Post Fertilisation Structures and Events

Post Fertilisation: Structure and Events-

Post-fertilisation refers to the series of events that occur after the fusion of the male and female gametes during sexual reproduction in plants. These events lead to the development of the embryo and endosperm within the ovule. 

● Endosperm Development:

  ○ After fertilisation, the zygote divides and forms an embryo, and the primary endosperm nucleus (PEN) is formed by the fusion of two polar nuclei in the central cell of the embryo sac. The PEN undergoes multiple rounds of mitosis without cytokinesis, resulting in the formation of a large, triploid endosperm. The endosperm serves as a nutrient-rich tissue that supports the developing embryo.

 ○ This results in the formation of a free-nuclear endosperm, where the endosperm is composed of numerous free nuclei without cell walls.

 ○ Following the free-nuclear stage, cell wall formation occurs, leading to the cellularization of the endosperm.
 ○ The number of free nuclei formed before cellularization varies among different plant species.

 ○ Endosperm in Coconut:

    ■ The tender coconut water that is commonly consumed is actually the free-nuclear endosperm.
    ■ It is made up of thousands of nuclei suspended in liquid form.
    ■ The white kernel surrounding the coconut water represents the cellular endosperm.

 ○ Endosperm Persistence in Seeds:

The fate of the endosperm varies among different plant species and can be categorised into two types: completely consumed and persistent endosperm.

Completely Consumed Endosperm:

In some plants like peas, groundnuts, beans, and certain other species, the endosperm is entirely consumed by the developing embryo before seed maturation.
Therefore, when these seeds are split open, no endosperm is visible as it has been utilised by the embryo for growth and development.

Persistent Endosperm: In other plant species like castor and coconut, the endosperm persists in the mature seed and is utilised during seed germination.
When seeds of castor are split open, the endosperm can be observed as a distinct tissue.
Similarly, the coconut fruit contains a persistent endosperm surrounding the liquid endosperm (tender coconut water).

Embryo Development (Embryogeny):The zygote develops into an embryo through a process known as embryogeny. It involves several stages:

Proembryo: The zygote undergoes divisions to form a proembryo, which consists of a few cells. These cells gradually differentiate into different regions of the embryo.

Globular Embryo: The proembryo develops into a globular embryo, characterised by the formation of a protoderm (gives rise to epidermis), ground meristem (gives rise to ground tissues), and procambium (gives rise to vascular tissues).

Heart-Shaped Embryo: The globular embryo further develops into a heart-shaped embryo, where the cotyledons (embryonic leaves) become distinguishable. The region above the cotyledons is called the epicotyl (future shoot), while the region below is called the hypocotyl (future root).

Mature Embryo: The heart-shaped embryo continues to develop, and the cotyledons grow larger. The embryonic axis, comprising the hypocotyl and epicotyl, elongates. The radicle (embryonic root) is formed at the lower end of the embryonic axis.



Typical Dicotyledonous and Monocotyledonous Embryos:

  ○ Dicotyledonous Embryo:

    ■ A typical dicotyledonous embryo consists of an embryonal axis and two cotyledons.
   ■ The embryonal axis has two distinct regions: the epicotyl and the hypocotyl.
   ■ The epicotyl is the portion of the embryonal axis above the level of cotyledons, terminating with the plumule or stem tip.
   ■ The hypocotyl is the cylindrical portion below the level of cotyledons.
   ■ At the lower end of the hypocotyl, there is the radicle or root tip covered with a root cap.
   ■ The root cap protects the root tip during growth and assists in penetrating the soil.


  ○ Monocotyledonous Embryo:

    ■ The embryo of monocotyledons possesses a single cotyledon.
   ■ In the grass family (monocots), the cotyledon is known as the scutellum.
   ■ The scutellum is situated towards one side (lateral) of the embryonal axis.
   ■ Below the scutellum, the embryonal axis has the radical (embryonic root) and root cap, enclosed in an undifferentiated sheath called the coleorrhiza.
   ■ The portion of the embryonal axis above the level of attachment of the scutellum is the epicotyl.
   ■ The epicotyl in monocots has a shoot apex and a few leaf primordia enclosed in a hollow foliar structure called the coleoptile.



  ○ Seed Formation:

   ■ Seeds are the final products of sexual reproduction in angiosperms, formed inside fruits.
   ■ A seed typically consists of seed coats, cotyledons, and an embryo axis.
   ■ Cotyledons are simple structures, often thick and swollen, storing food reserves.
   ■ Seeds can be non-albuminous (no residual endosperm) or albuminous (retain part of the endosperm).
   ■ Some seeds may also have a persistent nucellus known as perisperm.
   ■ Integuments of ovules harden to form tough seed coats with a small pore called the micropyle for oxygen and water entry during germination.
   ■ Seeds become relatively dry during maturation, with reduced water content (10-15% moisture).
   ■ The embryo may enter dormancy or germinate under suitable conditions.

 ○ Fruit Development:

   ■ Ovary of the flower develops into a fruit simultaneously with the transformation of ovules into seeds.
   ■ The wall of the ovary becomes the pericarp, the fruit wall.
   ■ Fruits can be fleshy (e.g., guava, mango) or dry (e.g., groundnut, mustard).
   ■ Fruits have mechanisms for seed dispersal to colonise new habitats.
   ■ In some species, thalamus (receptacle) contributes to fruit formation, known as false fruits (e.g., apple, strawberry).
   ■ Most fruits develop only from the ovary and are called true fruits.
   ■ Parthenocarpic fruits develop without fertilisation, often induced by growth hormones, and can be seedless (e.g., banana).


 ○ Advantages of Seeds:

   ■ Seeds offer advantages such as reliable reproduction independent of water.
   ■ They have adaptive strategies for dispersal and colonisation in new habitats.
   ■ Young seedlings are nourished by food reserves until capable of photosynthesis.
   ■ Hard seed coat protects the embryo, providing a survival advantage.
   ■ Seeds generate new genetic combinations, leading to variations and evolution.

 ○ Importance of Seeds:

   ■ Seeds are the basis of agriculture, providing food reserves and the ability to raise crops in the next season.
   ■ Dehydration and dormancy in mature seeds enable their storage for long periods.
   ■ Seed viability can vary from months to hundreds of years.
   ■ Records exist of very old yet viable seeds, such as a lupine (10,000 years) and date palm (2,000 years).

Apomixis and polyembryony

 ● Apomixis:

  ○ Apomixis is a type of asexual reproduction in plants where seeds are produced without fertilisation.
  ○ It involves the formation of embryos directly from the cells of the ovule, bypassing the need for pollination and double fertilisation.
  ○ In apomixis, the embryo develops from the nucellus or other cells within the ovule, without the involvement of male gametes.
  ○ The resulting seeds are genetically identical clones of the mother plant, maintaining the exact genetic composition.
  ○ Apomixis is a significant reproductive strategy in certain plant species, such as dandelions and some grasses.
  ○ It allows for the production of offspring that are genetically uniform and retain the desirable traits of the parent plant.

 ● Polyembryony:

  ○ Polyembryony refers to the phenomenon where multiple embryos develop from a single fertilised egg or from multiple eggs within the same        ovule.
  ○ It results in the production of multiple embryos within a single seed.
  ○ Polyembryony can occur through various mechanisms, including cleavage of the zygote, formation of multiple embryos from different egg cells, or the development of adventitious embryos from cells other than the egg.
  ○ The embryos in polyembryonic seeds can be genetically identical or may exhibit some genetic variation.
  ○ Polyembryony is observed in many plant species, including citrus fruits, mangoes, and certain conifers.
  ○ It can enhance the chances of survival and dispersal of the species, as well as contribute to genetic diversity within a single seed.