A phosphatidylserine decarboxylase activity in root cells of oat (Avena sativa) is involved in altering membrane phospholipid composition during drought stress acclimation.

During acclimation to drought stress, the lipid composition of oat root cell membranes is altered. The level of phosphatidylethanolamine (PE), a non-bilayer forming lipid, is increased relative to the bilayer-forming lipid phosphatidylcholine (PC). These changes are believed to increase stress tolerance by increasing the flexibility of the membranes. To elucidate if de novo lipid synthesis is involved in altering membrane lipid composition, oat plants, acclimated or non-acclimated, were incubated in vivo with radioactively labelled lipid precursors. The labelling pattern indicated that de novo synthesis, at least partly, is causing the alterations. In plants, phospholipids can be synthesized by the Kennedy pathway, with addition of activated head groups to diacylglycerol (DAG) or, alternatively, via the CDP-DAG pathway, where phospahtidylserine (PS) is decarboxylated to form PE. To reveal the importance of the respective pathways during acclimation, we studied the effect of a decarboxylase inhibitor and the relative incorporation of [(3)H]-serine and [(14)C]-ethanolamine in vivo. Activities of CTP:ethanolaminephosphate cytidyltransferase (EC 2.7.7.14), phosphatidylserine decarboxylase (EC 4.1.1.65) and phosphatidylserine synthase; CDP-DAG:L-serine o-phosphatidyltransferase (EC 2.7.8.8) were measured and additionally, the presence of a PS decarboxylase (PSD1) in oat was confirmed by immunoblotting. The results suggest that PE synthesis via the Kennedy pathway is downregulated during acclimation and that synthesis by PS decarboxylation, via the CDP-DAG pathway, is increased, mainly through an increased activity of PS synthase.

Permeability behaviour of lipid vesicles prepared from plant plasma membranes--impact of compositional changes.

Exposure of oat seedlings to repeated moderate water deficit stress causes a drought acclimation of the seedlings. This acclimation is associated with changes in the lipid composition of the plasma membrane of root cells. Here, plasma membranes from root cells of acclimated and control plants were isolated using the two-phase partitioning method. Membrane vesicles were prepared of total lipids extracted from the plasma membranes. In a series of tests the vesicle permeability for glucose and for protons were analysed and compared with the permeability of model vesicles. Further, the importance of critical components for the permeability properties was analysed by modifying the lipid composition of the vesicles from acclimated and from control plants. The purpose was to add specific lipids to vesicles from acclimated plants to mimic the composition of the vesicles from control plants and vice versa. The plasma membrane lipid vesicles from acclimated plants had a significantly increased permeability for glucose and decreased permeability for protons as compared to control vesicles. The results point to the importance of the ratio phosphatidylcholine (PC)/phosphatidylethanolamine (PE), the levels of cerebrosides and free sterols and the possible interaction of these components for the plasma membrane as a permeability barrier.

Alterations of wheat root plasma membrane lipid composition induced by copper stress result in changed physicochemical properties of plasma membrane lipid vesicles.

A response when wheat is grown in excess copper is an altered lipid composition of the root plasma membrane (PM). With detailed characterisation of the root PM lipid composition of the copper-treated plants as a basis, in the present study, model systems were used to gain a wider understanding about membrane behaviour, and the impact of a changed lipid composition.PMs from root cells of plants grown in excess copper (50 microM Cu(2+)) and control (0.3 microM Cu(2+)) were isolated using the two-phase partitioning method. Membrane vesicles were prepared of total lipids extracts from the isolated PMs, and also reference vesicles of phosphatidylcholine (PC). In a series of tests, the vesicle permeability for glucose and for protons was analysed. The vesicles show that copper stress reduced the permeability for glucose of the lipid bilayer barrier. When vesicles from stressed plants were modified by addition of lipids to resemble vesicles from control plants, the permeability for glucose was very similar to that of vesicles from control plants. The permeability for protons did not change upon stress. Electron paramagnetic resonance (EPR) of the lipid vesicles spin probed with n-doxylstearic acid (nDSA) was used to explore the lipid rotational freedom at different depth of the bilayer. The EPR measurements supported the permeability data, indicating that the copper stress resulted in more tightly packed bilayers of the PMs with reduced acyl chain motion.

Phosphate-deficient oat replaces a major portion of the plasma membrane phospholipids with the galactolipid digalactosyldiacylglycerol.

The plasma membranes of oat normally resemble those of other eukaryotes in containing mainly phospholipids and sterols. We here report the novel finding that the galactolipid digalactosyldiacylglycerol (DGDG) can constitute a substantial proportion of oat plasma membrane lipids, in both shoots and roots. When oat was cultivated under severe phosphate limitation, up to 70% of the plasma membrane phosphoglycerolipids were replaced by DGDG. Our finding not only reflects a far more developed potential for plasticity in plasma membrane lipid composition than often assumed, but also merits interest in the context of the limited phosphate availability in many soils.

Phosphate-limited oat. The plasma membrane and the tonoplast as major targets for phospholipid-to-glycolipid replacement and stimulation of phospholipases in the plasma membrane.

We recently reported that cultivation of oat (Avena sativa L.) without phosphate resulted in plasma membrane phosphoglycerolipids being replaced to a large extent by digalactosyldiacylglycerol (DGDG) (Andersson, M. X., Stridh, M. H., Larsson, K. E., Liljenberg, C., and Sandelius, A. S. (2003) FEBS Lett. 537, 128-132). We report here that DGDG is not the only non-phosphorous-containing lipid that replaces phospholipids but that also the content of glucosylceramides and sterolglycosides increased in plasma membranes as a response to phosphate starvation. In addition, phosphate deficiency induced similar changes in lipid composition in the tonoplast. The phospholipid-to-glycolipid replacement apparently did not occur to any greater extent in endoplasmic reticulum, Golgi apparatus, or mitochondrial inner membranes. In contrast to the marked effects on lipid composition, the polypeptide patterns were largely similar between root plasma membranes from well-fertilized and phosphate-limited oat, although the latter condition induced at least four polypeptides, including a chaperone of the HSP80 or HSP90 family, a phosphate transporter, and a bacterial-type phosphoesterase. The latter polypeptide reacted with an antibody raised against a phosphate deficiency-induced phospholipase C from Arabidopsis thaliana (Nakamura, Y., Awai, K., Masuda, T., Yoshioka, Y., Takamiya, K., and Ohta, H. (2005) J. Biol. Chem. 280, 7469-7476). In plasma membranes from oat, however, a phospholipase D-type activity and a phosphatidic acid phosphatase were the dominant lipase activities induced by phosphate deficiency. Our results reflect a highly developed plasticity in the lipid composition of the plasma membrane and the tonoplast. In addition, phosphate deficiency-induced alterations in plasma membrane lipid composition may involve different sets of lipid-metabolizing enzymes in different plant tissues or species, at different stages of plant development and/or at different stages of stress adjustments.