The Arabidopsis thaliana genome has multiple divergent forms of phosphoinositol-specific phospholipase C1

Phosphoinositide-specific phospholipase C in oat roots: association with the actin cytoskeleton

Phosphoinositide-specific phospholipase C (PI-PLC) activities are involved in mediating plant cell responses to environmental stimuli. Two variants of PI-PLC have been partially purified from the roots of oat seedlings; one cytosolic and one particulate. Although the cytosolic enzyme was significantly purified, the activity still co-migrated with a number of other proteins on heparin HPLC and also on size-exclusion chromatography. The partially purified PI-PLC was tested by Western blotting, and we found that actin and actin-binding proteins, profilin and tropomyosin, co-purified with cytosolic phospholipase C. After a non-ionic detergent (Triton X-100) treatment, PI-PLC activities still remained with the actin cytoskeleton. The effects of phalloidin and F-buffer confirmed this association; these conditions, which favor actin polymerization, decreased the release of PI-PLC from the cytoskeleton. The treatments of latrunculin and G-buffer, the conditions that favor actin depolymerization, increased the release of PI-PLC from the cytoskeleton. These results suggest that oat PI-PLC associates with the actin cytoskeleton.

Expression and evolution of the phosphoinositide-specific phospholipase C gene family in Arabidopsis thaliana

Phosphoinositide-specific phospholipase C cleaves the substrate phosphatidylinositol 4,5-bisphosphate and generates inositol 1,4,5-trisphosphate and 1,2-diacylglycerol, both of which are second messengers in the phosphoinositide signal transduction pathways operative in animal cells. Five PI-PLC isoforms, beta, gamma, delta, epsilon and zeta, have been identified in mammals. Plant PI-PLCs are structurally close to the mammalian PI-PLC-zeta isoform. The Arabidopsis genome contains nine AtPLC genes. Expression patterns of all nine genes in different organs and in response to various environmental stimuli were studied by applying a quantitative RT-PCR approach. Multiple members of the gene family were differentially expressed in Arabidopsis organs, suggesting putative roles for this enzyme in plant development, including tissue and organ differentiation. This study also shows that a majority of the AtPLC genes are induced in response to various environmental stimuli, including cold, salt, nutrients Murashige-Skoog salts, dehydration, and the plant hormone abscisic acid. Results of this and previous studies strongly suggest that transcriptional activation of the PI-PLC gene family is important for adapting plants to stress environments. Expression patterns and phylogenetic relationships indicates that AtPLC gene members probably evolved through multiple rounds of gene duplication events, with AtPLC4 and AtPLC5 and AtPLC8 and AtPLC9 being duplicated in tandem in recent times.

The evolution of Ca2+ signalling in photosynthetic eukaryotes

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

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

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

Molecular cloning and expression analysis of phospholipase Cδ from mud loach, Misgurnus mizolepis

A gene encoding phosphoinositide-specific phospholipase C (PLC), designated ML-PLCdelta, was cloned from mud loach (Misgurnus mizolepis) liver. A complete cDNA encoding ML-PLCdelta was isolated by screening the cDNA library of mud loach liver and using the 5'-rapid amplification of cDNA ends (RACE) method. The full-length ML-PLCdelta gene contains an open reading frame of 2325 base pairs encoding a 774 amino acid protein with a molecular mass of 88,072 Da; this corresponds to the size of the protein expressed in Escherichia coli BL21 (DE3) using pET28a vector. It contains all of the characteristic domains found in mammalian PLCdelta isozymes (PH domain, EF-hands, X-Y catalytic region, and a C2 domain). A homology search revealed that ML-PLCdelta shares relatively high sequence identity with mammalian PLCdelta1 (51-52%) and catfish PLCdelta (64%). The recombinant ML-PLCdelta protein expressed as a histidine-tagged fusion protein in E. coli was purified to apparent homogeneity by Ni(2+)-NTA affinity chromatography. The recombinant ML-PLCdelta showed a concentration-dependent PLC activity to phosphatidylinositol 4,5-bis-phosphate (PIP(2)) and its activity was Ca(2+)-dependent, which was similar to mammalian PLCdelta isozymes.

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

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

Isolation and characterization of a phospholipase C delta isoform from pea that is regulated by light in a tissue specific manner

Phosphoinositide-specific phospholipases C (PLCs) play an important role in many cellular responses and are involved in the production of secondary messengers. We report the cloning and characterization of a cDNA encoding a PLC-delta from Pisum sativum (PsPLC). The amino acid sequence deduced from the cDNA sequence showed 75-80% identity to other plant PLCs and contained the characteristic X, Y and C2 domains. The genomic PLC clone from pea was also characterized and found to contain eight introns. The protein was expressed in Escherichia coli, but the recombinant product did not show any phosphoinositide (PI)- or phosphatidylinositol-4, 5-bisphosphate (PIP2)-specific activity, despite having all known residues required for such activity, and in spite of the fact that its C2 domain was shown to bind calcium. Under similar in vitro assay conditions the recombinant tobacco PLC used as a control showed calcium-dependent PI- and PIP2-specific activity. Though PsPLC did not show enzyme activity in vitro, and may represent an inactive form of PLC, such as those reported in some mammalian systems, analysis of the transcription of PsPLC showed that the gene is expressed in all pea tissues, and is regulated by light in a tissue-specific manner. Roots showed higher expression of PsPLC than shoots. A putative PsPLC promoter region (792 bp) was also cloned and found to contain root-specific and light-responsive cis elements, suggesting that this form of PLC may be involved in important functions in plants.

The G-protein-coupled receptor GCR1 regulates DNA synthesis through activation of phosphatidylinositol-specific phospholipase C

Different lines of evidence suggest that specific events during the cell cycle may be mediated by a heterotrimeric G-protein activated by a cognate G-protein coupled receptor. However, coupling between the only known Galpha-subunit of the heterotrimeric G-protein (GPA1) and the only putative G-protein coupled receptor (GCR1) of plants has never been shown. Using a variety of approaches, we show here that GCR1-enhanced thymidine incorporation into DNA depends on an increase in phosphatidylinositol-specific phospholipase C activity and an elevation of inositol 1,4,5-trisphosphate levels in the cells. Tobacco (Nicotiana tabacum) cells that overexpress either Arabidopsis GCR1 or GPA1 display this phenomenon. We suggest on the basis of these results that GCR1-controlled events during the cell cycle involve phosphatidylinositol-specific phospholipase C as an effector of GCR1 and inositol 1,4,5-trisphosphate as a second messenger, and that GCR1 and GPA1 are both involved in this particular signaling pathway.

Phospholipid-based signaling in plants

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

PLANT PHOSPHOLIPASES

Phospholipases are a diverse series of enzymes that hydrolyze phospholipids. Multiple forms of phospholipases D, C, and A have been characterized in plants. These enzymes are involved in a broad range of functions in cellular regulation, lipid metabolism, and membrane remodeling. In recent years, increasing attention has been paid to the many roles of phospholipases in signal transduction. This review highlights recent developments in the understanding of biochemical, molecular biological, and functional aspects of various phospholipases in plants.

Structure, function, and control of phosphoinositide-specific phospholipase C

Phosphoinositide-specific phospholipase C (PLC) subtypes beta, gamma, and delta comprise a related group of multidomain phosphodiesterases that cleave the polar head groups from inositol lipids. Activated by all classes of cell surface receptor, these enzymes generate the ubiquitous second messengers inositol 1,4, 5-trisphosphate and diacylglycerol. The last 5 years have seen remarkable advances in our understanding of the molecular and biological facets of PLCs. New insights into their multidomain arrangement and catalytic mechanism have been gained from crystallographic studies of PLC-delta(1), while new modes of controlling PLC activity have been uncovered in cellular studies. Most notable is the realization that PLC-beta, -gamma, and -delta isoforms act in concert, each contributing to a specific aspect of the cellular response. Clues to their true biological roles were also obtained. Long assumed to function broadly in calcium-regulated processes, genetic studies in yeast, slime molds, plants, flies, and mammals point to specific and conditional roles for each PLC isoform in cell signaling and development. In this review we consider each subtype of PLC in organisms ranging from yeast to mammals and discuss their molecular regulation and biological function.

Decreased inositol 1,4,5-trisphosphate content in pathogen-challenged soybean cells

Phosphoinositide-specific phospholipase C (PI-PLC) has been shown to be transiently activated when plant cells were treated with elicitors. We thus investigated the activity of PI-PLC when soybean cells were infected with the bacterial pathogen Pseudomonas syringae pv. glycinea, by measuring cellular cytosolic inositol 1,4,5-trisphosphate (IP3) levels. We observed that IP3 content decreased in both compatible and incompatible interactions. In vitro phosphatase activities were similar in both water control and infected cells with slightly lower IP3 degradation observed for infected cells, indicating that the reduced IP3 content in infected cells most likely results from reduced PI-PLC activity. We hypothesize that reduced IP3 content following infection may lead to suppression of various housekeeping activities of the cells, thus diverting the cellular resources either to the synthesis of defense-related compounds against pathogens, and/or to the growth of pathogens.

Map positions of SFR genes in relation to other freezing-related genes of Arabidopsis thaliana

We determined the map positions of seven SFR genes and compared them to the positions of 51 genes suspected of involvement in freezing tolerance in Arabidopsis thaliana. The SFR genes were recognized by the freezing sensitivity of mutants; the others (including 14 whose map positions we have determined) were genes whose expression is induced by low temperature, genes involved in abscisic acid (ABA) biosynthesis and perception, and genes involved in tolerance of oxidative stress. The comparison of map positions indicated a limited set of potential identities, some of which were eliminated by further mapping or by an allelism test.