PROTEOLYSIS IN THE QUIESCENT SEED

The importance of the proteolytic system of the quiescent seed in rega rd to mobilization of storage protein was assessed by identification o f proteases already present at this stage. Extracts of quiescent cotyl edons of white lupin (Lupinus albus) submitted to 45 h autolysis in vi tro displayed a higher degree of protein degradation than that observe d in vivo after two days of seed imbibition. Differences in the suscep tibility to proteolysis were verified by densitometric analysis of the polypeptides after electrophoretic separation. The pH dependence of t he proteolytic activities and the responses to specific protease inhib itors showed that the proteolytic systems vary from quiescent to 1- to 3-day-imbibed cotyledons. By labelling an endogenous globulin with I- 125 a sensitive radiometric assay allowed the identification of both a n acidic and a neutral proteolytic system in the quiescent cotyledon. Within the quiescent seed there already exists a high potential for in itiating proteolysis, so that the requirement for proteolysis by speci fic endopeptidases synthesized de novo upon imbibition only apples to part of the reserve proteins.