Sexual Reproduction In Plants, Sporogenesis (Microsporogenesis, Megasporogenesis) Gametogenesis (Microgametogenesis, Megagametogenesis)

SEXUAL REPRODUCTION IN PLANTS, SPOROGENESIS (MICROSPOROGENESIS, MEGASPOROGENESIS) GAMETOGENESIS (MICROGAMETOGENESIS, MEGAGAMETOGENESIS)
SEXUAL REPRODUCTION:-
Sexual reproduction involves fusion of male and female gametes to form a zygote, which develops into an embryo. In crop plants, male and female gametes are produced in specialised structures known as flowers.

Flower:- A flower usually consists of sepals, petals (or their modifications), stamens and/or pistil. A flower containing both stamens and pistil is a perfect or hermaphrodite flower. If it contains stamens, but not pistil, it is known as staminate, while a pistillate flower contains pistil, but not stamens, it is known as Pistillate flower.
In "crop plants, meiotic division of specific cells in stamen and pistil yields microspores and megaspores, respectively. This is followed by mitotic division of the spore nuclei to produce gametes; the male and female gametes are produced in microspores and megaspores, respectively.

(A). Sporogenesis:- Productions of microspores and megaspores is known as sporogenesis. Microspores are produced in anthers (microsporogenesis), while megsaspores are produced in ovules (megasporogenesis).
(1). Megasporogenesis:- Developmental process of megaspore by meiosis, is called Megasporogensis. Megasporogenesis occurs in ovules, which are present inside the ovary.
- Ovule is present in ovary (according to species).
- A cell from each ovule become differentiate into megaspore mother cell.
- This cell produces 4 (n) cells by meiosis and present in linear arrangement.
- Three of the cells become degenerated and remaining 1 haploid cells is called Megaspore.

(2). Microsporogenesis:- Developmental process of haploid (n) pollen grains (which are called microspore), is called Microsporogenesis.
- Each anther has four pollen sacs, which contain numerous pollen mother cells (PMCs).
- Each PMC undergoes meiosis to produce four haploid (n) cells or microspores.
- Each and every haploid cell develops into a pollen grain after maturation.

(B). Gametogenesis:- The production of male and female gametes in the microspores and the megaspores, respectively, is known as gametogenesis.

(1). Megagametogenesis:- The development of embryo sac from a megaspore is known as megagametogenesis.
- The nucleus of a functional megaspore divides mitotically three times and produce four or more nuclei.
- The exact number of nuclei and their arrangement vary considerably from one species to another. In most of the crop plants, megaspore nucleus undergoes three mitotic divisions to produce eight nuclei.
- Three of these nuclei move to one pole and produce a central egg cell and two synergid cells; one synergid is situated on either side of the egg cell.
- Another three nuclei migrate to the opposite pole to give rise to antipodal cells.
- The two nuclei remaining in the centre, the polar nuclei, fuse to form a secondary nucleus.
- The megaspore thus develops into a mature megagametophyte or embryo sac.
- The embryo sac generally contains one egg cell, two synergids, three antipodal cells (all haploid), and one diploid secondary nucleus.

(2). Microgametogenesis:- Developmental process of production of male gamete or sperm, is known as Microgametogenesis.
- During the maturation of pollen, the microspore nucleus divides mitotically to produce a generative and a vegetative or tube nucleus.
- One nucleus is called Vegetative or Tube Nucleus and another one is called Generative Nucleus.
- This condition is called Binucleate stage and the pollen is generally released in this binucleate stage. When the pollen lands onto the stigma of a flower, it is known as pollination.
- Shortly after pollination, the pollen germinates. The pollen tube enters the stigma and grows through the style.
- The generative nucleus now undergoes a mitotic division to produce two male gametes or sperms. The pollen, along with the pollen tube, is known as microgametophyte.
- The pollen tube finally enters the ovule through a small pore, micropyle, and discharges the two sperms into the embryo sac.