Alan Musgrave

PHOSPHOLIPID SIGNALLING IN PLANTS

book, 1996, p. 61. w x 424 H. Pfaffmann, E. Hartmann, A.O. Brightman, D.J. Morre, Ž . Plant Physiol. 85 1987 1151–1155. w x 425 C. Pical, A.S. Sandelius, P.M. Melin, M. Sommarin, Plant Ž . Physiol. 10

Hyperosmotic stress induces rapid synthesis of phosphatidyl-D-inositol 3,5-bisphosphate in plant cells

Abstract. Cells from several different plant species synthesised a polyphosphoinositide (PPI)-like lipid when osmo-stressed. Synthesis was maximal after about 10 min and was stimulated by a variety of osmolytes. Using NaCl, the strongest response centred around 200 mM. The lipid was shown to be the novel PPI isomer phosphatidyl-inositol 3,5-bisphosphate [PtdIns-(3,5)P2] by analytical thin-layer chromatography and conversion to PtdIns(3,4,5)P3 using recombinant phosphoinositide 4-OH kinase. The results indicate that PtdIns-(3,5)P2 plays a role in a general osmo-signalling pathway in plants. Its potential role is discussed.

Detailed analysis of the turnover of polyphosphoinositides and phosphatidic acid upon activation of phospholipases C and D in Chlamydomonas cells treated with non-permeabilizing concentrations of mastoparan

Abstract. Treating Chlamydomonas moewusii cells with non-permeabilizing concentrations of mastoparan (1–5 μM) increased inositol 1,4,5-trisphosphate (InsP3) levels up to 20-fold in a dose-dependent manner and rapidly induced deflagellation and mating-structure activation, two well-defined Ca2+-responses. When metabolism of the phospholipid precursors was monitored in 32Pi-labelled cells, as much as 70% of the radioactivity in phosphatidylinositol bisphosphate (PtdInsP2) was lost within 20 s. Thereafter, the 32P-label in PtdInsP2 increased to twice the control level within 10 min. A similar pattern of 32P-labelling was also exhibited by PtdInsP. An HPLC-headgroup analysis revealed that only PtdIns4P and PtdIns(4,5)P2 were involved and not the D3-phosphorylated isomers. Correlated with the increased polyphosphoinositide (PPI) turnover, there was a massive (5- to 10-fold) increase in 32P-labelled phosphatidic acid (PtdOH) and, slightly later, an increase in its metabolic product, diacylglycerol pyrophosphate (DGPP), reflecting the phosphorylation of the resulting diacylglycerol (DAG) and PtdOH, respectively. Mastoparan-treatment of 32P-labelled cells in the presence of 0.2% n-butanol increased the formation of radioactive phosphatidylbutanol (PtdBut), a specific reporter of phospholipase D (PLD) activity. This means that mastoparan activates both phospholipase C (PLC) and PLD, and thus both pathways could contribute to the increase in PtdOH. To distinguish between them, a differential labelling strategy was applied based on the fact that 32Pi-label is slowly incorporated into structural phospholipids but rapidly incorporated into ATP. Since PLD hydrolyses a structural lipid, radioactivity only appears slowly in PtdOHPLD (and PtdBut). In contrast, PtdOHPLC is synthesised by phosphorylation of DAG, and therefore should rapidly incorporate radioactivity. In practice, PtdOH formed on addition of mastoparan was rapidly labelled, reflecting the specific radioactivity of the [32P]ATP pool. Based on the production of [32P]PtdBut, we estimate that about 5–17% of the PtdOH was generated through the PLD pathway, while the majority originated from PLC activity. Together, this is the first demonstration (i) that PLC activation is correlated with increases in Ca2+, InsP3, PtdOH and DGPP, at the cost of PtdInsP and PtdInsP2, all in one and the same cell, (ii) of the characteristics of stimulated and unstimulated PPI turnover, (iii) that stimulated turnover affects the D-4 PPI and not the 3-isomers, (iv) that PLC and PLD are activated at the same time, (v) of a simple labelling method to discriminate between the two in terms of PtdOH production.

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.

Cyclic AMP is involved in sexual reproduction of Chlamydomonas eugametos

When plus and minus mating type gametes of Chlamydomonas eugametos were mixed, a rapid transient increase in the amount of cAMP was observed with a maximum at 20 s after the start of the sexual agglutination reaction. The transient increase only occurred within the cells and was also exhibited when cell suspensions of single mating type were presented with isolated flagella of the other mating type. Cyclic AMP‐dependent protein kinase and cyclic AMP‐phosphodiesterase activities were found in cell homogenates. Since the rise in cAMP concentration preceded all known morphological and physiological changes in the cells that prepare them for fusion, it might be a primary response, induced by sexual agglutination.

Evidence for a functional membrane barrier in the transition zone between the flagellum and cell body of Chlamydomonas eugametos gametes

Evidence is presented which supports the concept of a functional membrane barrier in the transition zone at the base of each flagellum of Chlamydomonas eugametos gametes. This makes it unlikely that agglutination factors present on the surface of the cell body can diffuse or be transported to the flagellar membrane. The evidence is as follows: 1) The glycoprotein composition of the flagellar membrane is very different to that of the cell-body plasma membrane. 2) The flagella of gametes treated with cycloheximide, tunicamycin or α, α′-dipyridyl become non-agglutinable but the source of agglutination factors on the cell body is not affected. 3) Even under natural conditions when the flagella are non-agglutinable, for example in vis-à-vis pairs or in appropriate cell strains that are non-agglutinable in the dark, the cell bodies maintain the normal complement of active agglutinins. 4) When flagella of living cells are labeled with antibodies bound to fluorescein, the label does not diffuse onto the cell-body surface. 5) When gametes fuse to form vis-à-vis pairs, the original mating-type-specific antigenicity of each cell body is slowly lost (probably due to the antigens diffusing over both cell bodies), while the specific antigenicity of the flagellar surface is maintained. Even when the flagella of vis-à-vis pairs are regenerated from cell bodies with mixed antigenicity, the antigenicity of the flagella remains matingtype-specific. 6) Evidence is presented for the existence of a pool of agglutination factors within the cell bodies but not on the outer surface of the cells.

Rapid turnover of polyphosphoinositides in carnation flower petals

Carnation (Dianthus caryophyllus L. cv. White Sim) petal discs were radiolabelled with [32P]orthophosphate and the lipids were extracted and analysed by thin-layer chromatography and autoradiography. Phospholipids were identified by co-migration with standards using thin-layer chromatography with different solvent systems. Results showed that [32P]orthophosphate was rapidly incorporated into the minor lipids phosphatidic acid (PtdOH), phosphatidylinositol monophosphate (PtdInsP) and phosphatidylinositol bisphosphate (PtdInsP2), and relatively slowly into the structural lipids phosphatidylcholine, -ethanolamine, -glycerol and -inositol. Pulse-chase experiments revealed that the label was rapidly lost from PtdOH, PtdInsP and PtdInsP2 while the structural lipids remained radiolabelled. The amount of PtdInsP and PtdInsP2 was found to constitute 0.45% and 0.013%, respectively, of the total phospholipids, on a molar basis. Together these results show that the turnover of the chemically low-abundant polyphosphoinositides is relatively high compared with the major structural phospholipids. Phosphatidylinositol monophosphate was further characterized by showing that it incorporates myo[3H]inositol and that its major fatty-acid constituents are palmitic acid and linoleic acid. Furthermore, we present evidence for the presence of both phosphatidylinositol 3-phosphate and phosphatidylinositol 4-phosphate isomers. The significance of these results is discussed with respect to plant phosphoinositide signal transduction.

SEXUAL AGGLUTINATION FACTOR FROM CHLAMYDOMONAS-EUGAMETOS

Chlamydomonas eugametos gametes can sexually agglutinate via their flagellar surfaces whereas vegetative cells cannot. Therefore, flagellar glycoproteins, present in gamete cells but absent from vegetative cells, were investigated as prospective mt-agglutination factors. They were identified as periodic acid Schiff (PAS) stained bands separated in sodium dodecyl sulphate-polyacrylamide electrophoresis gels. Gamete-specific bands were determined by comparison with equivalent gels of vegetative flagella and by immunological techniques using antisera raised against isolated mt- gamete flagella. Four high molecular weight flagellar glycoproteins proved to be gamete specific (PAS-1.2, PAS-1.3, PAS-3 and PAS-4). They were extracted from flagella by 3 M guanidine thiocyanate, separated in a column of Sepharose 2B, and tested for in vitro agglutination activity on mt+ gametes. A single peak of activity was found to be correlated with the presence of the PAS-1.2 band. It is shown that mt- agglutination activity is related to the concentration of this glycoprotein in flagellar membranes.

Ethylene and buoyancy control rachis elongation of the semi-aquatic fern Regnillidium diphyllum.

SummaryWhen fronds of the semi-aquatic fern Regnillidium diphyllum Lindman are submerged the rachi elongate much more than when in contact with the air. Submergence appears to have two consequences that account for the enhanced elongation. Firstly, ethylene accumulates in the tissue and secondly, the hollow fronds are buoyed upward from the rhizome, continuously creating a tension in the rachi. The effect of ethylene is a rapid enhancement of cell elongation. It is suggested that ethylene softens the cell walls which are then stretched by the tensile force resulting from buoyancy.

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.