Early embryogenesis in Arabidopsis thaliana. I. The mature embryo sac

Abstract

Arabidopsis thaliana has a seven-celled eight-nucleate megagametophyte of the Polygonum type; each cell type displays a different form of structural specialization. The egg apparatus cells are highly polarized; the egg has a large micropylar vacuole and chalazally sited nucleus, whereas the opposite is true for the synergids. At the chalazal region of the egg apparatus cells there are no cell wall boundaries, although their plasmalemmas are in intimate contact. The common wall between the two synergids is thin and irregular and contains plasmodesmatal connections. The synergid cytoplasm is rich in organelles; profiles of rough endoplasmic reticulum appear in masses of parallel stacked cisternae, and large accumulations of mitochondria occur adjacent to the filiform apparatus. The egg cell cytoplasm is quiescent; ribosome concentration and frequencies of dictyosomes and endoplasmic reticulum are noticeably lower and plastids are poorly differentiated. The central cell is long and vacuolate with a large diploid nucleus; fusion of the polar nuclei occurs prior to embryo sac maturity. The cytoplasm contains numerous starch-containing plastids accumulated in a shell around the nucleus. A high ribosome concentration and the absence of vacuoles and dictyosomes typifies the antipodal cell cytoplasm. All antipodal cells are interconnected by plasmodesmata as well as being connected to the nucellus and central cell. Key words: Arabidopsis, embryo sac, embryogenesis, cell specializations, stereology.

Ovule ontogeny of Tabebuia pulcherrima Sandwith (Bignoniaceae): embryo sac development

“…In this last case, only the plasmalemma delimits the cells (Cocucci & Jensen 1969, Jensen 1972, Kapil & Bhatnagar 1981, Willemse & van Went 1984, Folsom & Cass 1990, Mansfield et al 1991, Russell 1992, Huang & Russell 1992. The total or partial absence of cell walls in the chalazal portion of the egg apparatus is important for double fertilization, as the male gametes released by the pollen tube inside one of the synergids need to be transferred, one to the egg cell and another to the central cell.…”

“…The total or partial absence of cell walls in the chalazal portion of the egg apparatus is important for double fertilization, as the male gametes released by the pollen tube inside one of the synergids need to be transferred, one to the egg cell and another to the central cell. Moreover, according to Mansfield et al (1991), the absence of cell walls in that area allows the free translocation of nutrients from synergids to egg cell and central cell. In T. pulcherrima these cell walls become very thin, but they persist until the maturity of the embryo sac and no discontinuity was observed.…”

“…In Capsella bursa-pastoris the cell walls are also continuous in the chalazal region of the egg apparatus, but frequently a pore is observed in the walls between the degenerative synergid and the egg cell in fertilized megagametophytes (Schulz & Jensen 1968). This indicates that the presence of cell walls is not necessarily in contradiction with the fusion of the gametes (Willemse & van Went 1984, Mansfield et al 1991.…”

“…Both the synergids and the egg cell present an accentuated degree of polarity. Cytoplasmic polarity of the egg cell seems to be an almost universal character in the angiosperms, and may be an important factor (or even a requirement) for the subsequent development of the zygote (Mansfield et al 1991).…”

“…Many ultrastructural studies have demonstrated that the synergids are very active cells, probably involved in the secretion of substances -e.g., chemiotactic substances which guide the pollen tube to the synergids, as well as in the absorption of nutritious material originating from tissues of the micropylar region and its translocation to the central and egg cells (Schulz & Jensen 1968, Tilton 1981, Willemse & van Went 1984, Folsom & Cass 1990, Mansfield et al 1991, Russell 1992, Tirlapur et al 1993. According to this vision, the filiform apparatus exercises the important function of increasing diffusion efficiency, like wall ingrowths of transfer cells.…”