Microsporangium, structure and function of wall layers, ultra functional changes in tapetum and meiocytes, role of callose, role of tapetum in pollen development, development of male gametophyte

Microsporangium, structure and function of wall layers:-

Structure of Microsporangium:- 

> It is a structure in the plant’s male reproductive organ where the development of pollen takes place.

 > A microsporangium appears to have a circular outline when viewed transversely. It is surrounded by four layers:

i. Epidermis

ii. Endothecium

iii. Middle layers

iv. Tapetum

> The outer three layers protect the pollen and help in the splitting of the anther to release the pollen. 

> The tapetum nourishes the pollen grain. 

> The cells of the tapetum are multinucleated and have dense cytoplasm. 

> A young anther comprises a group of compactly arranged homogenous cells called sporogenous tissue.

Sporogenous Tissue:- These are located at the centre of each microsporangium in a young anther. With the development of the anther, the sporogenous cells undergo meiotic division to form microspore tetrads. Each sporogenous cell is known as a pollen mother cell or microspore mother cell. This process by which a microspore is formed from the pollen mother cell is known as microsporogenesis. The microspores are arranged in the form of a tetrad. As the anther matures it dehydrates and the microspores dissociate from each other and develop into a pollen grain.

Pollen Grain:- The matured pollen grain contains two cells: a generative cell and a pollen tube cell. The generative cell is present within the larger pollen tube cell. The tube cells form the pollen tube on germination. The generative cell migrates into the ovary through the pollen tube. Inside the pollen tube, the generative cell divides into two gametes or sperms. The anther matures and releases the pollen grains.

Ultra functional changes in tapetum:-

Tapetum:-

> A tapetum is the deepest layer of the anther, which encompasses the sporogenous tissue. The sporogenous tissue produces microspores that supply enzymes and nutrition for pollen development. Tapetum is mostly multinucleated due to mitotic cell division. Abnormal cell division can also lead to tapetum polyploidy.

> The tapetum is a huge structure, and thus can absorb food and provide nutrition to the development of the pollen grains. They also aid the regulatory molecules that help in the formation of the pollen grains. The cells of tapetum have dense cytoplasm.

Structure of Tapetum:-

Tapetum is a thick single-celled nutritive layer found behind the epidermis and endodermis. Their cells are initially diploid in nature but can become polyploid due to abnormal cell division. Their primary role is to absorb nutrition from the middle layer, and provide it to the microsporocytes. Also, the tapetum cells can secrete enzymes and hormones. The tapetum layer is almost lost in the mature anther.

Types of Tapetum:-

i. Secretory or Glandular Tapetum:- They surround the anther locule. This type of secretory tapetum is more common in angiosperms.

ii. Amoeboid or Plasmodial Tapetum:- After the disintegration of the tapetal cell wall, a multinucleate structure, along with the cell protoplast called the plasmodium tapetum, is formed.

Role of Callose:-

> After the discontinuity of the plasmodesmata connection among the microsporocytes, it is covered by a primary wall made up of cellulose, and shortly before meiosis this wall disintegrates and is replaced by a massive deposit of callose (β-1,3- glucan), outside the plasma membrane. Callose deposition starts at the corners of the cells between the plasma membrane and the original wall. However, the primary wall persists in Allium tuberosum and Cyclamen persicum until late tetrad stage. The possible reason for the delay is to form a barrier in the entry of macromolecules in the microsporocytes,thus ensuringautonomous development of microspores.

> The deposition of the callose is initially incomplete, leaving many gaps through which there is an establishment of massive cytoplasmic channels between the microsporocytes. These channels are 1 -2 pm in diameter and attain their maximum development in the zygotene- pachytene stage. Thus at this stage the highly interconnectedmass of microsporocytes in the locule, form a large meiocytic syncytium.

> This massive coenocyte provides a channel for the transport and distribution of metabolites.Further it imposes a mutual influence of one cell over the other, thus helping to maintain a close synchrony during meiosis among the large number of microsporocytes in the anther locule. At the close of meiotic prophase the callose walls of the microsporocyte lock up, and the cytoplasmic channels are cut off and

> Now the microsporocytes go through the rest of the meiosis as isolated cells. At the end of metaphase I or II, the callose wall around microspore mother cells become continuous.The isolation of the microspore mother cells and then the microspores, by a callose wall seems essential for the normal development of the pollen grains. Failure of callose wall development or its early breakdown results in pollen sterility. The accurate determination of the timing and pattern of DNA synthesis during microsporocyte meiosis is markedly complex. Studies involving autoradiography and microspectro- photometry have establishedthat the main DNA synthesis period for the microsporocytes begins shortly before leptotene and continues into leptotene. For example in Lilhim longiflorum autoradiography of 32P incorporation, followedby enzyme digestion and acid hydrolysis, for eliminatingRNA and phosphoproteins, identified a preleptotene period of DNA synthesis. Whereas microspectrophotometricmeasurement showeda 4C DNA amount in the microsporocytes before their entry into meiosis,a transient 2C amount after meiosis I, and a 1C amount in the microspores. What prompts sporogenous cells to cease mitotic divisions and enter the meiotic cycle is not known. There is a prediction that meiosis is triggered by the synthesis of some factors in tissues other than the sporogenous cells. These substances have trival names as meiosis determinants, or meiosis-inducingsubstances. Few attempts have been made to clone meiosis specific genes from microsporocytes and identify the protein products they encode.

> However, none of the genes so far characterized can be considered to be meiosis specific because they encode common proteins associated with cell metabolism such as HSP (heat shock proteins), serine proteases, proteins involved in DNA repair, and leucine zipper proteins. Thus the action of specific genes that control the entry of cells into and their exit from meiosis is still a matter of distant dream. The cytoplasm of meiocytes undergoes profound changes during meiosis and there is a significant fall in the cytoplasmic RNA. Functions of Callose Wall: The possible functionsof the callose envelope are: 

i. To control major features,such as the arrangement of apertures, which are probably related to the geometry imposed by the callose wall. 

ii. To isolate the youngmicrospore from influences of tapetal cells during early stages of development. This isolation enables the young microspores to deposit a primexine without interaction between them and the tapetum. Knox (1984) thus emphasizedthat the callose wall serves to separate the gametophyte from the sporophyte. 

iii. Participates in the development of wall ornamentation. Godwin (1968) described the action of callose wall as a template, defining the position of apertures and the deposition of the primexine in Ipomoea. 

iv. Protects the meiocytes from dehydration under condition of deficient water supply. 

v. It isolates and insulates meiocytes for the normal completion of meiosis.This isolation provides avenues for two major events, viz., transition from sporophytic phase to gametophytic phase, and the expression of the gametophytic genome which is essential for achieving the limited function of gamete formation and their discharge in the embryo sac.

Role of Tapetum in Pollen Development:-

i. Tapetum provides nutrition for the developing pollen grains.

ii. They also act as a precursor source for the pollen wall or pollen coat.

iii. It transports supplements to the anthers.

iv. Pollenkitt is also formed by the tapetal cells around the microsporocytes. This pollenkitt is responsible for nutrition and pollen development.

v. The tapetal cells act as a precursor to sporopollenin. The outer wall of pollen grains contains a chemical called sporopollenin that protects the pollen from harsh external factors.

vi. Tapetum expels Ubisch bodies, which helps thicken the pollen grain membrane. These ubisch bodies or orbicules are a part of the sporopollenin.

vii. Also, it secretes the callase enzyme that breaks the callose compounds used to unite four pollens (pollen tetrad). Thus, they allow pollens from a tetrad to be separated.

Development of male gametophyte:-

 1. Vegetative cell:- After pollen grains divide into two cells, the vegetative cell increases in size. The cell organelles increase in number and size. Vegetative cells includes organelles such as ribosomes, E.R., plastid, dictyosomes, and mitochondria. Nucleus is spherical or irregular in outline containing chromosomes and one or two nucleoli. In mature stages nucleus often lacks nucleolus.

2. Generative cell:- (A reproductive cell, especially a cell of an angiosperm pollen grain that divides to produce two male gamete nuclei) Initially the generative cell is attached to intine of the pollen wall but later on it gets detached from the pollen wall. It lies free in the cytoplasm of vegetative cell but there is no cytoplasmic connection between two cells. The generative cell cytoplasm contains the organells Such as mitochondria, ribosomes, endosplasmic reticulum, microtubule, and dictyosomes (In the embryo sac, the double fusion of the generative cells with the egg cell and the two nuclei of the central cell give rise to the diploid zygote and the triploid endosperm, respectively.)

3. Formation of male gametes:- In the anthers of a flower, certain diploid cells undergo meiosis, forming haploid spores (called microspores—"little spores"). These microspores divide by mitosis to form two-celled pollen, consisting of a vegetative cell and a generative cell. Or may be three celled (vegetative + 2 male gamets) at time of release from the anther. When the pollen grain release it landed on the stigma, The pollen grains has two cells (vegetative and generative cell). The generative cell forms two gametes. These are non motile. In angiosperms the female gametophyte (embryo sac) is situated at distance from the stigma and to transport the male gamets pollen tube is form. The pollen tube along with two male gamets grows down through the style and enters the ovule for the process of fertilization.