Somatic embryogenesis: cell biological aspects

Somatic embryogenesis is the development from somatic cells, through an orderly series of characteristic morphological stages, of structures that resemble zygotic embryos. Structures of somatic origin resembling embryos may form spontaneously on, for instance, leaf tips of Malaxis (Taylor 1967; for review see Vasil & Vasil 1980). Under experimental conditions somatic cells in tissue culture may enter a developmental pathway resembling that of zygotic embryos in seeds. The mature zygotic embryo typically contains the basic organs of the vegetative plant and one or two storage organs, the cotyledons. Normally it enters a period of metabolic quiescence and developmental arrest before germination, growth and the production of additional tissues and organs of the plant body take place. Walbot (1968) described zygotic embryo development as five consecutive developmental processes. (1) Cell division with little growth, and differentiation of all major tissues and organs: embryo specification. (2) Rapid cell expansion and division: embryo growth. (3) Little or no cell division and expansion, synthesis of storage molecules: embryo maturation. (4) Developmental arrest: embryo dormancy. (5) Renewed cell expansion and division: embryo germination.