The female gametophyte

Abstract

The angiosperm female gametophyte is critical for plant reproduction. It contains the egg cell and central cell that become fertilized and give rise to the embryo and endosperm of the seed, respectively. Female gametophyte development begins early in ovule development with the formation of a diploid megaspore mother cell that undergoes meiosis. One resulting haploid megaspore then develops into the female gametophyte. Genetic and epigenetic processes mediate specification of megaspore mother cell identity and limit megaspore mother cell formation to a single cell per ovule. Auxin gradients influence female gametophyte polarity and a battery of transcription factors mediate female gametophyte cell specification and differentiation. The mature female gametophyte secretes peptides that guide the pollen tube to the embryo sac and contains protein complexes that prevent seed development before fertilization. Post-fertilization, the female gametophyte influences seed development through maternal-effect genes and by regulating parental contributions. Female gametophytes can form by an asexual process called gametophytic apomixis, which involves formation of a diploid female gametophyte and fertilization-independent development of the egg into the embryo. These functions collectively underscore the important role of the female gametophyte in seed and food production.

Figure 1.

The Arabidopsis female gametophyte. (A) Ovule. (B) Female gametophyte. (C) Synergid cells. View in (B) and (C) is perpendicular to that in (A). The mature female gametophyte in Arabidopsis is approximately 105 µm in length and approximately 25 µm in width. In all panels, the black circles/ovals represent nuclei and the white areas represent vacuoles. The dashed line at the chalazal ends of the synergid cells in (C) represents a discontinuous or absent cell wall. Abbreviations: ac, antipodal cells; cc, central cell; ch, chalazal region of the ovule; ec, egg cell; f, funiculus; fa, filiform apparatus; mp, micropyle; sc, synergid cell; sn, synergid nucleus, sv, synergid vacuole.

Figure 2.

Megasporogenesis in Arabidopsis. (A) Apical region of a young, finger-like ovule primordium. The megaspore mother cell forms from a sub-epidermal cell at the distal end of the ovule primordium. L1 is the outer layer of cells. (B) Steps of megasporogenesis. Ovule primordia arise as finger-like projections from the placenta. The megaspore mother cell undergoes meiosis and forms four megaspores. Three of the megaspores undergo cell death. The chalazal-most megaspore survives, becomes the functional megaspore, and undergoes megagametogenesis. Black circles/ovals represent nuclei. Abbreviations: dm, degenerating megaspores; fm, functional megaspore; ii, inner integument; L1, L1 epidermal layer of the ovule primordium; mmc, megaspore mother cell; mt, meiotic tetrad; oi, outer integument.

Figure 3.

Megagametogenesis in Arabidopsis. (A) Steps of megagametogenesis emphasizing development within the ovule. (B) Stages of megagametogenesis (Christensen et al., 1998). The megaspore contains a single nucleus (stage FG1). This nucleus undergoes two rounds of mitosis, producing a four-nucleate coenocyte, with two nuclei at each pole separated by a large central vacuole (stage FG4). During a third mitosis, phragmoplasts and cell plates form between sister and non-sister nuclei and the nuclei become completely surrounded by cell walls (Stage FG5). During cellularization, the polar nuclei migrate toward the center of the female gametophyte and fuse before fertilization. These events produce a seven-celled structure consisting of three antipodal cells, one central cell, two synergid cells, and one egg cell. If the female gametophyte is not fertilized, the antipodal cells eventually degenerate (Stage FG7, not shown). White areas represent vacuoles and black circles/ovals represent nuclei. Abbreviations: ac, antipodal cells; cc, central cell; ccn; central cell nucleus; ch, chalazal region of the ovule; ec, egg cell; f, funiculus; fg, female gametophyte; fm, functional megaspore; ii, inner integument; m, megaspore; mp, micropyle; oi, outer integument; pn, polar nuclei; sc, synergid cells.

Figure 4.

Female gametophyte development in diplosporous and aposporous apomicts compared with Arabidopsis. (A) Steps of megasporogenesis and megagametogenesis in Arabidopsis ovules. Red and blue colors represent diploid and haploid cells, respectively. (B) Steps in diplosporous female gametophyte development. The megaspore mother cell enters meiosis and the process fails with the resultant diploid cell undergoing mitosis to form a diploid female gametophyte (red). Alternatively, the megaspore mother cell may directly undergo mitosis to form the diploid female gametophyte. In the latter it is unclear if the cell initiating diplospory is a functional megaspore mother cell. (C) Steps in aposporous female gametophyte development. Megaspore mother cell differentiatiation (red) occurs and it can undergo megasporogenesis and megagametogenesis to form a haploid female gametophyte (blue). Diploid aposporous initial cells differentiate during the events of megasporogenesis close to sexually programmed cells and undergo mitosis forming a diploid female gametophyte (yellow). Aposporous initial cell formation begins at different times in different apomictic species, either soon after megaspore mother cell formation, during meiotic tetrad development or functional megaspore selection. In some species, both haploid and aposporous gametophytes can co-exist in ovules while in others the sexual pathway terminates, usually during early mitotic divisions of the aposporous initial cell. Abbreviations: ai, aposporous initial cells; dfg, diploid female gametophyte; dm, degenerating megaspores; fm, functional megaspore; hfg, haploid female gametophyte; mmc, megaspore mother cell; mt, meiotic tetrad; (+/-), may be present or absent.