Characterization of Inositol Phosphates in Carrot (Daucus carota L.) Cells

Calcium channels in the vacuolar membrane of plants: multiple pathways for intracellular calcium mobilization

An increasing number of studies imply that Ca 2+ mobilization from intracellular stores plays an important role in stimulus evoked elevation of cytosolic free calcium during signal transduction in plants. It is believed that Ca 2+ is released mainly from the vacuole, which contains a high Ca 2+ concentration in a large volume, and can be regarded as the principal Ca 2+ pool in mature higher plant cells. The large size of the organelle confers unique experimental advantages to the study of endomembrane ion channels. The patch-clamp technique can be directly applied to isolated vacuoles to characterize Ca 2+ release pathways at the single channel level and confirm their membrane location. Using radiometric, ligand-binding and electrophysiological techniques we characterized two different pathways by which Ca 2+ can be mobilized from the vacuole of Beta vulgaris tap roots. Inositol 1,4,5 trisphosphate (Ins P 3 )-elicited Ca 2+ release from tonoplast enriched vesicles is dose-dependent, highly specific for Ins P 3 , and is competitively inhibited by low M r heparin ( K i = 34 nM). This striking resemblance to the animal counterpart which is probably located in the ER is further reflected by the binding properties of the solubilized Ins P 3 receptor from beet, which bears similarities to the Ins P 3 receptor of cerebellum. Thus, Ins P 3 and heparin bind to a single site with sub-micromolar K d s, whereas other inositol phosphates have affinities in the supra-micromolar range. The second Ca 2+ channel in the beet tonoplast is voltage-sensitive and channel openings are largely promoted by positive shifts in the vacuolar membrane potential over the physiological range. Channel activity is neither affected by Ins P 3 addition nor by alteration of cytosolic free calcium, and from a large range of Ca 2+ antagonists tested, only Zn 2+ and the lanthanide Gd 3+ proved to be effective inhibitors. With Ca 2+ as a charge carrier the maximum unitary slope conductance is about 12 pS and saturation occurs at < 5 mM vacuolar Ca 2+ . The channel has an approximately 20-fold higher selectivity for Ca 2+ over K + which is achieved by a Ca 2+ binding site in the channel pore. The unique properties of this novel Ca 2+ release pathway suggests that it is specific for plants. The presence of both Ins P 3 -gated and voltage-gated Ca 2+ channels at the vacuolar membrane implies flexibility in the mechanism of intracellular Ca 2+ mobilization in plant cells.

Evidence that generation of inositol 1,4,5-trisphosphate and hydrolysis of phosphatidylinositol 4,5-bisphosphate are rapid responses following addition of fungal elicitor which induces phytoalexin synthesis in lucerne (Medicago sativa) suspension culture cells

Treatment of lucerne suspension culture cells with glycoprotein elicitor from the phytopathogenic fungus Verticillium albo-atrum R & B triggers Ca(2+)-mediated induction of antimicrobial secondary metabolites termed phytoalexins. The present study investigated the possible role of polyphosphoinositide signal transduction in phytoalexin elicitation. Within 1 min of addition of elicitor to lucerne suspension culture cells we found a 100-160% (15-25 pmol/g fresh wt) increase in the level of compound with chromatographic and electrophoretic properties expected for an inositol trisphosphate (InsP3) and which was strongly bound by an inositol 1,4,5-trisphosphate (Ins(1,4,5)P3)-specific binding protein; after 3 min the level of this compound had fallen below that observed prior to elicitor challenge. In 32P-prelabelled cells, the relative proportion of radioactivity which cochromatographed with phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P2) was found to have decreased by 48% 1 min after elicitor addition and that rapid depletion of membrane lipid radioactivity was specific to this lipid fraction. The rapid, transient increase in level of Ins(1,4,5)P3 and concomitant fall in PtdIns(4,5)P2 suggests that Ins(1,4,5)P3 generated by hydrolysis of PtdIns(4,5)P2 may provide a Ca(2+)-mobilizing signal in phytoalexin elicitation in lucerne.

Lithium decreases cold-induced microtubule depolymerization in mesophyll cells of spinach

Freezing, dehydration, and supercooling cause microtubules in mesophyll cells of spinach (Spinacia oleracea L. cv Bloomsdale) to depolymerize (ME Bartolo, JV Carter [1991] Plant Physiol 97: 175-181). The objective of this study was to gain insight into the question of whether microtubules depolymerize as a direct response to environmental stresses or as an indirect response to cellular changes that accompany the stresses. Leaf sections of spinach were treated with Li(+) before and during exposure to low temperature. Treatment with Li(+) decreased the amount of microtubule depolymerization in cells subjected to low temperature, relative to a nontreated control, raising the possibility that the microtubules in these cells may not be inherently cold labile. Rather, microtubule depolymerization may be in response to cold-induced changes in concentration of cytoplasmic components.

Ethanol stimulates phospholipid turnover and inositol 1,4,5-trisphosphate production in Chlamydomonas eugametos gametes

Alcohols induce mating-structure activation in Chlamydomonas eugametos gametes. From the effect of ethanol on the (32)P-labelling of polyphosphoinositides, we conclude that the synthesis of these lipids is stimulated. Biologically inactive concentrations of ethanol (<6%) had no effect on synthesis, but 6-8% ethanol stimulated synthesis for upto 60 min. The (32)P incorporated into polyphosphoinositides and phosphatidic acid during ethanol treatment was readily chased out when 1 mM unlabelled Na3PO4 was added. Using a binding assay for inositol 1,4,5-trisphosphate, we show that the production of this phospholipid constituent is dramatically increased after ethanol treatment. This effect, coupled to a rise in intracellular calcium concentration, could explain gamete activation. The significance of these results in explaining other ethanol-induced phenomena in algae is discussed.

Ca2+ regulation of phosphatidylinositol turnover in the plasma membrane of tobacco suspension culture cells

The biochemical properties of the enzymes involved in phosphatidylinositol (PI) turnover in higher plants were investigated using the plasma membrane isolated from tobacco suspension culture cells by aqueous two-phase partitioning. Submicromolar concentrations of Ca2+ inhibited PI kinase and phosphatidylinositol 4-phosphate (PIP) kinase and stimulated phospholipase C. Diacylglycerol (DG) kinase was inhibited by Ca2+, but required a higher concentration than the physiological level. From the above results we postulate the following scheme: signal coupled activation of phospholipase C produces IP3 which induces Ca2+ release from the intracellular Ca2+ compartment, the increased cytoplasmic Ca2+ in turn activates phospholipase C and causes a further increase of the cytoplasmic Ca2+ level. This inhibits PI kinase and PIP kinase and brings about a limited supply of PIP2, the substrate of phospholipase C. Consequently, IP3 production decreases and Ca2+ mobilization ceases. Then cytosolic Ca2+ returns to the stationary level by the Ca2+ pump at the plasma membrane and at the endoplasmic reticulum and Ca2+/H+ antiporter at the plasma membrane and at the tonoplast.

Polyphosphoinositol lipids in Chlamydomonas eugametos gametes

In Chlamydomonas eugametos gametes, phosphatidylinositol 4-phosphate (PtdInsP) and phosphatidylinositol 4,5-bisphosphate (PtdInsP2) comprised 0.4 and 0.3% of the whole-cell phospholipids. They were concentrated in the plasma membrane around the cell body and were present in low concentrations in the flagellar membrane. When gametes were fed (32)PO 4 (-) , the label was rapidly incorporated into PtdInsP and PtdInsP2 and only slowly incorporated into structural lipids such as phosphatidylethanolamine and phosphatidylglycerol. Similarly, when a pulse of (32)PO 4 (-) was chased with PO 4 (-) , the label was rapidly lost from the polyphosphoinositol lipids but not from the structural lipids. The major fatty acids in the polyphosphoinositides were C-22 carbon polyenoic acids (70%). The significance of these results in relationship to intracellular signalling via inositol phosphates and Ca(2+) is discussed.

Short-term treatment with cell wall degrading enzymes increases the activity of the inositol phospholipid kinases and the vanadate-sensitive ATPase of carrot cells

Treating carrot (Daucus carota L.) suspension culture cells with a mixture of cell wall degrading enzymes, Driselase, resulted in an increase in the percentage of [(3)H]phosphatidylinositol bisphosphate. Analysis of the lipid kinase activities in the isolated plasma membranes after whole cell treatment indicated that treatment with Driselase (2% weight/volume; the equivalent of 340 units per milliliter of hemicellulase and 400 units per milliliter of cellulase activity) or treatment with hemicellulase (31.7% weight/volume, 20.7 units per milliliter) resulted in an increase in the inositol phospholipid kinase activity. However, treatment with cellulase alone had no effect at 0.5% (weight/volume, 17.2 units per milliliter) or inhibited the kinase activity at 1% (weight/volume, 34.4 units per milliliter). The active stimulus in Driselase was heat sensitive. The plasma membrane vanadate-sensitive ATPase activity also increased when the cells were treated with Driselase. A time course study indicated that both the inositol phospholipid kinases and the plasma membrane vanadate-sensitive ATPase responded to as little as 5 seconds of treatment with 2% Driselase. However, at the lowest concentration of Driselase (0.04%, weight/volume) that resulted in an increase in inositol phospholipid kinase activity, the ATPase activity was not affected. Because inositol phospholipids have been shown to activate the vanadate-sensitive ATPase from plants (AR Memon, Q Chen, WF Boss [1989] Biochem Biophys Res Commun 162: 1295-1301), a stimulus-response pathway involving both the inositol phospholipid kinases and the plasma membrane vanadate-sensitive ATPase activity is discussed.

Transmembrane Signaling via Phosphatidylinositol 4,5-Bisphosphate Hydrolysis in Plants

Recent investigations have confirmed the presence of the polyphosphoinositides, phosphatidylinositol 4-phosphate and phosphatidylinositol 4,5-bisphosphate (PIP(2)), as well as inositol phospholipid-specific phospholipase C in higher plant and microalgal cells. In addition, it has been shown that stimulation of some photosynthetic cell types by environmental or hormonal challenge is accompanied by degradation of the polyphosphoinositides. The products of phospholipase C-catalyzed PIP(2) hydrolysis, inositol 1,4,5-trisphosphate and diacylglycerol, appear to be capable of releasing organelle-bound Ca(2+) and stimulating protein kinase C-like activity in vitro. However, a direct cause and effect relationship between stimulated PIP(2) breakdown and changes in intracellular calcium, protein phosphorylation, or cell function has not yet been unequivocally established. Despite a number of technical difficulties slowing progress in this field, it is likely that photosynthetic organisms will soon be shown to transmit physiologically significant extracellular signals across their plasma membranes by a PIP(2)-mediated transduction mechanism.

Inositol Trisphosphate Metabolism in Carrot (Daucus carota L.) Cells

The metabolism of exogenously added d-myo-[1-(3)H]inositol 1,4,5-trisphosphate (IP(3)) has been examined in microsomal membrane and soluble fractions of carrot (Daucus carota L.) cells grown in suspension culture. When [(3)H]IP(3) was added to a microsomal membrane fraction, [(3)H]IP(2) was the primary metabolite consisting of approximately 83% of the total recovered [(3)H] by paper electrophoresis. [(3)H]IP was only 6% of the [(3)H] recovered, and 10% of the [(3)H]IP(3) was not further metabolized. In contrast, when [(3)H]IP(3) was added to the soluble fraction, approximately equal amounts of [(3)H]IP(2) and [(3)H]IP were recovered. Ca(2+) (100 micromolar) tended to enhance IP(3) dephosphorylation but inhibited the IP(2) dephosphorylation in the soluble fraction by about 20%. MoO(4) (2-) (1 millimolar) inhibited the dephosphorylation of IP(3) by the microsomal fraction and the dephosphorylation of IP(2) by the soluble fraction. MoO(4) (2-), however, did not inhibit the dephosphorylation of IP(3) by the soluble fraction. Li(+) (10 and 50 millimolar) had no effect on IP(3) metabolism in either the soluble or membrane fraction; however, Li(+) (50 millimolar) inhibited IP(2) dephosphorylation in the soluble fraction about 25%.

Inositol phospholipids activate plasma membrane ATPase in plants

Phosphatidylinositol-4-monophosphate and phosphatidylinositol-4,5-bisphosphate increased the activity of the vanadate-sensitive ATPase associated with plasma membranes isolated from both sunflower hypocotyls and carrot suspension culture cells. The response was not due to the metabolism of the polyphosphoinositides since diacylglycerol, inositol-1,4-bisphosphate, inositol-1,4,5-trisphosphate, glycerophosphoinositol monophosphate and glycerophosphoinositol bisphosphate had no effect. These data suggest that activation of the inositol phospholipid kinases could be a critical step in signal transduction in plants.