Triple Fusion in Flowering Plants Explained

Triple Fusion is defined as fusion taking place between two polar nuclei and sperm nucleus. This happens during double fertilization in a seed plant, and therefore gives origin to a triploid nucleus known as the Primary Endosperm Nucleus(PEN). The PEN later evolves into an endosperm. This fusion primarily takes place in sexually reproducing plants called angiosperms.

To understand reproduction in angiosperms and to know more about what triple fusion is, we must first understand what is the morphology of the reproductive parts of the male and female plants. And further understanding, where, in the process of fertilization, do we come across triple fusion, its formation, and the fate of the gamete formed after triple fusion.

How does it Take Place?

Let's discuss the process in which triple fusion takes place in the following steps –

• The pollen grains that fall in the stigma of a flower germinates and give rise to the pollen tube.

• The pollen tube enters the embryo sac from the micropylar end passing through style.

• Zygote(syngamy) is formed when one of the male gametes fuses with the egg cell.

• The second male gamete fuses with the two polar nuclei present in the central cell and forms a triploid primary endosperm nucleus.

• Triple process is hence, the process that involves the fusion of three haploid nuclei.

In this process one main gamete and two polar nuclei are involved.

Where Does it Start?

Now we already know the steps of the process in which triple fusion takes place, let us now discuss where it starts.

The zygote is formed by the fusion of the male gamete with the nucleus of the egg cell. The zygote remains dormant for some time and waits for the endosperm to form, which develops from the primary endosperm cell resulting from triple fusion.  Before the development of the embryo from the zygote starts the endosperm provides food for the growing employo  until its formation.

Pollination 

Pollination is the process by which the male anther transfers the pollen grain onto the stigma in the female flower. Pollination can occur either by wind or air(abiotic agents) or by the animals(biotic agents). In wind pollination, the pollen grains are taken by the wind and if the pollen lands on a suitable stigma, the pollination process takes place. This process requires more pollen grain production and also most of the pollen grains go unfertilized as the probability of landing on a stigma remains a problem. Insect driven pollination is more specific as flowers producing nectars are attractive and only a few pollen grains stick to the insects which land on the stigma of another plant when the insect takes nectar from another plant. In this case, the flowers are more attractive and produce nectar for different insects.

Double Fertilization

As the pollen grains land on the stigma, the tube cell of the pollen starts growing which causes the pollen-tube germination. The pollen-tube penetrates the stigma and grows towards the style and reaches the base of the ovule. The two germ cells travel through the course of the pollen-tube formation and reach the ovule.

In the ovule, one germ cell fertilizes the egg cell of the female flower and forms the zygote. Both the nuclei are haploid and become diploid after fertilization. The zygote later forms the embryo and is the main product of the fertilization.

However, another fertilization takes place in angiosperms, where 2 polar nuclei and a sperm cell fuse together to form a triploid cell. Since there are three cells fusing together to form another cell, the phenomenon is called Triple Fusion.

The product of the fusion of secondary nuclei results in the formation of Primary Endosperm Nucleus. 

Endosperm

Endosperm development is faster than the development of an embryo so that the embryo has a hostile environment for its growth. It divides and forms a triploid endosperm tissue. It has reserve food materials and is used for the nutrition of the embryo. The Primary endosperm nucleus forms a successive nuclear division to give rise to free nuclei. This stage of development is called free nuclear endosperm. It eventually forms the cell wall. In the example of tender coconut, the white kernel is the cellular endosperm and the water is the free nuclear endosperm.

The Function of Endosperm 

The primary function of the endosperm is to provide nutrition for the developing embryo after double fertilization. 

Embryo

The fertilization that takes place between the egg cell and the male germ cell of the plant results in the formation of zygote. The zygote later proliferates into the embryo which is the main cell that carries the information of the parent plants.

Conclusion

This article is focused on Triple fusion and pollination. It elaborates on double fertilization and examples on the same. Go through the article to understand the concept better. This would be highly beneficial from an exam point of view.