What is Tapetum?
In the anther of flowering plants, tapetum is a specialised layer of nutritive cells situated between the sporogenous tissue and the anther wall. The anther is the part of the stamen where pollen is produced. Tapetum is a source of precursors for the pollen coat and is crucial for the nourishment and growth of pollen grains. It constructs the pollen sac's innermost layer and provides the pollen grain's surface with a lipid coating. It is an anther's diploid tissue. In the following sections, we will study the structure of the tapetum and its different functions.
What is a Tapetum?
Tapetum is the anther's innermost cell layer surrounding developing pollen mother cells (PMCs) and/or microspores, supplying nutrition and enzymes needed for microsporogenesis and pollen maturation. Two separate sexes engage in sexual reproduction in plants. Gametes are the blooming plant's reproductive cells. The flower reproduces sexually since there are both male and female gametes. The male gamete pollen grains develop in the anther and pollen sac in which pollen grains are generated. The epidermis, endothecium, middle layer, and tapetum are the four layers that surround each pollen sac.
The tapetum is the innermost layer, while the epidermis is the outer layer. The tapetum is present as a uniform layer surrounding the sporogenous tissue. The tapetum passes the food from the middle layer to sporogenous tissues. These sporogenous tissues form pollen grains and get nutrition from tapetum.
The Anther Lobe
Tapetal Cells
The cells that are present in the tapetum layer are known as tapetal cells. The tapetal cells may be uni-, bi-, or multinucleated. The tapetal cells have dense cytoplasm and conspicuous nuclei. These cells are large because they have to provide nutrients to develop pollen grains inside the pollen sac. These tapetal cells store food for developing pollen grains. Pro-ubisch is spherical bodies that are present in these tapetal cells. These pro-ubisch bodies are helpful for exine thickening.
Types of Tapetum
Tapetum is divided based on their behaviour, such as amoeboid (plasmodial) tapetum and secretory (glandular) tapetum.
Amoeboid Tapetum
In the amoeboid tapetum, the tapetal cells grow and fuse with each other. Their protoplasts fuse but not their nuclei. So it is a multinucleated layer. Then these cells form a plasmodium-like structure and move between MMC and provide nutrition. As pollen grains mature and the anther lobe dehisces, these plasmodia become dehydrated due to absorption of pollen grains and deposit as debris on the wall of pollen grains and help form some parts of exine (outer layer of pollen grains).
Alisma orientale, lily(Lilium), and Typha latifolia have these types of tapetum.
Secretory Tapetum
These tapeta remain in their original shape till the maturation of pollen grains. They are intact with the middle layer and do not change their shape throughout the development of pollen grains. These cells fuse just after the formation of the tapetum and remain multinucleated at the end of the maturation of the pollen grain. Tapetal cells become thick-walled and increase their number of pro-ubisch bodies just before the microspore mother cells (MMC) undergo the meiosis process. The ubisch bodies are only produced by tapetum and help in the exine formation.
The cells in this layer secrete their secretions slowly, so-called secretory tapetum, and help in the exine formation. In angiosperms, it is the most common type of tapetum. Helleborus foetidus and Nigella damascena are some species that have these types of tapetum.
Tapetum Brain
The corpus callosum, a nerve tract under the cerebral cortex of the brain, is present in placental animals, and the tapetum is a component of this framework. To improve night vision, the choroid's tapetum lucidum, a reflective region, bounces light back into the retina. The tapetum lucidum is a reflecting region of the choroid layer that is pigment-free. Choroid coat is another name for it.
Are Tapetal Cells Haploid or Diploid?
The innermost layer full of nutritive cells within the anther acts as a harbinger of sporopollenin, which is a chemical that protects pollen from external adverse conditions. Tapetal cells go through endomitosis, where the nucleus and nuclear membrane splits, but cytokinesis doesn’t happen, so they become multinucleated, having more than one nucleus per cell.
The tapetal cells are metabolically active as their division happens through endomitosis, and they contain dense cytoplasm thus, they are polyploidy, which happens when a cell possesses one or more additional sets of chromosomes. In this, mitosis normally happens till cytokinesis doesn’t happen. Two sets of chromosomes are thus constituted into a common nuclear membrane forming a restitution nucleus. So, tapetal cells are polyploid.
Function of Tapetum
The tapetum provides various enzymes, hormones, amino acids, and nutritive materials to microsporocytes during the development of pollen grains. Tapetal cells of tapetum store the food in the early stage of microsporogenesis. It transfers food from the middle layer to the spore mother cells. It produces ubisch bodies coated with sporopollenin to cause thickening of the exine. It secretes an oily material that forms a pollenkitt of mature pollen grains. It adds some proteins to the exine that help in compatibility with stigma.
Interesting Fact
A salient process tapetum acts out is the secretion of callose enzyme, which sub-divides the callose compounds used to form four pollen (pollen tetrad).
Important Questions
Is tapetum dependent on male meiosis for progression in plants? Ans: In higher plants, male meiosis is an important process during microsporogenesis and is vital for male fertility and seed set. Meiosis involves the organising of chromosomes and cytoskeleton, which takes place in sexual cells. In the normal development of the tapetum, the somatic layer surrounding male meiosis is important for completing the cycle. Disruptions in tapetum development cause changes in the range of genes involved in male reproduction.
What is the role of pollen in the protection of male sperm from harsh conditions?
Ans: The pollen acts as a biological protector of the male sperm from harsh conditions as it's covered by a cell wall made of exine, whose major constituent is sporopollenin. Tapetum is in close contact with the developing gametophytes and acts well in pollen coat and pollen wall formation. Evidence indicates that sporopollenin is built through catalytic enzyme reactions in the tapetum.
Key Features
Tapetum is vital for the growth and development of pollen grains.
Tapetum is the specialised innermost layer found within the anther of flowering plants.
The radically large nucleus of the tapetum helps in the storage of the necessary nutrients.
Types of tapetum include secretory and plasmodium.
Conclusion
The tapetum is the innermost layer of the anther wall and covers the sporogenous tissue. It is a layer that stores food and provides for developing microspore mother cells. This is a diploid tissue of the male part of the plants. The above information helps to understand the concept of the tapetum layer of the anther wall in angiosperms.
FAQs on Tapetum
1. Why is sporopollenin the biosphere's most resistant substance?
The substance known as sporopollenin is found in the outermost layer of pollen grains. Sporopollenin is a strong, highly cross-linked, and remarkably stable polymer made of carbon, hydrogen, and oxygen. It is virtually completely resistant to environmental deterioration, which safeguards plants in a hard terrestrial environment. It cannot be broken down by any biological or physical process and is resistant to all enzymes. It is discovered in spore and pollen walls, from where it derives its name. It aids the fossilisation of pollen grains.
2. What exactly are microspores?
Any of the heterosporous plants' spores that produce male gametophytes. The microspore mother cell splits through meiosis inside the microsporangium to create four microspores, each of which will eventually develop into a pollen grain. The growing microspores receive nutrients from an inner layer of cells called the tapetum, which also supplies important elements to the pollen wall. Within each anther of the stamens are four clusters of cells known as microsporangia (pollen sacs). To create microspores, the microsporangia go through meiosis.
3. How do bi-nucleated tapetal cells form?
The tapetal cell feeds the growing microspores in the pollen sac. Uni-, bi-, and multinucleated cells make up these tapetal cells. During the microspore growth process, these cells produce these circumstances. The tapetal cell's nucleus has three ways to divide: endomitosis, regular mitosis, and restitution nuclei. The process of endomitosis, in which the cells divide only nuclearly and not cytoplasmically, occurs in these cells. Then, a cell is said to be bi-nucleated if two nuclei float inside a single cytoplasm. When it undergoes several nuclear divisions without cytokinesis, it sometimes develops many nuclei.
4. Discuss the formation of pollen grains.
Being multinucleated, tapetum is radically enlarged, thus enabling the massive storage of food and nutrients needed. They are a harbinger of sporopollenin. Pollen grains are found in the anthers of flowering plants. The tapetal cells, due to their nature and size, provide nutrition to microsporocytes when meiosis occurs. Microspores are produced, which then undergo mitosis to form the pollen grains. The microspores are haploid in nature.
5. What are the functions of the tapetum?
Tapetum is essential for the growth, development, and nutrition of pollen grains. They act as harbingers or successors for the pollen. Secretion of enzyme callose degenerates the tetrad wall and synthesises the pollen kit and trephine, thus binding pollen grain together and helping in the pollination process. It provides the nutrition for the microspores resulting in the synthesis of sporopollenin, which in turn forms the pollen grain wall. It is a specialised layer found within the anther of flowering plants.
6. What are tapetal cells?
The tapetal cell is the layer of cells found in the anther deeply ensconced within it. It provides nutrition for the microspores resulting in the synthesis of sporopollenin. Tapetal cells are polyploid in nature. These cells are metabolically active as their division happens through endomitosis, and they contain dense cytoplasm. Thus, they are polyploidy when a cell possesses one or more additional sets of chromosomes.
