Introduction
Fertilisation, specifically double fertilisation in Angiosperms, is used to reproduce plants. Hence, flowers are believed to represent the reproductive sections of angiosperms, which are the most diverse kind of terrestrial plants. For example, these flowers might be bisexual or unisexual. Further, without microsporogenesis (pollen creation), megasporogenesis (embryo sac development) and pollination, the process of double fertilisation is impossible (the pollen transfers from the anther to the stigma, either of the same or distinct flowers of the similar species. In technical terms, the fusion of one female gametophyte with two male gametes is a complex process called double fertilisation.
Definition of double fertilisation
When it comes to flowering plants, double fertilisation is a defining characteristic. One female gamete joins two male gametes in this phenomenon. A single male gamete fertilises the egg, which results in zygote production, while the other forms an endosperm by joining with two polar nuclei. This process can be described as double fertilisation definition.
Double fertilisation stimulates the plant, resulting in ovarian growth into fruits and ovule development into seeds. The plant’s diploid condition is restored once the haploid male and female gametes combine.
Development of male gametophyte
- Microsporogenesis: Microsporogenesis is the process of the male gametophyte (microspore) forming.
- In this procedure, one pollen mother cell or microspore mother cell (PMC or MMC) passes through meiosis to create four haploid pollen or microspores.
- The pollen nucleus separates into two nuclei as the pollen matures: the smaller generative nucleus (male nucleus) and the larger tube nucleus (vegetative nucleus).
- Pollen grains are transmitted to the stigma during this two-celled stage in over 60% of angiosperms, whereas pollen is discharged at a three-celled stage in the remaining 40% of angiosperms.
Development of female gametophyte and megasporogenesis
- Megasporogenesis: Megasporogenesis is the process of forming a megaspore from a megaspore mother cell.
- An ovary can hold one or more ovules. There is a megaspore mother cell in each ovule, which is a huge, diploid cell.
- Megaspore mother cells go through meiosis and produce four haploid megaspores, among which one is functional and the other three are not.
- The functional haploid megaspore develops and divides into eight haploid nuclei after 33 mitotic cycles. The embryo sac also termed the female gametophyte, is the structure that forms.
Pollination
Pollination is the transport of pollen grains from the anther to the stigma of the same or distinct flowers of the same species. Pollen grains may be transferred spontaneously or with the assistance of pollination agents such as butterflies, honey bees, wind, birds, and water.
Process of double fertilisation
In angiosperms, the process of double fertilisation classification includes the following steps:
- The most varied category of terrestrial plants is angiosperms, i.e. flower-bearing plants. Flowers are the reproductive organs of angiosperms, containing distinct male and female reproductive organs. Gametes – egg cells and sperms, respectively – are found in both.
- Pollination aids pollen grains in reaching the stigma through the style. First, both the sperm cells join forces and penetrate the ovule-synergid cell, followed by fertilisation.
- Fertilisation in angiosperms produces two structures: endosperm and zygote, thus the term “double fertilisation.”
- Double fertilisation is a complicated process in which one sperm cell fuses with the cell of an egg while the other combines with both the polar nucleus, resulting in a diploid (2n), triploid (3n) and a zygote.
- The endosperm is known as triple fusion because it comprises three haploid nuclei fused. The primary endosperm nucleus eventually matures into a primary endosperm cell (PEC) and subsequently endosperm.
- After several cell divisions, the zygote becomes an embryo.
Events after double fertilisation
In the flower, the following events take place once the double fertilisation procedure is completed:
- Endosperm development: Mitotic divisions generate endosperm from the triploid primary endosperm nucleus (PEN). Endosperm formation occurs soon before embryo development and there are three forms of endosperm: free nuclear, cellular and helobial.
- Development of zygote into an embryo: At the micropylar end of the embryo sac, the zygote generated by the union of the haploid male gamete with the haploid female gamete develops. Mitotic divisions transform the zygote into an embryo. A developed embryo has one (monocot) or two (dicot) cotyledons, a plumule at the apical end and a radicle at the micropylar end.
- Formation of seed: After fertilisation, an ovule passes through a sequence of modifications that culminate in seed formation. The two integuments, testa and tegmen, grow into the two seed coats. In certain seeds, a completely fertilised ovule containing an embryo stores food material in the form of endosperm and a protective seed coat may be characterised as a seed.
- Formation of fruit: A mature ovary produces a natural fruit. The ovary wall grows, forming a pericarp and the majority of the floral elements, such as petals, style, sepals, stamens and stigma, wither and fall off while the seed develops. As a result, developed ovary-bearing seeds might be characterised as fruit.
Significance of double fertilisation
In angiosperms, the major benefit of double fertilisation is energy conservation. The plants do not spend energy on seed nutritive tissue until an egg is fertilised. In addition, the endosperm nucleus is particularly active and divides fast, resulting in the formation of nutritive tissue very quickly.
The following are some further implications of double fertilisation:
- Produces a food supply for the seed: The secondary result of fertilisation, PEN, develops into endosperm, a nutritious tissue that feeds the developing embryo. Some plants even utilise this food supply amid adverse environmental circumstances when a new plant arises.
- Provides extra protection for the species: This method of fertilising ensures the longevity of a blooming plant. Because there are two male gametes, they have a significantly better chance of interacting with a female gametophyte. As previously stated, after being fertilised by a sperm cell, the endosperm stores nutrients. Because the plants do not have to devote energy-intensive resources to an egg that will not be fertilised.
- Conserves energy: The blooming plant conserves energy as the embryo begins to form since all of the necessary nutrients are contained inside the seed. Consequently, the plant will concentrate on its survival for as long as possible.
- Allows for rapid seed development: Seed development is faster than other modes of reproduction because two male gametes are engaged in the reproductive process. In addition, the endosperm nucleus splits fast to provide nutrient-rich tissue, which allows the embryo to expand rapidly.
Conclusion
The fusing of haploid male gametes with the haploid egg nucleus generates the diploid zygote. The fusion of another haploid male gamete with the diploid polar nucleus forms the triploid main endosperm nucleus, known as double fertilisation. Before double fertilisation can begin, megasporogenesis, microsporogenesis and pollination must occur. The pollen-pistil interaction is responsible for positively chemotropic and negatively aerotropic pollen tube germination.
Siphonogamy is a kind of fertilisation that involves conveying male gametes via a pollen tube. The development of endosperm and zygote occurs after the double fertilisation process is completed, resulting in the creation of seeds and fruit.