THE LIPID-COMPOSITION OF CYTOSOLIC PARTICLES ISOLATED FROM SENESCING BEAN COTYLEDONS

Non-sedimentable particles have been isolated from the cytosol of youn g two-day old and senescing seven-day-old bean cotyledons by ultrafilt ration. The cytosolic particles have an average diameter of 240 nm and , when visualized in thin section by electron microscopy, appear osmio philic and uniformly stained. Particles of similar size and morphology are also discernible in the cytoplasm of thin-sectioned cotyledon tis sue. The cytosolic particles of both young and senescing tissue contai n phospholipids with all of the fatty acids detectable in correspondin g microsomal membrane phospholipids, and they also contain the same fr ee sterols present in microsomal membranes. Free fatty acids and stery l/wax esters, phospholipid catabolites that are known to cause bilayer destabilization, are enriched in cytosolic particles relative to memb ranes by eight- to 23-fold and 41- to 213-fold for young and senescing cotyledon tissue, respectively. Thus phospholipid catabolite removal appears to be higher for the older membranes, but this notwithstanding , steryl/wax esters and free fatty acids still accumulate in microsoma l membranes with advancing senescence. The data are consistent with th e contention that blebbing of cytosolic particles from membranes is a means of removing phospholipid catabolites that would otherwise destab ilize the bilayer, and suggest that the efficiency of catabolite remov al is lower for senescing membranes.