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The study of human anatomy and physiology is one of the crucial Biology Topics for medical professionals and researchers.
Pollination and Fertilization – Double Fertilization in Plants
The process of union between the male gamete and the female gamete is called fertilization. Fertilization is the process where the male and female gametes fuse to produce a zygote. Sexual reproduction takes place by the union of male and female gametes in flowering plants. The process of union between the gametes is known as fertilization or fecundation. The diploid cell or zygote (2n) which is formed by the fertilization process grows into an embryo within the seed. When the seed germinates, the embryo gives rise to an adult plant.
Process of Fertilization
- Pollen grains are transferred from anther to the stigma and it germinates.
- After germination, a pollen tube emerges from the pollen grain and divides to form male gametes.
- The male gametes produced by pollen grains in the embryo sac fuse with the female gamete present in the ovule.
- This process of fusion of germ cells is called fertilization and forms a diploid zygote.
- After fertilization, the zygote undergoes a series of mitotic divisions to form a multicellular embryo, within the ovule.
- Ovule gradually converts into seed and the ovary becomes grows and ripens to form a fruit.
- The fruit bears a seed that contains a future plant or embryo which develops into a seedling under appropriate conditions.
Double Fertilization
Double fertilization is a complex fertilization mechanism of flowering plants. This process involves the joining of a female gametophyte (also called the embryo sac) with two male gametes (also called sperm). As a result of fertilization, the zygote (2n) is formed which grows into an embryo within the seed.
The union, in flowering plants, of two sperm nuclei, where one sperm nucleus unites with the egg to form the diploid zygote, from which the embryo develops and the other unites with a diploid secondary nucleus and forms endosperm. (OR) Fertilization is the characteristic of seed plants in which one sperm nucleus fuses with the egg nucleus to form an embryo and the other fuses with polar nuclei to form an endosperm.
Process of Double Fertilization in Flowering Plants
The events of the double fertilization process are described below:
1. Transference of mature pollen grains from anther takes place to the stigma of the same flower or different flowers.
2. The pollen grain becomes turgid and swollen by absorbing sugary stigmatic fluid. The inner wall or intine of the pollen grain is stretched and comes out in the form of a pollen tube through the germ pore.
3. Before the formation of the pollen tube, the nucleus of the pollen grain divides to form two daughter nuclei, of which the larger one is called the tube nucleus and the smaller one is called the generative nucleus.
4. The growth of the pollen tube is influenced by the sugary substances secreted by the stigma.
5. The pollen tube grows in length through the style, the tube nucleus lies at the tip of the pollen tube. The generative nucleus which lies behind the tube nucleus now divides to form two male gametes.
6. The entry of the pollen tube within the ovule depends upon the types of the ovule and the point through which it enters within the ovule. Entry through the micropyle is called porogamy, entry through the chalaza is called chalazogamy, and if it enters through the integuments the process is called mesogamy.
7. After entering the ovule, it makes its way by destroying the nucellar tissue toward the embryo sac. The pollen tube penetrates the embryo sac, the tip of the pollen tube gets disorganized and the male gametes are discharged into it.
8. At this time, the embryo sac consists of 8 haploid nuclei of which three nuclei lie at the chalazal end, called the antipodal cells or antipodals whose functions are unknown. The three cells at the micropylar end are collectively called the egg apparatus, in which the central cell is the egg or ovum, and the two neighbouring cells are called synergids. A single nucleus from each pole moves to the centre of the embryo sac and fuses to form a diploid (2n) secondary nucleus or definitive nucleus. This type of embryo sac is known as monosporic.
9. In the double fertilization process, involves the joining of a female gametophyte (megagametophyte, also called the embryo sac) with two male gametes (sperm). It beings when a pollen grain ad¬heres to the stigma of the carpel, the female repro¬ductive structure of a flower. The pollen grain then takes in moisture and begins to germinate, forming a pollen tube that extends down toward the ovary through the style. The tip of the pollen tube then enters the ovary and penetrates through the micropyle opening in the ovule. The pollen tube proceeds to release the two sperm in the megagametophyte.
One sperm fertilizes the egg cell and the other sperm combines with the two polar nuclei of the large central cell of the megagametophyte. The haploid sperm and haploid egg combine to form a diploid zygote, while the other sperm and the two haploid polar nuclei of the large central cell of the megagametophyte form a triploid nucleus (triple fusion). Some plants may form polyploid nuclei. The large cell of the gametophyte will then develop into the endosperm, a nutrient-rich tissue that provides nourishment to the developing embryo. Double fertilization is the rule in angiosperms plants and was observed first by the scientist Nawaschin in 1898.
10. The large triploid cell of the gametophyte will then develop into the endosperm, a nutrient-rich tissue that provides nourishment to the developing embryo.
11. The two central maternal nuclei (polar nuclei) that contribute to the endosperm, arise by mitosis from the same single meiotic product that gave rise to the egg. The maternal contribution to the genetic constitution of the triploid endosperm is double that of the embryo. The ovary, surrounding the ovules, develops into the fruit, which protects the seeds and may function to disperse them.
Transformation of Ovary after Fertilization:
| Before Fertilization | After Fertilization |
| 1. Ovary | 1. Fruit |
| 2. Ovary wall | 2. Pericarp |
| 3. Ovule | 3. Seed |
| 4. Integument | 4. Seed coat |
| 5. Egg/Ovum | 5. Embryo |
| 6. Endosperm nucleus | 6. Endosperm |
| 7. Micropyle | 7. Micropyle |
In a recent study done on the plant Arabidopsis thaliana, the migration of male nuclei inside the female gamete and fusion with the female nuclei has been documented for the first time using in vivo imaging. Some of the genes involved in the migration and fusion process have also been determined.
Significance of Double Fertilization
- It gives stimulus to the plant due to which the ovary develops into fruit and ovules develop into seeds.
- It restores the diploid condition by the fusion of haploid male and female gametes.
- It results in the formation of the diploid zygote, which develops into an embryo and gives rise to a new plant.
- It results in the formation of a triploid primary endosperm nucleus (PEN) which develops into endosperm in the seed.
- It provides nourishment to the developing embryo.
- It brings about the recombination of characters resulting in variation among the offspring.
- The variations thus caused, contribute to the mechanism of evolution.