Endosperm

Endosperm is a type of tissue located within seeds that provide nutrients to the developing embryo. This tissue is rich in essential nutrients, such as starch, proteins, and oils, which aid in the growth of the seedling once germination starts. It develops generally  after fertilization and surrounds the embryo.

It is generally triploid tissue. In flowering plants, it is the result of a double fertilization, where one sperm cell fertilizes the egg cell to form the embryo, while the other sperm cell fertilizes two polar nuclei in the central cell of the embryo sac, creating the triploid endosperm.

1.0Types of endosperm 

Depending upon mode of development there are three types of endosperm found in nature.

1. Nuclear Endosperm or Free Nuclear Endosperm :

• Free Nuclear Endosperm refers to a type of endosperm in plants where the nuclei resulting from the division of the primary endosperm nucleus (also called the primary endosperm cell) do not get enclosed within individual cell walls. Instead, these nuclei freely float within a shared cytoplasm.
• This type of endosperm lacks cellular boundaries, meaning that the nuclei resulting from the division of the primary endosperm nucleus do not become compartmentalized within separate cell membranes. As a result, a multinucleate, cellularly undivided mass of endosperm develops, containing numerous nuclei in a common cytoplasm.
• Free nuclear endosperm is found in some plant species and is characterized by a syncytial structure, where the nuclei remain free in a common cytoplasm without cellular boundaries, distinct from the cellular endosperm where the nuclei are enclosed within individual cell walls.

2. Cellular Endosperm : 

• This type of endosperm is found in the Gamopetalae group. During the development each division of the primary endosperm nucleus is followed by cytokinesis. The endosperm is formed by successive mitotic divisions that are accompanied by cell wall formation, resulting in a multicellular structure. Each cell contains a single nucleus. This cellular endosperm configuration is a notable feature of seed development.

3. Helobial Endosperm: 

• This type of endosperm is characteristic of plants in the Order Helobiales, which are a group of monocots. The developmental process lies between the nuclear and cellular endosperm types. In this type, the initial division of the primary endosperm nucleus is accompanied by the formation of a transverse wall. This division results in an unequal split within the embryo sac, creating two compartments: a smaller chalazal chamber and a larger micropylar chamber.
• Following this partition, both chambers undergo successive free nuclear divisions, but the micropylar chamber experiences relatively more nuclear divisions compared to the chalazal chamber. Typically, the chalazal chamber deteriorates. In the micropylar chamber, the free nuclear divisions are succeeded by the formation of cell walls, resulting in the creation of a cellular endosperm tissue.

2.0Distinct feature of endosperm

1. Endosperm Formation and Characteristics:

• Endosperm is typically triploid, containing three sets of chromosomes (3n), in contrast to the diploid (2n) nature of most plant tissues. This triploidy is a result of the double fertilization process in flowering plants, where two sperm cells join with the egg and the central cell, resulting in the development of the embryo and the endosperm, respectively.
• Results from triple fusion in angiosperms, containing both maternal and paternal chromosomes, showcasing hybrid vigor.
• Endosperm is absent in some plant families like Orchidaceae, Podostemaceae, and Trapaceae.

2. Significance for Human Nutrition:

• Endosperm is a vital source of calories for human beings, contributing to a major portion of the human diet.  Food products like cornflakes, popcorn, beer, and various cooking and industrial oils originate from endosperm.

3. Utilization as food :

• Various cereals and their products, including rice, wheat, and maize, consist of endosperm, forming a significant part of the global food supply. Cooking oils such as coconut oil and corn oil, as well as industrial oils like palm oil and castor bean oil, are products derived from endosperm.

4. Comparison with gymnosperms : 

• Functionally endosperm is comparable to the female gametophyte in gymnosperms but has a unique origin. In gymnosperms endosperm forms before fertilization and is haploid whereas in angiosperm mostly it is a product of triple fusion and is triploid. 

3.0Functions of endosperm

• It provides nutrition to the developing embryo during its early stages of development. (from zygote to globular embryo).
• Apart from being a nutritive tissue, endosperm also regulates the precise mode of embryo development.
• The endosperm acts as a protective layer for the developing embryo, cushioning it against mechanical damage and providing a supportive structure.
• In certain plant species, the endosperm is involved in various metabolic activities, hormone production, and signaling processes critical for seed development and plant growth.