Transport and dynamics of molecules dissolved in maize root cortex membranes

Translational diffusion of a fluorescent sterol probe was measured in the plasma membranes of protoplasts isolated from cortical cells of the primary root of maize seedlings. The apparent lateral diffusion coefficient was typically observed to be nearly insensitive to temperature, while the mobile fraction increased with increasing temperature. These fluorescence photobleaching recovery (FPR) measurements were compared with the electron paramagnetic resonance (EPR) spectra of the methyl ester of 13-doxyl palmitic acid in membranes of corn root tissue in situ. The complex spectra observed with this probe were analyzed as weighted sums of simpler spectra of various order parameters and rotational correlation times. The reconstituted spectra calculated from the model show that EPR also detects a mobile (less ordered, fluid) fraction, distinguished by the order parameter S = 0.1 to 0.2, which becomes more abundant as temperature increases and is qualitatively comparable to the mobile fraction determined by the FPR method. The observed results on the mobile fractions and the diffusion rates for translational (FPR) as well as rotational (EPR) motions are interpreted in terms of membrane organization, thus providing information on the population and structural patterns of the coexisting domains with a special emphasis on the response of the membrane to temperature changes.