Structures and Events Involved in Post-Fertilisation

Post-fertilisation events are the series of events involved in reproduction after male and female gamete fusion, also called fertilisation. This event occurs in every sexually reproducing organism after external or internal fertilisation. After fertilisation, supportive structures like the sepal and petal fall off, including the female reproductive organs that are stigma and style. Ovules present within the ovary mature after fertilisation and become seeds while the ovary transforms into fruit.

Stages of post-fertilisation events 

The events occurring in the post-fertilisation process are listed below. 

• Formation of the endosperm.
• Formation of the embryo.
• Disorganisation of the nucellus.
• Formation of seed.
• Formation of fruit.

Formation of endosperm

Endosperms are nutritive tissue that forms after fertilisation in plants and plays a vital role in developing embryos. Based on development, there are three types of endosperms:

• Nuclear endosperms: In this type of endosperm, the endosperm nucleus is divided multiple times and forms multiple cores. This division is called the free nuclear division.
• Cellular endosperms: This type of endosperm is very uncommon in plants. The primary endosperm nucleus (PEN) division occurs first in the cellular endosperm, followed by cytoplasm division (cytokinesis). The formation of two cells occurs due to the transverse division, thus creating a chamber for chalaza and micropyle. Further division leads to the construction of the cellular endosperm.
• Helobial endosperm: Helobial endosperm is most often observed in monocotyledons. This type is an intermediate of both endosperms (nuclear and cellular endosperms). The first division in the global endosperm results in the formation of micropylar and chalazal chambers like cellular endosperm. The chalazal section does not increase and functions as a base cell. On the other hand, the micropylar cell divides similarly as in the nuclear endosperm. 

The function of endosperm

• It is essential for the growth and development of the embryo as it stores the food material. 
• It consists of plant hormones such as cytokinin that helps in cell differentiation. 
• Embryonic growth can be regulated according to favourable environmental conditions by the signals induced by the endosperm. 

Formation of embryo

Forming and developing an embryo from a zygote (zygotic embryogenesis) or a somatic cell (somatic embryogenesis) is called embryogenesis. The development of a source starts after the endosperm is well developed, as the endosperm has to provide nutrition and nurture the seed. Usually, the zygote divides immediately after or before the first division of the primary endosperm nucleus.

Embryogenesis in dicotyledons

Embryogenesis of a dicotyledon was first observed by Hanstein (1870) on Capsella bursa-pastoris, a member belonging to Cruciferae. A typical dicot zygote initially elongates and undergoes a transverse division providing two unequally divided cells. The large cell is called a suspensor cell, and the smaller cell is called an embryo cell or terminal cell. The suspensor cell further undergoes transverse division and produces 6-10 filamentous suspensor cells; this pushes the embryo into the endosperm—the first cell at the micropylar end increases in size and functions as haustorium. Haustorium is a modified stem or root with a unique water and nutrient absorbing capacity. The last cell adjacent to the embryo is called hypophysis, which gives rise to the radicle and root cap on development. On the other hand, the embryo cell also undergoes two vertical divisions and one transverse division and forms an 8 cell, a structure arranged in two tiers. The upper tier of cells is called epibasal, and the lower basal cells are called hypo basal. 

The epibasal cells develop and form two cotyledons and the plumule, while the hypo basal cells produce the hypocotyl except for its tip. The eight embryonic cells or octants undergo division to have an outer layer of protoderm or dermatogen, and the inner cells differentiate further into procambium and ground meristem. Initially, the embryo is globular and undifferentiated. An early seed with radial symmetry is called a proembryo. It is transformed into a source with radicle, plumule, and cotyledons development. Separating the two cotyledons, a faint plumule is present, and the embryo is now heart-shaped. At this stage, the rate of growth of the cotyledons is very high so that they elongate tremendously while the plumule remains as a small mound of undifferentiated tissue.

Development of monocotyledon embryo

The early development of a monocot embryo is like dicots; as the story continues, monocots’ embryogenesis becomes more complex than the dicots. The stages of embryogenesis in monocots include the proembryo, globular, scutella, and coleoptile stages. The first cell division after fertilisation is asymmetrical and leads to the formation of apical and basal cells. The proembryo is like the dicots in the globular stage, except that suspensor cells are less differentiated. The group of cells on one side of the proembryo divides rapidly and gives rise to the embryo axis. In the scutella stage, the remnant of cotyledon is visible, and monocots have reduced the pair of cotyledons to a single modified cotyledon, now called scutellum. It acts as connective tissue between the endosperm and embryo axis. 

In the coleoptile stage, the embryo axis differentiates to form plumule and radicle. The specialised tissue around the shoot and root that helps germinate is called coleoptile and coleorhiza. 

Formation of seeds

After double fertilisation, the embryo and endosperm develop gradually. During this period, the ovary and ovule turn into fruit and seeds. Seeds are the end product of the sexual reproduction of flowering plants. The external layer of the ovule becomes dry and hard and converts into testa. The internal layer of the ovule forms taxman. Most often, seeds are surrounded by a thin layer called perisperms. Funicles are attached to ovules through the hilum. Based on the presence or absence of endosperms, the classification of seeds can be done in the following manner: 

Endospermic seeds

During the development of an embryo, some seeds do not use whole endosperms for their development; hence few amounts of endosperms are left in mature seeds. This condition is called endospermic albuminous—for example, rice and wheat.  

Non-Endospermic seeds 

Many plants use the whole endosperm during the development of their seeds; hence no endosperm was found in these types of mature seeds. This condition is called non- endospermic albuminous—for example, bean and pea. 

Part of the seeds that appear after fertilisation

• The stalk of seed as a scar
• Nucellus end otherwise as a perisperm
• Found embryo
• The internal layer becomes tegmen
• The external layer becomes testa

Development of fruits

After fertilisation, the ovary changes into the pericarp. The fruit, which is the change forms of ovaries like mango, tomato, etc., is called an actual fruit.

Some fruits are derived from structures other than the ovary, such as the sepal, petal, and bud; the thalamus is called false fruit. Examples include – apple and coconut. 

Most often, fruits are formed without the fertilisation process; such fruits are called parthenocarpic fruits. Examples include – grapes and bananas.  

Conclusion 

Fertilisation is when male and female gametes fuse to form a diploid zygote. This zygote undergoes further development to create an embryo, and the process is called embryogenesis. The events that occur after the fertilisation process is called post-fertilisation events. This development phase gives rise to several structures and processes and, finally, a healthy new individual.