Describe different types of endosperm development in angiosperms. What is the significance of aleurone tissue? Mention the function of endosperm in seed.

Types of Endosperm Development

Endosperm development primarily occurs after fertilization and can be categorized into three main types:

1. Nuclear Endosperm Development

This is the most common type of endosperm development, observed in species like wheat and maize. The primary endosperm nucleus (PEN) undergoes successive free nuclear divisions without cell wall formation, resulting in a large number of free nuclei within the central cell. These nuclei are dispersed throughout the cytoplasm and later, cellularization occurs by the formation of cell walls around the nuclei, leading to a cellular endosperm. Initially, the endosperm is liquid and milky, but it becomes solid as cellularization progresses.

2. Cellular Endosperm Development

In this type, observed in species like balsam (Impatiens) and beetroot, the PEN divides mitotically, and each division is immediately followed by cell wall formation. This results in a solid, cellular endosperm from the very beginning. Cellularization occurs simultaneously with nuclear division, leading to a structured, cellular tissue. This type of development is slower than nuclear development.

3. Helobial Endosperm Development

This is an intermediate type, found in species like Helobias and Asphodelus. The first division of the PEN is unequal, forming a large micropylar chamber cell and a small chalazal chamber cell. The micropylar cell undergoes further nuclear divisions followed by cellularization, forming the main body of the endosperm. The chalazal cell remains undivided for some time and later undergoes cellularization, contributing to the haustorium, which absorbs nutrients from the surrounding tissues.

The following table summarizes the key differences between these three types:

Feature	Nuclear	Cellular	Helobial
Initial Nuclear Divisions	Free nuclear divisions	Followed by cell wall formation	First division unequal
Cellularization	Late	Simultaneous with division	Micropylar cell cellularizes, chalazal forms haustorium
Endosperm Texture (Initial)	Liquid/Milky	Solid	Intermediate
Examples	Wheat, Maize	Balsam, Beetroot	Helobias, Asphodelus

Significance of Aleurone Tissue

The aleurone layer is a proteinaceous layer surrounding the endosperm in many seeds, particularly cereals like wheat and maize. It is derived from the peripheral cells of the endosperm. The aleurone cells are typically single-layered and contain protein bodies (aleurone grains) rich in proteins, enzymes (like amylase), and phytates.

Significance:

• Nutrient Mobilization: The aleurone layer plays a crucial role in mobilizing nutrients from the endosperm during seed germination. Upon receiving hormonal signals (primarily gibberellins) from the embryo, the aleurone layer synthesizes and secretes enzymes like α-amylase, which break down starch in the endosperm into sugars, providing energy for the growing embryo.
• Protection: The aleurone layer provides a protective barrier against mechanical damage and fungal attack.
• Phytate Storage: It stores phytates, which act as a reserve of phosphorus and also have antioxidant properties.

Function of Endosperm in Seed

The endosperm serves several vital functions in seed development and germination:

• Nutrient Storage: The primary function of the endosperm is to store nutrients – primarily starch, proteins, and oils – that are essential for the growth and development of the embryo.
• Embryo Nourishment: During germination, the endosperm provides the embryo with the energy and building blocks required for initial growth before the seedling can establish its own photosynthetic capacity.
• Hormonal Regulation: The endosperm can also synthesize and regulate plant hormones that influence embryo development and germination.
• Seed Size and Viability: The size of the endosperm often correlates with seed size and can influence seed viability and seedling vigor.