Development of endosperm and embryo

Introduction

In seed biology, double fertilisation results in endosperm and embryo development. Double fertilisation occurs when two male gametes of a pollen grain fuse with two distinct parts of the ovary. The endosperm plays a pivotal role in supporting embryonic growth. It supplies nutrients, protects the embryo, and controls embryonic growth by acting as a barrier during seed development and germination. 

Development of endosperm and embryo

The ovule present inside the ovary of the flower encloses female gametophyte or embryo sac. This sac consists of female gamete (egg cell) and central cell along with other cells which are not significant in the development of endosperm and embryo. Both these are part of the seed in angiosperms.

 A pollen grain is made of two male gametes. During double fertilisation, one of the male gametes fuses with the female gamete (gametes are always haploid), resulting in zygote (diploid cell) formation and later as an embryo. This consists of a tiny embryonic axis which on germination of the seed forms a plant.

 The other male gamete fuses with the central diploid cell resulting in the formation of a triploid endosperm nucleus. The developing embryo receives its nourishment from the endosperm. Further repeated mitotic cell divisions result in the formation of an endosperm and is a triploid tissue. Recent studies have shown that the endosperm can sense its surrounding environment and secrete signals to regulate the growth of embryo.

 Definition of endosperm

The endosperm is the surrounding nutritive tissue that develops from the primary endosperm nucleus. Endosperms can be diploid, triploid, and polyploid. These provide nourishment to the embryo.

Definition of embryo

An embryo refers to the initial stage of development of a multicellular diploid organism. In seeds like angiosperms and gymnosperms, an embryo develops inside the seed.

The embryo bears an embryonic axis that has radicle and plumule which later grows into roots and shoots respectively.

Basic features of endosperm and embryo

• The endosperm provides nutrition to developing plant embryos. In angiosperms, endosperm is  triploid and in gymnosperms, it is haploid.
• During germination, the embryo secretes signals to degrade the starch as food reserves and promote endosperm weakening.
• Endosperm is found mostly in monocot seeds while in dicot seeds it is replaced by cotyledons.
• The outer layer of the endosperm is called the aleurone layer in monocot seeds. This layer secretes the amylase enzyme, which helps break starch into sugar for utilisation by the embryo.
• The developing seedling gets its food from the endosperm in case of monocot seed while in dicot seed the germinating seedling gets nourishment from the cotyledons.
• The endosperm contains several hormones like cytokines which helps in cell differentiation and embryo growth.
• Coconut water is an example of a liquid endosperm.

Development of endosperm and embryo classification

The endosperms are an important part of the seed. An endosperm forms the surrounding tissue of providing primary food and nutrients to the growing embryo. The endosperm and embryo are broadly categorised as under.

Types of endosperm

Based on development, the endosperm is of three types:

• Nuclear endosperm: This is a common kind of endosperm development, where the nucleus division repeatedly occurs without the formation of cell walls (cytokinesis). It forms many free nuclei in the central cell of the embryo sac, pushing all the nuclei to the peripheral cytoplasm. Finally, the cell wall formation takes place from the periphery towards the center leading to the formation of multicellular endospermic tissue. Examples of nuclear endosperms are commonly found in coconuts, maize, rice, wheat, cotton, and sunflower, to name a few.

• Cellular endosperm: In this type of endosperm development, each nuclear division of the primary endosperm nucleus is consecutively followed by cell wall formation. It involves the division of the endosperm nucleus into many segments, which are usually not coherent. Such development is not common. Examples of cellular endosperms are commonly found in petunia and datura.

• Helobial endosperm: This development is commonly associated with monocot embryo development. This development is a combination of both cellular and nuclear types. The process involves cell wall formation following the first cell division, similar to cellular endosperm development. However, subsequent divisions do not lead to the formation of cell walls. Instead, it forms a large micropylar cell with further division similar to nuclear endosperm. Examples of helobial endosperms are commonly found in foxtail lilies.

Types of embryo

Embryogeny, or the development of an embryo from the diploid zygote, is of following two major types:

• Dicotyledons – This is a type of development where the apex is downward or present towards the inside part of the zygote. The first division transversely produces a basal cell towards the micropylar and a terminal cell towards the chalaza. Dicots have two cotyledons present inside them. Examples of dicot seeds are beans, pea, and mango, to name a few.
• Monocotyledons – In monocot embryos, the initial development is similar to dicot embryos. Later on, the division happens repeatedly in both peripheral and central groups of cells. Examples of monocot seeds are grasses, orchids, and bananas, to name a few.

Similarities between endosperm and embryo

• Endosperm and embryo are two distinct structures in seed biology that occur in angiosperms. 
• Both develop from a process called double fertilisation.  
• Two sperm nuclei in the pollen grain are responsible for double fertilisation.

Differences between endosperm and embryo

• An endosperm provides food and nourishment to the developing plant embryo in angiosperms. An embryo is a part of the seed that develops into a new individual of the species.
• An endosperm occurs in angiosperms, while an embryo occurs in every multicellular organism that undergoes fertilisation.
• The endosperm and embryo develop by triple fusion and fertilisation, respectively. 
• The endosperm is triploid in angiosperms, while the embryo is diploid.

Conclusion

Endosperm and embryo are important parts of a seed and responsible for germination. Endosperm in the seed is developed due to the triple fusion of a male gamete with the central cell of the embryo sac. On the other hand, an embryo is developed through fertilization in all multicellular organisms that undergo sexual reproduction. It develops when a male gamete fuses with an egg cell in the ovary. Endosperms provide food and nourishment to the developing embryo. There are three major types of endosperms and two major types of embryo development. Commonly endosperms are present in cereals like maize, wheat, and coconuts too.