Metabolic engineering of seeds can achieve levels of omega-7 fatty acids comparable with the highest levels found in natural plant sources

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.

MALDI-MS screening of microbial colonies with isomer resolution to select fatty acid desaturase variants

Creating controlled lipid unsaturation locations in oleochemicals can be a key to many bioengineered products. However, evaluating the effects of modifications to the acyl-ACP desaturase on lipid unsaturation is not currently amenable to high-throughput assays, limiting the scale of redesign efforts to <200 variants. Here, we report a rapid MS assay for profiling the positions of double bonds on membrane lipids produced by Escherichia coli colonies after treatment with ozone gas. By MS measurement of the ozonolysis products of Δ6 and Δ8 isomers of membrane lipids from colonies expressing recombinant Thunbergia alata desaturase, we screened a randomly mutagenized library of the desaturase gene at 5 s per sample. Two variants with altered regiospecificity were isolated, indicated by an increase in 16:1 Δ8 proportion. We also demonstrated the ability of these desaturase variants to influence the membrane composition and fatty acid distribution of E. coli strains deficient in the native acyl-ACP desaturase gene, fabA. Finally, we used the fabA deficient chassis to concomitantly express a non-native acyl-ACP desaturase and a medium-chain thioesterase from Umbellularia californica, demonstrating production of only saturated free fatty acids.

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.

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.

Producing high value unsaturated fatty acid by whole-cell catalysis using microalga: A case study with Tribonema minus

High value unsaturated fatty acids can be produced by de novo synthesis in microalgal cells, especially via heterotrophic cultivation. Unfortunately, the lipid accumulation of heterotrophic microalgae cannot be improved efficiently in conventional ways. Here we reported heterotrophic Tribonema minus, a promising resource for the production of palmitoleic acid which has increasing demands in health service for patients with metabolic syndrome, as whole-cell biocatalyst to develop a novel way of shifting low value exogenous saturated fatty acids to high value ones. Results showed that myristic acid is the best precursor for whole-cell catalysis; it elevated the lipid content of T. minus to 42.2%, the highest among the tried precursors. The influences of cultivation condition on the utilization of extrinsic myristic acid and lipid accumulation were also determined. Under the optimized condition, the lipid content reached as high as 48.9%. In addition, our findings showed that ~13.0% of C16:1 in T. minus is derived from extrinsic myristic acid, and 30.1% of metabolized precursor is converted into heterologous fatty acids. Thus, a feasible approach for both increasing the value of low value saturated fatty acid by bioconversion and enhancing the lipid accumulation in microalgae is proposed by supplementing extrinsic myristic acid.

Molecular Approaches Reduce Saturates and Eliminate trans Fats in Food Oils

Vegetable oils composed of triacylglycerols (TAG) are a major source of calories in human diets. However, the fatty acid compositions of these oils are not ideal for human nutrition and the needs of the food industry. Saturated fatty acids contribute to health problems, while polyunsaturated fatty acids (PUFA) can become rancid upon storage or processing. In this review, we first summarize the pathways of fatty acid metabolism and TAG synthesis and detail the problems with the oil compositions of major crops. Then we describe how transgenic expression of desaturases and downregulation of the plastid FatB thioesterase have provided the means to lower oil saturates. The traditional solution to PUFA rancidity uses industrial chemistry to reduce PUFA content by partial hydrogenation, but this results in the production of trans fats that are even more unhealthy than saturated fats. We detail the discoveries in the biochemistry and molecular genetics of oil synthesis that provided the knowledge and tools to lower oil PUFA content by blocking their synthesis during seed development. Finally, we describe the successes in breeding and biotechnology that are giving us new, high-oleic, low PUFA varieties of soybean, canola and other oilseed crops.

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.

Signatures of selection in recently domesticated macadamia

Macadamia is a high value nut crop that is recently domesticated, ideal for testing the effect of artificial selection. Here, we sequence the genome of Hawaiian cultivar ‘Kau’ and assemble into 794 Mb in 14 pseudo-chromosomes with 37,728 genes. Genome analysis reveals a whole-genome duplication event, occurred 46.8 million years ago. Gene expansions occurred in gene families involves in fatty acid biosynthesis. Gene duplication of MADS-Box transcription factors in proanthocyanidin biosynthesis are relevant for seed coat development. Genome re-sequencing of 112 accessions reveals the origin of Hawaiian cultivars from Mount Bauple in southeast Queensland in Australia. Selective sweeps are detected in macadamia cultivars, including genes involved in fatty acid biosynthesis, seed coat development, and heat stress response. Such strong effects of artificial selection in few generations reveals the genomic basis for ‘one-step operation’ for clonal crop domestication. The knowledge gained could accelerate domestication of new crops from wild species.

Macadamia is a recently domesticated nut crop. Here, the authors report the genome assembly of Hawaiian cultivar ‘Kau’ and conduct population genomic analyses to reveal the origin of Hawaiian cultivars and the genomic basis for one-step operation for the clonal crop domestication.

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.

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.

Comparison of Lipid and Palmitoleic Acid Induction of Tribonema minus under Heterotrophic and Phototrophic Regimes by Using High-Density Fermented Seeds

Palmitoleic acid, one scarce omega-7 monounsaturated fatty acid, has important applications in the fields of medicine and health products. Tribonema has been considered as a promising candidate for the production of palmitoleic acid due to its high lipid and palmitoleic acid content and remarkable heterotrophic ability. The high-density heterotrophic cultivation of Tribonema minus was conducted in this work, and the highest biomass of 42.9 g L⁻¹ and a relatively low lipid content of 28.7% were observed. To further enhance the lipid and palmitoleic acid accumulation, induction strategies under two regimes of phototrophy and heterotrophy with different conditions were investigated and compared. Results demonstrated encouraging promotions both by heterotrophic and phototrophic ways, and the final lipid contents reached 41.9% and 49.0%, respectively. In consideration of the time cost, however, the induction under heterotrophic conditions was much more advantageous, by which the highest lipid and palmitoleic acid productivities of 1.77 g L⁻¹ d⁻¹ and 924 mg L⁻¹ d⁻¹ were obtained respectively, with the lipid yield on glucose of 0.26 g g⁻¹.

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.

RNA-seq data reveals a coordinated regulation mechanism of multigenes involved in the high accumulation of palmitoleic acid and oil in sea buckthorn berry pulp

BACKGROUND

Sea buckthorn is a woody oil crop in which palmitoleic acid (C16:1n7, an omega-7 fatty acid (FA)) contributes approximately 40% of the total FA content in berry pulp (non-seed tissue). However, the molecular mechanisms contributing to the high accumulation of C16:1n7 in developing sea buckthorn berry pulp (SBP) remain poorly understood.

RESULTS

We identified 1737 unigenes associated with lipid metabolism through RNA-sequencing analysis of the four developmental stages of berry pulp in two sea buckthorn lines, 'Za56' and 'TF2-36'; 139 differentially expressed genes were detected between the different berry pulp developmental stages in the two lines. Analyses of the FA composition showed that the C16:1n7 contents were significantly higher in line 'Za56' than in line 'TF2-36' in the mid-late developmental stages of SBP. Additionally, qRT-PCR analyses of 15 genes involved in FA and triacylglycerol (TAG) biosynthesis in both lines revealed that delta9-ACP-desaturase (ACP-Δ9D) competed with 3-ketoacyl-ACP-synthase II (KASII) for the substrate C16:0-ACP and that ACP-Δ9D and delta9-CoA-desaturase (CoA-Δ9D) gene expression positively correlated with C16:1n7 content; KASII and fatty acid elongation 1 (FAE1) gene expression positively correlated with C18:0 content in developing SBP. Specifically, the abundance of ACP-Δ9D and CoA-Δ9D transcripts in line 'Za56', which had a higher C16:1n7 content than line 'TF2-36', suggests that these two genes play an important role in C16:1n7 biosynthesis. Furthermore, the high expressions of the glycerol-3-phosphate dehydrogenase (GPD1) gene and the WRINKLED1 (WRI1) transcription factor contributed to increased biosynthesis of TAG precursor and FAs, respectively, in the early developmental stages of SBP, and the high expression of the diacylglycerol O-acyltransferase 1 (DGAT1) gene increased TAG assembly in the later developmental stages of SBP. Overall, we concluded that increased ACP-Δ9D and CoA-Δ9D levels coupled with decreased KASII and FAE1 activity is a critical event for high C16:1n7 accumulation and that the coordinated high expression of WRI1, GPD1, and DGAT1 genes resulted in high oil accumulation in SBP.

CONCLUSION

Our results provide a scientific basis for understanding the mechanism of high C16:1n7 accumulation in berry pulp (non-seed tissue) and are valuable to the genetic breeding programme for achieving a high quality and yield of SBP oil.

Tandem Mass Tag Based Quantitative Proteomics of Developing Sea Buckthorn Berries Reveals Candidate Proteins Related to Lipid Metabolism

Sea buckthorn ( Hippophae L.) is an economically important shrub or small tree distributed in Eurasia. Most of its well-recognized medicinal and nutraceutical products are derived from its berry oil, which is rich in monounsaturated omega-7 (C16:1) fatty acid and polyunsaturated omega-6 (C18:2) and omega-3 (C18:3) fatty acids. In this study, tandem mass tags (TMT)-based quantitative analysis was used to investigate protein profiles of lipid metabolism in sea buckthorn berries harvested 30, 50, and 70 days after flowering. In total, 8626 proteins were identified, 6170 of which were quantified. Deep analysis results for the proteins identified and related pathways revealed initial fatty acid accumulation during whole-berry development. The abundance of most key enzymes involved in fatty acid and triacylglycerol (TAG) biosynthesis peaked at 50 days after flowering, but TAG synthesis through the PDAT (phospholipid: diacylglycerol acyltransferase) pathway mostly occurred early in berry development. In addition, the patterns of proteins involved in lipid metabolism were confirmed by combined quantitative real-time polymerase chain reaction, enzyme-linked immunosorbent assay, and parallel reaction monitoring analyses. Our data on the proteomic spectrum of sea buckthorn berries provide a scientific basic for understanding lipid metabolism and related pathways in the developing berries.

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.

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.

Increasing genetic variability in oilseed rape (Brassica napus) - Genotypes and phenotypes of oilseed rape transformed by wild type Agrobacterium rhizogenes

Brassica napus (oilseed rape) is a major oil crop worldwide. Due to the short domestication period of oilseed rape the genetic variability is limited compared to other crops. Transfer of rol and aux genes from Agrobacterium rhizogenes is used in horticulture to increase genetic variability. In the current study, we explore transformation by A. rhizogenes as a biotechnological approach in breeding for more branched and shorter oilseed rape. In the 2nd generation of transformed oilseed rape, branch numbers increased significantly by 49% from 7.7 ± 0.4 to 11.5 ± 1.9 when comparing rol+/aux+ plants with WT. Simultaneously, the apical height of plants was reduced by 25% from 81.3 ± 1.9 cm to 62.4 ± 6.7 cm in rol+/aux+ plants at the onset of flowering. Reproductive parameters affecting yield as seed size and number were negatively affected in rol+/aux+ plants. Interestingly, oil composition was changed in rol+/aux+ seeds. Oleic acid (ω9) contents were reduced by more than 3% whereas α-linolenic acid (ω6) increased by more than 25% in mature seeds. To obtain shorter and more branched breeding material of oilseed rape we suggest crossing plants with the rol+/aux+ genotype back into the parental breeding line. This could reduce the negative impact of rol+/aux+ on yield.

Biotin Attachment Domain-Containing Proteins Irreversibly Inhibit Acetyl CoA Carboxylase

The first committed step in fatty acid synthesis is mediated by acetyl-CoA carboxylase (ACCase), a biotin-dependent enzyme that carboxylates acetyl-CoA to produce malonyl-CoA. ACCase can be feedback regulated by short-term or long-term exposure to fatty acids in the form of Tween 80 (predominantly containing oleic acid), which results in reversible or irreversible ACCase inhibition, respectively. Biotin attachment domain-containing (BADC) proteins are inactive analogs of biotin carboxyl transfer proteins that lack biotin, and their incorporation into ACCase down-regulates its activity by displacing active (biotin-containing) biotin carboxyltransferase protein subunits. Arabidopsis (Arabidopsis thaliana) lines containing T-DNA insertions in BADC1, BADC2, and BADC3 were used to generate badc1 badc2 and badc1 badc3 double mutants. The badc1 badc3 mutant exhibited normal growth and development; however, ACCase activity was 26% higher in badc1 badc3 and its seeds contained 30.1% more fatty acids and 32.6% more triacylgycerol relative to wild-type plants. To assess whether BADC contributes to the irreversible phase of ACCase inhibition, cell suspension cultures were generated from the leaves of badc1 badc3 and wild-type plants and treated with 10 mm Tween 80. Reversible ACCase inhibition was similar in badc1 badc3 and wild-type cultures after 2 d of Tween 80 treatment, but irreversible inhibition was reduced by 50% in badc1 badc3 relative to wild-type plants following 4 d of Tween 80 treatment. In this study, we present evidence for two important homeostatic roles for BADC proteins in down-regulating ACCase activity: by acting during normal growth and development and by contributing to its long-term irreversible feedback inhibition resulting from the oversupply of fatty acids.

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.

Common and Species-Specific Effects of Phosphate on Marine Microalgae Fatty Acids Shape Their Function in Phytoplankton Trophic Ecology

The use of fatty acids (FA) to infer structure of phytoplankton assemblages and as indicators of microalgae nutritional value is acquiring relevance in modern phytoplankton ecology and new advances concerning factors influencing FA variability among microalgae are demanded. In this regard, the relationship between phosphorus and FA remains particularly little studied in marine phytoplankton. In the present study, we focus on phosphate effects on FA from a diversified set of marine microalgae and provide new insights into the applicability of FA in phytoplankton trophic ecology. Phosphate deprivation mainly induced monounsaturated FA production in eight out of nine microalgae and their changes were species-specific, with palmitoleic acid exhibiting extreme variation and discriminating between haptophyte classes. The important phosphate-induced and interspecific variability found for oleic acid was perceived as a concern for the current application of this FA as a trophic position indicator in grazers. Chloroplast C-16 and C-18 polyunsaturated FA were more affected by phosphate than C-20 and C-22 highly unsaturated FA (HUFA). The relative stability of stearidonic acid to phosphate in cryptophytes and haptophytes pinpointed this FA as a suited marker for both microalgae groups. Taken all species together, phosphate deprivation and taxonomy accounted for 20.8 and 50.7% of total FA variation, respectively. HUFA were minimally affected by phosphate indicating their suitability as indicators of phytoplankton trophic value. The asymptotic relationship between HUFA and phosphorus cell content suggested mineral composition (phosphorus) could be more important than HUFA content as attribute of marine microalgae nutritional value at the species level.

Heterotrophy of filamentous oleaginous microalgae Tribonema minus for potential production of lipid and palmitoleic acid

Heterotrophic fermentation and high valuable co-product producing are thought to be effective ways to improve the economic viability and feasibility of commercial production of microalgae biofuels. This work reported the heterotrophic cultivation of Tribonema minus for lipid and palmitoleic acid (a novel functional fatty acid) production. Firstly, the heterotrophic ability of T. minus was identified for the first time with significant promotion in biomass and lipid productivity, and glucose and urea were then selected as the optimal carbon and nitrogen sources. Moreover, nutrient concentrations and culture conditions were optimized. Highest biomass and lipid productivity of 30.8gL^-1 and 730mgL^-1d^-1 were obtained respectively by adding 80gL^-1 glucose at once. In addition, 2gL^-1 urea, 0.8gL^-1 K₂HPO₄, 24mgL^-1 ammonium ferric citrate, initial pH of 6, and temperature of 27°C were determined as the appropriate conditions for heterotrophic growth and lipid production.

Spatial and Temporal Mapping of Key Lipid Species in Brassica napus Seeds

The regulation of lipid synthesis in oil seeds is still not fully understood. Oilseed rape (Brassica napus) is the third most productive vegetable oil crop on the global market; therefore, increasing our understanding of lipid accumulation in oilseed rape seeds is of great economic, as well as intellectual, importance. Matrix-assisted laser/desorption ionization-mass spectrometry imaging (MALDI-MSI) is a technique that allows the mapping of metabolites directly onto intact biological tissues, giving a spatial context to metabolism. We have used MALDI-MSI to study the spatial distribution of two major lipid species, triacylglycerols and phosphatidylcholines. A dramatic, heterogenous landscape of molecular species was revealed, demonstrating significantly different lipid compositions between the various tissue types within the seed. The embryonic axis was found to be particularly enriched in palmitic acid, while the seed coat/aleurone layer accumulated vaccenic, linoleic, and α-linoleic acids. Furthermore, the lipid composition of the inner and outer cotyledons differed from each other, a remarkable discovery given the supposed identical functionality of these two tissues. Triacylglycerol and phosphatidylcholine molecular species distribution was analyzed through a developmental time series covering early seed lipid accumulation to seed maturity. The spatial patterning of lipid molecular species did not vary significantly during seed development. Data gathered using MALDI-MSI was verified through gas chromatography analysis of dissected seeds. The distinct lipid distribution profiles observed imply differential regulation of lipid metabolism between the different tissue types of the seed. Further understanding of this differential regulation will enhance efforts to improve oilseed rape productivity and quality.

Genetic enhancement of palmitic acid accumulation in cotton seed oil through RNAi down-regulation of ghKAS2 encoding β-ketoacyl-ACP synthase II (KASII)

Palmitic acid (C16:0) already makes up approximately 25% of the total fatty acids in the conventional cotton seed oil. However, further enhancements in palmitic acid content at the expense of the predominant unsaturated fatty acids would provide increased oxidative stability of cotton seed oil and also impart the high melting point required for making margarine, shortening and confectionary products free of trans fatty acids. Seed-specific RNAi-mediated down-regulation of β-ketoacyl-ACP synthase II (KASII) catalysing the elongation of palmitoyl-ACP to stearoyl-ACP has succeeded in dramatically increasing the C16 fatty acid content of cotton seed oil to well beyond its natural limits, reaching up to 65% of total fatty acids. The elevated C16 levels were comprised of predominantly palmitic acid (C16:0, 51%) and to a lesser extent palmitoleic acid (C16:1, 11%) and hexadecadienoic acid (C16:2, 3%), and were stably inherited. Despite of the dramatic alteration of fatty acid composition and a slight yet significant reduction in oil content in these high-palmitic (HP) lines, seed germination remained unaffected. Regiochemical analysis of triacylglycerols (TAG) showed that the increased levels of palmitic acid mainly occurred at the outer positions, while C16:1 and C16:2 were predominantly found in the sn-2 position in both TAG and phosphatidylcholine. Crossing the HP line with previously created high-oleic (HO) and high-stearic (HS) genotypes demonstrated that HP and HO traits could be achieved simultaneously; however, elevation of stearic acid was hindered in the presence of high level of palmitic acid.

ACYL-ACYL CARRIER PROTEIN DESATURASE2 and 3 Are Responsible for Making Omega-7 Fatty Acids in the Arabidopsis Aleurone

Omega-7 monounsaturated fatty acids (ω-7s) are specifically enriched in the aleurone of Arabidopsis (Arabidopsis thaliana) seeds. We found significant natural variation in seed ω-7 content and used a Multiparent Advanced Generation Inter-Cross population to fine-map a major quantitative trait loci to a region containing ACYL-ACYL CARRIER PROTEIN DESATURASE1 (AAD1) and AAD3 We found that AAD3 expression is localized to the aleurone where mutants show an approximately 50% reduction in ω-7 content. By contrast, AAD1 is localized to the embryo where mutants show a small reduction in ω-9 content. Enzymatic analysis has previously shown that AAD family members possess both stearoyl- and palmitoyl-ACP Δ(9) desaturase activity, including the predominant isoform SUPPRESSOR OF SALICYLIC ACID INSENSITIVE2. However, aad3 ssi2 aleurone contained the same amount of ω-7s as aad3 Within the AAD family, AAD3 shares the highest degree of sequence similarity with AAD2 and AAD4. Mutant analysis showed that AAD2 also contributes to ω-7 production in the aleurone, and aad3 aad2 exhibits an approximately 85% reduction in ω-7s Mutant analysis also showed that FATTY ACID ELONGASE1 is required for the production of very long chain ω-7s in the aleurone. Together, these data provide genetic evidence that the ω-7 pathway proceeds via Δ(9) desaturation of palmitoyl-ACP followed by elongation of the product. Interestingly, significant variation was also identified in the ω-7 content of Brassica napus aleurone, with the highest level detected being approximately 47% of total fatty acids.

Synthetic redesign of plant lipid metabolism

Plant seed lipid metabolism is an area of intensive research, including many examples of transgenic events in which oil composition has been modified. In the selected examples described in this review, progress towards the predictive manipulation of metabolism and the reconstitution of desired traits in a non-native host is considered. The advantages of a particular oilseed crop, Camelina sativa, as a flexible and utilitarian chassis for advanced metabolic engineering and applied synthetic biology are considered, as are the issues that still represent gaps in our ability to predictably alter plant lipid biosynthesis. Opportunities to deliver useful bio-based products via transgenic plants are described, some of which represent the most complex genetic engineering in plants to date. Future prospects are considered, with a focus on the desire to transition to more (computationally) directed manipulations of metabolism.

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.

50 years of Arabidopsis research: highlights and future directions

922 I. 922 II. 922 III. 925 IV. 925 V. 926 VI. 927 VII. 928 VIII. 929 IX. 930 X. 931 XI. 932 XII. 933 XIII. Natural variation and genome-wide association studies 934 XIV. 934 XV. 935 XVI. 936 XVII. 937 937 References 937 SUMMARY: The year 2014 marked the 25(th) International Conference on Arabidopsis Research. In the 50 yr since the first International Conference on Arabidopsis Research, held in 1965 in Göttingen, Germany, > 54 000 papers that mention Arabidopsis thaliana in the title, abstract or keywords have been published. We present herein a citational network analysis of these papers, and touch on some of the important discoveries in plant biology that have been made in this powerful model system, and highlight how these discoveries have then had an impact in crop species. We also look to the future, highlighting some outstanding questions that can be readily addressed in Arabidopsis. Topics that are discussed include Arabidopsis reverse genetic resources, stock centers, databases and online tools, cell biology, development, hormones, plant immunity, signaling in response to abiotic stress, transporters, biosynthesis of cells walls and macromolecules such as starch and lipids, epigenetics and epigenomics, genome-wide association studies and natural variation, gene regulatory networks, modeling and systems biology, and synthetic biology.

Half-of-the-Sites Reactivity of the Castor Δ9-18:0-Acyl Carrier Protein Desaturase

Fatty acid desaturases regulate the unsaturation status of cellular lipids. They comprise two distinct evolutionary lineages, a soluble class found in the plastids of higher plants and an integral membrane class found in plants, yeast (Saccharomyces cerevisiae), animals, and bacteria. Both classes exhibit a dimeric quaternary structure. Here, we test the functional significance of dimeric organization of the soluble castor Δ9-18:0-acyl carrier protein desaturase, specifically, the hypothesis that the enzyme uses an alternating subunit half-of-the-sites reactivity mechanism whereby substrate binding to one subunit is coordinated with product release from the other subunit. Using a fluorescence resonance energy transfer assay, we demonstrated that dimers stably associate at concentrations typical of desaturase assays. An active site mutant T104K/S202E, designed to occlude the substrate binding cavity, was expressed, purified, and its properties validated by x-ray crystallography, size exclusion chromatography, and activity assay. Heterodimers comprising distinctly tagged wild-type and inactive mutant subunits were purified at 1:1 stoichiometry. Despite having only one-half the number of active sites, purified heterodimers exhibit equivalent activity to wild-type homodimers, consistent with half-of-the-sites reactivity. However, because multiple rounds of turnover were observed, we conclude that substrate binding to one subunit is not required to facilitate product release from the second subunit. The observed half-of-the-sites reactivity could potentially buffer desaturase activity from oxidative inactivation. That soluble desaturases require only one active subunit per dimer for full activity represents a mechanistic difference from the membrane class of desaturases such as the Δ9-acyl-CoA, Ole1p, from yeast, which requires two catalytically competent subunits for activity.

Characterization of soluble acyl-ACP desaturases from Camelina sativa, Macadamia tetraphylla and Dolichandra unguis-cati

Acyl-acyl carrier protein (ACP) desaturases (EC 1.14.19.2) are soluble enzymes that catalyse the insertion of a double bond into saturated fatty acid bound in saturated acyl chains bound to ACP in higher plants, producing cis-monounsaturated fatty acids. Three types of soluble acyl-ACP desaturases have been described: Δ(9)-acyl-ACP, Δ(6)-acyl-ACP and Δ(4)-acyl-ACP desaturases, which differ in the substrate specificity and the position in which the double bond is introduced. In the present work, Camelina sativa (CsSAD), Macadamia tetraphylla (MtSAD) and Dolichandra unguis-cati (DuSAD) desaturases were cloned, sequenced and characterized. Single copies of CsSAD, MtSAD and DuSAD with three, one and two different alleles, respectively, were found. The corresponding mature proteins were heterologously expressed in Escherichia coli for biochemical characterization in protein extracts. The recombinant CsSAD enzyme showed 300-fold higher specificity towards 18:0-ACP than 16:0-ACP. Similar profile exhibited MtSAD although the differences in the specificity were lower, around 170-fold higher for 18:0-ACP than 16:0-ACP. Furthermore, DuSAD presented a profile showing preference towards 16:0-ACP against 18:0-ACP, around twice more, being so a Δ(9) palmitoyl-ACP desaturase. Also, we reported the expression profile of CsSAD, which showed the highest levels of expression in expanding tissues that typically are very active in lipid biosynthesis such as developing seed endosperm. Moreover, the possibility to express a new desaturase in C. sativa (oilseed crop that store high levels of oil and is easy to transform) to create a new line rich in short monounsaturated fatty acid is discussed.

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

Seed oils enriched in omega-7 monounsaturated fatty acids, including palmitoleic acid (16:1∆9) and cis-vaccenic acid (18:1∆11), have nutraceutical and industrial value for polyethylene production and biofuels. Existing oilseed crops accumulate only small amounts (<2%) of these novel fatty acids in their seed oils. We demonstrate a strategy for enhanced production of omega-7 monounsaturated fatty acids in camelina (Camelina sativa) and soybean (Glycine max) that is dependent on redirection of metabolic flux from the typical ∆9 desaturation of stearoyl (18:0)-acyl carrier protein (ACP) to ∆9 desaturation of palmitoyl (16:0)-acyl carrier protein (ACP) and coenzyme A (CoA). This was achieved by seed-specific co-expression of a mutant ∆9-acyl-ACP and an acyl-CoA desaturase with high specificity for 16:0-ACP and CoA substrates, respectively. This strategy was most effective in camelina where seed oils with ~17% omega-7 monounsaturated fatty acids were obtained. Further increases in omega-7 fatty acid accumulation to 60-65% of the total fatty acids in camelina seeds were achieved by inclusion of seed-specific suppression of 3-keto-acyl-ACP synthase II and the FatB 16:0-ACP thioesterase genes to increase substrate pool sizes of 16:0-ACP for the ∆9-acyl-ACP desaturase and by blocking C18 fatty acid elongation. Seeds from these lines also had total saturated fatty acids reduced to ~5% of the seed oil versus ~12% in seeds of nontransformed plants. Consistent with accumulation of triacylglycerol species with shorter fatty acid chain lengths and increased monounsaturation, seed oils from engineered lines had marked shifts in thermotropic properties that may be of value for biofuel applications.

Fatty acid biosynthesis revisited: structure elucidation and metabolic engineering

Fatty acids are primary metabolites synthesized by complex, elegant, and essential biosynthetic machinery. Fatty acid synthases resemble an iterative assembly line, with an acyl carrier protein conveying the growing fatty acid to necessary enzymatic domains for modification. Each catalytic domain is a unique enzyme spanning a wide range of folds and structures. Although they harbor the same enzymatic activities, two different types of fatty acid synthase architectures are observed in nature. During recent years, strained petroleum supplies have driven interest in engineering organisms to either produce more fatty acids or specific high value products. Such efforts require a fundamental understanding of the enzymatic activities and regulation of fatty acid synthases. Despite more than one hundred years of research, we continue to learn new lessons about fatty acid synthases' many intricate structural and regulatory elements. In this review, we summarize each enzymatic domain and discuss efforts to engineer fatty acid synthases, providing some clues to important challenges and opportunities in the field.

Biohydrogenation of C20 polyunsaturated fatty acids by anaerobic bacteria

The PUFAs include many bioactive lipids. The microbial metabolism of C18 PUFAs is known to produce their bioactive isomers, such as conjugated FAs and hydroxy FAs, but there is little information on that of C20 PUFAs. In this study, we aimed to obtain anaerobic bacteria with the ability to produce novel PUFAs from C20 PUFAs. Through the screening of ∼100 strains of anaerobic bacteria, Clostridium bifermentans JCM 1386 was selected as a strain with the ability to saturate PUFAs during anaerobic cultivation. This strain converted arachidonic acid (cis-5,cis-8,cis-11,cis-14-eicosatetraenoic acid) and EPA (cis-5,cis-8,cis-11,cis-14,cis-17-EPA) into cis-5,cis-8,trans-13-eicosatrienoic acid and cis-5,cis-8,trans-13,cis-17-eicosatetraenoic acid, giving yields of 57% and 67% against the added PUFAs, respectively. This is the first report of the isolation of a bacterium transforming C20 PUFAs into corresponding non-methylene-interrupted FAs. We further investigated the substrate specificity of the biohydrogenation by this strain and revealed that it can convert two cis double bonds at the ω6 and ω9 positions in various C18 and C20 PUFAs into a trans double bond at the ω7 position. This study should serve to open up the development of novel potentially bioactive PUFAs.

Expression of yeast acyl-CoA-∆9 desaturase leads to accumulation of unusual monounsaturated fatty acids in soybean seeds

An acyl-CoA-Δ9 desaturase from Saccharomyces cerevisiae was expressed by subcellular-targeting in soybean (Glycine max) seeds with the goal of increasing palmitoleic acid (16:1Δ9), a high-valued fatty acid (FA), and simultaneously decreasing saturated FA in oil. The expression resulted in the conversion of palmitic acid (16:0) to 16:1Δ9 in soybean seeds. 16:1Δ9 and its elongation product cis-vaccenic acid (18:1Δ11) were increased to 17 % of the total fatty acids by plastid-targeted expression of the enzyme. Other lipid changes include the decrease of polyunsaturated FA and saturated FA, suggesting that a mechanism exists downstream in oil biosynthesis to compensate the FA alternation. This is the first time a cytosolic acyl-CoA-∆9 desaturase is functionally expressed in plastid and stronger activity was achieved than its cytosolic expression. The present study provides a new strategy for converting 16:0 to 16:1Δ9 by engineering acyl-CoA-Δ9 desaturase in commercialized oilseeds.

Reducing saturated fatty acids in Arabidopsis seeds by expression of a Caenorhabditis elegans 16:0-specific desaturase

Plant oilseeds are a major source of nutritional oils. Their fatty acid composition, especially the proportion of saturated and unsaturated fatty acids, has important effects on human health. Because intake of saturated fats is correlated with the incidence of cardiovascular disease and diabetes, a goal of metabolic engineering is to develop oils low in saturated fatty acids. Palmitic acid (16:0) is the most abundant saturated fatty acid in the seeds of many oilseed crops and in Arabidopsis thaliana. We expressed FAT-5, a membrane-bound desaturase cloned from Caenorhabditis elegans, in Arabidopsis using a strong seed-specific promoter. The FAT-5 enzyme is highly specific to 16:0 as substrate, converting it to 16:1∆9; expression of fat-5 reduced the 16:0 content of the seed by two-thirds. Decreased 16:0 and elevated 16:1 levels were evident both in the storage and membrane lipids of seeds. Regiochemical analysis of phosphatidylcholine showed that 16:1 was distributed at both positions on the glycerolipid backbone, unlike 16:0, which is predominately found at the sn-1 position. Seeds from a plant line homozygous for FAT-5 expression were comparable to wild type with respect to seed set and germination, while oil content and weight were somewhat reduced. These experiments demonstrate that targeted heterologous expression of a desaturase in oilseeds can reduce the level of saturated fatty acids in the oil, significantly improving its nutritional value.

Metabolic engineering of plant oils and waxes for use as industrial feedstocks

Society has come to rely heavily on mineral oil for both energy and petrochemical needs. Plant lipids are uniquely suited to serve as a renewable source of high-value fatty acids for use as chemical feedstocks and as a substitute for current petrochemicals. Despite the broad variety of acyl structures encountered in nature and the cloning of many genes involved in their biosynthesis, attempts at engineering economic levels of specialty industrial fatty acids in major oilseed crops have so far met with only limited success. Much of the progress has been hampered by an incomplete knowledge of the fatty acid biosynthesis and accumulation pathways. This review covers new insights based on metabolic flux and reverse engineering studies that have changed our view of plant oil synthesis from a mostly linear process to instead an intricate network with acyl fluxes differing between plant species. These insights are leading to new strategies for high-level production of industrial fatty acids and waxes. Furthermore, progress in increasing the levels of oil and wax structures in storage and vegetative tissues has the potential to yield novel lipid production platforms. The challenge and opportunity for the next decade will be to marry these technologies when engineering current and new crops for the sustainable production of oil and wax feedstocks.

Control limits for accumulation of plant metabolites: brute force is no substitute for understanding

Which factors limit metabolite accumulation in plant cells? Are theories on flux control effective at explaining the results? Many biotechnologists cling to the idea that every pathway has a rate limiting enzyme and target such enzymes first in order to modulate fluxes. This often translates into large effects on metabolite concentration, but disappointing small increases in flux. Rate limiting enzymes do exist, but are rare and quite opposite to what predicted by biochemistry. In many cases however, flux control is shared among many enzymes. Flux control and concentration control can (and must) be distinguished and quantified for effective manipulation. Flux control for several 'building blocks' of metabolism is placed on the demand side, and therefore increasing demand can be very successful. Tampering with supply, particularly desensitizing supply enzymes, is usually not very effective, if not dangerous, because supply regulatory mechanisms function to control metabolite homeostasis. Some important, but usually unnoticed, metabolic constraints shape the responses of metabolic systems to manipulation: mass conservation, cellular resource allocation and, most prominently, energy supply, particularly in heterotrophic tissues. The theoretical basis for this view shall be explored with recent examples gathered from the manipulation of several metabolites (vitamins, carotenoids, amino acids, sugars, fatty acids, polyhydroxyalkanoates, fructans and sugar alcohols). Some guiding principles are suggested for an even more successful engineering of plant metabolism.

A galactoglycerolipid lipase is required for triacylglycerol accumulation and survival following nitrogen deprivation in Chlamydomonas reinhardtii

Following N deprivation, microalgae accumulate triacylglycerols (TAGs). To gain mechanistic insights into this phenomenon, we identified mutants with reduced TAG content following N deprivation in the model alga Chlamydomonas reinhardtii. In one of the mutants, the disruption of a galactoglycerolipid lipase-encoding gene, designated PLASTID GALACTOGLYCEROLIPID DEGRADATION1 (PGD1), was responsible for the primary phenotype: reduced TAG content, altered TAG composition, and reduced galactoglycerolipid turnover. The recombinant PGD1 protein, which was purified from Escherichia coli extracts, hydrolyzed monogalactosyldiacylglycerol into its lyso-lipid derivative. In vivo pulse-chase labeling identified galactoglycerolipid pools as a major source of fatty acids esterified in TAGs following N deprivation. Moreover, the fatty acid flux from plastid lipids to TAG was decreased in the pgd1 mutant. Apparently, de novo-synthesized fatty acids in Chlamydomonas reinhardtii are, at least partially, first incorporated into plastid lipids before they enter TAG synthesis. As a secondary effect, the pgd1 mutant exhibited a loss of viability following N deprivation, which could be avoided by blocking photosynthetic electron transport. Thus, the pgd1 mutant provides evidence for an important biological function of TAG synthesis following N deprivation, namely, relieving a detrimental overreduction of the photosynthetic electron transport chain.

Feedback regulation of plastidic acetyl-CoA carboxylase by 18:1-acyl carrier protein in Brassica napus

Plant seed oil represents a major renewable source of reduced carbon, but little is known about the biochemical regulation of its synthesis. The goal of this research was to identify potential feedback regulation of fatty acid biosynthesis in Brassica napus embryo-derived cell cultures and to characterize both the feedback signals and enzymatic targets of the inhibition. Fatty acids delivered via Tween esters rapidly reduced the rate of fatty acid synthesis in a dose-dependent and reversible manner, demonstrating the existence of feedback inhibition in an oil-accumulating tissue. Tween feeding did not affect fatty acid elongation in the cytosol or the incorporation of radiolabeled malonate into nascent fatty acids, which together pinpoint plastidic acetyl-CoA carboxylase (ACCase) as the enzymatic target of feedback inhibition. To identify the signal responsible for feedback, a variety of Tween esters were tested for their effects on the rate of fatty acid synthesis. Maximum inhibition was achieved upon feeding oleic acid (18:1) Tween esters that resulted in the intracellular accumulation of 18:1 free fatty acid, 18:1-CoA, and 18:1-acyl-carrier protein (ACP). Direct, saturable inhibition of ACCase enzyme activity was observed in culture extracts and in extracts of developing canola seeds supplemented with 18:1-ACP at physiological concentrations. A mechanism for feedback inhibition is proposed in which reduced demand for de novo fatty acids results in the accumulation of 18:1-ACP, which directly inhibits plastidic ACCase, leading to reduced fatty acid synthesis. Defining this mechanism presents an opportunity for mitigating feedback inhibition of fatty acid synthesis in crop plants to increase oil yield.

Biosynthesis and metabolic engineering of palmitoleate production, an important contributor to human health and sustainable industry

Palmitoleate (cis-Δ9-16:1) shows numerous health benefits such as increased cell membrane fluidity, reduced inflammation, protection of the cardiovascular system, and inhibition of oncogenesis. Plant oils containing this unusual fatty acid can also be sustainable feedstocks for producing industrially important and high-demand 1-octene. Vegetable oils rich in palmitoleate are the ideal candidates for biodiesel production. Several wild plants are known that can synthesize high levels of palmitoleate in seeds. However, low yields and poor agronomic characteristics of these plants limit their commercialization. Metabolic engineering has been developed to create oilseed crops that accumulate high levels of palmitoleate or other unusual fatty acids, and significant advances have been made recently in this field, particularly using the model plant Arabidopsis as the host. The engineered targets for enhancing palmitoleate synthesis include overexpression of Δ9 desaturase from mammals, yeast, fungi, and plants, down-regulating KASII, coexpression of an ACP-Δ9 desaturase in plastids and CoA-Δ9 desaturase in endoplasmic reticulum (ER), and optimizing the metabolic flux into triacylglycerols (TAGs). This review will mainly describe the recent progress towards producing palmitoleate in transgenic plants by metabolic engineering along with our current understanding of palmitoleate biosynthesis and its regulation, as well as highlighting the bottlenecks that require additional investigation by combining lipidomics, transgenics and other "-omics" tools. A brief review of reported health benefits and non-food uses of palmitoleate will also be covered.

Fatty acid composition of developing sea buckthorn (Hippophae rhamnoides L.) berry and the transcriptome of the mature seed

Background

Sea buckthorn (Hippophae rhamnoides L.) is a hardy, fruit-producing plant known historically for its medicinal and nutraceutical properties. The most recognized product of sea buckthorn is its fruit oil, composed of seed oil that is rich in essential fatty acids, linoleic (18∶2ω-6) and α-linolenic (18∶3ω-3) acids, and pulp oil that contains high levels of monounsaturated palmitoleic acid (16∶1ω-7). Sea buckthorn is fast gaining popularity as a source of functional food and nutraceuticals, but currently has few genomic resources; therefore, we explored the fatty acid composition of Canadian-grown cultivars (ssp. mongolica) and the sea buckthorn seed transcriptome using the 454 GS FLX sequencing technology.

Results

GC-MS profiling of fatty acids in seeds and pulp of berries indicated that the seed oil contained linoleic and α-linolenic acids at 33–36% and 30–36%, respectively, while the pulp oil contained palmitoleic acid at 32–42%. 454 sequencing of sea buckthorn cDNA collections from mature seeds yielded 500,392 sequence reads, which identified 89,141 putative unigenes represented by 37,482 contigs and 51,659 singletons. Functional annotation by Gene Ontology and computational prediction of metabolic pathways indicated that primary metabolism (protein>nucleic acid>carbohydrate>lipid) and fatty acid and lipid biosynthesis pathways were highly represented categories. Sea buckthorn sequences related to fatty acid biosynthesis genes in Arabidopsis were identified, and a subset of these was examined for transcript expression at four developing stages of the berry.

Conclusion

This study provides the first comprehensive genomic resources represented by expressed sequences for sea buckthorn, and demonstrates that the seed oil of Canadian-grown sea buckthorn cultivars contains high levels of linoleic acid and α-linolenic acid in a close to 1∶1 ratio, which is beneficial for human health. These data provide the foundation for further studies on sea buckthorn oil, the enzymes involved in its biosynthesis, and the genes involved in the general hardiness of sea buckthorn against environmental conditions.

The significance of different diacylgycerol synthesis pathways on plant oil composition and bioengineering

The unique properties of vegetable oils from different plants utilized for food, industrial feedstocks, and fuel is dependent on the fatty acid (FA) composition of triacylglycerol (TAG). Plants can use two main pathways to produce diacylglycerol (DAG), the immediate precursor molecule to TAG synthesis: (1) De novo DAG synthesis, and (2) conversion of the membrane lipid phosphatidylcholine (PC) to DAG. The FA esterified to PC are also the substrate for FA modification (e.g., desaturation, hydroxylation, etc.), such that the FA composition of PC-derived DAG can be substantially different than that of de novo DAG. Since DAG provides two of the three FA in TAG, the relative flux of TAG synthesis from de novo DAG or PC-derived DAG can greatly affect the final oil FA composition. Here we review how the fluxes through these two alternate pathways of DAG/TAG synthesis are determined and present evidence that suggests which pathway is utilized in different plants. Additionally, we present examples of how the endogenous DAG synthesis pathway in a transgenic host plant can produce bottlenecks for engineering of plant oil FA composition, and discuss alternative strategies to overcome these bottlenecks to produce crop plants with designer vegetable oil compositions.