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

The early developmental sequences in the formation of the Zoneof Separation and Secretion in a hexaploid species of Solanumnigrum L. are described. Ultrastructural changes which occurredduring the development of the embryo/endosperm interface couldbe related to the different stages in the embryo's development.The first step was the completion of the cell wall around thechalazal end of the zygote; a thin wall was formed along theendosperm cell(s) abutting the zygote. From the mature zygotestage to the quadrant stage, minute plasmalemma invaginationsoccurred along the endosperm wall facing the zygote. These invaginationsenlarged, and from the mid-globular stage onwards became filledwith a fine fibrillar material; this material accumulated betweenthe endosperm cell wall and the plasmalemma before being releasedinto the developing periembryonic and intercellular spaces tobecome the extracellular matrix. Cell wall development in theendosperm cells abutting the embryo followed an unusual path.During the quadrant stage, whilst the outer embryo wall increasedin thickness due to vesicle fusion, the endosperm cell wallfacing the embryo showed a loosening of the wall fibrils aswell as partial separation of these same endosperm cells fromeach other. From the early-globular stage, the endosperm cellwalls opposite the embryo became electron-translucent, disappearinginto the extracellular matrix. Enzymic secretions by the embryomay account for the alteration in the abutting endosperm cellwalls. Enzymic activity may also explain the development ofa homogenous electron-opaque layer over the outer embryo wallas well as the differences in the width of the fibrillar layerwhich accumulated around the cotyledons as the embryo grew throughthe Zone of Separation and Secretion. The potential roles ofthe extracellular matrix are briefly discussed. Solanum nigrum L.; embryo/endosperm interface; Zone of Separation and Secretion; embryo development; cellular endosperm