WHAT IS MICROSPOROGENESIS? - BIOLOGY - 2025

The microsporogenesis in botany, is one of the stages of formation of pollen grains. Specifically, it comprises the formation of a microspore. It occurs in the anthers of the flower, starting from cells called microsporocytes.

In essence, the process involves the meiotic division of microsporocytes, leading to the formation of cells called microspores. As the microsporocyte undergoes meiosis, for each initial cell, four daughters are obtained with a reduction in genetic load.

Source: André Karwath aka Aka

The fate of the microspore is to transform into a cell with two nuclei. During its development, the microspore progressively increases its volume, an event that is associated with the formation of a vacuole. In parallel, the displacement of the cell nucleus occurs.

This process gives rise to the pollen grain, which undergoes successive transformations after fertilization. The process by which a microspore turns into pollen is called microgametogenesis.

Once the pollen descends the stigma, the duplication of one of the nuclei, known as sperm nuclei, occurs again. In this way, the male gametophyte is made up of a cell with three nuclear structures.

Microsporogenesis

The pollen

A pollen grain is a structure of more or less microscopic size that corresponds to the male gametophyte of plants with seeds or spermatophytes.

The portion of the anther that contains the grains is called the pollen sac, located in the male zone of the flower: the stamens.

Microsporogenesis: first stage of pollen development

The life of this male gametophyte occurs in three well-defined phases: an initial stage of development that within the male sporophytic tissue called microsporogenesis; followed by an independent stage of travel to a compatible stigma and finally a rapid growth phase in female sporophytic tissue.

The first stage is microsporogenesis and occurs inside the anthers. This comprises a series of meiotic divisions of cells called microsporocytes or "mother pollen", which are encapsulated in a thick wall of callose.

Tetrad formation

The result of this division is a tetrad of cells, where each one will develop into a male gametophyte. Each of these cells is encapsulated in a second wall of callose.

Remember that meiosis is a process of cell division with reducing properties. The genetic load of the stem cell is not the same as that found in daughters.

In the case of microsporocytes, these are diploid, so the daughter cells resulting from the initial division are haploid. The number of resulting chromosomes depends on the species.

Cytokinesis

Nuclear meiotic division is followed by cytokinesis. This step is decisive for the final formation of the tetrad, since there are multiple patterns or types of dispositions for it.

Successive cytokinesis occurs when each cell division is accompanied by a division of the cytoplasm, a phenomenon typical of monocots. When this happens, we will see that the microspores are arranged in a single plane, either in the form of a tetrad, a rhombus or forming a letter T.

The alternate division is known as simultaneous cytokinesis, where the walls form at the end of meiosis. It occurs in the group of dicotyledons. This pattern results in the dispersion of microspores in multiple planes.

Pollen wall formation

The pollen walls begin to form while the microspores are still in the tetrad arrangement and encapsulated by the callose walls.

The first step involves the deposition of a substance called primexin on the surface of the microspore. This is followed by the deposition of the sporopolenin precursors. The process ends with the deposition of sporopolenin, a molecule of an impermeable nature, resistant to attack by chemicals.

The openings develop in the regions where the deposition of primexin has been prevented, by the action of the endoplasmic reticulum.

Role of the mat in pollen development and transport

During pollen formation the mat plays a crucial role. This comprises a layer of cells located in the anther that surrounds the stem cells of the microspore. There are two cell types on the mat: secretory and amoeboid.

These cells are very specialized and have a fairly short life span. With the passage of time, cells lose their organization and are finally reabsorbed.

Its main role in pollen development involves providing nutrition to the microspore. In addition, they have the ability to synthesize a series of enzymes and to produce the "cement" of pollen or pollenkit.

The pollenkit is a material of heterogeneous nature (lipids, flavonoids, carotenoids, proteins, polysaccharides, etc.) and of sticky consistency that helps to hold the pollen grains together during transport and protects them from desiccation, ultraviolet light, and other factors that may affect its quality.

Microgametogenesis

To conclude, we will briefly explain what microgametogenesis consists of, in order to expose conclusively how the genesis of a pollen grain occurs. This process varies in angiosperms and gymnosperms, namely:

Angiosperms

In angiosperms, microgametogenesis comprises the first and second mitotic divisions of pollen, leading to the formation of male gametes.

This process begins with the formation of a vacuole located in the center of the cell, an event that forces the nucleus to move. This nuclear movement marks the transition from a microspore to a young pollen grain.

The first mitotic division is followed by a second asymmetric division, where a generative and a vegetative portion are formed. The latter comprises a larger volume and contains a large, diffuse nucleus. In the case of the generative portion it contains a smaller and condensed nucleus.

Subsequently, a symmetric division occurs where the generative cell gives rise to two sperm cells.

Gymnosperms

In contrast, microgametogenesis in gymnosperms develops through multiple mitotic divisions. Most of the pollen grains in this plant lineage are made up of more than one cell.

References

1. Blackmore, S., & Knox, RB (Eds.). (2016). Microspores Evolution and Ontogeny: Evolution and Ontogeny. Academic Press.
2. Davies, PJ (Ed.). (2013). Plant hormones: physiology, biochemistry and molecular biology. Springer Science & Business Media.
3. Hesse, M., Halbritter, H., Weber, M., Buchner, R., Frosch-Radivo, A., Ulrich, S., & Zetter, R. (2009). Pollen terminology: an illustrated handbook. Springer Science & Business Media.
4. López, BP, Calvarro, LM, & Garay, AG (2014). Pollen embryogenesis (gametic embryogenesis). REDUCA (Biology), 7 (2).
5. Smith H. & Grierson D. (Ed.). (1982) The Molecular Biology of Plant Development. University of California Press.