Tapetum Type: 2 Main Types of Tapetum | Anther Wall

The tapetum on the basis of its behaviour is divided into two main types: 1. Amoeboid 2. Secretory Tapetum.

Type # 1. Amoeboid or Invasive or Periplasmodial Tapetum:

This type of tapetum is seen in Alisma, Butomus, Tradescantia, Typha, etc. where, the tapetal cells fuse among themselves to form a tapetal periplasmodium. The protoplast of the fused tapetal cells move into the locule, where they surround the pollen mother cells or the developing pollen grains. This protoplast movement into the locule may take place during meiotic prophase or may be delayed until the tetrad stage.

In Tradescantia during pre-meiotic stage of sporogenous cell development the tapetal cells become progressively more vacuolated, and vesicles possibly derived from dictyosomes, begin to be discharged from the cell. These vesicles contain enzymes which speed up the breakdown and rapid dissolution of the tapetal cell wall. At about the same time the pollen mother cells are undergoing meiotic divisions to form the microspores.

A wave of lysis extends through the anther locules, which strips the tapetal cells of their carbohydrates. The net effect of these activities is an anther cavity containing microspore tetrads infiltrated by tapetal protoplast which now contain significantly more polysaccharide material.

Near the end of pollen mother cell meiosis, the new periplasmodial tapetal cytoplasm undergoes a further reorganization. Callases derived from the tapetal protoplasts, progressively dissociate the microspore tetrads of their callose coat and the long amoeboid-like tapetal cells penetrate between the newly released immature pollen grains.

The tapetal cell metabolism then changes from carbohydrate formation and degeneration to the synthesis of lipid material. Lipid globules appear within the plastids and are progressively extruded from there into the tapetal cytoplasm.

At about final mitosis, the tapetal cytoplasm appear more vacuolated and a second phase of polysaccharide synthesis appears. Finally, changes in the anther cuticle and connective cause an extensive degradation of the tapetal cytoplasm, and at anthesis it is deposited as a thin tryphine (complex mixture of hydrophilic substances) layer on the surface of the pollen grains.

Mepham and Lane (1969) studied the ultrastructure of this tapetum type in Tradescantia and showed that it possessed an organized and functional structure. The periplasmodium infiltrates between the microspore mother cells and eventually, engulf each pollen mother cell and plays an important role in the subsequent development of microspores.

Along with development, the cell organelles in the periplasmodium undergo reorganization and division of the nuclei. Hydrolytic enzymes secreted by the dictyosmes in the tapetal cells break the tapetal cell walls at thepremeiotic stage of sporogenous tissue.

The hydrolytic enzymes may also reach and digest the walls of some sporogenous cells in the anther locule. Following the completion of meiosis the callose wall around the spore degenerate enzymetically derived from the tapetal Plasmodium, as a consequence prior to anthesis the spores are bathed in the tapetal cytoplasm.

Pacini and Juniper (1983) studied the behaviour of the periplasmodium tapetum in Arum italicum, through meiosis and microspore developments as shown whose events are brief outlined below:

i) The species has a two layered amoeboid tapetum whose cells are interconnected at pre- meiotic phase by the plasmodesmata (Fig 1.4 a).

ii) At leptotene stage cell wall of the innermost tapetal cells begin to dissolve and the plasmodesmata connection widen to form cytomictic channels (Fig 1.4 b).

iii) At pachytene stage the tangential walls common to the cells of the tapetal layers disappear and microtubules are visible as lying parallel to the radial walls (Fig 1.4 c).

iv) By the early anaphase-I most of the radial walls between the tapetal cells disappear and the microtubules lie parallel to its inner tangential wall (Fig 1.4 d).

v) At telophase-I the tapetal cell’s protoplast fuses to form a mass that gradually penetrate the microspore mother cells (Fig 1.4 e).

vi) Before the onset of the tetrad stage there is disappearance of the inner tangential wall of the tapetal cells and the amoeboid tapetum gradually envelope the tetrads. The tapetal cell membrane which now surrounds the tetrads establishes an intimate contact with their callose wall. Some of the microtubules are seen to run as arcs, parallel to the tetrad wall (Fig 1.4 f).

vii) The inner layer of the tapetum degenerate which is followed by the release of the microspores from the tetrads, and finally the cells of the outer tapetum disintegrate. Thus the tapetum’s bilayered identity is no longer recognizable (Fig 1.4 g).

viii) The tapetal plasma membrane now retracts in a convolute fashion from the exine and leaves behind cone shaped spaces (Fig 1.4 h). These spaces are filled with materials forming spines on the exine (Fig 1.4 i).

Type # 2. Glandular or Secretory Tapetum:

It is the most common type of tapetum in angiosperms, where the cells remain in their original position and later break down progressively. Among the substance released from the degenerating tapetum are pollenkitt and tryphine.

The details of event in the secretory tapetal cells of Helleborus foetidus from the premeiotic stage to the pollen maturation have been described in detail by Echlin and Godwin (1968) which are briefly stated below:

i. The cell wall is relatively thin, being composed of middle lamella and a primary wall with low content of cellulose. The cytoplasm possesses mitochondria, plastids, number of pro-Ubisch bodies and dictyosomes.

ii. Prior to meiosis the tapetal cell wall becomes thick and the cytoplasm becomes denser due to the increased number of ribosomes and pro- Ubisch bodies. Thickening is however, irregular on the wall facing the locule.

iii. During meiosis there is further increase in the number of pro-Ubisch bodies together with increase in nuclear size.

iv. During the tetrad stage the pro-Ubisch bodies are surrounded by a radiating zone of ribosomes. After separation from the tetrad the pro-Ubisch bodies pass through the tapetal cell membrane and are extruded into the space between the membrane and the cell wall where they are immediately coated with sporopollenin, and are now referred as Ubisch bodies.

v. The inner tangential wall of the tapetal cell now appears thinner and a space appears between the cell membrane and the wall. This is followed by the disappearance of the wall and a new membrane is formed around the tapetal protoplast along the thecal phase.

This layer is called the tapetal membrane. This layer is acetolysis resistant, studded with Ubisch granules and encloses the developing pollen grains, in the anther locule, like a sac.