An Evaluation of the Role of Membrane Self-Digestion in Cotyledon Senescence

Summary The protein complement of plasma membrane from bean cotyledon undergoes substantive deteriorative changes as the tissue senesces, and these can be visualized by polyacrylamide gel electrophoresis of the isolated membranes. In particular, sharp clearly defined peaks of protein characterizing the polypeptide profile of young membrane are replaced by broad diffuse regions of protein stain in the gels for senescent membrane. It is conceivable that this deterioration of membrane protein arises from both intrinsic and extrinsic proteolytic activity, the former being part of the membranes themselves and the latter present in the cytosol. However, a major role for intrinsic protease activity appears to be ruled out by the fact that in vitro incubation of isolated plasma membrane at various stages of senescence in buffer for periods of up to 60 h under asceptic conditions fails to simulate the changes in protein complement that occur in situ. By contrast, cytosol protease activity changes during germination in a manner that is consistent with its prospective role in membrane deterioration.