Microsporogenesis: Definition, Diagram, Process

Microsporogenesis: Have you ever handled a flower’s dehisced anthers, whether Hibiscus or another? The golden dusty or powdery substance on your fingertips is pollen grains, which are created by a process known as microsporogenesis. Microsporogenesis takes place within the microsporangia or (the anther) of flowering plants. Spores are a plant’s reproductive components.

Microsporogenesis is the process in which the microspores are developed within seed plant microsporangia (or pollen sacs). A diploid cell in the microsporangium, known as a microsporocyte or pollen mother cell, goes through meiosis and produces four haploid microspores. Each microspore grows into a pollen grain (the microgametophyte). Read on to find complete details on Microsporogenesis.

Define Microsporogenesis

Microsporogenesis is the process of formation of pollen grains (or microspores) inside the pollen sacs (or microsporangium) of flowering plants by meiotic or reduction division. Each microspore is the first cell of the male gametophyte that produces male gametes.

In other words, meiosis, or reduction division, occurs in the nucleus of each microspore mother cell, resulting in four haploid nuclei, which is known as microsporogenesis. During the microsporogenesis process, diploid sporogenous cells develop into microsporocytes (pollen mother cells or meiocytes) that divide to create four haploid microspores during meiosis.

Each diploid meiocyte produces a tetrad of four haploid microspores, and microsporogenesis is completed when unique single-celled haploid microspores appear.

Microsporogenesis Diagram

Refer below to better understand the process of microsporogenesis with a diagram.

Fig: Microsporogenesis

Microsporogenesis in Hindi

परागमातृ कोशिका (Pollen mother cell) से लघुबीजाणु (Microspore) बनने की प्रक्रिया लघुबीजाणुजनन (Microsporogenesis) कहलाती है।

Structure of Pollen Sac or Microsporangium

Stamen is the male reproductive organ of the flower, which is having a bilobed anther that is attached to the filament. The anther is bilobed and each lobe encloses two pollen sacs or microsporangia. The microsporangia contain thousands of pollen grains. The pollen sacs or the microsporangia of a bilobed anther appear to lie at four corners of the anther. Thus, the anther in Angiosperms are tetrasporangiate.

The two lobes of the anther are joined by a connective that contains the vascular bundle which carries the nutrients. Each microsporangium (or pollen sac) has two parts, i.e., the outer wall and central homogenous sporogenous tissue. The wall of Microsporangium has four types of layers, i.e., the epidermis (common anther covering), endothecium, \({\rm{1 – 3}}\) layer thick middle layers and innermost tapetum.

The epidermis, endothecium, and middle layers are protective in function. The endothecium helps in the dehiscence of ripe anther to release pollen grains. The middle layer degenerates at maturity. Tapetum is the innermost wall layer that nourishes the developing pollen grains.

The cells of tapetum have dense cytoplasm and generally possess more than one nucleus. The centre of microsporangium has compactly arranged homogenous cells called sporogenous cells or microsporocytes. A microsporocyte has abundant cytoplasm and a prominent nucleus.

Fig: A-Stamen, B- T.S. of Anther, C- T.S. of Microsporangium

Microsporogenesis Notes

Describe the Process of Microsporogenesis?

Sporogenous tissue or microsporocytes fill the whole interior of a microsporangium whose cells divide with the growth of the anther and increase their number. Each microsporocyte undergoes mitotic division and produces a number of microspore mother cells (MMC) or pollen mother cells (PMC) which are diploid.

The microspore mother cells develop an internal layer of callose (\(\beta {\rm{ – 1,}}\,{\rm{3}}\) glucan) which breaks the plasmodesmatal connections among them. The separated mother cells round off and undergo meiosis to produce tetrads of haploid microspore or pollen grains. This phenomenon is called microsporogenesis.

There are five types of a tetrad are called isobilateral, tetrahedral, decussate, T-shaped and linear. Pollen grains of a tetrad grow and separate from one another. Usually, the arrangement of microspores in a tetrad is tetrahedral (the most common type) or isobilateral. However, decussate, linear and T-shaped tetrads are also found. The pollen grains are the first cells of the male gametophyte.

Fig: Microsporogenesis in detail

Dehiscence of Anther

The mature anther dries up and the sterile strip present between the two pollen sacs of each anther lobe disintegrates to form a single cavity. With the loss of water, the differentially thickened dead cells of endothecium contract from their outer thin walls and become concave. It brings the outer radial walls nearer.

As a result, the endothecium shortens and ruptures the anther lobe wall in the region of the stomium. The exposed spores are picked up by various agencies for pollination.

Structure of Pollen Grain

Pollen grains are generally spherical in a structure whose diameter is about \(25 – 50\,{\rm{\mu m}}{\rm{.}}\) There is a highly resistant wall on the outside and cellular contents inside. Its cytoplasm is rich in starch and unsaturated oils. Pollen grain protoplast is uninucleate in the beginning but at the time of liberation, it becomes \(2 – 3\) celled.

The wall or covering of pollen grain is called sporoderm which has two layers, i.e., the outer exine and the inner intine.

Exine: It is made up of a highly resistant fatty substance called sporopollenin (one of the most resistant biological materials that are not degraded by an enzyme). It enables the pollen grains to survive in unfavourable conditions like high temperatures, strong acids and alkalis for a long time. The regions where exine is thin or absent are known as germ pores from where the pollen tube emerges at the time of pollen germination.

Intine: It is a thin and smooth layer present inner to exine made up of cellulose and pectin. It is a continuous layer that gives rise to the pollen tube at the time of pollen germination.

When the pollen grain becomes mature, it contains two cells, i.e., the vegetative cell and the generative cell. The vegetative cell is bigger and has an abundant food reserve with a large irregularly shaped nucleus.

The generative cell is comparatively smaller and floats in the cytoplasm of the vegetative cell. It is spindle-shaped and has dense cytoplasm and a nucleus. In over \(60\% \) of angiosperms, the pollen grains are shed at this two-celled stage. In the remaining species, the generative cell divides mitotically to give rise to two male gametes. Hence, the pollen grains are shed at the three-celled stage.

Fig: Structure of a Pollen grain

Fig: Pollen grain and Microgametogenesis process (diagrammatic)

Pollen Development Process

There are 4 stages of pollen development. Pollen tube cells and generative cells are the two types of cells that make up a mature pollen grain. The tube cells grow into the pollen tube on successful germination, while the generative cell goes into the ovary through the pollen tube.

Generous cells are found within big pollen tube cells and divide to produce a few gametes or sperm cells inside the tube. When the anther matures, it releases all of the pollen grains for fertilisation.

Difference between Microsporogenesis and Megasporogenesis

The following table shows the difference between microsporogenesis and megasporogenesis:

Summary

Thus, microsporogenesis is the process of formation of pollen grains (or microspores) inside the pollen sacs (or microsporangium) of flowering plants by meiotic or reduction division. Pollen grain represents the first cell of the male gametophyte.

The microspores or the pollen grains undergo mitotic division to form vegetative and generative cells. The generative cell further divides to form the male gametes.

FAQs on Microsporogenesis

Q.1: What is Microsporogenesis?
Ans: Microsporogenesis is the process of formation of pollen grains (or microspores) inside the pollen sacs (or microsporangium) of flowering plants by meiotic or reduction division.

Q.2: What is Microsporangia?
Ans: Microsporangia are bi-lobed structures that operate as pollen sacs and are found in the plant’s anther, near the end of the long filament-like stamen. Male plant gametophytes grow and mature in a plant’s anther. The microsporangia are the parts of the anther where pollen or microspores form.

Q.3: What is the significance of Microsporogenesis?
Ans: Microsporogenesis plays an important role in sexually reproducing plants. In this process, the pollen grains are produced in the anther which contains two haploid male gametes so that after the process of fertilisation, the original chromosome number is attained.

Q.4: Where does Microsporogenesis occur in Angiosperms?
Ans: Microsporogenesis occurs in the Microsporangium or anther of the flowering plants.

Q.5: What is the basic difference between Microsporogenesis and Megasporogenesis?
Ans: Megasporogenesis is the process of formation of haploid megaspores from megaspore mother cells (or megasporangium) whereas microsporogenesis refers to the process of formation of haploid microspores from microspore mother cells (or microsporangium).

Q.6: What is the end product of Microsporogenesis?
Ans: The end product of microsporogenesis is a pollen grain that further develops into the male gametes.

Q.7: What type of cell division is involved in Microsporogenesis?
Ans: Microsporogenesis is the formation of a haploid microspore from the diploid microspore mother cell (MMC). The division involved in this process is meiosis.

Q.8: What is Pollination by insects called?
Ans: Pollination by insects is called Entomophily.