Effects of plant sterols on the hydration and phase behavior of DOPE/DOPC mixtures

Freeze-induced injury of protoplasts of non-acclimated rye and oat is associated with the formation of the inverted hexagonal (HII) phase in regions where the plasma membrane and various endomembranes are brought into close apposition as a result of freeze-induced dehydration. The influence of lipid composition and hydration on the propensity of mixtures of DOPE:DOPC containing either sterols or acylated steryl glucosides to form the HII phase was determined by DSC, freeze-fracture electron microscopy and X-ray diffraction. The addition of plant sterols to a mixture of DOPE/DOPC (either 1:1:1 or 1:1:2 mole ratio of DOPE/DOPC/sterols) reduced the total hydration of the mixture (expressed as wt% water) after desorption over a range of osmotic pressures of 2.8 to 286 MPa. However, most or all of the water remaining in the dehydrated lipid mixtures was associated predominantly with the phospholipids. Both sterols and acylated steryl glucosides significantly promoted both the dehydration-induced and thermally induced L alpha-->HII phase transitions in DOPE/DOPC mixtures however, acylated steryl glucosides were much more effective. In mixtures containing plant sterols, the HII phase occurred after dehydration at 20 MPa (20 degrees C), which resulted in a water content of 11.7 wt%. In contrast, mixtures containing acylated steryl glucosides were in the HII phase in excess water, i.e., they did not require dehydration to effect the L alpha-->HII phase transition. The results indicate that genotypic differences in the lipid composition of the plasma membrane of rye and oat leaves have a significant influence on the propensity for formation of the HII phase during freeze-induced dehydration.