Protein Degradation and Proteolytic Activities in Germinating Field Beans (Vicia faba L., var. minor)

Abstract

Protein degradation in germinating seeds of field beans ( Vicia faba L., var. minor) was investigated biochemically and by electron microscopy. Rapid nitrogen transfer from cotyledons into the germ starts at the 7th day of germination simultaneously with the beginning of rapid protein degradation. Decrease of nitrogen in the cotyledons and increase in the germ, respectively, accompany corresponding changes in the dry weights of these organs. Legumin (11 S storage protein) degradation commences between 7th and 8th day previous to the breakdown of vicilin (7 S storage protein). On acid polyacrylamide gels no changes in amount and mobility of vicilin and legumin were observed before the 9th and 7th days respectively, whereas under denaturing conditions degradation products were already visible at earlier germination stages. It indicates hidden endoproteolytic cleavages. In immunological as well as in basic electrophoresis mobility changes were observed previous to the rapid degradation of the respective proteins. Amido-N of the proteins does not change parallel to the mobility shifts. Two peaks of acid endopeptidase activity were measured during germination, 1 at the 3th and 2 at the 9th day, the latter being much more pronounced than the former. BAPAase 1) activity starts to increase at the 7th day. Changes in the electron density of protein bodies were already visible previous to the onset of rapid protein degradation in the cotyledons, but the translucent areas appearing inside the protein bodies from the 4th day on may also be interpreted in terms of preparation artefacts. The results are in accordance with a two-step hypothesis of storage protein breakdown in field bean cotyledons.