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 tonoplastfrom cultured cells of chilling-sensitive (CS) and chilling-insensitive(CI) strains of rice (Oryza sativa L.) were analyzed by monitoringthe fluorescence polarization of an embedded fluorophore, 1,6-diphenyl-1,3,5-hexatriene(DPH), and their relationship to the degree of unsaturationof fatty acids in phospholipids was examined. Polarization values (P) for the tonoplast from cultured cellsof CI rice, in contrast to those from CS rice, exceeded thosefor the plasma membrane. The values for the tonoplast and plasmamembrane from CI cells were somewhat higher than those fromCS cell. Thus, the tonoplast of CI cells has the lowest fluidity,while the fluidity of the tonoplast and plasma membrane of CIcells shows greater dependence on temperature. Arrhenius plotsof the fluorescence anisotropy parameter {(r_o/r)–1}–1of DPH in the plasma membrane and tonoplast from CI cells gavea slope that was virtually linear throughout the entire rangeof temperatures from 50°C to 10°C. However, in the caseof CS cells, a discontinuity was sometimes noted in the curvebetween 35°C and 30°C for tonoplast membranes. The activationenergy (E_a) of the anisotropy parameter of DPH in both the plasmamembrane and tonoplast from CI cells was greater than that fromCS cells. E_a in both cases for CS cells increased with increasingduration of exposure to low tempera ture (5°C), becomingnearly the same as that for CI cells. The proportion of unsaturated fatty acids, such as linoleicacid (18:2) and linolenic acid (18:3), in the total phospholipidsof the plasma membrane and tonoplast from CI cells was muchhigher than that from CS cells. In membranes from CS cells,this proportion also increased with increasing duration of exposureto low temperature and reached the value for membranes fromCI cells. In particular, in CS cells, the most dramatic changewas the change in PE and PC that in volved a sharp decreasein levels of 18:1, accompanied by an increase in 18:3. The proportionof unsaturated fatty acids was increased by exposure to lowtemperature, with an accompanying in crease in values of E_a. (Received April 10, 1991; Accepted May 9, 1992)