Effect of Abscisic Acid, Osmoticum, and Desiccation on Synthesis of Storage Proteins during the Development of White Spruce Somatic Embryos

Abstract

The effect of abscisic acid (ABA), non-permeating osmoticum and desiccation treatment on storage protein synthesis during maturation of somatic embryos of Picea glauca (Moench) Voss. was examined. Sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDS-PAGE) and Western blot analysis demonstrated that some of the major crystalloid and matrix polypeptides were absent from somatic embryos maturing on medium containing ABA and low osmoticum. However, treatment with polyethylene glycol-4000 (PEG) in combination with ABA resulted in the synthesis of a spectrum of storage polypeptides resembling that of mature zygotic embryos. These storage proteins accumulated throughout an 8-week culture period, resulting in a threefold higher protein content than somatic embryos maturing for the same time in the absence of PEG. The structure and distribution of protein bodies in cells of these osmotically treated somatic embryos was similar to that in cells of mature zygotic embryos. Treatment with 5·0-7·5% PEG prevented catabolism of the accumulated storage polypeptides during desiccation. The optimal culture conditions for somatic embryo maturation and storage protein deposition was 16 μM ABA and 7·5% PEG for 8 weeks followed by desiccation. Analysis of mRNAs by in vitro translation and immunoprecipitation of translated products showed that the crystalloid protein mRNA profiles of zygotic and those of somatic embryos maturing on 16 μM ABA in the absence of PEG were similar. The differences observed in the pattern of accumulated polypeptides in these somatic embryos and those of mature zygotic embryos, therefore, indicates that storage-protein synthesis in response to osmoticum is in part regulated at the translational level. During regeneration of somatic embryos to plantlets the storage polypeptides were rapidly utilized in a manner similar to that in zygotic seedlings.