Sexual Reproduction in Flowering Plants

Learning Outcomes

1. Understand the structure and function of flower as a reproductive organ in angiosperms.
2. Recognize the roles of androecium and gynoecium in reproductive processes.
3. Grasp the concept of double fertilization and its uniqueness in angiosperms.
4. Comprehend post-fertilization changes, seed formation, and fruit development.
5. Understand apomixis and polyembryony and their significance in plant breeding.

FLOWER – A FASCINATING ORGAN OF ANGIOSPERMS

Flowers have long been valued for their aesthetic, ornamental, and social significance. Beyond human appreciation, they serve as vital structures for sexual reproduction in angiosperms. The androecium and gynoecium develop as reproductive organs in the flower. Flowers are the sites where gametes form and fertilization occurs, leading to seed and fruit formation.

PRE-FERTILISATION: STRUCTURES AND EVENTS

Before a plant flowers, hormonal and structural changes initiate the development of floral primordium. Floral buds emerge on inflorescences, leading to androecium and gynoecium development, the male and female reproductive structures.

Stamen, Microsporangium, and Pollen Grain

1. Stamen structure: Each stamen consists of a filament and a bilobed anther attached to the thalamus or petal. Anthers are typically dithecous, containing two theca with microsporangia that develop into pollen sacs.
2. Mature anther: A mature anther shows four microsporangia where pollen grains form.
3. Microsporangium: Surrounded by four wall layers—the epidermis, endothecium, middle layers, and tapetum—the sporogenous tissue undergoes microsporogenesis. Cells divide meiotically to form microspore tetrads, which later dissociate into pollen grains.
4. Pollen grain wall: Exine, the outer wall made of sporopollenin, ensures protection, while the inner wall, intine, is composed of cellulose and pectin.
5. Pollen maturity: In most angiosperms, pollen grains shed at a 2-celled stage, with vegetative and generative cells. Some shed as 3-celled, with two male gametes.

The Pistil, Megasporangium (Ovule), and Embryo Sac

1. Pistil structure: The pistil consists of stigma, style, and ovary. Inside the ovary, ovules are attached to the placenta.
2. Ovule structure: The ovule contains funicle, hilum, integuments, micropyle, nucellus, and embryo sac.
3. Megasporogenesis: The megaspore mother cell (MMC) undergoes meiosis, resulting in one functional megaspore, which forms the embryo sac.
4. Embryo sac development: The functional megaspore divides mitotically into an 8-nucleate, 7-celled embryo sac, including antipodals, synergids, egg cell, and polar nuclei in the central cell.

Important Note: In angiosperms, sporopollenin, present in the pollen grain wall, is highly resistant to environmental factors such as high temperatures and acids.

POLLINATION

Pollination ensures the transfer of pollen grains to the stigma of the pistil, vital for fertilization. It can be classified based on the source of pollen:

1. Autogamy: Pollination occurs within the same flower.
2. Geitonogamy: Pollen transfers from one flower to another on the same plant.
3. Xenogamy: Pollen comes from a different plant, ensuring genetic diversity.

Agents of Pollination

Pollination agents include abiotic (wind and water) and biotic (animals).

1. Wind pollination: Pollen grains are lightweight and non-sticky, adapted for air-borne dispersal.
2. Water pollination: Occurs in aquatic plants like Vallisneria, where pollen floats to the female flower.
3. Animal pollination: Insects, birds, and even mammals participate in pollination by transferring pollen while seeking nectar.

POLLEN-PISTIL INTERACTION

Once pollen reaches the stigma, pollen-pistil interaction determines if the pollen is compatible. If accepted, the pollen grain germinates, forming a pollen tube that carries the male gametes to the embryo sac.

1. Pollen tube formation: The vegetative cell extends into a pollen tube, while the generative cell divides to form two male gametes.
2. Pollen tube entry: The pollen tube enters the ovule through the micropyle, guided by the filiform apparatus of the synergids.
3. Double fertilization: One male gamete fuses with the egg (syngamy), forming a zygote, while the other fuses with the polar nuclei (triple fusion), forming the primary endosperm nucleus.

DOUBLE FERTILISATION

Double fertilization is a unique feature in angiosperms. Two fertilization events occur in the embryo sac:

1. Syngamy: One male gamete fuses with the egg cell, resulting in a diploid zygote.
2. Triple fusion: The second male gamete fuses with the polar nuclei, forming the triploid primary endosperm nucleus.

POST-FERTILISATION: STRUCTURES AND EVENTS

After fertilization, multiple post-fertilization events take place, including the development of endosperm, embryo, seed, and fruit.

Endosperm Development

1. Endosperm formation: The primary endosperm nucleus divides, forming a triploid endosperm tissue, which provides nutrition to the developing embryo.
2. Types: Endosperm development can be nuclear, where free nuclei form before cellularization. The coconut is a classic example, with free-nuclear endosperm (coconut water) and cellular endosperm (kernel).

Embryo Development

1. Embryo stages: The zygote undergoes stages from proembryo, globular, heart-shaped, to a mature embryo.
2. Dicot embryos: Have two cotyledons, a plumule, and radicle.
3. Monocot embryos: Have one cotyledon, called scutellum, and a coleoptile enclosing the shoot apex.

Seed and Fruit Development

1. Seed structure: Seeds consist of a seed coat, cotyledons, and an embryo. They can be albuminous (with remaining endosperm) or non-albuminous (without endosperm).
2. Fruit development: The ovary develops into the fruit, while the ovules become seeds. True fruits develop from the ovary, while false fruits involve other floral parts.

Concept Note: The process of double fertilization is unique to angiosperms and leads to the simultaneous formation of a zygote and endosperm, both essential for seed development.

APOMIXIS AND POLYEMBRYONY

Apomixis refers to the formation of seeds without fertilization and mimics sexual reproduction. Common in species like Citrus and Mango, it bypasses the typical fertilization process:

1. Diploid egg develops into an embryo without meiosis.
   2. Nucellar cells may develop into embryos, leading to polyembryony (multiple embryos in one seed).

Polyembryony ensures the formation of genetically identical embryos, akin to clones. This phenomenon has agricultural advantages, especially in the development of hybrid seeds.

Important Note: Apomixis offers a significant advantage in agriculture as it allows the preservation of hybrid vigor across generations without the need for annual hybrid seed production.

MCQ: What is the primary difference between syngamy and triple fusion in angiosperms?
Answer: Syngamy involves the fusion of a male gamete with an egg, while triple fusion involves a male gamete fusing with two polar nuclei to form the primary endosperm nucleus.