Biochemical and ultrastructural trends in proteolysis of the beta-subunit of 7S protein in the cotyledons during germination of soybean seeds

摘要

Antibodies raised against the purified beta-subunit of beta-conglycinin were used in immunohistochemical studies to monitor the pattern of beta-conglycinin mobilization in the cotyledons during soybean [Glycine max (L.) Merr.] seed germination. Western blot analysis revealed that the break down of the beta-subunit of beta-conglycinin commenced as early as 2 days after seed imbibition (DAI). Concurrent with the degradation of the beta-subunit of beta-conglycinin, accumulation of 48, 28, and 26 kD proteolytic intermediates was observed from 2 to 6 DAI. Western blot analysis also revealed that the acidic subunit of glycinin was mobilized earlier than the basic subunit. The basic glycinin subunit was subjected to proteolysis within 2 DAI resulting inthe appearance of an intermediate product approximately 2 kD smaller than the native basic glycinin subunit. In contrast to the major seed storage proteins, lipoxygenase was subjected to limited proteolysis and was detected even after 8 DAI. The first sign of beta-conglycinin breakdown was observed near the vascular strands and proceeded from the vascular strands towards the epidermis. Protein A-gold localization studies using thin sections of soybean cotyledons and antibodies raised against the beta-subunit of beta-conglycinin revealed intense labeling over protein bodies. A pronounced decrease in the protein A-gold labeling intensity over protein bodies was observed at later stages of seed germination. The protein bodies, which were converted into alarge central vacuole by 8 DAI, contained very little 7S protein as evidenced by sparse protein A-gold labeling in the vacuoles.