Lodgepole pine seed germination. II. The seed proteins and their mobilization in the megagametophyte and embryonic axis

Abstract

Insoluble crystalloid proteins formed the major storage reserve in the mature lodgepole (Pinus contorta Dougl.) seed where they were found mainly in the megagametophyte. Smaller amounts were identified in the embryonic axis. These proteins had a subunit molecular mass in the presence of sodium dodecylsulphate of 50–55 kDa. Crystalloid subunits were heterodimers with subunit polypeptide molecular masses of 33–36 and 22–23.5 kDa. Following the emergence of the radicle from the seed coat, this reserve was rapidly mobilized in both the megagametophyte and the embryonic axis, and clearly played a major nutritional role in the germinated seed. In the megagametophyte, crystalloid protein hydrolysis appeared to involve the production of a buffer-soluble protein with a molecular mass of 34 kDa that was a heterodimer. Several soluble megagametophyte and embryonic axis proteins were identified that were also rapidly hydrolyzed following germination. These too could play a storage role. Crystalloid protein hydrolysis in the megagametophyte was not accompanied by a significant increase in the soluble megagametophyte amino acid pool. In contrast, the soluble amino acid pool of the embryonic axis increased markedly following germination. Isolated germinated embryonic axes did not show this increase. Thus, amino acids generated by crystalloid protein hydrolysis appeared to be immediately exported to the developing embryonic axis, presumably to support the many metabolic events that are occurring during its early growth.