Lamellar-to-hexagonal HII phase transitions in liposomes of rye plasma membrane lipids after osmotic dehydration

The plasma membrane of isolated protoplasts from non-acclimated but not from acclimated rye leaves undergoes lamellar-to-hexagonal H II phase transitions after freeze-induced dehydration (Gordon-Kamm, W.J. and P.L. Steponkus (1984) Proc. Natl. Acad. Sci. USA 81, 6373–6377). An effort was made to determine whether plasma membrane lipid components show similar behavior when dehydrated. Liposomes were prepared from the total lipid mixture extracted from plasma membrane fractions isolated from non-acclimated (NA) or cold acclimated (ACC) rye leaves ( Secale cereale L. cv Puma). Freeze-fracture electron microscopy was used to identify phase changes in liposomes after osmotic dehydration. When NA liposomes prepared in 1.3 wt% sorbitol were incubated at 4°C in 73 wt% sorbitol for 15 h (overnight), hexagonal H II phase was observed in aggregates of apparently fused liposomes with H II observed in approximately 75% of the fracture faces. In contrast, neither aggregates of apparently fused liposomes nor H II phase was observed in ACC liposomes after dehydration under the same conditions Dehydration of NA or ACC liposomes in 40 or 60 wt% sorbitol resulted in the formation of intermediate morphologies such as loosely ordered cylinders and ripple structures. Lipidic particles and pits of various sizes and morphologies were observed after dehydraton of both NA and ACC liposomes in 40, 60, or 73 wt% sorbitol. These results indicate that lipid composition determines the propensity of the plasma membrane to undergo dehydration-induced phase transitions and that alterations in the lipid composition of the plasma membrane play a causal role in the process of cold acclimation.