An Ultrastructural Study of The Embryo/Endosperm Interface in the Developing Seeds of Solanum nigrumL. Zygote to Mid Torpedo Stage

Abstract

The early developmental sequences in the formation of the Zone of Separation and Secretion in a hexaploid species of Solanum nigrumL. are described. Ultrastructural changes which occurred during the development of the embryo/endosperm interface could be related to the different stages in the embryo's development. The first step was the completion of the cell wall around the chalazal end of the zygote; a thin wall was formed along the endosperm cell(s) abutting the zygote. From the mature zygote stage to the quadrant stage, minute plasmalemma invaginations occurred along the endosperm wall facing the zygote. These invaginations enlarged, and from the mid-globular stage onwards became filled with a fine fibrillar material; this material accumulated between the endosperm cell wall and the plasmalemma before being released into the developing periembryonic and intercellular spaces to become the extracellular matrix. Cell wall development in the endosperm cells abutting the embryo followed an unusual path. During the quadrant stage, whilst the outer embryo wall increased in thickness due to vesicle fusion, the endosperm cell wall facing the embryo showed a loosening of the wall fibrils as well as partial separation of these same endosperm cells from each other. From the early-globular stage, the endosperm cell walls opposite the embryo became electron-translucent, disappearing into the extracellular matrix. Enzymic secretions by the embryo may account for the alteration in the abutting endosperm cell walls. Enzymic activity may also explain the development of a homogenous electron-opaque layer over the outer embryo wall as well as the differences in the width of the fibrillar layer which accumulated around the cotyledons as the embryo grew through the Zone of Separation and Secretion. The potential roles of the extracellular matrix are briefly discussed.