Megasporangia and Megasporogenesis

Introduction

The megaspore is produced inside the nucellus of the ovule in gymnosperms and flowering plants. During megasporogenesis, a diploid precursor cell, known as the megasporocyte or megaspore mother cell, goes through meiosis to produce four haploid cells at first (the megaspores). Angiosperms have three megasporogenesis patterns: monosporic, bisporic, and tetrasporic, also known as the Polygonum, Alisma, and Drusa types, respectively. The monosporic pattern is found in many economically and biologically important groups, including Brassicaceae (Arabidopsis, Capsella, Brassica), Gramineae (maize, rice, wheat), Malvaceae (cotton), Leguminosae (beans, soybean), and Solanaceae (pepper, tobacco, tomato, potato, petunia).

Megasporogenesis

Megasporogenesis is the process by which megaspores form. Megaspores are formed in the megaspore mother cell through meiosis or reduction division. At the micropylar region, a single nucellus cell differentiates into the megaspore mother cell, or MMC. The cell expands in size and has a dense cytoplasm and nucleus. It divides by meiosis to produce four haploid megaspores.

In most cases, one of the four megaspores is active and germinates into a female gametophyte or embryo sac. This is referred to as monosporic development. The nucleus of a functional megaspore divides mitotically to form a 7-celled, 8-nucleate embryo sac. The embryo sac’s central cell has two polar nuclei.

Megasporangia

The funicle connects the ovule to the placenta. The hilum is the attachment point between the ovule and the funicle. Each ovule has a protective covering called an integument that surrounds the nucellus. The integuments are absent at the tip, leaving a small opening known as the micropyle. 

The chalaza is found opposite the micropylar end and forms the ovule’s basal part. The nucellus is the ovule’s main body, which is composed of parenchymatous mass. Inside the nucellus is an embryo sac, also known as a female gametophyte. Megasporogenesis occurs when the diploid megaspore mother cell undergoes meiosis to produce four haploid megaspores. 

Three of the four megaspores have degenerated, leaving only one functional.

The female gametophyte develops from the functional megaspore. It is a seven-celled, eight-nucleated structure made up of two synergids, three antipodal cells, two polar nuclei, and one egg cell.

Microsporogenesis 

Microsporogenesis is the process by which a microspore mother cell goes through meiosis and forms a haploid microspore tetrad. It enters the pollen sac of the anther. Megasporogenesis is the process by which the megaspore mother cell goes through meiosis while also forming megaspores in the nucleus region. It enters the ovary and enters the ovule.

Female Gametophyte

The female gametophyte of angiosperms is essential for plant reproduction. It contains the egg cell and the central cell, which become fertilised and give rise to the seed’s embryo and endosperm, respectively. Female gametophyte development begins with the formation of a diploid megaspore mother cell that undergoes meiosis early in ovule development. The female gametophyte develops from a single haploid megaspore. Megaspore mother cell identity is specified by genetic and epigenetic processes, which limit megaspore mother cell formation to a single cell per ovule. 

Female gametophyte polarity is influenced by auxin gradients, and a slew of transcription factors mediate female gametophyte cell specification and differentiation.The mature female gametophyte secretes peptides that guide the pollen tube to the embryo sac, as well as protein complexes that prevent seed development prior to fertilisation. The female gametophyte influences seed development after fertilisation by regulating parental contributions and maternal-effect genes. 

Female gametophytes can be formed through an asexual process known as gametophytic apomixis, which involves the formation of a diploid female gametophyte and the development of the egg into the embryo without the need for fertilisation.

Development of Megasporogenesis

Megasporogenesis is the process by which megaspores develop from megaspore mother cells. Megaspores are produced by the female reproductive organs.

• The female flowering plant’s ovules contain a layer of sporogenous tissue
• The sporogenous tissue gives rise to the megaspore mother cells (MMC). It’s a big diploid cell with a big nucleus and a lot of cytoplasm
• The MMC divides into four haploid megaspores via meiotic nuclear divisions
• The megaspores are arranged axially in a pattern known as the linear tetrad
• Only one of the four megaspores is active, while the other three have degenerated
• The megaspore at the chalazal end is active, but the three towards the micropyle have degenerated
• The embryo sac is formed when the single functional megaspore divides mitotically
• Megagametogenesis is the process by which a single functional megaspore divides mitotically to form an embryo sac
• It divides several times to produce eight haploid nuclei for the embryo sac, also known as a megagametophyte

Conclusion

The megaspore is produced inside the nucellus of the ovule in gymnosperms and flowering plants. In most cases, one of the four megaspores is active and germinates into a female gametophyte or embryo sac. The chalaza is found opposite the micropylar end and forms the ovule’s basal part. The nucellus is the ovule’s main body, which is composed of parenchymatous mass. Inside the nucellus is an embryo sac, also known as a female gametophyte. Megasporogenesis is the process by which the megaspore mother cell goes through meiosis while also forming megaspores in the nucleus region.