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Pollen wall development may be considered under exine and intine growth phases.
1. Exine Growth Phase:
After meiosis of microsporocyte (microspore mother cells) the tetrads of haploid microspores are enveloped by a callosic wall. The callose wall can be detected around the microspore mother cells during initiation of meiosis (Fig. 2.1a and 2.2a). It forms a layer between the cytoplasm and pollen mother cell wall.
Additional callose is formed after the second meiotic division which isolates the young microspores from each other. The callose special cell envelope, persists until it is enzymetically digested at the end of the tetrad stage. The pollen wall formation is discussed under two models, viz., the primexine model and the undulation model.
a) Primexine Model:
Heslop-Harrison (1971) considers the primexine as the blue print for the exine. The primexine has a matrix presumably made of cellulose microfibrils, and is deposited in between the spore and the callose wall. Scott (1994) believes that “the primexine acts as a loose scaffold on to which sporopollenin monomers (fatty acids and phenols) are covalently attached by the localized action of super oxide radicals generated at the plasmalemma”.
Beside its function as a controlled deposition of sporopollenin it also functions as a pathway for the diffusion of other substances such as enzymes, including those responsible for digestion of the callose wall.
In the beginning the primexine is discontinuous at certain specific regions of the plasmalemma (Fig. 2.1b and 2.2b) which marks the position of the future germ pore. During the later period of primexine deposition additional gaps appear and these are occupied by columns of electron dense intricate tubular lamellae of 70 nm diameter, called probaculae that rests on the plasmalemma (Fig 2.1c and 2.2 c).
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The probaculae appear to condense round the plasmalemma and become more evident by the deposition and polymerization of the sporopollenin precursors. Later during the late tetrad stages, the probaculae become disjointedly differentiated into electron dense baculae.
The baculae become laterally expanded at the base to form the foot layer (Fig 2.1d and 2.2 d). Depending upon the taxon they may remain free above or increase in electron density due to rapid deposition of sporopollenin, and their heads expand laterally to form the tectum (roof), over the primexine matrix.
The foot layer represents the future nexine 1, while the baculae and the roof layer, the future sexine. Further deposition of sporopollenin continues and the whole pollen grain expands laterally and radially as the pollen grain enlarges.
The chemical constituent of the probaculae and the foot layer is not clear and their electron opacity is different from that of the sporopollenin of mature pollen, for which it has been described as protosporopollenin. In the initial stages these layers are not resistant to acetolysis, however, they become resistant with the development of the tetrad stage.
In many marine angiosperms primexine is absent and consequently do not develop normal exine. The nexine2 is deposited below the sexine. In the initial process a number of very thin electron transparent lamellae appear to arise from the cytoplasm and provide a locus around which sporopollenin is deposited. With the progress of deposition the lamellae thicken and merge with each other to from the nexine2.
The presumptive germinal apertures of pollen grain are already demarcated during the microspore tetrad stage. The sites of pre-aperture are distinguished by the absence of the primexine matrix and is associated with an underlying plate of endoplasmic reticulum oriented parallely to the plasma membrane.
This endoplasmic reticulum may physically prevent the movement of membranous structures (coated with primexine material) to the cell surface. Sheldon and Dickinson (1986) believed that the meiotic spindle plays a role in aperture positioning.
Later the callose wall dissolves, thus releasing the tetrads. The spores now expand rapidly and the primexine matrix is largely disrupted, and only the residue can be observed between the baculae of the mature exine. A rapid conversion of the protosporopollenin occurs, and the primexine acquires the staining properties of the exine.
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b) Undulation Model:
Studies on exine development by Takahashi, (1989, 1993, 1995) in Caesalpinia and Lilium gives little support to the primexine model. It explains that the exine formation commences with the tetrad stage by the invagination or undulation of the plasma membrane which is possibly under the control of cytoskeleton elements. These invaginated localized regions match to the regions of future lumina and distensions that correspond to the muri of the mature exine.
Takahashi (1995) observed that in Lilium the plasma membrane assumes a reticulate pattern which matches the pattern of the mature exine. Fibrous threads (10- 20nm diameter) together with granules (10 nm diameter) aggregate at the regions of protuberances of the plasma membrane and slowly these aggregates develop into a smooth protectum of 0.5 to 0.7 µm diameter (Fig 2.3).
During the later part of the tetrad stage the probaculae and the protectum are more distinguishable beneath the callose wall and the plasma membrane assumes a smooth outline. Probaculae are later formed in between the plasma membrane and the protectum.
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At this time the cellulosic fibrous primexine is distinguishable in spaces between the probaculae and the callose wall dissolves, thus releasing the microspores and further differentiation of the exine continues. Thus the undulated plasmamembrane plays an important role in pollen wall development and in fact the protectum is the first exine layer that is deposited on this membrane.
Christensen (1972) outlines the following steps in the development of the pollen wall:
a) In the tetrad stage a new wall called primexine, is deposited around the microspore protoplast within the wall of callose. The primexine appears to contain cellulose microfibrils.
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b) In the transition of primexine to exine elements of primexine produces precursors of rod like bacula, which form the sexine. As a result of rapid deposition of sporopollenin the baculae enlarge in electron density and their heads expand laterally to form the tectum.
This is simultaneously followed by the lateral expansion of the base of the bacula to form the foot-layer. Deposition of sporopollenin continues and the whole wall expands laterally and radially. It also witnesses the dissolution of the cellulose wall as a consequence the pollen grain lie free in the pollen sac.
c) Nexine is deposited below the sexine. This is initiated by the deposition of number of electron transparent lamellae originating from the cytoplasm, around which sporopollenin is deposited. Deposition is followed by the thickening of the lamellae which unite with each other to form the nexine. (Fig. 2.4).
d) In the aperturate region the sexine has very short bacula, while the nexine becomes much thicker and discontinuous in this region than the rest of the pollen wall.
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e) At later stage cellulosic intine is formed inside the nexine.
2. Intine Growth Phase:
The intine usually starts to develop at the vacuolated stage, beneath the apertures. It increases in thickness under the pores and later on starts to develop under the interapertural parts as a thin layer.
Golgi bodies are frequent during intine synthesis e.g., Ranunculaceae while in others, E.R. and polyribosomes are abundant e.g. Cosmos. The intine beneath the pore become comparatively very thick and is provided with fibrillar material and radially arranged membranous units.
In the final period of wall development, additional lipoidal and pigmented substances may accumulate on or/and within the outer exine. This material called the pollenkitt, imparts colour or odour to the pollen and may cause the pollen grains to adhere together during dehiscence.
