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

MORN motifs in plant PIPKs are involved in the regulation of subcellular localization and phospholipid binding

Multiple repeats of membrane occupation and recognition nexus (MORN) motifs were detected in plant phosphatidylinositl monophosphate kinase (PIPK), a key enzyme in PI-signaling pathway. Structural analysis indicates that all the MORN motifs (with varied numbers at ranges of 7-9), which shared high homologies to those of animal ones, were located at N-terminus and sequentially arranged, except those of OsPIPK1 and AtPIPK7, in which the last MORN motif was separated others by an approximately 100 amino-acid "island" region, revealing the presence of two kinds of MORN arrangements in plant PIPKs. Through employing a yeast-based SMET (sequence of membrane-targeting) system, the MORN motifs were shown being able to target the fusion proteins to cell plasma membrane, which were further confirmed by expression of fused MORN-GFP proteins. Further detailed analysis via deletion studies indicated the MORN motifs in OsPIPK1, together with the 104 amino-acid "island" region are involved in the regulation of differential subcellular localization, i.e. plasma membrane or nucleus, of the fused proteins. Fat Western blot analysis of the recombinant MORN polypeptide, expressed in Escherichia coli, showed that MORN motifs could strongly bind to PA and relatively slightly to PI4P and PI(4,5)P2. These results provide informative hints on mechanisms of subcellular localization, as well as regulation of substrate binding, of plant PIPKs.

Autophagy in the control of programmed cell death

Programmed cell death (PCD) is essential for plant development and immunity. Localized PCD is associated with the hypersensitive response (HR), which is a constituent of a successful plant innate immune response. Plants have developed mechanisms to meticulously prevent HR-PCD lesions from spreading. Our understanding of these mechanisms is still in its incipient stages. A recent study demonstrated that autophagy, a universally conserved process of macromolecule turnover, plays a pivotal role in controlling HR-PCD. The molecular identity of the mediators between the PCD and HR pathways is still obscure, but recent work has begun to shed light on the relationship between HR-PCD and autophagy and to suggest possible mechanisms for the regulation of these pathways.

Phospholipid molecular profiles in the seed kernel from different sunflower (Helianthus annuus) mutants

Phospholipids are essential components of plant cell membranes whose acyl composition appears to be influenced by oil composition in the sunflower. In the current study, we have determined the diacylglycerol profile of the main phospholipids using phospholipase C degradation and separation of the diacylglycerols by HPLC and GLC. The main polar lipid molecular species were defined in different classes of sunflower kernel: PC, PE, and PI. The proportions of each were determined at different stages of development in order to define the point at which the mutations carried by each sunflower line affected the phospholipid composition of the seeds. The results indicated that modifications to intraplastidial de novo FA synthesis affected the seed phospholipid profile during the whole period of the seed formation, including accumulation and maturation, whereas the influence of mutations in the endoplasmic reticulum desaturases were more readily detected at later stages of development. These results are discussed in terms of the pathways involved in glycerolipid synthesis and phospholipid conversion in sunflower seeds.

Phosphatidylinositol 4-kinases: old enzymes with emerging functions

Phosphoinositides account for only a tiny fraction of cellular phospholipids but are extremely important in the regulation of the recruitment and activity of many signaling proteins in cellular membranes. Phosphatidylinositol (PtdIns) 4-kinases generate PtdIns 4-phosphate, the precursor of important regulatory phosphoinositides but also an emerging regulatory molecule in its own right. The four mammalian PtdIns 4-kinases regulate a diverse array of signaling events, as well as vesicular trafficking and lipid transport, but the mechanisms by which their lipid product PtdIns 4-phosphate controls these processes is only beginning to unfold.

A role for the RabA4b effector protein PI-4Kbeta1 in polarized expansion of root hair cells in Arabidopsis thaliana

The RabA4b GTPase labels a novel, trans-Golgi network compartment displaying a developmentally regulated polar distribution in growing Arabidopsis thaliana root hair cells. GTP bound RabA4b selectively recruits the plant phosphatidylinositol 4-OH kinase, PI-4Kβ1, but not members of other PI-4K families. PI-4Kβ1 colocalizes with RabA4b on tip-localized membranes in growing root hairs, and mutant plants in which both the PI-4Kβ1 and -4Kβ2 genes are disrupted display aberrant root hair morphologies. PI-4Kβ1 interacts with RabA4b through a novel homology domain, specific to eukaryotic type IIIβ PI-4Ks, and PI-4Kβ1 also interacts with a Ca²⁺ sensor, AtCBL1, through its NH₂ terminus. We propose that RabA4b recruitment of PI-4Kβ1 results in Ca²⁺-dependent generation of PI-4P on this compartment, providing a link between Ca²⁺ and PI-4,5P₂–dependent signals during the polarized secretion of cell wall components in tip-growing root hair cells.

Petunia phospholipase c1 is involved in pollen tube growth

Although pollen tube growth is essential for plant fertilization and reproductive success, the regulators of the actin-related growth machinery and the cytosolic Ca2+ gradient thought to determine how these cells elongate remain poorly defined. Phospholipases, their substrates, and their phospholipid turnover products have been proposed as such regulators; however, the relevant phospholipase(s) have not been characterized. Therefore, we cloned cDNA for a pollen-expressed phosphatidylinositol 4,5-bisphosphate (PtdInsP2)-cleaving phospholipase C (PLC) from Petunia inflata, named Pet PLC1. Expressing a catalytically inactive form of Pet PLC1 in pollen tubes caused expansion of the apical Ca2+ gradient, disruption of the organization of the actin cytoskeleton, and delocalization of growth at the tube tip. These phenotypes were suppressed by depolymerizing actin with low concentrations of latrunculin B, suggesting that a critical site of action of Pet PLC1 is in regulating actin structure at the growing tip. A green fluorescent protein (GFP) fusion to Pet PLC1 caused enrichment in regions of the apical plasma membrane not undergoing rapid expansion, whereas a GFP fusion to the PtdInsP2 binding domain of mammalian PLC delta1 caused enrichment in apical regions depleted in PLC. Thus, Pet PLC1 appears to be involved in the machinery that restricts growth to the very apex of the elongating pollen tube, likely through its regulatory action on PtdInsP2 distribution within the cell.

Petunia phospholipase c1 is involved in pollen tube growth

Although pollen tube growth is essential for plant fertilization and reproductive success, the regulators of the actin-related growth machinery and the cytosolic Ca2+ gradient thought to determine how these cells elongate remain poorly defined. Phospholipases, their substrates, and their phospholipid turnover products have been proposed as such regulators; however, the relevant phospholipase(s) have not been characterized. Therefore, we cloned cDNA for a pollen-expressed phosphatidylinositol 4,5-bisphosphate (PtdInsP2)-cleaving phospholipase C (PLC) from Petunia inflata, named Pet PLC1. Expressing a catalytically inactive form of Pet PLC1 in pollen tubes caused expansion of the apical Ca2+ gradient, disruption of the organization of the actin cytoskeleton, and delocalization of growth at the tube tip. These phenotypes were suppressed by depolymerizing actin with low concentrations of latrunculin B, suggesting that a critical site of action of Pet PLC1 is in regulating actin structure at the growing tip. A green fluorescent protein (GFP) fusion to Pet PLC1 caused enrichment in regions of the apical plasma membrane not undergoing rapid expansion, whereas a GFP fusion to the PtdInsP2 binding domain of mammalian PLC delta1 caused enrichment in apical regions depleted in PLC. Thus, Pet PLC1 appears to be involved in the machinery that restricts growth to the very apex of the elongating pollen tube, likely through its regulatory action on PtdInsP2 distribution within the cell.

The cold-induced early activation of phospholipase C and D pathways determines the response of two distinct clusters of genes in Arabidopsis cell suspensions

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

Molecular mechanisms of phytochrome signal transduction in higher plants

Phytochromes in higher plants play a great role in development, responses to environmental stresses and signal transduction, which are the fundamental principles for higher plants to be adapted to changing environment. Deep and systematic understanding of the phytochrome in higher plants is of crucial importance to molecular biology, purposeful improvement of environment in practice, especially molecular mechanism by which higher plants perceive UV-B stress. The last more than 10 years have seen rapid progress in this field with the aid of a combination of molecular, genetic and cell biological approaches. No doubt, what is the most important, is the application of Arabidopsis experimental system and the generation of various mutants regarding phytochromes (phy A-E). Increasing evidence demonstrates that phytochrome signaling transduction constitutes a highly ordered multidimensional network of events. Some phytochromes and signaling intermediates show light-dependent nuclear-cytoplasmic partitioning, which implies that early signaling events take place in the nucleus and that subcellular localization patterns most probably represent an important signaling control point. The main subcellular localization includes nucleus, cytosol and chloroplasts, respectively. Additionally, proteasome-mediated degradation of signaling intermediates most possibly function in concert with subcellular partitioning events as an integrated checkpoint. What higher plants do in this way is to execute accurate responses to the changes in the light environment on the basis of interconnected subcellular organelles. By integrating the available data, at the molecular level and from the angle of eco-environment, we should be able to construct a solid foundation for further dissection of phytochrome signaling transduction in higher plants.

P5644 interacts with phosphatidylinositol-4-phosphate adaptor protein-1 associated protein-1

The human novel gene pp5644 (GeneBank Accession No. AF289559) coding for 124 amino acids was recently cloned. Overexpression of pp5644 in Hela cells significantly inhibited the growth and colony formation. The pp5644-interacting protein FAPP1 (phosphatidylinositol-four-phosphate adaptor protein1) associated protein-1, called FASP1, was obtained by using yeast two-hybrid system. The interaction between pp5644 and FASP1 was experimentally confirmed by GST pull-down assay in vitro and co-immunoprecipitation assay in vivo. Co-localization of pp5644 and FASP1 in cytoplasm in Hela cells could further support the interaction. Based on the experimental results, it is suggested that pp5644 physically bind to FASP1 and the biological significance of this kind of interaction in vivo is discussed.

Characterization and comparative analysis of Arabidopsis phosphatidylinositol phosphate 5-kinase 10 reveals differences in Arabidopsis and human phosphatidylinositol phosphate kinases

Arabidopsis phosphatidylinositol phosphate (PtdInsP) kinase 10 (AtPIPK10; At4g01190) is shown to be a functional enzyme of the subfamily A, type I AtPtdInsP kinases. It is biochemically distinct from AtPIPK1 (At1g21980), the only other previously characterized AtPtdInsP kinase which is of the B subfamily. AtPIPK10 has the same K(m), but a 10-fold lower V(max) than AtPIPK1 and it is insensitive to phosphatidic acid. AtPIPK10 transcript is most abundant in inflorescence stalks and flowers, whereas AtPIPK1 transcript is present in all tissues. Comparative analysis of recombinant AtPIPK10 and AtPIPK1 with recombinant HsPIPKIalpha reveals that the Arabidopsis enzymes have roughly 200- and 20-fold lower V(max)/K(m), respectively. These data reveal one explanation for the longstanding mystery of the relatively low phosphatidylinositol-(4,5)-bisphosphate:phosphatidylinositol-4-phosphate ratio in terrestrial plants.

Phosphatidic acid: a multifunctional stress signaling lipid in plants

Phosphatidic acid (PA) has only recently been identified as an important signaling molecule in both plants and animals. Nonetheless, it already promises to rival the importance of the classic second messengers Ca(2+) and cAMP. In plants, its formation is triggered in response to various biotic and abiotic stress factors, including pathogen infection, drought, salinity, wounding and cold. In general, PA signal production is fast (minutes) and transient. Recently, our understanding of the role of PA formation in stress responses as a result of phospholipases C and D activity has greatly increased. Moreover, the first protein targets of PA have been identified. Based on this recent work, potential mechanisms by which PA provokes downstream effects are emerging.

Mutations in the Arabidopsis phosphoinositide phosphatase gene SAC9 lead to overaccumulation of PtdIns(4,5)P2 and constitutive expression of the stress-response pathway

Phosphoinositides (PIs) are signaling molecules that regulate cellular events including vesicle targeting and interactions between membrane and cytoskeleton. Phosphatidylinositol (PtdIns)(4,5)P(2) is one of the best characterized PIs; studies in which PtdIns(4,5)P(2) localization or concentration is altered lead to defects in the actin cytoskeleton and exocytosis. PtdIns(4,5)P(2) and its derivative Ins(1,4,5)P(3) accumulate in salt, cold, and osmotically stressed plants. PtdIns(4,5)P(2) signaling is terminated through the action of inositol polyphosphate phosphatases and PI phosphatases including supressor of actin mutation (SAC) domain phosphatases. In some cases, these phosphatases also act on Ins(1,4,5)P(3). We have characterized the Arabidopsis (Arabidopsis thaliana) sac9 mutants. The SAC9 protein is different from other SAC domain proteins in several ways including the presence of a WW protein interaction domain within the SAC domain. The rice (Oryza sativa) and Arabidopsis SAC9 protein sequences are similar, but no apparent homologs are found in nonplant genomes. High-performance liquid chromatography studies show that unstressed sac9 mutants accumulate elevated levels of PtdIns(4,5)P(2) and Ins(1,4,5)P(3) as compared to wild-type plants. The sac9 mutants have characteristics of a constitutive stress response, including dwarfism, closed stomata, and anthocyanin accumulation, and they overexpress stress-induced genes and overaccumulate reactive-oxygen species. These results suggest that the SAC9 phosphatase is involved in modulating phosphoinsitide signals during the stress response.

Phosphate-limited oat. The plasma membrane and the tonoplast as major targets for phospholipid-to-glycolipid replacement and stimulation of phospholipases in the plasma membrane

We recently reported that cultivation of oat (Avena sativa L.) without phosphate resulted in plasma membrane phosphoglycerolipids being replaced to a large extent by digalactosyldiacylglycerol (DGDG) (Andersson, M. X., Stridh, M. H., Larsson, K. E., Liljenberg, C., and Sandelius, A. S. (2003) FEBS Lett. 537, 128-132). We report here that DGDG is not the only non-phosphorous-containing lipid that replaces phospholipids but that also the content of glucosylceramides and sterolglycosides increased in plasma membranes as a response to phosphate starvation. In addition, phosphate deficiency induced similar changes in lipid composition in the tonoplast. The phospholipid-to-glycolipid replacement apparently did not occur to any greater extent in endoplasmic reticulum, Golgi apparatus, or mitochondrial inner membranes. In contrast to the marked effects on lipid composition, the polypeptide patterns were largely similar between root plasma membranes from well-fertilized and phosphate-limited oat, although the latter condition induced at least four polypeptides, including a chaperone of the HSP80 or HSP90 family, a phosphate transporter, and a bacterial-type phosphoesterase. The latter polypeptide reacted with an antibody raised against a phosphate deficiency-induced phospholipase C from Arabidopsis thaliana (Nakamura, Y., Awai, K., Masuda, T., Yoshioka, Y., Takamiya, K., and Ohta, H. (2005) J. Biol. Chem. 280, 7469-7476). In plasma membranes from oat, however, a phospholipase D-type activity and a phosphatidic acid phosphatase were the dominant lipase activities induced by phosphate deficiency. Our results reflect a highly developed plasticity in the lipid composition of the plasma membrane and the tonoplast. In addition, phosphate deficiency-induced alterations in plasma membrane lipid composition may involve different sets of lipid-metabolizing enzymes in different plant tissues or species, at different stages of plant development and/or at different stages of stress adjustments.

An essential function of phosphatidylinositol phosphates in activation of plant shaker-type K+ channels

A prominent regulatory property of plant shaker-type K+ channels is the 'rundown' that causes channel closure upon membrane excision from the cell, implicating intracellular factor(s) in maintaining channel activity. One such factor has been identified as hydrolysable ATP-Mg although the mechanism for ATP function remains unknown. Here we report identification of phosphatidylinositol (PI) phosphates (PIPs) as essential regulators for the voltage-dependent and -independent activation of plant shaker-type channels such as SKOR, an outward rectifying K+ channel. Inhibition of PI kinase activity abolished the function of ATP-Mg in restoration of rundown channel activity, demonstrating that PIPs production by PI kinases and ATP-Mg underlies ATP-induced activation of the rundown channel. We also identified aluminum block as a common feature of the plant shaker-type channels and provided evidence that aluminum block of these channels may result from Al interaction with PIPs.

Mutation of SAC1, an Arabidopsis SAC domain phosphoinositide phosphatase, causes alterations in cell morphogenesis, cell wall synthesis, and actin organization

SAC (for suppressor of actin) domain proteins in yeast and animals have been shown to modulate the levels of phosphoinositides, thereby regulating several cellular activities such as signal transduction, actin cytoskeleton organization, and vesicle trafficking. Nine genes encoding SAC domain-containing proteins are present in the Arabidopsis thaliana genome, but their roles in plant cellular functions and plant growth and development have not been characterized. In this report, we demonstrate the essential roles of one of the Arabidopsis SAC domain proteins, AtSAC1, in plant cellular functions. Mutation of the AtSAC1 gene in the fragile fiber7 (fra7) mutant caused a dramatic decrease in the wall thickness of fiber cells and vessel elements, thus resulting in a weak stem phenotype. The fra7 mutation also led to reduced length and aberrant shapes in fiber cells, pith cells, and trichomes and to an alteration in overall plant architecture. The AtSAC1 gene was found to be expressed in all tissues in elongating organs; however, it showed predominant expression in vascular tissues and fibers in nonelongating parts of stems. In vitro activity assay demonstrated that AtSAC1 exhibited phosphatase activity toward phosphatidylinositol 3,5-biphosphate. Subcellular localization studies showed that AtSAC1 was colocalized with a Golgi marker. Truncation of the C terminus by the fra7 mutation resulted in its localization in the cytoplasm but had no effect on phosphatase activity. Furthermore, examination of the cytoskeleton organization revealed that the fra7 mutation caused the formation of aberrant actin cables in elongating cells but had no effect on the organization of cortical microtubules. Together, these results provide genetic evidence that AtSAC1, a SAC domain phosphoinositide phosphatase, is required for normal cell morphogenesis, cell wall synthesis, and actin organization.

The evolution of Ca2+ signalling in photosynthetic eukaryotes

It is likely that cytosolic Ca2+ elevations have played a part in eukaryotic signal transduction for about the last 2 Gyr, being mediated by a group of molecules which are collectively known as the [Ca2+]cyt signalling toolkit. Different eukaryotes often display strikingly similar [Ca2+]cyt signalling elevations, which may reflect conservation of toolkit components (homology) or similar constraints acting on different toolkits (homoplasy). Certain toolkit components, which are presumably ancestral, are shared by plants and animals, but some components are unique to photosynthetic organisms. We propose that the structure of modern plant [Ca2+]cyt signalling toolkits may be explained by their modular adaptation from earlier pathways.

Cloning of Brassica napus phospholipase C2 (BnPLC2), phosphatidylinositol 3-kinase (BnVPS34) and phosphatidylinositol synthase1 (BnPtdIns S1)--comparative analysis of the effect of abiotic stresses on the expression of phosphatidylinositol signal transduction-related genes in B. napus

The cloning and identification of full-length cDNA fragments coding for the Brassica napus phosphatidylinositol-specific phospholipase C2 (BnPLC2), phosphatidylinositol 3-kinase (BnVPS34) and phosphatidylinositol synthase (BnPtdIns S1) is described. In addition, two complementary fragments (120 nucleotides long) corresponding to Arabidopsis PtdIns 4-kinase (PtdIns 4-K) and PtdIns-4-phosphate 5-kinase (PtdIns4P 5-K) sequences were chemically synthesized. These, as well as the cDNA clones, were used as probes to study the corresponding steady state mRNA levels in different tissues and developmental stages of B. napus, as well as in response to different environmental conditions. Transcripts corresponding to BnPLC2, BnPtdIns S1, BnVPS34 and PtdIns 4-K were found constitutively expressed at different levels in most tissues, with young leaves, siliques, and developing seeds showing the lowest levels. No detectable PtdIns4P 5-K transcripts were found in buds or flowers. Up-regulation of BnPLC2 was seen in response to low temperature stress, which was notably accompanied by a parallel down-regulation of BnPtdIns S1, while BnVPS34 and PtdIns 4-K remained at control levels. A moderate increase in PtdIns4P 5-K levels was noted. In high salinity conditions BnPtdIns S1, BnVPS34 and BnPLC2 transcripts had similar responses but at different levels, with no major changes detected for PtdIns 4-K or PtdIns4P 5-K. Significantly, all five transcripts increased under drought stress conditions and all stressed plants clearly showed relatively higher levels of total inositol trisphosphate.

Plants: the latest model system for G-protein research

In humans, heterotrimeric G proteins couple stimulus perception by G-protein-coupled receptors (GPCRs) with numerous downstream effectors. By contrast, despite great complexity in their signal-transduction attributes, plants have a simpler repertoire of G-signalling components. Nonetheless, recent studies on Arabidopsis thaliana have shown the importance of plant G-protein signalling in such fundamental processes as cell proliferation, hormone perception and ion-channel regulation.

DNA chip-based expression profile analysis indicates involvement of the phosphatidylinositol signaling pathway in multiple plant responses to hormone and abiotic treatments

The phosphatidylinositol (PI) metabolic pathway is considered critical in plant responses to many environmental factors, and previous studies have indicated the involvement of multiple PI-related gene families during cellular responses. Through a detailed analysis of the Arabidopsis thaliana genome, 82 polypeptides were identified as being involved in PI signaling. These could be grouped into different families including PI synthases (PIS), PI-phosphate kinases (PIPK), phospholipases (PL), inositol polyphosphate phosphatases (IPPase), inositol polyphosphate kinases (IPK), PI transfer proteins and putative inositol polyphosphate receptors. The presence of more than 10 isoforms of PIPK, PLC, PLD and IPPase suggested that these genes might be differentially expressed during plant cellular responses or growth and development. Accordingly, DNA chip technology was employed to study the expression patterns of various isoforms. In total, 79 mRNA clones were amplified and used for DNA chip generation. Expression profile analysis was performed using samples that represented multiple tissues or cellular responses. Tested samples included normal leaf, stem and flower tissues, and leaves from plants treated with various hormones (auxin, cytokinin, gibberellin, abscisic acid and brassinosteroid) or environmental factors (temperature, calcium, sodium, drought, salicylic acid and jasmonic acid). Results showed that many PI pathway-related genes were differentially expressed under these experimental conditions. In particular, the different isoforms of each family were specifically expressed in many cases, suggesting their involvement in tissue specificity and cellular responses to environmental conditions. This work provides a starting point for functional studies of the relevant PI-related proteins and may help shed light onto the role of PI pathways in development and cellular responses.

A role of Arabidopsis inositol polyphosphate kinase, AtIPK2alpha, in pollen germination and root growth

Inositol polyphosphates, such as inositol trisphosphate, are pivotal intracellular signaling molecules in eukaryotic cells. In higher plants the mechanism for the regulation of the type and the level of these signaling molecules is poorly understood. In this study we investigate the physiological function of an Arabidopsis (Arabidopsis thaliana) gene encoding inositol polyphosphate kinase (AtIPK2alpha), which phosphorylates inositol 1,4,5-trisphosphate successively at the D-6 and D-3 positions, and inositol 1,3,4,5-tetrakisphosphate at D-6, resulting in the generation of inositol 1,3,4,5,6-pentakisphosphate. Semiquantitative reverse transcription-PCR and promoter-beta-glucuronidase reporter gene analyses showed that AtIPK2alpha is expressed in various tissues, including roots and root hairs, stem, leaf, pollen grains, pollen tubes, the flower stigma, and siliques. Transgenic Arabidopsis plants expressing the AtIPK2alpha antisense gene under its own promoter were generated. Analysis of several independent transformants exhibiting strong reduction in AtIPK2alpha transcript levels showed that both pollen germination and pollen tube growth were enhanced in the antisense lines compared to wild-type plants, especially in the presence of nonoptimal low Ca(2+) concentrations in the culture medium. Furthermore, root growth and root hair development were also stimulated in the antisense lines, in the presence of elevated external Ca(2+) concentration or upon the addition of EGTA. In addition, seed germination and early seedling growth was stimulated in the antisense lines. These observations suggest a general and important role of AtIPK2alpha, and hence inositol polyphosphate metabolism, in the regulation of plant growth most likely through the regulation of calcium signaling, consistent with the well-known function of inositol trisphosphate in the mobilization of intracellular calcium stores.

FRAGILE FIBER3, an Arabidopsis gene encoding a type II inositol polyphosphate 5-phosphatase, is required for secondary wall synthesis and actin organization in fiber cells

Type II inositol polyphosphate 5-phosphatases (5PTases) in yeast and animals have been known to regulate the level of phosphoinositides and thereby influence various cellular activities, such as vesicle trafficking and actin organization. In plants, little is known about the phosphatases involved in hydrolysis of phosphoinositides, and roles of type II 5PTases in plant cellular functions have not yet been characterized. In this study, we demonstrate that the FRAGILE FIBER3 (FRA3) gene of Arabidopsis thaliana, which encodes a type II 5PTase, plays an essential role in the secondary wall synthesis in fiber cells and xylem vessels. The fra3 mutations caused a dramatic reduction in secondary wall thickness and a concomitant decrease in stem strength. These phenotypes were associated with an alteration in actin organization in fiber cells. Consistent with the defective fiber and vessel phenotypes, the FRA3 gene was found to be highly expressed in fiber cells and vascular tissues in stems. The FRA3 protein is composed of two domains, an N-terminal localized WD-repeat domain and a C-terminal localized 5PTase catalytic domain. In vitro activity assay demonstrated that recombinant FRA3 exhibited phosphatase activity toward PtdIns(4,5)P2, PtdIns(3,4,5)P3, and Ins(1,4,5)P3, with the highest substrate affinity toward PtdIns(4,5)P2. The fra3 missense mutation, which caused an amino acid substitution in the conserved motif II of the 5PTase catalytic domain, completely abolished the FRA3 phosphatase activity. Moreover, the endogenous levels of PtdIns(4,5)2 and Ins(1,4,5)P3 were found to be elevated in fra3 stems. Together, our findings suggest that the FRA3 type II 5PTase is involved in phosphoinositide metabolism and influences secondary wall synthesis and actin organization.

Pharmacological evidence that multiple phospholipid signaling pathways link Rhizobium nodulation factor perception in Medicago truncatula root hairs to intracellular responses, including Ca2+ spiking and specific ENOD gene expression

Rhizobium nodulation (Nod) factors are specific lipochito-oligosaccharide signals essential for initiating in root hairs of the host legume developmental responses that are required for controlled entry of the microsymbiont. In this article, we focus on the Nod factor signal transduction pathway leading to specific and cell autonomous gene activation in Medicago truncatula cv Jemalong in a study making use of the Nod factor-inducible MtENOD11 gene. First, we show that pharmacological antagonists that interfere with intracellular ion channel and Ca2+ pump activities are efficient blockers of Nod factor-elicited pMtENOD11-beta-glucuronidase (GUS) expression in root hairs of transgenic M. truncatula. These results indicate that intracellular Ca2+ release and recycling activities, essential for Ca2+ spiking, are also required for specific gene activation. Second, pharmacological effectors that inhibit phospholipase D and phosphoinositide-dependent phospholipase C activities are also able to block pMtENOD11-GUS activation, thus underlining a central role for multiple phospholipid signaling pathways in Nod factor signal transduction. Finally, pMtENOD11-GUS was introduced into all three Nod-/Myc- dmi M. truncatula mutant backgrounds, and gene expression was evaluated in response to the mastoparan peptide agonist Mas7. We found that Mas7 elicits root hair MtENOD11 expression in dmi1 and dmi2 mutants, but not in the dmi3 mutant, suggesting that the agonist acts downstream of DMI1/DMI2 and upstream of DMI3. In light of these results and the recently discovered identities of the DMI gene products, we propose an integrated cellular model for Nod factor signaling in legume root hairs in which phospholipids play a key role in linking the Nod factor perception apparatus to downstream components such as Ca2+ spiking and ENOD gene expression.

Phosphoinositide-specific phospholipase C is involved in cytokinin and gravity responses in the moss Physcomitrella patens

The phosphoinositide signalling pathway is important in plant responses to extracellular and intracellular signals. To elucidate the physiological functions of phosphoinositide-specific phopspholipase C, PI-PLC, targeted knockout mutants of PpPLC1, a gene encoding a PI-PLC from the moss Physcomitrella patens, were generated via homologous recombination. Protonemal filaments of the plc1 lines show a dramatic reduction in gametophore formation relative to wild type: this was accompanied by a loss of sensitivity to cytokinin. Moreover, plc1 appeared paler than the wild type, the result of an altered differentiation of chloroplasts and reduced chlorophyll levels compared with wild type filaments. In addition, the protonemal filaments of plc1 have a strongly reduced ability to grow negatively gravitropically in the dark. These effects imply a significant role for PpPLC1 in cytokinin signalling and gravitropism.

Patellin1, a novel Sec14-like protein, localizes to the cell plate and binds phosphoinositides

Membrane trafficking is central to construction of the cell plate during plant cytokinesis. Consequently, a detailed understanding of the process depends on the characterization of molecules that function in the formation, transport, targeting, and fusion of membrane vesicles to the developing plate, as well as those that participate in its consolidation and maturation into a fully functional partition. Here we report the initial biochemical and functional characterization of patellin1 (PATL1), a novel cell-plate-associated protein that is related in sequence to proteins involved in membrane trafficking in other eukaryotes. Analysis of the Arabidopsis genome indicated that PATL1 is one of a small family of Arabidopsis proteins, characterized by a variable N-terminal domain followed by two domains found in other membrane-trafficking proteins (Sec14 and Golgi dynamics domains). Results from immunolocalization and biochemical fractionation studies suggested that PATL1 is recruited from the cytoplasm to the expanding and maturing cell plate. In vesicle-binding assays, PATL1 bound to specific phosphoinositides, important regulators of membrane trafficking, with a preference for phosphatidylinositol(5)P, phosphatidylinositol(4,5)P(2), and phosphatidylinositol(3)P. Taken together, these findings suggest a role for PATL1 in membrane-trafficking events associated with cell-plate expansion or maturation and point to the involvement of phosphoinositides in cell-plate biogenesis.

Cyclodextrins enhance recombinant phosphatidylinositol phosphate kinase activity

Inositol lipid kinases have been studied extensively in both plant and animal systems. However, major limitations for in vitro studies of recombinant lipid kinases are the low specific activity and instability of the purified proteins. Our goal was to determine if cyclodextrins would provide an effective substrate delivery system and enhance the specific activity of lipid kinases. For these studies, we have used recombinant Arabidopsis thaliana phosphatidylinositol phosphate kinase 1 (At PIPK1). At PIPK1 was produced as a fusion protein with glutathione-S-transferase and purified on glutathione-Sepharose beads. A comparison of lipid kinase activity using substrate prepared in alpha-, beta-, or gamma-cyclodextrin indicated that beta-cyclodextrin was most effective and enhanced lipid kinase activity 6-fold compared with substrate prepared in Triton X-100-mixed micelles. We have optimized reaction conditions and shown that product can be recovered from the cyclodextrin-treated recombinant protein, which reveals a potential method for automating the assay for pharmacological screening.

The involvement of cytosolic lipases in converting phosphatidyl choline to substrate for galactolipid synthesis in the chloroplast envelope

Here we report that cytosolic phospholipases are involved in the utilization of phosphatidylcholine (PC) as substrate for chloroplast-localized synthesis of monogalactosyldiacylglycerol (MGDG). Isolated chloroplasts were pre-incubated with lysoPC and [14C]18:0-CoA to form [14C]PC. When soluble plant proteins (cytosol) and UDP-galactose were added, [14C] MGDG was formed. An inhibitor of phospholipase D markedly lowered the formation of [14C]MGDG, whereas thermolysin pretreatment of the chloroplasts was without effect. The cytosolic activity resided in the >100-kDa fraction. In a second approach, [14C]PC-containing lipid mixtures were incubated with cytosol. Degradation of [14C]PC to [14C]diacylglycerol was highest when the lipid composition of the mixture mimicked that of the outer chloroplast envelope. We also investigated whether PC of extraplastidic origin could function as substrate for MGDG synthesis. Isolated chloroplasts were incubated with enriched endoplasmic reticulum containing radiolabelled acyl lipids. In the presence of cytosol and UDP-galactose, there was a time-dependent transfer of [14C]PC from this fraction to chloroplasts, where [14C]MGDG was formed. We conclude that chloroplasts recruit cytosolic phospholipase D and phosphatidic acid phosphatase to convert PC to diacylglycerol. Apparently, these lipases do not interact with chloroplast surface proteins, but rather with outer membrane lipids, either for association to the envelope or for substrate presentation.

NAD - new roles in signalling and gene regulation in plants

The pyridine nucleotides, NAD⁺ , NADH, NADP⁺ , and NADPH have long-established and well-characterised roles as redox factors in processes such as oxidative phosphorylation, the TCA cycle, and as electron acceptors in photosynthesis. Recent years have seen an increase in the number of signalling and gene regulatory processes where NAD⁺ or NADP⁺ are metabolised. Cyclic ADP-ribose (cADPR) and nicotinic acid adenine dinucleotide phosphate (NAADP) are metabolites of NAD⁺ and NADP⁺ , respectively, and now have widely accepted roles as potent intracellular calcium releasing agents in animals, but are less well characterised in plants. NAD kinases catalyse the transfer of a phosphate group from ATP to NAD to form NADP and are well characterised in plants in their requirement for the calcium binding protein calmodulin, thereby putatively linking their regulation to stress-induced intracellular calcium release. A second group of proteins unrelated to those above, the sirtuins (Sir2) and poly ADP-ribose polymerases (PARPs), cleave NAD and transfer the ADP-ribose group to acetyl groups and proteins, respectively. These have roles in transcriptional control and DNA repair in eukaryotes. Contents Summary I. Introduction 32 II. NAD synthesis and breakdown 32 III. cADPR in plants 34 IV. NAADP in plants 35 V. NAD kinases 35 VI. NAD and gene regulation 37 VII. Sir2 is an NAD dependant histone deacetylase 37 VIII. Nicotinamidases 38 IX. Poly ADP-ribosylation 39 X. Poly(ADP-ribose) glycohydrolase (PARG) 40 XI. Subcellular compartmentation of NAD and NADP in plants 41 XII. Conclusions 41 Acknowledgements 41 References 41.

Lipid signaling

Various lipids are involved in mediating plant growth, development and responses to biotic and abiotic cues, and their production is regulated by lipid-signaling enzymes. Lipid-hydrolyzing enzymes play a pivotal role both in the production of lipid messengers and in other processes, such as cytoskeletal rearrangement, membrane trafficking, and degradation. Studies on the downstream targets and modes of action of lipid signals in plants are still in their early stages but distinguishing features of plant lipid-based signaling are being recognized. Phospholipase D enzymes and phosphatidic acid may play a broader role in lipid signaling in plants than in other systems.

Gene-specific expression and calcium activation of Arabidopsis thaliana phospholipase C isoforms

•  PI-PLCs synthesise the calcium releasing second messenger IP₃ . We investigated the expression patterns of the Arabidopsis PI-PLC gene family and measured in vitro activity of encoded enzymes. •  Gene specific RT-PCR and promoter-GUS fusions were used to analyse AtPLC gene expression patterns. The five available AtPLC cDNAs were expressed as fusion proteins in Escherichia coli. •  All members of the AtPLC gene family were expressed in multiple organs of the plant. AtPLC1, and AtPLC5 expression was localized to the vascular cells of roots and leaves with AtPLC5::GUS also detected in the guard cells. AtPLC4::GUS was detected in pollen and cells of the stigma surface. In seedlings, AtPLC2 and AtPLC3 were constitutively expressed, while AtPLCs 1, 4 and 5 were induced by abiotic stresses. AtPLC1-5 were all shown to have phospholipase C activity in the presence of calcium ions. •  AtPLCs showed limited tissue specific expression and expression of at least three genes was increased by abiotic stress. The differing calcium sensitivities of recombinant AtPLC protein activities may provide a mechanism for generating calcium signatures.

Rapid accumulation and metabolism of polyphosphoinositol and its possible role in phytoalexin biosynthesis in yeast elicitor-treated Cupressus lusitanica cell cultures

Inositol 1,4,5-trisphosphate [Ins(1,4,5)P(3)] rapidly accumulates in elicited Cupressus lusitanica Mill. cultured cells by 4- to 5-fold over the control, and then it is metabolized. Correspondingly, phospholipase C (PLC) activity toward phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P(2)] is stimulated to high levels by the elicitor and then decreases whereas Ins(1,4,5)P(3) phosphatase activity declines at the beginning of elicitation and increases later. These observations indicate that elicitor-induced biosynthesis and dephosphorylation of Ins(1,4,5)P(3) occur simultaneously and that the Ins(1,4,5)P(3) level may be regulated by both PtdIns(4,5)P(2)-PLC and Ins(1,4,5)P(3) phosphatases. Studies on the properties of PLC and Ins(1,4,5)P(3) phosphatases indicate that PLC activity toward PtdIns(4,5)P(2) was optimal at a lower Ca(2+) concentration than activity toward phosphatidylinositol whereas Ins(1,4,5)P(3) phosphatase activity is inhibited by high Ca(2+) concentration. This suggests that Ins(1,4,5)P(3) biosynthesis and degradation may be regulated by free cytosolic Ca(2+). In addition, a relationship between Ins(1,4,5)P(3) signaling and accumulation of a phytoalexin (beta-thujaplicin) is suggested because inhibition or promotion of Ins(1,4,5)P(3) accumulation by neomycin or LiCl affects elicitor-induced production of beta-thujaplicin. Moreover, ruthenium red inhibits elicitor-induced accumulation of beta-thujaplicin while thapsigargin alone induces beta-thujaplicin accumulation. These results suggest that Ca(2+) released from intracellular calcium stores may mediate elicitor-induced accumulation of beta-thujaplicin via an Ins(1,4,5)P(3) signaling pathway, since it is widely accepted that Ins(1,4,5)P(3) can mobilize Ca(2+) from intracellular stores. This work demonstrates an elicitor-triggered Ins(1,4,5)P(3) turnover, defines its enzymatic basis and regulation, and suggests a role for Ins(1,4,5)P(3) in elicitor-induced phytoalexin accumulation via a Ca(2+) signaling pathway.

Phytohormones participate in an S6 kinase signal transduction pathway in Arabidopsis

Addition of fresh medium to stationary cells of Arabidopsis suspension culture induces increased phosphorylation of the S6 ribosomal protein and activation of its cognate kinase, AtS6k. Analysis of the activation response revealed that medium constituents required for S6 kinase activation were the phytohormones 1-naphthylacetic acid (auxin) and kinetin. Pretreatment of cells with anti-auxin or PI3-kinase drugs inhibited this response. Consistent with these findings, LY294002, a PI3-kinase inhibitor, efficiently suppressed phytohormone-induced S6 phosphorylation and translational up-regulation of ribosomal protein S6 and S18A mRNAs without affecting global translation. These data indicate that (1) activation of AtS6k is regulated by phytohormones, at least in part, via a lipid kinase-dependent pathway, that (2) the translational regulation of ribosomal proteins appears to be conserved throughout the plant and animal kingdom, and that (3) these events are hallmarks of a growth-related signal transduction pathway novel in plants.

The Vr-PLC3 gene encodes a putative plasma membrane-localized phosphoinositide-specific phospholipase C whose expression is induced by abiotic stress in mung bean (Vigna radiata L.)

Phosphoinositide-specific phospholipase C (PI-PLC) catalyzes the hydrolysis of phosphatidylinositol 4,5-bisphosphate to generate inositol 1,4,5-trisphosphate and diacylglycerol, both of which act as secondary messengers in animal cells. In this report, we identified in Vigna radiata L. (mung bean) three distinct partial cDNAs (pVr-PLC1, pVr-PLC2, and pVr-PLC3), which encode forms of putative PI-PLC. All three Vr-PLC genes were transcriptionally active and displayed unique patterns of expression. The Vr-PLC1 and Vr-PLC2 transcripts were constitutively expressed to varying degrees in every tissue of mung bean plants examined. In contrast, the Vr-PLC3 mRNA level was very low under normal growth conditions and was rapidly induced in an abscisic acid-independent manner under environmental stress conditions (drought and high salinity). An isolated genomic clone, about 8.2 kb in length, showed that Vr-PLC1 and Vr-PLC3 are in tandem array in the mung bean genome. The predicted primary sequence of Vr-PLC3 (M(r)=67.4 kDa) is reminiscent of the delta-isoform of animal enzymes which contain core sequences found in typical PI-PLCs, such as the catalytic domain comprising X and Y motifs, a lipid-binding C2 domain, and the less conserved EF-hand domain. Results of in vivo targeting experiment using a green fluorescent protein (GFP) showed that the GFP-Vr-PLC3 fusion protein was localized primarily to the plasma membrane of the Arabidopsis protoplast. The C2 domain was essential for Vr-PLC3 to be targeted to the plasma membrane. The possible biological functions of stress-responsive Vr-PLC3 in mung bean plants are discussed.

3-methyladenine inhibits autophagy in tobacco culture cells under sucrose starvation conditions

Tobacco (Nicotiana tabacum) culture cells perform autophagy and degrade cellular proteins in response to sucrose starvation. When protein degradation is blocked by the cysteine protease inhibitor E-64c, lysosomes containing particles of cytoplasm (autolysosomes) accumulate in the cells. Therefore, using light microscopy, we can determine whether cells have performed autophagy. In this study, we investigated whether or not 3-methyladenine (3-MA), which is a known inhibitor of autophagy in mammalian cells, blocks autophagy in tobacco culture cells. The accumulation of autolysosomes was blocked by the addition to the culture media of 5 mM 3-MA together with E-64c. We did not detect autolysosomes or structures thought to be involved with autophagy, such as autophagosomes, accumulating in these cells, as observed by electron microscopy. 3-MA blocked cellular protein degradation without any effect on cellular protease activity. In mammalian cells, phosphatidylinositol 3-kinase (PtdIns 3-kinase) is a putative target of 3-MA. The PtdIns 3-kinase inhibitors wortmannin and LY294002 also inhibited the accumulation of autolysosomes in tobacco culture cells. These results suggest that (1) 3-MA inhibits autophagy by blocking the formation of autophagosomes in tobacco culture cells, and (2) PtdIns 3-kinase is essential for autophagy in tobacco cells.

An introduction to plant sphingolipids and a review of recent advances in understanding their metabolism and function

Sphingolipids are ubiquitous constituents of eukaryotic cells, and have been intensively investigated in mammals and yeast for decades. Aspects of sphingolipid biochemistry in plants have been explored only recently. To date, progress has been made in determining the structure and occurrence of sphingolipids in plant tissues; in characterizing the enzymatic steps involved in production and turnover of sphingolipids (and, in some cases, the genes encoding the relevant enzymes); and in identifying a variety of biological functions for sphingolipids in plants. Given that these efforts are far from complete and much remains to be learned, this review represents a status report on the burgeoning field of plant sphingolipid biochemistry. Contents Summary 677 I. Introduction 678 II. Plant sphingolipid structure 678 III. Sphingolipid metabolism in plants 683 IV. Sphingolipid functions in plants 693 V. Conclusions 696 Acknowledgements 696 References 696.

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.

A protein kinase target of a PDK1 signalling pathway is involved in root hair growth in Arabidopsis

Here we report on a lipid-signalling pathway in plants that is downstream of phosphatidic acid and involves the Arabidopsis protein kinase, AGC2-1, regulated by the 3'-phosphoinositide-dependent kinase-1 (AtPDK1). AGC2-1 specifically interacts with AtPDK1 through a conserved C-terminal hydrophobic motif that leads to its phosphorylation and activation, whereas inhibition of AtPDK1 expression by RNA interference abolishes AGC2-1 activity. Phosphatidic acid specifically binds to AtPDK1 and stimulates AGC2-1 in an AtPDK1-dependent manner. AtPDK1 is ubiquitously expressed in all plant tissues, whereas expression of AGC2-1 is abundant in fast-growing organs and dividing cells, and activated during re-entry of cells into the cell cycle after sugar starvation-induced G1-phase arrest. Plant hormones, auxin and cytokinin, synergistically activate the AtPDK1-regulated AGC2-1 kinase, indicative of a role in growth and cell division. Cellular localisation of GFP-AGC2-1 fusion protein is highly dynamic in root hairs and at some stages confined to root hair tips and to nuclei. The agc2-1 knockout mutation results in a reduction of root hair length, suggesting a role for AGC2-1 in root hair growth and development.

OsPIPK 1, a rice phosphatidylinositol monophosphate kinase, regulates rice heading by modifying the expression of floral induction genes

A rice gene, OsPIPK 1, encoding a 792-aa putative phosphatidylinositol 4-phosphate 5-kinase (PIPK), was identified and characterized. Comparison between the cDNA and genomic sequences revealed the presence of 10 exons (39-1050 bp) and 9 introns (88-745 bp) in OsPIPK 1 gene. The deduced amino acid sequence of OsPIPK1 contains a lipid kinase domain that is highly homologous to those of previously isolated PIPKs, and structural analysis revealed the intriguing presence of multiple MORN motifs at the N-terminus. The MORN motifs have also been detected in PIPKs from Arabidopsis thaliana and Oryza sativa, but not in the well-characterized PIPKs from animal and yeast cells. RT-PCR analysis indicated that OsPIPK 1 was expressed almost constitutively in roots, shoots, stems, leaves and flowers, and up-regulated following treatment with plant hormones or application of various stresses. An antisense transgenic strategy was used to suppress the expression of OsPIPK 1, and homozygous transgenic plants showed earlier heading (7-14 days earlier) than control plants, suggesting that OsPIPK 1 negatively regulates floral initiation. This was further confirmed by morphologic observation showing earlier floral development in antisense plants, as well as leaf emergence measurement indicating delayed leaf development under OsPIPK 1 deficiency, a common phenotype observed with earlier flowering. RT-PCR analysis and cDNA chip technology were used to examine transcripts of various genes in the transgenic plants and the results showed altered transcriptions of several flowering-time or -identity related genes, suggesting that OsPIPK 1 is involved in rice heading through regulation of floral induction genes, signaling and metabolic pathways.

AtDGK2, a novel diacylglycerol kinase from Arabidopsis thaliana, phosphorylates 1-stearoyl-2-arachidonoyl-sn-glycerol and 1,2-dioleoyl-sn-glycerol and exhibits cold-inducible gene expression

Diacylglycerol kinase (DGK) phosphorylates diacylglycerol (DAG) to generate phosphatidic acid (PA). Both DAG and PA are implicated in signal transduction pathways. DGKs have been widely studied in animals, but their analysis in plants is fragmentary. Here, we report the cloning and biochemical characterization of AtDGK2, encoding DGK from Arabidopsis thaliana. AtDGK2 has a predicted molecular mass of 79.4 kDa and, like AtDGK1 previously reported, harbors two copies of a phorbol ester/DAG-binding domain in its N-terminal region. AtDGK2 belongs to a family of seven DGK genes in A. thaliana. AtDGK3 to AtDGK7 encode approximately 55-kDa DGKs that lack a typical phorbol ester/DAG-binding domain. Phylogenetically, plant DGKs fall into three clusters. Members of all three clusters are widely expressed in vascular plants. Recombinant AtDGK2 was expressed in Escherichia coli and biochemically characterized. The enzyme phosphorylated 1,2-dioleoyl-sn-glycerol to yield PA, exhibiting Michaelis-Menten type kinetics. Estimated K(m) and V(max) values were 125 microm for DAG and 0.25 pmol of PA min(-1) microg(-1), respectively. The enzyme was maximally active at pH 7.2. Its activity was Mg(2+)-dependent and affected by the presence of detergents, salts, and the DGK inhibitor R59022, but not by Ca(2+). AtDGK2 exhibited substrate preference for unsaturated DAG analogues (i.e. 1-stearoyl-2-arachidonoyl-sn-glycerol and 1,2-dioleoyl-sn-glycerol). The AtDGK2 gene is expressed in various tissues of the Arabidopsis plant, including leaves, roots, and flowers, as shown by Northern blot analysis and promoter-reporter gene fusions. We found that AtDGK2 is induced by exposure to low temperature (4 degrees C), pointing to a role in cold signal transduction.

Localization of a highly active pool of type II phosphatidylinositol 4-kinase in a p97/valosin-containing-protein-rich fraction of the endoplasmic reticulum

Different phosphoinositides are synthesized in cell membranes in order to perform a variety of functions. One of the most abundant of these lipids is phosphatidylinositol (PI) 4-phosphate (PI4P), which is formed in human eukaryotes by type II and type III phosphatidylinositol 4-kinase (PI4K II and III) activities. PI4K II activity occurs in many different subcellular membranes, although no detailed analysis of the distribution of this activity has been reported. Using density gradient ultracentrifugation, we have previously found that in A431 cells the predominant PI4K activity arises from a type II alpha enzyme that is localized to a buoyant membrane fraction of unknown origin [Waugh, Lawson, Tan and Hsuan (1998) J. Biol. Chem. 273, 17115-17121]. We show here that these buoyant membranes contain an activated form of PI4K II alpha that can be separated from the bulk of the PI4K II alpha protein in A431 and COS-7 cells. Proteomic analysis revealed that the buoyant membrane fraction contains numerous endoplasmic reticulum (ER)-marker proteins, although it was separated from the bulk of the ER, ER-Golgi intermediate compartment, transitional ER, Golgi and other major subcellular membranes. Furthermore, the majority of the cytoplasmic valosin-containing protein (VCP), an AAA+ATPase implicated in various ER, transitional ER, Golgi and nuclear functions, was almost completely localized to the same buoyant membrane fraction. Co-localization of VCP and PI4K activity was confirmed by co-immunoprecipitation. These results suggest the previously unsuspected existence of an ER-related domain in which the bulk of the cellular PI4P synthesis and VCP are localized.

Differential regulation of two Arabidopsis type III phosphatidylinositol 4-kinase isoforms. A regulatory role for the pleckstrin homology domain

Here, we compare the regulation and localization of the Arabidopsis type III phosphatidylinositol (PtdIns) 4-kinases, AtPI4Kalpha1 and AtPI4Kbeta1, in Spodoptera frugiperda (Sf9) insect cells. We also explore the role of the pleckstrin homology (PH) domain in regulating AtPI4Kalpha1. Recombinant kinase activity was found to be differentially sensitive to PtdIns-4-phosphate (PtdIns4P), the product of the reaction. The specific activity of AtPI4Kalpha1 was inhibited 70% by 0.5 mm PtdIns4P. The effect of PtdIns4P was not simply due to charge because AtPI4Kalpha1 activity was stimulated approximately 50% by equal concentrations of the other negatively charged lipids, PtdIns3P, phosphatidic acid, and phosphatidyl-serine. Furthermore, inhibition of AtPI4Kalpha1 by PtdIns4P could be alleviated by adding recombinant AtPI4Kalpha1 PH domain, which selectively binds to PtdIns4P (Stevenson et al., 1998). In contrast, the specific activity of AtPI4Kbeta1, which does not have a PH domain, was stimulated 2-fold by PtdIns4P but not other negatively charged lipids. Visualization of green fluorescent protein fusion proteins in insect cells revealed that AtPI4Kalpha1 was associated primarily with membranes in the perinuclear region, whereas AtPI4Kbeta1 was in the cytosol and associated with small vesicles throughout the cytoplasm. Expression of AtPI4Kalpha1 without the PH domain in the insect cells compromised PtdIns 4-kinase activity and caused mislocalization of the kinase. The green fluorescent protein-PH domain alone was associated with intracellular membranes and the plasma membrane. In vitro, the PH domain appeared to be necessary for association of AtPI4Kalpha1 with fine actin filaments. These studies support the idea that the Arabidopsis type III PtdIns 4-kinases are responsible for distinct phosphoinositide pools.

The SAC domain-containing protein gene family in Arabidopsis

The SAC domain was first identified in the yeast (Saccharomyces cerevisiae) Sac1p phosphoinositide phosphatase protein and subsequently found in a number of proteins from yeast and animals. The SAC domain is approximately 400 amino acids in length and is characterized by seven conserved motifs. The SAC domains of several proteins have been recently demonstrated to possess phosphoinositide phosphatase activities. Sac1p has been shown to regulate the levels of various phosphoinositides in the phosphoinositide pool and affect diverse cellular functions such as actin cytoskeleton organization, Golgi function, and maintenance of vacuole morphology. The Arabidopsis genome contains a total of nine genes encoding SAC domain-containing proteins (AtSACs). The SAC domains of the AtSACs possess the conserved amino acid motifs that are believed to be important for the phosphoinositide phosphatase activities of yeast and animal SAC domain proteins. AtSACs can be divided into three subgroups based on their sequence similarities, hydropathy profiles, and phylogenetic relationship. Gene expression analysis demonstrated that the AtSAC genes exhibited differential expression patterns in different organs and, in particular, the AtSAC6 gene was predominantly expressed in flowers. Moreover, the expression of the AtSAC6 gene was highly induced by salinity. These results provide a foundation for future studies on the elucidation of the cellular functions of SAC domain-containing proteins in Arabidopsis.

Three SAC1-like genes show overlapping patterns of expression in Arabidopsis but are remarkably silent during embryo development

In Saccharomyces cerevisiae, the SAC1 gene encodes a polyphosphoinositide phosphatase (PPIPase) that modulates the levels of phosphoinositides, which are key regulators of a number of signal transduction processes. SAC1p has been implicated in multiple cellular functions: actin cytoskeleton organization, secretory functions, inositol metabolism, ATP transport, and multiple-drug sensitivity. Here, we describe the characterization of three genes in Arabidopsis thaliana, AtSAC1a, AtSAC1b, and AtSAC1c, encoding proteins similar to those of yeast SAC1p. We demonstrated that the three AtSAC1 proteins are functional homologs of the yeast SAC1p because they can rescue the cold-sensitive and inositol auxotroph yeast sac1-null mutant strain. The fact that Arabidopsis and yeast SAC1 genes derived from a common ancestor suggests that this plant multigenic family is involved in the phosphoinositide pathway and in a range of cellular functions similar to those in yeast. Using GFP fusion experiments, we demonstrate that the three AtSAC1 proteins are targeted to the endoplasmic reticulum. Their expression patterns are overlapping, with at least two members expressed in each organ. Remarkably, AtSAC1 genes are not expressed during seed development, and therefore additional phosphatases are required to control phosphoinositide levels in seeds.

Phospholipase C is required for the control of stomatal aperture by ABA

The calcium-releasing second messenger inositol 1,4,5-trisphosphate is involved in the regulation of stomatal aperture by ABA. In other signalling pathways, inositol 1,4,5-trisphosphate is generated by the action of phospholipase C. We have studied the importance of phospholipase C in guard cell ABA-signalling pathways. Immunolocalisation of a calcium-activated phospholipase C confirmed the presence of phospholipase C in tobacco guard cells. Transgenic tobacco plants with considerably reduced levels of phospholipase C in their guard cells were only partially able to regulate their stomatal apertures in response to ABA. These results suggest that phospholipase C is involved in the amplification of the calcium signal responsible for reductions in stomatal aperture in response to ABA. As full ABA-induced inhibition of stomatal opening was not observed, our results support a role for the action of other calcium-releasing second messengers in the guard cell ABA-signalling pathway. It is not known whether these different calcium-releasing second messengers act in the same or parallel ABA-signalling pathways.

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.