In this article we will discuss about the functions of tapetum.
Many physiological functions have been accredited to the tapetum, but they all congregate on the ability of its cells to synthesize metabolites for the nourishment of microsporocytes, regulation of meiosis, pollen wall formation, synthesis of pollenkitt, etc.
Experimental approach to establish tapetal function in a convincing way has come in recent years. The immunoelectron microscopic methods have aided to localize the proteins in the boundary between the tapetum and the middle layers of the anther, and in the exine of the early vacuolated microspores of Sinapis alba.
This evidence provided support to the idea that some of the components of the exine are synthesized in the tapetum. In another experiment by Mariani (1990) the tapetum-specific TA29 gene from tobacco was engineered to programme the expression of an RNase gene in the tapetum of transgenic tobacco.
The result was the total annihilation of the tapetum in the anthers of transgenic plants, which was followed by the inhibition in pollen formation and promotion of male sterility.
The main function of tapetum tissue is briefly outlined below:
i. Secretion of the enzyme callase (β-1,3- glucanase) to dissolve the callosic wall of the tetrad and set them free. Stieglitz and Stern (1973) measured the callase activity separately in anther wall and the microsporocytes from early meiosis to tetrad dissociation.
It was seen that the activity was all together absent in the developing microsporocytes but was present in wall tissue that reached its peak at the time of microspore release from the tetrads.
Concomitantly, support for the stability of the callose envelope around microsporocytes is regulated by the tapetum has come from the experiments of Worrall (1992) where premature secretion of callase in the tapetum of transgenic tobacco plants was engineered by introducing a (1,3)-β-glucanase gene under the control of tapetum-specific promoters from Brassica napus.
This caused the premature dissolution of the callose sheath around microsporocytes and production of plants showing varying degrees of male sterility, and pollen grains lacking normal wall configuration.
ii. Secretion of polysaccharides into the locules during the free microspore stage, which are absorbed by microspores.
iii. The role of the tapetal cells in the secretion of sporopollenin precursor is non-ambiguous however, its role in the synthesis of sporopollenin is not clear. In taxa with secretory tapetum, sporopollenin is deposited in the form of orbicules on the inner phase of the tapetal cells. In taxa with plasmodial tapetum sporopollenin material is present in the thecal fluid.
In a number of taxa the bulk of the exine develops only after the release of tetrads, thus the tapetal cells supplies the necessary precursor for sporopollenin deposits after the release of the tetrads (Figure 1.6). According to the results of degradation experiments, formation of exine precursors could be phenols or p-coumaric acid.
iv. Tapetal cells have other activities that result in formation of different structures, which characterize some families. For instance in Onagraceae tapetal cells play a role in formation of fine flexible threads, known as viscin threads, in continuation with the outer layer of the exine. In Asteraceae the tapetum forms an acetolysis resistant membrane outside the sporogenous tissue. This membrane is known as the culture sac or peritapetal membrane.
v. Formation of Ubisch bodies.
vi. Formation of pollenkitt, and tryphine, which are deposited on the pollen surface and helps to bind pollen grains together, and for efficient insect pollination. Ultrastructural studies have shown that both of these substances are synthesized in a special population of plastids in the tapetal cells.
vii. Formation of pollen wall during post- meiotic period.