The synthesis of acyl lipids in plant tissues

Plant Acyl-CoA-Binding Proteins-Their Lipid and Protein Interactors in Abiotic and Biotic Stresses

Plants are constantly exposed to environmental stresses during their growth and development. Owing to their immobility, plants possess stress-sensing abilities and adaptive responses to cope with the abiotic and biotic stresses caused by extreme temperatures, drought, flooding, salinity, heavy metals and pathogens. Acyl-CoA-binding proteins (ACBPs), a family of conserved proteins among prokaryotes and eukaryotes, bind to a variety of acyl-CoA esters with different affinities and play a role in the transport and maintenance of subcellular acyl-CoA pools. In plants, studies have revealed ACBP functions in development and stress responses through their interactions with lipids and protein partners. This review summarises the roles of plant ACBPs and their lipid and protein interactors in abiotic and biotic stress responses.

Using lipidomics to reveal details of lipid accumulation in developing seeds from oilseed rape (Brassica napus L.)

With dwindling available agricultural land, concurrent with increased demand for oil, there is much current interest in raising oil crop productivity. We have been addressing this issue by studying the regulation of oil accumulation in oilseed rape (Brassica napus L). As part of this research we have carried out a detailed lipidomic analysis of developing seeds.

The molecular species distribution in individual lipid classes revealed quite distinct patterns and showed where metabolic connections were important. As the seeds developed, the molecular species distributions changed, especially in the period of early (20 days after flowering, DAF) to mid phase (27DAF) of oil accumulation. The patterns of molecular species of diacylglycerol, phosphatidylcholine and acyl-CoAs were used to predict the possible relative contributions of diacylglycerol acyltransferase (DGAT) and phospholipid:diacylglycerol acyltransferase to triacylglycerol production. Our calculations suggest that DGAT may hold a more important role in influencing the molecular composition of TAG. Enzyme selectivity had an important influence on the final molecular species patterns.

Our data contribute significantly to our understanding of lipid accumulation in the world's third most important oil crop.

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Lipidomic analysis of developing rapeseed seeds is reported

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Results show distinct differences between lipid classes

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Changes in molecular species distributions were found during development

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The data were used to evaluate the contribution of different synthetic pathways

Plant Phospholipid Diversity: Emerging Functions in Metabolism and Protein-Lipid Interactions

Phospholipids are essential components of biological membranes and signal transduction cascades in plants. In recent years, plant phospholipid research was greatly advanced by the characterization of numerous mutants affected in phospholipid biosynthesis and the discovery of a number of functionally important phospholipid-binding proteins. It is now accepted that most phospholipids to some extent have regulatory functions, including those that serve as constituents of biological membranes. Phospholipids are more than an inert end product of lipid biosynthesis. This review article summarizes recent advances on phospholipid biosynthesis with a particular focus on polar head group synthesis, followed by a short overview on protein-phospholipid interactions as an emerging regulatory mechanism of phospholipid function in arabidopsis (Arabidopsis thaliana).

Plasticity and constraints on fatty acid composition in the phospholipids and triacylglycerols of Arabidopsis accessions grown at different temperatures

BACKGROUND

Natural selection acts on multiple traits in an organism, and the final outcome of adaptive evolution may be constrained by the interaction of physiological and functional integration of those traits. Fatty acid composition is an important determinant of seed oil quality. In plants the relative proportions of unsaturated fatty acids in phospholipids and seed triacylglycerols often increases adaptively in response to lower growing temperatures to increase fitness. Previous work produced evidence of genetic constraints between phospholipids and triacylglycerols in the widely studied Arabidopsis lines Col and Ler, but because these lines are highly inbred, the correlations might be spurious. In this study, we grew 84 wild Arabidopsis accessions at two temperatures to show that genetic correlation between the fatty acids of the two lipid types is not expected and one should not influence the other and seed oil evolution and also tested for the adaptive response of fatty acids to latitude and temperature.

RESULTS

As expected no significant correlations between the two lipids classes at either growing temperature were observed. The saturated fatty acids and erucic acid of triacylglycerols followed a significant latitudinal cline, while the fatty acids in phospholipids did not respond to latitude as expected. The expected plastic response to temperature was observed for all the triacylglycerol fatty acids whereas only oleic acid showed the expected pattern in phospholipids. Considerable phenotypic variation of the fatty acids in both the lipid types was seen.

CONCLUSION

We report the first evidence supporting adaptive evolution of seed triacylglycerols in Arabidopsis on a latitudinal cline as seen in other species and also their plastic adaptive response to growing temperature. We show that as expected there is no genetic correlations between the fatty acids in triacylglycerols and phospholipids, indicating selection can act on seed triacylglycerols without being constrained by the fatty acid requirements of the phospholipids. Phospholipid fatty acids do not respond to latitude and temperature as seen elsewhere and needs further investigation. Thus, the adaptive response of Arabidopsis and the genetic tools available for manipulating Arabidopsis, makes it an excellent system for studying seed oil evolution and also for breeding seed oil crops especially the Brassica species.

Diacylglycerol acyltransferase: A key mediator of plant triacylglycerol synthesis

Many plants deposit TAG in seeds and fruits as the major form of storage lipid. TAG production is of tremendous socioeconomic value in food, nutraceutical, and industrial applications, and thus numerous conventional and molecular genetic strategies have been explored in attempts to increase TAG content and modify the FA composition of plant seed oils. Much research has focused on the acyl-CoA-dependent reaction catalyzed by diacylglycerol acyltransferase (DGAT), which is an integral endoplasmic reticulum protein and has also been shown to be present in oil bodies and plastids. DGAT enzymes exhibit diverse biochemical properties among different plant species, many of which are summarized here. In addition to catalyzing a critical step in TAG biosynthesis, there is evidence that DGAT has roles in lipid metabolism associated with germination and leaf senescence. TAG can also be formed in plants via two different acyl-CoA-independent pathways, catalyzed by phospholipid: diacylglycerol acyltransferase and diacylglycerol transacylase. The current understanding of the terminal step in TAG formation in plants and the development of molecular genetic approaches aimed at altering TAG yield and FA composition of TAG are discussed.

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.

Alternative pathways for phosphatidylcholine synthesis in olive (Olea europaea L.) callus cultures

Olive (Olea europaea L.) callus cultures were incubated with [2-14C]ethanolamine and [Me-14C]choline in order to study phospholipid synthesis. Radioactivity from [Me-14C]choline was shown to be incorporated into the phosphatidylcholine via the CDP-base pathway. [2-14C]Ethanolamine was primarily incorporated into phosphatidylethanolamine, but significant radio-activity was also detected in phosphatidylcholine, indicating the operation of a methylation route. Incubation with [2-14C]ethanolamine indicated that phosphatidylcholine and phosphatidylethanolamine incorporated radiolabel over a similar time course. This led us to investigate the possibility that phosphatidylcholine was being synthesized by a methylation pathway distinct from the direct methylation of phosphatidylethanolamine. There was extensive incorporation of [2-14C]ethanolamine into different components of the aqueous phase of the incubations, within which phospho-base derivatives of ethanolamine were prominent. These intermediates were identified and provided evidence for the operation of an alternative methylation pathway via phosphodimethylethanolamine for the biosynthesis of phosphatidylcholine in olives.

Neutral lipid transfer and lipolysis convert high molecular weight LDL from cholesterol-fed nonhuman primates towards normal: a molecular analysis

In cynomolgus monkeys (Macaca fascicularis) fed an atherogenic diet, large, cholesterol ester-rich LDL (Mr greater than 3.5.10(6] are found at the same time that the plasma triacylglycerol levels are low. We studied whether the presence of higher concentrations of triacylglycerol-rich lipoproteins (VLDL) during in vitro incubations would allows depletion from LDL of cholesterol ester and a decreased LDL molecular weight. Three high Mr LDL (Mr = (3.7-4.8).10(6)), rich in cholesterol ester (50 +/- 1.4% by weight), were isolated from three animals by zonal ultracentrifugation, and were then incubated with human VLDL at 37 degrees C for 18 h in lipoprotein-deficient human plasma containing neutral lipid transfer activity. After incubation, modified LDL (M-LDL) was isolated by zonal ultracentrifugation. M-LDL was triacylglycerol-rich (36 +/- 5% by weight) and cholesterol ester-poor (20 +/- 3%), and cholesterol ester had transferred into VLDL. Purified lipoprotein lipase was added to the M-LDL, and triacylglycerol was hydrolyzed. The size of the post-lipolysis M-LDL (Mp-LDL) particles became smaller (mean diameters of 253 A and 228 A for two native LDLs and 215 A and 193 A for Mp-LDL, respectively). Both analytical and zonal ultracentrifugation showed Mp-LDL to be more dense than native LDL. Estimated molecular weights for Mp-LDL were 40%-50% less than that of the original LDL, and fell within the molecular weight range for normal human and monkey LDL. Lipid exchanges, but not apoprotein transfers, were responsible for LDL remodelling, as supported by three separate methods of analysis. Cholesterol ester losses accounted for about two-thirds of the molecular weight decrease. These in vitro results suggest that cholesterol ester enrichment of apoprotein B lipoprotein particles can be reversed by providing adequate levels of VLDL in the presence of neutral lipid transfer processes and lipolytic activity.

Evidence for separate elongation enzymes for very-long-chain-fatty-acid synthesis in potato (Solanum tuberosum)

Aging potato (Solanum tuberosum) tuber discs in a Ca2+-containing medium resulted in increased rates of fatty acid labelling from [1-14C]acetate with time. Maximal labelling rates were seen after 6-8 h aging in a number of varieties. Saturated very-long-chain fatty acids (C20 and particularly C22 and C24) were very poorly labelled in freshly cut tissue. They were synthesized in increasing amounts and in a homologous sequence with progressive aging times. Use of increasing induction times and cycloheximide or puromycin as protein-synthesis inhibitors indicated that the sequence of fatty acid elongation was dependent on protein synthesis de novo and was controlled by three separate specific elongase enzymes.

Ether glycerolipids: novel substrates for studying specificity of enzymes involved in glycerolipid biosynthesis in higher plants

Ether glycerolipids, predominantly alkylacylglycerols and alkylacylglycerophosphocholines, are synthesized in photomixotrophic rape (Brassica napus) suspension cells from various exogenous monoalkylglycerols. The stereospecific distribution of acyl moieties was studied in these ether glycerolipids with regard to chain-length and degree of unsaturation of alkyl moieties and compared with the distribution of acyl moieties in the corresponding endogenous acyl glycerolipids. The results show the following: (1) Alkylacylglycerophosphocholines replaced up to one-half of the corresponding physiological membrane lipids, i.e. diacylglycerophosphocholines, without changing the total amount of cholineglycerophospholipids as compared to untreated cells. (2) The composition of acyl moieties in total lipids of rape cells was practically unaltered by fatty acids derived via oxidative cleavage from the various alkyl moieties of either glycerolipids. (3) In 1-O-alkyl-2-acylglycerols derived from exogenous alkylglycerols and in endogenous 1,2-diacylglycerols compositions of acyl moieties were found to be different indicating that different pathways were operative in the biosynthesis of these two neutral glycerolipids. (4) Enzymes involved in synthesizing molecular species of 1-O-alkyl-2-acylglycerophosphocholines or 2-O-alkyl-1-acylglycerophosphocholines as well as 1,2-diacylglycerophosphocholines showed similar specificities with regard to chain-length and degree of unsaturation of both alkyl and corresponding acyl moieties. Thus, ether glycerolipids formed by plant cells from exogenous alkylglycerols are suitable metabolites for studying the specificity of enzymes involved in the biosynthesis of glyerolipids.

The effect of hypolipidemic drugs on plant lipid metabolism

The effect of hypolipidemic drugs, WY14643 and DH990, on plant lipid metabolism has been studied. The total incorporation of [14C]acetate into lipids was inhibited by addition of both drugs to aged potato (Solanum tuberosum) tuber discs, spinach (Spinacia oleracea) leaves, and spinach chloroplasts, while the incorporation in Chlorella vulgaris cells was affected only by DH990. Moreover, DH990 inhibited the incorporation of 14C-labeled fatty acids into phosphatidylcholine and phosphatidylethanolamine of potato discs, and decreased the incorporation into phosphatidylglycerol of Chlorella cells. DH990 inhibited the formation of polyunsaturated fatty acids in potato discs, Chlorella cells, and spinach leaves, whereas WY14643 had no effect on the formation of these fatty acids. Stearoyl-ACP desaturase from safflower (Carthamus tinctorius) seeds was very sensitive to both drugs, especially DH990, which completely blocked the activity at 2 mM levels. When safflower lysophospholipid acyltransferases were solubilized by detergent treatment, only DH990 inhibited the incorporation of [14C]oleoyl-CoA into lysophosphatidylcholine or lysophosphatidylethanolamine. Both drugs inhibited fatty acid synthesis from [14C]malonyl-CoA in the microsomal fraction from safflower seeds, but only DH990 inhibited FAS activity in the soluble fraction; both drugs inhibited severely the formation of stearic acid. Both acetyl-CoA carboxylase and acetyl-CoA synthetase were sensitive to both drugs.

sn-Glycerol-3-phosphate acyltransferase in a particulate fraction from maturing safflower seeds

The properties of the acyl-CoA:sn-glycerol-3-phosphate O-acyltransferase in a 20,000g particulate fraction from maturing safflower seeds were investigated. The optimum pH of the reaction was 7.2. The apparent Km for glycerophosphate was 0.54 mM. Only monoacylglycerophosphate was accumulated in the particulate fraction under normal conditions. Position 1 of glycerophosphate was exclusively esterified with either palmitoyl-CoA or linoleoyl-CoA as acyl donor, while 2-acylglycerophosphate was the minor product. The specificity and selectivity of the acyltransferase for acyl-CoA were broad and somewhat affected by temperature. The concentration of glycerophosphate did not affect the selectivity. These observations suggested that the fatty acid composition of position 1 of safflower triacylglycerol must primarily depend on the composition of the acyl-CoA pool in the site of synthesis, and that growth temperature and the acyl-CoA selectivity of the glycerophosphate acyltransferase may be rather minor factors regarding regulation of the fatty acid composition of position 1 in triacylglycerol.

Lipid metabolism in fungi

So far, reviews that have appeared on fungal lipids present data mainly on the lipid composition of these organisms and the influence of lipids on their physiology. These reviews provide little information about the enzymes of lipid metabolism in these organisms and it is assumed, by most workers, that lipid synthesis in all fungi takes place as in Saccharomyces cervesiae, the only fungus in which the complete pathways of phospholipid biosynthesis have been worked out. During the last few years, literature has accumulated on lipid metabolic enzymes of other fungi, as investigators became increasingly interested in this area of research. The present review, after an introduction, will be divided into different sections and each section will deal, comparatively, with various aspects of fungal lipid metabolism and physiology. This review will, therefore, bring out the differences or similarities of lipid metabolism in diverse fungal species.

Hormonal regulation of phosphatidylcholine synthesis in plants. The inhibition of cytidylyltransferase activity by indol-3-ylacetic acid

Indol-3-ylacetic acid stimulated stem elongation within 1 h of treatment of Pisum sativum L. cv. Feltham First stem sections. This elongation was accompanied by an increase in the endogenous level of phosphocholine and a decrease in that of CDP-choline. Measurements in vitro of the CDP-base pathway enzymes showed an increase in choline phosphotransferase and a decrease in cytidylyltransferase activity on hormone treatment. These results indicate that the decrease in phosphatidylcholine labelling from [14C]choline that is observed on indol-3-ylacetic acid treatment of pea stem sections is caused by the decrease in cytidylyltransferase activity.

Formation of complex ether lipids from 1-O-alkylglycerols in cell suspension cultures of rape

Photomixotrophic cell suspension cultures of rape, Brassica napus, were incubated with rac-1-O-[1'-(14)C]hexadecylglycerol. Radioactivity was incorporated predominantly into choline glycerophospholipids. Prolonged incubation led also to considerable proportions of labeled ethanolamine glycerophospholipids. In addition to these ionic lipids,isomeric hexadecylacylglycerols as well as hexadecyldiacylglycerols were formed. About a third of the hexadecylglycerol supplied as substrate was cleaved within 48 h incubation. The palmitic acid formed by oxidative cleavage of the substrate was incorporated predominantly into choline glycerophospholipids, ethanolamine glycerophospholipids, and triacylglycerols. Incubation of an equimolar mixture of homologous saturated rac-1-O-[1'(14)C]alkylglycerols (C14, C16, C18, C20) with rape cells showed that alkylglycerols with alkyl moieties having 16 and 18 carbon atoms were incorporated preferentially. Incubation of labeled hexadecyglycerol with a homogenate of rape cells led also predominantly to choline glycerophospholipids; highest yields were obtained at pH 7. Neither the 1-O-alkyl moieties in choline glycerophospholipis nor those in ethanolamine glycerophospholipids were desaturated to 1-O-alk-1'-enylmoieties. The results of these experiments led to the following conclusions: (1) The acylation of 1-O-alkylglycerols to isomeric alkylacylglycerols is catalyzed by two acyltransferases differing in their specificity with regard to the chain length of the alkyl moiety in the substrate. (2) CDP-Choline: diacylglycerol cholinephosphotransferase and CDP-ethanolamine: diacylglycerol ethanolaminephosphotransferase are two enzymes differing in various respects. Cholinephosphotransferase exhibits a much higher affinity for 1-O-alkyl-2-O-acylglycerols than ethanolaminephosphotransferase. The two enzymes show marked differences with regard to their specificity for 1-O-alkyl-2-O-acylglycerols differing in the chain lengths of their alkyl moieties.

Lipase-induced alterations of fatty acid synthesis by subcellular fractions from germinating pea (Pisum sativum L.)

1. The effect of exogenous lipases on fatty acid synthesis from [14C]malonyl-CoA by the microsomal and soluble fractions from germinating peas was studied. 2. Addition of phospholipase A2 or the lipase from Rhizopus arrhizus had no effect on total fatty acid synthesis by the soluble fraction but caused severe inhibition of that by the microsomal fraction. 3. The addition of enzymes with phospholipase activity particularly inhibited the microsomal stearate elongase. 4. Control studies indicated that the phospholipase-induced inhibition of fatty acid synthesis was due to the location of fatty acid synthetase, palmitate elongase and stearate elongase on the outside of the microsomal vesicles. 5. Experiments with a trypsin-like proteinase showed that approximately half the microsomal fatty acid synthesis was resistant to proteolysis. 6. Although addition of exogenous phospholipases had no effect on total fatty acid synthesis by the soluble fraction, it did increase alpha-hydroxylation of newly-formed palmitate and stearate. 7. The results provide further evidence for differences between the soluble and particulate fatty acid synthetase and palmitate elongase activities of germinating pea.

Products of fatty acid synthesis by a particulate fraction from germinating pea (Pisum sativum L.)

The synthesis of lipids and acyl thioesters was studied in microsomal preparations from germinating pea (Pisum sativum cv. Feltham First) seeds. Under conditions of maximal synthesis (in the presence of exogenous acyl-carrier protein) acyl-acyl-carrier proteins accounted for about half the total incorporation from [14C]malonyl-CoA. Decreasing the concentrations of exogenous acyl-carrier protein lowered the overall synthesis of fatty acids by decreasing, almost exclusively, the radioactivity associated with acyl-acyl-carrier proteins. A time-course experiment showed that acyl-acyl-carrier proteins accumulated most of the radioactive label at the beginning of the incubation but, eventually, the amount of radioactivity in that fraction decreased, while a simultaneous increase in the acyl-CoA and lipid fractions was noticed. Addition of exogenous CoA (1 mM) produced a decrease of total incorporation, but an increase in the radioactivity incorporated into acyl-CoA. The microsomal preparations synthesized saturated fatty acids up to C20, including significant proportions of pentadecanoic acid and heptadecanoic acid. Synthesis of these 'odd-chain' fatty acids only took place in the microsomal fraction. In contrast, when the 18,000g supernatant (containing the microsomal and soluble fractions) was incubated with [14C]malonyl-CoA, the radioactive fatty acid and acyl classes closely resembled the patterns produced by germinating in the presence of [14C]acetate in vivo. The results are discussed in relation to the role of acyl thioesters in the biosynthesis of plant lipids.

Thylakoid membrane biogenesis in Chlamydomonas reinhardtii 137+: cell cycle variations in the synthesis and assembly of polar glycerolipid

The synthesis and assembly of thylakoid membrane polar glycerolipid (glycolipid, phospholipid, and ether lipid) have been monitored in synchronous cultures of the green alga Chlamydomonas reinhardtii 137+. A "pulse" protocol using radioactive acetate as the lipogenic precursor was devised to allow assessment of both processes during the 24-h (12-h light/12-h dark) vegetative cell cycle. Under these conditions, acetate incorporation into each chromatographically resolved lipid at the cellular level reliably reflects lipid synthesis, and the appearance of radiolabeled lipid in purified photosynthetic membrane is indicative of the lipid assembly attendant to thylakoid biogenesis. Our results demonstrate that polar glycerolipid is synthesized by the alga and is assembled into its thylakoid membrane continuously, but differentially, with respect to cell cycle time. Synthesis and assembly are most rapid during the photoperiod (mid-to-late G1), reach maximum rates at mid-photoperiod, and are comparatively negligible in the dark (S, M, and early-to-mid G1). The extent to which synthesis and assembly vary within this general kinetic pattern, though, is characteristic of each thylakoid lipid, suggesting that the processes take place in a multistep manner with some temporal coordination among the different lipid types. Parallelism between the cyclic patterns of polar lipid synthesis at the cellular level and of polar lipid assembly into photosynthetic membrane at the subcellular level indicates that lipid production is not only essential to continuing thylakoid biogenesis but is also the critical determinant of the kinetics of thylakoid lipid assembly.

Fatty acid elongation by a particulate fraction from germinating pea

The synthesis of fatty acids from [14C]malonyl-CoA was studied with a high-speed particulate fraction from germinating pea (Pisum sativum). The variety used (Feltham First) produced mainly saturated fatty acids with palmitate (30--40%) and stearate (40--60%) predominating. Several palmitate-containing lipids stimulated overall synthesis and, in addition, increased the percentage of label in stearate. The production of stearate was severely inhibited by preincubation of the microsomal fraction with snake venom phospholipase A2 or by incubation with Rhizopus arrhizus lipase. Addition of a series of di-saturated phosphatidylcholines, with different acyl constituents, resulted in stimulation of overall fatty acid synthesis as well as an increase in the radiolabelling of the fatty acid two carbon atoms longer than the acyl chain added. This chain lengthening of fatty acids donated from phosphatidylcholine was due to the action of both fatty acid synthetase and palmitate elongase. The latter would utilize dipalmitoyl phosphatidylcholine and was sensitive to arsenite whereas fatty acid synthetase would use dilauroyl phosphatidylcholine and was sensitive to cerulenin. The results are discussed in relation to previous data obtained in vivo on plant fatty acid synthesis and current suggestions for the role of phosphatidylcholine in this process.

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.