MICROSPOROGENESIS

Microsporogenesis

The process of the formation and differentiation of microspores (pollen grains) from microspore mother cells (MMC) by reductional division is called microsporogenesis.

Microsporogenesis is well studied under following heads:

(1) Structure of anther:

The fertile portion of stamens is called anther. Each anther is usually made up of two lobes connected by a connective. In turn each anther lobe contains two pollen chambers placed longitudinally. Each pollen chamber represents a microsporangium and is filled with a large number of pollen grains or microspores.

The pollen sacs are surrounded by following 4 layers :

(i) Epidermis: This is the outermost single layered and protective. In Arceuthobium, cells of epidermis develop a fibrous thickening and the epidermis is designated as exothecium.

(ii) Endothecium: Inner to epidermis, there is a single layer of radially elongated cells. Cells of endothecium develop fibrous thickening (made up of cellulose with a little pectin and lignin) which help in the dehiscence of anther. In between these cells, a few cells without thickening are also present. These thick walled cells collectively form the stomium.

(iii) Middle layer: Three to four layers of thin walled cells situated just below the endothecium are known as middle layers. Cells of this layer are ephemeral and degenerate to provide nourishment to growing microspore mother cells.

(iv) Tarentum: This is the innermost layer of the wall. The cells are multinucleate (undergo endopolyploidy) and polyploid. Tapetal cells are nutritive.

(2) Development of anther and formation of microspores (Pollen grains):

(a) The young anther consists of homogenous mass of paranchymatous cells surrounded by epidermis. It soon becomes four lobed.

(b) In each of the four lobes, some of the hypodermal cells begin to act as archesporial initials.

(c) Each archesporial initial divides into an outer primary parietal cell and an inner primary sporogenous cell.

(d) The primary parietal cell divides to form 3-5 wall layers, i.e., endothecium, middle layers and tapetum.

(e) The primary sporogenous cells divide to produce a mass of sporogenous cells or microsporocytes.

(f) Each microspore mother cell divides meiotically to form four haploid microspores or pollen grains and remains arranged in tetrads.

(3) Development of male gametophyte (Microgametogenesis):

(a) Microspore or pollen grain is the first cell of male gametophyte (partially developed).

(b) The wall of the pollen grain is made of two layers.The outer layer is called exine. It is made up of sporopollenin (derived from carotenoid). The inner intine is thin, delicate and is made of cellulose and pectose.

(4) Pre-pollination development:

(a) Microspores start germinating in situ (i.e. while enclosed inside the microsporangium or pollen sac) and are called precocious.

(b) Microspore nucleus divides mitotically to form a smaller generative cell lying next to spore wall and a much larger vegetative cell (or tube cell).

(c) A callose layer is deposited around the generative cell. The generative cell loses its contact with the wall of microspore and becomes free in the cytoplasm.

(d) The callose layer then dissolves. The pollen grains are shed from the anther at this bicelled stage (rarely three celled).

(5) Post-pollination development:

(a) The liberated pollen grains are transferred to the receptive surface of the carpel (i.e., stigma) by the process called pollination.

(b) On the stigma, the pollen grain absorbs water and swells within a few minutes.

(c) The vegetative (or tube) cell enlarges and comes out through one of the apertures in the form of a pollen tube.

(d) The wall of pollen tube is the extension of intine. The tube secretes exogenous pectinases and other hydrolytic enzymes to create a passage for its entry.

(e) The vegetative and generative nuclei are carried by the pollen tube, the farmer lying at its tip.

(f) The generative cell divides to form two non-motile male gametes.

(g) The tube nucleus has no important function and may disintegrate.