Early Development of the Seed Coat of Soybean (Glycine max)

Abstract

Although the development of the soybean ovule has been fairly well studied, knowledge of the sequence of events in the seed coat during the first 3 weeks after flowering is incomplete. The goal of the present study was to document, using light microscopy, the early development of the soybean seed coat with respect to changes in structure and histochemistry. At anthesis, the seed coat consists of an outer layer of cuboidal epidermal cells surrounding several layers of undifferentiated parenchyma (which together constitute the outer integument), and an inner layer of cuboidal endothelial cells (the inner integument). At 3 d post anthesis (dpa), the inner integument has expanded to include three to five layers of relatively large cells with thick, heavily-staining cell walls immediately adjacent to the endothelium. By 18 dpa, the outer integument has developed into a complex of tissues comprised of an inner layer of thick-walled parenchyma, an outer layer of thin-walled parenchyma containing vascular tissue which has grown down from the lateral vascular bundles in the hilum region, a hypodermis of hourglass cells, and palisade layer (epidermis). The thick-walled parenchyma of the inner integument has become completely stretched and compressed, leaving a single, deeply staining wall layer directly above the endothelium. At 21 dpa, the outermost cells of the endosperm have begun to compress the endothelium. At 45 dpa (physiological maturity) the seed coat retains only the palisade layer, hourglass cells, and a few layers of thin-walled parenchyma. The innermost layer of the endosperm, the aleurone layer, adheres to the inside of the seed coat. This knowledge will be invaluable in future studies of manipulation of gene expression in the seed coat to modify seed or seed coat characteristics.