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Pathways of Development in Pollen Grains - The early divisions in responding pollen grains may occur in one of the following four ways.
(i) The unicleate pollen grain may divide symmetrically to yield two equal daughter cells both of which undergo further divisions, e.g., Datura innoxia (Pathway I).
(ii) In some other cases, e.g., N. tabacum, Datura metel, barley, wheat, triticale; chillies, etc., the unicleate pollen divides unequally (as it does in nature). The generative cell degenerates callus/embryo originates due to successive divisions of the vegetative cell (Pathway. II).
(iii) But in few species, e.g., Hyoscyamus niger, the pollen embryos originate from the generative cell alone; the vegetative cell either does not divide or divides only to a limited extent forming a suspensor like structure (Pathway III).

(iv) Finally, in some species, e.g. Datura innoxia, the uninucleate pollen grains divide unequally, producing generative and vegetative cells, but both these cells divide repeatedly to contribute to the developing embryo/callus (Pathway IV).
Most of the pollen grains of many crop species, e.g., tobacco, barley, wheat, etc., are bigger, stain deeply with acetocarmine and contain plenty of starch. But a small proportion (Ca. 0.7%) of the pollen grains are smaller and stain faintly with acetocarmine; these are called S-grains.
This phenomenon is known as pollen dimorphism. It is these S-grains, which respond during anther culture; the frequency of responding pollen grains can be enhanced over that of S-grains by certain pretreatments, e.g., chilling.

Pollen grains of the cultured anthers show remarkable cytological changes during the first 6-12 days, called the inductive period. In tobacco, the gametophytic cytoplasm of binucleate pollen grains is degraded, ribosomes are eliminated and only few mitochondria and plastids remain. New ribosomes are synthesized following the first sporophytic division of the vegetative cell.
The responsive pollen grains become multicellular and ultimately burst open to release the cell mass. This cell mass may either assume the shape of a globular embryo and undergo the developmental stages of embryogeny, or it may develop into a callus depending on the plant species.

In some species, e.g., rice, wheat, rye, maize, etc., the pollen grains can be induced to produce embryos or calli by simply altering the medium composition. Careful studies in tobacco have shown that
(i) over 80% of well developed embryos are associated with their radicular ends to the anther wall and that
(ii) the exine of pollen grains must rupture in such a way as to expose the putative plumular ends of the developing cell mass for their further differentiation into SEs.
The latter ensures that the exine lies between the would be radicular end and the supporting tissue to which the cell mass is adhered; this is believed to be important in establishing polarity, which appears to be essential for SE differentiation.
 

 
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