Biochemical characterization of a high-palmitoleic acid Helianthus annuus mutant

Transcriptomics of developing wild sunflower seeds from the extreme ends of a latitudinal gradient differing in seed oil composition

Seed oil composition, an important agronomic trait in cultivated sunflower, varies latitudinally across the native range of its wild progenitor. This pattern is thought to be driven by selection for a higher proportion of saturated fatty acids in southern populations compared with northern populations, likely due to the different temperatures experienced during seed germination. To investigate whether these differences in fatty acid composition between northern and southern populations correspond to transcriptional variation in the expression of genes involved in fatty acid metabolism, we sequenced RNA from developing seeds of sunflowers from Texas, USA, and Saskatchewan, Canada (the extreme ends of sunflower's latitudinal range) grown in a common garden. We found 4,741 genes to be differentially expressed between Texas and Canada, including several genes involved in lipid metabolism. Several differentially expressed lipid metabolism genes also colocalized with known oil quantitative trait loci (QTL). The genes producing stearoyl-ACP-desaturases (SAD) were of particular interest because of their known role in the conversion of fully saturated into unsaturated fatty acids. Two SAD genes were more highly expressed in seeds from Canadian populations, consistent with the observation of increased levels of unsaturated fatty acids in seeds from that region. We also constructed a gene co-expression network to investigate regional variation in network modules. The results of this analysis revealed regional differentiation for eight of 12 modules but no clear relationship with oil biosynthesis. Overall, the differential expression of SAD genes offers a partial explanation for the observed differences in seed oil composition between Texas and Canada, while the expression patterns of other metabolic genes suggest complex regulation of fatty acid production and usage across latitudes.

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.

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.

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.

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.

Sunflower HaGPAT9-1 is the predominant GPAT during seed development

In oil crops, triacylglycerol biosynthesis is an important metabolic pathway in which glycerol-3-phosphate acyltransferase (GPAT) performs the first acylation step. Mass spectrometry analysis of developing sunflower (Helianthus annuus) seed membrane fractions identified an abundant GPAT, HaGPAT9 isoform 1, with a N-terminal peptide that possessed two phosphorylated residues with possible regulatory function. HaGPAT9-1 belongs to a broad eukaryotic GPAT family, similar to mammalian GPAT3, and it represents one of the two sunflower GPAT9 isoforms, sharing 90% identity with HaGPAT9-2. Both sunflower genes are expressed during seed development and in vegetative tissues, with HaGPAT9-1 transcripts accumulating at relatively higher levels than those for HaGPAT9-2. Green fluorescent protein tagging of HaGPAT9-1 confirmed its subcellular accumulation in the endoplasmic reticulum. Despite their overall sequence similarities, the two sunflower isoforms displayed significant differences in their enzymatic activities. For instance, HaGPAT9-1 possesses in vivo GPAT activity that rescues the lethal phenotype of the cmy228 yeast strain, while in vitro assays revealed a preference of HaGPAT9-1 for palmitoyl-, oleoyl- and linoleoyl-CoAs of one order of magnitude, with the highest increase in yield for oleoyl- and linoleoyl-CoAs. By contrast, no enzymatic activity could be detected for HaGPAT9-2, even though its over-expression modified the TAG profile of yeast.

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.

Changes in acyl-coenzyme A pools in sunflower seeds with modified fatty acid composition

Triacylglycerols (TAGs) are the main reserve product accumulated by oilseeds and they are synthesized by the successive esterification of acyl-CoA derivatives to glycerol molecules through a series of reactions occurring in the endoplasmic reticulum. Acyl-CoA derivatives produced in developing seeds are derived from the de novo plastidial synthesis of fatty acids. This pool of metabolites is also implicated in the elongation of acyl chains due to the action of extraplastidial fatty acid elongases and the incorporation of polyunsaturated fatty acids into TAGs by reticular transacylase enzymes. Analyzing the composition of this pool of metabolites could help us better understand how plant lipid metabolism is regulated. In the present study, we analyzed the size and composition of the acyl-CoA pools in tissues from three sunflower mutants that accumulate oils with modified fatty acid composition. Acyl-CoAs were transformed into their corresponding acyl-etheno-CoA derivatives and analyzed by high performance liquid chromatography with fluorescence detection. We studied developing seeds, germinating cotyledons and leaf tissue to determine how mutations responsible for these traits alter the acyl-CoA pool and hence, the glycerolipid composition of the seeds. Likewise, we analyzed the metabolism of modified TAGs by cotyledons during germination. The metabolic responses of the plant and the effects of the modifications in lipid metabolism that occurred in these mutants are also discussed.

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.

Acyl-ACP thioesterases from macadamia (Macadamia tetraphylla) nuts: cloning, characterization and their impact on oil composition

The mechanisms by which macadamia nuts accumulate the unusual palmitoleic and asclepic acyl moieties, which constitute up to 20% of the fatty acids in some varieties, are still unknown. Acyl-acyl carrier protein (ACP) thioesterases (EC 3.1.2.14) are intraplastidial enzymes that terminate the synthesis of fatty acids in plants and that facilitate the export of the acyl moieties to the endoplasmic reticulum where they can be used in the production of glycerolipids. Here, we have investigated the possible role of acyl-ACP thioesterase activity in the composition of macadamia kernel oil. Accordingly, two acyl-ACP thioesterases were cloned from developing macadamia kernels, one of the FatA type and the other of the FatB type. These enzymes were heterologously expressed in Escherichia coli, and the recombinant thioesterases were purified, characterized kinetically and assayed with a variety of substrates, demonstrating the high specificity of macadamia FatA towards 16:1-ACP. Acyl-ACP thioesterase activity was also characterized in crude extracts from two different varieties of macadamia, Cate and Beaumont, which accumulate different amounts of n-7 fatty acids. The impact of acyl-ACP thioesterase activities on the oil composition of these kernels is discussed in the light of these results.

Effect of the ferredoxin electron donor on sunflower (Helianthus annuus) desaturases

Ferredoxins are proteins that participate in photosynthesis and in other processes that require reducing equivalents, such as the reduction of nitrogen or fatty acid desaturation. Two classes of ferredoxins have been described in plants: light-regulated photosynthetic ferredoxins and heterotrophic ferredoxins whose activity is not influenced by light. Genes encoding the two forms of ferredoxin have been cloned and characterized in developing sunflower cotyledons. Here, these genes were overexpressed in Escherichia coli and they were purified by ion exchange and size exclusion chromatography to study their capacity to supply electrons to two different sunflower desaturases: soluble stearoyl-ACP desaturase from sunflower cotyledons, and membrane bound desaturase FAD7 expressed in yeast. In both cases photosynthetic ferredoxin was the form that promoted the strongest desaturase activity.

The biochemical characterization of a high-stearic acid sunflower mutant reveals the coordinated regulation of stearoyl-acyl carrier protein desaturases

In this study, we have biochemically characterized the high-stearic sunflower (Helianthus annuus L.) mutant CAS-14. This mutant displays an abnormal fatty acid composition along the length of the seed when grown at high temperatures. Thus, at the expense of oleate and linoleate, CAS-14 seeds present an increasing axial gradient of stearic acid from the embryo to the distal extreme of the seed. The accumulation of oil in this mutant was initially characterized by analysing the incorporation of radiolabelled acetate, a fatty acid synthetic precursor, into the developing seed tissues in vivo. These experiments indicated that the mutant phenotype was associated with a decrease in the soluble stearoyl-acyl carrier protein desaturase (SAD) activity, as later confirmed when assessing this activity in cell-free extracts from developing sunflower kernels. Furthermore, SAD enzyme gene transcription was also examined in this tissue, identifying the coordinated decrease in the transcription of the sad17 and sad6 genes as underlying the decrease in enzyme activity. On the basis of these results and those previously obtained on the inheritance of the CAS-14 trait, we discuss the possible regulatory mechanisms acting on plant soluble desaturases.

Phospholipid molecular profiles in the seed kernel from different sunflower (Helianthus annuus) mutants

Phospholipids are essential components of plant cell membranes whose acyl composition appears to be influenced by oil composition in the sunflower. In the current study, we have determined the diacylglycerol profile of the main phospholipids using phospholipase C degradation and separation of the diacylglycerols by HPLC and GLC. The main polar lipid molecular species were defined in different classes of sunflower kernel: PC, PE, and PI. The proportions of each were determined at different stages of development in order to define the point at which the mutations carried by each sunflower line affected the phospholipid composition of the seeds. The results indicated that modifications to intraplastidial de novo FA synthesis affected the seed phospholipid profile during the whole period of the seed formation, including accumulation and maturation, whereas the influence of mutations in the endoplasmic reticulum desaturases were more readily detected at later stages of development. These results are discussed in terms of the pathways involved in glycerolipid synthesis and phospholipid conversion in sunflower seeds.

Cloning, characterization and structural model of a FatA-type thioesterase from sunflower seeds (Helianthus annuus L.)

The substrate specificity of acyl-acyl carrier protein (ACP) thioesterases (EC 3.1.2.14) determines the fatty acids available for the biosynthesis of storage and membrane lipids in seeds. In order to determine the mechanisms involved in the biosynthesis of fatty acids in sunflower seeds (Helianthus annuus L.), we isolated, cloned and sequenced a cDNA clone of acyl-ACP thioesterase from developing sunflower seeds, HaFatA1. Through the heterologous expression of HaFatA1 in Escherichia coli we have purified and characterized this enzyme, showing that sunflower HaFatA1 cDNA encodes a functional thioesterase with preference for monounsaturated acyl-ACPs. The HaFatA1 thioesterase was most efficient (kcat/K(m)) in catalyzing oleoyl-ACP, both in vivo and in vitro. By comparing this sequence with those obtained from public databases, we constructed a phylogenetic tree that included FatA and FatB thioesterases, as well as related prokaryotic proteins. The phylogenetic relationships support the endosymbiotic theory of the origin of eukaryotic cells and the suggestion that eubacteria from the delta-subdivision were the guest cells in the symbiosis with archaea. These prokaryotic proteins are more homologous to plant FatB, suggesting that the ancient thioesterases were more similar to FatB. Finally, using the available structure prediction methods, a 3D model of plant acyl-ACP thioesterases is proposed that reflects the combined data from direct mutagenesis and chimera studies. In addition, the model was tested by mutating the residues proposed to interact with the ACP protein in the FatA thioesterase by site-directed mutagenesis. The results indicate that this region is involved in the stabilization of the substrate at the active site.

Very long chain fatty acid synthesis in sunflower kernels

Most common seed oils contain small amounts of very long chain fatty acids (VLCFAs), the main components of oils from species such as Brassica napus or Lunnaria annua. These fatty acids are synthesized from acyl-CoA precursors in the endoplasmic reticulum through the activity of a dissociated enzyme complex known as fatty acid elongase. We studied the synthesis of the arachidic, behenic, and lignoceric VLCFAs in sunflower kernels, in which they account for 1-3% of the saturated fatty acids. These VLCFAs are synthesized from 18:0-CoA by membrane-bound fatty acid elongases, and their biosynthesis is mainly dependent on NADPH equivalents. Two condensing enzymes appear to be responsible for the synthesis of VLCFAs in sunflower kernels, beta-ketoacyl-CoA synthase-I (KCS-I) and beta-ketoacyl-CoA synthase-II (KCS-II). Both of these enzymes were resolved by ion exchange chromatography and display different substrate specificities. While KCS-I displays a preference for 20:0-CoA, 18:0-CoA was more efficiently elongated by KCS-II. Both enzymes have different sensitivities to pH and Triton X-100, and their kinetic properties indicate that both are strongly inhibited by the presence of their substrates. In light of these results, the VLCFA composition of sunflower oil is considered in relation to that in other commercially exploited oils.