Dual role for phospholipid:diacylglycerol acyltransferase: enhancing fatty acid synthesis and diverting fatty acids from membrane lipids to triacylglycerol in Arabidopsis leaves

Triacylglycerol Accumulation in Photosynthetic Cells in Plants and Algae

Plant and algal oils are some of the most energy-dense renewable compounds provided by nature. Triacylglycerols (TAGs) are the major constituent of plant oils, which can be converted into fatty acid methyl esters commonly known as biodiesel. As one of the most efficient producers of TAGs, photosynthetic microalgae have attracted substantial interest for renewable fuel production. Currently, the big challenge of microalgae based TAGs for biofuels is their high cost compared to fossil fuels. A conundrum is that microalgae accumulate large amounts of TAGs only during stress conditions such as nutrient deprivation and temperature stress, which inevitably will inhibit growth. Thus, a better understanding of why and how microalgae induce TAG biosynthesis under stress conditions would allow the development of engineered microalgae with increased TAG production during conditions optimal for growth. Land plants also synthesize TAGs during stresses and we will compare new findings on environmental stress-induced TAG accumulation in plants and microalgae especially in the well-characterized model alga Chlamydomonas reinhardtii and a biotechnologically relevant genus Nannochloropsis.

Identification of Characteristic Fatty Acids to Quantify Triacylglycerols in Microalgae

The fatty acid profiles of lipids from microalgae are unique. Polyunsaturated fatty acids are generally enriched in polar lipids, whereas saturated and monounsaturated fatty acids constitute the majority of fatty acids in triacylglycerols (TAG). Each species has characteristic fatty acids, and their content is positively or negatively correlated with TAGs. The marine oleaginous diatom Phaeodactylum tricornutum was used as the paradigm to determine the quantitative relationship between TAG and characteristic fatty acid content. Fatty acid profiles and TAG content of Phaeodactylum tricornutum were determined in a time course. C16:0/C16:1 and eicosapentaenoic acid (EPA, C20:5n3) were identified as characteristic fatty acids in TAGs and polar lipids, respectively. The percentage of those characteristic fatty acids in total fatty acids had a significant linear relationship with TAG content, and thus, the correlation coefficient presenting r (2) were 0.96, 0.94, and 0.97, respectively. The fatty acid-based method for TAG quantification could also be applied to other microalgae such as Nannochloropsis oceanica in which the r (2) of C16:0 and EPA were 0.94 and 0.97, respectively, and in Chlorella pyrenoidosa r (2)-values for C18:1 and C18:3 with TAG content were 0.91 and 0.99, repectively. This characteristic fatty acid-based method provided a distinct way to quantify TAGs in microalgae, by which TAGs could be measured precisely by immediate transesterification from wet biomass rather than using conventional methods. This procedure simplified the operation and required smaller samples than conventional methods.

Arabidopsis TRIGALACTOSYLDIACYLGLYCEROL5 Interacts with TGD1, TGD2, and TGD4 to Facilitate Lipid Transfer from the Endoplasmic Reticulum to Plastids

The biogenesis of photosynthetic membranes in the plastids of higher plants requires an extensive supply of lipid precursors from the endoplasmic reticulum (ER). Four TRIGALACTOSYLDIACYLGLYCEROL (TGD) proteins (TGD1,2,3,4) have thus far been implicated in this lipid transfer process. While TGD1, TGD2, and TGD3 constitute an ATP binding cassette transporter complex residing in the plastid inner envelope, TGD4 is a transmembrane lipid transfer protein present in the outer envelope. These observations raise questions regarding how lipids transit across the aqueous intermembrane space. Here, we describe the isolation and characterization of a novel Arabidopsis thaliana gene, TGD5. Disruption of TGD5 results in similar phenotypic effects as previously described in tgd1,2,3,4 mutants, including deficiency of ER-derived thylakoid lipids, accumulation of oligogalactolipids, and triacylglycerol. Genetic analysis indicates that TGD4 is epistatic to TGD5 in ER-to-plastid lipid trafficking, whereas double mutants of a null tgd5 allele with tgd1-1 or tgd2-1 show a synergistic embryo-lethal phenotype. TGD5 encodes a small glycine-rich protein that is localized in the envelope membranes of chloroplasts. Coimmunoprecipitation assays show that TGD5 physically interacts with TGD1, TGD2, TGD3, and TGD4. Collectively, these results suggest that TGD5 facilitates lipid transfer from the outer to the inner plastid envelope by bridging TGD4 with the TGD1,2,3 transporter complex.

Transcriptional transitions in Nicotiana benthamiana leaves upon induction of oil synthesis by WRINKLED1 homologs from diverse species and tissues

BACKGROUND

Carbon accumulation and remobilization are essential mechanisms in plants to ensure energy transfer between plant tissues with different functions or metabolic needs and to support new generations. Knowledge about the regulation of carbon allocation into oil (triacylglycerol) in plant storage tissue can be of great economic and environmental importance for developing new high-yielding oil crops. Here, the effect on global gene expression as well as on physiological changes in leaves transiently expressing five homologs of the transcription factor WRINKLED1 (WRI1) originating from diverse species and tissues; Arabidopsis thaliana and potato (Solanum tuberosum) seed embryo, poplar (Populus trichocarpa) stem cambium, oat (Avena sativa) grain endosperm, and nutsedge (Cyperus esculentus) tuber parenchyma, were studied by agroinfiltration in Nicotiana benthamiana.

RESULTS

All WRI1 homologs induced oil accumulation when expressed in leaf tissue. Transcriptome sequencing revealed that all homologs induced the same general patterns with a drastic shift in gene expression profiles of leaves from that of a typical source tissue to a source-limited sink-like tissue: Transcripts encoding enzymes for plastid uptake and metabolism of phosphoenolpyruvate, fatty acid and oil biosynthesis were up-regulated, as were also transcripts encoding starch degradation. Transcripts encoding enzymes in photosynthesis and starch synthesis were instead down-regulated. Moreover, transcripts representing fatty acid degradation were up-regulated indicating that fatty acids might be degraded to feed the increased need to channel carbons into fatty acid synthesis creating a futile cycle. RT-qPCR analysis of leaves expressing Arabidopsis WRI1 showed the temporal trends of transcripts selected as 'markers' for key metabolic pathways one to five days after agroinfiltration. Chlorophyll fluorescence measurements of leaves expressing Arabidopsis WRI1 showed a significant decrease in photosynthesis, even though effect on starch content could not be observed.

CONCLUSIONS

This data gives for the first time a general view on the transcriptional transitions in leaf tissue upon induction of oil synthesis by WRI1. This yields important information about what effects WRI1 may exert on global gene expression during seed and embryo development. The results suggest why high oil content in leaf tissue cannot be achieved by solely transcriptional activation by WRI1, which can be essential knowledge in the development of new high-yielding oil crops.

PHOSPHATIDIC ACID PHOSPHOHYDROLASE Regulates Phosphatidylcholine Biosynthesis in Arabidopsis by Phosphatidic Acid-Mediated Activation of CTP:PHOSPHOCHOLINE CYTIDYLYLTRANSFERASE Activity

Regulation of membrane lipid biosynthesis is critical for cell function. We previously reported that disruption of PHOSPHATIDIC ACID PHOSPHOHYDROLASE1 (PAH1) and PAH2 stimulates net phosphatidylcholine (PC) biosynthesis and proliferation of the endoplasmic reticulum (ER) in Arabidopsis thaliana. Here, we show that this response is caused specifically by a reduction in the catalytic activity of the protein and positively correlates with an accumulation of its substrate, phosphatidic acid (PA). The accumulation of PC in pah1 pah2 is suppressed by disruption of CTP:PHOSPHOCHOLINE CYTIDYLYLTRANSFERASE1 (CCT1), which encodes a key enzyme in the nucleotide pathway for PC biosynthesis. The activity of recombinant CCT1 is stimulated by lipid vesicles containing PA. Truncation of CCT1, to remove the predicted C-terminal amphipathic lipid binding domain, produced a constitutively active enzyme. Overexpression of native CCT1 in Arabidopsis has no significant effect on PC biosynthesis or ER morphology, but overexpression of the truncated constitutively active version largely replicates the pah1 pah2 phenotype. Our data establish that membrane homeostasis is regulated by lipid composition in Arabidopsis and reveal a mechanism through which the abundance of PA, mediated by PAH activity, modulates CCT activity to govern PC content.

Tracking the metabolic pulse of plant lipid production with isotopic labeling and flux analyses: Past, present and future

Metabolism is comprised of networks of chemical transformations, organized into integrated biochemical pathways that are the basis of cellular operation, and function to sustain life. Metabolism, and thus life, is not static. The rate of metabolites transitioning through biochemical pathways (i.e., flux) determines cellular phenotypes, and is constantly changing in response to genetic or environmental perturbations. Each change evokes a response in metabolic pathway flow, and the quantification of fluxes under varied conditions helps to elucidate major and minor routes, and regulatory aspects of metabolism. To measure fluxes requires experimental methods that assess the movements and transformations of metabolites without creating artifacts. Isotopic labeling fills this role and is a long-standing experimental approach to identify pathways and quantify their metabolic relevance in different tissues or under different conditions. The application of labeling techniques to plant science is however far from reaching it potential. In light of advances in genetics and molecular biology that provide a means to alter metabolism, and given recent improvements in instrumentation, computational tools and available isotopes, the use of isotopic labeling to probe metabolism is becoming more and more powerful. We review the principal analytical methods for isotopic labeling with a focus on seminal studies of pathways and fluxes in lipid metabolism and carbon partitioning through central metabolism. Central carbon metabolic steps are directly linked to lipid production by serving to generate the precursors for fatty acid biosynthesis and lipid assembly. Additionally some of the ideas for labeling techniques that may be most applicable for lipid metabolism in the future were originally developed to investigate other aspects of central metabolism. We conclude by describing recent advances that will play an important future role in quantifying flux and metabolic operation in plant tissues.

Tracking synthesis and turnover of triacylglycerol in leaves

Triacylglycerol (TAG), typically represents <1% of leaf glycerolipids but can accumulate under stress and other conditions or if leaves are supplied with fatty acids, or in plants transformed with regulators or enzymes of lipid metabolism. To better understand the metabolism of TAG in leaves, pulse-chase radiolabelling experiments were designed to probe its synthesis and turnover. When Arabidopsis leaves were incubated with [(14)C]lauric acid (12:0), a major initial product was [(14)C]TAG. Thus, despite low steady-state levels, leaves possess substantial TAG biosynthetic capacity. The contributions of diacylglycerol acyltransferase1 and phospholipid:diacylglycerol acyltransferase1 to leaf TAG synthesis were examined by labelling of dgat1 and pdat1 mutants. The dgat1 mutant displayed a major (76%) reduction in [(14)C]TAG accumulation whereas pdat1 TAG labelling was only slightly reduced. Thus, DGAT1 has a principal role in TAG biosynthesis in young leaves. During a 4h chase period, radioactivity in TAG declined 70%, whereas the turnover of [(14)C]acyl chains of phosphatidylcholine (PC) and other polar lipids was much lower. Sixty percent of [(14)C]12:0 was directly incorporated into glycerolipids without modification, whereas 40% was elongated and desaturated to 16:0 and 18:1 by plastids. The unmodified [(14)C]12:0 and the plastid products of [(14)C]12:0 metabolism entered different pathways. Although plastid-modified (14)C-labelled products accumulated in monogalactosyldiacylglycerol, PC, phosphatidylethanolamine, and diacylglcerol (DAG), there was almost no accumulation of [(14)C]16:0 and [(14)C]18:1 in TAG. Because DAG and acyl-CoA are direct precursors of TAG, the differential labelling of polar glycerolipids and TAG by [(14)C]12:0 and its plastid-modified products provides evidence for multiple subcellular pools of both acyl-CoA and DAG.

Genome-wide analysis of PHOSPHOLIPID:DIACYLGLYCEROL ACYLTRANSFERASE (PDAT) genes in plants reveals the eudicot-wide PDAT gene expansion and altered selective pressures acting on the core eudicot PDAT paralogs

PHOSPHOLIPID:DIACYLGLYCEROL ACYLTRANSFERASE (PDAT) is an enzyme that catalyzes the transfer of a fatty acyl moiety from the sn-2 position of a phospholipid to the sn-3-position of sn-1,2-diacylglyerol, thus forming triacylglycerol and a lysophospholipid. Although the importance of PDAT in triacylglycerol biosynthesis has been illustrated in some previous studies, the evolutionary relationship of plant PDATs has not been studied in detail. In this study, we investigated the evolutionary relationship of the PDAT gene family across the green plants using a comparative phylogenetic framework. We found that the PDAT candidate genes are present in all examined green plants, including algae, lowland plants (a moss and a lycophyte), monocots, and eudicots. Phylogenetic analysis revealed the evolutionary division of the PDAT gene family into seven major clades. The separation is supported by the conservation and variation in the gene structure, protein properties, motif patterns, and/or selection constraints. We further demonstrated that there is a eudicot-wide PDAT gene expansion, which appears to have been mainly caused by the eudicot-shared ancient gene duplication and subsequent species-specific segmental duplications. In addition, selection pressure analyses showed that different selection constraints have acted on three core eudicot clades, which might enable paleoduplicated PDAT paralogs to either become nonfunctionalized or develop divergent expression patterns during evolution. Overall, our study provides important insights into the evolution of the plant PDAT gene family and explores the evolutionary mechanism underlying the functional diversification among the core eudicot PDAT paralogs.

Understanding the biochemical basis of temperature-induced lipid pathway adjustments in plants

Glycerolipid biosynthesis in plants proceeds through two major pathways compartmentalized in the chloroplast and the endoplasmic reticulum (ER). The involvement of glycerolipid pathway interactions in modulating membrane desaturation under temperature stress has been suggested but not fully explored. We profiled glycerolipid changes as well as transcript dynamics under suboptimal temperature conditions in three plant species that are distinctively different in the mode of lipid pathway interactions. In Arabidopsis thaliana, a 16:3 plant, the chloroplast pathway is upregulated in response to low temperature, whereas high temperature promotes the eukaryotic pathway. Operating under a similar mechanistic framework, Atriplex lentiformis at high temperature drastically increases the contribution of the eukaryotic pathway and correspondingly suppresses the prokaryotic pathway, resulting in the switch of lipid profile from 16:3 to 18:3. In wheat (Triticum aestivum), an 18:3 plant, low temperature also influences the channeling of glycerolipids from the ER to chloroplast. Evidence of differential trafficking of diacylglycerol moieties from the ER to chloroplast was uncovered in three plant species as another layer of metabolic adaptation under temperature stress. We propose a model that highlights the predominance and prevalence of lipid pathway interactions in temperature-induced lipid compositional changes.

Expression of Mouse MGAT in Arabidopsis Results in Increased Lipid Accumulation in Seeds

Worldwide demand for vegetable oil is projected to double within the next 30 years due to increasing food, fuel, and industrial requirements. There is therefore great interest in metabolic engineering strategies that boost oil accumulation in plant tissues, however, efforts to date have only achieved levels of storage lipid accumulation in plant tissues far below the benchmark to meet demand. Monoacylglycerol acyltransferase (MGAT) is predominantly associated with lipid absorption and resynthesis in the animal intestine where it catalyzes monoacylglycerol (MAG) to form diacylglycerol (DAG), and then triacylglycerol (TAG). In contrast plant lipid biosynthesis routes do not include MGAT. Rather, DAG and TAG are either synthesized from glycerol-3-phosphate by a series of three subsequent acylation reactions, or originated from phospholipids via an acyl editing pathway. Mouse MGATs 1 and 2 have been shown to increase oil content transiently in Nicotiana benthamiana leaf tissue by 2.6 fold. Here we explore the feasibility of this approach to increase TAG in Arabidopsis thaliana seed. The stable MGAT2 expression resulted in a significant increase in seed oil content by 1.32 fold. We also report evidence of the MGAT2 activity based on in vitro assays. Up to 3.9 fold increase of radiolabeled DAG were produced in seed lysate which suggest that the transgenic MGAT activity can result in DAG re-synthesis by salvaging the MAG product of lipid breakdown. The expression of MGAT2 therefore creates an independent and complementary TAG biosynthesis route to the endogenous Kennedy pathway and other glycerolipid synthesis routes.

Wrinkled1 Accelerates Flowering and Regulates Lipid Homeostasis between Oil Accumulation and Membrane Lipid Anabolism in Brassica napus

Wrinkled1 (WRI1) belongs to the APETALA2 transcription factor family; it is unique to plants and is a central regulator of oil synthesis in Arabidopsis. The effects of WRI1 on comprehensive lipid metabolism and plant development were unknown, especially in crop plants. This study found that BnWRI1 in Brassica napus accelerated flowering and enhanced oil accumulation in both seeds and leaves without leading to a visible growth inhibition. BnWRI1 decreased storage carbohydrates and increased soluble sugars to facilitate the carbon flux to lipid anabolism. BnWRI1 is localized to the nucleus and directly binds to the AW-box at proximal upstream regions of genes involved in fatty acid (FA) synthesis and lipid assembly. The overexpression (OE) of BnWRI1 resulted in the up-regulation of genes involved in glycolysis, FA synthesis, lipid assembly, and flowering. Lipid profiling revealed increased galactolipids monogalactosyldiacylglycerol (MGDG), digalactosyldiacylglycerol (DGDG), and phosphatidylcholine (PC) in the leaves of OE plants, whereas it exhibited a reduced level of the galactolipids DGDG and MGDG and increased levels of PC, phosphatidylethanolamide, and oil [triacylglycerol (TAG)] in the siliques of OE plants during the early seed development stage. These results suggest that BnWRI1 is important for homeostasis among TAG, membrane lipids and sugars, and thus facilitates flowering and oil accumulation in B. napus.

Deciphering the roles of Arabidopsis LPCAT and PAH in phosphatidylcholine homeostasis and pathway coordination for chloroplast lipid synthesis

Phosphatidylcholine (PC) is a key intermediate in the metabolic network of glycerolipid biosynthesis. Lysophosphatidylcholine acyltransferase (LPCAT) and phosphatidic acid phosphatase (PAH) are two key enzymes of PC homeostasis. We report that LPCAT activity is markedly induced in the Arabidopsis pah mutant. The quadruple pah lpcat mutant, with dual defects in PAH and LPCAT, had a level of lysophosphatidylcholine (LPC) that was much higher than that in the lpcat mutants and a PC content that was higher than that in the pah mutant. Comparative molecular profile analysis of monogalactosyldiacylglycerol and digalactosyldiacylglycerol revealed that both the pah and pah lpcat mutants had increased proportions of 34:6 from the prokaryotic pathway despite differing levels of LPCAT activity. We show that a decreased representation of the C16:0 C18:2 diacylglycerol moiety in PC was a shared feature of pah and pah lpcat, and that this change in PC metabolic profile correlated with the increased prokaryotic contribution to chloroplast lipid synthesis. We detected increased PC deacylation in the pah lpcat mutant that was attributable at least in part to the induced phospholipases. Increased LPC generation was also evident in the pah mutant, but the phospholipases were not induced, raising the possibility that PC deacylation is mediated by the reverse reaction of LPCAT. We discuss possible roles of LPCAT and PAH in PC turnover that impacts lipid pathway coordination for chloroplast lipid synthesis.

Preferred carbon precursors for lipid labelling in the heterotrophic endosperm of developing oat (Avena sativa L.) grains

Oat (Avena sativa L.) is unusual among the cereal grains in storing high amounts of oil in the endosperm; up to 90% of total grain oil. By using oat as a model species for oil metabolism in the cereal endosperm, we can learn how to develop strategies to redirect carbon from starch to achieve high-oil yielding cereal crops. Carbon precursors for lipid synthesis were compared in two genetically close oat cultivars with different endosperm oil content (about 6% and 10% of grain dw, medium-oil; MO, and high-oil; HO cultivar, respectively) by supplying a variety of (14)C-labelled substrates to the grain from both up- and downstream parts of glycolysis, either through detached oat panicles in vitro or by direct injection in planta. When supplied by direct injection, (14)C from acetate was identified to label the lipid fraction of the grain to the highest extent among substrates tested; 46% of net accumulated (14)C, demonstrating its applicability as a marker for lipids in the endosperm. Time course analyses of injected (14)C acetate during grain development suggested a more efficient transfer of fatty acids from polar lipids to triacylglycerol in the HO as compared to the MO cultivar, and turnover of triacylglycerol was suggested to not play a major role for the final oil content of oat grain endosperm despite the low amount of protective oleosins in this tissue. Moreover, availability of light was shown to drastically affect grain net carbon accumulation from (14)C-sucrose when supplied through detached panicles for the HO cultivar.

Arabidopsis lipins, PDAT1 acyltransferase, and SDP1 triacylglycerol lipase synergistically direct fatty acids toward β-oxidation, thereby maintaining membrane lipid homeostasis

Triacylglycerol (TAG) metabolism is a key aspect of intracellular lipid homeostasis in yeast and mammals, but its role in vegetative tissues of plants remains poorly defined. We previously reported that PHOSPHOLIPID:DIACYLGLYCEROL ACYLTRANSFERASE1 (PDAT1) is crucial for diverting fatty acids (FAs) from membrane lipid synthesis to TAG and thereby protecting against FA-induced cell death in leaves. Here, we show that overexpression of PDAT1 enhances the turnover of FAs in leaf lipids. Using the trigalactosyldiacylglycerol1-1 (tgd1-1) mutant, which displays substantially enhanced PDAT1-mediated TAG synthesis, we demonstrate that disruption of SUGAR-DEPENDENT1 (SDP1) TAG lipase or PEROXISOMAL TRANSPORTER1 (PXA1) severely decreases FA turnover, leading to increases in leaf TAG accumulation, to 9% of dry weight, and in total leaf lipid, by 3-fold. The membrane lipid composition of tgd1-1 sdp1-4 and tgd1-1 pxa1-2 double mutants is altered, and their growth and development are compromised. We also show that two Arabidopsis thaliana lipin homologs provide most of the diacylglycerol for TAG synthesis and that loss of their functions markedly reduces TAG content, but with only minor impact on eukaryotic galactolipid synthesis. Collectively, these results show that Arabidopsis lipins, along with PDAT1 and SDP1, function synergistically in directing FAs toward peroxisomal β-oxidation via TAG intermediates, thereby maintaining membrane lipid homeostasis in leaves.

The characteristics and potential applications of structural lipid droplet proteins in plants

Plant cytosolic lipid droplets are storage organelles that accumulate hydrophobic molecules. They are found in many tissues and their general structure includes an outer lipid monolayer with integral and associated proteins surrounding a hydrophobic core. Two distinct types can be distinguished, which we define here as oleosin-based lipid droplets (OLDs) and non-oleosin-based lipid droplets (NOLDs). OLDs are the best characterized lipid droplets in plants. They are primarily restricted to seeds and other germinative tissues, their surface is covered with oleosin-family proteins to maintain stability, they store triacylglycerols (TAGs) and they are used as a source of energy (and possibly signaling molecules) during the germination of seeds and pollen. Less is known about NOLDs. They are more abundant than OLDs and are distributed in many tissues, they accumulate not only TAGs but also other hydrophobic molecules such as natural rubber, and the structural proteins that stabilize them are unrelated to oleosins. In many species these proteins are members of the rubber elongation factor superfamily. NOLDs are not typically used for energy storage but instead accumulate hydrophobic compounds required for environmental interactions such as pathogen defense. There are many potential applications of NOLDs including the engineering of lipid production in plants and the generation of artificial oil bodies.

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.

Understanding and manipulating plant lipid composition: Metabolic engineering leads the way

The manipulation of plant seed oil composition so as to deliver enhanced fatty acid compositions suitable for feed or fuel has long been a goal of metabolic engineers. Recent advances in our understanding of the flux of acyl-changes through different key metabolic pools such as phosphatidylcholine and diacylglycerol have allowed for more targeted interventions. When combined in iterative fashion with further lipidomic analyses, significant breakthroughs in our capacity to generate plants with novel oils have been achieved. Collectively these studies, working at the interface between metabolic engineering and synthetic biology, demonstrate the positive fundamental and applied outcomes derived from such research.

Multigene Engineering of Triacylglycerol Metabolism Boosts Seed Oil Content in Arabidopsis

Increasing the yield of oilseed crops is an important objective for biotechnologists. A number of individual genes involved in triacylglycerol metabolism have previously been reported to enhance the oil content of seeds when their expression is altered. However, it has yet to be established whether specific combinations of these genes can be used to achieve an additive effect and whether this leads to enhanced yield. Using Arabidopsis (Arabidopsis thaliana) as an experimental system, we show that seed-specific overexpression of WRINKLED1 (a transcriptional regulator of glycolysis and fatty acid synthesis) and DIACYLGLYCEROL ACYLTRANSFERASE1 (a triacylglycerol biosynthetic enzyme) combined with suppression of the triacylglycerol lipase SUGAR-DEPENDENT1 results in a higher percentage seed oil content and greater seed mass than manipulation of each gene individually. Analysis of total seed yield per plant suggests that, despite a reduction in seed number, the total yield of oil is also increased.

Comparative analyses of three Chlorella species in response to light and sugar reveal distinctive lipid accumulation patterns in the Microalga C. sorokiniana

While photosynthetic microalgae, such as Chlorella, serve as feedstocks for nutritional oils and biofuels, heterotrophic cultivation can augment growth rates, support high cell densities, and increase triacylglycerol (TAG) lipid content. However, these species differ significantly in their photoautotrophic and heterotrophic characteristics. In this study, the phylogeny of thirty Chlorella strains was determined in order to inform bioprospecting efforts and detailed physiological assessment of three species. The growth kinetics and lipid biochemistry of C. protothecoides UTEX 411, C. vulgaris UTEX 265, and C. sorokiniana UTEX 1230 were quantified during photoautotrophy in Bold's basal medium (BBM) and heterotrophy in BBM supplemented with glucose (10 g L⁻¹). Heterotrophic growth rates of UTEX 411, 265, and 1230 were found to be 1.5-, 3.7-, and 5-fold higher than their respective autotrophic rates. With a rapid nine-hour heterotrophic doubling time, Chlorella sorokiniana UTEX 1230 maximally accumulated 39% total lipids by dry weight during heterotrophy compared to 18% autotrophically. Furthermore, the discrete fatty acid composition of each strain was examined in order to elucidate lipid accumulation patterns under the two trophic conditions. In both modes of growth, UTEX 411 and 265 produced 18∶1 as the principal fatty acid while UTEX 1230 exhibited a 2.5-fold enrichment in 18∶2 relative to 18∶1. Although the total lipid content was highest in UTEX 411 during heterotrophy, UTEX 1230 demonstrated a two-fold increase in its heterotrophic TAG fraction at a rate of 28.9 mg L⁻¹ d⁻¹ to reach 22% of the biomass, corresponding to as much as 90% of its total lipids. Interestingly, UTEX 1230 growth was restricted during mixotrophy and its TAG production rate was suppressed to 18.2 mg L⁻¹ d⁻¹. This constraint on carbon flow raises intriguing questions about the impact of sugar and light on the metabolic regulation of microalgal lipid biosynthesis.