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International audience; Diacylglycerol kinases (DGKs) catalyze the phosphorylation of diacylglycerol into phosphatidic acid. To fulfill their role in many signalling processes, DGKs must be located at, or in, membranes. Most mammalian DGKs are cytosolic and are recruited to membranes upon stimulation, except for e type DGKs that are permanently membrane-associated through a hydrophobic segment. Nothing is known about the mechanism(s) involved in the membrane localization of plant DGKs. By fusion to fluorescent proteins, we show that two DGKs from cluster I in Arabidopsis thaliana possess amino-terminal hydrophobic segments that are sufficient to address them to endoplasmic reticulum membranes.

Salicylic acid (SA) has a central role in defense against pathogen attack. In addition, its role in such diverse processes as germination, flowering, senescence, and thermotolerance acquisition has been documented. However, little is known about the early signaling events triggered by SA. Using Arabidopsis (Arabidopsis thaliana) suspension cells as a model, it was possible to show by in vivo metabolic phospholipid labeling with 33Pi that SA addition induced a rapid and early (in few minutes) decrease in a pool of phosphatidylinositol (PI). This decrease paralleled an increase in PI 4-phosphate and PI 4,5-bisphosphate. These changes could be inhibited by two different inhibitors of type III PI 4-kinases, phenylarsine oxide and 30 μ m wortmannin; no inhibitory effect was seen with 1 μ m wortmannin, a concentration inhibiting PI 3-kinases but not PI 4-kinases. We therefore undertook a study of the effects of wortmannin on SA-responsive transcriptomes. Using the Complete Arabidopsis Transcriptome MicroArray chip, we could identify 774 genes differentially expressed upon SA treatment. Strikingly, among these genes, the response to SA of 112 of them was inhibited by 30 μ m wortmannin, but not by 1 μ m wortmannin.

In plants, a temperature downshift represents a major stress that will lead to the induction or repression of many genes. Therefore, the cold signal has to be perceived and transmitted to the nucleus. In response to a cold exposure, we have shown that the phospholipase D (PLD) and the phospholipase C (PLC)/diacylglycerol kinase pathways are simultaneously activated. The role of these pathways in the cold response has been investigated by analyzing the transcriptome of cold-treated Arabidopsis (Arabidopsis thaliana) suspension cells in the presence of U73122 or ethanol, inhibitors of the PLC/diacylglycerol kinase pathway and of the phosphatidic acid produced by PLD, respectively. This approach showed that the expression of many genes was modified by the cold response in the presence of such agents. The cold responses of most of the genes were repressed, thus correlating with the inhibitory effect of U73122 or ethanol. We were thus able to identify 58 genes that were regulated by temperature downshift via PLC activity and 87 genes regulated by temperature downshift via PLD-produced phosphatidic acid. Interestingly, each inhibitor appeared to affect different cold response genes. These results support the idea that both the PLC and PLD pathways are upstream of two different signaling pathways that lead to the activation of the cold response. The connection of these pathways with the CBF pathway, currently the most understood genetic system playing a role in cold acclimation, is discussed.

Lipid transfer proteins (LTPs) are small, basic and abundant proteins in higher plants. They are capable of binding fatty acids and of transferring phospholipids between membranes in vitro. LTPs from this family contain a signal peptide and are secreted in the cell wall. Their biological function is presently unknown. LTPs have been suggested to participate to cutin assembly and to the defense of the plants against pathogens. A genetic approach should prove useful to provide clues on their in vivo functions. Here, the characterization of the LTP gene family in Arabidopsis thaliana is described. At least 15 genes were identified, their map position determined and the expression pattern characterized for six of them. All the sequences exhibit the typical features of plant LTPs. The molecular weight is close to 9 kDa, the isoelectric point is near 9 (except for three acidic LTPs), and typical amino acid residues such as cysteines are conserved. Genomic DNA blotting hybridization experiments performed using ltp1 to ltp6 as probes indicate that ltps form distinct 1-3 gene subfamilies which do not cross hybridize. Expression studies indicate that all the genes tested are expressed in flowers and siliques, but not in roots. Ltp1, ltp5 and ltp2 are expressed significantly in leaves, while ltp6 is detected only in 2-4-week-old leaves. In addition, ltp4 and ltp3 are strongly upregulated by abscisic acid (ABA). Tandem repeats can be noted concerning ltp1 and ltp2 on chromosome 2, ltp3 and ltp4 on chromosome 5 and ltp5 and ltp12 on chromosome 3. While ltp7, ltp8 and ltp9 map at the same position on chromosome 2, the other genes are dispersed throughout the genome. The characterization of the Arabidopsis ltp gene family will permit to initiate a genetic approach for determining the in vivo function(s) of these proteins.

International audience; Phosphoinositide-dependent phospholipases C (PI-PLCs) are activated in response to various stimuli. They utilize substrates provided by type III-Phosphatidylinositol-4 kinases (PI4KIII) to produce inositol triphosphate and diacylglycerol (DAG) that is phosphorylated into phosphatidic acid (PA) by DAG-kinases (DGKs). The roles of PI4KIIIs, PI-PLCs, and DGKs in basal signaling are poorly understood. We investigated the control of gene expression by basal PI-PLC pathway in Arabidopsis thaliana suspension cells. A transcriptome-wide analysis allowed the identification of genes whose expression was altered by edelfosine, 30 μM wortmannin, or R59022, inhibitors of PI-PLCs, PI4KIIIs, and DGKs, respectively. We found that a gene responsive to one of these molecules is more likely to be similarly regulated by the other two inhibitors. The common action of these agents is to inhibit PA formation, showing that basal PI-PLCs act, in part, on gene expression through their coupling to DGKs. Amongst the genes up-regulated in presence of the inhibitors, were some DREB2 genes, in suspension cells and in seedlings. The DREB2 genes encode transcription factors with major roles in responses to environmental stresses, including dehydration. They bind to C-repeat motifs, known as Drought-Responsive Elements that are indeed enriched in the promoters of genes up-regulated by PI-PLC pathway inhibitors. PA can also be produced by phospholipases D (PLDs). We show that the DREB2 genes that are up-regulated by PI-PLC inhibitors are positively or negatively regulated, or indifferent, to PLD basal activity. Our data show that the DREB2 genetic pathway is constitutively repressed in resting conditions and that DGK coupled to PI-PLC is active in this process, in suspension cells and seedlings. We discuss how this basal negative regulation of DREB2 genes is compatible with their stress-triggered positive regulation.

International audience; BACKGROUND: Phospholipases D (PLD) are major components of signalling pathways in plant responses to some stresses and hormones. The product of PLD activity is phosphatidic acid (PA). PAs with different acyl chains do not have the same protein targets, so to understand the signalling role of PLD it is essential to analyze the composition of its PA products in the presence and absence of an elicitor. METHODOLOGY/PRINCIPAL FINDINGS: Potential PLD substrates and products were studied in Arabidopsis thaliana suspension cells treated with or without the hormone salicylic acid (SA). As PA can be produced by enzymes other than PLD, we analyzed phosphatidylbutanol (PBut), which is specifically produced by PLD in the presence of n-butanol. The acyl chain compositions of PBut and the major glycerophospholipids were determined by multiple reaction monitoring (MRM) mass spectrometry. PBut profiles of untreated cells or cells treated with SA show an over-representation of 160/18∶2- and 16∶0/18∶3-species compared to those of phosphatidylcholine and phosphatidylethanolamine either from bulk lipid extracts or from purified membrane fractions. When microsomal PLDs were used in in vitro assays, the resulting PBut profile matched exactly that of the substrate provided. Therefore there is a mismatch between the acyl chain compositions of putative substrates and the in vivo products of PLDs that is unlikely to reflect any selectivity of PLDs for the acyl chains of substrates. CONCLUSIONS: MRM mass spectrometry is a reliable technique to analyze PLD products. Our results suggest that PLD action in response to SA is not due to the production of a stress-specific molecular species, but that the level of PLD products per se is important. The over-representation of 160/18∶2- and 16∶0/18∶3-species in PLD products when compared to putative substrates might be related to a regulatory role of the heterogeneous distribution of glycerophospholipids in membrane sub-domains.

From rape (Brassica napus) seedlings proteins able to bind fatty acids and their CoA-esters were purified by gel filtration and cation-exchange chromatography. Among the four proteins detected, one of them (peak IV) appeared purified to homogeneity. This protein is a monomer with a molecular mass of about 9 kDa, as estimated by gel filtration and by polyacrylamide gel electrophoresis. The isoelectric point of the rape protein was higher than 10.5 as determined by chromatofocusing. The pure rape protein appeared furthermore to be able to transfer several phospholipids (phosphatidylcholine, phosphatidylinositol and phosphatidylethanolamine) between membranes. The rape protein, having a multifunctional property, was thus called acyl-binding/lipid-transfer protein (AB-LTP). In order to compare this protein to plant lipid-transfer proteins (LTPs), its structure was determined. The amino acid analysis of the rape AB-LTP revealed a high amount of alanine, an absence of histidine and tryptophan and the presence of eight cysteine residues. The N-terminal amino acid sequence of the rape protein revealed a high homology to plant LTPs. These observations led us to propose that the rape AB-LTPs belong to a category of plant proteins interacting with lipids and playing a role in the fatty acid dynamics.

Salicylic acid (SA) plays a central role in defense against pathogen attack, as well as in germination, flowering, senescence, and the acquisition of thermotolerance. In this report we investigate the involvement of phospholipase D (PLD) in the SA signaling pathway. In presence of exogenous primary alcohols, the production of phosphatidic acid by PLD is diverted toward the formation of phosphatidylalcohols through a reaction called transphosphatidylation. By in vivo metabolic phospholipid labeling with 33Pi, PLD activity was found to be induced 45 min after addition of SA. We show that incubation of Arabidopsis (Arabidopsis thaliana) cell suspensions with primary alcohols inhibited the induction of two SA-responsive genes, PATHOGENESIS-RELATED1 and WRKY38, in a dose-dependent manner. This inhibitory effect was more pronounced when the primary alcohols were more hydrophobic. Secondary or tertiary alcohols had no inhibitory effect. These results provide compelling arguments for PLD activity being upstream of the induction of these genes by SA. A subsequent study of n-butanol effects on the SA-responsive transcriptome identified 1,327 genes differentially expressed upon SA treatment. Strikingly, the SA response of 380 of these genes was inhibited by n-butanol but not by tert-butanol. A detailed analysis of the regulation of these genes showed that PLD could act both positively and negatively, either on gene induction or gene repression. The overlap with the previously described phosphatidylinositol-4-kinase pathway is discussed.

1. 1. We have isolated from mycelia of Mucor mucedo, a filamentous fungus, a phospholipid transfer protein. 2. 2. The purification steps wer gel filtration, hydroxyapatite chromatography, blue affinity column and fast protein liquid chromatography on anion exchanger. 3. 3. A purified protein was obtained with a molecular mass of 24 kDa and a pI of 5.05 and its N-terminal sequence was established. 4. 4. This protein transfers phosphatidylinositol, as well as phosphatidylcholine and phosphatidyl-ethanolamine.

The SEC14 gene of Saccharomyces cerevisiae codes for a phosphatidylinositol-transfer protein (Sec14p(sc)) which is capable of transferring both phosphatidylinositol and phosphatidylcholine between membranes in vitro. Genetic and biochemical studies conducted in S. cerevisiae have shown that this protein acts as an inhibitor of phosphatidylcholine biosynthesis via the so-called Kennedy pathway only. This inhibition is controlled by the binding of phospholipids to the Sec14p(sc) protein. Here we describe the isolation of a cDNA from Arabidopsis thaliana by functional complementation of a sec14(ts) mutant of S. cerevisiae. This cDNA, designated AtSEC14, is capable of restoring the growth of the sec14(ts) mutant at the restrictive temperature of 37 degrees C. Extracellular invertase measurements indicated that the cDNA can partly restore protein secretion. In addition, the phosphatidylinositol-transfer activity measured in protein extracts is greatly enhanced in the complemented mutant strain when compared with the sec14(ts) mutant. The best sequence similarity at the amino acid level is found with the Sec14p protein of S. cerevisiae (36.5% similarity), and most of the amino acids that are thought to be involved in the binding of phospholipids in the yeast protein are conserved in the AtSEC14 gene product. Southern analysis suggests the presence of a single gene in the Arabidopsis genome, although the existence of distantly related sequences cannot be excluded. This gene is expressed in roots, leaves, flowers and siliques of Arabidopsis.

Three cDNA clones encoding lipid transfer proteins (LTPs) were isolated by applying the rapid amplification of cDNA ends (RACE) protocol to imbibed seeds and germinating seedlings of Brassica napus. The deduced amino-acid sequences show a great degree of homology and they exhibit the common features shared by all LTPs. Their expression pattern indicates a strong developmental, hormonal, and environmental regulation. They are expressed only in cotyledons and hypocotyls of germinating seedlings and their levels of expression increase upon treatment with cis-abscisic acid and NaCl. Their distribution in the cotyledons of young seedlings is suggestive of a role related to the mobilization of lipid reserves.

Publisher Summary This chapter discusses the phospholipid transfer proteins (PLTP) from higher plants. Plant cells contain soluble proteins that are able to facilitate in vitro bidirectional movements of phospholipids between membranes. These proteins, called “PLTP,” have been purified and characterized from plant tissues and from animal tissues or yeast. PLTPs are assumed to participate in the intracellular distribution of phospholipids and could be involved in membrane biogenesis or in the function of membrane-bound enzymes using lipids as substrates. PLTP can thus modify the lipid composition of a membrane. This studying the phospholipid–protein interactions within the membranes and the effects of changes in lipid concentrations on the functional properties of membranes. PLTP can also been used as mild agents to determine the asymmetry of the lipid composition of the membrane leaflets. Also, these proteins are useful tools for inserting exogenous phospholipids, for example, those containing fluorescent compounds, to determine the mobility of lipids within membranes.

A cytosolic protein, able to facilitate intermembrane movements of phospholipids in vitro, has been purified to homogeneity from sunflower seedlings. This protein, which has the properties of a lipid-transfer protein (LTP), is also able to bind oleoyl-CoA, as shown by FPLC chromatography. This finding, in addition to previous observations suggesting that a lipid-transfer protein from spinach leaves can bind oleic acid and that oat seedlings contain a fatty acid-binding protein with similar features than lipid transfer proteins, provides a clear demonstration that plant cells contain bifunctional fatty acid/lipid transfer proteins. These proteins can play an active role in fatty acid metabolism which involves movements of oleyl-CoA between intracellular membranes.

In order to study the biosynthesis of a plant phospholipid transfer protein (PLTP), poly (A)+RNAs have been prepared from maize seedlings and translated in vitro with a rabbit reticulocyte lysate. The newly synthesized proteins were then separated by fast protein liquid chromatography (FPLC) followed by SDS-PAGE or by high performance liquid chromatography (HPLC) coupled to a radioactivity detector monitor. It has been showed that a radioactive band comigrating with a 14C methylated pure PLTP was detected by SDS-PAGE. This result, confirmed by direct radioactive monitoring, indicates that PLTP is actively synthesized in vitro .

The complete amino acid sequence phospholipid-transfer protein isolated from spinach leaves has been determined. The primary structure of the spinach protein was elucidated by analyses by HPLC of cyanogen bromide fragments and peptides obtained by tryptic digestions. The single polypeptide chain of the spinach protein consists of 91 amino acid residues. The protein contains six cysteines whereas phenylalanine and glutamine are absent. The present data, which are the first to be obtained with a phospholipid-transfer protein from a photosynthetic tissue, are compared to the amino acid sequences determined with plant and animal proteins involved in the intracellular transport of hydrophobic compounds.

The polypeptide composition of plasmalemma, mitochondria and endoplasmic reticulum, isolated from maize coleoptile, was determined using polyacrylamide gel electrophoresis in the presence of lithium dodecyl sulfate. A polypeptide with an apparent molecular weight of 90,000 was found to be the major polypeptide associated with plasmalemma. This polypeptide is firmly attached to the membrane since it was not extracted by KCl. However, after a treatment with Triton X100, the polypeptide was detached from the membrane. It was also found that a heat treatment of the membrane partly destroyed this polypeptide. This effect of heat treatment was also observed after isolation of the polypeptide by electrophoresis.

A low-molecular-weight protein catalysing the transfer of phosphatidylcholine from liposomes to mitochondria and chloroplasts has been isolated from spinach (Spinacia oleracea) by chromatography on Sephadex G-75.

Membrane rigidification could be the first step of cold perception in poikilotherms. We have investigated its implication in diacylglycerol kinase (DAGK) activation by cold stress in suspension cells from Arabidopsis mutants altered in desaturase activities. By lateral diffusion assay, we showed that plasma membrane rigidification with temperature decrease was steeper in cells deficient in oleate desaturase than in wild type cells and in cells overexpressing linoleate desaturase. The threshold for the activation of the DAGK pathway in each type of cells correlated with this order of rigidification rate, suggesting that cold induced-membrane rigidification is upstream of DAGK pathway activation.

Publisher Summary This chapter describes procedures for the purification of a phospholipid transfer protein, as well as the assays for measuring the activity from spinach protein. The spinach transfer protein is basic, with an isoelectric point near 9.0 determined by chromatofocusing. Maize and castor bean transfer proteins are also basic. In addition to phosphatidylcholine, the spinach transfer protein is also able to transfer phosphatidylinositol, phosphatidylglycerol, and to a lesser extent, phosphatidylethanolamine. A similar nonspecific characteristic has also been found for other plant or animal transfer proteins. The spinach protein is active with various mixtures of natural and artificial membranes: liposome–mitochondria, liposome–liposome, and liposome–microsome. The recent observations that the spinach protein is active with intact chloroplasts and with chloroplast envelope membranes open new perspectives for the study of the physiological roles of these proteins in photosynthetic cells.

The synthesis of phospholipid transfer proteins has been studied in vitro after isolation of poly(A)+RNAs from maize seedlings and by in vivo labelling of coleoptiles. After immunoprecipitation of translation products in wheat germ or in reticulocyte lysate systems, the analysis by electrophoresis revealed two bands of molecular mass 9 kDa and 12 kDa. The in vitro synthesized 12 kDa protein is a precursor of the 9 kDa purified protein from maize seedlings as suggested by competition experiments with the pure protein. After immunoprecipitation of in vivo labelled proteins, two bands were detected. One of them, having a molecular mass of 7 kDa, could be related to the in vitro synthesized 9 kDa protein, the other corresponding to the purified protein. Furthermore, biosynthesis of both precursors occurs on membrane-bound polysomes. Presumably a post translational process occurs, yielding to the mature forms.

A phospholipid transfer protein has been purified 195-fold from an extract of spinach leaves. This protein is capable of transferring phosphatidylcholine, phosphatidylinositol, phosphatidylglycerol and phosphatidyl-ethanolamine from liposomes to mitochondria. In addition to this protein, a minor part of the total activity was associated with a less purified fraction. The pure protein has an isoelectric point of 9.0 ± 0.2 determined by a chromatofocusing technique. Electrophoresis on sodium dodecyl sulfate/polyacrylamide gel showed that the protein is homogeneous and has an apparent molecular weight of 9000 ± 1000, in agreement with the value (8832) calculated from the amino acid composition. This composition is characterized by a high amount of alanine and glycine and by the absence of phenylalanine, whereas arginine, glutamine, histidine and methionine are minor components. The spinach protein is also able to transfer phosphatidylcholine and phosphatidylglycerol from liposomes to intact chloroplasts. This observation reinforces the hypothesis that plastid phospholipids are partly imported from outside the organelle by a transfer process.

Among the cytosolic non enzymatic proteins, some are able to stimulate phospholipid transfer between two distinct membranes. These proteins are named Phospholipid Transfer Proteins (PLTP).

Phospholipid transfer proteins (PLTP) have been isolated to homogeneity from various plant sources: spinach leaves1, maize seedlings2,3 and castor bean endosperm4. These proteins are able to facilitate an in vitro transfer of phospholipids between membranes. This led to the hypothesis that these proteins participate in vivo in the turnover and biogenesis of membranes5. In order to study the function of these proteins, it is essential to know their biochemical properties as well as their biogenesis. In this paper, we compare the properties of the transfer proteins isolated from the three tissues and we indicate new informations concerning the in vitro synthesis of the protein isolated from maize seedlings.