Female gametophyte (Embryo sac of Angiosperm)

The female gametophyte of angiosperms is essential for plant reproduction. It contains the fertilised egg cell and the central cell, which give rise to the embryo and endosperm of the seed, respectively. Female gametophyte development begins with the creation of a diploid megaspore mother cell that undergoes meiosis early in ovule development. 

The female gametophyte develops from one of the haploid megaspores that arise. Megaspore mother cell identity is determined by genetic and epigenetic processes, which limit megaspore mother cell production to a single cell per ovule.

Female Gametophyte

In angiosperms, female gametophyte creation is essential for both sexual and asexual seed development. Seed development in sexually reproducing angiosperms begins when pollen is transported from the anther to the stigma of the carpel. The male gametophyte then creates a pollen tube that develops through the interior tissues of the carpel and into the ovule, delivering its two sperm cells to the female gametophyte. 

The egg is fertilised by one sperm, and the central cell is fertilised by the other. The egg cell gives rise to the seed’s embryo, which is the start of the sporophyte generation, the central cell develops into the seed’s endosperm, which surrounds and feeds the developing embryo, and the surrounding sporophytic cells give rise to the seed coat after double fertilisation.

Flowering plants, or angiosperms, are heterosporous, meaning they produce two types of spores that develop into two forms of unisexual gametophytes. The megaspore is the first type of spore. Megasporogenesis occurs when diploid megaspore mother cells conduct meiosis and produce haploid megaspores, which then mature into haploid female gametophytes during megagametogenesis. 

The microspore is the second form of spore. Microspores are produced during microsporogenesis by diploid microspore mother cells, which then undergo microgametogenesis and develop into male gametophytes.

Female Gametophyte development in angiosperms

The ovary, a female reproductive organ, completely encloses one or more ovules, which are the location of the female gametophyte, also known as the embryo sac. With a micropyle at the distal end, the ovule is connected to the inner surface of the ovary.

Each ovule contains a single megaspore mother cell, which is surrounded by integuments, sporophytic cells’ protective and nutritive layers. The megaspore mother cell goes through meiosis to produce four haploid offspring. Only the one near the proximal end of the haploid products degenerates and becomes a functioning megaspore.

Four forms of female gametophyte development have been identified based on the pattern of following cell divisions. The megaspore traverses a polygonum-type pathway in most flowering plant species, including Arabidopsis and maize, resulting in a seven-celled female gametophyte.

Three rounds of mitotic divisions occur without cytokinesis during this form of embryo sac development. A huge central vacuole divides the eight nuclei into two groups, each of which has four nuclei and is placed near one end of an extended embryo sac. The embryo sac is then cellularized, with three antipodal cells produced at the proximal end and two synergid cells and one egg cell produced at the distal end.

A huge central cell is formed by the remaining two nuclei and much of the cytoplasm in the centre.

Angiosperms have two types of fertilisation. The pollen tube extends into the micropyle and penetrates one of the two synergids when it reaches the ovule’s distal end. The two sperm cells are released: one merges with the egg cell to form the zygote, while the other fuses with the central cell to form the endosperm precursor. 

The endosperm in a polygonum-type embryo sac is a triploid organ. The embryo sac is generated by two rounds of mitosis following meiosis in the water lily Nuphar polysepalum, as well as several other basal angiosperms, and contains four haploid cells, including the egg and central cells.

As a result, both the zygote and the endosperm produced by double fertilisation are diploid.

The endosperm is a unique angiosperm organ that is a key nutritional structure that supports embryo and/or seedling development, regardless of ploidy. Angiosperms’ success is assumed to have been helped by the presence of endosperm, animal-attractive blooms, and protective fruit.

Structure of Mature Female Gametophyte

Transmission electron microscopy was used to study the structure of the mature female gametophyte in Arabidopsis. The cell types of the female gametophyte have a wide range of structural specialisations, according to this research. The importance of specialisations in the fertilisation process in angiosperms is discussed.

The nuclei of the egg and central cells are polarised so that they are very near to one another. Since these two nuclei are the targets of the two sperm nuclei, this characteristic is critical for double fertilisation. Furthermore, the cell walls of the egg, synergid, and central cells are missing or discontinuous in the locations where they intersect, and the plasma membranes of these cells are in direct contact with one another. 

The synergid cell wall has been further refined. The filiform apparatus is a structure formed by the thickening and extensive invagination of the synergid cell wall near the micropylar pole. The filiform apparatus increases the surface area of the plasma membrane in this region and has a high concentration of secretory organelles, implying that it may help transport chemicals into and out of synergid cells.

Conclusion

The female gametophyte of angiosperms is essential for plant reproduction. It contains the fertilised egg cell and the central cell, which give rise to the embryo and endosperm of the seed, respectively. Female gametophyte development begins with the creation of a diploid megaspore mother cell that undergoes meiosis early in ovule development. In angiosperms, female gametophyte creation is essential for both sexual and asexual seed development. Seed development in sexually reproducing angiosperms begins when pollen is transported from the anther to the stigma of the carpel.