, respectively. In Metazoa, research investigating the distribution and evolution of these gene families has been restricted largely to Bilateria. Here, we provide insights into the phylogenetic and molecular evolutionary histories of the Fad and Elovl gene families in Cnidaria, the sister phylum to Bilateria. Four model cnidarian genomes and six actiniarian transcriptomes were interrogated. Analysis of the fatty acid composition of a candidate cnidarian species, Actinia tenebrosa, was performed to determine the baseline profile of this species. Phylogenetic analysis revealed lineage-specific gene duplication in actiniarians for both the Fad and Elovl gene families. Two distinct cnidarian Fad clades clustered with functionally characterized Δ5 and Δ6 proteins from fungal and plant species, respectively. Alternatively, only a single cnidarian Elovl clade clustered with functionally characterized Elovl proteins (Elovl4), while two additional clades were identified, one actiniarian-specific (Novel ElovlA) and the another cnidarian-specific (Novel ElovlB). In actiniarians, selection analyses revealed pervasive purifying selection acting on both gene families. However, codons in the Elovl gene family show patterns of nucleotide variation consistent with the action of episodic diversifying selection following gene duplication events. Significantly, these codons may encode amino acid residues that are functionally important for Elovl proteins to target and elongate different precursor fatty acids. In A. tenebrosa, the fatty acid analysis revealed an absence of LC-PUFAs > C20 molecules and implies that the Elovl enzymes are not actively contributing to the elongation of these LC-PUFAs. Overall, this study has revealed that actiniarians possess Fad and Elovl genes required for the biosynthesis of some LC-PUFAs, and that these genes appear to be distinct from bilaterians.

The Linoleic Acid: Dihomo-γ-Linolenic Acid Ratio (LA:DGLA)-An Emerging Biomarker of Zn Status

Zinc (Zn) deficiency is a common aliment predicted to affect 17% of the world's population. Zinc is a vital micronutrient used for over 300 enzymatic reactions and multiple biochemical and structural processes in the body. Although whole blood, plasma, and urine zinc decrease in severe zinc deficiency, accurate assessment of zinc status, especially in mild to moderate deficiency, is difficult as studies with these biomarkers are often contradictory and inconsistent. Hence, as suggested by the World Health Organization, sensitive and specific biological markers of zinc status are still needed. In this review, we provide evidence to demonstrate that the LA:DGLA ratio (linoleic acid:dihomo-γ-linolenic acid ratio) may be a useful additional indicator for assessing Zn status more precisely. However, this biomarker needs to be tested further in order to determine its full potential.

In Silico Analysis of Fatty Acid Desaturase Genes and Proteins in Grasses

Fatty acid desaturases (FADs) catalyze the introduction of a double bond into acyl chains. Two FAD groups have been identified in plants: acyl-acyl carrier proteins (ACPs) and acyl-lipid or membrane-bound FAD. The former catalyze the conversion of 18:0 to 18:1 and to date have only been identified in plants. The latter are found in eukaryotes and bacteria and are responsible for multiple desaturations. In this study, we identified 82 desaturase gene and protein sequences from 10 grass species deposited in GenBank that were analyzed using bioinformatic approaches. Subcellular localization predictions of desaturase family revealed their localization in plasma membranes, chloroplasts, endoplasmic reticula, and mitochondria. The in silico mapping showed multiple chromosomal locations in most species. Furthermore, the presence of the characteristic histidine domains, the predicted motifs, and the finding of transmembrane regions strongly support the protein functionality. The identification of putative regulatory sites in the promotor and the expression profiles revealed the wide range of pathways in which fatty acid desaturases are involved. This study is an updated survey on desaturases of grasses that provides a comprehensive insight into diversity and evolution. This characterization is a necessary first step before considering these genes as candidates for new biotechnological approaches.

Towards the development of a sustainable soya bean-based feedstock for aquaculture

Soya bean (Glycine max (L.) Merr.) is sought after for both its oil and protein components. Genetic approaches to add value to either component are ongoing efforts in soya bean breeding and molecular biology programmes. The former is the primary vegetable oil consumed in the world. Hence, its primary usage is in direct human consumption. As a means to increase its utility in feed applications, thereby expanding the market of soya bean coproducts, we investigated the simultaneous displacement of marine ingredients in aquafeeds with soya bean-based protein and a high Omega-3 fatty acid soya bean oil, enriched with alpha-linolenic and stearidonic acids, in both steelhead trout (Oncorhynchus mykiss) and Kampachi (Seriola rivoliana). Communicated herein are aquafeed formulations with major reduction in marine ingredients that translates to more total Omega-3 fatty acids in harvested flesh. Building off of these findings, subsequent efforts were directed towards a genetic strategy that would translate to a prototype design of an optimal identity-preserved soya bean-based feedstock for aquaculture, whereby a multigene stack approach for the targeted synthesis of two value-added output traits, eicosapentaenoic acid and the ketocarotenoid, astaxanthin, were introduced into the crop. To this end, the systematic introduction of seven transgenic cassettes into soya bean, and the molecular and phenotypic evaluation of the derived novel events are described.

Targeted metabolomics shows plasticity in the evolution of signaling lipids and uncovers old and new endocannabinoids in the plant kingdom

The remarkable absence of arachidonic acid (AA) in seed plants prompted us to systematically study the presence of C20 polyunsaturated fatty acids, stearic acid, oleic acid, jasmonic acid (JA), N-acylethanolamines (NAEs) and endocannabinoids (ECs) in 71 plant species representative of major phylogenetic clades. Given the difficulty of extrapolating information about lipid metabolites from genetic data we employed targeted metabolomics using LC-MS/MS and GC-MS to study these signaling lipids in plant evolution. Intriguingly, the distribution of AA among the clades showed an inverse correlation with JA which was less present in algae, bryophytes and monilophytes. Conversely, ECs co-occurred with AA in algae and in the lower plants (bryophytes and monilophytes), thus prior to the evolution of cannabinoid receptors in Animalia. We identified two novel EC-like molecules derived from the eicosatetraenoic acid juniperonic acid, an omega-3 structural isomer of AA, namely juniperoyl ethanolamide and 2-juniperoyl glycerol in gymnosperms, lycophytes and few monilophytes. Principal component analysis of the targeted metabolic profiles suggested that distinct NAEs may occur in different monophyletic taxa. This is the first report on the molecular phylogenetic distribution of apparently ancient lipids in the plant kingdom, indicating biosynthetic plasticity and potential physiological roles of EC-like lipids in plants.

Internalization and Phytotoxic Effects of CuO Nanoparticles in Arabidopsis thaliana as Revealed by Fatty Acid Profiles

Internalization and phytotoxic effects of CuO nanoparticles (nCuO) in plants were studied at the cellular level. Arabidopsis thaliana was hydroponically challenged by nCuO (100 mg/L), as compared to Cu²⁺ ions (1.2 mg/L), to account for nCuO dissolution for 96 h and 28 days to monitor Cu accumulation in the plant as well as the fatty acid (FA) profiles of the plant cell membrane. Under the same growing conditions, the nCuO exposure resulted in more Cu accumulation than did the Cu²⁺ exposure. Multiple microscopic techniques confirmed the internalization and sequestration of nCuO in root cell vacuoles, where transformation of Cu(II) to Cu(I)Cl occurred. Short and long exposures (96 h versus 28 days) to both nCuO and Cu²⁺ elevated FA saturation degrees in plant cells through oxidative stress, as verified by in situ detection of superoxide radicals, with conversions mostly from C18:3, C16:3, and C18:2 to C16:0. Only the long exposure to nCuO significantly brought about an additional elevation of FA saturation degree in root cells. These results demonstrated that the acute effects of plant exposure to nCuO were mainly produced from the stress of Cu²⁺ ions released from nCuO dissolution, while the chronic effects in roots were significantly developed by the nCuO particle stress. The findings in this work are novel and may offer significant implications in better understanding nanoparticle-induced phytotoxicity and potential risks in ecosystems.

Novel Genes Encoding Hexadecanoic Acid Δ6-Desaturase Activity in a Rhodococcus sp

cis-6-Hexadecenoic acid, a major component of human sebaceous lipids, is involved in the defense mechanism against Staphylococcus aureus infection in healthy skin and closely related to atopic dermatitis. Previously, Koike et al. (Biosci Biotechnol Biochem 64:1064-1066, 2000) reported that a mutant strain of Rhodococcus sp. produced cis-6-hexadecenoate derivatives from palmitate alkyl esters. From the mutant Rhodococcus strain, we identified and sequenced two open reading frames present in an amplified 5.7-kb region; these open reading frames encoded tandemly repeated Δ6-desaturase-like genes, Rdes1 and Rdes2. A phylogenetic tree indicated that Rdes1 and Rdes2 were different from previously known Δ6-desaturase genes, and that they formed a new cluster. Rdes1 and Rdes2 were each introduced into vectors and then expressed separately in Escherichia coli, and the fatty acid composition of the transformed cells was analyzed by gas chromatography and mass spectrometry. The amount of cis-6-hexadecenoic acid was significantly higher in Rdes1- or Rdes2-transformed E. coli cells (twofold and threefold, respectively) than in vector-only control cells. These results showed that cis-6-hexadecenoic acid was produced in E. coli cells by the rhodococcal Δ6-desaturase-like proteins.

Vegetable Oil: Nutritional and Industrial Perspective

Oils of plant origin have been predominantly used for food-based applications. Plant oils not only represent a non-polluting renewable resource but also provide a wide diversity in fatty acids (FAs) composition with diverse applications. Besides being edible, they are now increasingly being used in industrial applications such as paints, lubricants, soaps, biofuels etc. In addition, plants can be engineered to produce fatty acids which are nutritionally beneficial to human health. Thus these oils have potential to 1) substitute ever increasing demand of non –renewable petroleum sources for industrial application and 2) also spare the marine life by providing an alternative source to nutritionally and medically important long chain polyunsaturated fatty acids or ‘Fish oil’. The biochemical pathways producing storage oils in plants have been extensively characterized, but the factors regulating fatty acid synthesis and controlling total oil content in oilseed crops are still poorly understood. Thus understanding of plant lipid metabolism is fundamental to its manipulation and increased production. This review on oils discusses fatty acids of nutritional and industrial importance, and approaches for achieving future designer vegetable oil for both edible and non-edible uses. The review will discuss the success and bottlenecks in efficient production of novel FAs in non-native plants using genetic engineering as a tool.

Detection of Acyl-CoA Derivatized with Butylamide for in vitro Fatty Acid Desaturase Assay

Membrane-bound fatty acid desaturases acting on acyl-CoA contribute to the biosynthesis of unsaturated fatty acids, such as arachidonic acid and docosahexaenoic acid in higher organisms. We propose a simplified method for measuring the desaturase activity that combines the in vitro reaction by desaturase-expressing yeast cell homogenate and the detection of acyl-CoA product as butylamide derivatives by gas chromatography. To set up the in vitro reaction, we traced the in vivo activity of rat liver ∆6 fatty acid desaturase (D6d) expressed in the yeast, Saccharomyces cerevisiae, and determined the time taken for the D6d activity to reach its maximum level. The cell homogenate of yeast expressing the maximum D6d activity was made to react in vitro with linoleoyl-CoA to generate the D6d product, γlinolenoyl-CoA. This product was successfully detected as a peak corresponding to γ-linolenoyl butylamide on gas chromatography. This procedure, with low background expression, using non-labeled acyl-CoA as substrate, will contribute toward developing a simple in vitro desaturase assay. It will also help in elucidating the functions of membrane-bound fatty acid desaturases with various substrate specificities and regioselectivities.

Fatty Acid Desaturases, Polyunsaturated Fatty Acid Regulation, and Biotechnological Advances

Polyunsaturated fatty acids (PUFAs) are considered to be critical nutrients to regulate human health and development, and numerous fatty acid desaturases play key roles in synthesizing PUFAs. Given the lack of delta-12 and -15 desaturases and the low levels of conversion to PUFAs, humans must consume some omega-3 and omega-6 fatty acids in their diet. Many studies on fatty acid desaturases as well as PUFAs have shown that fatty acid desaturase genes are closely related to different human physiological conditions. Since the first front-end desaturases from cyanobacteria were cloned, numerous desaturase genes have been identified and animals and plants have been genetically engineered to produce PUFAs such as eicosapentaenoic acid and docosahexaenoic acid. Recently, a biotechnological approach has been used to develop clinical treatments for human physiological conditions, including cancers and neurogenetic disorders. Thus, understanding the functions and regulation of PUFAs associated with human health and development by using biotechnology may facilitate the engineering of more advanced PUFA production and provide new insights into the complexity of fatty acid metabolism.

Enhancement of α-linolenic acid content in transgenic tobacco seeds by targeting a plastidial ω-3 fatty acid desaturase (fad7) gene of Sesamum indicum to ER

KEY MESSAGE

Expression of sesame plastidial FAD7 desaturase modified with the endoplasmic reticulum targeting and retention signals, enhances the α-linolenic acid accumulation in seeds of Nicotiana tabacum. In plants, plastidial ω-3 fatty acid desaturase-7 (FAD7) catalyzes the formation of C16 and C18 trienoic fatty acids using organellar glycerolipids and participate in the membrane lipid formation. The plastidial ω-3 desaturases (FAD7) share high sequence homology with the microsomal ω-3 desaturases (FAD3) at the amino acid level except the N-terminal organelle transit peptide. In the present study, the predicted N-terminal plastidial signal peptide of fad7 gene was replaced by the endoplasmic reticulum signal peptide and an endoplasmic reticulum retention signal was placed at the C-terminal. The expression of the modified sesame ω-3 desaturase increases the α-linolenic acid content in the range of 4.78-6.77 % in the seeds of transgenic tobacco plants with concomitant decrease in linoleic acid content. The results suggested the potential of the engineered plastidial ω-3 desaturase from sesame to influence the profile of α-linolenic acid in tobacco plant by shifting the carbon flux from linoleic acid, and thus it can be used in suitable genetic engineering strategy to increase the α-linolenic acid content in sesame and other vegetable oils.

Biosynthesis of Essential Polyunsaturated Fatty Acids in Wheat Triggered by Expression of Artificial Gene

The artificial gene D6D encoding the enzyme ∆⁶desaturase was designed and synthesized using the sequence of the same gene from the fungus Thamnidium elegans. The original start codon was replaced by the signal sequence derived from the wheat gene for high-molecular-weight glutenin subunit and the codon usage was completely changed for optimal expression in wheat. Synthesized artificial D6D gene was delivered into plants of the spring wheat line CY-45 and the gene itself, as well as transcribed D6D mRNA were confirmed in plants of T₀ and T₁ generations. The desired product of the wheat genetic modification by artificial D6D gene was the γ-linolenic acid. Its presence was confirmed in mature grains of transgenic wheat plants in the amount 0.04%-0.32% (v/v) of the total amount of fatty acids. Both newly synthesized γ-linolenic acid and stearidonic acid have been detected also in leaves, stems, roots, awns, paleas, rachillas, and immature grains of the T₁ generation as well as in immature and mature grains of the T₂ generation. Contents of γ-linolenic acid and stearidonic acid varied in range 0%-1.40% (v/v) and 0%-1.53% (v/v) from the total amount of fatty acids, respectively. This approach has opened the pathway of desaturation of fatty acids and production of essential polyunsaturated fatty acids in wheat.

The codon-optimized Δ(6)-desaturase gene of Pythium sp. as an empowering tool for engineering n3/n6 polyunsaturated fatty acid biosynthesis

Background

The ∆⁶-desaturase gene, encoding a key enzyme in the biosynthesis of polyunsaturated fatty acids, has potential in pharmaceutical and nutraceutical applications.

Results

The ∆⁶-desaturase gene has been isolated from a selected strain of Oomycetes, Pythium sp. BCC53698. The cloned gene (PyDes6) contained an open reading frame (ORF) of 1401 bp encoding 466 amino acid residues. The deduced amino acid sequence shared a high similarity to those of other ∆⁶-desaturases that contained the signature features of a membrane-bound ∆⁶-desaturase, including a cytochrome b₅ and three histidine-rich motifs and membrane-spanning regions. Heterologous expression in Saccharomyces cerevisiae showed that monoene, diene and triene fatty acids having ∆⁹-double bond were substrates for PyDes6. No distinct preference between the n-3 and n-6 polyunsaturated fatty acyl substrates was found. The ∆⁶-desaturated products were markedly increased by codon optimization of PyDes6.

Conclusion

The codon-optimized ∆⁶-desaturase gene generated in this study is a promising tool for further reconstitution of the fatty acid profile, in a host system of choice, for the production of economically important fatty acids, particularly the n-3 and n-6 polyunsaturated fatty acids.

Electronic supplementary material

The online version of this article (doi:10.1186/s12896-015-0200-6) contains supplementary material, which is available to authorized users.

Modifying the lipid content and composition of plant seeds: engineering the production of LC-PUFA

Omega-3 fatty acids are characterized by a double bond at the third carbon atom from the end of the carbon chain. Latterly, long chain polyunsaturated omega-3 fatty acids such as eicosapentaenoic acid (EPA; 20:5Δ5,8,11,14,17) and docosahexanoic acid (DHA; 22:6 Δ4,7,10,13,16,19), which typically only enter the human diet via the consumption of oily fish, have attracted much attention. The health benefits of the omega-3 LC-PUFAs EPA and DHA are now well established. Given the desire for a sustainable supply of omega-LC-PUFA, efforts have focused on enhancing the composition of vegetable oils to include these important fatty acids. Specifically, EPA and DHA have been the focus of much study, with the ultimate goal of producing a terrestrial plant-based source of these so-called fish oils. Over the last decade, many genes encoding the primary LC-PUFA biosynthetic activities have been identified and characterized. This has allowed the reconstitution of the LC-PUFA biosynthetic pathway in oilseed crops, producing transgenic plants engineered to accumulate omega-3 LC-PUFA to levels similar to that found in fish oil. In this review, we will describe the most recent developments in this field and the challenges of overwriting endogenous seed lipid metabolism to maximize the accumulation of these important fatty acids.

Metabolic engineering of microorganisms to produce omega-3 very long-chain polyunsaturated fatty acids

Omega-3 long-chain polyunsaturated fatty acids (LC-PUFAs) have received growing attention due to their significant roles in human health. Currently the main source of these nutritionally and medically important fatty acids is marine fish, which has not met ever-increasing global demand. Microorganisms are an important alternative source also being explored. Although many microorganisms accumulate omega-3 LC-PUFAs naturally, metabolic engineering might still be necessary for significantly improving their yields. Here, we review recent research involving the engineering of microorganisms for production of omega-3 LC-PUFAs, including eicospentaenoic acid and docosohexaenoic acid. Both reconstitution of omega-3 LC-PUFA biosynthetic pathways and modification of existing pathways in microorganisms have demonstrated the potential to produce high levels of omega-3 LC-PUFAs. However, the yields of omega-3 LC-PUFAs in host systems have been substantially limited by potential metabolic bottlenecks, which might be caused partly by inefficient flux of fatty acid intermediates between the acyl-CoA and different lipid class pools. Although fatty acid flux in both native and heterologous microbial hosts might be controlled by several acyltransferases, evidence has suggested that genetic manipulation of one acyltransferase alone could significantly increase the accumulation of LC-PUFAs. The number of oleaginous microorganisms that can be genetically transformed is increasing, which will advance engineering efforts to maximize LC-PUFA yields in microbial strains.

Identification of the substrate recognition region in the Δ⁶-fatty acid and Δ⁸-sphingolipid desaturase by fusion mutagenesis

Δ⁸-sphingolipid desaturase and Δ⁶-fatty acid desaturase share high protein sequence identity. Thus, it has been hypothesized that Δ⁶-fatty acid desaturase is derived from Δ⁸-sphingolipid desaturase; however, there is no direct proof. The substrate recognition regions of Δ⁶-fatty acid desaturase and Δ⁸-sphingolipid desaturase, which aid in understanding the evolution of these two enzymes, have not been reported. A blackcurrant Δ⁶-fatty acid desaturase and a Δ⁸-sphingolipid desaturase gene, RnD6C and RnD8A, respectively, share more than 80 % identity in their coding protein sequences. In this study, a set of fusion genes of RnD6C and RnD8A were constructed and expressed in yeast. The Δ⁶- and Δ⁸-desaturase activities of the fusion proteins were characterized. Our results indicated that (1) the exchange of the C-terminal 172 amino acid residues can lead to a significant decrease in both desaturase activities; (2) amino acid residues 114-174, 206-257, and 258-276 played important roles in Δ⁶-substrate recognition, and the last two regions were crucial for Δ⁸-substrate recognition; and (3) amino acid residues 114-276 of Δ⁶-fatty acid desaturase contained the substrate recognition site(s) responsible for discrimination between ceramide (a substrate of Δ⁸-sphingolipid desaturase) and acyl-PC (a substrate of Δ⁶-fatty acid desaturase). Substituting the amino acid residues 114-276 of RnD8A with those of RnD6C resulted in a gain of Δ⁶-desaturase activity in the fusion protein but a loss in Δ⁸-sphingolipid desaturase activity. In conclusion, several regions important for the substrate recognition of Δ⁸-sphingolipid desaturase and Δ⁶-fatty acid desaturase were identified, which provide clues in understanding the relationship between the structure and function in desaturases.

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.

A linear concatenation strategy to construct 5'-enriched amplified cDNA libraries using multiple displacement amplification

In various experimental systems, limiting available amounts of RNA may prevent a researcher from performing large-scale analyses of gene transcripts. One way to circumvent this is to 'pre-amplify' the starting RNA/cDNA, so that sufficient amounts are available for any downstream analysis. In the present study, we report the development of a novel protocol for constructing amplified cDNA libraries using the Phi29 DNA polymerase based multiple displacement amplification (MDA) system. Using as little as 200 ng of total RNA, we developed a linear concatenation strategy to make the single-stranded cDNA template amenable for MDA. The concatenation, made possible by the template switching property of the reverse transcriptase enzyme, resulted in the amplified cDNA library with intact 5' ends. MDA generated micrograms of template, allowing large-scale polymerase chain reaction analyses or other large-scale downstream applications. As the amplified cDNA library contains intact 5' ends, it is also compatible with 5' RACE analyses of specific gene transcripts. Empirical validation of this protocol is demonstrated on a highly characterized (tomato) and an uncharacterized (corn gromwell) experimental system.

Identification and functional characterization of genes encoding omega-3 polyunsaturated fatty acid biosynthetic activities from unicellular microalgae

In order to identify novel genes encoding enzymes involved in the biosynthesis of nutritionally important omega-3 long chain polyunsaturated fatty acids, a database search was carried out in the genomes of the unicellular photoautotrophic green alga Ostreococcus RCC809 and cold-water diatom Fragilariopsis cylindrus. The search led to the identification of two putative “front-end” desaturases (Δ6 and Δ4) from Ostreococcus RCC809 and one Δ6-elongase from F. cylindrus. Heterologous expression of putative open reading frames (ORFs) in yeast revealed that the encoded enzyme activities efficiently convert their respective substrates: 54.1% conversion of α-linolenic acid for Δ6-desaturase, 15.1% conversion of 22:5n-3 for Δ4-desaturase and 38.1% conversion of γ-linolenic acid for Δ6-elongase. The Δ6-desaturase from Ostreococcus RCC809 displays a very strong substrate preference resulting in the predominant synthesis of stearidonic acid (C18:4Δ^6,9,12,15). These data confirm the functional characterization of omega-3 long chain polyunsaturated fatty acid biosynthetic genes from these two species which have until now not been investigated for such activities. The identification of these new genes will also serve to expand the repertoire of activities available for metabolically engineering the omega-3 trait in heterologous hosts as well as providing better insights into the synthesis of eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) in marine microalgae.

Metabolic engineering of Phaeodactylum tricornutum for the enhanced accumulation of omega-3 long chain polyunsaturated fatty acids

We have engineered the diatom Phaeodactylum tricornutum to accumulate the high value omega-3 long chain polyunsaturated fatty acid docosahexaenoic acid (DHA). This was achieved by the generation of transgenic strains in which the Δ5-elongase from the picoalga Ostreococcus tauri was expressed to augment the endogenous fatty acid biosynthetic pathway. Expression of the heterologous elongase resulted in an eight-fold increase in docosahexaenoic acid content, representing a marked and valuable change in the fatty acid profile of this microalga. Importantly, DHA was shown to accumulate in triacylglycerols, with several novel triacylglycerol species being detected in the transgenic strains. In a second iteration, co-expression of an acyl-CoA-dependent Δ6-desaturase with the Δ5-elongase further increased DHA levels. Together, this demonstrates for the first time the potential of using iterative metabolic engineering to optimise omega-3 content in algae.

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First example of using metabolic engineering in microalgae to modify the accumulation of high value omega-3 long chain polyunsaturated fatty acids.

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First example of multigene expression in a diatom.

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Detailed lipidomic analysis of diatom Phaeodactylum tricornutum.

Survey of the total fatty acid and triacylglycerol composition and content of 30 duckweed species and cloning of a Δ6-desaturase responsible for the production of γ-linolenic and stearidonic acids in Lemna gibba

BACKGROUND

Duckweeds, i.e., members of the Lemnoideae family, are amongst the smallest aquatic flowering plants. Their high growth rate, aquatic habit and suitability for bio-remediation make them strong candidates for biomass production. Duckweeds have been studied for their potential as feedstocks for bioethanol production; however, less is known about their ability to accumulate reduced carbon as fatty acids (FA) and oil.

RESULTS

Total FA profiles of thirty duckweed species were analysed to assess the natural diversity within the Lemnoideae. Total FA content varied between 4.6% and 14.2% of dry weight whereas triacylglycerol (TAG) levels varied between 0.02% and 0.15% of dry weight. Three FA, 16:0 (palmitic), 18:2Δ9,12 (Linoleic acid, or LN) and 18:3Δ9,12,15 (α-linolenic acid, or ALA) comprise more than 80% of total duckweed FA. Seven Lemna and two Wolffiela species also accumulate polyunsaturated FA containing Δ6-double bonds, i.e., GLA and SDA. Relative to total FA, TAG is enriched in saturated FA and deficient in polyunsaturated FA, and only five Lemna species accumulate Δ6-FA in their TAG. A putative Δ6-desaturase designated LgDes, with homology to a family of front-end Δ6-FA and Δ8-spingolipid desaturases, was identified in the assembled DNA sequence of Lemna gibba. Expression of a synthetic LgDes gene in Nicotiana benthamiana resulted in the accumulation of GLA and SDA, confirming it specifies a Δ6-desaturase.

CONCLUSIONS

Total accumulation of FA varies three-fold across the 30 species of Lemnoideae surveyed. Nine species contain GLA and SDA which are synthesized by a Δ6 front-end desaturase, but FA composition is otherwise similar. TAG accumulates up to 0.15% of total dry weight, comparable to levels found in the leaves of terrestrial plants. Polyunsaturated FA is underrepresented in TAG, and the Δ6-FA GLA and SDA are found in the TAG of only five of the nine Lemna species that produce them. When present, GLA is enriched and SDA diminished relative to their abundance in the total FA pool.

Site-directed mutagenesis of a fatty acid elongase ELO-like condensing enzyme

The condensation step of fatty acid elongation is the addition of a C2 unit from malonyl-CoA to an acyl primer catalyzed by one of two families of enzymes, the 3-ketoacyl-CoA synthases and the ELO-like condensing enzymes. 3-Ketoacyl-CoA synthases use a Claisen-like reaction mechanism while the mechanism of the ELO-catalyzed condensation reaction is unknown. We have used site-directed mutagenesis of Dictyostelium discoideum EloA to identify residues important to catalytic activity and/or structure. Mutation of highly conserved polar residues to alanine resulted in an inactive enzyme strongly suggesting that these residues play a role in the condensation reaction.

Fusarium oxysporum f.sp. ciceri race 1 induced redox state alterations are coupled to downstream defense signaling in root tissues of chickpea (Cicer arietinum L.)

Reactive oxygen species are known to play pivotal roles in pathogen perception, recognition and downstream defense signaling. But, how these redox alarms coordinate in planta into a defensive network is still intangible. Present study illustrates the role of Fusarium oxysporum f.sp ciceri Race1 (Foc1) induced redox responsive transcripts in regulating downstream defense signaling in chickpea. Confocal microscopic studies highlighted pathogen invasion and colonization accompanied by tissue damage and deposition of callose degraded products at the xylem vessels of infected roots of chickpea plants. Such depositions led to the clogging of xylem vessels in compatible hosts while the resistant plants were devoid of such obstructions. Lipid peroxidation assays also indicated fungal induced membrane injury. Cell shrinkage and gradual nuclear adpression appeared as interesting features marking fungal ingress. Quantitative real time polymerase chain reaction exhibited differential expression patterns of redox regulators, cellular transporters and transcription factors during Foc1 progression. Network analysis showed redox regulators, cellular transporters and transcription factors to coordinate into a well orchestrated defensive network with sugars acting as internal signal modulators. Respiratory burst oxidase homologue, cationic peroxidase, vacuolar sorting receptor, polyol transporter, sucrose synthase, and zinc finger domain containing transcription factor appeared as key molecular candidates controlling important hubs of the defense network. Functional characterization of these hub controllers may prove to be promising in understanding chickpea-Foc1 interaction and developing the case study as a model for looking into the complexities of wilt diseases of other important crop legumes.

Identification and functional analysis of a Δ6-desaturase gene and the effects of temperature and wounding stresses on its expression in Microula sikkimensis leaves

A Δ6-desaturase gene was isolated from Microula sikkimensis. Sequence analysis indicated that the gene, designated MsD6DES, had an open reading frame of 1,357 bp and encoded 448 amino acids. Heterologous expression in tobacco indicated that MsD6DES can use endogenous substrates to synthesize γ-linolenic acid (GLA, 18:3(Δ 6,9,12)) and octadecatetraenoic acid (OTA, 18:4(Δ 6,9,12,15)). MsD6DES transcripts were distributed in all tested tissues, with high expression levels in seeds and young leaves. The effects of temperature and wounding stresses on MsD6DES expression were analyzed. The results indicated that temperature regulates MsD6DES at the transcriptional level. MsD6DES expression increased first, reaching a maximum 4 h after low-temperature treatment. A slight increase in MsD6DES transcript levels was also observed under high temperature. We found that the response of MsD6DES to temperature stress was different from those of fungi and algae. In addition, MsD6DES was found to be wound-inducible.

The thermostability of two kinds of recombinant ∆6-fatty acid desaturase with different N-terminal sequence lengths in low temperature

Two recombinant Rhizopus stolonifer ∆6-fatty acid desaturase enzymes with different-length N-termini were cloned and expressed in Saccharomyces cerevisiae strain INVScl: LRsD6D begins with the sequence of the N-terminal of the R. stolonifer ∆6-fatty acid desaturase native, encoding a deduced polypeptide of 459 amino acids (M-S-T-L-D-R-Q-S-I-F-T-I-K-E-L-E-S-I-S-Q-R-I-H-D-G-D-E-E-A-M-K-F), whereas SRsD6D begins with the amino acid sequence of the predicted ORF, encoding a deduced polypeptide of 430 amino acids (M-K-F) and LRsD6D is longer than SRsD6D by 29 amino acids (M-S-T-L-D-R-Q-S-I-F-T-I-K-E-L-E-S-I-S-Q-R-I-H-D-G-D-E-E-A). Bioinformatic analysis characterized the two recombinant ∆6-fatty acid desaturase enzymes with different-length N-termini, including three conserved histidine-rich motifs, hydropathy profile, and a cytochrome b5-like domain in the N-terminus. When the coding sequence was expressed in S. cerevisiae strain INVScl, the coding produced ∆6-fatty acid desaturase activity exhibited by RsD6D, leading to a novel peak corresponding to γ-linolenic acid methyl ester standards, which was detected with the same retention time. The residual activity of LRsD6D was 74 % at 15 °C for 4 h and that of SRsD6D was 43 %. Purified recombinant LRsD6D was more stable than SRsD6D, indicating that the N-terminal extension, containing mostly hydrophobic residues, affected the overall stability of recombinant LRsD6D.

Reconstitution of EPA and DHA biosynthesis in arabidopsis: iterative metabolic engineering for the synthesis of n-3 LC-PUFAs in transgenic plants

An iterative approach to optimising the accumulation of non-native long chain polyunsaturated fatty acids in transgenic plants was undertaken in Arabidopsis thaliana. The contribution of a number of different transgene enzyme activities was systematically determined, as was the contribution of endogenous fatty acid metabolism. Successive iterations were informed by lipidomic analysis of neutral, polar and acyl-CoA pools. This approach allowed for a four-fold improvement on levels previously reported for the accumulation of eicosapentaenoic acid in Arabidopsis seeds and also facilitated the successful engineering of the high value polyunsaturated fatty acid docosahexaenoic acid to 10-fold higher levels. Our studies identify the minimal gene set required to direct the efficient synthesis of these fatty acids in transgenic seed oil.

The modification of plant oil composition via metabolic engineering--better nutrition by design

This article will focus on the modification of plant seed oils to enhance their nutritional composition. Such modifications will include C18 Δ6-desaturated fatty acids such as γ-linolenic and stearidonic acid, omega-6 long-chain polyunsaturated fatty acids such as arachidonic acid, as well as the omega-3 long-chain polyunsaturated fatty acids (often named 'fish oils') such as eicosapentaenoic acid and docosahexaenoic acid. We will consider how new technologies (such as synthetic biology, next-generation sequencing and lipidomics) can help speed up and direct the development of desired traits in transgenic oilseeds. We will also discuss how manipulating triacylglycerol structure can further enhance the nutritional value of 'designer' oils. We will also consider how advances in model systems have translated into crops and the potential end-users for such novel oils (e.g. aquaculture, animal feed, human nutrition).