Lateral domain diversity in membranes of callus and root cells of potato as revealed by EPR spectroscopy

Lateral heterogeneity in terms of co-existing domains with a distinct molec ular organization is an area of increasing interest in membrane biology, Th e structural and dynamic aspects of the in-plane domain organization of lip ids are becoming well documented, especially for model membrane systems. Po tato (Solanum tuberosum L, cv, Desiree) callus cells and roots of plantlets from stem node culture were doped with a spin-labeled analog of the methyl ester of palmitic acid bearing the paramagnetic nitroxide group at positio n C-5 of the acyl chain, which serves as a monitor of membrane fluidity of the region close to the polar phospholipid head groups of the bilayer, Mode l reconstruction of the line-shapes of the expert mental spectra revealed t he co-existence of two types of membrane domains with different ordering an d dynamics of lipids in the membranes of both callus and root cells, With c hanges in temperature, relatively small differences were detected in either type of domain in the lipid ordering of the bilayer as characterized by or der parameter S, However, the relative population of domains in the bilayer exhibited stronger temperature dependence, Typically, the relative proport ion of disordered domains with less molecular order (smaller S) was larger in the membranes of callus cells compared to those of root cells, indicatin g higher fluidity throughout the measured temperature range (5-35 degrees C ). The Arrhenius activation energies for rearrangement of lipid molecules w ithin the bilayer were found to be higher for root tissue membranes, indica ting the ability of root cells to oppose actively any drastic changes of me mbrane structuring under temperature stress. The distinctions in organizati on of lateral domains between the callus and root cell membranes may be cor related with differences in growth rate and metabolic activity between thes e two types of tissue.