Using genetic tools to understand plant phosphoinositide signalling

Functional Analyses of the Two Distinctive Types of Two-Pore Channels and the Slow Vacuolar Channel in Marchantia polymorpha

The two-pore channel (TPC) family is widely conserved in eukaryotes. Many vascular plants, including Arabidopsis and rice, possess a single TPC gene which functions as a slow vacuolar (SV) channel-voltage-dependent cation-permeable channel located in the vacuolar membrane (tonoplast). On the other hand, a liverwort Marchantia polymorpha genome encodes three TPC homologs: MpTPC1 is similar to TPCs in vascular plants (type 1 TPC), while MpTPC2 and MpTPC3 are classified into a distinctive group (type 2 TPC). Phylogenetic analysis suggested that the type 2 TPC emerged before the land colonization in plant evolution and was lost in vascular plants and hornworts. All of the three MpTPCs were shown to be localized at the tonoplast. We generated knockout mutants of tpc1, tpc2, tpc3 and tpc2 tpc3 double mutant by clustered regularly interspaced short palindromic repeats/Cas9 genome editing and performed patch-clamp analyses of isolated vacuoles. The SV channel activity was abolished in the Mptpc1 loss-of-function mutant (Mptpc1-1KO), while Mptpc2-1KO, Mptpc3-1KO and Mptpc2-2/tpc3-2KO double mutant exhibited similar activity to the wild type, indicating that MpTPC1 (type 1) is solely responsible for the SV channel activity. Activators of mammalian TPCs, phosphatidylinositol-3,5-bisphosphate and nicotinic acid adenine dinucleotide phosphate, did not affect the ion channel activity of any MpTPCs. These results indicate that the type 1 TPCs, which are well conserved in all land plant species, encode the SV channel, while the type 2 TPCs likely encode other tonoplast cation channel(s) distinct from the SV channel and animal TPCs.

Enhancing single-cell bioconversion efficiency by harnessing nanosecond pulsed electric field processing

Nanosecond pulsed electric field (nsPEF) processing is gaining momentum as a physical means for single-cell bioconversion efficiency enhancement. The technology allows biomass yields per substrate (Y_X/S) to be leveraged and poses a viable option for stimulating intracellular compound production. NsPEF processing thus resonates with myriad domains spanning the pharmaceutical and medical sectors, as well as food and feed production. The exact working mechanisms underlying nsPEF-based enhancement of bioconversion efficiency, however, remain elusive, and a better understanding would be pivotal for leveraging process control to broaden the application of nsPEF and scale-up industrial implementation. To bridge this gap, the study provides the electrotechnological and metabolic fundamentals of nsPEF processing in the bio-based domain to enable a critical evaluation of pathways underlying the enhancement of single-cell bioconversion efficiency. Evidence suggests that treating cells during the rapid proliferating and thus the early to mid-exponential state of cellular growth is critical to promoting bioconversion efficiency. A combined effect of transient intracellular and sublethal stress induction and effects caused on the plasma membrane level result in an enhancement of cellular bioconversion efficiency. Congruency exists regarding the involvement of transient cytosolic Ca²⁺ hubs in nsPEF treatment responses, as well as that of reactive oxygen species formation culminating in the onset of cellular response pathways. A distinct assignment of single effects and their contributions to enhancing bioconversion efficiency, however, remains challenging. Current applications of nsPEF processing comprise microalgae, bacteria, and yeast biorefineries, but these endeavors are in their infancies with limitations associated with a lack of understanding of the underlying treatment mechanisms, an incomplete reporting, insufficient characterization, and control of processing parameters. The study aids in fostering the upsurge of nsPEF applications in the bio-based domain by providing a basis to gain a better understanding of cellular mechanisms underlying an nsPEF-based enhancement of cellular bioconversion efficiency and suggests best practice guidelines for nsPEF documentation for improved knowledge transfer. Better understanding and reporting of processes parameters and consequently improved process control could foster industrial-scale nsPEF realization and ultimately aid in perpetuating nsPEF applicability within the bio-based domain.

Link between Lipid Second Messengers and Osmotic Stress in Plants

Plants are subject to different types of stress, which consequently affect their growth and development. They have developed mechanisms for recognizing and processing an extracellular signal. Second messengers are transient molecules that modulate the physiological responses in plant cells under stress conditions. In this sense, it has been shown in various plant models that membrane lipids are substrates for the generation of second lipid messengers such as phosphoinositide, phosphatidic acid, sphingolipids, and lysophospholipids. In recent years, research on lipid second messengers has been moving toward using genetic and molecular approaches to reveal the molecular setting in which these molecules act in response to osmotic stress. In this sense, these studies have established that second messengers can transiently recruit target proteins to the membrane and, therefore, affect protein conformation, activity, and gene expression. This review summarizes recent advances in responses related to the link between lipid second messengers and osmotic stress in plant cells.

Acclimation to salt modifies the activation of several osmotic stress-activated lipid signalling pathways in Chlamydomonas

Osmotic stress rapidly activates several phospholipid signalling pathways in the unicellular alga Chlamydomonas. In this report, we have studied the effects of salt-acclimation on growth and phospholipid signalling. Growing cells on media containing 100 mM NaCl increased their salt-tolerance but did not affect the overall phospholipid content, except that levels of phosphatidylinositol phosphate (PIP) and phosphatidylinositol 4,5-bisphosphate [PI(4,5)P₂] were reduced by one-third. When these NaCl-acclimated cells were treated with increasing concentrations of salt, the same lipid signalling pathways as in non-acclimated cells were activated. This was witnessed as increases in phosphatidic acid (PA), lyso-phosphatidic acid (L-PA), diacylglycerol pyrophosphate (DGPP), PI(4,5)P₂ and its isomer PI(3,5)P₂. However, all dose-dependent responses were shifted to higher osmotic-stress levels, and the responses were lower than in non-acclimated cells. When NaCl-acclimated cells were treated with other osmotica, such as KCl and sucrose, the same effects were found, illustrating that they were due to hyperosmotic rather than hyperionic acclimation. The results indicate that acclimation to moderate salt stress modifies stress perception and the activation of several downstream pathways.

Phospholipid composition and a polybasic motif determine D6 PROTEIN KINASE polar association with the plasma membrane and tropic responses

Polar transport of the phytohormone auxin through PIN-FORMED (PIN) auxin efflux carriers is essential for the spatiotemporal control of plant development. The Arabidopsis thaliana serine/threonine kinase D6 PROTEIN KINASE (D6PK) is polarly localized at the plasma membrane of many cells where it colocalizes with PINs and activates PIN-mediated auxin efflux. Here, we show that the association of D6PK with the basal plasma membrane and PINs is dependent on the phospholipid composition of the plasma membrane as well as on the phosphatidylinositol phosphate 5-kinases PIP5K1 and PIP5K2 in epidermis cells of the primary root. We further show that D6PK directly binds polyacidic phospholipids through a polybasic lysine-rich motif in the middle domain of the kinase. The lysine-rich motif is required for proper PIN3 phosphorylation and for auxin transport-dependent tropic growth. Polybasic motifs are also present at a conserved position in other D6PK-related kinases and required for membrane and phospholipid binding. Thus, phospholipid-dependent recruitment to membranes through polybasic motifs might not only be required for D6PK-mediated auxin transport but also other processes regulated by these, as yet, functionally uncharacterized kinases.

Two-ligand priming mechanism for potentiated phosphoinositide synthesis is an evolutionarily conserved feature of Sec14-like phosphatidylinositol and phosphatidylcholine exchange proteins

The two-ligand priming mechanism for stimulated phosphoinositide synthesis described for Saccharomyces Sec14 is also a conserved feature of Sec14-like phosphatidylinositol- and phosphatidylcholine-transfer proteins of the most evolutionarily advanced plants.

Lipid signaling, particularly phosphoinositide signaling, plays a key role in regulating the extreme polarized membrane growth that drives root hair development in plants. The Arabidopsis AtSFH1 gene encodes a two-domain protein with an amino-terminal Sec14-like phosphatidylinositol transfer protein (PITP) domain linked to a carboxy-terminal nodulin domain. AtSfh1 is critical for promoting the spatially highly organized phosphatidylinositol-4,5-bisphosphate signaling program required for establishment and maintenance of polarized root hair growth. Here we demonstrate that, like the yeast Sec14, the AtSfh1 PITP domain requires both its phosphatidylinositol (PtdIns)- and phosphatidylcholine (PtdCho)-binding properties to stimulate PtdIns-4-phosphate [PtdIns(4)P] synthesis. Moreover, we show that both phospholipid-binding activities are essential for AtSfh1 activity in supporting polarized root hair growth. Finally, we report genetic and biochemical evidence that the two-ligand mechanism for potentiation of PtdIns 4-OH kinase activity is a broadly conserved feature of plant Sec14-nodulin proteins, and that this strategy appeared only late in plant evolution. Taken together, the data indicate that the PtdIns/PtdCho-exchange mechanism for stimulated PtdIns(4)P synthesis either arose independently during evolution in yeast and in higher plants, or a suitable genetic module was introduced to higher plants from a fungal source and subsequently exploited by them.

Plant Phosphoglycerolipids: The Gatekeepers of Vascular Cell Differentiation

In higher plants, the plant vascular system has evolved as an inter-organ communication network essential to deliver a wide range of signaling factors among distantly separated organs. To become conductive elements, phloem and xylem cells undergo a drastic differentiation program that involves the degradation of the majority of their organelles. While the molecular mechanisms regulating such complex process remain poorly understood, it is nowadays clear that phosphoglycerolipids display a pivotal role in the regulation of vascular tissue formation. In animal cells, this class of lipids is known to mediate acute responses as signal transducers and also act as constitutive signals that help defining organelle identity. Their rapid turnover, asymmetrical distribution across subcellular compartments as well as their ability to rearrange cytoskeleton fibers make phosphoglycerolipids excellent candidates to regulate complex morphogenetic processes such as vascular differentiation. Therefore, in this review we aim to summarize, emphasize and connect our current understanding about the involvement of phosphoglycerolipids in phloem and xylem differentiation.

Long-Distance Lipid Signaling and its Role in Plant Development and Stress Response

Lipids are important signaling compounds in plants. They can range from small lipophilic molecules like the dicarboxylic acid Azelaic acid to complex phosphoglycerolipids and regulate plant development as well as the response to biotic and abiotic stress. While their intracellular function is well described, several lipophilic signals are known to be found in the plant phloem and can, thus, also play a role in long-distance signaling. Mostly, they play a role in the pathogen response and systemic acquired resistance. This is particularly true for oxylipins, dehydroabietinal, and azelaic acid. However, several phospholipids have now been described in phloem exudates. Their intracellular function as well as implications and a model for long-distance signaling are discussed in this chapter.

Sec14-nodulin proteins and the patterning of phosphoinositide landmarks for developmental control of membrane morphogenesis

A Sec14-nodulin protein model is used to identify the nodulin domain as a novel phosphoinositide effector module with a role in controlling lateral organization of phosphoinositide. The domain organization of Sec14-nodulin proteins suggests a versatile principle for the bit mapping of membrane surfaces into high-definition lipid-signaling screens.

Polarized membrane morphogenesis is a fundamental activity of eukaryotic cells. This process is essential for the biology of cells and tissues, and its execution demands exquisite temporal coordination of functionally diverse membrane signaling reactions with high spatial resolution. Moreover, mechanisms must exist to establish and preserve such organization in the face of randomizing forces that would diffuse it. Here we identify the conserved AtSfh1 Sec14-nodulin protein as a novel effector of phosphoinositide signaling in the extreme polarized membrane growth program exhibited by growing Arabidopsis root hairs. The data are consistent with Sec14-nodulin proteins controlling the lateral organization of phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P₂) landmarks for polarized membrane morphogenesis in plants. This patterning activity requires both the PtdIns(4,5)P₂ binding and homo-oligomerization activities of the AtSfh1 nodulin domain and is an essential aspect of the polarity signaling program in root hairs. Finally, the data suggest a general principle for how the phosphoinositide signaling landscape is physically bit mapped so that eukaryotic cells are able to convert a membrane surface into a high-definition lipid-signaling screen.

Do phosphoinositides regulate membrane water permeability of tobacco protoplasts by enhancing the aquaporin pathway?

MAIN CONCLUSION

Enhancing the membrane content of PtdInsP 2 , the already-recognized protein-regulating lipid, increased the osmotic water permeability of tobacco protoplasts, apparently by increasing the abundance of active aquaporins in their membranes. While phosphoinositides are implicated in cell volume changes and are known to regulate some ion channels, their modulation of aquaporins activity has not yet been reported for any organism. To examine this, we compared the osmotic water permeability (P f) of protoplasts isolated from tobacco (Nicotiana tabacum) cultured cells (NT1) with different (genetically lowered or elevated relative to controls) levels of inositol trisphosphate (InsP3) and phosphatidyl inositol [4,5] bisphosphate (PtdInsP2). To achieve this, the cells were transformed with, respectively, the human InsP3 5-phosphatase ('Ptase cells') or human phosphatidylinositol (4) phosphate 5-kinase ('PIPK cells'). The mean P f of the PIPK cells was several-fold higher relative to that of controls and Ptase cells. Three results favor aquaporins over the membrane matrix as underlying this excessive P f: (1) transient expression of the maize aquaporin ZmPIP2;4 in the PIPK cells increased P f by 12-30 μm s(-1), while in the controls only by 3-4 μm s(-1). (2) Cytosol acidification-known to inhibit aquaporins-lowered the P f in the PIPK cells down to control levels. (3) The transcript of at least one aquaporin was elevated in the PIPK cells. Together, the three results demonstrate the differences between the PIPK cells and their controls, and suggest a hitherto unobserved regulation of aquaporins by phosphoinositides, which could occur through direct interaction or indirect phosphoinositides-dependent cellular effects.

Rapid identification of a natural knockout allele of ARMADILLO REPEAT-CONTAINING KINESIN1 that causes root hair branching by mapping-by-sequencing

In Arabidopsis (Arabidopsis thaliana), branched root hairs are an indicator of defects in root hair tip growth. Among 62 accessions, one accession (Heiligkreuztal2 [HKT2.4]) displayed branched root hairs, suggesting that this accession carries a mutation in a gene of importance for tip growth. We determined 200- to 300-kb mapping intervals using a mapping-by-sequencing approach of F2 pools from crossings of HKT2.4 with three different accessions. The intersection of these mapping intervals was 80 kb in size featuring not more than 36 HKT2.4-specific single nucleotide polymorphisms, only two of which changed the coding potential of genes. Among them, we identified the causative single nucleotide polymorphism changing a splicing site in ARMADILLO REPEAT-CONTAINING KINESIN1. The applied strategies have the potential to complement statistical methods in high-throughput phenotyping studies using different natural accessions to identify causative genes for distinct phenotypes represented by only one or a few accessions.

The phosphoinositide PI(3,5)P₂ mediates activation of mammalian but not plant TPC proteins: functional expression of endolysosomal channels in yeast and plant cells

Two-pore channel proteins (TPC) encode intracellular ion channels in both animals and plants. In mammalian cells, the two isoforms (TPC1 and TPC2) localize to the endo-lysosomal compartment, whereas the plant TPC1 protein is targeted to the membrane surrounding the large lytic vacuole. Although it is well established that plant TPC1 channels activate in a voltage- and calcium-dependent manner in vitro, there is still debate on their activation under physiological conditions. Likewise, the mode of animal TPC activation is heavily disputed between two camps favoring as activator either nicotinic acid adenine dinucleotide phosphate (NAADP) or the phosphoinositide PI(3,5)P₂. Here, we investigated TPC current responses to either of these second messengers by whole-vacuole patch-clamp experiments on isolated vacuoles of Arabidopsis thaliana. After expression in mesophyll protoplasts from Arabidopsis tpc1 knock-out plants, we detected the Arabidopsis TPC1-EGFP and human TPC2-EGFP fusion proteins at the membrane of the large central vacuole. Bath (cytosolic) application of either NAADP or PI(3,5)P₂ did not affect the voltage- and calcium-dependent characteristics of AtTPC1-EGFP. By contrast, PI(3,5)P₂ elicited large sodium currents in hTPC2-EGFP-containing vacuoles, while NAADP had no such effect. Analogous results were obtained when PI(3,5)P₂ was applied to hTPC2 expressed in baker's yeast giant vacuoles. Our results underscore the fundamental differences in the mode of current activation and ion selectivity between animal and plant TPC proteins and corroborate the PI(3,5)P₂-mediated activation and Na(+) selectivity of mammalian TPC2.

Plant phosphoinositides-complex networks controlling growth and adaptation

Plants differ in many ways from mammals or yeast. However, plants employ phosphoinositides for the regulation of essential cellular functions as do all other eukaryotes. In recent years the plant phosphoinositide system has been linked to the control of cell polarity. Phosphoinositides are also implicated in plant adaptive responses to changing environmental conditions. The current understanding is that plant phosphoinositides control membrane trafficking, ion channels and the cytoskeleton in similar ways as in other eukaryotic systems, but adapted to meet plant cellular requirements and with some plant-specific features. In addition, the formation of soluble inositol polyphosphates from phosphoinositides is important for the perception of important phytohormones, as the relevant receptor proteins contain such molecules as structural cofactors. Overall, the essential nature of phosphoinositides in plants has been established. Still, the complexity of the phosphoinositide networks in plant cells is only emerging and invites further study of its molecular details. This article is part of a special issue entitled Phosphoinositides.

Regulatory roles of phosphoinositides in membrane trafficking and their potential impact on cell-wall synthesis and re-modelling

BACKGROUND

Plant cell walls are complex matrices of carbohydrates and proteins that control cell morphology and provide protection and rigidity for the plant body. The construction and maintenance of this intricate system involves the delivery and recycling of its components through a precise balance of endomembrane trafficking, which is controlled by a plethora of cell signalling factors. Phosphoinositides (PIs) are one class of signalling molecules with diverse roles in vesicle trafficking and cytoskeleton structure across different kingdoms. Therefore, PIs may also play an important role in the assembly of plant cell walls.

SCOPE

The eukaryotic PI pathway is an intricate network of different lipids, which appear to be divided in different pools that can partake in vesicle trafficking or signalling. Most of our current understanding of how PIs function in cell metabolism comes from yeast and mammalian systems; however, in recent years significant progress has been made towards a better understanding of the plant PI system. This review examines the current state of knowledge of how PIs regulate vesicle trafficking and their potential influence on plant cell-wall architecture. It considers first how PIs are formed in plants and then examines their role in the control of vesicle trafficking. Interactions between PIs and the actin cytoskeleton and small GTPases are also discussed. Future challenges for research are suggested.

Bipolar Plasma Membrane Distribution of Phosphoinositides and Their Requirement for Auxin-Mediated Cell Polarity and Patterning in Arabidopsis

Cell polarity manifested by asymmetric distribution of cargoes, such as receptors and transporters, within the plasma membrane (PM) is crucial for essential functions in multicellular organisms. In plants, cell polarity (re)establishment is intimately linked to patterning processes. Despite the importance of cell polarity, its underlying mechanisms are still largely unknown, including the definition and distinctiveness of the polar domains within the PM. Here, we show in Arabidopsis thaliana that the signaling membrane components, the phosphoinositides phosphatidylinositol 4-phosphate (PtdIns4P) and phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P₂] as well as PtdIns4P 5-kinases mediating their interconversion, are specifically enriched at apical and basal polar plasma membrane domains. The PtdIns4P 5-kinases PIP5K1 and PIP5K2 are redundantly required for polar localization of specifically apical and basal cargoes, such as PIN-FORMED transporters for the plant hormone auxin. As a consequence of the polarity defects, instructive auxin gradients as well as embryonic and postembryonic patterning are severely compromised. Furthermore, auxin itself regulates PIP5K transcription and PtdIns4P and PtdIns(4,5)P₂ levels, in particular their association with polar PM domains. Our results provide insight into the polar domain-delineating mechanisms in plant cells that depend on apical and basal distribution of membrane lipids and are essential for embryonic and postembryonic patterning.

Preferred orientations of phosphoinositides in bilayers and their implications in protein recognition mechanisms

Phosphoinositides (PIPs), phosphorylated derivatives of phosphatidylinositol (PI), are essential regulatory lipids involved in various cellular processes, including signal transduction, membrane trafficking, and cytoskeletal remodeling. To gain insight into the protein-PIPs recognition process, it is necessary to study the inositol ring orientation (with respect to the membrane) of PIPs with different phosphorylation states. In this study, 8 PIPs (3 PIP, 2 PIP2, and 3 PIP3) with different phosphorylation and protonation sites have been separately simulated in two mixed bilayers (one with 20% phosphatidylserine (PS) lipids and another with PS lipids switched to phosphatidylcholine (PC) lipids), which roughly correspond to yeast membranes. Uniformity of the bilayer properties including hydrophobic thickness, acyl chain order parameters, and heavy atom density profiles is observed in both PS-contained and PC-enriched membranes due to the same hydrophobic core composition. The relationship between the inositol ring orientation (tilt and rotation angles) and its solvent-accessible surface area indicates that the orientation is mainly determined by its solvation energy. Different PIPs exhibit a clear preference in the inositol ring rotation angle. Surprisingly, a larger proportion of PIPs inositol rings stay closer to the surface of PS-contained membranes compared to PC-enriched ones. Such a difference is rationalized with the formation of more hydrogen bonds between the PS/PI headgroups and the PIPs inositol rings in PS-contained membranes. This hydrogen bond network could be functionally important; thus, the present results can potentially add important and detailed features into the existing protein-PIPs recognition mechanism.

Increasing Phosphatidylinositol (4,5)-Bisphosphate Biosynthesis Affects Basal Signaling and Chloroplast Metabolism in Arabidopsis thaliana

One challenge in studying the second messenger inositol(1,4,5)-trisphosphate (InsP₃) is that it is present in very low amounts and increases only transiently in response to stimuli. To identify events downstream of InsP₃, we generated transgenic plants constitutively expressing the high specific activity, human phosphatidylinositol 4-phosphate 5-kinase Iα (HsPIPKIα). PIP5K is the enzyme that synthesizes phosphatidylinositol (4,5)-bisphosphate (PtdIns(4,5)P₂); this reaction is flux limiting in InsP₃ biosynthesis in plants. Plasma membranes from transgenic Arabidopsis expressing HsPIPKIα had 2-3 fold higher PIP5K specific activity, and basal InsP₃ levels in seedlings and leaves were >2-fold higher than wild type. Although there was no significant difference in photosynthetic electron transport, HsPIPKIα plants had significantly higher starch (2-4 fold) and 20% higher anthocyanin compared to controls. Starch content was higher both during the day and at the end of dark period. In addition, transcripts of genes involved in starch metabolism such as SEX1 (glucan water dikinase) and SEX4 (phosphoglucan phosphatase), DBE (debranching enzyme), MEX1 (maltose transporter), APL3 (ADP-glucose pyrophosphorylase) and glucose-6-phosphate transporter (Glc6PT) were up-regulated in the HsPIPKIα plants. Our results reveal that increasing the phosphoinositide (PI) pathway affects chloroplast carbon metabolism and suggest that InsP₃ is one component of an inter-organelle signaling network regulating chloroplast metabolism.

Phosphatidylinositol 4,5-bisphosphate influences PIN polarization by controlling clathrin-mediated membrane trafficking in Arabidopsis

The functions of the minor phospholipid phosphatidylinositol-4,5-bisphosphate [PtdIns(4,5)P2] during vegetative plant growth remain obscure. Here, we targeted two related phosphatidylinositol 4-phosphate 5-kinases (PI4P 5-kinases) PIP5K1 and PIP5K2, which are expressed ubiquitously in Arabidopsis thaliana. A pip5k1 pip5k2 double mutant with reduced PtdIns(4,5)P2 levels showed dwarf stature and phenotypes suggesting defects in auxin distribution. The roots of the pip5k1 pip5k2 double mutant had normal auxin levels but reduced auxin transport and altered distribution. Fluorescence-tagged auxin efflux carriers PIN-FORMED (PIN1)-green fluorescent protein (GFP) and PIN2-GFP displayed abnormal, partially apolar distribution. Furthermore, fewer brefeldin A-induced endosomal bodies decorated by PIN1-GFP or PIN2-GFP formed in pip5k1 pip5k2 mutants. Inducible overexpressor lines for PIP5K1 or PIP5K2 also exhibited phenotypes indicating misregulation of auxin-dependent processes, and immunolocalization showed reduced membrane association of PIN1 and PIN2. PIN cycling and polarization require clathrin-mediated endocytosis and labeled clathrin light chain also displayed altered localization patterns in the pip5k1 pip5k2 double mutant, consistent with a role for PtdIns(4,5)P2 in the regulation of clathrin-mediated endocytosis. Further biochemical tests on subcellular fractions enriched for clathrin-coated vesicles (CCVs) indicated that pip5k1 and pip5k2 mutants have reduced CCV-associated PI4P 5-kinase activity. Together, the data indicate an important role for PtdIns(4,5)P2 in the control of clathrin dynamics and in auxin distribution in Arabidopsis.

Arranged marriage in lipid signalling? The limited choices of PtdIns(4,5)P2 in finding the right partner

Inositol-containing phospholipids (phosphoinositides, PIs) control numerous cellular processes in eukaryotic cells. For plants, a key involvement of PIs has been demonstrated in the regulation of membrane trafficking, cytoskeletal dynamics and in processes mediating the adaptation to changing environmental conditions. Phosphatidylinositol-4,5-bisphosphate (PtdIns(4,5)P(2)) mediates its cellular functions via binding to various alternative target proteins. Such downstream targets of PtdIns(4,5)P(2) are characterised by the possession of specific lipid-binding domains, and binding of the PtdIns(4,5)P(2) ligand exerts effects on their activity or localisation. The large number of potential alternative binding partners - and associated cellular processes - raises the question how alternative or even contrapuntal effects of PtdIns(4,5)P(2) are orchestrated to enable cellular function. This article aims to provide an overview of recent insights and new views on how distinct functional pools of PtdIns(4,5)P(2) are generated and maintained. The emerging picture suggests that PtdIns(4,5)P(2) species containing different fatty acids influence the lateral mobility of the lipids in the membrane, possibly enabling specific interactions of PtdIns(4,5)P(2) pools with certain downstream targets. PtdIns(4,5)P(2) pools with certain functions might also be defined by protein-protein interactions of PI4P 5-kinases, which pass PtdIns(4,5)P(2) only to certain downstream partners. Individually or in combination, PtdIns(4,5)P(2) species and specific protein-protein interactions of PI4P 5-kinases might contribute to the channelling of PtdIns(4,5)P(2) signals towards specific functional effects. The dynamic nature of PI-dependent signalling complexes with specific functions is an added challenge for future studies of plant PI signalling.

Overexpression of the phosphatidylinositol synthase gene (ZmPIS) conferring drought stress tolerance by altering membrane lipid composition and increasing ABA synthesis in maize

Phosphatidylinositol (PtdIns) synthase is a key enzyme in the phospholipid pathway and catalyses the formation of PtdIns. PtdIns is not only a structural component of cell membranes, but also the precursor of the phospholipid signal molecules that regulate plant response to environment stresses. Here, we obtained transgenic maize constitutively overexpressing or underexpressing PIS from maize (ZmPIS) under the control of a maize ubiquitin promoter. Transgenic plants were confirmed by PCR, Southern blotting analysis and real-time RT-PCR assay. The electrospray ionization tandem mass spectrometry (ESI-MS/MS)-based lipid profiling analysis showed that, under drought stress conditions, the overexpression of ZmPIS in maize resulted in significantly elevated levels of most phospholipids and galactolipids in leaves compared with those in wild type (WT). At the same time, the expression of some genes involved in the phospholipid metabolism pathway and the abscisic acid (ABA) biosynthesis pathway including ZmPLC, ZmPLD, ZmDGK1, ZmDGK3, ZmPIP5K9, ZmABA1, ZmNCED, ZmAAO1, ZmAAO2 and ZmSCA1 was markedly up-regulated in the overexpression lines after drought stress. Consistent with these results, the drought stress tolerance of the ZmPIS sense transgenic plants was enhanced significantly at the pre-flowering stages compared with WT maize plants. These results imply that ZmPIS regulates the plant response to drought stress through altering membrane lipid composition and increasing ABA synthesis in maize.

The Ca(2+) -binding protein PCaP2 located on the plasma membrane is involved in root hair development as a possible signal transducer

Plasma membrane-associated Ca(2+) -binding protein-2 (PCaP2) of Arabidopsis thaliana is a novel-type protein that binds to the Ca(2+) /calmodulin complex and phosphatidylinositol phosphates (PtdInsPs) as well as free Ca(2+) . Although the PCaP2 gene is predominantly expressed in root hair cells, it remains unknown how PCaP2 functions in root hair cells via binding to ligands. From biochemical analyses using purified PCaP2 and its variants, we found that the N-terminal basic domain with 23 amino acids (N23) is necessary and sufficient for binding to PtdInsPs and the Ca(2+) /calmodulin complex, and that the residual domain of PCaP2 binds to free Ca(2+) . In mutant analysis, a pcap2 knockdown line displayed longer root hairs than the wild-type. To examine the function of each domain in root hair cells, we over-expressed PCaP2 and its variants using the root hair cell-specific EXPANSIN A7 promoter. Transgenic lines over-expressing PCaP2, PCaP2(G2A) (second glycine substituted by alanine) and ∆23PCaP2 (lacking the N23 domain) exhibited abnormal branched and bulbous root hair cells, while over-expression of the N23 domain suppressed root hair emergence and elongation. The N23 domain was necessary and sufficient for the plasma membrane localization of GFP-tagged PCaP2. These results suggest that the N23 domain of PCaP2 negatively regulates root hair tip growth via processing Ca(2+) and PtdInsP signals on the plasma membrane, while the residual domain is involved in the polarization of cell expansion.

Assaying plant phosphatidic acid phosphatase activity

Phosphatidic acid phosphatase (PAP; EC 3.1.3.4) catalyzes the dephosphorylation of phosphatidic acid (PA) to produce diacylglycerol (DAG) and inorganic phosphate. In seed plants, PA plays pivotal roles both as a precursor to membrane lipids and as a signaling molecule. As more information on the roles of PAP in plants becomes available and the importance of PAP is revealed, protocols for assaying plant PAP activity are of interest to an increasing audience. This chapter describes procedures to assay plant PAP activity that are based on recent publications.

Mass measurement of polyphosphoinositides by thin-layer and gas chromatography

Phosphoinositides derive from the phospholipid, phosphatidylinositol (PtdIns), by phosphorylation of the inositol ring in the lipid head group. The determination of phosphoinositide species is a particular challenge, because the structurally similar inositolphosphate-head groups must be analyzed as well as the lipid-associated fatty acids. The method presented in this chapter consists of two steps: First phosphoinositides are separated by thin-layer chromatography (TLC) according to their characteristic head groups and the individual lipids are isolated. Second, the fatty acids associated with each isolated lipid are analyzed using a gas-chromatograph (GC). The combination of these two classical methods for lipid analysis, TLC and GC, provides a cost-efficient and reliable alternative to lipidomics approaches requiring more extensive instrumentation.

Overlapping and divergent signaling pathways for ARK1 and AGD1 in the control of root hair polarity in Arabidopsis thaliana

We previously showed that seedlings harboring mutations in genes encoding ARK1, an armadillo repeat-containing kinesin, or AGD1, a class 1 ARF-GAP, have root hairs that exhibit wavy/spiral growth and two tips originating from one initiation site. These root hair defects were accompanied by bundling of endoplasmic microtubules and filamentous actin (F-actin) that extended to the extreme root hair apex. The similar phenotypes of ark1 and agd1 mutants suggest a tight coordination between the cytoskeleton and membrane trafficking in the control of root hair polarity. Indeed, cell biological and genetic studies of the agd1 mutant provided evidence that AGD1's involvement in root hair development involves cross-talk among phosphoinositides (PIs), the actin cytoskeleton and other small GTPases such as ROP2 and RABA4b. Here we show that ark1 root hairs mirror those of agd1 with regard to altered targeting of ROP2 and RABA4b, as well as abnormal tonoplast organization. Furthermore, like agd1, enhanced root hair defects in double mutants in ARK1 and genes encoding a type B phosphatidylinositol-4-phosphate 5-kinase 3 (PIP5K3), a phosphatidylinositol-4-phosphate (PI-4P) phosphatase (RHD4), a phosphatidylinositol transfer protein (COW1), and a vegetative actin isoform (ACT2), were observed. However, root hair shape of some ark1 double mutant combinations, particularly those with act2, pip5k3 and rhd4 (ark1 act2, ark1 pip5k3, ark1 rhd4), differed in some respects from agd1 act2, agd1 pip5k3, and agd1 rhd4. Taken together our results continue to point to commonalities between ARK1 and AGD1 in specifying root hair polarity, but that these two modulators of tip-growth can also regulate root hair development through divergent signaling routes with AGD1 acting predominantly during root hair initiation and ARK1 functioning primarily in sustained tip growth.

The essential phosphoinositide kinase MSS-4 is required for polar hyphal morphogenesis, localizing to sites of growth and cell fusion in Neurospora crassa

Fungal hyphae and plant pollen tubes are among the most highly polarized cells known and pose extraordinary requirements on their cell polarity machinery. Cellular morphogenesis is driven through the phospholipid-dependent organization at the apical plasma membrane. We characterized the contribution of phosphoinositides (PIs) in hyphal growth of the filamentous ascomycete Neurospora crassa. MSS-4 is an essential gene and its deletion resulted in spherically growing cells that ultimately lyse. Two conditional mss-4-mutants exhibited altered hyphal morphology and aberrant branching at restrictive conditions that were complemented by expression of wild type MSS-4. Recombinant MSS-4 was characterized as a phosphatidylinositolmonophosphate-kinase phosphorylating phosphatidylinositol 4-phosphate (PtdIns4P) to phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P₂). PtdIns3P was also used as a substrate. Sequencing of two conditional mss-4 alleles identified a single substitution of a highly conserved Y750 to N. The biochemical characterization of recombinant protein variants revealed Y750 as critical for PI4P 5-kinase activity of MSS-4 and of plant PI4P 5-kinases. The conditional growth defects of mss-4 mutants were caused by severely reduced activity of MSS-4(Y750N), enabling the formation of only trace amounts of PtdIns(4,5)P₂. In N. crassa hyphae, PtdIns(4,5)P₂ localized predominantly in the plasma membrane of hyphae and along septa. Fluorescence-tagged MSS-4 formed a subapical collar at hyphal tips, localized to constricting septa and accumulated at contact points of fusing N. crassa germlings, indicating MSS-4 is responsible for the formation of relevant pools of PtdIns(4,5)P₂ that control polar and directional growth and septation. N. crassa MSS-4 differs from yeast, plant and mammalian PI4P 5-kinases by containing additional protein domains. The N-terminal domain of N. crassa MSS-4 was required for correct membrane association. The data presented for N. crassa MSS-4 and its roles in hyphal growth are discussed with a comparative perspective on PI-control of polar tip growth in different organismic kingdoms.

A role for phosphoinositides in regulating plant nuclear functions

Nuclear localized inositol phospholipids and inositol phosphates are important for regulating many essential processes in animal and yeast cells such as DNA replication, recombination, RNA processing, mRNA export and cell cycle progression. An overview of the current literature indicates the presence of a plant nuclear phosphoinositide (PI) pathway. Inositol phospholipids, inositol phosphates, and enzymes of the PI pathway have been identified in plant nuclei and are implicated in DNA replication, chromatin remodeling, stress responses and hormone signaling. In this review, the potential functions of the nuclear PI pathway in plants are discussed within the context of the animal and yeast literature. It is anticipated that future research will help shed light on the functional significance of the nuclear PI pathway in plants.

AGD1, a class 1 ARF-GAP, acts in common signaling pathways with phosphoinositide metabolism and the actin cytoskeleton in controlling Arabidopsis root hair polarity

The Arabidopsis thaliana AGD1 gene encodes a class 1 adenosine diphosphate ribosylation factor-gtpase-activating protein (ARF-GAP). Previously, we found that agd1 mutants have root hairs that exhibit wavy growth and have two tips that originate from a single initiation point. To gain new insights into how AGD1 modulates root hair polarity we analyzed double mutants of agd1 and other loci involved in root hair development, and evaluated dynamics of various components of root hair tip growth in agd1 by live cell microscopy. Because AGD1 contains a phosphoinositide (PI) binding pleckstrin homology (PH) domain, we focused on genetic interactions between agd1 and root hair mutants altered in PI metabolism. Rhd4, which is knocked-out in a gene encoding a phosphatidylinositol-4-phosphate (PI-4P) phosphatase, was epistatic to agd1. In contrast, mutations to PIP5K3 and COW1, which encode a type B phosphatidylinositol-4-phosphate 5-kinase 3 and a phosphatidylinositol transfer protein, respectively, enhanced the root hair defects of agd1. Enhanced root hair defects were also observed in double mutants to AGD1 and ACT2, a root hair-expressed vegetative actin isoform. Consistent with our double-mutant studies, targeting of tip growth components involved in PI signaling (PI-4P), secretion (RABA4b) and actin regulation (ROP2), were altered in agd1 root hairs. Furthermore, tip cytosolic calcium ([Ca²⁺](cyt) ) oscillations were disrupted in root hairs of agd1. Taken together, our results indicate that AGD1 links PI signaling to cytoskeletal-, [Ca²⁺](cyt-) , ROP2-, and RABA4b-mediated root hair development.

A novel class of PTEN protein in Arabidopsis displays unusual phosphoinositide phosphatase activity and efficiently binds phosphatidic acid

PTEN (phosphatase and tensin homologue deleted on chromosome ten) proteins are dual phosphatases with both protein and phosphoinositide phosphatase activity. They modulate signalling pathways controlling growth, metabolism and apoptosis in animals and are implied in several human diseases. In the present paper we describe a novel class of PTEN pro-teins in plants, termed PTEN2, which comprises the AtPTEN (Arabidopsis PTEN) 2a and AtPTEN2b proteins in Arabidopsis. Both display low in vitro tyrosine phosphatase activity. In addition, AtPTEN2a actively dephosphorylates in vitro the 3' phosphate group of PI3P (phosphatidylinositol 3-phosphate), PI(3,4)P2 (phosphatidylinositol 3,4-bisphosphate) and PI(3,5)P2 (phosphatidylinositol 3,5-bisphosphate). In contrast with animal PTENs, PI(3,4,5)P3 (phosphatidylinositol 3,4,5-trisphosphate) is a poor substrate. Site-directed mutagenesis of AtPTEN2a and molecular modelling of protein-phosphoinositide interactions indicated that substitutions at the PTEN2 core catalytic site of the Lys267 and Gly268 residues found in animals, which are critical for animal PTEN activity, by Met267 and Ala268 found in the eudicot PTEN2 are responsible for changes in substrate specificity. Remarkably, the AtPTEN2a protein also displays strong binding activity for PA (phosphatidic acid), a major lipid second messenger in plants. Promoter::GUS (β-glucuronidase) fusion, transcript and protein analyses further showed the transcriptional regulation of the ubiquitously expressed AtPTEN2a and AtPTEN2b by salt and osmotic stress. The results of the present study suggest a function for this novel class of plant PTEN proteins as an effector of lipid signalling in plants.

Lipid profiling by electrospray ionization tandem mass spectrometry and the identification of lipid phosphorylation by kinases in potato stolons

There is limited information about the involvement of lipids and esterified fatty acids in signaling pathways during plant development. The purpose of this study was to evaluate the lipid composition and molecular species of potato (Solanum tuberosum L., cv. Spunta) stolons and to identify phosphorylated lipids in the first two developmental stages of tuber formation. Lipid profiling was determined using ESI-MS/MS, a useful method for the determination of the biosynthesis and catabolism of lipids based on their fatty acid composition. The most prevalent compound identified in this study was phosphatidic acid (PA); digalactosyldiacylglycerol (DGDG) was the second most abundant compound. A 34:2 species was identified in PA, phosphatidylcholine (PC), phosphatidylinositol (PI), and phosphatidylethanolamine (PE). The identification of lipid phosphorylation by kinases was revealed by the presence of the phosphorylated lipids. PA was metabolized to diacylglycerol pyrophosphate (DGPP) by phosphatidic acid kinase (PAK). This work establishes a correlation between lipid fatty acid composition and lipid metabolism enzymes at the beginning of tuber formation and is the first report of PAK activity in the early events of potato tuber formation.

Overexpression of the phosphatidylinositol synthase gene from Zea mays in tobacco plants alters the membrane lipids composition and improves drought stress tolerance

Phosphatidylinositol (PtdIns) is an important lipid because it serves as a key membrane constituent and is the precursor of the inositol-containing lipids that are found in all plants and animals. It is synthesized from cytidine-diphosphodiacylglycerol (CDP-DG) and myo-inositol by PtdIns synthase (PIS). We have previously reported that two putative PIS genes from maize (Zea mays L.), ZmPIS and ZmPIS2, are transcriptionally up-regulated in response to drought (Sui et al., Gene, 426:47-56, 2008). In this work, we report on the characterization of ZmPIS in vitro and in vivo. The ZmPIS gene successfully complemented the yeast pis mutant BY4743, and the determination of PIS activity in the yeast strain further confirmed the enzymatic function of ZmPIS. An ESI-MS/MS-based lipid profiling approach was used to identify and quantify the lipid species in transgenic and wild-type tobacco plants before and after drought treatment. The results show that the overexpression of ZmPIS significantly increases lipid levels in tobacco leaves under drought stress compared to those of wild-type tobacco, which correlated well with the increased drought tolerance of the transgenic plants. Further analysis showed that, under drought stress conditions, ZmPIS overexpressors were found to exhibit increased membrane integrity, thereby enabling the retention of more solutes and water compared with the wild-type and the vector control transgenic lines. Our findings give us new insights into the role of the ZmPIS gene in the response of maize to drought/osmotic stress and the mechanisms by which plants adapt to drought stress.

Phosphoinositide signaling

"All things flow and change…even in the stillest matter there is unseen flux and movement." Attributed to Heraclitus (530-470 BC), from The Story of Philosophy by Will Durant. Heraclitus, a Greek philosopher, was thinking on a much larger scale than molecular signaling; however, his visionary comments are an important reminder for those studying signaling today. Even in unstimulated cells, signaling pathways are in constant metabolic flux and provide basal signals that travel throughout the organism. In addition, negatively charged phospholipids, such as the polyphosphorylated inositol phospholipids, provide a circuit board of on/off switches for attracting or repelling proteins that define the membranes of the cell. This template of charged phospholipids is sensitive to discrete changes and metabolic fluxes-e.g., in pH and cations-which contribute to the oscillating signals in the cell. The inherent complexities of a constantly fluctuating system make understanding how plants integrate and process signals challenging. In this review we discuss one aspect of lipid signaling: the inositol family of negatively charged phospholipids and their functions as molecular sensors and regulators of metabolic flux in plants.

Myo-inositol and beyond--emerging networks under stress

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

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

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

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

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

Assessment of mitochondria as a compartment for phosphatidylinositol synthesis in Solanum tuberosum

The outer mitochondrial membrane is particularly rich in phosphatidylinositol (PtdIns), a phospholipid found in different amounts in all eukaryotic membranes, but not synthesized in situ by all. PtdIns is therefore subjected to traffic from the synthesizing membranes to the non-synthesizing ones. The contribution of mitochondria to the cell PtdIns pool has never been the focus of a specific study in plants, whereas in yeast, the presence of the enzyme responsible for synthesis, PtdIns synthase (PIS, cytidine 5'-diphospho-1,2-diacyl-sn-glycerol:myo-inositol 3-phosphatidyltransferase, EC 2.7.8.11), has clearly been demonstrated in mitochondria. As these organelles have now been shown to be responsible for the synthesis of several lipids, the present work aimed at evaluating mitochondria as a compartment for the synthesis of PtdIns in plants. The sub-cellular localization of PIS was studied in Solanum tuberosum L. by membrane fractionation, enzymatic analysis and by confocal microscopy in living cells. In potato, beside the endoplasmic reticulum, the activity of PIS was found to be tightly associated to mitochondria. Using a fluorescent reporter fusion, the enzyme was also found to be associated to these organelles. The enzyme was not present at the plasma membrane. A comparison of the localization in other cell systems suggests that the mitochondrial localization could be regulated.

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

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

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.

At the poles across kingdoms: phosphoinositides and polar tip growth

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