The seed proteins of white spruce and their mobilization following germination

Buffer‐soluble proteins that have subunit molecular weights, in the presence of sodium dodecyl sulphate (SDS), of 47, 31 and 27 kilodaltons (kDa) form the major storage proteins in the mature white spruce [ Picea glauca (Moench) Voss] seed. These proteins were found mainly in the megagametophyte. but smaller amounts were identified in the embryo. Following the completion of germination, this reserve was rapidly hydrolyzed in both tissues and probably plays a major nutritional role in the germinated seed. Buffer‐insoluble proteins were also found in megagametophytes and embryos from the mature seed. These proteins were soluble in buffer only if SDS was present. Predominant in this class of proteins were several that have a subunit molecular weight and structure that is characteristic of seed crystalloid storage proteins; the subunits were shown to be heterodimers with polypeptide molecular weights in the 33 kDa to 37 kDa and 23.5 kDa to 25 kDa ranges. This reserve was rapidly hydrolyzed in the germinated seed. Storage protein hydrolysis was accompanied by a significant increase in the soluble amino acid pool in both megagametophytes and embryos. Cell‐free extracts of mature seed megagametophytes and embryos contained leucine‐naphthylamidase (leuNAase) activity. Following germination. this activity was maintained at a constant level in megagametophytes but increased substantially in embryos.