Double Fertilization in Plants

Recently I’ve been researching all of the different sweet corn varieties and how to grow them. Some have to be isolated from other corn; some don’t. As I was thinking about writing on this topic, I realized that there was some information that I needed to share before addressing the issue of corn and that’s the phenomenon of double fertilization.

When plants produce pollen, the pollen lands on the pistil of the flower, the pollen tube grows down to the female gamete and the female flowers are fertilized.  However in most plants there’s an additional step called double fertilization.

I need to begin this post by saying that what I’ll be writing about is what generally happens in seed bearing plants. Since this is botany, there’s sure to be an exception to everything that I write!

When pollen develops in the anther of a flower, diploid cells undergoes meiosis to become haploid cells. As one of these haploid cells (called a microspore) develops into a pollen grain, the cell divided once by mitosis to create two haploid cells. When the pollen lands of the pistil of a plant, one cell in the pollen becomes the pollen tube that grows down the pistil of the flower. The other cell divides to form two sperm that travel down the pollen tube.

In the ovules of the flower, cells divide to form four haploid megaspores. Three of these die leaving one megaspore which divides by mitosis to form eight nuclei divided between seven cells – six cells with one haploid nucleus in them and one cell with two haploid nuclei in it. One of the cells with one nucleus becomes the egg which when fertilized by one of the sperm cells becomes the zygote. This now diploid cell multiplies and develops into the embryo. This is the first fertilization.

But there’s still one more sperm cell that was delivered by the pollen tube and it fertilizes the cell within the ovule that has two nuclei. This results in a cell that is now triploid (1N from the sperm + 2(1N nuclei from the female cell) = 3N). This triploid cell develops into the endosperm of the future seed. This is the double fertilization in plants.

Inside a Corn Kernel from Botany for Gardeners by Brian Capon

Inside a Corn Kernel
from Botany for Gardeners by Brian Capon

Endosperm is part of a seed’s energy storage system. Plants with a lot endosperm in their seeds are the grains. The majority of a wheat, corn or rice kernel is made up of the floury endosperm tissue. When these seeds germinate, the endosperm provides the energy that the plant needs to emerge from the soil and begin growing. But from a human perspective, the endosperm of these cereals provides a large part of the human diet. When you eat bread, tortillas, rice, pastries, or sweet corn, you’re eating endosperm that developed from double fertilization.

While endosperm makes up the majority of a cereal crop’s seeds, in other plants the endosperm is absorbed during the seeds development. For example, there is no endosperm in a pea or bean seed – it’s been absorbed during the development of the seed. In these cases the energy for the germinating seed is contained within structures called cotyledons. Evolutionary biologists suggest that the development of a large endosperm is a later evolutionary step and that the cereal crops are more recent developments in the evolutionary process.

What does all of this matter? First, I just think this process of double fertilization is fascinating. Also it tells us that the only part of a plant that’s affected by the pollen that fertilizes it is the embryo and the endosperm.

There are stories out there about plants cross breeding and changing the taste of the vegetable or fruit that develop around this seed. I read that if a hot pepper pollen gets on a sweet pepper blossom, the sweet pepper will be hot. Others say that a squash being pollinated by a pumpkin will change the taste of the squash. But when you know about double fertilization in a plant, you realize that the only things that can be changed by the pollination process is the embryo and the endosperm.

That means that while those myths might be out there about cross-pollinated peppers and squash, what people are reporting is biologically impossible. The fruit of a pepper grows from the cells of the plant and isn’t affected by the pollination process. All the cells in the squash you eat came from the squash vine that you planted. The only part of the plant that’s been changed by pollination is the seeds. Peas, lima beans and shell bean could be affected by cross-pollination but since they self-pollinate, it’s not a problem. No vegetables that you grow in the garden this year will be affected by cross-pollination except one – sweet corn.

In sweet corn, the plant is wind-pollinated and the seed is eaten. The bulk of that seed is endosperm so what pollinated it makes a huge difference. In the next post I want to write about the different corn varieties that are available at this time and the different kind of endosperm that’s found in them.

But for new, the simple truth of double fertilization just shows how complex and amazing plants can be. Something as seemingly simple as fertilization has an unexpected step. While there are still debates about why this happens, the fact that it does shows the amazing process of seed development. I can’t look at a kernel of corn and not be in awe of the steps necessary to create it.

 

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