Inositol phospholipid metabolism in Arabidopsis. Characterized and putative isoforms of inositol phospholipid kinase and phosphoinositide-specific phospholipase C

The cellular language of myo-inositol signaling

The simple polyol, myo-inositol, is used as a building block of a cellular language that plays various roles in signal transduction. This review describes the terminology used to denote myo-inositol-containing molecules, with an emphasis on how phosphate and fatty acids are added to create second messengers used in signaling. Work in model systems has delineated the genes and enzymes required for synthesis and metabolism of many myo-inositol-containing molecules, with genetic mutants and measurement of second messengers playing key roles in developing our understanding. There is increasing evidence that molecules such as myo- inositol(1,4,5)trisphosphate and phosphatidylinositol(4,5)bisphosphate are synthesized in response to various signals plants encounter. In particular, the controversial role of myo-inositol(1,4,5)trisphosphate is addressed, accompanied by a discussion of the multiple enzymes that act to regulate this molecule. We are also beginning to understand new connections of myo-inositol signaling in plants. These recent discoveries include the novel roles of inositol phosphates in binding to plant hormone receptors and that of phosphatidylinositol(3)phosphate binding to pathogen effectors.

Myo-inositol and beyond--emerging networks under stress

Myo-inositol is a versatile compound that generates diversified derivatives upon phosphorylation by lipid-dependent and -independent pathways. Phosphatidylinositols form one such group of myo-inositol derivatives that act both as membrane structural lipid molecules and as signals. The significance of these compounds lies in their dual functions as signals as well as key metabolites under stress. Several stress- and non-stress related pathways regulated by phosphatidylinositol isoforms and associated enzymes, kinases and phosphatases, appear to function in parallel to coordinatively adapt growth and stress responses in plants. Recent evidence also postulates their crucial roles in nuclear functions as they interact with the key players of chromatin structure, yet other nuclear functions remain largely unknown. Phosphatidylinositol monophosphate 5-kinase interacts with and represses a cytosolic neutral invertase, a key enzyme of sugar metabolism suggesting a crosstalk between lipid and sugar signaling. Besides phosphatidylinositol, myo-inositol derived galactinol and associated raffinose-family oligosaccharides are emerging as antioxidants and putative signaling compounds too. Importantly, myo-inositol polyphosphate 5-phosphatase (5PTase) acts, depending on sugar status, as a positive or negative regulator of a global energy sensor, SnRK1. This implies that both myo-inositol- and sugar-derived (e.g. trehalose 6-phosphate) molecules form part of a broad regulatory network with SnRK1 as the central regulator. Recently, it was shown that the transcription factor bZIP11 also takes part in this network. Moreover, a functional coordination between neutral invertase and hexokinase is emerging as a sweet network that contributes to oxidative stress homeostasis in plants. In this review, we focus on myo-inositol, its direct and more downstream derivatives (galactinol, raffinose), and the contribution of their associated networks to plant stress tolerance.

Mechanisms of xylanase-induced nitric oxide and phosphatidic acid production in tomato cells

The second messenger nitric oxide (NO), phosphatidic acid (PA) and reactive oxygen species (ROS) are involved in the plant defense response during plant-pathogen interactions. NO has been shown to participate in PA production in response to the pathogen-associated molecular pattern xylanase in tomato cell suspensions. Defense responses downstream of PA include ROS production. The goal of this work was to study the signaling mechanisms involved in PA production during the defense responses triggered by xylanase and mediated by NO in the suspension-cultured tomato cells. We analyzed the participation of protein kinases, guanylate cyclase and the NO-mediated posttranslational modification S-nitrosylation, by means of pharmacology and biochemistry. We showed that NO, PA and ROS levels are significantly diminished by treatment with the general protein kinase inhibitor staurosporine. This indicates that xylanase-induced protein phosphorylation events might be the important components leading to NO formation, and hence for the downstream regulation of PA and ROS levels. When assayed, a guanylate cyclase inhibitor or a cGMP analog did not alter the PA accumulation. These results suggest that a cGMP-mediated pathway is not involved in xylanase-induced PA formation. Finally, the inhibition of protein S-nitrosylation did not affect NO formation but compromised PA and ROS production. Data collectively indicate that upon xylanase perception, cells activate a protein kinase pathway required for NO formation and that, S-nitrosylation-dependent mechanisms are involved in downstream signaling leading to PA and ROS.

Arabidopsis AtVPS15 is essential for pollen development and germination through modulating phosphatidylinositol 3-phosphate formation

Arabidopsis thaliana phosphatidylinositol 3-kinase (AtVPS34) functions in the development and germination of pollen by catalyzing the biosynthesis of phosphatidylinositol 3-phosphate (PI3P). In yeast, Vps15p is required for the membrane targeting and activity of Vps34. The expression of Arabidopsis thaliana VPS15 (AtVPS15), an ortholog of yeast Vps15, is mainly detected in pollen grains and pollen tubes. To determine its role in pollen development and pollen tube growth, we attempted to isolate the T-DNA insertion mutants of AtVPS15; however, homozygous lines of atvps15 were not obtained from the progeny of atvps15/+ heterozygotes. Genetic analysis revealed that the abnormal segregation is due to the failure of transmission of the atvps15 allele through pollen. Most pollen grains from the atvps15/+ genotype are viable, with normal exine structure and nuclei, but some mature pollen grains are characterized with unusual large vacuoles that are not observed in pollen grains from the wild AtVPS15 genotype. The germination ratio of pollen from the atvps15/+ genotype is about half when compared to that from the wild AtVPS15 genotype. When supplied with PI3P, in vitro pollen germination of the atvps15/+ genotype is greatly improved. Presumably, AtVPS15 functions in pollen development and germination by regulating PI3P biosynthesis in Arabidopsis.

Arabidopsis phosphatidylinositol monophosphate 5-kinase 2 is involved in root gravitropism through regulation of polar auxin transport by affecting the cycling of PIN proteins

Phosphatidylinositol monophosphate 5-kinase (PIP5K) catalyzes the synthesis of PI-4,5-bisphosphate (PtdIns(4,5)P(2)) by phosphorylation of PI-4-phosphate at the 5 position of the inositol ring, and is involved in regulating multiple developmental processes and stress responses. We here report on the functional characterization of Arabidopsis PIP5K2, which is expressed during lateral root initiation and elongation, and whose expression is enhanced by exogenous auxin. The knockout mutant pip5k2 shows reduced lateral root formation, which could be recovered with exogenous auxin, and interestingly, delayed root gravity response that could not be recovered with exogenous auxin. Crossing with the DR5-GUS marker line and measurement of free IAA content confirmed the reduced auxin accumulation in pip5k2. In addition, analysis using the membrane-selective dye FM4-64 revealed the decelerated vesicle trafficking caused by PtdIns(4,5)P(2) reduction, which hence results in suppressed cycling of PIN proteins (PIN2 and 3), and delayed redistribution of PIN2 and auxin under gravistimulation in pip5k2 roots. On the contrary, PtdIns(4,5)P(2) significantly enhanced the vesicle trafficking and cycling of PIN proteins. These results demonstrate that PIP5K2 is involved in regulating lateral root formation and root gravity response, and reveal a critical role of PIP5K2/PtdIns(4,5)P(2) in root development through regulation of PIN proteins, providing direct evidence of crosstalk between the phosphatidylinositol signaling pathway and auxin response, and new insights into the control of polar auxin transport.

PIPKs are essential for rhizoid elongation and caulonemal cell development in the moss Physcomitrella patens

PtdIns-4,5-bisphosphate is a lipid messenger of eukaryotic cells that plays a critical role in processes such as cytoskeleton organization, intracellular vesicular trafficking, secretion, cell motility, regulation of ion channels and nuclear signalling pathways. The enzymes responsible for the synthesis of PtdIns(4,5)P₂ are phosphatidylinositol phosphate kinases (PIPKs). The moss Physcomitrella patens contains two PIPKs, PpPIPK1 and PpPIPK2. To study their physiological role, both genes were disrupted by targeted homologous recombination and as a result mutant plants with lower PtdIns(4,5)P₂ levels were obtained. A strong phenotype for pipk1, but not for pipk2 single knockout lines, was obtained. The pipk1 knockout lines were impaired in rhizoid and caulonemal cell elongation, whereas pipk1-2 double knockout lines showed dramatic defects in protonemal and gametophore morphology manifested by the absence of rapidly elongating caulonemal cells in the protonemal tissue, leafy gametophores with very short rhizoids, and loss of sporophyte production. pipk1 complemented by overexpression of PpPIPK1 fully restored the wild-type phenotype whereas overexpression of the inactive PpPIPK1E885A did not. Overexpression of PpPIPK2 in the pipk1-2 double knockout did not restore the wild-type phenotype demonstrating that PpPIPK1 and PpPIPK2 are not functionally redundant. In vivo imaging of the cytoskeleton network revealed that the shortened caulonemal cells in the pipk1 mutants was the result of the absence of the apicobasal gradient of cortical F-actin cables normally observed in wild-type caulonemal cells. Our data indicate that both PpPIPKs play a crucial role in the development of the moss P. patens, and particularly in the regulation of tip growth.

Phosphatidylinositol transfer proteins: negotiating the regulatory interface between lipid metabolism and lipid signaling in diverse cellular processes

Phosphoinositides represent only a small percentage of the total cellular lipid pool. Yet, these molecules play crucial roles in diverse intracellular processes such as signal transduction at membrane-cytosol interface, regulation of membrane trafficking, cytoskeleton organization, nuclear events, and the permeability and transport functions of the membrane. A central principle in such lipid-mediated signaling is the appropriate coordination of these events. Such an intricate coordination demands fine spatial and temporal control of lipid metabolism and organization, and consistent mechanisms for specifically coupling these parameters to dedicated physiological processes. In that regard, recent studies have identified Sec14-like phosphatidylcholine transfer protein (PITPs) as "coincidence detectors," which spatially and temporally link the diverse aspects of the cellular lipid metabolome with phosphoinositide signaling. The integral role of PITPs in eukaryotic signal transduction design is amply demonstrated by the mammalian diseases associated with the derangements in the function of these proteins, to stress response and developmental regulation in plants, to fungal dimorphism and pathogenicity, to membrane trafficking in yeast, and higher eukaryotes. This review updates the recent advances made in the understanding of how these proteins, specifically PITPs of the Sec14-protein superfamily, operate at the molecular level and further describes how this knowledge has advanced our perception on the diverse biological functions of PITPs.

Misregulation of phosphoinositides in Arabidopsis thaliana decreases pollen hydration and maternal fertility

Phosphoinositides are important lipids involved in membrane identity, vesicle trafficking, and intracellular signaling. In recent years, phosphoinositides have been shown to play a critical role in polarized secretion in plants, as perturbations of phosphoinositide metabolism, through loss of function mutants, result in defects in root hair elongation and pollen tube growth, where polarized secretion occurs rapidly. In the Brassicaceae, responses of stigmatic papillae to compatible pollen are also thought to involve highly regulated secretory events to facilitate pollen hydration and penetration of the pollen tube through the stigmatic surface. We therefore sought to analyze the female sporophyte fertility of the root hair defective4-1 mutant and the PI 4-kinase β1/β2 double mutant, which differentially affect phosphatidylinositol-4-phosphate (PI4P) levels. Stigmas from both mutants supported slower rates of pollen grain hydration, and the fecundity of these mutants was also diminished as a result of failed pollination events. This study therefore concludes that PI4P is integral to appropriate pistil responses to compatible pollen.

Circadian clock-regulated phosphate transporter PHT4;1 plays an important role in Arabidopsis defense

The Arabidopsis accelerated cell death 6-1 (acd6-1) mutant shows constitutive defense, cell death, and extreme dwarf phenotypes. In a screen for acd6-1 suppressors, we identified a mutant that was disrupted by a T-DNA in the PHOSPHATE TRANSPORTER 4;1 (PHT4;1) gene. The suppressor mutant pht4;1-1 is dominant, expresses truncated PHT4;1 transcripts, and is more susceptible to virulent Pseudomonas syringae strains but not to several avirulent strains. Treatment with a salicylic acid (SA) agonist induced a similar level of resistance in Col-0 and pht4;1-1, suggesting that PHT4;1 acts upstream of the SA pathway. Genetic analysis further indicates that PHT4;1 contributes to SID2-dependent and -independent pathways. Transgenic expression of the DNA fragment containing the PHT4;1-1 region or the full-length PHT4;1 gene in wild-type conferred enhanced susceptibility to Pseudomonas infection. Interestingly, expression of PHT4;1 is regulated by the circadian clock. Together, these data suggest that the phosphate transporter PHT4;1 is critical for basal defense and also implicate a potential role of the circadian clock in regulating innate immunity of Arabidopsis.

Phosphatidylinositol-4,5-bisphosphate influences Nt-Rac5-mediated cell expansion in pollen tubes of Nicotiana tabacum

The regulation of pollen tube growth by the phospholipid phosphatidylinositol-4,5-bisphosphate (PtdIns(4,5)P(2) ) is not well understood. The Arabidopsis genome encodes two type A phosphatidylinositol-4-phosphate (PI4P) 5-kinases, PIP5K10 and PIP5K11, which are exclusively expressed in pollen and produce PtdIns(4,5)P(2) in vitro. Fluorescence-tagged PIP5K10 and PIP5K11 localized to lateral subapical plasma membrane microdomains in tobacco pollen tubes in a pattern closely resembling the distribution of PtdIns(4,5)P(2,) with the exception of notably weaker association at the extreme apex. Overexpression of PIP5K10 or PIP5K11 in tobacco pollen tubes resulted in severe tip swelling and altered actin fine structure similar to that reported for overexpression of tobacco Nt-Rac5, a monomeric GTPase known to regulate the actin cytoskeleton. Increased sensitivity of Arabidopsis pip5k10 pip5k11 double mutant pollen tubes to Latrunculin B (LatB) further supports a role for type A PI4P 5-kinases in controlling the actin cytoskeleton. Despite the disruption of both its type A PI4P 5-kinases, the pip5k10 pip5k11 double mutant was fertile, indicating that one of the remaining type B PI4P 5-kinase isoforms might be functionally redundant with PIP5K10 and PIP5K11. Antagonistic effects of PIP5K11 and the Nt-Rac5-specific guanine nucleotide dissociation inhibitor, Nt-RhoGDI2, on tip swelling observed in coexpression-titration experiments indicate a link between PtdIns(4,5)P(2) and Rac-signaling in pollen tubes. The data suggest that type A PI4P 5-kinases influence the actin cytoskeleton in pollen tubes in part by counteracting Nt-RhoGDI2, possibly contributing to the control of the pool of plasma membrane-associated Nt-Rac5.

Reverse-genetic analysis of the two biotin-containing subunit genes of the heteromeric acetyl-coenzyme A carboxylase in Arabidopsis indicates a unidirectional functional redundancy

The heteromeric acetyl-coenzyme A carboxylase catalyzes the first and committed reaction of de novo fatty acid biosynthesis in plastids. This enzyme is composed of four subunits: biotin carboxyl-carrier protein (BCCP), biotin carboxylase, α-carboxyltransferase, and β-carboxyltransferase. With the exception of BCCP, single-copy genes encode these subunits in Arabidopsis (Arabidopsis thaliana). Reverse-genetic approaches were used to individually investigate the physiological significance of the two paralogous BCCP-coding genes, CAC1A (At5g16390, codes for BCCP1) and CAC1B (At5g15530, codes for BCCP2). Transfer DNA insertional alleles that completely eliminate the accumulation of BCCP2 have no perceptible effect on plant growth, development, and fatty acid accumulation. In contrast, transfer DNA insertional null allele of the CAC1A gene is embryo lethal and deleteriously affects pollen development and germination. During seed development the effect of the cac1a null allele first becomes apparent at 3-d after flowering, when the synchronous development of the endosperm and embryo is disrupted. Characterization of CAC1A antisense plants showed that reducing BCCP1 accumulation to 35% of wild-type levels, decreases fatty acid accumulation and severely affects normal vegetative plant growth. Detailed expression analysis by a suite of approaches including in situ RNA hybridization, promoter:reporter transgene expression, and quantitative western blotting reveal that the expression of CAC1B is limited to a subset of the CAC1A-expressing tissues, and CAC1B expression levels are only about one-fifth of CAC1A expression levels. Therefore, a likely explanation for the observed unidirectional redundancy between these two paralogous genes is that whereas the BCCP1 protein can compensate for the lack of BCCP2, the absence of BCCP1 cannot be tolerated as BCCP2 levels are not sufficient to support heteromeric acetyl-coenzyme A carboxylase activity at a level that is required for normal growth and development.

Variable Regions of PI4P 5-Kinases Direct PtdIns(4,5)P(2) Toward Alternative Regulatory Functions in Tobacco Pollen Tubes

The apical plasma membrane of pollen tubes contains different PI4P 5-kinases that all produce phosphatidylinositol-4,5-bisphosphate [PtdIns(4,5)P(2)] but exert distinct cellular effects. In the present example, overexpression of Arabidopsis AtPIP5K5 or tobacco NtPIP5K6-1 caused growth defects previously attributed to increased pectin secretion. In contrast, overexpression of Arabidopsis AtPIP5K2 caused apical tip swelling implicated in altering actin fine structure in the pollen tube apex. AtPIP5K5, NtPIP5K6-1, and AtPIP5K2 share identical domain structures. Domains required for correct membrane association of the enzymes were identified by systematic deletion of N-terminal domains and subsequent expression of fluorescence-tagged enzyme truncations in tobacco pollen tubes. A variable linker region (Lin) contained in all PI4P 5-kinase isoforms of subfamily B, but not conserved in sequence, was recognized to be necessary for correct subcellular localization of AtPIP5K5, NtPIP5K6-1, and AtPIP5K2. Deletion of N-terminal domains including the Lin domain did not impair catalytic activity of recombinant AtPIP5K5, NtPIP5K6-1, or AtPIP5K2 in vitro; however, the presence of the Lin domain was necessary for in vivo effects on pollen tube growth upon overexpression of truncated enzymes. Overexpression of catalytically inactive variants of AtPIP5K5, NtPIP5K6-1, or AtPIP5K2 did not influence pollen tube growth, indicating that PtdIns(4,5)P(2) production rather than structural properties of PI4P 5-kinases was relevant for the manifestation of growth phenotypes. When Lin domains were swapped between NtPIP5K6-1 and AtPIP5K2 and the chimeric enzymes overexpressed in pollen tubes, the chimeras reciprocally gained the capabilities to invoke tip swelling or secretion phenotypes, respectively. The data indicate that the Lin domain directed the enzymes into different regulatory contexts, possibly contributing to channeling of PtdIns(4,5)P(2) at the interface of secretion and actin cytoskeleton.

Phosphoinositides regulate clathrin-dependent endocytosis at the tip of pollen tubes in Arabidopsis and tobacco

Using the tip-growing pollen tube of Arabidopsis thaliana and Nicotiana tabacum as a model to investigate endocytosis mechanisms, we show that phosphatidylinositol-4-phosphate 5-kinase 6 (PIP5K6) regulates clathrin-dependent endocytosis in pollen tubes. Green fluorescent protein-tagged PIP5K6 was preferentially localized to the subapical plasma membrane (PM) in pollen tubes where it apparently converts phosphatidylinositol 4-phosphate (PI4P) to phosphatidylinositol 4,5-bisphosphate [PI(4,5)P(2)]. RNA interference-induced suppression of PIP5K6 expression impaired tip growth and inhibited clathrin-dependent endocytosis in pollen tubes. By contrast, PIP5K6 overexpression induced massive aggregation of the PM in pollen tube tips. This PM abnormality was apparently due to excessive clathrin-dependent membrane invagination because this defect was suppressed by the expression of a dominant-negative mutant of clathrin heavy chain. These results support a role for PI(4,5)P(2) in promoting early stages of clathrin-dependent endocytosis (i.e., membrane invagination). Interestingly, the PIP5K6 overexpression-induced PM abnormality was partially suppressed not only by the overexpression of PLC2, which breaks down PI(4,5)P(2), but also by that of PI4Kβ1, which increases the pool of PI4P. Based on these observations, we propose that a proper balance between PI4P and PI(4,5)P(2) is required for clathrin-dependent endocytosis in the tip of pollen tubes.

Is membrane occupation and recognition nexus domain functional in plant phosphatidylinositol phosphate kinases?

Phosphatidylinositol phosphate kinase (PIPK) catalyzes a key step controlling cellular contents of phosphatidylinositol-4,5-bisphosphate [PtdIns(4,5)P2], a critical intracellular messenger involved in vesicle trafficking and modulation of actin cytoskeleton and also a substrate of phospholipase C to produce the two intracellular messengers, diacylglycerol and inositol-1,4,5-trisphosphate. In addition to the conserved C-terminal PIPK catalytic domain, plant PIPKs contain a unique structural feature consisting of a repeat of membrane occupation and recognition nexus (MORN) motifs, called the MORN domain, in the N-terminal half. The MORN domain has previously been proposed to regulate plasma membrane localization and phosphatidic acid (PA)-inducible activation. Recently, the importance of the catalytic domain, but not the MORN domain, in these aspects was demonstrated. These conflicting data raise the question about the function of the MORN domain in plant PIPKs. We therefore performed analyses of PpPIPK1 from the moss Physcomitrella patens to elucidate the importance of the MORN domain in the control of enzymatic activity; however, we found no effect on either enzymatic activity or activation by PA. Taken together with our previous findings of lack of function in plasma membrane localization, there is no positive evidence indicating roles of the MORN domain in enzymatic and functional regulations of PpPIPK1. Therefore, further biochemical and reverse genetic analyses are necessary to understand the biological significance of the MORN domain in plant PIPKs.

Inhibition of phospholipase C disrupts cytoskeletal organization and gravitropic growth in Arabidopsis roots

The phospholipase protein superfamily plays an important role in hormonal signalling and cellular responses to environmental stimuli. There is also growing evidence for interactions between phospholipases and the cytoskeleton. In this report we used a pharmacological approach to investigate whether inhibiting a member of the phospholipase superfamily, phospholipase C (PLC), affects microtubules and actin microfilaments as well as root growth and morphology of Arabidopsis thaliana seedlings. Inhibiting PLC activity using the aminosteroid U73122 significantly inhibited root elongation and disrupted root morphology in a concentration-dependent manner, with the response being saturated at 5 μM, whereas the inactive analogue U73343 was ineffective. The primary root appeared to lose growth directionality accompanied by root waving and formation of curls. Immunolabelling of roots exposed to increasingly higher U73122 concentrations revealed that the normal transverse arrays of cortical microtubules in the elongation zone became progressively more disorganized or depolymerized, with the disorganization appearing within 1 h of incubation. Likewise, actin microfilament arrays also were disrupted. Inhibiting PLC using an alternative inhibitor, neomycin, caused similar disruptions to both cytoskeletal organization and root morphology. In seedlings gravistimulated by rotating the culture plates by 90°, both U73122 and neomycin disrupted the normal gravitropic growth of roots and etiolated hypocotyls. The effects of PLC inhibitors are therefore consistent with the notion that, as with phospholipases A and D, PLC likewise interacts with the cytoskeleton, alters growth morphology, and is involved in gravitropism.

Functional cooperativity of enzymes of phosphoinositide conversion according to synergistic effects on pectin secretion in tobacco pollen tubes

The Arabidopsis phosphoinositide kinases PI4Kβ1 and PIP5K5 have been implicated in the control of directional vesicle trafficking underlying polar tip growth in pollen tubes. PI4Kβ1 and PIP5K5 catalyze key consecutive steps of phosphoinositide conversion, and it appears obvious that phosphatidylinositol-4-phosphate formed by PI4Kβ1 might act as a substrate for phosphatidylinositol-4,5-bisphosphate formation by PIP5K5. However, this hypothesis has not been experimentally addressed and distinct localization patterns of PI4Kβ1, PIP5K5, and also PI-synthases (PIS) generating phosphatidylinositol suggest additional complexity. Here, the synergistic functionality of enzymes of phosphoinositide conversion was assessed. In tobacco and Arabidopsis pollen tubes, phosphoinositides influence the apical secretion of pectin, and increased pectin deposition results in characteristic morphological alterations. Catalytically active and dominant negative variants of PI4Kβ1 and PIP5K5 were systematically co-expressed in tobacco pollen tubes and the incidence of morphologies related to enhanced pectin secretion was evaluated. The data support a proposed functional interplay of PI4Kβ1 and PIP5K5 at the trans-Golgi network, mediating directional vesicle trafficking. Co-expression experiments additionally including PIS isoforms, PIS1 or PIS2, indicate that pectin secretion is synergistically mediated by PI4Kβ1 and PIP5K5 acting on PtdIns formed by PIS2 rather than PIS1. Possible ramifications for the preferential channeling of phosphoinositide intermediates between particular isoforms of PI pathway enzymes are discussed.

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.

A genome-wide survey of maize lipid-related genes: candidate genes mining, digital gene expression profiling and co-location with QTL for maize kernel oil

Lipids play an important role in plants due to their abundance and their extensive participation in many metabolic processes. Genes involved in lipid metabolism have been extensively studied in Arabidopsis and other plant species. In this study, a total of 1003 maize lipid-related genes were cloned and annotated, including 42 genes with experimental validation, 732 genes with full-length cDNA and protein sequences in public databases and 229 newly cloned genes. Ninety-seven maize lipid-related genes with tissue-preferential expression were discovered by in silico gene expression profiling based on 1984483 maize Expressed Sequence Tags collected from 182 cDNA libraries. Meanwhile, 70 QTL clusters for maize kernel oil were identified, covering 34.5% of the maize genome. Fifty-nine (84%) QTL clusters co-located with at least one lipid-related gene, and the total number of these genes amounted to 147. Interestingly, thirteen genes with kernel-preferential expression profiles fell within QTL clusters for maize kernel oil content. All the maize lipid-related genes identified here may provide good targets for maize kernel oil QTL cloning and thus help us to better understand the molecular mechanism of maize kernel oil accumulation.

A dibasic amino acid pair conserved in the activation loop directs plasma membrane localization and is necessary for activity of plant type I/II phosphatidylinositol phosphate kinase

Phosphatidylinositol phosphate kinase (PIPK) is an enzyme involved in the regulation of cellular levels of phosphoinositides involved in various physiological processes, such as cytoskeletal organization, ion channel activation, and vesicle trafficking. In animals, research has focused on the modes of activation and function of PIPKs, providing an understanding of the importance of plasma membrane localization. However, it still remains unclear how this issue is regulated in plant PIPKs. Here, we demonstrate that the carboxyl-terminal catalytic domain, which contains the activation loop, is sufficient for plasma membrane localization of PpPIPK1, a type I/II B PIPK from the moss Physcomitrella patens. The importance of the carboxyl-terminal catalytic domain for plasma membrane localization was confirmed with Arabidopsis (Arabidopsis thaliana) AtPIP5K1. Our findings, in which substitution of a conserved dibasic amino acid pair in the activation loop of PpPIPK1 completely prevented plasma membrane targeting and abolished enzymatic activity, demonstrate its critical role in these processes. Placing our results in the context of studies of eukaryotic PIPKs led us to conclude that the function of the dibasic amino acid pair in the activation loop in type I/II PIPKs is plant specific.

[Lipid signaling pathways in plants and their roles in response to water constraints]

Plants are sessile organisms that have developed the capacity to detect slight variations of their environment. They are able to perceive these environmental signals and to transduce them by signaling pathways in order to trigger adaptative responses. Lipid signaling elements play a central role in these pathways in plants. A key element is phosphatidic acid (PA), which can be produced by two pathways. In the first one, phospholipids are hydrolysed by phospholipase D (PLD) to release PA. In the second one, PA is produced through the activity of phospholipase C (PLC) to produce diacylglycerol (DAG) which is then phosphorylated by DAG kinase (DAGK). The amount of PA in the cell is regulated by PA kinase, which phosphorylates PA to produce diacylglycerolpyrophosphate (DGPP), considered as a second messenger as well. PLCs play a dual role in cell signaling by regulating the amount of intracellular Ca(2+), another essential second messenger. Phosphoinositides, such as PI3P, PI4P and PI(4,5)P(2), are substrates of PLCs and PLDs and are considered as second messengers also. In this review, we present recent data regarding the specific features of these lipid signaling pathways in plant compared with other eukaryotes.

Spatial and temporal integration of signalling networks regulating pollen tube growth

The overall function of a cell is determined by its contingent of active signal transduction cascades interacting on multiple levels with metabolic pathways, cytoskeletal organization, and regulation of gene expression. Much work has been devoted to analysis of individual signalling cascades interacting with unique cellular targets. However, little is known about how cells integrate information across hierarchical signalling networks. Recent work on pollen tube growth indicates that several key signalling cascades respond to changes in cell hydrodynamics and apical volume. Combined with known effects on cytoarchitecture and signalling from other cell systems, hydrodynamics has the potential to integrate and synchronize the function of the broader signalling network in pollen tubes. This review will explore recent work on cell hydrodynamics in a variety of systems including pollen, and discuss hydrodynamic regulation of cell signalling and function including exocytosis and endocytosis, actin cytoskeleton reorganization, cell wall deposition and assembly, phospholipid and inositol polyphosphate signalling, ion flux, small G-proteins, fertilization, and self-incompatibility. The combined data support a newly emerging model of pollen tube growth.

Polyphosphoinositides are enriched in plant membrane rafts and form microdomains in the plasma membrane

In this article, we analyzed the lipid composition of detergent-insoluble membranes (DIMs) purified from tobacco (Nicotiana tabacum) plasma membrane (PM), focusing on polyphosphoinositides, lipids known to be involved in various signal transduction events. Polyphosphoinositides were enriched in DIMs compared with whole PM, whereas all structural phospholipids were largely depleted from this fraction. Fatty acid composition analyses suggest that enrichment of polyphosphoinositides in DIMs is accompanied by their association with more saturated fatty acids. Using an immunogold-electron microscopy strategy, we were able to visualize domains of phosphatidylinositol 4,5-bisphosphate in the plane of the PM, with 60% of the epitope found in clusters of approximately 25 nm in diameter and 40% randomly distributed at the surface of the PM. Interestingly, the phosphatidylinositol 4,5-bisphosphate cluster formation was not significantly sensitive to sterol depletion induced by methyl-beta-cyclodextrin. Finally, we measured the activities of various enzymes of polyphosphoinositide metabolism in DIMs and PM and showed that these activities are present in the DIM fraction but not enriched. The putative role of plant membrane rafts as signaling membrane domains or membrane-docking platforms is discussed.

At the poles across kingdoms: phosphoinositides and polar tip growth

Phosphoinositides (PIs) are minor, but essential phospholipid constituents of eukaryotic membranes, and are involved in the regulation of various physiological processes. Recent genetic and cell biological advances indicate that PIs play important roles in the control of polar tip growth in plant cells. In root hairs and pollen tubes, PIs control directional membrane trafficking required for the delivery of cell wall material and membrane area to the growing tip. So far, the exact mechanisms by which PIs control polarity and tip growth are unresolved. However, data gained from the analysis of plant, fungal and animal systems implicate PIs in the control of cytoskeletal dynamics, ion channel activity as well as vesicle trafficking. The present review aims at giving an overview of PI roles in eukaryotic cells with a special focus on functions pertaining to the control of cell polarity. Comparative screening of plant and fungal genomes suggests diversification of the PI system with increasing organismic complexity. The evolutionary conservation of the PI system among eukaryotic cells suggests a role for PIs in tip growing cells in models where PIs so far have not been a focus of attention, such as fungal hyphae.

Identification of tomato phosphatidylinositol-specific phospholipase-C (PI-PLC) family members and the role of PLC4 and PLC6 in HR and disease resistance

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.

Physcomitrella patens: a model to investigate the role of RAC/ROP GTPase signalling in tip growth

Polarized cell expansion plays an important role in plant morphogenesis. Tip growth is a dramatic form of this process, which is widely used as a model to study its regulation by RAC/ROP GTPase signalling. During the dominant haploid phase of its life cycle, the moss Physcomitrella patens contains different types of cells that expand by tip growth. Physcomitrella is a highly attractive experimental system because its genome has been sequenced, and transgene integration by homologous recombination occurs in this plant at frequencies allowing effective gene targeting. Furthermore, together with the vascular spikemoss Selaginella moellendorffii, whose genome has also been sequenced, the non-vascular moss Physcomitrella provides an evolutionary link between green algae and angiosperms. BLAST searches established that the Physcomitrella and Selaginella genomes encode not only putative RAC/ROP GTPases, but also homologues of all known regulators of polarized RAC/ROP signalling, as well as of key effectors acting in signalling cascades downstream of RAC/ROP activity. Nucleotide sequence relationships within seven different families of Physcomitrella, Selaginella, Arabidopsis thaliana and Nicotiana tabacum (tobacco) genes with distinct functions in RAC/ROP signalling were characterized based on extensive maximum likelihood and Neighbor-Joining analyses. The results of these analyses are interpreted in the light of current knowledge concerning expression patterns and molecular functions of RAC/ROP signalling proteins in angiosperms. A key aim of this study is to facilitate the use of Physcomitrella as a model to investigate the molecular control of tip growth in plants.

Involvement of phospholipase C in the responses triggered by extracellular phosphatidylinositol 4-phosphate

Phosphatidylinositol 4-phosphate (PI4P) is a minor phospholipid signal molecule involved in diverse biological processes. It is also the main precursor of phosphatidylinositol 4,5-bisphosphate [PI(4,5)P(2)], which is well described as the substrate of the phospholipase C (PLC)-diacylglycerol kinase (DGK) signalling pathway. Recently, we have demonstrated that PI4P plays a role as an extracellular signalling molecule involved in the activation of xylanase-induced defence responses in tomato cell suspensions. Here we make an approach to the possible mode of action of extracellular PI4P by analysing the involvement of PLC/DGK signalling pathway. We show that extracellular PI4P can be incorporated into tomato cells and further metabolized to PI, although its conversion to PI(4,5)P(2) could not be detected. In addition, treatment of tomato cells with the PLC inhibitor U73122 inhibited the oxidative burst triggered by PI4P treatment, suggesting the involvement of PLC in the induction of this response. Nevertheless, we demonstrate that PI4P treatment does not induce DGK activity. These results indicate that PLC but not DKG activation would be triggered by extracellular PI4P. In this sense, a possible mode of action of extracellular PI4P as a direct substrate of PLC to induce this signalling pathway is discussed.

An Arabidopsis hydrophilic Ca2(+) -binding protein with a PEVK-rich domain, PCaP2, is associated with the plasma membrane and interacts with calmodulin and phosphatidylinositol phosphates

We found a new hydrophilic protein in Arabidopsis thaliana. Real-time PCR demonstrated that the protein was expressed in roots. Histochemical analysis of promoter-beta-glucuronidase fusions demonstrated its extensive expression in root hairs. The protein is rich in proline, glutamate, valine and lysine residues (PEVK-rich domain), and bound Ca(2+) even in the presence of Mg(2+) and K(+) when examined by the (45)Ca overlay assay. Treatment of seedlings with K(+), Mn(2+), Zn(2+), Na(+), ABA and gibberellic acid, and cold and drought stresses enhanced the transcription. Expression of the protein linked to green fluorescent protein in A. thaliana showed its plasma membrane localization and cell-specific expression in the epidermal cells including root hairs and the elongating pollen tubes. Therefore, we named the protein PCaP2 (plasma membrane-associated Ca(2+)-binding protein-2). The substitution of glycine at position 2 with alanine resulted in cytoplasmic localization of PCaP2. These results and the N-terminal characteristic motif suggest that PCaP2 is N-myristoylated at Gly2. We examined the capacity for binding to phosphatidylinositol phosphates (PtdInsPs), and found that PCaP2 interacts strongly with PtdIns(3,5)P(2), PtdIns(4,5)P(2) and PtdIns(3,4,5)P(3), and weakly with PtdIns(3,4)P(2). Furthermore, calmodulin was associated with PCaP2 in a Ca(2+)-dependent manner, and its association weakened the interaction of PCaP2 with PtdInsPs. These results indicate that PCaP2 is involved in intracellular signaling through interaction with PtdInsPs and calmodulin in growing root hairs. PCaP2 was previously reported as microtubule-associated protein-18. We discuss the physiological roles of PCaP2 in relation to microtubules in cells.

The Arabidopsis stem cell factor POLTERGEIST is membrane localized and phospholipid stimulated

Stem cell maintenance and differentiation are tightly regulated in multicellular organisms. In plants, proper control of the stem cell populations is critical for extensive postembryonic organogenesis. The Arabidopsis thaliana protein phosphatase type 2C proteins POLTERGEIST (POL) and PLL1 are essential for maintenance of both the root and shoot stem cells. Specifically, POL and PLL1 are required for proper specification of key asymmetric cell divisions during stem cell initiation and maintenance. POL and PLL1 are known to be integral components of the CLE/WOX signaling pathways, but the location and mechanisms by which POL and PLL1 are regulated within these pathways are unclear. Here, we show that POL and PLL1 are dual-acylated plasma membrane proteins whose membrane localization is required for proper function. Furthermore, this localization places POL and PLL1 in proximity of the upstream plasma membrane receptors that regulate their activity. Additionally, we find that POL and PLL1 directly bind to multiple lipids and that POL is catalytically activated by phosphatidylinositol (4) phosphate [PI(4)P] in vitro. Based on these results, we propose that the upstream receptors in the CLE/WOX signaling pathways may function to either limit PI(4)P availability or antagonize PI(4)P stimulation of POL/PLL1. Significantly, the findings presented here suggest that phospholipids play an important role in promoting stem cell specification.

Overexpression of BrMORN, a novel 'membrane occupation and recognition nexus' motif protein gene from Chinese cabbage, promotes vegetative growth and seed production in Arabidopsis

Proteins that contain membrane occupation and recognition nexus (MORN) motifs regulate various aspects of cellular metabolism by localizing proteins in different cellular organelles. The full-length Brassica rapa MORN motif protein (BrMORN) cDNA consists of 1,510 bp encoding 502 deduced amino acids with a predicted molecular mass of 55.8 kDa and an isoelectric point of 9.72. BrMORN is a novel protein composed of two N-terminal transmembrane helices and seven C-terminal MORN motifs and it appears to be localized on the plastid envelope. BrMORN expression was relatively high in actively-growing tissues, but low in mature tissues and under some abiotic stresses. Arabidopsis thaliana plants overexpressing BrMORN showed an enhanced rate of growth, hypocotyl elongation, and increases in the size of vegetative organs and seed productivity under normal growth conditions. In addition, cell size in Arabidopsis plants overexpressing BrMORN was 24% larger than that of wild-type plants, implying that the increase in the size of vegetative organs is due to cell enlargement. The increased size of the vegetative organs also led to increased seed production. Our data suggest that the MORN motif of BrMORN may act at the plastid envelope and facilitate plant growth via cell enlargement.

Arabidopsis deadenylases AtCAF1a and AtCAF1b play overlapping and distinct roles in mediating environmental stress responses

To maintain homeostasis in an ever-changing environment organisms have evolved mechanisms to reprogram gene expression. One central mechanism regulating gene expression is messenger RNA (mRNA) degradation, which is initiated by poly(A) tail shortening (deadenylation). The carbon catabolite repressor 4-CCR4 associated factor1 (CCR4-CAF1) complex is the major enzyme complex that catalyzes mRNA deadenylation and is conserved among eukaryotes. However, the components and functions of this global regulatory complex have not been well characterized in plants. Here we investigate the CAF1 family in Arabidopsis (Arabidopsis thaliana). We identify 11 AtCAF1 homologs and show that a subset of these genes are responsive to mechanical wounding, among them are AtCAF1a and AtCAF1b whose expression levels are rapidly and transiently induced by wounding. The differential expression profiles of the various AtCAF1s suggest that not all AtCAF1 genes are involved in stress-responsive regulation of transcript levels. Comparison of misexpressed genes identified via transcript profiling of Atcaf1a and Atcaf1b mutants at different time points before and after wounding suggests that AtCAF1a and AtCAF1b target shared and unique transcripts for deadenylation with temporal specificity. Consistent with the AtPI4Kgamma3 transcript exhibiting the largest increase in abundance in Atcaf1b, AtCAF1b targets AtPI4Kgamma3 mRNA for deadenylation. Stress-tolerance assays demonstrate that AtCAF1a and AtCAF1b are involved in mediating abiotic stress responses. However, AtCAF1a and AtCAF1b are not functionally redundant in all cases, nor are they essential for all environmental stresses. These findings demonstrate that these closely related proteins exhibit overlapping and distinct roles with respect to mRNA deadenylation and mediation of stress responses.

Arabidopsis FAB1/PIKfyve proteins are essential for development of viable pollen

Phosphatidylinositol 3,5-bisphosphate [PtdIns(3,5)P(2)] is a phospholipid that has a role in controlling membrane trafficking events in yeast and animal cells. The function of this lipid in plants is unknown, although its synthesis has been shown to be up-regulated upon osmotic stress in plant cells. PtdIns(3,5)P(2) is synthesized by the PIKfyve/Fab1 family of proteins, with two orthologs, FAB1A and FAB1B, being present in Arabidopsis (Arabidopsis thaliana). In this study, we attempt to address the role of this lipid by analyzing the phenotypes of plants mutated in FAB1A and FAB1B. It was not possible to generate plants homozygous for mutations in both genes, although single mutants were isolated. Both homozygous single mutant plant lines exhibited a leaf curl phenotype that was more marked in FAB1B mutants. Genetic transmission analysis revealed that failure to generate double mutant lines was entirely due to inviability of pollen carrying mutant alleles of both FAB1A and FAB1B. This pollen displayed severe defects in vacuolar reorganization following the first mitotic division of development. The presence of abnormally large vacuoles in pollen at the tricellular stage resulted in the collapse of the majority of grains carrying both mutant alleles. This demonstrates a crucial role for PtdIns(3,5)P(2) in modulating the dynamics of vacuolar rearrangement essential for successful pollen development. Taken together, our results are consistent with PtdIns(3,5)P(2) production being central to cellular responses to changes in osmotic conditions.

Function and regulation of phospholipid signalling in plants

As an important metabolic pathway, phosphatidylinositol metabolism generates both constitutive and signalling molecules that are crucial for plant growth and development. Recent studies using genetic and molecular approaches reveal the important roles of phospholipid molecules and signalling in multiple processes of higher plants, including root growth, pollen and vascular development, hormone effects and cell responses to environmental stimuli plants. The present review summarizes the current progress in our understanding of the functional mechanism of phospholipid signalling, with an emphasis on the regulation of Ins(1,4,5)P3-Ca2+ oscillation, the second messenger molecule phosphatidic acid and the cytoskeleton.

Phospholipid signaling responses in salt-stressed rice leaves

Salinity is one of the major environmental factors limiting growth and productivity of rice plants. In this study, the effect of salt stress on phospholipid signaling responses in rice leaves was investigated. Leaf cuts were radiolabeled with 32P-orthophosphate and the lipids extracted and analyzed by thin-layer chromatography, autoradiography and phosphoimaging. Phospholipids were identified by co-migration of known standards. Results showed that 32P(i) was rapidly incorporated into the minor lipids, phosphatidylinositol bisphosphate (PIP2) and phosphatidic acid (PA) and, interestingly, also into the structural lipids phosphatidylethanolamine (PE) and phosphatidylglycerol (PG), which normally label relatively slowly, like phosphatidylcholine (PC) and phosphatidylinositol (PI). Only very small amounts of PIP2 were found. However, in response to salt stress (NaCl), PIP2 levels rapidly (<30 min) increased up to 4-fold, in a time- and dose-dependent manner. PA and its phosphorylated product, diacylglycerolpyrophosphate (DGPP), also increased upon NaCl stress, while cardiolipin (CL) levels decreased. All other phospholipid levels remained unchanged. PA signaling can be generated via the combined action of phospholipase C (PLC) and diacylglycerol kinase (DGK) or directly via phospholipase D (PLD). The latter can be measured in vivo, using a transphosphatidylation assay. Interestingly, these measurements revealed that salt stress inhibited PLD activity, indicating that the salt stress-induced PA response was not due to PLD activity. Comparison of the 32P-lipid responses in salt-tolerant and salt-sensitive cultivars revealed no significant differences. Together these results show that salt stress rapidly activates several lipid responses in rice leaves but that these responses do not explain the difference in salt tolerance between sensitive and tolerant cultivars.

Phosphatidylinositol 3,5-bisphosphate and Fab1p/PIKfyve underPPIn endo-lysosome function

PtdIns(3,5)P(2) is one of the seven regulatory PPIn (polyphosphoinositides) that are ubiquitous in eukaryotes. It controls membrane trafficking at multiple points in the endosomal/lysosomal system and consequently regulates the size, shape and acidity of at least one endo-lysosomal compartment. PtdIns(3,5)P(2) appears to exert this control via multiple effector proteins, with each effector specific for a subset of the various PtdIns(3,5)P(2)-dependent processes. Some putative PtdIns(3,5)P(2) effectors have been identified, including Atg18p-related PROPPIN [beta-propeller(s) that bind PPIn] proteins and the epsin-like proteins Ent3p and Ent5p, whereas others remain to be defined. One of the principal functions of PtdIns(3,5)P(2) is to regulate the fission/fragmentation of endo-lysosomal sub-compartments. PtdIns(3,5)P(2) is required for vesicle formation during protein trafficking between endo-lysosomes and also for fragmentation of endo-lysosomes into smaller compartments. In yeast, hyperosmotic stress accelerates the latter process. In the present review we highlight and discuss recent studies that reveal the role of the HOPS-CORVET complex and the vacuolar H(+)-ATPase in the process of endo-lysosome fission, and speculate on connections between these machineries and the Fab1p pathway. We also discuss new evidence linking PtdIns(3,5)P(2) and PtdIns5P to the regulation of exocytosis.

Characterization of phosphatidylinositol phosphate kinases from the moss Physcomitrella patens: PpPIPK1 and PpPIPK2

Phosphoinositides (PIs) play a major role in eukaryotic cells, despite being a minor component of most membranes. This is the first report on PI metabolism in a bryophyte, the moss Physcomitrella patens. Moss PI composition is similar to that of other land plants growing under normal conditions. In contrast to the large number of PIPK genes present in flowering plants, the P. patens genome encodes only two type I/II PIPK genes, PpPIPK1 and PpPIPK2, which are very similar at both the nucleotide and protein product levels. However, the expression of the two genes is differentially regulated, and in vitro biochemical characterization shows that the resulting enzymes have different substrate specificities. PpPIPK1 uses PtdIns4P and PtdIns3P with similar preference and also metabolizes PtdIns(3,4)P(2) to produce PtdIns(3,4,5)P(3), a PI not yet detected in intact plant cells. PpPIPK2 prefers PtdIns as substrate and is much less active towards PtdIns4P and PtdIns3P. Thus, PpPIPK2 shows properties reminiscent of both PtdInsP-kinase and PtdIns-kinases. Moreover, a substitution of glutamic acid by alanine in the activation loop drastically reduced PpPIPK1 activity and altered the substrate specificity to PtdIns5P being the preferred substrate compared with PtdIns4P and PtdIns3P. These findings demonstrate that the substrate specificity of plant PIPKs is determined in a plant-specific manner, which provides new insights into the regulatory modes of PIPK activity in plants.

Using genetic tools to understand plant phosphoinositide signalling

Phosphoinositides (PIs) are regulatory lipids that control various physiological processes in eukaryotic organisms. As in other eukaryotes, the plant PI system is a central regulator of metabolism. The analysis of mutant plants that lack certain PI species has revealed their physiological relevance; however, knowledge of the factors controlling the distribution of PIs and the effects on their target proteins is still limited. To understand PI functions better, genetic approaches should be combined with biochemical analyses and cell biology, as has been done in several recent publications. Here, I highlight plant-specific physiological processes that are controlled by PIs and suggest future avenues of research. A detailed understanding of the functions and effects of PIs might offer new opportunities for modulating plant growth and hardiness against environmental influences.

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.

A plasma-membrane linker for the phosphoinositide-specific phospholipase C in tobacco plants

We previously screened genes that were transcriptionally activated during the early stage of wound response in tobacco plants (Nicotiana tabacum), and isolated a particular clone, which encoded a membrane-located protein, designated as NtC7. Upon overexpression in tobacco plants, NtC7 conferred a marked tolerance to osmotic stress, suggesting it to be involved in maintenance of osmotic adjustments. In this study, we searched for proteins which interact with NtC7 by the yeast two-hybrid screening, and isolated a clone encoding phosphoinositide-specific phospholipase C, designated as NtPI-PLC. Physical interaction between NtC7 and C2 domain of NtPI-PLC was confirmed by the pull-down assay. Expression of fused protein to green-fluorescence protein in onion epidermal cell layers indicated both proteins to predominantly localize to the plasma membrane. Their interaction in planta was shown by the bimolecular fluorescence complementation, which exhibited a clear fluorescence of reconstituted yellow fluorescence protein. Transcripts of NtC7 and NtPI-PLC were markedly increased 30 to 60 min after wounding. PI-PLC is one of key enzymes in metabolism of inositol phospholipids, which function in signal transduction and also in response to stresses including osmotic changes. It was shown to localize to plasma-membrane and, to a lesser extent, to cytosol. However, molecular mechanism of membrane localization has remained to be determined, because of the apparent lack of domains for membrane association. The present results suggest that one of such mechanisms is tethering NtPI-PLC to the plasma membrane through interaction with NtC7, which possesses a transmembrane domain at the C-terminus.

Plant phospholipid signaling: "in a nutshell"

Since the discovery of the phosphoinositide/phospholipase C (PI/PLC) system in animal systems, we know that phospholipids are much more then just structural components of biological membranes. In the beginning, this idea was fairly straightforward. Receptor stimulation activates PLC, which hydrolyses phosphatidylinositol4,5-bisphosphate [PtdIns(4,5)P2] into two second messengers: inositol 1,4,5-trisphosphate (InsP3) and diacylglycerol (DG). While InsP3 difuses into the cytosol and triggers the release of calcium from an internal store via ligand-gated calcium channels, DG remains in the membrane where it recruits and activates members of the PKC family. The increase in calcium, together with the change in phosphorylation status, (in)activates a variety of protein targets, leading to a massive reprogramming, allowing the cell to appropriately respond to the extracellular stimulus. Later, it became obvious that not just PLC, but a variety of other phospholipid-metabolizing enzymes were activated, including phospholipase A, phospholipase D, and PI 3-kinase. More recently, it has become apparent that PtdIns4P and PtdIns(4,5)P2 are not just signal precursors but can also function as signaling molecules themselves. While plants contain most of the components described above, and evidence for their role in cell signaling is progressively increasing, major differences between plants and the mammalian paradigms exist. Below, these are described "in a nutshell."