Growth in excess copper induces changes in the lipid composition and fluidity of PSII-enriched membranes in wheat

Changes in the lipid composition and fluidity of PSII-enriched thylakoids w ere studied in seedlings of wheat (Triticum Desf, cv. Adamello) grown in nu trient solution supplemented with CuSO4 to achieve a final concentration of 10 and 50 mu M Cu, Metal content increased in the chloroplasts of the 50 m u M Cu-grown plants. PSII isolated from wheat supplied with 10 mu M Cu did not show any alteration in the lipid composition or in the lipid and protei n levels of the membranes, nor was any change in the ultrastructure of the membranes detected. The 50 mu M Cu-grown plants showed thylakoid swelling, particularly in the stroma and terminal grana thylakoids, Furthermore, an a lteration in the lipid composition of PSII preparations was observed togeth er with a decrease in the lipid content, which resulted in a reduction in t he lipid to protein ratio. The monogalactosyldiacylglycerol (MGDG) to digal actosyldiacylglycerol (DGDG) molar ratio decreased, whereas the degradation of the polar lipids caused an accumulation of free fatty acids (FFA). The total amount of unsaturated lipids associated with the PSII-enriched membra nes of wheat was not affected by excess copper supplies, even though change s in the individual fatty acids occurred. The effect of copper on the fluid ity of PSII membranes was evaluated by electron paramagnetic resonance (EPR ) measurements, using spin-probed fatty acids as probes, The PSII membranes , spin probed by means of 5- and 16-doxylstearic acids, showed that only th e fluidity of the surface region of the bilayer close to the polar head gro up was reduced following the 50 mu M Cu supply. In contrast, the fluidity o f the inner membrane region of the bilayer did not show any change. The imp lications of changes in the lipid composition and lipid-protein interaction s on the fluidity of specific transversal membrane regions are discussed.