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

Abstract

The major 2S, 7S and 11S storage proteins are synthesized at the midto late stages during development of alfalfa seeds, mainly within the embryos. Mature dry seeds cannot synthesize these proteins upon subsequent germination and growth. When embryos were isolated from the seed during development (stages VII and VIII) and placed on water or nutrient medium, they germinated. They exhibited a pattern of protein synthesis which was identifiable as germinative/post-germinative, and was identical to that pattern synthesized by the embryos of germinated dry seeds, and of embryos from seeds that were subjected to drying prematurely at stages VII and VIII. When, after 48 h from the start of imbibition, abscisic acid or osmoticum was introduced to germinated embryos which were isolated at stage VII and not desiccated, the synthesis the 11S storage protein was restored to a level comparable to that of stage VII embryos before isolation. This was accompanied by an increase in transcription of the 11S storage-protein gene, even at a time when storage-protein mobilization was occurring. Thus storage-protein synthesis is not terminated temporally during alfalfa seed development, but is maintained well beyond the time of physiological maturity if the embryo is maintained under conditions which are favorable to development. Transfer of germinated stage VII embryos, which had first been desiccated, to abscisic acid or osmoticum 48 h after imbibition started did not result in any restoration of storage-protein synthesis. Thus desiccation, either prematurely or at maturation, causes storageprotein synthesis to cease and within the embryos this process is no longer responsive to abscisic acid or osmoticum.