Megasporophyll is the term used to describe the carpels of an angiosperm. It is subdivided into three regions: the ovary, the style, and the stigma. The ovary includes ovules, which are also known as megasporangium. It is the process of forming megaspores from the mother cell of a megaspore that is known as megasporogenesis (MMC).
Different stages of development of female gametophyte are:
1) One diploid cell evolves into a diploid megaspore mother cell within the nucleus of a growing ovule.
2) The meiotic division of this megaspore mother cell results in the formation of four haploid megaspores that are organised in a linear tetrad.
3) In angiosperms, only one of the megaspores is functional, while the other three are degenerate or nonfunctional. The female gametophyte is produced by the megaspore in the lower chalazal region (embryo sac).
4) The nucleus of the functioning megaspore undergoes mitotic division, resulting in the formation of two nuclei.
These are referred to as primary micropylar and primary chalazal nuclei, respectively.
5) The second division creates two nuclei, one micropylar and one chalazal. The third division results in the production of four nuclei at each pole as a result of the process.
6) A nucleus from each pole migrates to the centre, where it joins with other polar nuclei to form a secondary nucleus (2n). The central cell is known as the egg cell, and the two lateral cells are referred to as synergids (synergisms).
7) The three nuclei that are closest to the chalazal end are also arranged into three thin-walled cells known as antipodal cells.
8) The female gametophyte, also known as the embryo sac, is a structure that contains the egg machinery, secondary nucleus, and antipodals.
9) A mature angiosperm embryo sac has 8 nuclei and 7 cells.
Megasporogenesis
Megasporogenesis is comprised of three primary events: the development of the megaspore mother cell, meiosis to produce haploid megaspores, and the selection of megaspores (i.e., selection of the megaspore that develops into the female gametophyte).
In Arabidopsis, ovule primordia are finger-like projections from the placental tissue of the ovary that develop into fertilised seeds. The archesporial cell is formed during the early stages of ovule development by a subepidermal cell located at the distal end of the ovule primordium. As a result, in Arabidopsis and most other species, the archesporial cell differentiates directly into the megaspore mother cell (also known as the female meiocyte or megasporocyte); therefore, there is no functional difference between an archesporial cell and a megaspore mother cell in these organisms. In the absence of development into the megaspore mother cell, sporophytic cells are referred to as somatic cells, and they are found in all ovule cells. The megaspore mother cell is larger than the somatic cells and has a thicker cytoplasm as well as a larger nucleus when compared to them. A substantial increase in the size and length of the megaspore mother cell occurs just before meiosis. The megaspore mother cell then undergoes meiosis, resulting in the formation of four haploid megaspores, each with one nucleus. As a result, three of the megaspores deteriorate and only one is left to live. During megaspore selection in Arabidopsis and most other species, the megaspore with the highest chalazal number survives.
Callose (-1,3-glucan) is a histology marker that is connected with the megaspore mother cell in the megaspore. In the course of megasporogenesis, the megaspore mother cell, as well as cells undergoing meiosis, deposit callose in the walls of their respective cell walls. While growing in size, the callose in the cell walls of the selected megaspore is removed as the organism progresses through meiosis and into the process of megagametogenesis. Callose plays an unknown role in megasporogenesis and megaspore selection, according to current knowledge.
Structure of the mature female gametophyte
Transmission electron microscopy has been used to study the structure of the adult female gametophyte in Arabidopsis, and the results have been published. These studies have revealed that the cell types of the female gametophyte are all highly specialised in terms of structural specialisations. Those angiosperm specialisations that are critical for the fertilisation process are discussed in this chapter. The nuclei of the egg and central cells are polarised in such a way that they are very near to one another. When it comes to double fertilisation, this characteristic is critical since the two nuclei in question are the targets of the two sperm nuclei. The cell walls of the egg, synergid, and central cells are missing or discontinuous at the places where these cells come into touch with one another, and the plasma membranes of these cells are in direct contact with one another. In this location, there are no cell walls, which allows sperm cells to reach their fertilisation targets directly since the pollen tube discharges its two sperm cells into one of the synergid cells where they can fertilise them.
The filiform apparatus is a structure formed by the thickening and extensive invagination of the synergid cell wall at the micropylar pole, which is referred to as the filiform apparatus. Moreover, the filiform apparatus in this region significantly increases the surface area of the plasma membrane and has a high concentration of secretory organelles, indicating that it may expedite the movement of chemicals into and out of the synergid cells. In plants other than Arabidopsis, the filiform apparatus appears to be made of a number of different components, including cellulose and hemicellulose as well as pectin, callose, and protein, according to the staining properties of the cells. It is believed that the filiform apparatus performs at least two tasks related to the fertilisation process. First, pollen tube attractants are secreted by the synergid cells through the filiform apparatus (discussed below). As mentioned above, the pollen tube reaches the synergid cell by developing through the filiform apparatus, indicating that this structure is essential for pollen tube reception.
The antipodal cells of Arabidopsis, on the other hand, have no obvious specialisations and are thought to have no purpose. Antipodal cells in other species are characterised by the presence of finger-like cell wall extensions that resemble the filiform apparatus. In cereals, antipodal cells can multiply into as many as 100 cells in a single cell division. These findings show that antipodal cells have a role in transferring chemicals from surrounding ovule cells into the female gametophyte.
These findings show that antipodal cells have a role in transferring chemicals from surrounding ovule cells into the female gametophyte.
Conclusion
The ovary includes ovules, which are also known as megasporangium. It is the process of forming megaspores from the mother cell of a megaspore that is known as megasporogenesis (MMC). The following are the phases of development of a female gametophyte: In the nucleus of a growing ovule, one diploid cell develops into a megaspore mother cell, which is a diploid megaspore cell.
When it comes to reproduction, the female gametophyte is a very essential structure since it is involved in a variety of activities, including pollen tube guidance and fertilisation, induction of seed development, and maternal control of seed development.