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The exocyst is a well-known complex which tethers vesicles at the cell membrane before fusion. Whether an individual subunit can execute a unique function is largely unknown. Using yeast-two-hybrid (Y2H) analysis, we found that EXO70A1 interacted with the GOLD domain of Patellin3 (PATL3). The direct EXO70A1-PATL3 interaction was supported by in vitro and in vivo experiments. In Arabidopsis, PATL3-GFP colocalized with EXO70A1 predominantly at the cell membrane, and PATL3 localization was insensitive to BFA and TryA23. Remarkably, in the exo70a1 mutant, PATL3 proteins accumulated as punctate structures within the cytosol, which did not colocalize with several endomembrane compartment markers, and was insensitive to BFA. Furthermore, PATL3 localization was not changed in the exo70e2, PRsec6 or exo84b mutants. These data suggested that EXO70A1, but not other exocyst subunits, was responsible for PATL3 localization, which is independent of its role in secretory/recycling vesicle-tethering/fusion. Both EXO70A1 and PATL3 were shown to bind PI4P and PI(4,5)P2 in vitro. Evidence was obtained that the other four members of the PATL family bound to EXO70A1 as well, and shared a similar localization pattern as PATL3. These findings offered new insights into exocyst subunit-specific function, and provided data and tools for further characterization of PATL family proteins.

Phosphoinositides play an important role in various membrane trafficking events in eukaryotes. One of them, however, phosphatidylinositol 3,5-bisphosphate [PI(3,5)P2], has not been studied widely in plants. Using a combination of fluorescent reporter proteins and the PI(3,5)P2-specific inhibitor YM202636, here we demonstrated that in Arabidopsis thaliana, PI(3,5)P2 affects various membrane trafficking events, mostly in the post-Golgi routes. We found that YM201636 treatment effectively reduced PI(3,5)P2 concentration not only in the wild type but also in FAB1A-overexpressing Arabidopsis plants. In particular, reduced PI(3,5)P2 levels caused abnormal membrane dynamics of plasma membrane proteins, AUX1 and BOR1, with different trafficking patterns. Secretion and morphological characteristics of late endosomes and vacuoles were also affected by the decreased PI(3,5)P2 production. These pleiotropic defects in the post-Golgi trafficking events were caused by the inhibition of PI(3,5)P2 production. This effect is probably mediated by the inhibition of maturation of FAB1-positive late endosomes, thereby impairing late endosome function. In conclusion, our results imply that in Arabidopsis, late endosomes are involved in multiple post-Golgi membrane trafficking routes including not only vacuolar trafficking and endocytosis but also secretion.

Lipidomic analyses through LC-, GC-, and ESI-MS/MS can detect numerous lipid species based on headgroup and fatty acid compositions but usually miss the minor phospholipids involved in cell signaling because of their low chemical abundancy. Due to their high turnover, these signaling lipids are, however, readily picked up by labeling plant material with (32)P-orthophosphate and subsequent analysis of the lipid extracts by thin layer chromatography. Here, protocols are described for suspension-cultured tobacco BY-2 cells, young Arabidopsis seedlings, Vicia faba roots, and Arabidopsis leaf disks, which can easily be modified for other plant species and tissues.

In plants, phosphatidic acid (PA) functions as a metabolic precursor in the biosynthesis of glycerolipids, but it also acts as a key signaling lipid in the response to environmental stress conditions (Testerink and Munnik, J Exp Bot 62:2349-2361, 2011). In vivo (32)P-radiolabeling assays have shown the level of PA to increase within seconds/minutes of exposure to a stimulus. This response can be due to the activity of diacylglycerol kinase (DGK) and/or phospholipase D (PLD). A method is described to investigate which of the pathways is responsible for PA accumulation under a particular stress condition. First, a differential (32)P-radiolabeling protocol is used to discriminate (32)P-PA pools that are rapidly labeled versus those requiring long prelabeling times, reflecting DGK and PLD activities, respectively. Second, to specifically monitor the contribution of PLD, a transphosphatidylation assay is applied, which makes use of the artificial lipid phosphatidylbutanol as an in vivo marker of PLD activity.

The discovery of the green fluorescent protein has revolutionized cell biology as it allowed researchers to visualize dynamic processes in living cells. The fusion of fluorescent protein variants with lipid binding domains that bind to specific phospholipids have been very instrumental in investigating the role of these molecules in living plants. Here, we describe the use of these reporters to image lipids in living Arabidopsis seedlings using fluorescence microscopy.

Biological lipid extracts often contain small amounts of lysophospholipids (LPLs). Since different functions are emerging for LPLs in lipid metabolism and signalling, there is need for a reliable and cost-effective method for their identification. For this purpose, authentic LPL standards have to be synthesized from phosphoglycerides by PLA2 digestion in vitro. PLA2 specifically hydrolyzes the fatty acid ester linkage in the sn-2-position of phospholipids to liberate sn-2-linked fatty acids and the corresponding LPL. Due to this specificity, the reaction is also useful to analyze the positional distribution of fatty acids within membrane phospholipids. This chapter describes the in vitro generation of LPLs from diacyl-phosphoglycerides and their TLC analysis.

Historically, phosphoinositide-specific phospholipase C (PI-PLC) catalyzes the hydrolysis of phosphatidylinositol 4,5-bisphosphate (PIP2) to generate the second messengers, inositol 1,4,5-trisphosphate (InsP3) and diacylglycerol (DAG), which release Ca2+ from intracellular stores and activate members of the protein kinase C (PKC) family, respectively. While this signaling system is well understood in animal cells, for plants this is still far from clear, as they lack homologs for the InsP3 receptor and PKC, and display extremely low PIP2 levels in their membranes under normal conditions. Plant genomes do contain numerous genes coding for phosphatidylinositol kinase (PIK) to make phosphatidylinositol 4-phosphate (PIP), PIP kinase (PIPK) to synthesize PIP2, and PI-PLC to hydrolyze these lipids. Data is also emerging that not InsP3 or DAG but their phosphorylated products, i.e., inositolpolyphosphates (IPPs) such as InsP5 and InsP6, and phosphatidic acid (PA), are functioning as plant signaling molecules. The goal of this chapter is to provide a critical overview of what is currently known about plant PI-PLC signaling and to indicate directions for future research.

Lipid affinity beads can be used to identify novel proteins with lipid binding capacity or to determine binding prerequisites of known lipid-binding proteins. Here we describe several applications for which this tool can be used and which considerations have to be taken into account. In addition to a precise protocol, several suggestions are made for experimental setups to facilitate identification of in vivo lipid binding targets.

Phospholipase D (PLD) hydrolyzes structural phospholipids like phosphatidylcholine (PC) and phosphatidylethanolamine (PE) into phosphatidic acid (PA) and free choline/ethanolamine. In plants, this activity can be stimulated by a wide variety of biotic and abiotic stresses (Li et al., Biochim Biophys Acta 1791:927-935, 2009; Testerink and Munnik, J Exp Bot 62(7):2349-2361, 2011). This chapter describes a protocol for the measurement of PLD activity in vivo. The protocol takes advantage of a unique property of PLD, i.e., its ability to substitute a primary alcohol, such as 1-butanol, for water in the hydrolytic reaction. This transphosphatidylation reaction results in the formation of phosphatidylbutanol (PBut), which is a specific and unique reporter for PLD activity. The assay is highly sensitive for detecting PLD activity in vivo, following stimulation of intact plant cells, seedlings, and tissues, being a valuable method for studying the regulation of plant PLD activity in vivo.

Protein-lipid interactions play an important role in cellular protein relocation, activation and signal transduction. The liposome-binding assay is a simple and inexpensive method to examine protein-lipid binding in vitro. The phospholipids used for liposome production are dried and hydrated. Subsequent extrusion of the phospholipid mixture ensures the production of large unilamellar vesicles (LUV) filled with raffinose. Those LUVs can be easily separated from the aqueous solution by centrifugation. By incubating a protein of interest with the LUVs and subsequent centrifugation steps, the bound protein fraction can be determined using Western Blot or Coomassie staining. This technique enables analysis of protein-lipid binding affinity and specificity.

Protein-lipid interactions play an important role in lipid metabolism, membrane trafficking and cell -signaling by regulating protein localization, activation, and function. The Fat Blot assay is a relatively simple and inexpensive method to examine these interactions using nitrocellulose membrane-immobilized lipids. The assay is adapted from the method by Dowler et al. (Sci STKE 129:pl6, 2002) and provides qualitative and quantitative information on the relative affinity with which a protein binds to a particular lipid. To perform a Fat Blot assay, serial dilutions of different phospholipids are spotted onto a nitrocellulose membrane. These membranes are then incubated with a lipid-binding protein possessing a GST (or other epitope) tag. The membranes are washed and the protein, which is bound to the membrane by virtue of its interaction with the lipid's head group, is detected by immunoblotting with an antibody against GST (or other epitope). The procedure only requires a few micrograms of protein and is quick, simple and cheap to perform.

We have recently detected phosphatidylinositol-4-phosphate (PI4P) in the extracellular medium of tomato cell suspensions. Extracellular PI4P was shown to trigger the activation of defence responses induced by the fungal elicitor xylanase. In this study, by applying a differential centrifugation technique, we found that extracellular PI4P is associated with fractions composed of diverse phospholipids and proteins, which were pelleted from the extracellular medium of tomato cell suspensions grown under basal conditions. Using mass spectrometry, we identified the proteins present in these pelleted fractions. Most of these proteins have previously been characterised as having a role in defence responses. Next, we evaluated whether PI4P could also be detected in an entire plant system. For this, apoplastic fluids of tomato plants grown under basal conditions were analysed using a lipid overlay assay. Interestingly, PI4P could be detected in intercellular fluids obtained from tomato leaflets and xylem sap of tomato plants. By employing electrospray ionisation tandem mass spectrometry (ESI-MS/MS), other phospholipids were also found in intercellular fluids of tomato plants. These had a markedly different profile from the phospholipid pattern identified in entire leaflets. Based on these results, the potential role of extracellular phospholipids in plant intercellular communication is discussed. Fil: Gonorazky, Ana Gabriela. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Mar del Plata. Instituto de Investigaciones Biológicas; Argentina. Universidad Nacional de Mar del Plata; Argentina Fil: Laxalt, Ana Maria. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Mar del Plata. Instituto de Investigaciones Biológicas; Argentina. Universidad Nacional de Mar del Plata; Argentina Fil: Dekker, H. L.. University Of Amsterdam; Países Bajos Fil: Rep, M.. University Of Amsterdam; Países Bajos Fil: Munnik, T.. University Of Amsterdam; Países Bajos Fil: Testerink, C.. University Of Amsterdam; Países Bajos Fil: de la Canal, L. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Mar del Plata. Instituto de Investigaciones Biológicas; Argentina. Universidad Nacional de Mar del Plata; Argentina

The perception of pathogen-derived elicitors by plants has been suggested to involve phosphatidylinositol-specific phospholipase-C (PI-PLC) signalling. Here we show that PLC isoforms are required for the hypersensitive response (HR) and disease resistance. We characterised the tomato [Solanum lycopersicum (Sl)] PLC gene family. Six Sl PLC-encoding cDNAs were isolated and their expression in response to infection with the pathogenic fungus Cladosporium fulvum was studied. We found significant regulation at the transcriptional level of the various SlPLCs, and SlPLC4 and SlPLC6 showed distinct expression patterns in C. fulvum-resistant Cf-4 tomato. We produced the encoded proteins in Escherichia coli and found that both genes encode catalytically active PI-PLCs. To test the requirement of these Sl PLCs for full Cf-4-mediated recognition of the effector Avr4, we knocked down the expression of the encoding genes by virus-induced gene silencing. Silencing of SlPLC4 impaired the Avr4/Cf-4-induced HR and resulted in increased colonisation of Cf-4 plants by C. fulvum expressing Avr4. Furthermore, expression of the gene in Nicotiana benthamiana enhanced the Avr4/Cf-4-induced HR. Silencing of SlPLC6 did not affect HR, whereas it caused increased colonisation of Cf-4 plants by the fungus. Interestingly, Sl PLC6, but not Sl PLC4, was also required for resistance to Verticillium dahliae, mediated by the transmembrane Ve1 resistance protein, and to Pseudomonas syringae, mediated by the intracellular Pto/Prf resistance protein couple. We conclude that there is a differential requirement of PLC isoforms for the plant immune response and that Sl PLC4 is specifically required for Cf-4 function, while Sl PLC6 may be a more general component of resistance protein signalling.

Diacylglycerol kinase (DGK, EC 2.7.1.107) is a lipid kinase that phosphorylates diacylglycerol (DAG) to generate phosphatidic acid (PA). DGK belongs to a well-conserved family of proteins found in diverse species, such as Saccharomyces cerevisiae, Drosophila melanogaster, Caenorhabditis elegans, mammals, plants, and bacteria (Merida et al. Biochem J 409:1-18, 2008). In eukaryotes, DGK can be activated in response to various stimuli, and its product PA is emerging as a novel lipid second messenger (Munnik, Trends Plant Sci 6:227-233, 2001; Testerink and Munnik, Trends Plant Sci 10:368-375, 2005). Despite similarities to the mammalian system, plant DGKs display several distinctive features.

Phosphatidic acid (PA) has been shown to be an important bioactive lipid that is specifically recognized by various proteins. As such, it plays a crucial role in cellular signaling in all eukaryotes. An important determinant for PA's role in its diverse functions is its anionic headgroup that resides very close to the hydrophobic interior of the lipid bilayer. In this chapter, we describe a new model, the electrostatic/ hydrogen-bond switch that describes PA's ionization properties and its specific interaction with proteins. Furthermore, we will allude to the broader implications of the model for all phosphomonoester moieties found in biological compounds. Recent data in support of the model, as well as biological predictions arising from it, are also discussed.

Phosphatidylinositol 3-kinase (PI3K) phosphorylates the D-3 position of phosphoinositides. In Arabidopsis, only one PI3K exists, which belongs to the class-III PI3K subfamily which makes phosphatidylinositol 3-phosphate (PtdIns3P). The single AtPI3K gene is essential for survival, since loss of its expression results in lethality. Although not much is known about the molecular mechanism of its function, recent studies show that plant PI3K is important for development and signaling, similar to yeast and animal systems. This includes involvement in endocytosis, reactive oxygen species (ROS) production, and transcriptional activity. Many more interesting stories about the role of this enzyme in the core of cellular activities of plants will be unfold as refined technologies are applied to study this important enzyme.

Phospholipids are important constituents of biological membranes, most of them fulfilling a structural role. However, it has become clear that in plants, just as in mammalian and yeast cells, some minor phospholipids, e.g. phosphoinositides, are important regulators of cellular function, providing docking sites for target proteins via lipid-binding domains, and/or modulating their enzymatic activity. The application of fluorescent proteins fused to lipid-binding domains to create the so-called, lipid biosensors, sheds new light on lipid molecules in living plant cells. Here, an overview is presented regarding their application.

Plants respond to wounding by means of a multitude of reactions, with the purpose of stifling herbivore assault. Phospholipase D (PLD) has previously been implicated in the wounding response. Arabidopsis (Arabidopsis thaliana) AtPLDa1 has been proposed to be activated in intact cells, and the phosphatidic acid (PA) it produces to serve as a precursor for jasmonic acid (JA) synthesis and to be required for wounding-induced gene expression. Independently, PLD activity has been reported to have a bearing on wounding-induced MAPK activation. However, which PLD isoforms are activated, where this activity takes place (in the wounded or non-wounded cells) and what exactly the consequences are is a question that has not been comprehensively addressed. Here, we show that PLD activity during the wounding response is restricted to the ruptured cells using 32Pi-labelled phospholipid analyses of Arabidopsis pld knock-out mutants and PLD-silenced tomato cell-suspension cultures. plda1 knock-out lines have reduced wounding-induced PA production, and the remainder is completely eliminated in a plda1/d double knock-out line. Surprisingly, wounding-induced protein kinase activation, AtLOX2 gene expression and JA biosynthesis were not affected in these knock-out lines. Moreover, larvae of the Cabbage White butterfly (Pieris rapae) grew equally well on wild-type and the pld knock-out mutants. Fil: Bargmann, Bastiaan O. R.. University of Amsterdam; Países Bajos Fil: Laxalt, Ana Maria. University of Amsterdam; Países Bajos. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Instituto de Investigaciones Biológicas y Tecnológicas. Universidad Nacional de Córdoba. Facultad de Ciencias Exactas, Físicas y Naturales. Instituto de Investigaciones Biológicas y Tecnológicas; Argentina Fil: Riet, Bas Ter. University of Amsterdam; Países Bajos Fil: Testerink, Christa. University of Amsterdam; Países Bajos Fil: Merquiol, Emmanuelle. Vrije Universiteit Amsterdam; Países Bajos Fil: Mosblech, Alina. Georg August University; Alemania Fil: Leon Reyes, Antonio. Utrecht University; Países Bajos Fil: Pieterse, Corné M. J.. Utrecht University; Países Bajos Fil: Haring, Michel A.. University of Amsterdam; Países Bajos Fil: Heilmann, Ingo. Georg August University; Alemania Fil: Bartels, Dorothea. Vrije Universiteit Amsterdam; Países Bajos Fil: Munnik, Teun. Universitat Bonn; Alemania

Phosphatidic acid (PA) is a minor but important phospholipid that, through specific interactions with proteins, plays a central role in several key cellular processes. The simple yet unique structure of PA, carrying just a phosphomonoester head group, suggests an important role for interactions with the positively charged essential residues in these proteins. We analyzed by solid-state magic angle spinning 31P NMR and molecular dynamics simulations the interaction of low concentrations of PA in model membranes with positively charged side chains of membrane-interacting peptides. Surprisingly, lysine and arginine residues increase the charge of PA, predominantly by forming hydrogen bonds with the phosphate of PA, thereby stabilizing the protein-lipid interaction. Our results demonstrate that this electrostatic/hydrogen bond switch turns the phosphate of PA into an effective and preferred docking site for lysine and arginine residues. In combination with the special packing properties of PA, PA may well be nature’s preferred membrane lipid for interfacial insertion of positively charged membrane protein domains.

Phospholipids play an important role in many signaling pathways in animal cells. Signaling cascades are triggered by the activation of phospholipid cleaving enzymes such as phospholipases C, D (PLD), and A2. Their activities result in the formation of second messengers and amplification of the initial signal. In this study, we provide experimental evidence that PLD is involved in the early events of dehydration in the resurrection plant Craterostigma plantagineum. The enzymatic activity of the PLD protein was activated within minutes after the onset of dehydration, and although it was not inducible by abscisic acid, PLD activity did increase in response to mastoparan, which suggests a role for heterotrimeric G proteins in PLD regulation. Two cDNA clones encoding PLDs, CpPLD-1 and CpPLD-2, were isolated. The CpPLD-1 transcript was constitutively expressed, whereas CpPLD-2 was induced by dehydration and abscisic acid. Immunological studies revealed changes in the subcellular localization of the PLD protein in response to dehydration. Taken together, the data on enzymatic activity as well as transcript and protein distributions allowed us to propose a role for PLD in the events leading to desiccation tolerance in C. plantagineum.

In mammalian cells, phospholipase D (PLD) and its product phosphatidic acid (PA) are involved in a number of signalling cascades, including cell proliferation, membrane trafficking and defence responses. In plant cells a signalling role for PLD and PA is also emerging. Plants have the extra ability to phosphorylate PA to produce diacylglycerol pyrophosphate (DGPP), a newly discovered phospholipid whose formation attenuates PA levels, but which could itself be a second messenger. Here we report that increases in PA and its conversion to DGPP are common stress responses to water deficit. Increases occur within minutes of treatment and are dependent on the level of stress. Part of the PA produced is due to PLD activity as measured by the in vivo transphosphatidylation of 1-butanol, and part is due to diacylglycerol kinase activity as monitored via 32P-PA formation in a differential labelling protocol. Increases in PA and DGPP are found not only in the green alga Chlamydomonas moewusii and cell-suspension cultures of tomato and alfalfa when subjected to hyperosmotic stress, but also in dehydrated leaves of the resurrection plant Craterostigma plantagineum. These results provide further evidence that PLD and PA play a role in plant signalling, and provide the first demonstration that DGPP is formed during physiological conditions that evoke PA synthesis.