Anther Structure & Microsporogenesis

Structure and Development of Anther & Microsporogenesis

• A bisexual or perfect flower contains both male (stamens) and female (pistil) parts.
• In contrast, a unisexual flower contains either a pistil or stamens, but not both.

Stamen

• It's the male reproductive part of a flower; its unit is the stamen and collectively known as androecium.
• Stamen is also referred to as microsporophyll.
• The number and length of stamens can vary among different species.

Parts of a Typical Stamen

• Anther: The upper fertile part, broader and knob-like, is a bi-lobed structure.
• Connective: The median sterile part is known as the connective, which joins the lobes.
• Each lobe has two pollen sacs in a mature anther.
• Filament: A long, thin structure that joins the stamen to the thalamus or petal of the flower.

Anther

• Anther is the part of a stamen that contains the pollen.
• The anther's function is to produce and provide the pollen required for reproduction.

Structure of Anther

• A normal bithecous or dithecous anther consists of two anther lobes connected by a connective.
• The lobes contain four elongated cavities or pollen sacs (microsporangia), where pollen grains are produced.
• A normal or complete anther has four microsporangia called Tetrasporangiate.
• Some plants have a single lobe with two microsporangia, known as monothecous, such as Hibiscus and Moringa.
• Rarely, an anther has only one lobe with a single microsporangium, called unisporangiate, like Arceuthiobium.

Development of Anther

• Anther development originates from the eusporangiate type, arising from multiple archesporial cells.
• A cross-section (T.S.) of the anther reveals parietal layers (wall layers) and pollen chambers, or microsporangia, containing sporogenous tissue or archesporial cells.

Parietal Layers (Wall Layers) of Anther

• From the outside to the inner side, the anther wall consists of the following layers:

Epidermis

• The outermost layer is single-celled and continuous but not archesporial in origin.
• It forms the outermost protective layer.

Endothecium

• Present below the epidermis; it's a single-celled thick layer.
• During maturation, the cells undergo changes in different walls.
• The outer wall remains thin, while inner and radial walls thicken due to α-cellulose fibers.
• Callose bands are present along the radial walls, but are absent in some places called stomium.
• Dehiscence occurs only from these places.
• It becomes hygroscopic due to fibrous thickening, aiding dehiscence.

Middle Layer

• Consists of parenchymatous cells.
• This layer is one to three cells thick and stores food.
• It's ephemeral or short-lived in nature and absent in mature anthers.
• In Holoptelia plants, it's 3 to 4 cells thick, while it's absent in Najadaceae & Lemnaceae families.

Tapetum

• It's the innermost, single layer of cells with dense cytoplasm and prominent nuclei.
• Provides nutrition to developing microspores.
• Tapetum cells are initially diploid but become polyploid due to endomitosis.
• Tapetum absorbs food from the middle layer, providing nutrition to microspore mother cells or microspores.
• Tapetum cells secrete hormones and enzymes and disappear in the mature anther, but can be multilayered in Nicodia and Costum plants.

Types of Tapetum

Amoeboid / Invasive tapetum / Periplasmodial tapetum

• This type is found in primitive Angiosperms.
• It absorbs all foods from the middle layer, which immediately degenerates.
• Tapetal cells convert absorbed food into special food granules called protoplast bodies.
• The innermost layer dissolves, releasing protoplast bodies into the microsporangium cavity.
• Protoplast bodies become known as periplasmodium, surrounding and nourishing the microspore mother cells.

Glandular / Secretary tapetum

• It is more developed and does not degenerate quickly.
• It absorbs nutrients from the middle layer and secretes them into the microsporangia (pollen sacs).
• Found in most flowering plants.
• It secretes enzymes and hormones, forming special granules called Pro ubisch bodies in the cytoplasm before degeneration.
• Pro ubisch bodies transfer between the cell wall and cell membrane of tapetal cells and are surrounded by sporopollenin.
• These are now called Ubisch bodies/orbicules, and they release into pollen sacs when tapetum degenerates.
• Sporopollenin participates in the formation of other coverings (exine) of pollen grains.
• The tapetum helps in the transfer and storage of food, formation of sporopollenin, pollen kitt materials, and tryphine.

Functions of Tapetum

• Secretion of callase enzyme (β-1,3- glucanase) to dissolve the callosic wall of the tetrad and release them.
• Secretion of polysaccharides into the locules during the free microspore stage, which are absorbed by the microspores.
• The tapetal cells secrete a sporopollenin precursor, though its role in sporopollenin synthesis is unclear.
• In secretory tapetum, sporopollenin is deposited as orbicules on the inner phase of the tapetal cells.
• Formation of Ubisch bodies, pollenkitt, and tryphine, where later two are deposited on the pollen surface to bind pollen grains together for efficient insect pollination.
• Ultrastructural studies show that a special population of plastids synthesizes these substances in the tapetal cells.
• Pollen wall is formed during the post-meiotic period.

Microsporogenesis

• Primary sporogenous cells give rise to microspore mother cells.
• Each microspore mother cell undergoes reduction division to produce four microspores or pollens.
• The process of microspore or pollen formation is microsporogenesis.
• A young anther consists of a mass of meristematic homogenous cells covered by a single layer of epidermis.
• It becomes four-lobed, and each lobe forms a microsporangium with microspores or pollen grains.
• Vascular tissue is formed in the middle region, and simultaneously, four cells below the epidermis in vertical rows become large with visible nuclei and dense cytoplasm.
• These are called archesporial cells.
• Archesporial cells divide periclinally to form primary parietal cells below the epidermis and primary sporogenous cells towards the center.
• Both cells undergo further division to form the anther's complete structure, excluding the epidermis.
• Primary parietal cells undergo further periclinal and anticlinal divisions to form a series of 3-5 layers, forming the anther walls.
• The outermost layer just below the epidermis formed by the primary parietal cells is called the endothecium or fibrous layer, followed by a 1-3 celled middle layer and the innermost layer called tapetum.
• Tapetal cells play a significant role during meiotic cell division in microsporogenous cells and pollen development.
• The primary sporogenous cells divide twice or more by mitotic division to form sporogenous cells, which later differentiate into microspore mother cells during pollen sac wall formation.
• Each microspore mother cell divides to form four haploid microspores or pollen grains by meiotic division or reduction division.
• During, this period, spherical bodies are formed inside the tapetal cells which are known as Ubisch-body.
• Ubisch body is made up of a complex substance called sporopollenin, a polymer of carotenoids.
• The tapetum layer degenerates after formation with Ubisch bodies participating in the formation of the exine of microspores inside pollen sacs.
• Then, thick-walled microspores are called pollen grains.
• At the initial stage, all four microspores are attached together with the help of a callose layer, forming something called a tetrad.
• The callose layer dissolves by callase enzyme, which the tapetum secretes.