Double Fertilization in Angiosperms

The process of double fertilization kickstarts seed formation in flowering plants (angiosperms) by fusing two male gametes (sperm cells) with two female gametes (egg and central cell) to generate the precursor cells for the embryo and endosperm, respectively. By species-specific pollen tube guidance and attraction processes, immobile sperm cells are carried by the pollen tube toward the ovule housing the female gametophyte. The two sperm cells unite with the egg and central cell after the pollen tube bursts inside the female gametophyte, causing seed formation. The endosperm is formed by the fertilized central cell, while the embryo and suspensor are formed by the fertilized egg cell, the zygote. The latter component connects the embryo to the developing seed's sporophytic maternal tissues. To ensure the transport of viable sperm cells and the production of both a functional zygote and endosperm, the underlying mechanisms of double fertilization are strictly regulated. The current state of knowledge about directed pollen tube growth and its communication with synergid cells resulting in pollen tube burst, the interaction of the four gametes resulting in cell fusion, and mechanisms by which flowering plants prevent multiple sperm cell entry (polyspermy) to maximize reproductive success will be discussed in this review.

Double Fertilization Definition

Reproduction in plants is usually achieved by fertilization or to be more precise, double fertilization. Double fertilization is a complex mechanism of fertilization which consists of the fusion of a single female gametophyte (megagametophyte, also referred to as the embryo sac) with two male gametes (sperm). 

What is Double Fertilization?

Double fertilization is a process that has developed in the flowering plants (or angiosperms). In general terms, the process consists of a couple of sperm cells, where the egg cell is fertilized by one sperm cell to produce the zygote and the other sperm cell fuses with the two polar nuclei or secondary nuclei that result in endosperm. 

Reproductive Structures in Angiosperms

Angiosperms are plant life that produce flora and end result, and they're the maximum common styles of plant life on earth. plants are the reproductive structures of angiosperms. They consist of 4 primary components:

• Carpel: female reproductive structure that makes eggs.

• Stamen: Male reproductive shape that makes sperm cells.

• Petals: surround the carpel and stamen and are regularly brightly coloured.

• Sepals: occupy the flower petals before it has opened.

In the course of flowering plant replica, two systems are created: a zygote and the endosperm. A zygote will develop and change into an embryo, or child plant. The endosperm is the plant embryo's meal supply. This process of forming a zygote and endosperm is called double fertilization, and it's miles precise to angiosperms.

Pollination 

Pollination is required before double fertilization may occur. When a pollen grain produced by a flower's stamen settles on the carpel of another flower, this is known as pollination. The pollen grain must specifically settle on the sticky platform, or stigma, of a carpel. A pollen grain is a sperm-containing protective structure. It grows a pollen tube once it falls on the stigma. This structure will develop from a carpel's style to its ovary. Double fertilization will take place in the ovary.

Double Fertilization Process

The pollen has to develop and evolve throughout the style to make way into the ovule after it is deposited on the stigma. The pollen or the microspores consist of two cells, namely, the generative cell and the pollen tube cell. 

The cell of the pollen tube develops into a pollen tube via which the developing cell passes through. The evolution of the pollen tube demands oxygen, water, and specific chemical indications. As the pollen tube passes through the style to make through the embryo sac, the tissues of the style support its further development. 

Throughout the procedure, in case the generative cell has not yet divided into two cells, it dissociates to produce two sperm cells. The synergids existing in the embryo sac discharge certain chemicals that provide the pathway for the pollen tube. Further, the pollen tube goes through the micropyle into the ovule sac. 

Either of the two sperm cells fertilizes the egg cell to create a diploid zygote and the alternative sperm cell associated with the other two secondary nuclei (polar nuclei) to develop a triploid cell that evolves into the endosperm. 

These two individual fertilization methods in angiosperms, when combined together, are referred to as double fertilization. After the completion of the fertilization mechanism, there is no room left for other sperms to make through. The ovule that has been fertilized produces the seed, while the tissues of the ovary evolve into the fruits that normally have the seeds within them. 

After the fertilization process, the development of the embryo starts. The zygote splits to produce two cells, namely, the upper cell or terminal cell and the lower cell or basal cell. The dissociation of the basal cell leads to the suspensor that ultimately creates a link with the maternal tissue. The suspensor offers a path for the food to be transferred to the developing embryo from the parent plant. 

The terminal cell also dissociates resulting in an earth-shaped pro-embryo. In the case of eudicots or dicots, the shape of the growing embryo is similar to a heart because of the availability of the two rudimentary cotyledons. In the case of non-endospermic dicots, including Capsella bursa, the endosperm grows during the initial stage yet is then digested. In this event, the nutrition storages are transported into the two cotyledons.

The cotyledons and embryos get congested within the growing seed and are compelled to twist as they gain size. Eventually, the cotyledons and embryo occupy the seed fully, during which, the seed becomes available for dispersion. The development of the embryo is kept on hold after a certain period of time and resumes only when the seed grows. The evolving seedling depends on the nutrition storages kept in the cotyledons unless the initial pair of leaves start photosynthesis.

Double Fertilization in Angiosperms

• Angiosperms are flower-bearing flora and are the maximum various institutions of terrestrial plant life. The flowers shape the reproductive part of angiosperms with separate male and female reproductive organs. each carries gametes – sperm and egg cells, respectively.

• Pollination helps the pollen grains to reach stigma through style. The 2 sperm cells input the ovule-synergid cell. This proceeds to fertilization.

• Fertilization results in two structures, particularly, zygote and endosperm, for this reason, the name “double fertilization.”

• Double fertilization is a complex manner in which out of sperm cells, one fuses with the egg mobile and the opposite fuses with two polar nuclei which result in a diploid (2n) zygote and a triploid (3n) number one endosperm nucleus (PEN) respectively.

• Considering that endosperm is fabricated from the fusion of three haploid nuclei, it is referred to as triple fusion. Ultimately, the number one endosperm nucleus develops into the number one endosperm mobile (p.c) after which into the endosperm.

• The zygote will become an embryo after several cell divisions.

• The embryo starts to grow and it gets crowded and begins to bend.

• In the final stage, the embryo fills up the seed completely.

Development of Embryo in Angiosperms

The moment the process of fertilization is accomplished, the development of the embryo begins and no further sperm cells are allowed to make through the ovary. The fertilized ovule grows into a seed and ovary tissues evolve as a fleshy fruit that encompasses the seed.

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After the process of fertilization, the zygote splits into the upper end cell and lower basal cell. The latter evolves into a suspensor that aids in the transportation of food to the developing embryo. The upper terminal cell grows into pro-embryo.

Phases of Development of Embryo in Angiosperms

The different phases involved in the development of an embryo are as follows:

• During the initial phase of development, the terminal cell dissociates producing a globe-shaped pro-embryo. In addition, the basal cell also splits into a suspensor.

• The developing embryo acquires a heart-like shape because of the existence of cotyledons.

• The developing embryo becomes congested and starts to adjust in different shapes.

• The embryo occupies all of the space of the seed.

Significance of Double Fertilization

The significance of double fertilization in angiosperms can be elaborated in the following points:

• As there are only two fusions; only two products are formed.

• The secondary product of fertilization – triploid primary endosperm nucleus (PEN) – evolves into a nutritive tissue known as the endosperm that is responsible for providing nutrition to the developing embryo.

• The diploid condition in the life cycle is retained in the product of the fusion process. The diploid zygote evolves into an embryo that, as a consequence, grows into a new plant.

• The process of double fertilization consists of the usage of both the male gametes formed by a pollen grain.

• As such, the chances of poly-embryology increases and this also leads to increased possibilities of the survival of the new plant.

• Double fertilization offers stimulus to the plants that are responsible for ovary developing into fruits and ovules developing into seeds.

• As double fertilization is a distinctive characteristic of angiosperms, the seeds of angiosperms are highly accessible.

• Double fertilization gives rise to the re-fusion of characteristics leading to the variation among the off-springs.