Summary of Embryogenesis
Embryogenesis is the process of initiation and development of an embryo from a zygote (zygotic embryogenesis) or a somatic cell (somatic embryogenesis). Embryo development occurs through an exceptionally organized sequence of cell division, enlargement and differentiation. Zygotic and somatic embryos share the same gross pattern of development. Both types of embryos develop as passing through typical developmental stages, such as globular, scutellar and coleoptilar stages for monocots, or globular, heart, torpedo and cotyledonary stages for dicots and conifers. Embryo development is bipolar, having a shoot and a radicular pole at opposite ends.
The physical, observable transition from a nonembryogenic cell to an embryogenic cell in somatic embryogenesis appears to occur when the progenitor cell undergoes an unequal division, resulting in a larger vacuolate cell and a small, densely cytoplasmic (embryogenic) cell . The embryogenic cell then either continues to divide irregularly to form a proembryonal complex or divides in a highly organized manner to form a somatic embryo.
The formation of the root apex involves derivatives of both the basal and apical cells of the two-cell embryo. The hypophyseal region, derived from the topmost cell of the suspensor, is incorporated into the embryo proper, giving rise to part of the root cap, its initial cells and the ground meristeminitial cells. The remainder, including the ground meristem and procambium, is contributed by the apical cell.
The delineation of the embryonic shoot apex is much more cryptic than that of root apex. The shoot apical meristem, the epiphysis, is determined in an early globular stage embryo before the cell cleavage that delineates the protoderm. The storage protein mRNA are not detected in epiphyseal cells but in cotyledonary cells at one embryonic stage, indicating a functional difference, and the O line is the lower boundary of the epiphysis. At the transition stage, the centrally localized epiphyseal cells divide more slowly than the cotyledon cell progenitors, resulting in the formation of a bilaterally symmetrical heart stage embryo.
Morphological changes during the transition from the globular stage to the heart stage are the first visible sign of the formation of the two embryonic organ systems: the cotyledons and the axis. The emergence of the cotyledons from a radically symmetrical globular embryo indicates that groups of cells in the apical region are induced to proliferate at specific sites. Polar auxin transport may be involved in directing these localized cell divisions.
Maturation is the terminal event of embryogenesis. In zygotic embryogenesis, maturation is characterized by attainment of mature embryo morphology, accumulation of storage carbohydrates, lipids and proteins, reduction in water content and a gradual decline or cessation of metabolism. Somatic embryos usually do not mature properly. Instead, due to environmental factors such as keeping a constant contact with inducing medium, somatic embryos often deviate from the normal developmental pattern by bypassing embryo maturation producing callus, undergoing direct secondary embryogenesis and/or germinating precociously. Somatic embryos growing from proembryonal complexes tend to develop asynchronously so that several stages are present in culture at any given time. Therefore, the most obvious developmental difference between zygotic and somatic embryos is perhaps that the latter lacks a quiescent resting phase.