Question

Important function of tapetum

Answer 1

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Tapetum is important for the nutrition and development of pollen grains, as well as a source of precursors for the pollen coat.

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Answer 2

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.