Low temperature-induced changes in the thermotropic properties and fatty acid composition of the plasma membrane and tonoplast of cultured rice (Oryza sativa L.) cells

The thermotropic transitions of the plasma membrane and tonoplast from cultured cells of chilling-sensitive (CS) and chilling-insensitive (CI) strains of rice (Oryza sativa L.) were analyzed by monitoring the fluorescence polarization of an embedded fluorophore, 1,6-diphenyl-1,3,5-hexatriene (DPH), and their relationship to the degree of unsaturation of fatty acids in phospholipids was examined. Polarization values (P) for the tonoplast from cultured cells of CI rice, in contrast to those from CS rice, exceeded those for the plasma membrane. The values for the tonoplast and plasma membrane from CI cells were somewhat higher than those from CS cell. Thus, the tonoplast of CI cells has the lowest fluidity, while the fluidity of the tonoplast and plasma membrane of CI cells shows greater dependence on temperature. Arrhenius plots of the fluorescence anisotropy parameter {(ro/r)−1}−1 of DPH in the plasma membrane and tonoplast from CI cells gave a slope that was virtually linear throughout the entire range of temperatures from 50°C to 10°C. However, in the case of CS cells, a discontinuity was sometimes noted in the curve between 35°C and 30°C for tonoplast membranes. The activation energy (Ea) of the anisotropy parameter of DPH in both the plasma membrane and tonoplast from CI cells was greater than that from CS cells. Ea in both cases for CS cells increased with increasing duration of exposure to low tempera ture (5°C), becoming nearly the same as that for CI cells. The proportion of unsaturated fatty acids, such as linoleic acid (18:2) and linolenic acid (18:3), in the total phospholipids of the plasma membrane and tonoplast from CI cells was much higher than that from CS cells. In membranes from CS cells, this proportion also increased with increasing duration of exposure to low temperature and reached the value for membranes from CI cells. In particular, in CS cells, the most dramatic change was the change in PE and PC that in volved a sharp decrease in levels of 18:1, accompanied by an increase in 18:3. The proportion of unsaturated fatty acids was increased by exposure to low temperature, with an accompanying in crease in values of Ea.