Incorporation of choline and ethanolamine into phospholipids in germinating soya bean

Functional divergence of a pair of Arabidopsis phospho-base methyltransferases, PMT1 and PMT3, conferred by distinct N-terminal sequences

In seed plants, phospho-base N-methyltransferase (PMT) catalyzes a key step in the biosynthesis pathway of phosphatidylcholine (PC), the most abundant phospholipid class. Arabidopsis thaliana possesses three copies of PMT, with PMT1 and PMT3 play a primary role because the pmt1 pmt3 double mutant shows considerably reduced PC content with a pale seedling phenotype. Although the function of PMT1 and PMT3 may be redundant because neither of the parental single mutants showed a similar mutant phenotype, major developmental defects and possible functional divergence of these PMTs underlying the pale pmt1 pmt3 seedling phenotype are unknown. Here, we show the major developmental defect of the pale seedlings in xylem of the hypocotyl with partial impairments in chloroplast development and photosynthetic activity in leaves. Although PMT1 and PMT3 are localized at the endoplasmic reticulum, their tissue-specific expression pattern was distinct in hypocotyls and roots. Intriguingly, the function of PMT3 but not PMT1 requires its characteristic N-terminal sequence in addition to the promoter because truncation of the N-terminal sequence of PMT3 or substitution with PMT1 driven by the PMT3 promoter failed to rescue the pale pmt1 pmt3 seedling phenotype. Thus, PMT3 function requires the N-terminal sequence in addition to its promoter, whereas the PMT1 function is defined by the promoter.

Headgroup biosynthesis of phosphatidylcholine and phosphatidylethanolamine in seed plants

Phospholipid biosynthesis is crucial for plant growth and development. It involves attachment of fatty acids to a phospho-diacylglycerol backbone and modification of the phospho-group into an amino alcohol. The biochemistry and molecular biology of the former has been well established, but a number of enzymes responsible for the latter have only recently been cloned and functionally characterized in Arabidopsis and some other model plant species. The metabolism involving the polar head groups of phospholipids established by past biochemical studies can now be validated by available gene knockout models. Moreover, gene knockout studies have revealed emerging functions of phospholipids in regulating plant growth and development. This review aims to revisit the old questions of polar headgroup biosynthesis of plant phosphatidylcholine and phosphatidylethanolamine by giving an overview of recent advances in the field and beyond.

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

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.

The widespread plant-colonizing bacterial species Pseudomonas syringae detects and exploits an extracellular pool of choline in hosts

The quaternary ammonium compound (QAC) choline is a major component of membrane lipids in eukaryotes and, if available to microbial colonists of plants, could provide benefits for growth and protection from stress. Free choline is found in homogenized plant tissues, but its subcellular location and availability to plant microbes are not known. Whole-cell bacterial bioreporters of the phytopathogen Pseudomonas syringae were constructed that couple a QAC-responsive transcriptional fusion with well-characterized bacterial QAC transporters. These bioreporters demonstrated the presence of abundant free choline compounds released from germinating seeds and seedlings of the bean Phaseolus vulgaris, and a smaller but consistently detectable amount of QACs, probably choline, from leaves. The localization of P. syringae bioreporter cells to the surface and intercellular sites of plant tissues demonstrated the extracellular location of these QAC pools. Moreover, P. syringae mutants that were deficient in the uptake of choline compounds exhibited reduced fitness on leaves, highlighting the importance of extracellular choline to P. syringae on leaves. Our data support a model in which this choline pool is derived from the phospholipid phosphatidylcholine through plant-encoded phospholipases that release choline into the intercellular spaces of plant tissues, such as for membrane lipid recycling. The consequent extracellular release of choline compounds enables their interception and exploitation by plant-associated microbes, and thus provides a selective advantage for microbes such as P. syringae that are adapted to maximally exploit choline.

Membrane lipid biosynthesis in Chlamydomonas reinhardtii: ethanolaminephosphotransferase is capable of synthesizing both phosphatidylcholine and phosphatidylethanolamine

Phosphatidylethanolamine, but not phosphatidylcholine, is found in Chlamydomonas reinhardtii. A cDNA coding for diacylglycerol: CDP-ethanolamine ethanolaminephosphotransferase (EPT) was cloned from C. reinhardtii. The C. reinhardtii EPT appears phylogenetically more similar to mammalian aminoalcoholphosphotransferases than to those of yeast and the least close to those of plants. Similar membrane topography was found between the C. reinhardtii EPT and the aminoalcoholphosphotransferases from mammals, yeast, and plants. A yeast mutant deficient in both cholinephosphotransferase and ethanolaminephosphotransferase was complemented by the C. reinhardtii EPT gene. Enzymatic assays of C. reinhardtii EPT from the complemented yeast microsomes demonstrated that the C. reinhardtii EPT synthesized both PC and PE in the transformed yeast. The addition of either unlabeled CDP-ethanolamine or CDP-choline to reactions reduced incorporation of radiolabeled CDP-choline and radiolabeled CDP-ethanolamine into phosphatidylcholine and phosphatidylethanolamine. EPT activity from the transformed yeast or C. reinhardtii cells was inhibited nearly identically by unlabeled CDP-choline, CDP-ethanolamine, and CMP when [14C]CDP-choline was used as the primary substrate, but differentially by unlabeled CDP-choline and CDP-ethanolamine when [14C]CDP-ethanolamine was the primary substrate. The Km value of the enzyme for CDP-choline was smaller than that for CDP-ethanolamine. This provides evidence that C. reinhardtii EPT, similar to plant aminoalcoholphosphotransferase, is capable of catalyzing the final step of phosphatidylcholine biosynthesis, as well as that of phosphatidylethanolamine in the Kennedy pathway.

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.

The AAPT1 gene of soybean complements a cholinephosphotransferase-deficient mutant of yeast

Aminoalcoholphosphotransferases (AAPTases) utilize diacylglycerols and cytidine diphosphate (CDP)-aminoalcohols as substrates in the synthesis of the abundant membrane lipids phosphatidylcholine and phosphatidylethanolamine. A soybean cDNA encoding an AAPTase that demonstrates high levels of CDP-choline:sn-1,2-diacylglycerol cholinephosphotransferase activity was isolated by complementation of a yeast strain deficient in this function and was designated AAPT1. The deduced amino acid sequence of the soybean cDNA showed nearly equal similarity to each of the two characterized AAPTase sequences from yeast, cholinephosphotransferase and ethanolaminephosphotransferase (CDP-ethanolamine:sn-1,2-diacylglycerol ethanolaminephosphotransferase). Moreover, assays of soybean AAPT1-encoded enzyme activity in yeast microsomal membranes revealed that the addition of CDP-ethanolamine to the reaction inhibited incorporation of 14C-CDP-choline into phosphatidylcholine in a manner very similar to that observed using unlabeled CDP-choline. Although DNA gel blot analysis suggested that AAPT1-like sequences are represented in soybean as a small multigene family, the same AAPT1 isoform isolated from a young leaf cDNA library was also recovered from a developing seed cDNA library. Expression assays in yeast using soybean AAPT1 cDNAs that differed only in length suggested that sequences in the 5'leader of the transcript were responsible for the negative regulation of gene activity in this heterologous system. The inhibition of translation mediated by a short open reading frame located 124 bp upstream of the AAPT1 reading frame is one model proposed for the observed down-regulation of gene activity.

Synthesis of Ethanolamine and Its Regulation in Lemna paucicostata

The metabolism of ethanolamine and its derivatives in Lemna paucicostata has been investigated, with emphasis on the path-way for synthesis of phosphoethanolamine, a precursor of phosphatidylcholine in higher plants. In experiments involving labeling of intact plants with radioactive serine, ambiguities of interpretation due to entry of radioactivity into methyl groups of methylated ethanolamine derivatives were mitigated by pregrowth of plants with methionine. Difficulties due to labeling of diacylglyceryl moieties of phospholipids were avoided by acid hydrolysis of crucial samples and determination of radioactivity in isolated serine or ethanolamine moieties. The results obtained from such experiments are most readily reconciled with the biosynthetic sequence: serine --> ethanolamine --> phosphoethanolamine --> phosphatidylethanolamine. A possible alternative is: serine --> phosphatidylserine --> phosphatidylethanolamine --> ethanolamine --> phosphoethanolamine. Cell-free extracts of L. paucicostata were shown to produce CO(2) from the carbon originating as C-1 of serine at a rate sufficient to satisfy the demand for ethanolamine moieties. A number of experiments produced no support for a hypothetical role for phosphoserine in phosphoethanolamine formation. Uptake of exogenous ethanolamine commensurately down-regulates the synthesis of ethanolamine moieties (considered as a whole, and regardless of their state of derivatization at the time of their formation). In agreement with previous observations, uptake of exogenous choline down-regulates the methylation of phosphoethanolamine, without being accompanied by secondary accumulation of a marked excess of ethanolamine derivatives.

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.

Lipid metabolism in germinating seeds. Purification of ethanolamine kinase from soya bean

Ethanolamine kinase has been purified to homogeneity from germinating soya bean (Glycine max L.) seeds. The purified enzyme had a molecular weight of 17--19 000 as estimated by gel filtration and sodium dodecyl suphate-polyacrylamide gel electrophoresis. It would not phosphorylate choline, had a Km for ethanolamine of 8 microM and utilised Mg-ATP. The kinase could be purified in a 37 000 molecular weight form (dimer) which would easily dissociate on storage. In contrast to ethanolamine kinase whose activity was unaffected by the presence of choline in the assay system, soya bean choline kinase, although not phosphorylating ethanolamine, was competitively inhibited by the latter. The purification of specific choline and ethanolamine kinases from germinating soya bean confirmed in vivo observations which had indicated separate enzymes.