Rapid turnover of phosphatidylinositol 3-phosphate in the green alga Chlamydomonas eugametos: signs of a phosphatidylinositide 3-kinase signalling pathway in lower plants?

Stress- and phospholipid signalling responses in Arabidopsis PLC4-KO and -overexpression lines under salt- and osmotic stress

Several drought and salt tolerant phenotypes have been reported when overexpressing (OE) phospholipase C (PLC) genes across plant species. In contrast, a negative role for Arabidopsis PLC4 in salinity stress was recently proposed, showing that roots of PLC4-OE seedlings were more sensitive to NaCl while plc4 knock-out (KO) mutants were more tolerant. To investigate this apparent contradiction, and to analyse the phospholipid signalling responses associated with salinity stress, we performed root growth- and phospholipid analyses on plc4-KO and PLC4-OE seedlings subjected to salinity (NaCl) or osmotic (sorbitol) stress and compared these with wild type (WT). Only very minor differences between PLC4 mutants and WT were observed, which even disappeared after normalization of the data, while in soil, PLC4-OE plants were clearly more drought tolerant than WT plants, as was found earlier when overexpressing Arabidopsis PLC2, -3, -5, -7 or -9. We conclude that PLC4 plays no opposite role in salt-or osmotic stress and rather behaves like the other Arabidopsis PLCs.

DIACYLGLYCEROL KINASE 5 participates in flagellin-induced signaling in Arabidopsis

Flagellin perception is a keystone of pattern-triggered immunity in plants. The recognition of this protein by a plasma membrane (PM) receptor complex is the beginning of a signaling cascade that includes protein phosphorylation and the production of reactive oxygen species (ROS). In both Arabidopsis (Arabidopsis thaliana) seedlings and suspension cells, we found that treatment with flg22, a peptide corresponding to the most conserved domain of bacterial flagellin, caused a rapid and transient decrease in the level of phosphatidylinositol (PI) 4,5-bisphosphate along with a parallel increase in phosphatidic acid (PA). In suspension cells, inhibitors of either phosphoinositide-dependent phospholipases C (PLC) or diacylglycerol kinases (DGKs) inhibited flg22-triggered PA production and the oxidative burst. In response to flg22, receptor-like kinase-deficient fls2, bak1, and bik1 mutants (FLAGELLIN SENSITIVE 2, BRASSINOSTEROID INSENSITIVE 1-associated kinase 1, and BOTRYTIS-INDUCED KINASE 1, respectively) produced less PA than wild-type (WT) plants, whereas this response did not differ in NADPH oxidase-deficient rbohD (RESPIRATORY BURST OXIDASE HOMOLOG D) plants. Among the DGK-deficient lines tested, the dgk5.1 mutant produced less PA and less ROS after flg22 treatment compared with WT seedlings. In response to flg22, dgk5.1 plants showed lower callose accumulation and impaired resistance to Pseudomonas syringae pv. tomato DC3000 hrcC-. Transcriptomics revealed that the basal expression of defense-related genes was altered in dgk5.1 seedlings compared with the WT. A GFP-DGK5 fusion protein localized to the PM, where RBOHD and PLC2 (proteins involved in plant immunity) are also located. The role of DGK5 and its enzymatic activity in flagellin signaling and fine-tuning of early immune responses in plant-microbe interactions is discussed.

The functions of phospholipases and their hydrolysis products in plant growth, development and stress responses

Cell membranes are the initial site of stimulus perception from environment and phospholipids are the basic and important components of cell membranes. Phospholipases hydrolyze membrane lipids to generate various cellular mediators. These phospholipase-derived products, such as diacylglycerol, phosphatidic acid, inositol phosphates, lysophopsholipids, and free fatty acids, act as second messengers, playing vital roles in signal transduction during plant growth, development, and stress responses. This review focuses on the structure, substrate specificities, reaction requirements, and acting mechanism of several phospholipase families. It will discuss their functional significance in plant growth, development, and stress responses. In addition, it will highlight some critical knowledge gaps in the action mechanism, metabolic and signaling roles of these phospholipases and their products in the context of plant growth, development and stress responses.

The Ancient Phosphatidylinositol 3-Kinase Signaling System Is a Master Regulator of Energy and Carbon Metabolism in Algae

Algae undergo a complete metabolic transformation under stress by arresting cell growth, inducing autophagy and hyper-accumulating biofuel precursors such as triacylglycerols and starch. However, the regulatory mechanisms behind this stress-induced transformation are still unclear. Here, we use biochemical, mutational, and "omics" approaches to demonstrate that PI3K signaling mediates the homeostasis of energy molecules and influences carbon metabolism in algae. In Chlamydomonas reinhardtii, the inhibition and knockdown (KD) of algal class III PI3K led to significantly decreased cell growth, altered cell morphology, and higher lipid and starch contents. Lipid profiling of wild-type and PI3K KD lines showed significantly reduced membrane lipid breakdown under nitrogen starvation (-N) in the KD. RNA-seq and network analyses showed that under -N conditions, the KD line carried out lipogenesis rather than lipid hydrolysis by initiating de novo fatty acid biosynthesis, which was supported by tricarboxylic acid cycle down-regulation and via acetyl-CoA synthesis from glycolysis. Remarkably, autophagic responses did not have primacy over inositide signaling in algae, unlike in mammals and vascular plants. The mutant displayed a fundamental shift in intracellular energy flux, analogous to that in tumor cells. The high free fatty acid levels and reduced mitochondrial ATP generation led to decreased cell viability. These results indicate that the PI3K signal transduction pathway is the metabolic gatekeeper restraining biofuel yields, thus maintaining fitness and viability under stress in algae. This study demonstrates the existence of homeostasis between starch and lipid synthesis controlled by lipid signaling in algae and expands our understanding of such processes, with biotechnological and evolutionary implications.

Vacuolar Trafficking Protein VPS38 Is Dispensable for Autophagy

Phosphatidylinositol 3-P (PI3P) is a signaling molecule that controls a variety of processes in endosomal, autophagic, and vacuolar/lysosomal trafficking in yeasts and mammals. Vacuolar protein sorting 34 (Vps34) is a conserved PI3K present in multiple complexes with specific functions and regulation. In yeast, the PI3K complex II consists of Vps34p, Vps15p, Vps30p/Atg6p, and Vps38p, and is essential for vacuolar protein sorting. Here, we describe the Arabidopsis (Arabidopsis thaliana) homolog of yeast Vps38p and human UV radiation resistance-associated gene protein. Arabidopsis VPS38 interacts with VPS30/ATG6 both in yeast and in planta. Although the level of PI3P in Arabidopsis vps38 mutants is similar to that in wild type, vps38 cells contain enlarged multivesicular endosomes and fewer organelles enriched in PI3P than the wild type. The vps38 mutants are defective in the trafficking of vacuolar cargo and its receptor VACUOLAR SORTING RECEPTOR2;1. The mutants also exhibit abnormal cytoplasmic distributions of endocytic cargo, such as auxin efflux carriers PINFORMED1 (PIN1) and PIN2. Constitutive autophagy is normal in the mutants but starvation-induced autophagy was slightly inhibited. We conclude that Arabidopsis VPS38 is dispensable for autophagy but essential for efficient targeting of biosynthetic and endocytic cargo to the vacuole.

Requirement of Phosphoinositides Containing Stearic Acid To Control Cell Polarity

Phosphoinositides (PIPs) are present in very small amounts but are essential for cell signaling, morphogenesis, and polarity. By mass spectrometry, we demonstrated that some PIPs with stearic acyl chains were strongly disturbed in a psi1Δ Saccharomyces cerevisiae yeast strain deficient in the specific incorporation of a stearoyl chain at the sn-1 position of phosphatidylinositol. The absence of PIPs containing stearic acid induced disturbances in intracellular trafficking, although the total amount of PIPs was not diminished. Changes in PIPs also induced alterations in the budding pattern and defects in actin cytoskeleton organization (cables and patches). Moreover, when the PSI1 gene was impaired, a high proportion of cells with bipolar cortical actin patches that occurred concomitantly with the bipolar localization of Cdc42p was specifically found among diploid cells. This bipolar cortical actin phenotype, never previously described, was also detected in a bud9Δ/bud9Δ strain. Very interestingly, overexpression of PSI1 reversed this phenotype.

SEC14 phospholipid transfer protein is involved in lipid signaling-mediated plant immune responses in Nicotiana benthamiana

We previously identified a gene related to the SEC14-gene phospholipid transfer protein superfamily that is induced in Nicotiana benthamiana (NbSEC14) in response to infection with Ralstonia solanacearum. We here report that NbSEC14 plays a role in plant immune responses via phospholipid-turnover. NbSEC14-silencing compromised expression of defense-related PR-4 and accumulation of jasmonic acid (JA) and its derivative JA-Ile. Transient expression of NbSEC14 induced PR-4 gene expression. Activities of diacylglycerol kinase, phospholipase C and D, and the synthesis of diacylglycerol and phosphatidic acid elicited by avirulent R. solanacearum were reduced in NbSEC14-silenced plants. Accumulation of signaling lipids and activation of diacylglycerol kinase and phospholipases were enhanced by transient expression of NbSEC14. These results suggest that the NbSEC14 protein plays a role at the interface between lipid signaling-metabolism and plant innate immune responses.

Salicylic acid modulates levels of phosphoinositide dependent-phospholipase C substrates and products to remodel the Arabidopsis suspension cell transcriptome

Basal phosphoinositide-dependent phospholipase C (PI-PLC) activity controls gene expression in Arabidopsis suspension cells and seedlings. PI-PLC catalyzes the production of phosphorylated inositol and diacylglycerol (DAG) from phosphoinositides. It is not known how PI-PLC regulates the transcriptome although the action of DAG-kinase (DGK) on DAG immediately downstream from PI-PLC is responsible for some of the regulation. We previously established a list of genes whose expression is affected in the presence of PI-PLC inhibitors. Here this list of genes was used as a signature in similarity searches of curated plant hormone response transcriptome data. The strongest correlations obtained with the inhibited PI-PLC signature were with salicylic acid (SA) treatments. We confirm here that in Arabidopsis suspension cells SA treatment leads to an increase in phosphoinositides, then demonstrate that SA leads to a significant 20% decrease in phosphatidic acid, indicative of a decrease in PI-PLC products. Previous sets of microarray data were re-assessed. The SA response of one set of genes was dependent on phosphoinositides. Alterations in the levels of a second set of genes, mostly SA-repressed genes, could be related to decreases in PI-PLC products that occur in response to SA action. Together, the two groups of genes comprise at least 40% of all SA-responsive genes. Overall these two groups of genes are distinct in the functional categories of the proteins they encode, their promoter cis-elements and their regulation by DGK or phospholipase D. SA-regulated genes dependent on phosphoinositides are typical SA response genes while those with an SA response that is possibly dependent on PI-PLC products are less SA-specific. We propose a model in which SA inhibits PI-PLC activity and alters levels of PI-PLC products and substrates, thereby regulating gene expression divergently.

Involvement of Phosphatidylinositol 3-kinase in the regulation of proline catabolism in Arabidopsis thaliana

Plant adaptation to abiotic stresses such as drought and salinity involves complex regulatory processes. Deciphering the signaling components that are involved in stress signal transduction and cellular responses is of importance to understand how plants cope with salt stress. Accumulation of osmolytes such as proline is considered to participate in the osmotic adjustment of plant cells to salinity. Proline accumulation results from a tight regulation between its biosynthesis and catabolism. Lipid signal components such as phospholipases C and D have previously been shown to be involved in the regulation of proline metabolism in Arabidopsis thaliana. In this study, we demonstrate that proline metabolism is also regulated by class-III Phosphatidylinositol 3-kinase (PI3K), VPS34, which catalyses the formation of phosphatidylinositol 3-phosphate (PI3P) from phosphatidylinositol. Using pharmacological and biochemical approaches, we show that the PI3K inhibitor, LY294002, affects PI3P levels in vivo and that it triggers a decrease in proline accumulation in response to salt treatment of A. thaliana seedlings. The lower proline accumulation is correlated with a lower transcript level of Pyrroline-5-carboxylate synthetase 1 (P5CS1) biosynthetic enzyme and higher transcript and protein levels of Proline dehydrogenase 1 (ProDH1), a key-enzyme in proline catabolism. We also found that the ProDH1 expression is induced in a pi3k-hemizygous mutant, further demonstrating that PI3K is involved in the regulation of proline catabolism through transcriptional regulation of ProDH1. A broader metabolomic analysis indicates that LY294002 also reduced other metabolites, such as hydrophobic and aromatic amino acids and sugars like raffinose.

Analysis of D3-,4-,5-phosphorylated phosphoinositides using HPLC

Detection of polyphosphoinositides (PPIs) is difficult due to their low chemical abundancy. This problem is further complicated by the fact that PPIs are present as various, distinct isomers, which are difficult, if not impossible, to separate by conventional thin layer chromatography (TLC) systems. PPIs in plants include PtdIns3P, PtdIns4P, PtdIns5P, PtdIns(3,5)P 2, and PtdIns(4,5)P 2. Here, a protocol is described analyzing plant PPIs using (32)P-orthophosphorus pre-labeled material. After extraction, lipids are deacylated and the resulting glycerophosphoinositol polyphosphates (GroPInsPs) separated by HPLC using a strong anion-exchange column and a shallow salt gradient. Alternatively, PPIs are first separated by TLC, the lipids reisolated, deacylated, and the GroPInsPs then separated by HPLC.

Phosphatidylinositol 3-kinase plays a vital role in regulation of rice seed vigor via altering NADPH oxidase activity

Phosphatidylinositol 3-kinase (PI3K) has been reported to be important in normal plant growth and stress responses. In this study, it was verified that PI3K played a vital role in rice seed germination through regulating NADPH oxidase activity. Suppression of PI3K activity by inhibitors wortmannin or LY294002 could abate the reactive oxygen species (ROS) formation, which resulted in disturbance to the seed germination. And then, the signal cascades that PI3K promoted the ROS liberation was also evaluated. Diphenylene iodonium (DPI), an NADPH oxidase inhibitor, suppressed most of ROS generation in rice seed germination, which suggested that NADPH oxidase was the main source of ROS in this process. Pharmacological experiment and RT-PCR demonstrated that PI3K promoted the expression of Os rboh9. Moreover, functional analysis by native PAGE and the measurement of the 2, 3-bis-(2-methoxy-4-nitro-5-sulfophenyl)-2H-tetrazo-lium-5- carboxanilide (XTT) formazan concentration both showed that PI3K promoted the activity of NADPH oxidase. Furthermore, the western blot analysis of OsRac-1 demonstrated that the translocation of Rac-1 from cytoplasm to plasma membrane, which was known as a key factor in the assembly of NADPH oxidase, was suppressed by treatment with PI3K inhibitors, resulting in the decreased activity of NADPH oxidase. Taken together, these data favored the novel conclusion that PI3K regulated NADPH oxidase activity through modulating the recruitment of Rac-1 to plasma membrane and accelerated the process of rice seed germination.

Arabidopsis type-III phosphatidylinositol 4-kinases β1 and β2 are upstream of the phospholipase C pathway triggered by cold exposure

Phosphatidylinositol-4-phosphate (PtdIns4P) is the most abundant phosphoinositide in plants and the precursor of phosphatidylinositol-4,5-bisphosphate [PtdIns(4,5)P(2)]. This lipid is the substrate of phosphoinositide-dependent phospholipase C (PI-PLC) that produces diacylglycerol (DAG) which can be phosphorylated to phosphatidic acid (PtdOH). In plants, it has been suggested that PtdIns4P may also be a direct substrate of PI-PLC. Whether PtdIns4P is the precursor of PtdIns(4,5)P(2) or a substrate of PI-PLC, its production by phosphatidylinositol-4-kinases (PI4Ks) is the first step in generating the phosphoinositides hydrolyzed by PI-PLC. PI4Ks can be divided into type-II and type-III. In plants, the identity of the PI4K upstream of PI-PLC is unknown. In Arabidopsis, cold triggers PI-PLC activation, resulting in PtdOH production which is paralleled by decreases in PtdIns4P and PtdIns(4,5)P(2). In suspension cells, both the PtdIns4P decrease and the PtdOH increase in response to cold were impaired by 30 μM wortmannin, a type-III PI4K inhibitor. Type-III PI4Ks include AtPI4KIIIα1, β1 and β2 isoforms. In this work we show that PtdOH resulting from the PI-PLC pathway is significantly lowered in a pi4kIIIβ1β2 double mutant exposed to cold stress. Such a decrease was not detected in single pi4kIIIβ1 and pi4kIIIβ2 mutants, indicating that AtPI4KIIIβ1 and AtPI4KIIIβ2 can both act upstream of the PI-PLC. Although several short-term to long-term responses to cold were unchanged in pi4kIIIβ1β2, cold induction of several genes was impaired in the double mutant and its germination was hypersensitive to chilling. We also provide evidence that de novo synthesis of PtdIns4P by PI4Ks occurs in parallel to PI-PLC activation.

Overexpression of Arabidopsis thaliana PTEN caused accumulation of autophagic bodies in pollen tubes by disrupting phosphatidylinositol 3-phosphate dynamics

Autophagy is a pathway in eukaryotes by which nutrient remobilization occurs through bulk protein and organelle turnover. Autophagy not only aides cells in coping with harsh environments but also plays a key role in many physiological processes that include pollen germination and tube growth. Most autophagic components are conserved among eukaryotes, but phylum-specific molecular components also exist. We show here that Arabidopsis thaliana PTEN, a protein and lipid dual phosphatase homologous to animal PTENs (phosphatase and tensin homologs deleted on chromosome 10), regulates autophagy in pollen tubes by disrupting the dynamics of phosphatidylinositol 3-phosphate (PI3P). The pollen-specific PTEN bound PI3P in vitro and was localized at PI3P-positive vesicles. Overexpression of PTEN caused accumulation of autophagic bodies and resulted in gametophytic male sterility. Such an overexpression effect was dependent upon its lipid phosphatase activity and was inhibited by exogenous PI3P or by expression of a class III phosphatidylinositol 3-kinase (PI3K) that produced PI3P. Overexpression of PTEN disrupted the dynamics of autophagosomes and a subpopulation of endosomes, as shown by altered localization patterns of respective fluorescent markers. Treatment with wortmannin, an inhibitor of class III PI3K, mimicked the effects by PTEN overexpression, which implied a critical role for PI3P dynamics in these processes. Despite sharing evolutionarily conserved catalytic domains, plant PTENs contain regulatory sequences that are distinct from those of animal PTENs, which might underlie their differing membrane association and thereby function. Our results show that PTEN regulates autophagy through phylum-specific molecular mechanisms.

The salt stress-induced LPA response in Chlamydomonas is produced via PLA₂ hydrolysis of DGK-generated phosphatidic acid

The unicellular green alga Chlamydomonas has frequently been used as a eukaryotic model system to study intracellular phospholipid signaling pathways in response to environmental stresses. Earlier, we found that hypersalinity induced a rapid increase in the putative lipid second messenger, phosphatidic acid (PA), which was suggested to be generated via activation of a phospholipase D (PLD) pathway and the combined action of a phospholipase C/diacylglycerol kinase (PLC/DGK) pathway. Lysophosphatidic acid (LPA) was also increased and was suggested to reflect a phospholipase A₂ (PLA₂) activity based on pharmacological evidence. The question of PA's and LPA's origin is, however, more complicated, especially as both function as precursors in the biosynthesis of phospho- and galactolipids. To address this complexity, a combination of fatty acid-molecular species analysis and in vivo ³²P-radiolabeling was performed. Evidence is provided that LPA is formed from a distinct pool of PA characterized by a high α-linolenic acid (18:3n-3) content. This molecular species was highly enriched in the polyphosphoinositide fraction, which is the substrate for PLC to form diacylglycerol. Together with differential ³²P-radiolabeling studies and earlier PLD-transphosphatidylation and PLA₂-inhibitor assays, the data were consistent with the hypothesis that the salt-induced LPA response is primarily generated through PLA₂-mediated hydrolysis of DGK-generated PA and that PLD or de novo synthesis [via endoplasmic reticulum - or plastid-localized routes] is not a major contributor.

Identification and structural characterization of FYVE domain-containing proteins of Arabidopsis thaliana

BACKGROUND

FYVE domains have emerged as membrane-targeting domains highly specific for phosphatidylinositol 3-phosphate (PtdIns(3)P). They are predominantly found in proteins involved in various trafficking pathways. Although FYVE domains may function as individual modules, dimers or in partnership with other proteins, structurally, all FYVE domains share a fold comprising two small characteristic double-stranded beta-sheets, and a C-terminal alpha-helix, which houses eight conserved Zn2+ ion-binding cysteines. To date, the structural, biochemical, and biophysical mechanisms for subcellular targeting of FYVE domains for proteins from various model organisms have been worked out but plant FYVE domains remain noticeably under-investigated.

RESULTS

We carried out an extensive examination of all Arabidopsis FYVE domains, including their identification, classification, molecular modeling and biophysical characterization using computational approaches. Our classification of fifteen Arabidopsis FYVE proteins at the outset reveals unique domain architectures for FYVE containing proteins, which are not paralleled in other organisms. Detailed sequence analysis and biophysical characterization of the structural models are used to predict membrane interaction mechanisms previously described for other FYVE domains and their subtle variations as well as novel mechanisms that seem to be specific to plants.

CONCLUSIONS

Our study contributes to the understanding of the molecular basis of FYVE-based membrane targeting in plants on a genomic scale. The results show that FYVE domain containing proteins in plants have evolved to incorporate significant differences from those in other organisms implying that they play a unique role in plant signaling pathways and/or play similar/parallel roles in signaling to other organisms but use different protein players/signaling mechanisms.

Reassessing the role of phospholipase D in the Arabidopsis wounding response

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) AtPLDalpha1 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 (32)P(i)-labelled phospholipid analyses of Arabidopsis pld knock-out mutants and PLD-silenced tomato cell-suspension cultures. pldalpha1 knock-out lines have reduced wounding-induced PA production, and the remainder is completely eliminated in a pldalpha1/delta 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.

Multiple PLDs required for high salinity and water deficit tolerance in plants

High salinity and drought have received much attention because they severely affect crop production worldwide. Analysis and comprehension of the plant's response to excessive salt and dehydration will aid in the development of stress-tolerant crop varieties. Signal transduction lies at the basis of the response to these stresses, and numerous signaling pathways have been implicated. Here, we provide further evidence for the involvement of phospholipase D (PLD) in the plant's response to high salinity and dehydration. A tomato (Lycopersicon esculentum) alpha-class PLD, LePLDalpha1, is transcriptionally up-regulated and activated in cell suspension cultures treated with salt. Gene silencing revealed that this PLD is indeed involved in the salt-induced phosphatidic acid production, but not exclusively. Genetically modified tomato plants with reduced LePLDalpha1 protein levels did not reveal altered salt tolerance. In Arabidopsis (Arabidopsis thaliana), both AtPLDalpha1 and AtPLDdelta were found to be activated in response to salt stress. Moreover, pldalpha1 and plddelta single and double knock-out mutants exhibited enhanced sensitivity to high salinity stress in a plate assay. Furthermore, we show that both PLDs are activated upon dehydration and the knock-out mutants are hypersensitive to hyperosmotic stress, displaying strongly reduced growth.

Imaging phosphatidylinositol 4-phosphate dynamics in living plant cells

Polyphosphoinositides represent a minor group of phospholipids, accounting for less than 1% of the total. Despite their low abundance, these molecules have been implicated in various signalling and membrane trafficking events. Phosphatidylinositol 4-phosphate (PtdIns4P) is the most abundant polyphosphoinositide. (32)Pi-labelling studies have shown that the turnover of PtdIns4P is rapid, but little is known about where in the cell or plant this occurs. Here, we describe the use of a lipid biosensor that monitors PtdIns4P dynamics in living plant cells. The biosensor consists of a fusion between a fluorescent protein and a lipid-binding domain that specifically binds PtdIns4P, i.e. the pleckstrin homology domain of the human protein phosphatidylinositol-4-phosphate adaptor protein-1 (FAPP1). YFP-PH(FAPP1) was expressed in four plant systems: transiently in cowpea protoplasts, and stably in tobacco BY-2 cells, Medicago truncatula roots and Arabidopsis thaliana seedlings. All systems allowed YFP-PH(FAPP1) expression without detrimental effects. Two distinct fluorescence patterns were observed: labelling of motile punctate structures and the plasma membrane. Co-expression studies with organelle markers revealed strong co-labelling with the Golgi marker STtmd-CFP, but not with the endocytic/pre-vacuolar marker GFP-AtRABF2b. Co-expression with the Ptdins3P biosensor YFP-2 x FYVE revealed totally different localization patterns. During cell division, YFP-PH(FAPP1) showed strong labelling of the cell plate, but PtdIns3P was completely absent from the newly formed cell membrane. In root hairs of M. truncatula and A. thaliana, a clear PtdIns4P gradient was apparent in the plasma membrane, with the highest concentration in the tip. This only occurred in growing root hairs, indicating a role for PtdIns4P in tip growth.

Phosphatidylinositol 4-phosphate accumulates extracellularly upon xylanase treatment in tomato cell suspensions

Various phosphoinositides have been implicated in plant defence signalling. Until now, such molecules have been exclusively related to intracellular signalling. Here, evidence is provided for the detection of extracellular phosphatidylinositol 4-phosphate (PI4P) in tomato cell suspensions. We have analysed and compared the intracellular and extracellular phospholipid profiles of [(32)P(i)]-prelabelled tomato cells, challenged with the fungal elicitor xylanase. These phospholipid patterns were found to be different, being phosphatidylinositol phosphate (PIP) the most abundant phospholipid in the extracellular medium. Moreover, while cells responded with a typical increase in phosphatidic acid and a decrease in intracellular PIP upon xylanase treatment, extracellular PIP level increased in a time- and dose-dependent manner. Using two experimental approaches, the extracellular PIP isoform was identified as PI4P. Addition of PI4P to tomato cell suspensions triggered the same defence responses as those induced by xylanase treatment. These include production of reactive oxygen species, accumulation of defence-related gene transcripts and induction of cell death. We demonstrate that extracellular PI4P is accumulated in xylanase-elicited cells and that exogenous application of PI4P mimics xylanase effects, suggesting its putative role as an intercellular signalling molecule.

Phosphatidylinositol kinases as regulators of GA-stimulated alpha-amylase secretion in barley (Hordeum vulgare)

Phosphorylated derivatives of phosphatidylinositol, in association with phosphatidylinositol 3-kinase (PI3 kinase, EC 2.7.1.137) and phosphatidylinositol 4-kinase (PI4 kinase, EC 2.7.1.67), play a key role in regulation of fundamental cell processes. We present evidence for a relationship between alpha-amylase (EC 3.2.1.1) secretion regulated by GA and levels of phosphatidylinositol 3-phosphate and phosphatidylinositol 4-phosphate (PtdIns(4)P) in barley (Hordeum vulgare). Microsomal membranes were incubated in the presence of [gamma-(32)P]ATP, and radiolabeled membrane lipids were extracted and separated by TLC using a boric acid system. Treatment of aleurone layers with GA for short or long periods of time increased PI4 kinase activity. To evaluate the effect of PtdIns(4)P levels on GA signaling, we used phenylarsine oxide (PAO), an inhibitor of PI4 kinase activity. PAO reversibly reduced the alpha-amylase secretion and protoplast cell vacuolation in a dose-dependent manner. Wortmannin showed a similar inhibitory effect on alpha-amylase secretion and PI4 kinase activity. GA evoked only a long-term increase in PI3 kinase activity, which was also affected by PAO. The effect of PAO was suppressed by the reducing agent 2,3-dimercapto-1-propanol (BAL), leading to restoration of secretion, vacuolation and PI4 kinase activity. In contrast, the effect of PAO on PI3 kinase activity was not abolished by BAL, suggesting that PI3 kinase is not involved in the secretion process. Likewise, the compound LY294002 inhibited PI3 kinase but had no effect on the secretion process. These findings indicate that PI4 kinase acts as a positive regulator of early GA signaling in aleurone.

Nitric oxide triggers phosphatidic acid accumulation via phospholipase D during auxin-induced adventitious root formation in cucumber

Auxin and nitric oxide (NO) play fundamental roles throughout plant life. NO is a second messenger in auxin signal transduction leading to root developmental processes. The mechanisms triggered by auxin and NO that direct adventitious root (AR) formation are beginning to be unraveled. The goal of this work was to study phospholipid (PL) signaling during the auxin- and NO-induced AR formation in cucumber (Cucumis sativus) explants. Explants were labeled with 32P-inorganic phosphate and treated with the auxins indole-3-acetic acid or 1-naphthylacetic acid, or the NO donor S-nitroso N-acetyl penicillamine, in the presence or absence of the specific NO scavenger 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide. PLs were separated by thin-layer chromatography and quantified. We report that the signaling PLs phosphatidic acid (PA), phosphatidylinositol phosphate, and phosphatidylinositol bisphosphate accumulated within 1 min after auxin or NO treatment. Both auxin and NO evoked similar and transient time course responses, since signaling PLs returned to control levels after 20 or 30 min of treatment. The results indicate that auxin relies on NO in inducing PA, phosphatidylinositol phosphate, and phosphatidylinositol bisphosphate accumulation. Furthermore, we demonstrate that auxin and NO trigger PA formation via phospholipase D (PLD) activity. Explants treated for 10 min with auxin or NO displayed a 200% increase in AR number compared with control explants. In addition, PLD activity was required for the auxin- and NO-induced AR formation. Finally, exogenously applied PA increased up to 300% the number of ARs. Altogether, our data support the idea that PLD-derived PA is an early signaling event during AR formation induced by auxin and NO in cucumber explants.

Visualization of phosphatidylinositol 4,5-bisphosphate in the plasma membrane of suspension-cultured tobacco BY-2 cells and whole Arabidopsis seedlings

Phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P(2)] is an important signalling lipid in mammalian cells, where it functions as a second-messenger precursor in response to agonist-dependent activation of phospholipase C (PLC) but also operates as a signalling molecule on its own. Much of the recent knowledge about it has come from a new technique to visualize PtdIns(4,5)P(2)in vivo, by expressing a green or yellow fluorescent protein (GFP or YFP) fused to the pleckstrin homology (PH) domain of human PLCdelta1 that specifically binds PtdIns(4,5)P(2). In this way, YFP-PH(PLCdelta1) has been shown to predominantly label the plasma membrane and to transiently translocate into the cytoplasm upon PLC activation in a variety of mammalian cell systems. In plants, biochemical studies have shown that PtdIns(4,5)P(2) is present in very small quantities, but knowledge of its localization and function is still very limited. In this study, we have used YFP-PH(PLCdelta1) to try monitoring PtdIns(4,5)P(2)/PLC signalling in stably-transformed tobacco Bright Yellow-2 (BY-2) cells and Arabidopsis seedlings. In both plant systems, no detrimental effects were observed, indicating that overexpression of the biosensor did not interfere with the function of PtdIns(4,5)P(2). Confocal imaging revealed that most of the YFP-PH(PLCdelta1) fluorescence was present in the cytoplasm, and not in the plasma membrane as in mammalian cells. Nonetheless, four conditions were found in which YFP-PH(PLCdelta1) was concentrated at the plasma membrane: (i) upon treatment with the PLC inhibitor U73122; (ii) in response to salt stress; (iii) as a gradient at the tip of growing root hairs; (iv) during the final stage of a BY-2 cell division. We conclude that PtdIns(4,5)P(2), as in animals, is present in the plasma membrane of plants, but that its concentration in most cells is too low to be detected by YFP-PH(PLCdelta1). Hence, the reporter remains unbound in the cytosol, making it unsuitable to monitor PLC signalling. Nonetheless, YFP-PH(PLCdelta1) is a valuable plant PtdIns(4,5)P(2) reporter, for it highlights specific cells and conditions where this lipid becomes abnormally concentrated in membranes, raising the question of what it is doing there. New roles for PtdIns(4,5)P(2) in plant cell signalling are discussed.

Visualization of PtdIns3P dynamics in living plant cells

To investigate PtdIns3P localization and function in plants, a fluorescent PtdIns3P-specific biosensor (YFP-2xFYVE) was created. On lipid dot blots it bound specifically and with high affinity to PtdIns3P. Transient expression in cowpea protoplasts labelled vacuolar membranes and highly motile structures undergoing fusion and fission. Stable expression in tobacco BY-2 cells labelled similar motile structures, but labelled vacuolar membranes hardly at all. YFP-2xFYVE fluorescence strongly co-localized with the pre-vacuolar marker AtRABF2b, partially co-localized with the endosomal tracer FM4-64, but showed no overlap with the Golgi marker STtmd-CFP. Treatment of cells with wortmannin, a PI3 kinase inhibitor, caused the YFP-2xFYVE fluorescence to redistribute into the cytosol and nucleus within 15 min. BY-2 cells expressing YFP-2xFYVE contained twice as much PtdIns3P as YFP-transformed cells, but this had no effect on cell-growth or stress-induced phospholipid signalling responses. Upon treatment with wortmannin, PtdIns3P levels were reduced by approximately 40% within 15 min in both cell lines. Stable expression of YFP-2xFYVE in Arabidopsis plants labelled different subcellular structures in root compared with shoot tissues. In addition labelling the motile structures common to all cells, YFP-2xFYVE strongly labelled the vacuolar membrane in leaf epidermal and guard cells, suggesting that cell differentiation alters the distribution of PtdIns3P. In dividing BY-2 cells, YFP-2xFYVE-labelled vesicles surrounded the newly formed cell plate, suggesting a role for PtdIns3P in cytokinesis. Together, these data show that YFP-2xFYVE may be used as a biosensor to specifically visualize PtdIns3P in living plant cells.

Auxin-induced reactive oxygen species production requires the activation of phosphatidylinositol 3-kinase

We recently reported that production of reactive oxygen species (ROS) is essential for auxin-induced gravitropic signaling. Here, we investigated the role of phosphatidylinositol 3-kinase and its product, PtdIns(3)P, in auxin-mediated ROS production and the root gravitropic response. Pretreatment with LY294002, an inhibitor of PtdIns 3-kinase activity, blocked auxin-mediated ROS generation, and reduced the sensitivity of root tissue to gravistimulation. The amount of PtdIns(3)P increased in response to auxin, and this effect was abolished by pretreatment with LY294002. In addition, sequestration of PtdIns(3)P by transient expression of the endosome binding domain in protoplasts abrogated IAA-induced ROS accumulation. These results indicate that activation of PtdIns 3-kinase and its product PtdIns(3)P are required for auxin-induced production of ROS and root gravitropism.

The xipotl mutant of Arabidopsis reveals a critical role for phospholipid metabolism in root system development and epidermal cell integrity

Phosphocholine (PCho) is an essential metabolite for plant development because it is the precursor for the biosynthesis of phosphatidylcholine, which is the major lipid component in plant cell membranes. The main step in PCho biosynthesis in Arabidopsis thaliana is the triple, sequential N-methylation of phosphoethanolamine, catalyzed by S-adenosyl-l-methionine:phosphoethanolamine N-methyltransferase (PEAMT). In screenings performed to isolate Arabidopsis mutants with altered root system architecture, a T-DNA mutagenized line showing remarkable alterations in root development was isolated. At the seedling stage, the mutant phenotype is characterized by a short primary root, a high number of lateral roots, and short epidermal cells with aberrant morphology. Genetic and biochemical characterization of this mutant showed that the T-DNA was inserted at the At3g18000 locus (XIPOTL1), which encodes PEAMT (XIPOTL1). Further analyses revealed that inhibition of PCho biosynthesis in xpl1 mutants not only alters several root developmental traits but also induces cell death in root epidermal cells. Epidermal cell death could be reversed by phosphatidic acid treatment. Taken together, our results suggest that molecules produced downstream of the PCho biosynthesis pathway play key roles in root development and act as signals for cell integrity.

Learning the lipid language of plant signalling

Plant cells respond to different biotic and abiotic stresses by producing various uncommon phospholipids that are believed to play key roles in cell signalling. We can predict how they work because animal and yeast proteins have been shown to have specific lipid-binding domains, which act as docking sites. When such proteins are recruited to the membrane locations where these phospholipids are synthesized, the phospholipids activate them directly, by inducing a conformational change, or indirectly, by juxtaposing them with an activator protein. The same lipid-binding domains are present in Arabidopsis proteins. We believe that they represent an untapped well of information about plant lipid signalling.

Activation of phospholipase D by osmotic cell swelling

In response to osmotic cell swelling, Intestine 407 cells react with a rapid and transient activation of phospholipase D (PLD). To investigate the role of PLD during the regulatory volume decrease, cells were treated with 1-butanol resulting in a depletion of PLD substrates. Activation of volume-regulated anion channels, but not the cell swelling-induced release of taurine, was largely inhibited in the presence of low concentrations of 1-butanol. In addition, hypotonicity-induced exocytosis, ATP release and subsequent endocytosis were found to be largely abrogated. The results support a model of cell volume regulation in which PLD plays an essential role in the cell swelling-induced vesicle cycling and in the activation of volume-sensitive anion channels.

The role of the actin cytoskeleton in plant cell signaling

The plant actin cytoskeleton provides a dynamic cellular component which is involved in the maintenance of cell shape and structure. It has been demonstrated recently that the actin cytoskeleton and its associated elements provide a key target in many signaling events. In addition to acting as a target, the actin cytoskeleton can also act as a transducer of signal information. In this review we describe some newly discovered aspects of the roles of the actin cytoskeleton in plant cell signaling. In addition to a summary of the roles played by actin-binding proteins, we also briefly review the progress made in understanding how the actin cytoskeleton participates in the self-incompatibility response in pollen tubes. Finally, the emerging importance of the actin cytoskeleton in the perception and responses to stimuli such as gravity, touch and cold stress exposure are discussed. Contents I. Introduction - the actin cytoskeleton 13 II. Actin-binding proteins 14 III. The actin cytoskeleton as a target and mediator of plant cell signaling 20 IV. Summary and conclusion 25 References 25 Acknowledgements 25.

Osmotically induced cell swelling versus cell shrinking elicits specific changes in phospholipid signals in tobacco pollen tubes

Pollen tube cell volume changes rapidly in response to perturbation of the extracellular osmotic potential. This report shows that specific phospholipid signals are differentially stimulated or attenuated during osmotic perturbations. Hypo-osmotic stress induces rapid increases in phosphatidic acid (PA). This response occurs starting at the addition of 25% (v/v) water to the pollen tube cultures and peaks at 100% (v/v) water. Increased levels of PA were detected within 30 s and reached maximum by 15 to 30 min after treatment. The pollen tube apical region undergoes a 46% increase in cell volume after addition of 100% water (v/v), and there is an average 7-fold increase in PA. This PA increase appears to be generated by phospholipase D because concurrent transphosphatidylation of n-butanol results in an average 8-fold increase in phosphatidylbutanol. Hypo-osmotic stress also induces an average 2-fold decrease in phosphatidylinositol phosphate; however, there are no detectable changes in the levels of phosphatidylinositol bisphosphates. In contrast, salt-induced hyperosmotic stress from 50 to 400 mm NaCl inhibits phospholipase D activity, reduces the levels of PA, and induces increases in the levels of phosphatidylinositol bisphosphate isomers. The pollen tube apical region undergoes a 41% decrease in cell volume at 400 mm NaCl, and there is an average 2-fold increase in phosphatidylinositol 3,5-bisphosphate and 1.4-fold increase in phosphatidylinositol 4,5-bisphosphate. The phosphatidylinositol 3,5-bisphosphate increase is detected within 30 s and reaches maximum by 15 to 30 min after treatment. In summary, these results demonstrate that hypo-osmotic versus hyperosmotic perturbation and the resultant cell swelling or shrinking differentially activate specific phospholipid signaling pathways in tobacco (Nicotiana tabacum) pollen tubes.

Phospholipid-based signaling in plants

Phospholipids are emerging as novel second messengers in plant cells. They are rapidly formed in response to a variety of stimuli via the activation of lipid kinases or phospholipases. These lipid signals can activate enzymes or recruit proteins to membranes via distinct lipid-binding domains, where the local increase in concentration promotes interactions and downstream signaling. Here, the latest developments in phospholipid-based signaling are discussed, including the lipid kinases and phospholipases that are activated, the signals they produce, the domains that bind them, the downstream targets that contain them and the processes they control.

Activation of phospholipases C and D is an early response to a cold exposure in Arabidopsis suspension cells

The signaling events generated by a cold exposure are poorly known in plants. We were interested in checking the possible activation of enzymes of the phosphoinositide signaling pathway in response to a temperature drop. In Arabidopsis suspension cells labeled with (33)PO(4)(3-), a cold treatment induces a rapid increase of phosphatidic acid (PtdOH) content. This production was due to the simultaneous activation of phospholipase C (through diacylglycerol kinase activity) and phospholipase D, as monitored by the production of inositol triphosphate and of transphosphatidylation product, respectively. Moreover, inhibitors of the phosphoinositide pathway and of diacylglycerol kinase reduced PtdOH production. Enzyme activation occurred immediately after cells were transferred to low temperature. The respective contribution of both kind of phospholipases in cold-induced production of PtdOH could be estimated. We created conditions where phospholipids were labeled with (33)PO(4)(3-), but with ATP being nonradioactive. In such conditions, the apparition of radioactive PtdOH reflected PLD activity. Thus, we demonstrated that during a cold stress, phospholipase D activity accounted for 20% of PtdOH production. The analysis of composition in fatty acids of cold-produced PtdOH compared with that of different phospholipids confirmed that cold-induced PtdOH more likely derived mainly from phosphoinositides. The addition of chemical reagents modifying calcium availability inhibited the formation of PtdOH, showing that the cold-induced activation of phospholipase pathways is dependent on a calcium entry.

KCl activates phospholipase D at two different concentration ranges: distinguishing between hyperosmotic stress and membrane depolarization

Hyperosmotic stress induces the rapid formation of phosphatidic acid (PA) in Chlamydomonas moewusii via the activation of two signalling pathways: phospholipase D (PLD) and phospholipase C (PLC), the latter in combination with diacylglycerol kinase (DGK) (Munnik et al., 2000). A concomitant increase in cell Ca(2+) becomes manifest as deflagellation. When KCl was used as osmoticum we found that two concentration ranges activated deflagellation: one between 50 and 100 mm and another above 200 mm. Deflagellation in low KCl concentrations was complete within 30 sec whereas in high concentrations it took 5 min. PLC was not activated, as it was by high KCl concentrations that cause hyperosmotic stress. Moreover PLD was activated more strongly by low than by high KCl concentrations. Potassium was the most potent monovalent cation based on the induction of deflagellation and the formation of PA and PBut. During treatment, the external medium acidified, indicating an increase in H(+)-ATPase activity in order to re-establish the membrane potential. Activation of PLD and deflagellation at low KCl concentrations were abrogated by treatment with La(3+), Gd(3+) and EGTA, indicating the dependency on extracellular Ca(2+). This suggests that low concentrations of KCl depolarize the plasma membrane, resulting in the activation of H(+)-ATPases and opening voltage-dependent Ca(2+) +/- channels, observed as deflagellation and an increase in PLD activity.

Identification of a new polyphosphoinositide in plants, phosphatidylinositol 5-monophosphate (PtdIns5P), and its accumulation upon osmotic stress

Polyphosphoinositides play an important role in membrane trafficking and cell signalling. In plants, two PtdInsP isomers have been described, PtdIns3P and PtdIns4P. Here we report the identification of a third, PtdIns5P. Evidence is based on the conversion of the endogenous PtdInsP pool into PtdIns(4,5)P(2) by a specific PtdIns5P 4-OH kinase, and on in vivo (32)P-labelling studies coupled to HPLC head-group analysis. In Chlamydomonas, 3-8% of the PtdInsP pool was PtdIns5P, 10-15% was PtdIns3P and the rest was PtdIns4P. In seedlings of Vicia faba and suspension-cultured tomato cells, the level of PtdIns5P was about 18%, indicating that PtdIns5P is a general plant lipid that represents a significant proportion of the PtdInsP pool. Activating phospholipase C (PLC) signalling in Chlamydomonas cells with mastoparan increased the turnover of PtdIns(4,5)P(2) at the cost of PtdIns4P, but did not affect the level of PtdIns5P. This indicates that PtdIns(4,5)P(2) is synthesized from PtdIns4P rather than from PtdIns5P during PLC signalling. However, when cells were subjected to hyperosmotic stress, PtdIns5P levels rapidly increased, suggesting a role in osmotic-stress signalling. The potential pathways of PtdIns5P formation are discussed.

Hyperosmotic stress rapidly generates lyso-phosphatidic acid in Chlamydomonas

Plant cells are continuously exposed to environmental stresses such as hyper-osmolarity, and have to respond in order to survive. When 32P-labelled Chlamydomonas moewusii cells were challenged with NaCl, the formation of a new radiolabelled phospholipid was stimulated, which was barely detectable before stimulation. The phospholipid was identified as lyso-phosphatidic acid (LPA), and was the only lyso-phospholipid to be accumulated. The increase in LPA was dose- and time-dependent. When other osmotically active compounds were used, the formation of LPA was also induced with similar kinetics, although salts were better inducers than non-salts. At least part of the LPA was generated by phospholipase A2 (PLA2) hydrolysing phosphatidic acid (PA). This claim is based on PA formation preceding LPA production, and PLA2 inhibitors decreasing the accumulation of LPA and promoting the conversion of PA to diacylglycerol pyrophosphate. The latter is another metabolic derivative of PA that is implicated in cell signalling. The involvement of multiple lipid-signalling pathways in hyperosmotic stress responses is discussed.

Polar glycerolipids of Chlamydomonas moewusii

The fatty acid and polar lipid compositions of the unicellular green alga Chlamydomonas moewusii were characterized. Since this organism is an important plant model for phospholipid-based signal transduction, interest was focused on the lipids phosphatidic acid, phosphatidylinositolphosphate and phosphatidylinositolbisphosphate. A phosphatidylinositol:phosphatidylinositolphosphate: phosphatidylinositolbisphosphate ratio of 100:1.7:1.3 was found. The polyphosphoinositides accounted for 0.8 mol% of the total phospholipids and their fatty acid compositions were similar to that of phosphatidylinositol except for the enrichment of linolenic acid in phosphatidylinositol phosphate. Phosphatidic acid accounted for 0.67 mol% of the phospholipids. Major structural glycerolipids were monogalactosyldiacylglycerol (35 mol%), digalactosyldiacylglycerol (15 mol%), sulfoquinovosyldiacylglycerol (10 mol%), diacylglyceryltrimethylhomoserine (16 mol%), phosphatidylglycerol (9 mol%), phosphatidylethanolamine (8 mol%) and phosphatidylinositol (6 mol%). Relative changes in the total fatty acid compositions found during growth on nutrient-limited medium reflected mainly alterations in the compositions of the chloroplast lipids phosphatidylglycerol and monogalactosyldiacylglycerol. [32P]Pi-incorporation studies revealed that it took 6 days before the amount of label in the major phospholipids was proportional to their abundance.

Salinity and hyperosmotic stress induce rapid increases in phosphatidylinositol 4,5-bisphosphate, diacylglycerol pyrophosphate, and phosphatidylcholine in Arabidopsis thaliana cells

In animal cells, phosphoinositides are key components of the inositol 1,4,5-trisphosphate/diacylglycerol-based signaling pathway, but also have many other cellular functions. These lipids are also believed to fulfill similar functions in plant cells, although many details concerning the components of a plant phosphoinositide system, and their regulation are still missing. Only recently have the different phosphoinositide isomers been unambiguously identified in plant cells. Another problem that hinders the study of the function of phosphoinositides and their derivatives, as well as the regulation of their metabolism, in plant cells is the need for a homogenous, easily obtainable material, from which the extraction and purification of phospholipids is relatively easy and quantitatively reproducible. We present here a thorough characterization of the phospholipids purified from [(32)P]orthophosphate- and myo-[2-(3)H]inositol-radiolabeled Arabidopsis thaliana suspension-cultured cells. We then show that NaCl treatment induces dramatic increases in the levels of phosphatidylinositol 4,5-bisphosphate and diacylglycerol pyrophosphate and also affects the turnover of phosphatidylcholine. The increase in phosphatidylinositol 4,5-bisphosphate was also observed with a non-ionic hyperosmotic shock. In contrast, the increase in diacylglycerol pyrophosphate and the turnover of phosphatidylcholine were relatively specific to salt treatments as only minor changes in the metabolism of these two phospholipids were detected when the cells were treated with sorbitol instead of NaCl.

Phosphoinositide kinases and the synthesis of polyphosphoinositides in higher plant cells

Phosphoinositides are a family of inositol-containing phospholipids which are present in all eukaryotic cells. Although in most cells these lipids, with the exception of phosphatidylinositol, constitute only a very minor proportion of total cellular lipids, they have received immense attention by researchers in the past 15-20 years. This is due to the discovery that these lipids, rather than just having structural functions, play key roles in a wide range of important cellular processes. Much less is known about the plant phosphoinositides than about their mammalian counterparts. However, it has been established that a functional phosphoinositide system exists in plant cells and it is becoming increasingly clear that inositol-containing lipids are likely to play many important roles throughout the life of a plant. It is not our intention to give an exhaustive overview of all aspects of the field, but rather we focus on the phosphoinositide kinases responsible for the synthesis of all phosphorylated forms of phosphatidylinositol. Also, we mention some of the aspects of current phosphoinositide research which, in our opinion, are most likely to provide a suitable starting point for further research into the role of phosphoinositides in plants.

Inositide signalling in Chlamydomonas: characterization of a phosphatidylinositol 3-kinase gene

Phosphoinositide (PI) 3-kinases, which phosphorylate the D-3 position of the inositol ring, function in several different signalling pathways. The phosphatidylinositol (PtdIns)-specific PI 3-kinase of yeast (Vps34p) is part of a receptor signalling protein complex associated with the trans-Golgi membranes, whereas PI 3-kinases that phosphorylate polyphosphoinositides in animal cells form a major receptor-controlled signalling pathway in the plasma membrane. Recent studies have indicated the presence of active PLC, PLD, and PI 3-kinase-dependent signalling systems in the unicellular green alga Chlamydomonas, and PtdIns-3P in Chlamydomonas shows a particularly high rate of turnover. Here we report the cloning of the Chlamydomonas Vps34p, and some characterisation of its properties, regulation and localisation. A single-copy 12 kb gene was present. The corresponding protein of 122 kDa had full-length homology with Vps34ps from other species, but it contained a novel spacer-like insert region of 148 amino acid residues between homology region 2 (HR2) and the C-terminal catalytic core domain, and three other shorter putative inserts. Available cDNAs were used to assemble a pBluescript clone expressing a recombinant protein which had PtdIns-specific 3-kinase activity. However, an unexpected observation was that recombinant proteins containing the complete catalytic core, but lacking HR2, had no lipid kinase activity, pointing to a previously unsuspected role for this domain, possibly in substrate binding. VPS34 mRNA and protein levels, as determined by RNAse protection assays and by immunological methods respectively, were low in all cell stages that were examined. Western blotting of subcellular fractions revealed that most of Vps34p in cell lysates of cw-15 (a cell wall-deficient mutant) could be recovered in a NP-40-resistant 100000 x g pellet, suggesting that the enzyme may have a location different from that found in higher plants.

Metabolic evidence for PtdIns(4,5)P2-directed phospholipase C in permeabilized plant protoplasts

Comparison of the sequences of the genes encoding phospholipase C (PLC) which have been cloned to date in plants with their mammalian counterparts suggests that plant PLC is similar to PLCdelta of mammalian cells. The physiological role and mechanism of activation of PLCdelta is unclear. It has recently been shown that Ins(1,4,5)P3 may not solely be the product of PtdIns(4,5)P2-directed PLC activity. Enzyme activities capable of producing Ins(1,4,5)P3 from endogenous inositol phosphates are present in Dictyostelium and also in rat liver. Significantly it has not been directly determined whether Ins(1,4,5)P3 present in higher plants is the product of a PtdIns(4, 5)P2-directed PLC activity. Therefore we have developed an experimental strategy for the identification of d-Ins(1,4,5)P3 in higher plants. By the use of a short-term non-equilibrium labelling strategy in permeabilized plant protoplasts, coupled to the use of a 'metabolic trap' to prevent degradation of [32P]Ins(1,4,5)P3, we were able to determine the distribution of 32P in individual phosphate esters of Ins(1,4,5)P3. The [32]Ins(1,4,5)P3 identified showed the same distribution of label in individual phosphate esters as that of [32P]PtdIns(4,5)P2 isolated from the same tissue. We thus provide in vivo evidence for the action of a PtdIns(4,5)P2-directed PLC activity in plant cells which is responsible for the production of Ins(1,4,5)P3 observed here. This observation does not, however, exclude the possibility that in other cells or under different conditions Ins(1,4,5)P3 can be generated by alternative routes.

Identification of diacylglycerol pyrophosphate as a novel metabolic product of phosphatidic acid during G-protein activation in plants

We provide evidence that phosphatidic acid (PtdOH) formed during signaling in plants is metabolized by a novel pathway. In much of this study, 32Pi-labeled Chlamydomonas cells were used, and signaling was activated by adding the G-protein activator mastoparan. Within seconds of activation, large amounts of [32P]PtdOH were formed, with peak production at about 4 min, when the level was 5-25-fold higher than the control. As the level of [32P]PtdOH subsequently decreased, an unknown phospholipid (PLX) increased in radiolabeling; before activation it was barely detectable. The chromatographic properties of PLX resembled those of lyso-PtdOH and CMP.PtdOH but on close inspection were found to be different. PLX was shown to be diacylglycerol pyrophosphate (DGPP), the product of a newly discovered enzyme, phosphatidate kinase, whose in vitro activity was described recently (Wissing, J. B., and Behrbohm, H. (1993) Plant Physiol. 102, 1243-1249). The identity of DGPP was established by co-chromatrography with a standard and by degradation analysis as follows: [32P]DGPP was deacylated, and the product (glycerolpyrophosphate, GroPP) was hydrolyzed by mild acid treatment or pyrophosphatase to produce GroP and Pi as the only radioactive products. Since DGPP is the pyrophosphate derivative of PtdOH and is formed as the concentration of PtdOH decreases, we assumed that PtdOH was converted in vivo to DGPP. This was confirmed by showing that during a short labeling protocol while the specific radioactivity of DGPP was increasing, the specific radioactivity of the 32Pi derived from DGPP as above was higher than that of [32P]GroP. DGPP was also formed in suspension cultures of tomato and potato cells, and its synthesis was activated by mastoparan. Moreover, it was also found in intact tissues of a number of higher plants, for example, carnation flower petals, vetch roots, leaves of fig-leaved goosefoot, and common persicaria and microspores of rape seed. Our results suggest that DGPP is a common but minor plant lipid that increases in concentration when signaling is activated. Possible functions of DGPP in phospholpase C and D signaling cascades are discussed.

G Protein Activation Stimulates Phospholipase D Signaling in Plants

We provide direct evidence for phospholipase D (PLD) signaling in plants by showing that this enzyme is stimulated by the G protein activators mastoparan, ethanol, and cholera toxin. An in vivo assay for PLD activity in plant cells was developed based on the use of a "reporter alcohol" rather than water as a transphosphatidylation substrate. The product was a phosphatidyl alcohol, which, in contrast to the normal product phosphatidic acid, is a specific measure of PLD activity. When 32P-labeled cells were treated with 0.1% n-butanol, 32P-phosphatidyl butanol (32P-PtdBut) was formed in a time-dependent manner. In cells treated with any of the three G protein activators, the production of 32P-PtdBut was increased in a dose-dependent manner. The G protein involved was pertussis toxin insensitive. Ethanol could activate PLD but was itself consumed by PLD as transphosphatidylation substrate. In contrast, secondary alcohols (e.g., sec-butyl alcohol) activated PLD but did not function as substrate, whereas tertiary alcohols did neither. Although most of the experiments were performed with the green alga Chlamydomonas eugametos, the relevance for higher plants was demonstrated by showing that PLD in carnation petals could also be activated by mastoparan. The results indicate that PLD activation must be considered as a potential signal transduction mechanism in plants, just as in animals.

G Protein Activation Stimulates Phospholipase D Signaling in Plants

We provide direct evidence for phospholipase D (PLD) signaling in plants by showing that this enzyme is stimulated by the G protein activators mastoparan, ethanol, and cholera toxin. An in vivo assay for PLD activity in plant cells was developed based on the use of a "reporter alcohol" rather than water as a transphosphatidylation substrate. The product was a phosphatidyl alcohol, which, in contrast to the normal product phosphatidic acid, is a specific measure of PLD activity. When 32P-labeled cells were treated with 0.1% n-butanol, 32P-phosphatidyl butanol (32P-PtdBut) was formed in a time-dependent manner. In cells treated with any of the three G protein activators, the production of 32P-PtdBut was increased in a dose-dependent manner. The G protein involved was pertussis toxin insensitive. Ethanol could activate PLD but was itself consumed by PLD as transphosphatidylation substrate. In contrast, secondary alcohols (e.g., sec-butyl alcohol) activated PLD but did not function as substrate, whereas tertiary alcohols did neither. Although most of the experiments were performed with the green alga Chlamydomonas eugametos, the relevance for higher plants was demonstrated by showing that PLD in carnation petals could also be activated by mastoparan. The results indicate that PLD activation must be considered as a potential signal transduction mechanism in plants, just as in animals.