What is Tapetum? Mention Its Significance

What is Tapetum?

Tapetum is the innermost wall layer of microsporangium or anther. It forms a wall layer and covers the sporogenous tissue all around. This single-layered structure forms a barrier between the sporogenous tissue and the anther wall. It is physiologically important to be nutritive in function as it facilitates the passage of food. Nutrition to the sporogenous tissue must pass through the tapetum.

Structure of Tapetum

Tapetum cells are large, teeth-shaped, and radially elongated.
The cells are usually homogenous but sometimes are of two types.
Cells of tapetum are uninucleate but will become multinucleate later.
The interior cells are larger and the outer cells are smaller. Such differential tapetum is called dimorphic tapetum.
Tapetum cells are generally diploid but can also be polyploids due to abnormal cell divisions. Such abnormality leads to male sterility.
It is present in all land plants but their origin may vary from one plant to another.
Tapetal cells degenerate in the later stages of pollen development.

Types of Tapetum

Based on their behavior and function, tapetum

Amoeboid
Glandular

Amoeboid Tapetum

Ampoeboid tapetum forms a mass of their cell contents when their cell walls disintegrate. These masses come from the receptor cells and unite to form a single mass known as amoeboid tapetum or tapetal plasmodium. This is further divided into four types.

Butamus type where the tapetal periplasmodium is formed before the meiosis of the microspore mother cell.
Triglochin type has the tapetal peri plasmodium formed much earlier and encroaches the space of spore mother cells.
Sparganium type is characterized by the amoeboid tapetum during the mother cell meiosis where it remains multinucleate.
Sagittaria type of tapetal cell detaches at the microspore tetrad stage and their protoplasts come out to form the periplasmodium.

Glandular Tapetum or Secretory Tapetum

Glandular tapetum is otherwise known as secretory tapetum. This type of tapetal cell makes its cell contents available for the spores. They surround the anther locule, keep their cell wall intact, and secret their substances into the anther sac.

Secretory tapetum plays an important role in pollen development as it remains till the pollen is fully developed. At the time of maturity of pollen grains, these cells break down completely. Their sticky secretions coat the pollens enabling insect pollination easier.

Functions of Tapetum

The tapetum is a nutritive tissue that nourishes the developing microspores.
Abnormal tapetal development causes male sterility in plants.
It forms a pathway for food conduction from outside into the sporogenous tissue.
After meiosis, nutrition is provided to the pollen.
It is the source of precursors for exine in the anther wall and contributes to exine formation through ubisch bodies.
In many plants, tapetum secretes sporophytic recognition proteins that help plants with their self-incompatibility systems.
Tapetum produces tryphine or pollenkitt, the sticky oily substance that covers the pollen grains.
It may also serve as starch storage in some plants.

References

Sukumaran O R. Pre-Degree Botany. Murali Publications.
Abraham P C. Anatomy, Embryology & Microtechnique. 1999. St. Mary’s Books & Publications.
Tapetum Structure and Ontogeny in Victoria (Nymphaeaceae)