Interrelationships between fatty acid biosynthesis and acyl-lipid synthesis in Chlorella vulgaris

Morphological, genotypic and metabolomic signatures confirm interfamilial hybridization between the ubiquitous kelps Macrocystis (Arthrothamnaceae) and Lessonia (Lessoniaceae)

Macrocystis pyrifera and Lessonia spicata are economically and ecologically relevant brown seaweeds that recently have been classified as members of two separated families within Laminariales (kelps). Here we describe for the first time the Macrocystis pyrifera x Lessonia spicata hybridization in the wild (Chiloe Island, Southeastern Pacific), where populations of the two parents exist sympatrically. Externally, this hybrid exhibited typical features of its parents M. pyrifera (cylindrical and flexible distal stipes, serrate frond margins and presence of sporophylls) and L. spicata (rigid and flat main stipe and first bifurcation), as well as intermediate features between them (thick unfused haptera in the holdfast). Histological sections revealed the prevalence of mucilage ducts within stipes and fronds (absent in Lessonia) and fully developed unilocular sporangia in the sporophylls. Molecular analyses confirmed the presence of the two parental genotypes for ITS1 nrDNA and the M. pyrifera genotype for two predominantly maternally inherited cytoplasmic markers (COI and rbcLS spacer) in the tissue of the hybrid. A metabolome-wide approach revealed that this hybrid is more chemically reminiscent to M. pyrifera. Nevertheless, several hits were identified as Lessonia exclusive or more remarkably, not present in any of the parent. Meiospores developed into apparently fertile gametophytes, which gave rise to F1 sporophytes that reached several millimeters before suddenly dying. In-vitro reciprocal crossing of Mar Brava gametophytes from both species revealed that although it is rare, interfamilial hybridization between the two species is possible but mostly overcome by pseudogamy of female gametophytes.

Genome-Scale Metabolic Model for the Green Alga Chlorella vulgaris UTEX 395 Accurately Predicts Phenotypes under Autotrophic, Heterotrophic, and Mixotrophic Growth Conditions

The green microalga Chlorella vulgaris has been widely recognized as a promising candidate for biofuel production due to its ability to store high lipid content and its natural metabolic versatility. Compartmentalized genome-scale metabolic models constructed from genome sequences enable quantitative insight into the transport and metabolism of compounds within a target organism. These metabolic models have long been utilized to generate optimized design strategies for an improved production process. Here, we describe the reconstruction, validation, and application of a genome-scale metabolic model for C. vulgaris UTEX 395, iCZ843. The reconstruction represents the most comprehensive model for any eukaryotic photosynthetic organism to date, based on the genome size and number of genes in the reconstruction. The highly curated model accurately predicts phenotypes under photoautotrophic, heterotrophic, and mixotrophic conditions. The model was validated against experimental data and lays the foundation for model-driven strain design and medium alteration to improve yield. Calculated flux distributions under different trophic conditions show that a number of key pathways are affected by nitrogen starvation conditions, including central carbon metabolism and amino acid, nucleotide, and pigment biosynthetic pathways. Furthermore, model prediction of growth rates under various medium compositions and subsequent experimental validation showed an increased growth rate with the addition of tryptophan and methionine.

Studies on the regulation of lipid biosynthesis in plants: application of control analysis to soybean

Although there is much knowledge of the enzymology (and genes coding the proteins) of lipid biosynthesis in higher plants, relatively little attention has been paid to regulation. We have demonstrated the important role for cholinephosphate cytidylyltransferase in the biosynthesis of the major extra-plastidic membrane lipid, phosphatidylcholine. We followed this work by applying control analysis to light-induced fatty acid synthesis. This was the first such application to lipid synthesis in any organism. The data showed that acetyl-CoA carboxylase was very important, exerting about half of the total control. We then applied metabolic control analysis to lipid accumulation in important oil crops - oilpalm, olive, and rapeseed. Recent data with soybean show that the block of fatty acid biosynthesis reactions exerts somewhat more control (63%) than lipid assembly although both are clearly very important. These results suggest that gene stacks, targeting both parts of the overall lipid synthesis pathway will be needed to increase significantly oil yields in soybean. This article is part of a Special Issue entitled: Membrane Structure and Function: Relevance in the Cell's Physiology, Pathology and Therapy.

Glycerolipid synthesis in Chlorella kessleri 11h. I. Existence of a eukaryotic pathway

The fatty acid distributions at the sn-1 and sn-2 positions in major chloroplast lipids of Chlorella kessleri 11h, monogalactosyl diacylglycerol (MGDG) and digalactosyl diacylglycerol (DGDG), were determined to show the coexistence of both C16 and C18 acids at the sn-2 position, i.e. of prokaryotic and eukaryotic types in these galactolipids. For investigation of the biosynthetic pathway for glycerolipids in C. kessleri 11h, cells were fed with [14C]acetate for 30 min, and then the distribution of the radioactivity among glycerolipids and their constituent fatty acids during the subsequent chase period was determined. MGDG and DGDG were labeled predominantly as the sn-1-C18-sn-2-C16 (C18/C16) species as early as by the start of the chase, which suggested the synthesis of these lipids within chloroplasts via a prokaryotic pathway. On the other hand, the sn-1-C18-sn-2-C18 (C18/C18) species of these galactolipids gradually gained radioactivity at later times, concomitant with a decrease in the radioactivity of the C18/C18 species of phosphatidylcholine (PC). The change at later times can be explained by the conversion of the C18/C18 species of PC into galactolipids through a eukaryotic pathway. The results showed that C. kessleri 11h, distinct from most of other green algal species that were postulated mainly to use a prokaryotic pathway for the synthesis of chloroplast lipids, is similar to a group of higher plants designated as 16:3 plants in terms of the cooperation of prokaryotic and eukaryotic pathways to synthesize chloroplast lipids. We propose that the physiological function of the eukaryotic pathway in C. kessleri 11h is to supply chloroplast membranes with 18:3/18:3-MGDG for their functioning, and that the acquisition of a eukaryotic pathway by green algae was favorable for evolution into land plants.

Isomeric sn-1-octadecenyl and sn-2-octadecenyl analogues of lysophosphatidylcholine as substrates for acylation and desaturation by plant microsomal membranes

To provide supporting and independent evidence for lipid-linked desaturation of acyl groups in plant microsomal membranes, ether-analogous substrates were synthesized and used for in-vitro desaturation studies. The substrates included 1-O-(9-cis-octadecenyl)-sn-glycero-3-phosphocholine and 2-O-(9-cis-octadecenyl)-sn-glycero-3-phosphocholine as well as labelled 1-O-(9-cis-[9,10-3H2]octadecenyl)-sn-glycero-3-phosphocholine. In experiments with microsomal membranes from developing fruits of sunflower, it was shown that both isomeric alkenyl ether phospholipids were acylated with [14C]oleoyl-CoA and [14C]palmitoyl-CoA. In the presence of O2 and NADH, the oleoyl groups incorporated into both compounds, i.e. at the sn-1 and sn-2 positions of the glycerol backbone of the substrate, were desaturated to linoleoyl residues in similar proportions. Under the same conditions, an additional double bond, but not an enol-ether double bond, was introduced into the ether-linked side chain of acylated 1-O-(9-cis-[9,10-3H2]octadecenyl)-sn-glycero-3-phosphocholine. This represents the first demonstration of this type of desaturation with an alkenyl ether phospholipid and confirms previous conclusions that plants introduce second and further double bonds into lipid-linked acyl groups.

Light control of fatty acid synthesis and diurnal fluctuations of fatty acid composition in leaves

1. Although isolated spinach chloroplasts were almost entirely (greater than 99%) dependent on light for fatty acid synthesis, leaf discs were capable of fatty acid synthesis in the dark (up to 500nmol of 3H/h per mg of chlorophyll equivalent to approx. 400nmol of carbon/h per mg of chlorophyll), which represented 12-20% of the corresponding 'light rates'. 2. Net fatty acid accumulation by greening maize leaves occurred largely or entirely during the light period. 3. There was a diurnal fluctuation in the proportions of C18 unsaturated fatty acids in the lipids of developing spinach leaves, where an increase in the concentration of oleate during the day and a subsequent decline at night was observed; a complementary change occurred in the concentration of alpha-linolenate. The rhythm is interpreted as reflecting the continuation of oleate and linoleate desaturation at high rates when oleate synthesis is markedly decreased at night. 4. Changes in the fatty acid composition of 3-sn-phosphatidylcholine accounted for at least 60% of the total decrease in oleate over the dark period. This result is consistent with suggestions that this lipid is the substrate for the leaf microsomal oleate desaturase and an intermediate in leaf glycerolipid biosynthesis.

In Vivo Pathway of Oleate and Linoleate Desaturation in Developing Cotyledons of Cucumis sativus L. Seedlings

Exogenous [1-(14)C]oleic acid and [1-(14)C]linoleic acid were taken up and esterified to complex lipids by greening cucumber (Cucumis sativus L.) cotyledons. Both (14)C-labeled fatty acids were initially esterified to phosphatidylcholine prior to eventual accumulation in triacylglycerols and galactolipids. Kinetic data suggest that esterification occurs prior to desaturation and that phosphatidylcholine is the initial site of both [(14)C]-oleate and [1-(14)C]linoleate esterification and of [1-(14)C]oleate desaturation to [1-(14)C]linoleate. [1-(14)C]Linoleic acid was esterified more rapidly than [(14)C]oleic acid and its desaturation product, [1-(14)C]alpha-linolenate, occurred mainly on monogalactosyl diacylglycerol, although some was also observed on the other major acyl lipids, including phosphatidylcholine.

Desaturation of linoleic acid from exogenous lipids by isolated chloroplasts

When [14C]diacylgalactosylglycerol was added to isolated pea or lettuce chloroplasts linolenate synthesis was seen. The desaturation of [14C]linoleate in diacylgalactosylglycerol to [14C]linolenate was stimulated by the addition of a soluble protein fraction containing lipid-exchange activity. Other [14C]acyl lipids were ineffective, except that [14C]phosphatidylcholine in the presence of UDP-galactose and sn-glycerol 3-phosphate could also supply [14C]linoleate for desaturation. These results are consistent with a role of diacylgalactosylglycerol in linolenate synthesis, as indirectly suggested by labelling experiments.

Fatty acid biosynthesis in the leaves of barley, wheat and pea

1. The incorporation of radioactivity from [1-14C]acetate into the leaf lipids of barley, pea and wheat has been studied in pulse-labelling experiments. 2. There was little increase in the total labelling of lipids after the leaves were transferred to non-radioactive medium. However, there was an increase in the relative labelling of unsaturated fatty acids. In addition, there was an increase in the relative labelling of diacylgalactosylglycerol. 3. The principal radioactively labelled acyl lipids were diacylgalactosylglycerol and phosphatidylcholine. Phosphatidylcholine showed a decreasing proportion of [14C]oleate and an increasing amount of [14C]linoleate with time. Diacylgalactosylglycerol also had decreasing amounts of [14C]oleate but, in addition, had an increasing proportion of [14C]linolenate with time. 4. The absence of significant amounts of [14C]linolenate in phosphatidylcholine appeared to exclude a role for this phospholipid in linoleate desaturation. 5. The specific radioactivities of oleate and linoleate in phosphatidylcholine, diacylgalactosylglycerol and diacylgalabiosylglycerol were very similar in any single experiment. It was concluded that these fatty acids can rapidly exchange between the three intact lipids.

Synthesis of chloroplast membrane lipids and chlorophyll in synchronous cultures of Chlamydomonas reinhardi

Chloroplast membrane lipid synthesis has been studied in synchronously growing cultures of Chlamydomonas reinhardi. The synthesis of sulfolipid and phospholipid were measured by incorporation of 35SO4(2-) and 32PO4(3-) during a 1-h pulse. Galactolipid synthesis was measured by H14CO3- incorporation into lipid fractions separated by thin layer chromatography. Lipid synthesis occurs principally during the light portion of the synchronous cycle. Phosphatidylglycerol is synthesized between 3-4 h in the light and sulfolipid is labeled between 7-9 h in the light. Galactolipid synthesis appears to reach maximal rates shortly after the lights go on and again at 7 h. Chlorophyll reaches maximal rates of synthesis after 7 h. These lipids are made and inserted into the chloroplast membrane prior to major increases in photosynthetic capacity. Our results also show that chloroplast membrane lipids are synthesized in a sequential or multistep process.

Labelling studies in vivo on the metabolism of the acyl and glycerol moieties of the glycerolipids in the developing maize leaf

1. When [2-3H]glycerol was supplied to developing maize-leaf laminae, label entered 3-sn-phosphatidycholine at a linear rate essentially from zero time, whereas other lipids were labelled at accelerating rates. On transfer of laminae from [3H]glycerol to unlabelled glycerol, radioactivity was rapidly lost from 3-sn-phosphatidylcholine and accumulated in other lipids, principally monogalactosyl diacyglycerol. 2. Degradation of these lipids showed that 3H was present only in the glycerol moiety of the lipids. 3. In double-labelling pulse-chase experiments with [14C]acetate, which labelled essentially only fatty acids and [3H]glycerol similar amounts of 14C and 3H radioactivity were lost from 3-sn-phosphatidylcholine and accumulated by monogalactosyl diacylglycerol. 4. The different molecular species of both lipids isolated from laminae during a double-labelled pulse-chase study were separated by argentation t.l.c., and the changes in the amount of radioactivity and the 14C/3H ratio in different species were compared. The greatest loss of radioactivity during the period in unlabelled substrates occurred from the 3-sn-phosphatidylcholine species containing oleate and from the dilinoleate species, and radioactivity accumulated by monogalactosyl diacyglycerol was mainly in the dilinolenate species. However, despite the considerable change in the radioactivity in these species during the chase, the 14C/3H ratio in each of them remained relatively unchanged. 5. It is proposed that 3-sn-phosphatidylcholine in the developing leaf may serve as a donor or linoleate-containing diacyl-glycerols which are incorporated into other lipids, principally monogalactosyl diacylglycerol.

Lipids of the yeast Hansenula anomala

An analysis of the free lipids of Hansenula anomala was performed. The main fatty acids obtained by saponification of whole cell crude lipids were palmitic, C18:1, C18:2 and C18:3 acids. In mitochondrial lipids the tri-unsaturated acid was present as traces. Fatty acid composition of each class of lipids was also determined. Phosphatidylcholine and phosphatidylethanolamine were the main phospholipids; phosphatidylserine, phosphatidylinositol and cardiolipin were also characterized. The most abundant sterols were ergosterol and lanosterol. An acetate of a 24-ethyl cholesterol was also isolated. Two glycolipids, a galactosyl diglyceride and a glucosyl ceramide were identified; concerning the galactosyl diglyceride, the content of C18:3 acid was higher than in other lipid classes. In the glucosyl ceramide, the main fatty acid was alpha-hydroxy C18:0 acid; C16:0, C18:1, C18:2 and C18:3 acids were present too. The long chain base was shown to be C18-phytosphingosine (4-hydroxy C18 sphinganine). Some similarities and differences with Saccharomyces cerevisiae are discussed.

The kinetics of incorporation in vivo of (14C)acetate and (14C)carbon dioxide into the fatty acids of glycerolipids in developing leaves

1. The patterns of incorporation of (14)C into glycerolipid fatty acids of developing maize leaf lamina from supplied [1-(14)C]acetate and from (14)CO(2) during steady-state photosynthesis were similar. Oleate of phosphatidylcholine and palmitate of phosphatidylglycerol attained linear rates of labelling more rapidly than did other fatty acids, particularly the linoleate and linolenate of monogalactosyl diacylglycerol. 2. After the transfer of lamina from labelled to unlabelled acetate, there was a decrease in labelled oleate and linoleate of phosphatidylcholine and a concomitant increase in the amount of radioactivity in the linoleate and linolenate of monogalactosyl diacylglycerol. 3. The rapidly labelled phospholipids, phosphatidylcholine and phosphatidylglycerol, were shown by differential and sucrose-density-gradient centrifugation to be associated with different organelles, the former being mainly in a low-density membrane fraction, probably microsomal, and the latter mainly in chloroplasts. 4. During a 48h period after supplying spinach leaves with [(14)C]acetate, radioactivity was lost from the oleate of phosphatidylcholine present in fractions sedimented at 12000g and 105000g, and accumulated in the linolenate of monogalactosyl diacylglycerol of the chloroplast. 5. It is proposed that the phosphatidylcholine of some non-plastid membranes is intimately involved in the process of oleate desaturation and that this lipid serves as a donor of unsaturated C(18) fatty acids to other lipids, principally monogalactosyl diacylglycerol, of the chloroplasts.

Metabolism of trans-3-hexadecenoic acid in broad bean

1. Broad bean (Vicia faba) leaves contain rather high concentrations (about 4% of total fatty acids) of the trans-3-hexadecenoic acid. 2. Amounts of the acid increase with the age of the leaves and are absent from etiolated tissue. 3. Changes in the levels of trans-delta-4-hexadecenoic acid can be produced by subjecting the intact plants to various light/dark periods. 4. Chloroplasts isolated from broad-bean leaves show high rates of fatty acid synthesis from [1-14C]acetate. Synthesis is dependent on coenzyme A and ATP but is insensitive to the addition of exogenous acyl carrier protein. 5. The pattern of acids made includes about 20% palmitic, 5% hexadeconoic, 10% stearic and 60% oleic. trans-3-Hexadecenoic acid synthesis was most active in chloroplasts from plants exposed to the dark for 5 days and light for 3 days. 6. Arsenite addition inhibited stearate formation by isolated chloroplasts but resulted in a two-fold stimulation of overall synthesis. 7. The rate of fatty acid synthesis by isolated chloroplasts paralleled the changes in endogenous trans-3-hexadecenoic acid levels in the leaves from which they were isolated.

Changes in Lipid Composition during Greening of Etiolated Pea Seedlings

After 7 days of germination in the dark, the three sections of pea seedlings studied (cotyledons, stems, and young leaves) are rich in linoleic acid; after illumination of the seedlings a very significant increase in linolenic acid is observed in the young leaves section, whereas only small variations are noted in the fatty acid composition of the other sections. The increase in linolenic acid results from the increase in galactolipid content of the young leaves; these already linolenic acid-rich galactolipids are present but only in small amounts in the etiolated seedlings (10% of total lipid).Variations in composition of the other lipid classes (phospholipids and neutral fats) were also studied. The possibility of fatty acid transport from the cotyledons toward the young leaves during the synthesis of the photosynthetic apparatus is discussed.

The composition of triglycerides from liver, egg yolk and adipose tissue of the laying hen

The composition of the triglycerides of liver, egg yolk and adipose tissue of laying hens fed on a standard diet were investigated by using argentation thin-layer chromatography to separate the triglycerides according to their degree of unsaturation. About 40% of liver triglycerides consisted of one saturated and two monoenoic fatty acids. Triglycerides containing linoleate were more abundant in adipose tissue than in either yolk or liver. Hydrolysis by pancreatic lipase of the tissue triglycerides and fractions obtained from these triglycerides showed that the triglycerides of adipose tissue had a less ordered arrangement of fatty acids at the 2-position than did either yolk or liver triglycerides. The labelling patterns of triglycerides formed in liver slices incubated in the presence of [1-(3)(14)C]glycerol indicated that triglycerides containing four or more double bonds are formed to a greater extent than are other triglyceride fractions. This is evidence for the concept that the type of triglyceride formed depends on the availability of fatty acids to the liver cells.

Turnover of the glycerolipids of pumpkin leaves. The importence of phosphatidylcholine

Between 1 and 5% of the (14)C recovered from pumpkin leaves within 15-60min after pulse-labelling with (14)CO(2) was in the lipids. The specific radioactivity of the phospholipids was higher than that of the glycolipids. Phosphatidylcholine had five times the specific radioactivity of monogalactosyl diglyceride, and the specific radioactivity of neither galactolipid changed significantly between 1 and 48h after labelling. It therefore seemed unlikely that the galactose moieties of the galactolipids were involved in the transport of assimilated compounds across the chloroplast membrane. Within 60min of the application of [1-(14)C]acetate to the surfaces of mature, intact pumpkin leaves 70% of the recovered (14)C was in the lipid fraction. Of the separated glycerolipids, phosphatidylcholine had by far the highest specific radioactivity at the shorter time-intervals, and the glycolipids again had the lowest specific radioactivities. Phosphatidylcholine was the only lipid to show a significant turnover of radiocarbon as judged by the decrease in specific radioactivity with time. From a comparison of the changes with time of the labelling of fatty acid fractions from phosphatidylcholine, phosphatidylethanolamine, phosphatidylglycerol and monogalactosyl diglyceride, it is suggested that the primary site of linolenic acid biosynthesis in leaf cells is within the phosphatidylcholine molecule.

Phospholipids of Nocardia coeliaca

The lipids of Nocardia coeliaca were separated into at least 10 components by the use of thin-layer chromatography. Phosphatidylcholine was the most abundant phospholipid in this organism, accounting for 25 to 40% of the total phospholipids. The major fatty acid components of the phosphatidylcholine were 14-methyl-pentadecanoic acid (41%), the other C(15) and C(17) iso- and anteiso-fatty acids (29%), and palmitic acid (13.5%). The next most abundant phospholipid was phosphatidylethanolamine (25 to 30%), followed by phosphatidylinositol (11 to 14%) and cardiolipin (7 to 15%). Phosphatidylethanolamine and phosphatidylinositol were very similar to the phosphatidylcholine in fatty acid composition, whereas cardiolipin was characterized by a higher content of palmitic acid (30%). In all of the phospholipids examined, only trace amounts of monounsaturated fatty acids were present. When washed cells of N. coeliaca were incubated with methionine-methyl-(14)C for 1 to 3 hr, the radioactivity was mainly incorporated into the choline moiety of the phosphatidylcholine. In contrast, acetate-1-(14)C or glycerol-1-(14)C was incorporated much more slowly into the phosphatidylcholine than into the other phospholipids and neutral lipids. No phosphatidylcholine was detected in 10 other species of Nocardia examined.

Fatty acid metabolism in the chloroplast lipids of green and blue-green algae

The pattern of uptake of radioactivity into chloroplast lipids when a green alga (Chlorella vulgaris) was incubated with sodium 2-(14)C-acetate differed appreciably from that obtained when two blue-green algae (Anabaena cylindrica andAnacystis nidulans) were incubated under similar conditions.The fatty acids of the digalactosyl diglyceride and sulphoquinovosyl diglyceride fractions from the blue-green algae were labeled more rapidly than were those of the corresponding fractions fromC. vulgaris, whereas the activity in the acids of the phosphatidyl glycerol fraction fromA. cylindrica andA. nidulans was relatively lower than that in the green alga. The results indicate that the metabolic behavior of chloroplast lipids may vary considerably according to the class of alga concerned.In all three alga, the evidence points to an intermediary function for the chloroplast lipids in fatty acid synthesis.Only limited exchange of acyl groups between the different chloroplast lipids seemed to occur during photoautotrophic growth.

The inhibition of unsaturated fatty acid biosynthesis in plants by sterculic acid

Sterculic acid, 8‐(2‐octyl‐1‐cyclopropenyl)octanoic acid, is a potent inhibitor of the enzyme system converting stearic acid to oleic acid in Chlorella vulgaris. No inhibition of oleic acid for mation from acetate, is, however, observed. With the C10, C12 or C14 acids as precursor, inhibition increases with chain length. The enzyme system converting oleic acid to linoleic acid is also less sensitive than the stearate‐oleate system. Stearic acid generated internally in leaf preparations anaerobically from acetate is still converted to oleic acid on transference to air in the presence of sterculic acid. Desaturation of added palmitic acid is readily inhibited by sterculic acid, so far as formation of the Δ7 and Δ9 acids are concerned. Desaturation of palmitic acid to the 3‐trans‐hexadecenoic acid is unaffected by sterculic acid.

Two possible schemes to account for the effects of sterculic acid on stearate desaturation are suggested.