Histopathology of Seed-Borne Infections - Applied Research Center ...

22 Histopathology of Seed-Borne Infectionsthree nuclei at the chalazal end form the antipodal cells. One nucleus from each endmoves to the center to form the polar nuclei. Thus an eight-nucleate, seven-celledfemale gametophyte or embryo sac is formed (Figure 2.7I, J). This type of embryosac development in which only one megaspore forms the embryo sac is known asthe monosporic type. When two or all four megaspore nuclei form the embryo sacs,they are known as bisporic and tetrasporic types, respectively. Each of these categorieshas a number of subtypes (Maheshwari, 1950, 1963).The female archesporium and megaspore mother cell have plasmodesmatalconnections with the nucellar cells. Callose is deposited during megasporogenesis.It develops in species with monosporic and bisporic embryo sacs, but is absent intetrasporic forms (Rodkiewicz, 1970). In the monosporic polygonum type of embryosac, the callose disappears completely from the chalazal functional megaspore.Plasmodesmata are usually absent during callose development between megasporesand the nucellus. Cytoplasmic connections are also absent in the transverse wallsof cells in dyads and tetrads. The functional megaspore and two- and four-nucleateembryo sacs have plasmodesmatal connections to the nucellus (Sehulz and Jensen,1986; Folsom and Cass, 1988, 1989).The ultrastructural studies of the embryo sac have shown that the egg andsynergids are surrounded by a wall at the micropylar end. The lower one third partof these cells is surrounded by the plasma membrane only (Jensen, 1965a, b; Folsomand Peterson, 1984; Maeda and Maeda, 1990; Willemse and van Went, 1984). Yan,Yang, and Jensen (1991), who have carried out detailed investigations on the developingembryo sac of Helianthus annuus, have observed that in the young embryosac, two days before anthesis, egg, synergids and central cell were completelysurrounded by walls. The chalazal portion of the walls of the egg, synergids, andthe micropylar part of the central cell disappeared one day before anthesis. Thechalazal and lateral wall of the central cell remained intact and became thick.The synergids are characterized by the presence of a filiform apparatus at themicropylar end. It consists of a mass of wall projections extending deep into thecytoplasm. The presence of plasmodesmata between the synergid and the centralcell are reported in a few species (Morgensen and Suthar, 1979; Wilms, 1981;Vijayraghavan and Bhat, 1983; Willemse and van Went, 1984; Folsom and Peterson,1984). In addition to plasmodesmata, Wang and Wang (1991) have observed somevesicles between the synergids and central cell and believe that these may be involvedin the transport of metabolites between synergids and the central cell.The plasma membrane and the cell wall of the egg are usually intact. Plasmodesmataare rarely observed between egg and the central cell cytoplasm in the micropylarregion in maize and rice (van Lammereen, 1986; Maeda and Maeda, 1990). Maedaand Maeda (1990) consider that the egg cell has transmembranal and symplasticconnections with the central cell, but apoplastic connections with nucellar cells.The antipodal cells are usually small and ephemeral but in Poaceae they multiply,become many-celled and persist during the early stages of grain formation. Thesecells develop wall invaginations on the side adjacent to nucellar cells (Newcomb,1973; Maze and Lin, 1975; Wilms, 1981; Engell, 1994). The presence of plasmodesmatalconnections between antipodal cells and the surrounding nucellar cells has