Protein Modification and Utilization of Starch in Soybean (Glycine maxL. Merr.) Seed Maturation

Abstract

Seeds of soybean (Glycine max L. Merr.) harvested at various stages of development and allowed to dry in intact pods undergo a maturation process and are viable. Defatted powders of seed harvested 24–66 d after flowering were extracted to yield buffer-soluble and alkali-soluble proteins. Imposition of a maturation process increased the level of buffer-soluble proteins but had no effect on the disulflde content of these proteins. After undergoing maturation, seeds showed an accumulation of buffer-soluble polypeptides in the molecular weight range of 43–94 kD. Maturation may be associated with the synthesis of specific polypeptides having a molecular weight of approximately 85 kD. Alkali-soluble proteins, which represents the storage proteins, did not show any responses to maturation. Their quantity increased substantially during seed development and the disulfide level was only half that of buffer-soluble proteins, attaining a maximum value of 10.9 mol S per 10⁵ g protein. Matured seed at all harvest dates had a final starch content close to that of normal seed, 10–20 mg g⁻¹, and soluble sugars were maintained at quite high levels, 51–83 mg g⁻¹. The metabolic program for synthesis and degradation of starch seems quite rigidly followed and is independent of harvest date or of attachment to the parent plant. Soybean seeds retain considerable soluble proteins and soluble sugars throughout maturation, and these collectively may be important in maintaining a desiccation resistant structure.