Conifer zygotic embryogenesis, somatic embryogenesis, and seed germination: Biochemical and molecular advances

The development of techniques for somatic embryogenesis in conifers has led to rapid advances in the ability to culture conifer tissue in vitro. Somatic embryogenesis now provides a means to clonally propagate commercially valuable conifers of several species and offers an in vitro experimental system for studying embryogenesis. As a result of these developments, the conifer zygotic and somatic embryo system has recently attracted the attention not only of the cell biologist but also of biochemists and molecular biologists. In this review I have attempted to consolidate the recent information on ultrastructure, biochemical and molecular aspects of conifer zygotic and somatic embryogenesis. In the first section, salient features of zygotic embryogenesis are highlighted followed by biochemical and ultrastructural characterization of development with reference to storage reserve deposition. In the second section, detailed characterization of seed storage proteins, changes in gene expression with emphasis on storage protein genes and Lea genes during zygotic and somatic embryogenesis are described. Effect of culture treatments such as abscisic acid, osmoticum and desiccation on storage protein gene expression in somatic embryos is discussed. Based on these studies, comparisons are presented between angiosperm and gymnosperm embryogeny regarding the unique morphological and developmental patterns of conifer embryogenesis. Finally, a brief discussion of recent progress in biochemical and molecular characterization of conifer seed germination is presented.