Ultrastructural Changes of the Egg Apparatus Associated with Fertilization and Proembryo Development of Soybean, Glycine max (Fabaceae)

Abstract

As part of a study involving pod retention in soybean, Glycine max (L.) Merr., we investigated changes occurring in the egg apparatus of non-abscised flowers from the time immediately preceding fertilization through early embryogeny. Prior to the entry of the pollen tube into the embryo sac, one of the synergids begins to degenerate as evidenced by increased electron density and a loss of volume. This cell serves as the site of entry for the pollen tube. The cytoplasm of the second, or persistent synergid, remains unaltered until after fertilization. Both synergids contain, in addition to a filiform apparatus, a single unidentified inclusion of flocculent material located in the chalazal portion of each cell. The zygote can be distinguished from the egg by its consistently narrow wall; and it divides to form a proembryo, a mass of cells not yet differentiated into embryo proper and suspensor. The basal cells of the proembryo are more vacuolate than the apical ones, characteristic of the basal vacuolation of both egg and zygote. Cells of the proembryo are connected to one another via plasmodesmata, and with the exception of the basal-most cell, are isolated symplastically from the surrounding endosperm. Wall ingrowths frequently occur in certain cells of the proembryo, notably those cells in contact with the degenerate synergid and embryo sac wall. At a later stage of ontogeny, by which time the globular embryo proper has become distinct from the suspensor, the wall ingrowths are concentrated in the suspensor.