Microsporogenesis: from sporogenous tissue to pollen

From one cell to four

A pollen grain is not made in one step. It comes from a chain of cell divisions that you can count, like the steps of a recipe.

The cells of the sporogenous tissue divide again and again by mitosis. Each cell of this tissue is a pollen mother cell, abbreviated PMC. It is also called the microspore mother cell. A PMC is diploid, with $2n$ chromosomes, where $n$ is the number in one set.

Each PMC then divides by meiosis. Meiosis halves the chromosome number. The result is a group of four cells, held together like four pieces of an orange. This group is the microspore tetrad. Each cell of the tetrad is a microspore, and it is haploid, with $n$ chromosomes.

The whole process of making microspores from a PMC by meiosis is called microsporogenesis.

As the anther matures and dries out, the four microspores of each tetrad come apart. Each one grows into a pollen grain. Inside one microsporangium thousands of microspores form. They are released when the anther dehisces.

The numbers to keep in mind

• One PMC makes four microspores. Not one, and not two.
• All four microspores of a tetrad can become pollen grains. None is thrown away. In the female side the story is different, and you will see why in the next course.
• To make $N$ pollen grains, you need $N/4$ PMCs. The meiosis happens once in each PMC.

A small example. A student wants 100 pollen grains. She needs 25 PMCs, and there are 25 meioses. This type of count is asked again and again in NEET.

The microspore divides once more

A microspore is a single cell with one nucleus. It does not stop there. Its nucleus divides by mitosis into two cells of different size.

• The vegetative cell is big. It has plenty of food and a large nucleus of irregular shape.
• The generative cell is small. It is spindle shaped, with dense cytoplasm and a nucleus. It floats inside the vegetative cell.

Now the pollen grain has two cells. In over 60 per cent of flowering plants, the pollen is shed at this two-celled stage. In the rest, the generative cell divides once more by mitosis before the pollen is shed, and it forms two male gametes. The grain is then three-celled.

If the pollen is shed at the two-celled stage, the generative cell divides later, when the pollen tube is growing. The result is the same: two male gametes.

Chromosome numbers along the way

Take a plant whose PMC has 24 chromosomes, which is the number in rice. Both mitoses keep the number the same, and meiosis halves it once.

All the cells from the microspore onwards are haploid, because mitosis does not change the number. This is a very common trap: the cells made by mitosis are not diploid.

You should find that every nucleus from the microspore onwards has half the number of the PMC. The three nuclei of a three-celled grain together hold one and a half times the number of the PMC.

Try it in Python

The program counts the divisions and the cells from a number of PMCs.

Practise with new numbers

Common mistakes

• Saying that a PMC makes one microspore. It makes four.
• Saying that the microspore is diploid. Meiosis made it haploid.
• Calling the generative cell diploid. It comes from a mitosis of a haploid cell, so it is haploid.
• Mixing up the two divisions. The first, in the PMC, is meiosis. The two later ones, in the microspore and the generative cell, are mitoses.
• Forgetting that two-celled pollen is still immature. The generative cell divides later, in the pollen tube.

Remember these

• Sporogenous tissue cells divide by mitosis. A pollen mother cell is $2n$ and divides by meiosis into four microspores of $n$.
• The four microspores form a tetrad. The tetrad separates as the anther dries.
• The microspore divides by mitosis into a vegetative cell and a small generative cell. Over 60 per cent of flowering plants shed pollen at this two-celled stage.
• The generative cell divides by mitosis into two male gametes, in the grain or in the pollen tube.
• All nuclei from the microspore onwards are haploid.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. How many microspores does one pollen mother cell make, and by which division?

2. How many pollen mother cells are needed to make 60 pollen grains?

3. A pollen mother cell has 20 chromosomes. How many chromosomes has a microspore, and how many has a male gamete?

4. How are the vegetative cell and the generative cell different?

5. When does the generative cell divide in a plant that sheds two-celled pollen?

References

• NCERT (2023). Biology, Textbook for Class XII, chapter 1, Sexual Reproduction in Flowering Plants.
• Clark M. A. et al. (2018). Biology 2e, OpenStax, chapter 32, Plant Reproduction.