Phospholipid signaling in plants: holding on to phospholipase D.

Plant cells contain various phospholipase-based signaling pathways. In fact, their repertoire of phospholipase D (PLD) molecules far outnumbers those of mammalian and yeast cells. Munnik and Musgrave take a broad look at PLD function in animal, yeast, and plant cells, and suggest that a PLD-based connection between membranes and microtubules is a biological property worth considering across species.

Identification of a new polyphosphoinositide in plants, phosphatidylinositol 5-monophosphate (PtdIns5P), and its accumulation upon osmotic stress.

Polyphosphoinositides play an important role in membrane trafficking and cell signalling. In plants, two PtdInsP isomers have been described, PtdIns3P and PtdIns4P. Here we report the identification of a third, PtdIns5P. Evidence is based on the conversion of the endogenous PtdInsP pool into PtdIns(4,5)P(2) by a specific PtdIns5P 4-OH kinase, and on in vivo (32)P-labelling studies coupled to HPLC head-group analysis. In Chlamydomonas, 3-8% of the PtdInsP pool was PtdIns5P, 10-15% was PtdIns3P and the rest was PtdIns4P. In seedlings of Vicia faba and suspension-cultured tomato cells, the level of PtdIns5P was about 18%, indicating that PtdIns5P is a general plant lipid that represents a significant proportion of the PtdInsP pool. Activating phospholipase C (PLC) signalling in Chlamydomonas cells with mastoparan increased the turnover of PtdIns(4,5)P(2) at the cost of PtdIns4P, but did not affect the level of PtdIns5P. This indicates that PtdIns(4,5)P(2) is synthesized from PtdIns4P rather than from PtdIns5P during PLC signalling. However, when cells were subjected to hyperosmotic stress, PtdIns5P levels rapidly increased, suggesting a role in osmotic-stress signalling. The potential pathways of PtdIns5P formation are discussed.

Characterization of five tomato phospholipase D cDNAs: rapid and specific expression of LePLDbeta1 on elicitation with xylanase.

Phospholipase D (PLD, EC 3.1.4.4.) has been implicated in a variety of plant processes, including signalling. In Arabidopsis thaliana a PLD gene family has been described and individual members classified into alpha-, beta- and gamma-classes. Here we describe a second PLD gene family in tomato (Lycopersicon esculentum) that includes three alpha- and two beta-classes. Different expression patterns in plant organs were observed for each PLD. In testing a variety of stress treatments on tomato cell suspensions, PLDbeta1 mRNA was found to rapidly and specifically accumulate in response to the fungal elicitor xylanase. The greatest increase was found 2 h after treatment with 100 microg m1(-1) xylanase (ninefold). In vivo PLD activity increased nearly threefold over a 1.5 h period of treatment. When the elicitor was injected into tomato leaves, PLDbeta1 mRNA accumulation peaked at 2 h (threefold increase), before decreasing to background levels within 72 h. Mutant, non-active xylanase was as effective as the active enzyme in eliciting a response, suggesting that xylanase itself, and not the products resulting from its activity, functioned as an elicitor. When chitotetraose was used as elicitor, no PLDbeta1 mRNA accumulation was observed, thus it is not a general response to elicitation. Together these data show that PLD genes are differentially regulated, reflecting potential differences in cellular function. The possibility that PLDbeta1 is a signalling enzyme is discussed.

Hyperosmotic stress rapidly generates lyso-phosphatidic acid in Chlamydomonas.

Plant cells are continuously exposed to environmental stresses such as hyper-osmolarity, and have to respond in order to survive. When 32P-labelled Chlamydomonas moewusii cells were challenged with NaCl, the formation of a new radiolabelled phospholipid was stimulated, which was barely detectable before stimulation. The phospholipid was identified as lyso-phosphatidic acid (LPA), and was the only lyso-phospholipid to be accumulated. The increase in LPA was dose- and time-dependent. When other osmotically active compounds were used, the formation of LPA was also induced with similar kinetics, although salts were better inducers than non-salts. At least part of the LPA was generated by phospholipase A2 (PLA2) hydrolysing phosphatidic acid (PA). This claim is based on PA formation preceding LPA production, and PLA2 inhibitors decreasing the accumulation of LPA and promoting the conversion of PA to diacylglycerol pyrophosphate. The latter is another metabolic derivative of PA that is implicated in cell signalling. The involvement of multiple lipid-signalling pathways in hyperosmotic stress responses is discussed.

Nod factor-induced phosphatidic acid and diacylglycerol pyrophosphate formation: a role for phospholipase C and D in root hair deformation.

Rhizobium-secreted nodulation factors are lipochitooligosaccharides that trigger the initiation of nodule formation on host legume roots. The first visible effect is root hair deformation, but the perception and signalling mechanisms that lead to this response are still unclear. When we treated Vicia sativa seedlings with mastoparan root hairs deformed, suggesting that G proteins are involved. To investigate whether mastoparan and Nod factor activate lipid signalling pathways initiated by phospholipase C (PLC) and D (PLD), seedlings were radiolabelled with [(32)P]orthophosphate prior to treatment. Mastoparan stimulated increases in phosphatidic acid (PA) and diacylglycerol pyrophosphate, indicative of PLD or PLC activity in combination with diacylglycerol kinase (DGK) and PA kinase. Treatment with Nod factor had similar effects, although less pronounced. The inactive mastoparan analogue Mas17 had no effect. The increase in PA was partially caused by the activation of PLD that was monitored by its in vivo transphosphatidylation activity. The application of primary butyl alcohols, inhibitors of PLD activity, blocked root hair deformation. Using different labelling strategies, evidence was provided for the activation of DGK. Since the PLC antagonist neomycin inhibited root hair deformation and the formation of PA, we propose that PLC activation produced diacylglycerol (DAG), which was subsequently converted to PA by DGK. The roles of PLC and PLD in Nod factor signalling are discussed.

Elicitation of suspension-cultured tomato cells triggers the formation of phosphatidic acid and diacylglycerol pyrophosphate.

Phosphatidic acid (PA) and its phosphorylated derivative diacylglycerol pyrophosphate (DGPP) are lipid molecules that have been implicated in plant cell signaling. In this study we report the rapid but transient accumulation of PA and DGPP in suspension-cultured tomato (Lycopersicon esculentum) cells treated with the general elicitors, N,N',N",N"'-tetraacetylchitotetraose, xylanase, and the flagellin-derived peptide flg22. To determine whether PA originated from the activation of phospholipase D or from the phosphorylation of diacylglycerol (DAG) by DAG kinase, a strategy involving differential radiolabeling with [(32)P]orthophosphate was used. DAG kinase was found to be the dominant producer of PA that was subsequently metabolized to DGPP. A minor but significant role for phospholipase D could only be detected when xylanase was used as elicitor. Since PA formation was correlated with the high turnover of polyphosphoinositides, we hypothesize that elicitor treatment activates phospholipase C to produce DAG, which in turn acts as substrate for DAG kinase. The potential roles of PA and DGPP in plant defense signaling are discussed.

Hyperosmotic stress stimulates phospholipase D activity and elevates the levels of phosphatidic acid and diacylglycerol pyrophosphate.

In mammalian cells, phospholipase D (PLD) and its product phosphatidic acid (PA) are involved in a number of signalling cascades, including cell proliferation, membrane trafficking and defence responses. In plant cells a signalling role for PLD and PA is also emerging. Plants have the extra ability to phosphorylate PA to produce diacylglycerol pyrophosphate (DGPP), a newly discovered phospholipid whose formation attenuates PA levels, but which could itself be a second messenger. Here we report that increases in PA and its conversion to DGPP are common stress responses to water deficit. Increases occur within minutes of treatment and are dependent on the level of stress. Part of the PA produced is due to PLD activity as measured by the in vivo transphosphatidylation of 1-butanol, and part is due to diacylglycerol kinase activity as monitored via 32P-PA formation in a differential labelling protocol. Increases in PA and DGPP are found not only in the green alga Chlamydomonas moewusii and cell-suspension cultures of tomato and alfalfa when subjected to hyperosmotic stress, but also in dehydrated leaves of the resurrection plant Craterostigma plantagineum. These results provide further evidence that PLD and PA play a role in plant signalling, and provide the first demonstration that DGPP is formed during physiological conditions that evoke PA synthesis.

Polar glycerolipids of Chlamydomonas moewusii.

The fatty acid and polar lipid compositions of the unicellular green alga Chlamydomonas moewusii were characterized. Since this organism is an important plant model for phospholipid-based signal transduction, interest was focused on the lipids phosphatidic acid, phosphatidylinositolphosphate and phosphatidylinositolbisphosphate. A phosphatidylinositol:phosphatidylinositolphosphate: phosphatidylinositolbisphosphate ratio of 100:1.7:1.3 was found. The polyphosphoinositides accounted for 0.8 mol% of the total phospholipids and their fatty acid compositions were similar to that of phosphatidylinositol except for the enrichment of linolenic acid in phosphatidylinositol phosphate. Phosphatidic acid accounted for 0.67 mol% of the phospholipids. Major structural glycerolipids were monogalactosyldiacylglycerol (35 mol%), digalactosyldiacylglycerol (15 mol%), sulfoquinovosyldiacylglycerol (10 mol%), diacylglyceryltrimethylhomoserine (16 mol%), phosphatidylglycerol (9 mol%), phosphatidylethanolamine (8 mol%) and phosphatidylinositol (6 mol%). Relative changes in the total fatty acid compositions found during growth on nutrient-limited medium reflected mainly alterations in the compositions of the chloroplast lipids phosphatidylglycerol and monogalactosyldiacylglycerol. [32P]Pi-incorporation studies revealed that it took 6 days before the amount of label in the major phospholipids was proportional to their abundance.

Chlamydomonas contains calcium stores that are mobilized when phospholipase C is activated.

Mastoparan induces Ca(2+)-dependent deflagellation of the unicellular green alga Chlamydomonas moewusii Gerloff, as well as the activation of phospholipase C and the production of inositol 1,4, 5-trisphosphate (InsP(3); T. Munnik et al., 1998, Planta 207: 133-145). Even in the absence of extracellular Ca(2+), mastoparan still induces deflagellation (L.M. Quarmby and H.C. Hartzell, 1994, J Cell Biol 124: 807-815; J.A.J. van Himbergen et al., 1999, J Exp Bot, in press) suggesting that InsP(3) mediates Ca(2+) release from intracellular stores. To test this hypothesis, cells were pre-loaded with (45)Ca(2+) and their plasma membranes permeabilized by digitonin. Subsequent treatment of the cells with mastoparan (3.5 microM) induced release of intracellular (45)Ca(2+). Mastoparan also activated phospholipase C in permeabilized cells, as demonstrated by the breakdown of (32)P-phosphatidylinositol 4,5-bisphosphate and the production of diacylglycerol. The mastoparan analogues mas7 and mas17 were also effective and their efficacy was correlated with their biological activity. X-ray microanalysis showed that electron-dense bodies (EDBs) are a major Ca(2+) store in C. moewusii. Analysis of digitonin-permeabilized cells showed that EDBs lost calcium at digitonin concentrations that released radioactivity from (45)Ca(2+)-labelled cells, suggesting that (45)Ca(2+) monitored the content of EDBs. X-ray microanaysis of living cells treated with mastoparan also revealed that calcium was released from EDBs.

Distinct osmo-sensing protein kinase pathways are involved in signalling moderate and severe hyper-osmotic stress

Plant growth is severely affected by hyper-osmotic salt conditions. Although a number of salt-induced genes have been isolated, the sensing and signal transduction of salt stress is little understood. We provide evidence that alfalfa cells have two osmo-sensing protein kinase pathways that are able to distinguish between moderate and extreme hyper-osmotic conditions. A 46 kDa protein kinase was found to be activated by elevated salt concentrations (above 125 mM NaCl). In contrast, at high salt concentrations (above 750 mM NaCl), a 38 kDa protein kinase, but not the 46 kDa kinase, became activated. By biochemical and immunological analysis, the 46 kDa kinase was identified as SIMK, a member of the family of MAPKs (mitogen-activated protein kinases). SIMK is not only activated by NaCl, but also by KCl and sorbitol, indicating that the SIMK pathway is involved in mediating general hyper-osmotic conditions. Salt stress induces rapid but transient activation of SIMK, showing maximal activity between 8 and 16 min before slow inactivation. When inactive, most mammalian and yeast MAPKs are cytoplasmic but undergo nuclear transloca- tion upon activation. By contrast, SIMK was found to be a constitutively nuclear protein and the activity of the kinase was not correlated with changes in its intra-cellular compartmentation, suggesting an intra-nuclear mechanism for the regulation of SIMK activity.

Characterisation and cloning of a calmodulin-like domain protein kinase from Chlamydomonas moewusii (Gerloff).

Calcium-stimulated protein kinase activity in the flagella of the green alga Chlamydomonas moewusii (Gerloff) was characterised. Using SDS-PAGE and an on-blot phosphorylation assay, a 65-kDa protein was identified as the major calcium-stimulated protein kinase. Its activity was directly stimulated by calcium, a characteristic of the calmodulin-like domain protein kinases (CDPKs). Monoclonal antibodies raised against the CDPK alpha from soybean cross-reacted with the 65-kDa protein in the flagella, and also with other proteins in the flagellum and cell body. The same monoclonal antibodies were used to screen a C. moewusii cDNA expression library in order to isolate CDPK cDNAs from C. moewusii. The CCK1 cDNA encodes a protein with a kinase and calmodulin-like domain linked by a junction domain typical of CDPKs. From Southern analyses, evidence was obtained for a CDPK gene family in C. moewusii and C. reinhardtii.

Identification of diacylglycerol pyrophosphate as a novel metabolic product of phosphatidic acid during G-protein activation in plants.

We provide evidence that phosphatidic acid (PtdOH) formed during signaling in plants is metabolized by a novel pathway. In much of this study, 32Pi-labeled Chlamydomonas cells were used, and signaling was activated by adding the G-protein activator mastoparan. Within seconds of activation, large amounts of [32P]PtdOH were formed, with peak production at about 4 min, when the level was 5-25-fold higher than the control. As the level of [32P]PtdOH subsequently decreased, an unknown phospholipid (PLX) increased in radiolabeling; before activation it was barely detectable. The chromatographic properties of PLX resembled those of lyso-PtdOH and CMP.PtdOH but on close inspection were found to be different. PLX was shown to be diacylglycerol pyrophosphate (DGPP), the product of a newly discovered enzyme, phosphatidate kinase, whose in vitro activity was described recently (Wissing, J. B., and Behrbohm, H. (1993) Plant Physiol. 102, 1243-1249). The identity of DGPP was established by co-chromatrography with a standard and by degradation analysis as follows: [32P]DGPP was deacylated, and the product (glycerolpyrophosphate, GroPP) was hydrolyzed by mild acid treatment or pyrophosphatase to produce GroP and Pi as the only radioactive products. Since DGPP is the pyrophosphate derivative of PtdOH and is formed as the concentration of PtdOH decreases, we assumed that PtdOH was converted in vivo to DGPP. This was confirmed by showing that during a short labeling protocol while the specific radioactivity of DGPP was increasing, the specific radioactivity of the 32Pi derived from DGPP as above was higher than that of [32P]GroP. DGPP was also formed in suspension cultures of tomato and potato cells, and its synthesis was activated by mastoparan. Moreover, it was also found in intact tissues of a number of higher plants, for example, carnation flower petals, vetch roots, leaves of fig-leaved goosefoot, and common persicaria and microspores of rape seed. Our results suggest that DGPP is a common but minor plant lipid that increases in concentration when signaling is activated. Possible functions of DGPP in phospholpase C and D signaling cascades are discussed.

G Protein Activation Stimulates Phospholipase D Signaling in Plants.

We provide direct evidence for phospholipase D (PLD) signaling in plants by showing that this enzyme is stimulated by the G protein activators mastoparan, ethanol, and cholera toxin. An in vivo assay for PLD activity in plant cells was developed based on the use of a "reporter alcohol" rather than water as a transphosphatidylation substrate. The product was a phosphatidyl alcohol, which, in contrast to the normal product phosphatidic acid, is a specific measure of PLD activity. When 32P-labeled cells were treated with 0.1% n-butanol, 32P-phosphatidyl butanol (32P-PtdBut) was formed in a time-dependent manner. In cells treated with any of the three G protein activators, the production of 32P-PtdBut was increased in a dose-dependent manner. The G protein involved was pertussis toxin insensitive. Ethanol could activate PLD but was itself consumed by PLD as transphosphatidylation substrate. In contrast, secondary alcohols (e.g., sec-butyl alcohol) activated PLD but did not function as substrate, whereas tertiary alcohols did neither. Although most of the experiments were performed with the green alga Chlamydomonas eugametos, the relevance for higher plants was demonstrated by showing that PLD in carnation petals could also be activated by mastoparan. The results indicate that PLD activation must be considered as a potential signal transduction mechanism in plants, just as in animals.

Tyrosine phosphatase signalling in a lower plant: cell-cycle and oxidative stress-regulated expression of the Chlamydomonas eugametos VH-PTP13 gene.

The first evidence for tyrosine phosphatase signalling pathways in plants is presented by characterizing a putative protein tyrosine phosphatase gene from the unicellular green alga Chlamydomonas eugametos. This cDNA, referred to as VH-PTP13, contains an open reading frame specifying a protein with a molecular weight of 30.3 kDa, that has significant homology with a distinct group of dual-specificity phosphatases. The highest homology is found with CL-100, a human stress-response gene that regulates MAPkinase activity. The purified VH-PTP13 protein expressed in E. coli had phosphatase activity and inactivated MAPkinases from alfalfa and tobacco. Nondividing C. eugametos gametes did not express the VH-PTP13 gene whereas synchronously dividing vegetative cells only expressed VH-PTP13 in the early G1-phase of the cycle, implying a function there. When vegetative cells were subjected to oxidative stress, expression of the VH-PTP13 gene was strongly induced, analogous to the human CL-100 gene. Its potential role in plant signalling pathways is discussed.

Rapid turnover of phosphatidylinositol 3-phosphate in the green alga Chlamydomonas eugametos: signs of a phosphatidylinositide 3-kinase signalling pathway in lower plants?

When Chlamydomonas eugametos gametes were incubated in carrier-free [32P]P1, the label was rapidly incorporated into PtdInsP and PtdInsP2 and, after reaching a maximum within minutes, was chased out by recirculating unlabelled P1 in the cell. This pulse-chase labelling pattern reflects their rapid turnover. In contrast, 32P incorporation into the structural lipids was slow and continued for hours. Of the radioactivity in the PtdInsP spot, 15% was in PtdIns3P and the rest in PtdIns4P, and of that in the PtdInsP2 spot, 1% was in PtdIns(3,4)P2 and the rest in PtdIns(4,5)P2, confirming the findings by Irvine, Letcher, Stephens and Musgrave [(1992) Biochem. J. 281, 269-266]. When cells were labelled with carrier-free [32P]P1, both PtdInsP isomers incorporated label in a pulse-chase-type pattern, demonstrating for the first time in a plant or animal system that D-3 poly-phosphoinositides turn over rapidly in non-stimulated cells, with kinetics similar to those shown by the D-4 isomers. In animal systems such lipids are already established as signalling molecules, and the data suggest that a similar role must be sought for them in lower plants such as Chlamydomonas.

Inositol polyphosphate metabolism and inositol lipids in a green alga, Chlamydomonas eugametos.

Swimming suspensions of Chlamydomonas eugametos were pelleted and homogenized, and the metabolism of inositol polyphosphates by cellular homogenates or supernatants was investigated. Ins(1,4,5)P3 was dephosphorylated under physiological conditions to yield a single InsP2, Ins(1,4]2. In the presence of ATP it was phosphorylated to give Ins(1,3,4,5)P3 as the only InsP4. The Ins(1,4,5)P3 3-kinase activity was predominantly soluble, was not detectably affected by calmodulin or Ca2+, and had a Km for Ins(1,4,5)P3 of 50 microM (two orders of magnitude higher than its mammalian counterpart). Ins(1,3,4,5)P4 was dephosphorylated by the cellular supernatants to Ins(1,3,4)P3 and Ins(1,4,5)P3, and could be phosphorylated to Ins(1,3,4,5,6)P4. No Ins(1,3,4)P3 6-kinase activity could be detected, and experiments with [3H]Ins(1,4,[32P]5)P3 revealed that Ins(1,3,4,5,6)P5 is formed from Ins(1,4,5)P3 with little loss of the 5-phosphate, i.e. the predominant route of synthesis is probably by a direct 6-phosphorylation of Ins(1,3,4,5)P4. Similar experiments with an (NH4)2SO4 fraction of turkey erythrocyte cytosol gave essentially the same result, i.e. direct phosphorylation of Ins(1,3,4,5)P4 in the 6 position is the predominant route of synthesis of InsP5 from that InsP4 in vitro. No InsP6 formation was detected in any of these experiments, but labelling of intact C. eugametos with [3H]inositol revealed that the cells do synthesize InsP6. The lipids of C. eugametos cells contain PtdIns, PtdIns(4)P and PtdIns(4,5)P2 [Irvine, Letcher, Lander, Drøbak, Dawson & Musgrave (1989) Plant Physiol. 64, 888-892]. Further examination of 32P-labelled lipids revealed that about 20% of the PtdInsP was the PtdIns(3)P isomer, and about 1% or less of the PtdInsP2 was the PtdIns(3,4)P2 isomer. The overall inositide metabolism of C. eugametos resembles that of a mammalian cell more closely than it does that of a plant cell or slime mould, and this suggests firstly that the known metabolism of inositol polyphosphates arose at an early time in eukaryotic evolution, and secondly that Chlamydomonas might prove a useful organism for genetic and comparative studies of inositide enzymology.

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.

Phototactic responses of Chlamydomonas eugametos gametes before and after fusion.

The phototactic behavior of Chlamydomonas eugametos gametes and vis-à-vis pairs was quantitated using a fully automated, computer-controlled microvideo image analysis system. Two different mt- (mating type minus) and one mt+ (mating type plus) strain, together with the two combinations of pairs were studied. One mt- strain of dark-adapted gametes was non-phototactic while the others were positively phototactic at all effective intensities of white light. The mt+ strain exhibited one of the strongest positive responses that has so far been reported in algae (r-values greater than 0.7). After sexual fusion, the mt+ cell powers the swimming vis-à-vis pair. Its phototactic behavior reversed on fusion, with the pairs swimming away from all effective light intensities, irrespective of whether its partner was formerly phototactic or not. However, when adapted to the dark for an hour or more, vis-à-vis pairs swam positively to the light. The ecological consequence could be that pairs settle and develop into zygotes under intermediate light intensities or at light-dark interfaces.