What is Plant Fertilization?
Fertilization is the process of fusion of two dissimilar sexual reproductive units, called gametes. In 1884 the process of plant fertilization was first discovered by Strasburger. The female gametophyte or embryo sac of angiosperms (the flowering plants) is situated in the ovule, at a distance from the stigma. Therefore in order to transport male gametes into the embryo sac the pollen, which is transferred to the stigma, produces a pollen tube. Plant fertilization is the union of male and female gametes (i.e. reproductive cells) to produce a zygote or fertilized egg.
Germination of pollen grains
When the pollen is shack from anther it has usually two cells, a generative cell and a tube cell (vegetative cell). The generative cell forms two male gametes. Once the pollen has landed on the receptive stigma, its germination starts. On the surface of the stigma the pollen absorbs water, swells and produces a pollen tube. The stigmatic fluid secreted by the stigma contains sugars, lipids, resins, etc. and thus provides a suitable medium for the germination of pollen grains. The pollen tube penetrates into the stigmatic papillae by the action of an enzyme, known as cutinase, present in the pollen grain and pollen tube. This enzyme degrades the cutin of the stigma at the place of contact with pollen tube. To perform the plant fertilization the entire contents of the pollen, following its germination, move into the pollen tube. The enlargement of the pollen tube is primarily restricted to its tip where most of the cytoplasm is concentrated. The budding pollen tube penetrates the stigma and pushes its way through the style and down the wall of the ovary.
How pollen tube enters into ovule in plant fertilization?
In the process of plant fertilization the pollen tube may enter the ovule through one of the following three routes: -
When pollen tube enters the ovule through the micropyle, the condition is known as porogamy . This is the most common mode of pollen tube entry into the ovule.
When pollen tube enters the ovule through the chalazal end, the condition is said to be chalazogamy.
When pollen tube enters the ovule through integument, the condition is described as mesogarny.
Entry of pollen tube in the embryo sac
Irrespective of the route of pollen tube into the ovule, it always enters the embryo sac from the micropylar end. The entry of pollen tube into the embryo sac may be as follows:
a) Between the egg cell and one of the synergids.
b) Between the wall of the embryo sac and one or both the synergids.
c) Between the two synergids.
d) Directly penetrates one of the synergids.
So in plant fertilization, Synergids thus not only play an important role in determining the entry of pollen tube in the embryo sac but they also affect propagation of male gametes in the embryo sac.
Discharge of male gametes
After entering into the embryo sac, the tip of the pollen tube bursts and the two male gametes are discharged. The tube nucleus disorganizes before bursting of the pollen tube. The male gametes, as they are released, show amoeboid movements. One of them moves towards the egg and the other goes to the polar nuclei.
Fusion of gametes
One male gamete fuses with the egg and this results in the formation of a diploid zygote. In case of plants this fusion of male and female gametes is known as plant fertilization. But what happens to the other male gamete? The other male gamete fuses with the two polar nuclei or secondary nucleus, and forms a triple fusion nucleus, called primary endosperm nucleus. The contact between one of the male gametes and the egg is established earlier than between the other male gamete and polar nuclei. But the primary endosperm is formed earlier than the zygote, perhaps because the cytoplasm of the central cell is more active than the egg. This type of plant fertilization which involves the fusion of one male nucleus with the egg and the other with the polar nuclei is unique to angiosperms.