Sexual Reproduction in Flowering Plants

Flower - An Organ of Sexual Reproduction

Flowers are the fascinating organs of angiosperms dedicated to sexual reproduction. Morphologically and embryologically, the flower is a modified shoot meant for sexual reproduction.

Parts of a Flower

A typical flower has four whorls of arrangements attached to the receptacle:

• Calyx: The outermost whorl consisting of sepals, protecting the flower in the bud stage.
• Corolla: Whorl consisting of petals, often brightly colored to attract pollinators.
• Androecium: The male reproductive organ consisting of stamens.
• Gynoecium: The female reproductive organ consisting of one or more carpels.

Androecium and Stamen

Each stamen represents the male reproductive organ and consists of two main parts:

• Anther: A terminal, typically bilobed structure. Each lobe has two pollen sacs (thecae), making them dithecous and tetrasporangiate.
• Filament: A long, slender stalk supporting the anther.

Structure of Microsporangium

Inside the anther, the microsporangia develop further into pollen sacs. A transverse section of a typical microsporangium appears circular and is surrounded by four wall layers:

• Epidermis: Outermost protective layer.
• Endothecium: Helps in anther dehiscence to release pollen.
• Middle Layers: 1-3 layers of cells providing nutrition initially.
• Tapetum: Innermost layer that nourishes the developing pollen grains and helps form the exine.

Microsporogenesis

The process of formation of microspores from a pollen mother cell (PMC) through meiosis is called microsporogenesis. As the anther develops, the sporogenous tissue inside each microsporangium undergoes meiotic divisions to form microspore tetrads.

Pollen Grain (Male Gametophyte)

Pollen grains represent the male gametophytes. They are generally spherical, measuring about 25-50 micrometers in diameter.

The pollen grain wall has two layers:

• Exine: The hard outer layer made of sporopollenin, one of the most resistant organic materials known. It can withstand high temperatures, strong acids, and alkali.
• Intine: The inner wall layer composed of cellulose and pectin.

A mature pollen grain contains two cells:

• Vegetative Cell: Bigger, has abundant food reserve and a large irregularly shaped nucleus.
• Generative Cell: Small and floats in the cytoplasm of the vegetative cell; divides mitotically to give rise to the two male gametes before pollen shed in many species.

Gynoecium and Megasporangium (Ovule)

The gynoecium represents the female reproductive organ. It may be syncarpous (carpels fused) or apocarpous (carpels free). Each carpel has three parts:

• Stigma: Landing platform for pollen grains.
• Style: Elongated slender part beneath the stigma.
• Ovary: Basal bulged part containing the ovarian cavity (locule).

The placenta is located inside the ovarian cavity, arising from which are the megasporangia, commonly called ovules.

Structure of an Anatropous Ovule

The most common type of ovule in angiosperms is the anatropous ovule. Its parts include:

• Funicle: Stalk by which the ovule is attached to the placenta.
• Hilum: Junction between ovule and funicle.
• Integuments: Protective envelopes surrounding the ovule (leaving a small opening called the micropyle).
• Chalaza: Basal part representing the opposite end of the micropyle.
• Nucellus: Central mass of cells enclosed by integuments containing reserve food material.
• Embryo Sac: Female gametophyte located inside the nucellus.

Megasporogenesis

The process of formation of megaspores from the megaspore mother cell (MMC) is termed megasporogenesis. The MMC differentiates in the micropylar region of the nucellus and undergoes meiosis to produce a linear tetrad of four megaspores, out of which three degenerate and only one remains functional.

Female Gametophyte (Embryo Sac)

The functional megaspore develops into the female gametophyte or embryo sac:

• The nucleus of the functional megaspore divides mitotically to form two nuclei, which move to opposite poles, forming a 2-nucleate stage.
• Sequential mitotic divisions result in 4-nucleate and subsequently 8-nucleate stages of the embryo sac.
• Cell wall formation occurs, organizing the 8-nucleate structure into a typical 7-celled and 8-nucleate embryo sac.
• At the micropylar end, an egg apparatus is formed consisting of two synergids and one egg cell. Synergids have cellular thickenings called filiform apparatus that guide pollen tubes.
• At the chalazal end, three cells are present called antipodals.
• The large central cell has two polar nuclei.

Pre-fertilization: Structures and Events

Pre-fertilization events include all the preparations and mechanisms required for fertilization to take place.

Pollination

Pollination is the transfer of pollen grains from the anther to the stigma of a pistil. It is classified into types based on the source of pollen:

Agents of Pollination

Plants use two abiotic (wind, water) and one biotic (animals) agents to achieve pollination.

• Abiotic Agents: Wind (Anemophily) is the most common among abiotic pollinations, producing lightweight, non-sticky pollen grains. Water pollination (Hydrophily) is rare, seen in about 30 genera, mostly monocots (e.g., Vallisneria, Hydrilla, Zostera).
• Biotic Agents: A majority of flowering plants use biological agents like bees, butterflies, flies, beetles, wasps, ants, birds, and bats. Floral rewards like nectar and edible pollen are provided to animal pollinators.

Outbreeding Devices

Continuous self-pollination results in inbreeding depression. Flowering plants have developed many devices to discourage self-pollination and encourage cross-pollination:

• Non-synchronization of pollen release and stigma receptivity.
• Different positions of anther and stigma in a flower.
• Self-incompatibility (a genetic mechanism preventing self-pollen from germinating).
• Production of unisexual flowers (dioecious plants).

Pollen-Pistil Interaction

Pollination does not guarantee the transfer of the right type of pollen. The pistil has the ability to recognize the pollen and promote or inhibit its germination. The continuous dialogue between pollen grain and pistil is called pollen-pistil interaction. Successful interaction results in pollen tube germination through the germ pores.

Double Fertilization

Events unique to angiosperms involve two types of fusion inside the embryo sac, termed double fertilization.

Double fertilization is a complex fertilization mechanism in which one female gamete is united with two male gametes.

Mechanism of Double Fertilization

• Syngamy: One of the male gametes moves toward the egg cell and fuses with its nucleus, producing a diploid zygote (2n).
• Triple Fusion: The second male gamete moves toward the two polar nuclei located in the central cell and fuses with them to produce a triploid primary endosperm nucleus (PEN) (3n).

Because two types of fusions—syngamy and triple fusion—take place in an embryo sac simultaneously, the phenomenon is termed double fertilization. The central cell becomes the primary endosperm cell (PEC) and develops into the endosperm.

Post-fertilization: Structures and Events

Following double fertilization, events of endosperm and embryo development, maturation of ovule(s) into seed(s), and ovary into fruit are collectively termed post-fertilization events.

Endosperm Development

Endosperm development precedes embryo development because the cells of the endosperm provide nutrition to the developing embryo. The primary endosperm cell divides repeatedly to form a triploid endosperm tissue. Endosperm may be:

• Non-albuminous (Ex-albuminous): Completely consumed during embryo development before seed maturation (e.g., pea, groundnut, bean).
• Albuminous: Retained in the mature seed and provides nourishment during seed germination (e.g., wheat, maize, castor, sunflower).

Embryo Development

Embryo development (embryogeny) starts at the micropylar end of the embryo sac where the zygote is situated. The zygote divides to form a proembryo and subsequently develops into globular, heart-shaped, and mature embryos.

• Dicotyledonous Embryo: Consists of an embryonal axis and two cotyledons. The portion of the embryonal axis above the level of cotyledons is the epicotyl, terminating with the plumule (stem tip). The portion below the level of cotyledons is the hypocotyl, terminating at the radicle (root tip).
• Monocotyledonous Embryo: Possesses only one cotyledon. In grass family, the cotyledon is called scutellum. The root apex is enclosed in a hollow sheath called coleorhiza, and the shoot apex is enclosed in a hollow foliar structure called coleoptile.

Seed

The seed is the fertilised ovule formed inside fruits. A seed typically consists of:

• Seed coat(s)
• Cotyledon(s)
• Embryo axis

Seeds can be non-albuminous or albuminous. Sometimes, remnants of nucellus are persistent in seeds, which is known as perisperm (e.g., black pepper, beet).

Fruit

The wall of the ovary develops into the wall of the fruit called pericarp. Fruits may be:

• True Fruits: Developed only from the ovary (e.g., mango, tomato).
• False Fruits: Thalamus contributes to fruit formation along with the ovary (e.g., apple, strawberry, cashew).
• Parthenocarpic Fruits: Developed without fertilization, rendering them seedless (e.g., banana).

Apomixis and Polyembryony

Though seeds are the products of fertilization, a few flowering plants have evolved special reproductive systems.

Apomixis

Apomixis is a mechanism of asexual reproduction that mimics sexual reproduction by producing seeds without fertilization.

Examples include species of Asteraceae and grasses. Apomictic seeds can be formed in several ways:

• In some species, the diploid egg cell is formed without reduction division and develops into the embryo without fertilization.
• In many species, some surrounding cells of the nucellus start dividing and develop into the embryos (e.g., Citrus, Mango).

Polyembryony

The occurrence of more than one embryo in a seed is referred to as polyembryony.

If you squeeze a seed of Citrus or mango varieties, you observe many embryos of different sizes. Polyembryony can arise due to:

• Cleavage of the proembryo.
• Development of multiple embryos from other cells of the embryo sac, such as synergids or antipodals.
• Activation of nucellar cells surrounding the embryo sac to form embryos.

Exam Note: Apomixis is commercially important because hybrid seeds produced annually are expensive for farmers; if hybrid varieties are made into apomicts, there is no segregation of characters in hybrid progeny, allowing farmers to reuse hybrid seeds year after year.