PHOSPHOLIPID-METABOLISM IN MEMBRANES OF SENESCING BEAN COTYLEDONS

Mechanisms underlying the depletion of phospholipid in senescing membr anes have been examined using microsomes isolated from bean cotyledons (Phaseolus vulgaris) at various stages of development. As the cotyled ons age, microsomal phospholipid levels relative to protein decrease b y 93% indicating that phospholipids are selectively depleted from sene scing membranes. This reflects active phospholipid catabolism, but can also be attributed to a reduction in phospholipid synthesis. Specific ally, the activities of choline phosphotransferase and ethanolamine ph osphotransferase, enzymes mediating the terminal step in the synthesis of phosphatidylcholine and phosphatidylethanolamine, respectively, de crease dramatically as the cotyledons senesce. Phosphatidylcholine and phosphatidylethanolamine comprise over 70% of the total phospholipid in these membranes, and this pronounced decline in their synthesis wit h advancing senescence will lead to phospholipid depletion. There is a lso a decrease with age in the activity of acyl-CoA synthetase, which generates acyl-CoA for use in phospholipid synthesis, Microsomal phosp holipid deacylation-reacylation activity declines as well as the cotyl edons senesce, but this can be accounted for in terms of decreased lev ers of phospholipid available for the reaction. Thus the depletion of phospholipid in senescing membranes can be attributed to active catabo lism in the face of declining synthesis.