Pollen Formation Development of the Pollen Wall and Tapetum

GENESIS OF POLLEN

Pollen is considered as the male gamete in flowering plants or angiosperms and gymnosperms through which genetic information is transmitted to the offspring. To the naked eye pollen grains appear mainly in the form of a yellow or cream coloured powder, which look alike, but they are quite different in their wall pattern. In fact each pollen grain bears a species-specific wall surface pattern, which helps immensely in identification and classification. These varied pollen wall patterns exhibit the beautiful art of nature. This becomes more apparent if the pollen walls are observed under

Typically, pollen has two cells known as the vegetative cell and generative cell. The latter gives rise to two sperm cells, which fuse with egg cells of the ovule during double fertilization. The vegetative cell comprises the bulk of the pollen cytoplasm, which is responsible for the development of the pollen tube (Fig. 3.1).

Unlike animals, sperm cells in plants are not motile and hence they have to be transferred towards the egg cells of ovules for fertilization. This

SEM.

Poll wall

Tu ce Ni

Poll wall

Tu ce Ni

Generative cell

Cytoplasm

Fig. 3.1 Structure of pollen contents.

Generative cell

Cytoplasm

Fig. 3.1 Structure of pollen contents.

is usually done by pollen tubes, which arise during germination of pollen on the stigma. When the pollen germinates, the pollen tube emerges through one of the apertures (pores) in the pollen grain wall.

The pollen tube acts as a channel of transport for the sperm cells to the embryo sac of the ovule to achieve fertilization. During fertilization one sperm cell fuses with the egg to form the diploid zygote and the other sperm cell fuses with two haploid polar nuclei to give rise to the endosperm, which serves to nourish the developing embryo during seed development. This is termed as double fertilization, which is characteristic of angiosperms only and not gymnosperms.

There is a tremendous variation in the size and shape of pollen grains. The range of size variation of pollen grains in terms of diameter is from 5200 mm with the average size being 30 mm. They vary considerably in the shape, which may be mostly spherical, oval or cubic, hexahedral, fibrous etc.

Fine structure of pollen grain walls revealed under SEM appears quite different from the one observed under the light microscope, for example a pollen grain of rice (Poaceae), Caryophyllaceae and Chenopodiaceae however appear as a vessel with a smooth wall surface. However, under SEM the black spots are clearly demonstrated as granulate concaves and plugs are identified at the aperture opposite the end from which the pollen tube elongates.

The structure of a flower, a sexual reproductive organ of a plant is shown in the Fig. 3.2. From the palynological point of view, the most

Pollen Wall Rice Sem

Seed

Fig. 3.2 Life cycle of a flowering plant showing the structure of a flower.

Seed

Fig. 3.2 Life cycle of a flowering plant showing the structure of a flower.

important part in the flowers is the stamen or male reproductive organ consisting of pollen sacs or anthers borne on elongated stalks or filaments.

POLLEN FORMATION

The male part of flowers is known as the androecium, which consists of structures called stamens. Each stamen comprises two parts: a stalk or filament, which terminates in the pollen-bearing structures, the anthers.

Pollen formation occurs in the anthers. Each anther is four lobed, each lobe is known as a loculus. The anther structure is composed of three parts: an outer wall, a lining layer of nourishing cells, the tapetum and a central mass of sporogenous tissue. The outer wall comprises relatively large cells with thin walls to allow loss of moisture at the onset of pollen release, anthesis, the cell wall which adjoins the ends of each lobe is thickened to resist increasing tension set up during the onset of pollen release anthesis. The tapetum or nourishing layer comprises a tissue in which, during its development is the depository of starch granules. These serve to provide essential nutriment and energy for the ongoing development of the innermost cell mass, the sporogenous tissue. The sporogenous tissue will eventually form tetrads of pollen grains (microspores) after a series of mitotic divisions followed by a final meiotic division, which produces each member of each tetrad having half of the parental chromosome number (n or haploid state).

Production of Pollen

Pollen grains or microspores are produced from pollen mother cells (microsporocytes), within the anthers (microsporangia or pollen sacs) of the flower. During its development two types of cells are differentiated within the anther; reproductive or sporangenous cells which give rise to pollen grains and the non-reproductive cells, which form the tissue layers such as epidermis, cortical and tapetal cell layers. During the formation of the pollen grains (microgametophyte) two distinct successive developmental phases are recognized, they are the microsporogenesis and microgametogenesis, which are illustrated in Fig. 3.3. The pollen mother cells are poorly attached to each other by plasma connections. Later, pollen mother cells assume a spherical shape and get detached followed by a thick callose layer around them. The pollen mother cells (2n) undergo meiosis to give rise to four haploid microspores (n).

During this phenomenon, a callose wall separates the members of the tetrad from one another, which is continuous with the callose surrounding the entire tetrad as seen in the illustration (Fig. 3.4).

Diploid (2n) nucleus

Microsorogenesis

Haploid <1n) nuclei Microspore

Meiosis I Microsporocyte

Meiosis II

Vegetative nucleus

Haploid <1n) nuclei Microspore

Tetrad

Microspore nucleus Vacuole Pollen Mitosis I

Generative cell

Vegetative nucleus

Generative cell

Microspore nucleus Vacuole Pollen Mitosis I

Released Polarised microapore mlcroapore

Bicellular Tricellular pollen Pollen

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  • LOUIS
    Which of the following contributes to pollen wall formation?
    11 months ago

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