Development of male gametophyte in Angiosperm

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Plants have a generational cycle in which the diploid sporophyte and the haploid gametophyte alternate. The haploid male and female gametes fuse to generate the diploid zygote. The zygote gives rise to the sporophyte generation. Males and females develop gametophytes in various ways. In angiosperms, a mature male gametophyte is a pollen grain, which is a 3-celled structure, whereas a female gametophyte is a 7-celled embryo sac. To learn more about male gametophytes, read this article.

Gametophyte

Archegonium and antheridium are two forms of gametophytes found in lower plants that produce eggs and sperms, respectively. Male and female gametophytes are found in higher plants, and they are made up of male and female gametes, respectively.

During the alternation of generation, the gametophyte is the sexual phase, whereas the sporophyte is the non-sexual phase. The sporophyte is a diploid generation created by the fusion of male and female sex cells, which results in a zygote (2n). Through meiosis, this sporophyte creates haploid spores, which give rise to a new gametophyte phase.

Male Gametophyte Structure

• In plants, flowers are the primary location of sexual reproduction. The calyx, corolla, androecium, and gynoecium are the primary components.
• A flower’s stamen depicts the male reproductive organ. The filament and the anther are the two sections.
• The anther is the fertile component of the stamen that looks like a knob.
• There is a long and narrow filament.
• Each anther has four lobes or microsporangia or pollen sacs, making it tetrasporangiate.
• Microsporangia produce pollen sacs, which become packed with pollen grains as they mature.
• Pollen grains have two layers of walls (sporoderm).
• The exine is a sporopollenin-based hard outer layer (one of the most resistant organic materials).
• High temperatures, enzymes, strong acids, and alkali have no effect on sporopollenin. As a result, it aids pollen grain fossilisation.
• It possesses conspicuous openings called germ pores, which are devoid of sporopollenin. Pollen tubes emerge from the pores of germs.
• The inner wall is made up of cellulose and pectin and is thin and continuous.
• The pollen grain’s cytoplasm is surrounded by a plasma membrane.
• In angiosperm, each mature pollen consists of two cells: the vegetative cell and the generative cell.
• The vegetative cell is bigger, with a huge irregularly shaped nucleus and considerable food reserves.
• The generative cell is the tiny cell that floats in the vegetative cell’s cytoplasm. It has a spindle-shaped nucleus and thick cytoplasm.

Male Gametophyte development in Angiosperms

Angiosperms are distinguished by the presence of seeds in the enclosing fruit, which are derived from a flower’s ovary. The flower is predominantly made up of sporophytic tissues, with male and female gametophytes that are much smaller than those found in other terrestrial plants.

In addition to the embryo, angiosperms have the rare property of multiple fertilisation, forming a triploid endosperm. The male gametophyte is found in the stamens’ anthers, whereas the female gametophyte is found in the pistil’s ovules.

Four pollen sacs (locules) in the anther contain many microspore mother cells, each of which goes through meiosis to create four microspores in a tetrad. The microspore is the beginning for male gametophyte production. The cytoplasm of the microspore is uniformly dispersed at first, with a haploid nucleus in the centre. Later on, a huge vacuole grows in the centre, pushing the nucleus to the side. During pollen development, many flowering plants, including the model species Arabidopsis and maize, undergo two mitotic divisions. A huge vegetative cell and a much smaller generative cell result from the initial division.

The vegetative cell inherits the majority of the cytoplasm from the microspore cell, contains a loose nucleus that is active in transcription, and completely contains the generative cells. The generative nucleus, on the other hand, is more densely organised and less active in transcription. A second mitosis occurs in the generative cell, resulting in the production of two sperm cells. Pollen formation is influenced by the function of tapetum, a sporophytic tissue that surrounds pollen.

Male Gametophyte

In higher plants, the formation of the male gametophyte is a complicated process. It primarily takes place as a result of the following events:

Development before pollination or Pre-Pollination development

• Male gametophytes begin to form inside the microsporangium or pollen sac before pollination. Precocious development is the term for this type of development.
• The microspore, also known as the pollen grain, is the male gametophyte’s initial cell.
• The nucleus of the microspore or pollen grain is pushed from the centre to one side near the wall during germination.
• The thick cytoplasm vacuoles greatly.
• The microspore divides for the first time, producing two uneven cells.
• The vegetative cell is the larger one, whereas the generative or germinative cell is the smaller thin one.
• The tube cell or vegetative cell has a big, irregular-shaped nucleus and a vacuolated cytoplasm rich in food reserve.
• Around the generative cell, a callose layer forms, separating the cell from the pollen grain wall. Later, the callose dissolves, allowing the naked generative cell to freely move around the tube cell’s cytoplasm.
• Pollen tubes are formed when the vegetative cell germinates.
• In the case of most angiosperms, pollen grains are discharged from the anther during this 2-celled stage.
• The generative cell divides twice to produce two non-motile male gametes.
• The pollen grain is shed at the 3-celled stage in some circumstances. Male gametes are produced prior to pollination in this situation.
• After pollination, the male gametophyte continues to develop on the stigma.

Development after pollination or Post-Pollination Development

• Pollen grains are placed on the stigma following pollination, and their germ pores absorb water and sugary compounds from the stigmatic fluid.
• The tube cell, also known as the vegetative cell, expands.
• As the volume of the cytoplasm rises, pressure on the intine increases.
• The intine emerges from the pollen grain in the shape of a pollen tube through one of the germ pores.
• The sugary material secreted by the stigma influences the pollen tube’s growth, as the pollen tube pierces the stigma and travels down the style and the ovary wall, carrying the two nuclei with it.
• The tube nucleus descends to the pollen tube’s tip and is followed by the generative nucleus.
• The generative nucleus divides quickly, resulting in the formation of two non-motile male gametes.
• Each male gamete has a big nucleus encased in a thin cytoplasmic sheath.
• Male gametes and the tube nucleus are carried by pollen tube to the female gametophyte.

Male Gametophyte Function

For sexual reproduction, the male gametophyte is required. The male gametophyte of flowering plants produces haploid male gametes.

Conclusion

A gametophyte is the multicellular haploid phase of every plant and algae’s life cycle. Archegonium and antheridium are two forms of gametophytes found in lower plants that produce eggs and sperms, respectively. Male and female gametophytes are found in higher plants, and they are made up of male and female gametes, respectively. In angiosperms, a mature male gametophyte is a pollen grain with three-celled structures. The male gametophyte grows in two stages, one before pollination and the other after pollination.