EMBRYOS OF ALFALFA (MEDICAGO-SATIVA L) CAN SYNTHESIZE STORAGE PROTEINS AFTER THE COMPLETION OF GERMINATION, IN RESPONSE TO ABSCISIC-ACID AND OSMOTICUM, IF MATURATION DRYING IS PREVENTED

The major 2S, 7S and 11S storage proteins are synthesized at the mid- to late stages during development of alfalfa seeds, mainly within the embryos. Mature dry seeds cannot synthesize these proteins upon subseq uent germination and growth. When embryos were isolated from the seed during development (stages VII and VIII) and placed on water or nutrie nt medium, they germinated. They exhibited a pattern of protein synthe sis which was identifiable as germinative/post-germinative, and was id entical to that pattern synthesized by the embryos of germinated dry s eeds, and of embryos from seeds that were subjected to drying prematur ely at stages VII and VIII. When, after 48 h from the start of imbibit ion, abscisic acid or osmoticum was introduced to germinated embryos w hich were isolated at stage VII and not desiccated, the synthesis the 11S storage protein was restored to a level comparable to that of stag e VII embryos before isolation. This was accompanied by an increase in transcription of the 11S storage-protein gene, even at a time when st orage-protein mobilization was occurring. Thus storage-protein synthes is is not terminated temporally during alfalfa seed development, but i s maintained well beyond the time of physiological maturity if the emb ryo is maintained under conditions which are favorable to development. Transfer of germinated stage VII embryos, which had first been desicc ated, to abscisic acid or osmoticum 48 h after imbibition started did not result in any restoration of storage-protein synthesis. Thus desic cation, either prematurely or at maturation, causes storage-protein sy nthesis to cease and within the embryos this process is no longer resp onsive to abscisic acid or osmoticum.