Copper excess reduces the fluidity of plasma membrane lipids of wheat roots: a spin probe EPR study

Electron paramagnetic resonance spectroscopy was applied to investigate changes in the organization and dynamics of plasma membrane (PM) lipids of roots caused by growing wheat (Triticum durum Desf. cv. Creso) seedlings under copper excess (50 muM Cu2+) . To this purpose, unilamellar vesicles were made from total lipid extracts of PMs isolated either from copper-stressed or control roots, and probed at different bilayer depth using stearic acids bearing a doxyl group at two different positions of the alkyl chain, namely at the fifth (5-DSA) and 16th (16-DSA) carbon. EPR spectra were recorded as a function of temperature in the range between 273 and 323 K and interpreted in terms of a microscopically ordered and macroscopically disordered model, in which orientational ordering and dynamics of the spin probes were described by means of the order parameter S and the rotational diffusion coefficient R, respectively. It was found that growth in Cu excess resulted in a decreased root PM fluidity. The results were discussed in relation to the changes observed in the lipid composition of PMs induced by Cu stress, paying particular attention to phosphatidyl-choline/phosphatidylethanolamine and phospholipids/free sterols ratios and to fatty acyl chain unsaturation degree of phospholipids.