Ultrastructure of Megasporogenesis and Early Megagametogenesis in Arundo formosana Hack. (Poaceae)

Abstract

The ovule of Arundo formosana Hack. is tenuinucellate, the megasporocyte originating from a hypodermal cell of the ovule primordium. Embryo sac development is of the Polygonum type. At its inception, the megasporocyte appears to be identical to the surrounding nucellar cells except for its larger size and dominant nucleus. The onset of megasporogenesis is indicated by the expansion of the megasporocyte, which is accompanied by a decrease in cytoplasmic density. Callose deposition in the expanded megasporocyte originates from the micropylar end and extends laterally. During megasporocyte expansion, the nucleus retains the micropylar location. Meiosis results in the formation of a linear, T-shaped, or intermediate tetrad: the chalazal megaspore is functional; all others will degenerate. The functional megaspore appears least dense among the cells of the tetrad and seldom contains vacuoles. As the megaspore enlarges, the organelles and the cytoplasmic density increase, the callose wall is gradually digested, and a electron-dense wall appears. The vacuoles are randomly distributed and then gradually migrate to both poles of the megaspore. A well-developed megaspore shows organelle polarity such that most plastids and dictyosomes are distributed in the micropylar region. The polarized dictyosome and plastid distributions persist through the two- and four-nucleate stage of the embryo sac, respectively. The two- and four-nucleate embryo sac is characterized by the presence of large central and chalazal vacuoles. Plasmodesmata persist in the chalazal wall during megasporogenesis and megagametogenesis to the four-nucleate stage.