Effects of a sub-lethal copper exposure on non-green Acer pseudoplatanus cell lipids

Non-green sycamore cells (Acer pseudoplatanus L.) treated with 50 μM Cu2+ showed a reversible arrest of cell growth after 5 d, indicating that the concentration used was sub-lethal for the cells. Growth arrest was accompanied by hexose and nucleotide-phosphate accumulation and, in contrast with previous reports, no polar-lipid degradation was found. Sub-lethal Cu2+ exposure resulted in an increase in phospholipid amounts, mostly due to an increase of phosphatidylethanolamine. Although the total amount of glycolipids was not significantly changed, Cu2+ exposure changed the relative concentrations of monogalactosyldiacylglycerol (MGDG), digalactosyldiacylglycerol (DGDG) and sulphoquinovosyldiacylglycerol (SQDG). There was a marked increase of DGDG whereas MGDG and SQDG declined. Our results suggest an increase of DGDG synthesis through a pathway similar to the recently described DGD2 pathway and an inhibition of SQDG synthesis. Furthermore, we observed modifications of fatty acid composition of the plastid synthesised lipids. A significant increase in linoleate (18:2) and a decrease in linolenate (18:3) were found in MGDG, DGDG, SQDG and phosphatidylglycerol (PG), reflecting a deficient desaturation of plastid-located 18:2 fatty acids. In SQDG and PG, Cu2+ treatment also induced a strong enrichment of palmitate (16:0). The effects of Cu2+ on DGDG synthesis and fatty acid desaturation are discussed.