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

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.

The essential role of anionic transport in plant cells: the pollen tube as a case study

Plasma membrane anion transporters play fundamental roles in plant cell biology, especially in stomatal closure and nutrition. Notwithstanding, a lot is still unknown about the specific function of these transporters, their specific localization, or molecular nature. Here the fundamental roles of anionic transport in plant cells are reviewed. Special attention will be paid to them in the control of pollen tube growth. Pollen tubes are extreme examples of cellular polarity as they grow exclusively in their apical extremity. Their unique cell biology has been extensively exploited for fundamental understanding of cellular growth and morphogenesis. Non-invasive methods have demonstrated that tube growth is governed by different ion fluxes, with different properties and distribution. Not much is known about the nature of the membrane transporters responsible for anionic transport and their regulation in the pollen tube. Recent data indicate the importance of chloride (Cl(-)) transfer across the plasma membrane for pollen germination and pollen tube growth. A general overview is presented of the well-known accumulated data in terms of biophysical and functional characterization, transcriptomics, and genomic description of pollen ionic transport, and the various controversies around the role of anionic fluxes during pollen tube germination, growth, and development. It is concluded that, like all other plant cells so far analysed, pollen tubes depend on anion fluxes for a number of fundamental homeostatic properties.

Molecular, cellular, and physiological responses to phosphatidic acid formation in plants

Phosphatidic acid (PA) is an essential phospholipid involved in membrane biosynthesis and signal transduction in all eukaryotes. This review focuses on its role as lipid second messenger during plant stress, metabolism, and development. The contribution of different individual isoforms of enzymes that generate and break down PA will be discussed and the downstream responses highlighted, with particular focus on proteins that bind PA. Through characterization of several of these PA targets, a molecular and genetic basis for PA's role in plant stress and development is emerging.

Osmoregulation in Lilium pollen grains occurs via modulation of the plasma membrane H+ ATPase activity by 14-3-3 proteins

To allow successful germination and growth of a pollen tube, mature and dehydrated pollen grains (PGs) take up water and have to adjust their turgor pressure according to the water potential of the surrounding stigma surface. The turgor pressure of PGs of lily (Lilium longiflorum) was measured with a modified pressure probe for simultaneous recordings of turgor pressure and membrane potential to investigate the relation between water and electrogenic ion transport in osmoregulation. Upon hyperosmolar shock, the turgor pressure decreased, and the plasma membrane (PM) hyperpolarizes in parallel, whereas depolarization of the PM was observed with hypoosmolar treatment. An acidification and alkalinization of the external medium was monitored after hyper- and hypoosmotic treatments, respectively, and pH changes were blocked by vanadate, indicating a putative role of the PM H(+) ATPase. Indeed, an increase in PM-associated 14-3-3 proteins and an increase in PM H(+) ATPase activity were detected in PGs challenged by hyperosmolar medium. We therefore suggest that in PGs the PM H(+) ATPase via modulation of its activity by 14-3-3 proteins is involved in the regulation of turgor pressure.

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.

Nonspecific phospholipase C NPC4 promotes responses to abscisic acid and tolerance to hyperosmotic stress in Arabidopsis

Diacyglycerol (DAG) is an important class of cellular lipid messengers, but its function in plants remains elusive. Here, we show that knockout of the Arabidopsis thaliana nonspecific phospholipase C (NPC4) results in a decrease in DAG levels and compromises plant response to abscisic acid (ABA) and hyperosmotic stresses. NPC4 hydrolyzes various phospholipids in a calcium-independent manner, producing DAG and a phosphorylated head group. NPC4 knockout (KO) plants display decreased ABA sensitivity in seed germination, root elongation, and stomatal movement and had decreased tolerance to high salinity and water deficiency. Overexpression of NPC4 renders plants more sensitive to ABA and more tolerant to hyperosmotic stress than wild-type plants. Addition of a short-chain DAG or a short-chain phosphatidic acid (PA) restores the ABA response of NPC4-KO to that of the wild type, but the addition of DAG together with a DAG kinase inhibitor does not result in a wild-type phenotype. These data suggest that NPC4-produced DAG is converted to PA and that NPC4 and its derived lipids positively modulate ABA response and promote plant tolerance to drought and salt stresses.

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.

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.

Heat stress activates phospholipase D and triggers PIP accumulation at the plasma membrane and nucleus

Heat stress induces an array of physiological adjustments that facilitate continued homeostasis and survival during periods of elevated temperatures. Here, we report that within minutes of a sudden temperature increase, plants deploy specific phospholipids to specific intracellular locations: phospholipase D (PLD) and a phosphatidylinositolphosphate kinase (PIPK) are activated, and phosphatidic acid (PA) and phosphatidylinositol 4,5-bisphosphate (PIP(2)) rapidly accumulate, with the heat-induced PIP(2) localized to the plasma membrane, nuclear envelope, nucleolus and punctate cytoplasmic structures. Increases in the steady-state levels of PA and PIP(2) occur within several minutes of temperature increases from ambient levels of 20-25 degrees C to 35 degrees C and above. Similar patterns were observed in heat-stressed Arabidopsis seedlings and rice leaves. The PA that accumulates in response to temperature increases results in large part from the activation of PLD rather than the sequential action of phospholipase C and diacylglycerol kinase, the alternative pathway used to produce this lipid. Pulse-labelling analysis revealed that the PIP(2) response is due to the activation of a PIPK rather than inhibition of a lipase or a PIP(2) phosphatase. Inhibitor experiments suggest that the PIP(2) response requires signalling through a G-protein, as aluminium fluoride blocks heat-induced PIP(2) increases. These results are discussed in the context of the diverse cellular roles played by PIP(2) and PA, including regulation of ion channels and the cytoskeleton.

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.

Uncovering hidden treasures in pollen tube growth mechanics

The long-standing model of tip growth in pollen tubes considers that exocytosis and growth occur at the apex and that the pool of very small vesicles in the apical dome contains secretory (exocytic) vesicles. However, recent work on vesicle trafficking dynamics in tobacco pollen tubes shows that exocytosis occurs in the subapical region. Taking these and other new results into account, we set out to resolve specific problems that are endemic in current models and present a two-part ACE (apical cap extension)-H (hydrodynamics) growth model. The ACE model involves delivery and recycling of materials required for new cell synthesis and the H model involves mechanisms that integrate and regulate key cellular pathways and drive cell elongation during growth.

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.

Comparative proteomic analysis of Arabidopsis mature pollen and germinated pollen

Proteomic analysis was applied to generating the map of Arabidopsis mature pollen proteins and analyzing the differentially expressed proteins that are potentially involved in the regulation of Arabidopsis pollen germination. By applying 2-D electrophoresis and silver staining, we resolved 499 and 494 protein spots from protein samples extracted from pollen grains and pollen tubes, respectively. Using the matrix-assisted laser desorption ionization time-of-flight mass spectrometry method, we identified 189 distinct proteins from 213 protein spots expressed in mature pollen or pollen tubes, and 75 new identified proteins that had not been reported before in research into the Arabidopsis pollen proteome. Comparative analysis revealed that 40 protein spots exhibit reproducible significant changes between mature pollen and pollen tubes. And 21 proteins from 17 downregulated and six upregulated protein spots were identified. Functional category analysis indicated that these differentially expressed proteins mainly involved in signaling, cellular structure, transport, defense/stress responses, transcription, metabolism, and energy production. The patterns of changes at protein level suggested the important roles for energy metabolism-related proteins in pollen tube growth, accompanied by the activation of the stress response pathway and modifications to the cell wall.

A pollen protein, NaPCCP, that binds pistil arabinogalactan proteins also binds phosphatidylinositol 3-phosphate and associates with the pollen tube endomembrane system

As pollen tubes grow toward the ovary, they are in constant contact with the pistil extracellular matrix (ECM). ECM components are taken up during growth, and some pistil molecules exert their effect inside the pollen tube. For instance, the Nicotiana alata 120-kD glycoprotein (120K) is an abundant arabinogalactan protein that is taken up from the ECM; it has been detected in association with pollen tube vacuoles, but the transport pathway between these compartments is unknown. We recently identified a pollen C2 domain-containing protein (NaPCCP) that binds to the carboxyl-terminal domain of 120K. As C2 domain proteins mediate protein-lipid interactions, NaPCCP could function in intracellular transport of 120K in pollen tubes. Here, we describe binding studies showing that the NaPCCP C2 domain is functional and that binding is specific for phosphatidylinositol 3-phosphate. Subcellular fractionation, immunolocalization, and live imaging results show that NaPCCP is associated with the plasma membrane and internal pollen tube vesicles. Colocalization between an NaPCCPgreen fluorescent protein fusion and internalized FM4-64 suggest an association with the endosomal system. NaPCCP localization is altered in pollen tubes rejected by the self-incompatibility mechanism, but our hypothesis is that it has a general function in the transport of endocytic cargo rather than a specific function in self-incompatibility. NaPCCP represents a bifunctional protein with both phosphatidylinositol 3-phosphate- and arabinogalactan protein-binding domains. Therefore, it could function in the transport of pistil ECM proteins in the pollen tube endomembrane system.

Still life: Pollen tube growth observed in millisecond resolution

Our recent work used novel methods to localize and track discrete vesicle populations in pollen tubes undergoing oscillatory growth. The results show that clathrin-dependent endocytosis occurs along the shank of the pollen tube, smooth vesicle endocytosis occurs at the tip, and exocytosis occurs in the subapical region. Here, growth of tobacco and lily pollen tubes is examined in greater temporal resolution using refraction-free high-resolution time-lapse differential interference contrast microscopy. Images were collected at 0.21 s intervals for 10 min, sequentially examined for millisecond details, compressed into video format and then examined for details of growth dynamics. The subapical growth zone is structurally fluid, with vesicle insertion into the plasma membrane, construction of new cell surface and cellular expansion. Incorporation of new membrane and wall materials causes localized disruption at the cell surface that precedes the start of the growth cycle by 3.44 +/- 0.39 s in tobacco, and 1.02 +/- 0.01 s in lily pollen tubes. Vesicle deposition increases after the start of the growth cycle and supports expansion of the growth zone. Growth reorientation involves a shift in the position and angle of the growth zone. In summary, these results support a new model of pollen tube growth.

Genome-wide identification, classification, evolutionary expansion and expression analyses of homeobox genes in rice

Homeobox genes play a critical role in regulating various aspects of plant growth and development. In the present study, we identified a total of 107 homeobox genes in the rice genome and grouped them into ten distinct subfamilies based upon their domain composition and phylogenetic analysis. A significantly large number of homeobox genes are located in the duplicated segments of the rice genome, which suggests that the expansion of homeobox gene family, in large part, might have occurred due to segmental duplications in rice. Furthermore, microarray analysis was performed to elucidate the expression profiles of these genes in different tissues and during various stages of vegetative and reproductive development. Several genes with predominant expression during various stages of panicle and seed development were identified. At least 37 homeobox genes were found to be differentially expressed significantly (more than two-fold; P < 0.05) under various abiotic stress conditions. The results of the study suggest a critical role of homeobox genes in reproductive development and abiotic stress signaling in rice, and will facilitate the selection of candidate genes of agronomic importance for functional validation.

Enhanced expression of phospholipase C 1 (ZmPLC1) improves drought tolerance in transgenic maize

Phosphatidylinositol-specific phospholipase C (PI-PLC) plays an important role in a variety of physiological processes in plants, including drought tolerance. It has been reported that the ZmPLC1 gene cloned from maize (Zea mays L.) encoded a PI-PLC and up-regulated the expression in maize roots under dehydration conditions (Zhai SM, Sui ZH, Yang AF, Zhang JR in Biotechnol Lett 27:799-804, 2005). In this paper, transgenic maize expressing ZmPLC1 transgenes in sense or antisense orientation were generated by Agrobacterium-mediated transformation and confirmed by Southern blot analysis. High-level expression of the transgene was confirmed by real-time RT-PCR and PI-PLC activity assay. The tolerance to drought stress (DS) of the homogenous transgenic maize plants was investigated at two developmental stages. The results demonstrated that, under DS conditions, the sense transgenic plants had higher relative water content, better osmotic adjustment, increased photosynthesis rates, lower percentage of ion leakage and less lipid membrane peroxidation, higher grain yield than the WT; whereas those expressing the antisense transgene exhibited inferior characters compared with the WT. It was concluded that enhanced expression of sense ZmPLC1 improved the drought tolerance of maize.

Constitutive expression of a meiotic recombination protein gene homolog, OsTOP6A1, from rice confers abiotic stress tolerance in transgenic Arabidopsis plants

Plant productivity is greatly influenced by various environmental stresses, such as high salinity and drought. Earlier, we reported the isolation of topoisomerase 6 homologs from rice and showed that over expression of OsTOP6A3 and OsTOP6B confers abiotic stress tolerance in transgenic Arabidopsis plants. In this study, we have assessed the function of nuclear-localized topoisomerase 6 subunit A homolog, OsTOP6A1, in transgenic Arabidopsis plants. The over expression of OsTOP6A1 in transgenic Arabidopsis plants driven by cauliflower mosaic virus-35S promoter resulted in pleiotropic effects on plant growth and development. The transgenic Arabidopsis plants showed reduced sensitivity to stress hormone, abscisic acid (ABA), and tolerance to high salinity and dehydration at the seed germination; seedling and adult stages as reflected by the percentage of germination, fresh weight of seedlings and leaf senescence assay, respectively. Concomitantly, the expression of many stress-responsive genes was enhanced under various stress conditions in transgenic Arabidopsis plants. Moreover, microarray analysis revealed that the expression of a large number of genes involved in various processes of plant growth and development and stress responses was altered in transgenic plants. Although AtSPO11-1, the homolog of OsTOP6A1 in Arabidopsis, has been implicated in meiotic recombination; the present study demonstrates possible additional role of OsTOP6A1 and provides an effective tool for engineering crop plants for tolerance to different environmental stresses.

Vesicle trafficking dynamics and visualization of zones of exocytosis and endocytosis in tobacco pollen tubes

Pollen tubes are one of the fastest growing eukaryotic cells. Rapid anisotropic growth is supported by highly active exocytosis and endocytosis at the plasma membrane, but the subcellular localization of these sites is unknown. To understand molecular processes involved in pollen tube growth, it is crucial to identify the sites of vesicle localization and trafficking. This report presents novel strategies to identify exocytic and endocytic vesicles and to visualize vesicle trafficking dynamics, using pulse-chase labelling with styryl FM dyes and refraction-free high-resolution time-lapse differential interference contrast microscopy. These experiments reveal that the apex is the site of endocytosis and membrane retrieval, while exocytosis occurs in the zone adjacent to the apical dome. Larger vesicles are internalized along the distal pollen tube. Discretely sized vesicles that differentially incorporate FM dyes accumulate in the apical, subapical, and distal regions. Previous work established that pollen tube growth is strongly correlated with hydrodynamic flux and cell volume status. In this report, it is shown that hydrodynamic flux can selectively increase exocytosis or endocytosis. Hypotonic treatment and cell swelling stimulated exocytosis and attenuated endocytosis, while hypertonic treatment and cell shrinking stimulated endocytosis and inhibited exocytosis. Manipulation of pollen tube apical volume and membrane remodelling enabled fine-mapping of plasma membrane dynamics and defined the boundary of the growth zone, which results from the orchestrated action of endocytosis at the apex and along the distal tube and exocytosis in the subapical region. This report provides crucial spatial and temporal details of vesicle trafficking and anisotropic growth.

Phospholipase C signaling involvement in macrotubule assembly and activation of the mechanism regulating protoplast volume in plasmolyzed root cells of Triticum turgidum

The role of phosphoinositide-specific phospholipase C (PI-PLC) signaling in the macrotubule-dependent protoplast volume regulation in plasmolyzed root cells of Triticum turgidum was investigated. At the onset of hyperosmotic stress, PI-PLC activation was documented. Inhibition of PI-PLC activity by U73122 blocked tubulin macrotubule formation in plasmolyzed cells and their protoplast volume regulatory mechanism. In neomycin-treated plasmolyzed cells, macrotubule formation and protoplast volume regulation were not affected. In these cells the PI-PLC pathway is down-regulated as neomycin sequesters the PI-PLC substrate, 4,5-diphosphate-phosphatidyl inositol (PtdInsP(2)). These phenomena were unaffected by R59022, an inhibitor of phosphatidic acic (PA) production via the PLC pathway. Taxol, a microtubule (MT) stabilizer, inhibited the hyperosmotic activation of PI-PLC, but oryzalin, which disorganized MTs, triggered PI-PLC activity. Taxol prevented macrotubule formation and inhibited the mechanism regulating the volume of the plasmolyzed protoplast. Neomycin partly relieved some of the taxol effects. These data suggest that PtdInspP(2) turnover via PI-PLC assists macrotubule formation and activation of the mechanism regulating the plasmolyzed protoplast volume; and the massive disorganization of MTs that is carried out at the onset of hyperosmotic treatment triggers the activation of this mechanism.

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

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

Cloning of phospholipase D from grape berry and its expression under heat acclimation

To investigate whether phospholipase D (PLD, EC 3.1.4.4) plays a role in adaptive response of post-harvest fruit to environment, a PLD gene was firstly cloned from grape berry (Vitis Vinifera L. cv. Chardonnay) using RT-PCR and 3'- and 5'-RACE. The deduced amino acid sequence (809 residues) showed 84.7% identity with that of PLD from Ricinus communis. The secondary structures of this protein showed the characteristic C2 domain and two active sites of a phospholipid-metabolizing enzyme. The PLD activity and its expression in response to heat acclimation were then assayed. The results indicated PLD was significantly activated at enzyme activity, as well as accumulation of PLD mRNA and synthesis of new PLD protein during the early of heat acclimation, primary suggesting that the grape berry PLD may be involved in the heat response in post-harvest grape berry. This work offers an important basis for further investigating the mechanism of post-harvest fruit adaptation to environmental stresses.

Deciphering the Possible Mechanism of GABA in Tobacco Pollen Tube Growth and Guidance

gamma-Aminobutyric acid (GABA) is an inhibitory transmitter in animal central and peripheral nervous systems, and also plays an important role in pollen tube growth and guidance. However, the mechanisms underlying these effects in plants are poorly understood, mainly because the GABA receptor in plants has not been elucidated. To address this issue, we recently created quantum dot probes to identify possible GABA receptors on the membrane surfaces of pollen protoplasts. We found that GABA bound to cell membranes and regulated downstream Ca(2+) oscillation in the cells. These results provide important clues to further specifying the nature of the binding sites and deciphering the role of GABA as a signal molecule in pollen tube growth and orientation.

Cell content of phosphatidylinositol (4,5)bisphosphate in Ehrlich mouse ascites tumour cells in response to cell volume perturbations in anisotonic and in isosmotic media

The labelling pattern of cellular phosphoinositides (PtdInsP(n)) was studied in Ehrlich ascites cells labelled in vivo for 24 h with myo-[2-(3)H]- or l-myo-[1-(3)H]inositol and exposed to anisotonic or isosmotic volume perturbations. In parallel experiments the cell volume ([(14)C]3-OMG space) was monitored. In hypotonic media the cells initially swelled osmotically and subsequently as expected showed a regulatory volume decrease (RVD) response. Concurrently, the cell content of PtdInsP(2) showed a marked, transient decrease and the content of PtdInsP a small, transient increase. The changes in PtdInsP(2) and PtdInsP content increased progressively with the extent of hypotonicity (in the range 1.00-0.50 relative osmolarity). No evidence was found for either hydrolysis of PtdInsP(2) or formation of PtdInsP(3). In hypertonic medium (relative osmolarity 1.50), cells initially shrank osmotically and subsequently as expected showed a small regulatory volume increase (RVI) response. Concurrently, the cell content of PtdInsP(2) showed a marked increase and the content of PtdInsP a small decrease, i.e. changes in the opposite direction of those seen in hypotonic media. In isosmotic media with high (100 mm) or low (0.8 mm) K(+) concentration, cells slowly swelled or shrank due to uptake or loss of isosmotic KCl. Under these conditions, with largely unchanged intracellular ionic strength, the cell content of PtdInsP(2) and PtdInsP remained constant. Our results show that PtdInsP(2) is not volume sensitive per se, and moreover that the regulatory volume adjustments in Ehrlich ascites cells are not mediated by PtdInsP(2) hydrolysis and its subsequent production of second messengers. The simplest interpretation of the observed effects would be that PtdInsP(2) is controlled by ionic strength, probably via activation/inhibition of phosphoinositide-specific phosphatases/kinases. In Ehrlich ascites cells, as shown previously, the opposing ion channels and transporters activated during RVD and RVI, respectively, are controlled with tight negative coordination by a common cell volume 'set-point' that is shifted in anisotonic media, but unchanged during cell swelling in isosmotic high K(+) medium. We hypothesize that PtdInsP(2) might orchestrate this 'set-point' shift.

F-box proteins in rice. Genome-wide analysis, classification, temporal and spatial gene expression during panicle and seed development, and regulation by light and abiotic stress

F-box proteins constitute a large family in eukaryotes and are characterized by a conserved F-box motif (approximately 40 amino acids). As components of the Skp1p-cullin-F-box complex, F-box proteins are critical for the controlled degradation of cellular proteins. We have identified 687 potential F-box proteins in rice (Oryza sativa), the model monocotyledonous plant, by a reiterative database search. Computational analysis revealed the presence of several other functional domains, including leucine-rich repeats, kelch repeats, F-box associated domain, domain of unknown function, and tubby domain in F-box proteins. Based upon their domain composition, they have been classified into 10 subfamilies. Several putative novel conserved motifs have been identified in F-box proteins, which do not contain any other known functional domain. An analysis of a complete set of F-box proteins in rice is presented, including classification, chromosomal location, conserved motifs, and phylogenetic relationship. It appears that the expansion of F-box family in rice, in large part, might have occurred due to localized gene duplications. Furthermore, comprehensive digital expression analysis of F-box protein-encoding genes has been complemented with microarray analysis. The results reveal specific and/or overlapping expression of rice F-box protein-encoding genes during floral transition as well as panicle and seed development. At least 43 F-box protein-encoding genes have been found to be differentially expressed in rice seedlings subjected to different abiotic stress conditions. The expression of several F-box protein-encoding genes is also influenced by light. The structure and function of F-box proteins in plants is discussed in light of these results and the published information. These data will be useful for prioritization of F-box proteins for functional validation in rice.

Life under pressure: hydrostatic pressure in cell growth and function

H(2)O is one of the most essential molecules for cellular life. Cell volume, osmolality and hydrostatic pressure are tightly controlled by multiple signaling cascades and they drive crucial cellular functions ranging from exocytosis and growth to apoptosis. Ion fluxes and cell shape restructuring induce asymmetries in osmotic potential across the plasma membrane and lead to localized hydrodynamic flow. Cells have evolved fascinating strategies to harness the potential of hydrodynamic flow to perform crucial functions. Plants exploit hydrodynamics to drive processes including gas exchange, leaf positioning, nutrient acquisition and growth. This paradigm is extended by recent work that reveals an important role for hydrodynamics in pollen tube growth.

A proteomics approach for identifying osmotic-stress-related proteins in rice

Osmotic stress can endanger the survival of plants. To investigate the mechanisms of how plants respond to osmotic stress, rice protein profiles from mannitol-treated plants, were monitored using a proteomics approach. Two-week-old rice seedlings were treated with 400mM mannitol for 48h. After separation of proteins from the basal part of leaf sheaths by two-dimensional polyacrylamide gel electrophoresis, 327 proteins were detected. The levels of 12 proteins increased and the levels of three proteins decreased with increasing concentration or duration, of mannitol treatment. Levels of a heat shock protein and a dnaK-type molecular chaperone were reduced under osmotic, cold, salt and drought stresses, and ABA treatment, whereas a 26S proteasome regulatory subunit was found to be responsive only to osmotic stress. Furthermore, proteins whose accumulation was sensitive to osmotic stress are present in an osmotic-tolerant cultivar. These results indicate that specific proteins expressed in the basal part of rice leaf sheaths show a coordinated response to cope with osmotic stress.

Conserved features of germination and polarized cell growth: a few insights from a pollen-fern spore comparison

BACKGROUND

The germination of both pollen and fern spores results in the emergence of a cell-pollen tube from pollen, rhizoid from spore-that grows in a polar fashion, primarily at its apical end. In both of these tip-growing cells, the delivery of secretory vesicles to the growing end is guided in part by a calcium gradient, with calcium entering at the tip where it is most highly concentrated. The similarities between the two systems extend beyond tip-focused calcium gradients to encompass signalling pathways and elements including calmodulin, nitric oxide, annexins and Rop-GTPases.

SCOPE AND AIMS

This review is limited to those pathways and elements that function similarly in fern and pollen systems based on currently available evidence. The aim is to illustrate the common mechanisms by which tip growth occurs, facilitate further investigations into this area, and examine the implications for the evolutionarily conserved control of tip growth.

CONCLUSIONS

The interplay of calcium, nitric oxide and other effectors in both pollen and fern spores suggests certain signalling pathways became important regulators of germination and growth early in the evolution of land plants. Both large- and small-scale comparative genomic methods have shown to be promising in their ability to find new and relevant comparisons for further research. Cross-species comparisons may serve to speed up this process by highlighting both basic pathways and system-specific deviations.

Aluminum inhibits phosphatidic acid formation by blocking the phospholipase C pathway

Aluminum (Al(3+)) has been recognized as a main toxic factor in crop production in acid lands. Phosphatidic acid (PA) is emerging as an important lipid signaling molecule and has been implicated in various stress-signaling pathways in plants. In this paper, we focus on how PA generation is affected by Al(3+) using Coffea arabica suspension cells. We pre-labeled cells with [(32)P]orthophosphate ((32)Pi) and assayed for (32)P-PA formation in response to Al(3+). Treating cells for 15 min with either AlCl(3) or Al(NO(3))(3) inhibited the formation of PA. In order to test how Al(3+) affected PA signaling, we used the peptide mastoparan-7 (mas-7), which is known as a very potent stimulator of PA formation. The Al(3+) inhibited mas-7 induction of PA response, both before and after Al(3+) incubation. The PA involved in signaling is generated by two distinct phospholipid signaling pathways, via phospholipase D (PLD; EC: 3.1.4.4) or via Phospholipase C (PLC; EC: 3.1.4.3), and diacylglycerol kinase (DGK; EC 2.7.1.107). By labeling with (32)Pi for short periods of time, we found that PA formation was inhibited almost 30% when the cells were incubated with AlCl(3) suggesting the involvement of the PLC/DGK pathway. Incubation of cells with PLC inhibitor, U73122, affected PA formation, like AlCl(3) did. PLD in vivo activation by mas-7 was reduced by Al(3+). These results suggest that PA formation was prevented through the inhibition of the PLC activity, and it provides the first evidence for the role of Al toxicity on PA production.

Phosphoinositide 5-phosphatase Fig 4p is required for both acute rise and subsequent fall in stress-induced phosphatidylinositol 3,5-bisphosphate levels

Phosphoinositide lipids regulate complex events via the recruitment of proteins to a specialized region of the membrane at a specific time. Precise control of both the synthesis and turnover of phosphoinositide lipids is integral to membrane trafficking, signal transduction, and cytoskeletal rearrangements. Little is known about the acute regulation of the levels of these signaling lipids. When Saccharomyces cerevisiae cells are treated with hyperosmotic medium the levels of phosphatidylinositol 3,5-bisphosphate (PI3,5P(2)) increase 20-fold. Here we show that this 20-fold increase is rapid and occurs within 5 min. Surprisingly, these elevated levels are transient. Fifteen minutes following hyperosmotic shock they decrease at a rapid rate, even though the cells remain in hyperosmotic medium. In parallel with the rapid increase in the levels of PI3,5P(2), vacuole volume decreases rapidly. Furthermore, concomitant with a return to basal levels of PI3,5P(2) vacuole volume is restored. We show that Fig 4p, consistent with its proposed role as a PI3,5P(2) 5-phosphatase, is required in vivo for this rapid return to basal levels of PI3,5P(2). Surprisingly, we find that Fig 4p is also required for the hyperosmotic shock-induced increase in PI3,5P(2) levels. These findings demonstrate that following hyperosmotic shock, large, transient changes occur in the levels of PI3,5P(2) and further suggest that Fig 4p is important in regulating both the acute rise and subsequent fall in PI3,5P(2) levels.

Modulation of hepatocellular swelling-activated K+currents by phosphoinositide pathway-dependent protein kinase C

K+channels participate in the regulatory volume decrease (RVD) accompanying hepatocellular nutrient uptake and bile formation. We recently identified KCNQ1 as a molecular candidate for a significant fraction of the hepatocellular swelling-activated K+current ( IKVol). We have shown that the KCNQ1 inhibitor chromanol 293B significantly inhibited RVD-associated K+flux in isolated perfused rat liver and used patch-clamp techniques to define the signaling pathway linking swelling to IKVolactivation. Patch-electrode dialysis of hepatocytes with solutions that maintain or increase phosphatidylinositol 4,5-bisphosphate (PIP2) increased IKVol, whereas conditions that decrease cellular PIP2decreased IKVol. GTP and AlF4−stimulated IKVoldevelopment, suggesting a role for G proteins and phospholipase C (PLC). Supporting this, the PLC blocker U-73122 decreased IKVoland inhibited the stimulatory response to PIP2or GTP. Protein kinase C (PKC) is involved, because K+current was enhanced by 1-oleoyl-2-acetyl- sn-glycerol and inhibited after chronic PKC stimulation with phorbol 12-myristate 13-acetate (PMA) or the PKC inhibitor GF 109203X. Both IKVoland the accompanying membrane capacitance increase were blocked by cytochalasin D or GF 109203X. Acute PMA did not eliminate the cytochalasin D inhibition, suggesting that PKC-mediated IKVolactivation involves the cytoskeleton. Under isotonic conditions, a slowly developing K+current similar to IKVolwas activated by PIP2, lipid phosphatase inhibitors to counter PIP2depletion, a PLC-coupled α1-adrenoceptor agonist, or PKC activators and was depressed by PKC inhibition, suggesting that hypotonicity is one of a set of stimuli that can activate IKVolthrough a PIP2/PKC-dependent pathway. The results indicate that PIP2indirectly activates hepatocellular KCNQ1-like channels via cytoskeletal rearrangement involving PKC activation.

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.

A system for studying the effect of mechanical stress on the elongation behavior of immobilized plant cells

The ability to apply controllable mechanical compressive force is essential for the study of plant cells responses to environmental stimulations. The work presented here aims towards establishing a system, which consists of a fabricated apparatus (including a loading unit, displacement sensor, data collector and processor, and a feedback control) and a protocol for test specimen preparation and force loading. By using a force-feedback control circuit coupled to a microchip, delivering the pre-defined and actual controlled stimulus is achieved. To calibrate the apparatus, the corresponding voltages are compared to the known weights. A linear regression is fit to the experimental data and a standardized coefficient of 0.998 is calculated. The morphological changes in response to mechanical stresses were investigated in agarose gel embedded chrysanthemum protoplasts, which tended to be elongated with a preferential axis oriented perpendicularly to the compressive stress direction. The results also indicated that there existed a certain dose-dependent relationship between the intensity of compressive force and the stress-induced responses. Additionally, the elongation response with preferential orientation was inhibited by application of RGD peptides, and its inverted sequence, DGR peptides failed to antagonize the effect of mechanical force on elongation performance.

Heterodimeric capping protein from Arabidopsis is regulated by phosphatidic acid

The cytoskeleton is a key regulator of morphogenesis, sexual reproduction, and cellular responses to extracellular stimuli. Changes in the cellular architecture are often assumed to require actin-binding proteins as stimulus-response modulators, because many of these proteins are regulated directly by binding to intracellular second messengers or signaling phospholipids. Phosphatidic acid (PA) is gaining widespread acceptance as a major, abundant phospholipid in plants that is required for pollen tube tip growth and mediates responses to osmotic stress, wounding, and phytohormones; however, the number of identified effectors of PA is rather limited. Here we demonstrate that exogenous PA application leads to significant increases in filamentous actin levels in Arabidopsis suspension cells and poppy pollen grains. To investigate further these lipid-induced changes in polymer levels, we analyzed the properties of a key regulator of actin filament polymerization, the heterodimeric capping protein from Arabidopsis thaliana (AtCP). AtCP binds to PA with a K(d) value of 17 muM and stoichiometry of approximately 1:2. It also binds well to PtdIns(4,5)P(2), but not to several other phosphoinositide or acidic phospholipids. The interaction with PA inhibited the actin-binding activity of CP. In the presence of PA, CP is unable to block the barbed or rapidly growing and shrinking end of actin filaments. Precapped filament barbed ends can also be uncapped by addition of PA, allowing rapid filament assembly from an actin monomer pool that is buffered with profilin. The findings support a model in which the inhibition of CP activity in cells by elevated PA results in the stimulation of actin polymerization from a large pool of profilin-actin. Such regulation may be important for the response of plant cells to extracellular stimuli as well as for the normal process of pollen tube tip growth.

The Vac14p-Fig4p complex acts independently of Vac7p and couples PI3,5P2 synthesis and turnover

Phosphoinositide-signaling lipids function in diverse cellular pathways. Dynamic changes in the levels of these signaling lipids regulate multiple processes. In particular, when Saccharomyces cerevisiae cells are exposed to hyperosmotic shock, PI3,5P2 (phosphatidylinositol [PI] 3,5-bisphosphate) levels transiently increase 20-fold. This causes the vacuole to undergo multiple acute changes. Control of PI3,5P2 levels occurs through regulation of both its synthesis and turnover. Synthesis is catalyzed by the PI3P 5-kinase Fab1p, and turnover is catalyzed by the PI3,5P2 5-phosphatase Fig4p. In this study, we show that two putative Fab1p activators, Vac7p and Vac14p, independently regulate Fab1p activity. Although Vac7p only regulates Fab1p, surprisingly, we find that Vac14 regulates both Fab1p and Fig4p. Moreover, Fig4p itself functions in both PI3,5P2 synthesis and turnover. In both the absence and presence of Vac7p, the Vac14p-Fig4p complex controls the hyperosmotic shock-induced increase in PI3,5P2 levels. These findings suggest that the dynamic changes in PI3,5P2 are controlled through a tight coupling of synthesis and turnover.

Macrotubule-dependent protoplast volume regulation in plasmolysed root-tip cells of Triticum turgidum: involvement of phospholipase D

The probable involvement of phospholipase D (PLD)/phosphatidic acid (PA) signalling in the hyperosmotic stress response of Triticum turgidum root cells was investigated by examining the effects of butanol-1, butanol-2, phosphatidylbutanol (PtdBut), N-acylethanolamine (NAE) and PA on the hyperosmotic response, the organization of the tubulin cytoskeleton and the accumulation of a phosphorylated p38-like mitogen-activated protein (MAP) kinase (phospho-p46) in plasmolysed root cells. The effects of all the treatments were assessed by differential interference contrast (DIC) microscopy of living cells, tubulin immunofluorescence, conventional transmission electron microscopy (TEM), tubulin immunogold localization, protoplast volume measurements and western blot analysis. Butanol-1 and NAE compromised the viability of plasmolysed cells, induced a marked reduction in the plasmolysed protoplast volume, and inhibited hyperosmotically induced tubulin macrotubule formation and the accumulation of phospho-p46. Exogenous PA reinforced the hyperosmotic response of T. turgidum root cells and positively affected tubulin macrotubule formation. Additionally, PA reduced the effects of butanol-1 in plasmolysed cells. Taken together, the data suggest that PLD-mediated PA synthesis occurs upstream of the accumulation of phospho-p46 to regulate hyperosmotically induced macrotubule formation in plasmolysed T. turgidum root cells.

Phosphatidylinositol 3,5-bisphosphate: metabolism and cellular functions

Polyphosphoinositides (PPIn) are low-abundance membrane phospholipids that each bind to a distinctive set of effector proteins and, thereby, regulate a characteristic suite of cellular processes. Major functions of phosphatidylinositol 3,5-bisphosphate [PtdIns(3,5)P(2)] are in membrane and protein trafficking, and in pH control in the endosome-lysosome axis. Recently identified PtdIns(3,5)P(2) effectors include a family of novel beta-propeller proteins, for which we propose the name PROPPINs [for beta-propeller(s) that binds PPIn], and possibly proteins of the epsin and CHMP (charged multi-vesicular body proteins) families. All eukaryotes, with the exception of some pathogenic protists and microsporidians, possess proteins needed for the formation, metabolism and functions of PtdIns(3,5)P(2). The importance of PtdIns(3,5)P(2) for normal cell function is underscored by recent evidence for its involvement in mammalian cell responses to insulin and for PtdIns(3,5)P(2) dysfunction in the human genetic conditions X-linked myotubular myopathy, Type-4B Charcot-Marie-Tooth disease and fleck corneal dystrophy.

Recruitment of myosin VIII towards plastid surfaces is root-cap specific and provides the evidence for actomyosin involvement in root osmosensing

The existence of a cell wall-plasma membrane-cytoskeleton (WMC) continuum in plants has long been postulated. However, the individual molecules building such a continuum are still largely unknown. We test here the hypothesis that the integrin-based multiprotein complexes of animal cells have been replaced in plants with more dynamic entities. Using an experimental approach based on protoplast digestion mixtures, and utilising specific antibodies against Arabidopsis ATM1 myosin, we reveal possible roles played by plant-specific unconventional myosin VIII in the functioning of WMC continuum. We demonstrate rapid relocation (less than 5 min) of myosin VIII to statolith surfaces in maize root-cap cells, which is accompanied by the reorganisation of actin cytoskeleton. Upon prolonged stimulation, myosin VIII is also recruited to plasmodesmata and pit-fields of plasmolysing root cap statocytes. The osmotic stimulus is the major factor inducing relocation, but the cell wall-cytoskeleton interactions also play an important role. In addition, we demonstrate the tight association of myosin VIII with the surfaces of chloroplasts, and provide an indication for the differences in the mechanisms of plastid movement in roots and leaves of plants. Overall, our data provide evidence for the active involvement of actomyosin complexes, rooted in the WMC continuum, in the cellular volume control and maintenance of spatial relationships between cellular compartments.

Microarray analysis reveals similarities and variations in genetic programs controlling pollination/fertilization and stress responses in rice (Oryza sativa L.)

Previously, we identified 253 cDNAs that are regulated by pollination/fertilization in rice by using a 10K cDNA microarray. In addition, many of them also appeared to be involved in drought and wounding responses. To investigate this relationship, we obtained their expression profiles after dehydration and wounding treatments in this study. Venn diagram analysis indicated that 53.8% (136/253) and 21% (57/253) of the pollination/fertilization-related genes are indeed regulated by dehydration and wounding, respectively, and nearly half of the genes expressed preferentially in unpollinated pistils (UP) are responsive to dehydration. These results indicated that an extensive gene set is shared among these responses, suggesting that the genetic programs regulating them are likely related. Among them, the genetic network of water stress control may be a key player in pollination and fertilization. Additionally, 39.5% (100/253) cDNAs that are related to pollination/fertilization appear not to be regulated by the stress treatments (dehydration and wounding), suggesting that the existence of additional genetic networks are involved in pollination/fertilization. Furthermore, comparative analysis of the expression profiles of the 253 cDNAs under 18 different conditions (various tissues, treatments and developmental status) revealed that the genetic networks regulating photosynthesis, starch metabolisms, GA- and defense-responses are involved in pollination and fertilization. Taken together, these results provided some clues to elucidate the molecular mechanisms of pollination and fertilization in rice.

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.

Phosphoinositides and phosphatidic acid regulate pollen tube growth and reorientation through modulation of [Ca2+]c and membrane secretion

The maintenance of a calcium gradient and vesicle secretion in the apex of pollen tubes is essential for growth. It is shown here that phosphatidylinositol-4,5-bisphosphate (PIP2) and D-myo-inositol-1,4,5-trisphosphate (IP3), together with phosphatidic acid (PA), play a vital role in the regulation of these processes. Changes in the intracellular concentration of both PIP2 and IP3 (induced by photolysis of caged-probes), modified growth and caused reorientation of the growth axis. However, measurements of cytosolic free calcium ([Ca2+]c) and apical secretion revealed significant differences between the photo-release of PIP2 or IP3. When released in the first 50 mum of the pollen tube, PIP2 led to transient growth perturbation, [Ca2+]c increases, and inhibition of apical secretion. By contrast, a concentration of IP3 which caused a [Ca2+]c transient of similar magnitude, stimulated apical secretion and caused severe growth perturbation. Furthermore, the [Ca2+]c transient induced by IP3 was spatially different causing a pronounced elevation in the sub-apical region. These observations suggest different targets for the two phosphoinositides. One of the targets is suggested to be PA, a product of PIP2 hydrolysis via phospholipase C (PLC) or phospholipase D (PLD) activity. It was found that antagonists of PA accumulation (e.g. butan-1-ol) and inhibitors of PLC and PLD reversibly halted polarity. Reduction of PA levels caused the dissipation of the [Ca2+]c gradient and inhibited apical plasma membrane recycling. It was also found to cause abolition of the apical zonation. These data suggest that phosphoinositides and phospholipids regulate tip growth through a multiple pathway system involving regulation of [Ca2+]c levels, endo/exocytosis, and vesicular trafficking.

Diacylglycerol pyrophosphate is a second messenger of abscisic acid signaling in Arabidopsis thaliana suspension cells

In plants, the importance of phospholipid signaling in responses to environmental stresses is becoming well documented. The involvement of phospholipids in abscisic acid (ABA) responses is also established. In a previous study, we demonstrated that the stimulation of phospholipase D (PLD) activity and plasma membrane anion currents by ABA were both required for RAB18 expression in Arabidopsis thaliana suspension cells. In this study, we show that the total lipids extracted from ABA-treated cells mimic ABA in activating plasmalemma anion currents and induction of RAB18 expression. Moreover, ABA evokes within 5 min a transient 1.7-fold increase in phosphatidic acid (PA) followed by a sevenfold increase in diacylglycerol pyrophosphate (DGPP) at 20 min. PA activated plasmalemma anion currents but was incapable of triggering RAB18 expression. By contrast, DGPP mimicked ABA on anion currents and was also able to stimulate RAB18 expression. Here we show the role of DGPP as phospholipid second messenger in ABA signaling.

Acquisition of Unprecedented Phosphatidylinositol 3,5-Bisphosphate Rise in Hyperosmotically Stressed 3T3-L1 Adipocytes, Mediated by ArPIKfyve-PIKfyve Pathway

Unlike yeast, where hyperosmotic stress induces a dramatic increase in phosphatidylinositol 3,5-bisphosphate (PtdIns 3,5-P(2)) synthesis, in mammalian cells, although activating a complex array of signaling events, hyperosmotic stress fails to up-regulate PtdIns 3,5-P(2), indicating the PtdIns 3,5-P(2) pathway is not involved in mammalian osmo-protective responses. Here we report an unexpected and marked PtdIns 3,5-P(2) increase in response to hyperosmotic stress in differentiated 3T3-L1 adipocytes. Because this effect was not observed in the precursor preadipocytes, a specific role during acquisition of the adipocyte phenotype and transition into insulin-responsive cells could be suggested. However, acute insulin action did not result in a measurable PtdIns 3,5-P(2) rise, indicating the PtdIns 3,5-P(2) pathway is a specific hyperosmotically activated signaling cascade selectively operating in differentiated 3T3-L1 adipocytes. Hyperosmolarity activates different components of several kinase cascades, including p38 mitogen-activated protein and tyrosine kinases, but these appear to be separate from the activated PtdIns 3,5-P(2) pathway. Because PtdIns 3,5-P(2) is primarily produced by PIKfyve-catalyzed synthesis and requires the upstream activator hVac14 (called herein ArPIKfyve) that physically associates with and activates PIKfyve, we examined the contribution of ArPIKfyve-PIKfyve for the hyperosmotic stress-induced rise in PtdIns 3,5-P(2). Small interfering RNA-directed gene silencing to selectively deplete ArPIKfyve or PIKfyve in 3T3-L1 adipocytes determined the ArPIKfyve-PIKfyve axis fully accountable for the hyperosmotically activated PtdIns 3,5-P(2). Together these results reveal a previously uncharacterized PtdIns 3,5-P(2) pathway activated selectively in hyperosmotically stressed 3T3-L1 adipocytes and suggest a plausible role for PtdIns 3,5-P(2) in the osmo-protective response mechanism in this cell type.