Gametogenesis in Plants: Sporogenesis and Gametogenesis

• In higher plants, the sexual reproduction completes in 2 steps:
  • Sporogenesis
  • Gametogenesis
• In angiosperm, flowers are the designated structures of sexual reproduction.
•  A flower is termed to be complete if it possesses all its four parts: calyx, corolla, androecium, and gynoecium.
• Androecium and gynoecium refer to the male and female organs of reproduction respectively whereas calyx and corolla aids in reproduction.
• Gametogenesis is defined as the biological process in which the formation of gametes take place.
•  The maturation of gametes take place in the reproductive cells of androecium and gynoecium.

Male gamete formation:

The formation of male gametes take place inside anther which consists of two steps:

1. Microsporogenesis
2. Micro gametogenesis

Microsporogenesis:

• The process of formation of microspores or pollen grains from microspore mother cell inside the microsporangium/pollen sac of anther is termed as microsporogenesis
• The formation of pollen grains take place in anther which is responsible for formation of male gametes.
• A typical anther is tetrasporangiate in nature. Connective, a column of sterile tissue connects the bilobed anther. Each lobe consists of two microsporangia divided by strip of sterile tissue.
• Externally these lobes can be splitted by longitudinal grooves.
• At maturity, due to breakdown of partition wall, two sporangia in a lobe fuse.
• The young anther consists of homogeneous cells that are bound by a well-defined epidermis.
• In course of development, the anther becomes four lobed, and in each lobe some hypodermal cells become more noticeable than the others because of their larger size and discrete nuclei.
• The cells are known as archesporium (or archesporial initials).
• The archesporial cells divide in a plane parallel to the outer wall of the anther lobe (periclinal division), cutting off parietal cells towards the epidermis and primary sporogenous cells towards the interior of the anther.
• The cells of the parietal layer go through a series of periclinal and anti-clinal division to form 2-5 concentric layers of the anther wall.
• The primary sporogenous cells, either directly or after a few mitoses, plays role as microspore mother cells.

T.S. of anther discloses the following structures:

1. Anther wall:

• The fully-fledged anther wall consists of an epidermis accompanied by a layer of endothecium, 2 or 3 middle layers and a single layered tapetum.

2. Epidermis:

• It is the outermost layer of anther.
• These cells divide frequently by anticlinal division so as to cope up with the rapidly enlarging internal tissue.
• Their role is to protect the anther.

3. Endothecium:

• Normally, the endothecium is single layered and emerges from the parietal cells.
• The cells adjacent to endothecium aids in the dehiscence of anther during the course of maturation.

4. Middle layer:

• It is 2-3 layered and emerges from parietal cells.
• The cells next to endothecium help in the dehiscence of anther and others act as storage centers for starch and other food materials essential for development of pollen.

5. Tapetum:

• This is the innermost layer of anther wall and arises from parietal cells. It completely surrounds the sporogenous tissue.
• It is made up of single layer of cells, each cell contains dense cytoplasm and obvious nucleus.
• It is physiologically vital layer as all the food materials to the sporogenous tissue must pass across it.

6. Sporogenous tissue:

• The sporogenous cells may either directly play role as microspore mother cells (or pollen mother cells) or they may go through a few mitoses before entering meiosis.
• In a meiotic division, each microspore mother cell yields four haploid microspores. Microspore tetrads represent the group of four microspor
• All the pollen mother cells in an anther locule are interlinked by plasmodesmata and with tapetal cells.
• When these cells go through meiosis the linkage between the tapetal cells and pollen mother cells are ruptured.
• The deposition of callose (glucans) makes the walls of pollen mother cells thick.
• The plasmodesmatal connections between the pollen mother cells are replaced by huge cytoplasmic channels.
• The channels allow passage for the transportation of cytoplasmic contents from one cell to the other is known as cytomixis.
• Because of this, a close synchrony is maintained during meiosis in the large number of pollen mother cells in anther locule.
• At the end of the first meiosis, the cytoplasmic channels are halted.
• Cytokinesis takes place either after first meiosis or after second meiosis yielding tetrad.
• Mostly all the four spores within a tetrad are totally segregated from one another and form the spores.
• Within the callose wall the microspores start to produce their individual wall.
• Then the breakdown of the common wall allows the microspores or pollen grains to set free in the anther locule.
• Each pollen grain bears haploid nucleus in the pollen cytoplasm surrounded externally by the plasmalemma and pollen wall.
• Pollen wall consists of outer uneven exine and inner even intine wall.

Micro-gametogenesis:

• The process of development of male gametes in the microspores or pollen grains is termed as Micro gametogenesis.
• Microspores symbolizes the initiation of the male gametophytic generation.
•  During gametogenesis, the pollen nucleus divides mitotically to yield the unequal cells:
  – The larger one is vegetative cell or tube cell which forms the pollen tube.
  -The smaller one, situated towards the wall is generative cell which again divides by mitosis to form two sperms (male gametes).
• After germination of pollen grain, the generative cell may divide inside the pollen grain or in the pollen tube.
•  At the same time when these alterations are going on, the microspore or pollen grain is also synthesizing its wall.
  – At one or more loci, the pollen wall is very thin.
  – These regions are called germpores.
•  Before pollen mitosis starts, 2 discrete changes in the pollen protoplast are observed:
• The nucleus is replaced from the center towards one side of the cell, which gives sign for the position of generative cell.
• The cytoplasm between the nucleus and the wall, on the side where vegetative cell is to be halted become highly vacuolated.
• Initially, the cytoplasm of the vegetative cell and that of generative cell is disconnected by two plasma membranes.
•  The wall of generative cell grows inwards between the plasma lemma of the generative cell and the intine, till the two ends of the wall connects and merge, then the cell is finally removed off and come to lie freely in the cytoplasm of the vegetative cell.
•  Afterwards, the wall of generative cell soon vanishes, and the cytoplasm of generative cell remains enclosed in two plasma membranes.
• The generative cell divides by a mitosis division.
• If this division takes place when they are in anther, the pollens are shed at 3 celled stage and if it takes place after pollen germination, they are shed at 2 celled stage.
• The exine ruptures and the intine forms a pollen tube.
• The vegetative nucleus enters the pollen tube. Now it is known as tube nucleus.
• The generative tube nucleus enters the tube and divides to form two generative nuclei which finally form the two male gametes.
• The tube nucleus disorganizes.
• One of the gamete fuses with the female gamete during fertilization.

Female gamete formation:

The formation of female gametes take place inside the ovules which consists of following steps:

1. Mega sporogenesis
2. Mega gametogenesis

Mega sporogenesis:

• The process of formation of megaspore, within the megasporangium or ovule is called as mega sporogenesis.
• Megaspores represents the female gametophyte.
• The ovule emerges as a small mass of homogenous tissue on the placenta in the ovary.
•  Integuments arise near to the base of this tissue which forms the nucellus in mature ovule.
  – Although the integuments commence later, they grow rapidly than the nucellus and soon covers it almost completely, omitting the region of the micropyle.
• The archesporium modifies immediately below the nucellar epidermis.
  -The archesporial cell directly plays role as the megaspore mother cell so that the sporagenous cell is hypodermal and the nucellar tissue around it stays single layered. -Such ovules are called tenuinucellate.
• In some others, the hypodermal archesporial cell divides obliquely, cutting an outer parietal cell and an inner sporogenous cell.
  -The parietal cell may either remain undivided or go through a few periclinal and anticlinal divisions so that the sporogenous cell becomes embedded in the nucellar tissue.
  – All such ovules where the sporogenous cell becomes sub-hypodermal, either due to the formation of parietal cells or due to the divisions in the nucellar epidermis, or both are called crassinucellate.
• The archaesporium turns more obvious than its surrounding cells due to its larger nucleus, denser cytoplasm and larger size.
• It either directly functions as the megaspore mother cell or it divides periclinally cutting an outer primary parietal cells and an inner sporogenous cell.
• The latter functions as the megaspore mother cell.
• Megaspore mother cell undergoes meiosis to yield four haploid megaspores.
• Only one of the four megaspores function and form the female gametophyte, the remaining upper three megaspores degenerate.
• Normally, the chalazal megaspore of tetrad is functional.
• The megasporangium together with the integuments, is called ovule.
• It is affixed to the placenta, on the inner wall of the ovary, by a stalk called funiculus.
• An ovule ready for fertilization constitutes of nucellar tissue covered almost completely by one or two integuments, leaving a small opening at the apical end.
• This opening termed as micropyle is the main passage for the entry of the pollen tube in the ovule.
• The basal region of the ovule where funiculus is affixed is called chalaza.
• In the nucellus, the female gametophyte is present, also called embryo sac.

Mega gametogenesis:

• The process of development of female gametes in the ovule or megaspore is known as the mega gametogenesis.
• The megaspore (n) marks the initiation of the megagametophyte generation.
•  The nucleus of the megaspore divides and grows into the female gametophyte or the embryo sac.
• The first division of the functional megaspore nucleus yields the two nuclei, one situated at the micropylar end and the other at the chalazal end inside the, megaspore.
• Both the nuclei divide and thus two nuclei lie at each pole.
• The third division yields 8 nuclei, 4 located at the micropylar end and the other four at the chalazal end.
• Out of the 4 nuclei at each pole, one moves to the centre and forms secondary nucleus after fusion.
• The three nuclei at the micropylar end forms the egg apparatus, and those at the chalazal end forms the three antipodal cells.
•  In the egg apparatus, larger cell is called the egg cell and the linked comparatively smaller cells are known as synergids.
• In this way, embryo sac is formed.
• At the same time, the tissue of nucellus elongates and forms funicle.
• The entire structure is now termed as ovule or megasporangium.

Gametogenesis in Plants: Sporogenesis and Gametogenesis