Sexual Reproduction in Flowering Plants
Biology of Reproduction
Biology is essentially the story of life. Species survive through reproduction, either asexually or sexually. Sexual reproduction creates new variants, enhancing survival.
Overview of Unit Chapters
This unit details reproduction in flowering plants and humans, using them as representative examples. It also addresses human reproductive health and its maintenance.
Chapter 1: Sexual Reproduction in Flowering Plants
Chapter 2: Human Reproduction
Chapter 3: Reproductive Health
Panchanan Maheshwari (1904-1966)
A distinguished Indian botanist who studied embryological aspects and promoted their use in taxonomy. He established the Department of Botany at the University of Delhi as a center for embryology and tissue culture research. His work on test tube fertilization and intra-ovarian pollination was globally recognized. Maheshwari also contributed to school education by leading the creation of biology textbooks for higher secondary schools in 1964.
Sexual Reproduction in Flowering Plants (Angiosperms)
Flowering plants use sexual reproduction, resulting in diverse flower structures adapted for fruit and seed formation.
1.1 Flower – A Fascinating Organ of Angiosperms
Flowers hold aesthetic, ornamental, social, religious, and cultural importance, symbolizing human emotions. Biologically, they are morphological and embryological marvels, serving as the sites for sexual reproduction.
1.2 Pre-fertilisation: Structures and Events
Before a flower appears, hormonal and structural changes lead to the development of the floral primordium. Inflorescences form, bearing floral buds and flowers, within which androecium (male) and gynoecium (female) reproductive structures differentiate.
1.2.1 Stamen, Microsporangium, and Pollen Grain
A typical stamen consists of a filament (stalk) and an anther (terminal structure). Anther: A typical angiosperm anther is bilobed (dithecous) with two theca in each lobe. A longitudinal groove separates the theca. The anther is four-sided (tetragonal) and contains four microsporangia at the corners, which develop into pollen sacs filled with pollen grains.
Structure of Microsporangium
A typical microsporangium appears circular in outline in transverse section and is surrounded by four wall layers: epidermis, endothecium, middle layers, and tapetum.The outer three layers protect and aid in anther dehiscence. The innermost layer, tapetum, nourishes developing pollen grains and has dense cytoplasm with multiple nuclei. When young, the center of each microsporangium is occupied by sporogenous tissue.
Microsporogenesis
As the anther develops, sporogenous tissue cells undergo meiosis to form microspore tetrads. Each sporogenous cell is a potential pollen or microspore mother cell (PMC). Microsporogenesis is the process of forming microspores from a PMC through meiosis. The microspores are arranged in a cluster of four cells (microspore tetrad). As the anthers mature and dehydrate, the microspores dissociate and develop into pollen grains, which are released upon anther dehiscence. This process is crucial for enabling fertilization as pollen grains carry male gametes to the stigma of the flower for successful pollination. Following pollination, the pollen grains germinate on the stigma, leading to the growth of the pollen tube, which travels down the style towards the ovary, where fertilization occurs with the ovule.
Pollen grains represent the male gametophytes. They vary in size, shape, color, and design among different species. Pollen grains are generally spherical, measuring about micrometers in diameter, with a two-layered wall.
Exine: The hard outer layer is made of sporopollenin, a highly resistant organic material that can withstand high temperatures, strong acids, and alkali. It has prominent apertures called germ pores where sporopollenin is absent. Sporopollenin preserves pollen grains as fossils. The exine exhibits diverse patterns and designs.
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ntine: The inner wall is a thin, continuous layer made of cellulose and pectin.
The cytoplasm of pollen grain is surrounded by a plasma membrane. When mature, the pollen grain contains two cells: a vegetative cell and a generative cell. The vegetative cell is larger with abundant food reserve and an irregularly shaped
nucleus. The generative cell is small and floats in the cytoplasm of the vegetative cell, with dense cytoplasm and a spindle-shaped nucleus. In over 60% of angiosperms, pollen grains are shed at the 2-celled stage. In the remaining species, the generative cell divides mitotically to form two male gametes before pollen grains are shed (3-celled stage).
Pollen Allergies
Pollen grains of many species can cause severe allergies and respiratory disorders like asthma and bronchitis. An example is Parthenium (carrot grass), which was imported with wheat and is now a common cause of pollen allergy.
Pollen Viability and Storage
Pollen grains must land on the stigma before losing viability to effect fertilization. Viability varies depending on temperature and humidity. Some cereals (rice, wheat) lose viability within 30 minutes, while some members of Rosaceae, Leguminoseae, and Solanaceae remain viable for months.Pollen grains can be stored for years in liquid nitrogen (), creating pollen banks for crop breeding programs, similar to seed banks.
1.2.2 The Pistil, Megasporangium (Ovule), and Embryo Sac
The gynoecium (female reproductive part) may consist of a single pistil (monocarpellary) or multiple pistils (multicarpellary), which may be fused (syncarpous) or free (apocarpous). Each pistil has three parts: stigma, style, and ovary. The stigma receives pollen grains. The style is the elongated part beneath the stigma. The ovary is the basal, bulged part containing the ovarian cavity (locule) and the placenta, from which megasporangia (ovules) arise.
The Megasporangium (Ovule)
The ovule is attached to the placenta by the funicle (stalk). The point of fusion between the ovule and funicle is the hilum. Each ovule has one or two protective envelopes called integuments, which encircle the nucellus except at the micropyle (small opening). Opposite the micropylar end is the chalaza (basal part of the ovule). The nucellus, enclosed within the integuments, is a mass of cells with abundant reserve food materials. Within the nucellus is the embryo sac (female gametophyte), typically formed from a single megaspore.
Megasporogenesis
Megasporogenesis is the process of forming megaspores from the megaspore mother cell (MMC). A single MMC differentiates in the micropylar region of the nucellus. It is a large cell with dense cytoplasm and a prominent nucleus. The MMC undergoes meiotic division, resulting in four megaspores. Meiosis results in the production of four megaspores.
Female Gametophyte
In most flowering plants, one megaspore is functional, while the other three degenerate. The functional megaspore develops into the female gametophyte (embryo sac). This is called monosporic development. The nucleus of the functional megaspore divides mitotically to form two nuclei, which move to opposite poles, forming the 2-nucleate embryo sac. Two more sequential mitotic nuclear divisions result in the formation of the 4-nucleate and 8-nucleate stages of the embryo sac. These mitotic divisions are free nuclear, meaning nuclear divisions are not immediately followed by cell wall formation.
After the 8-nucleate stage, cell walls form, organizing the typical female gametophyte or embryo sac. Six of the eight nuclei are surrounded by cell walls and organized into cells. The remaining two nuclei (polar nuclei) are situated below the egg apparatus in the large central cell. Three cells are grouped together at the micropylar end, forming the egg apparatus, which consists of two synergids and one egg cell. The synergids have filiform apparatus (cellular thickenings) at the micropylar tip, which guide pollen tubes into the synergid. Three cells are at the chalazal end and are called the antipodals. The large central cell has two polar nuclei. A typical angiosperm embryo sac at maturity is 8-nucleate but 7-celled.
1.2.3 Pollination
Pollination is the mechanism that brings male and female gametes together for fertilization, involving the transfer of pollen grains from the anther to the stigma of a pistil. Flowering plants have various adaptations to achieve pollination, often using external agents.
Kinds of Pollination
Depending on the pollen source, pollination is divided into three types:
Autogamy: Pollination occurs within the same flower, involving the transfer of pollen grains from the anther to the stigma of the same flower. Complete autogamy is rare in flowers that open and expose their anthers and stigma. It requires synchrony in pollen release and stigma receptivity, with the anthers and stigma located close to each other. Some plants like Viola, Oxalis, and Commelina produce chasmogamous flowers (normal flowers with exposed anthers and stigma) and cleistogamous flowers (flowers that do not open). Cleistogamous flowers are invariably autogamous, ensuring seed-set even without pollinators, because the anthers dehisce in the flower buds and pollen grains contact the stigma.
Geitonogamy: Pollination occurs between different flowers on the same plant. This is functionally cross-pollination, involving a pollinating agent, but genetically similar to autogamy.
Xenogamy: Pollination occurs between different plants. This is the only type of pollination that brings genetically different pollen grains to the stigma.
Agents of Pollination
Plants use abiotic (wind and water) and biotic (animals) agents for pollination. Most plants use biotic agents. Only a small proportion of plants use abiotic agents. Contact between pollen grains and the stigma is a chance event in wind and water pollination. To compensate for this, the flowers produce enormous amounts of pollen compared to the number of ovules.
Abiotic Agents
Wind Pollination: Common among abiotic pollinations, wind pollination requires light and non-sticky pollen grains for wind dispersal. Flowers often have well-exposed stamens and large, feathery stigmas to trap airborne pollen grains. Wind-pollinated flowers often have a single ovule in each ovary and numerous flowers packed into an inflorescence. Corn cob is a familiar example. Wind-pollination is common in grasses.
Water Pollination: Quite rare in flowering plants, limited to about 30 genera, mostly monocotyledons. Water is a regular mode of transport for male gametes in algae, bryophytes, and pteridophytes. Examples of water-pollinated plants include Vallisneria and Hydrilla (freshwater) and Zostera (marine sea-grasses). Not all aquatic plants use water for pollination. Water hyacinth and water lily have flowers that emerge above the water and are pollinated by insects or wind. In Vallisneria, female flowers reach the surface via a long stalk, and male flowers or pollen grains are released onto the surface and carried by water currents. In seagrasses, female flowers remain submerged, and pollen grains are released inside the water. Pollen grains are often long, ribbon-like, and protected from wetting by a mucilaginous covering. Wind and water-pollinated flowers are not very colorful and do not produce nectar.
Biotic Agents
Animal Pollination: Most flowering plants use animals as pollinating agents, including bees, butterflies, flies, beetles, wasps, ants, moths, birds (sunbirds and hummingbirds), and bats. Insects, particularly bees, are the dominant biotic pollinating agents. Larger animals like primates (lemurs), arboreal rodents, and reptiles (gecko lizards and garden lizards) also act as pollinators. Flowers of animal-pollinated plants are specifically adapted for particular animal species. Insect-pollinated flowers are often large, colorful, fragrant, and rich in nectar. When flowers are small, they are clustered into inflorescences to make them conspicuous. Animals are attracted by color and/or fragrance. Flowers pollinated by flies and beetles secrete foul odors. Flowers provide rewards like nectar and pollen grains. Animal visitors come into contact with the anthers and stigma while harvesting rewards, resulting in pollen grains coating the animal's body. The animal then carries pollen to the stigma of another flower, effecting pollination. Some species provide safe places to lay eggs as a floral reward, such as the tallest flower of Amorphophallus. A similar relationship exists between a species of moth and the plant Yucca, where both species cannot complete their life cycles without each other. The moth deposits its eggs in the locule of the ovary and pollinates the flower. The larvae of the moth emerge as the seeds develop.
Outbreeding Devices
Most flowering plants produce hermaphrodite flowers, leading to inbreeding depression from continued self-pollination. Plants have developed devices to discourage self-pollination and encourage cross-pollination:
Pollen release and stigma receptivity are not synchronized.
Anther and stigma are placed at different positions.
Self-incompatibility: A genetic mechanism preventing self-pollen from fertilizing ovules.
Production of unisexual flowers: Monoecious plants (castor and maize) prevent autogamy but not geitonogamy. Dioecious plants (papaya) have male and female flowers on different plants, preventing both autogamy and geitonogamy.
Pollen-Pistil Interaction
Pollination does not guarantee the transfer of compatible pollen. The pistil can recognize compatible and incompatible pollen. If compatible, the pistil accepts the pollen and promotes post-pollination events leading to fertilization. If incompatible, the pistil rejects the pollen, preventing pollen germination or pollen tube growth. Recognition is mediated by chemical components of the pollen interacting with those of the pistil.
Following compatible pollination, the pollen grain germinates on the stigma, producing a pollen tube through one of the germ pores. The pollen tube grows through the tissues of the stigma and style and reaches the ovary. In some plants, pollen grains are shed at a two-celled condition (vegetative cell and generative cell). In such plants, the generative cell divides and forms the two male gametes during the growth of the pollen tube. In plants that shed pollen in the three-celled condition, pollen tubes carry the two male gametes from the beginning. The pollen tube enters the ovule through the micropyle and then enters one of the synergids through the filiform apparatus. Filiform apparatus guides the entry of the pollen tube. All events from pollen deposition on the stigma until pollen tube entry into the ovule are referred to as pollen-pistil interaction. Pollen-pistil interaction is a dynamic process involving pollen recognition followed by promotion or inhibition of the pollen. Understanding this interaction helps breeders manipulate it to obtain desired hybrids.
Artificial Hybridisation
Breeders cross different species and genera to combine desirable traits. Artificial hybridization ensures only desired pollen grains are used and the stigma is protected from contamination. This involves emasculation and bagging techniques.
Emasculation: Removal of anthers from the flower bud of a bisexual flower before the anther dehisces.
Bagging: Covering emasculated flowers with a bag to prevent contamination of the stigma with unwanted pollen. When the stigma becomes receptive, mature pollen grains from the male parent are dusted on the stigma, and the flowers are rebagged until fruits develop. If the female parent has unisexual flowers, emasculation is unnecessary. The flower buds are bagged before the flowers open. When the stigma becomes receptive, pollination is carried out using the desired pollen and the flower rebagged.
1.3 Double Fertilisation
After entering one of the synergids, the pollen tube releases two male gametes into the cytoplasm of the synergid. One male gamete moves towards the egg cell and fuses with its nucleus (syngamy), forming a diploid zygote. The other male gamete moves towards the two polar nuclei in the central cell and fuses with them to produce a triploid primary endosperm nucleus (PEN) (triple fusion). Since both syngamy and triple fusion occur, the phenomenon is called double fertilization, unique to flowering plants. The central cell becomes the primary endosperm cell (PEC) and develops into the endosperm, while the zygote develops into an embryo.
1.4 Post-Fertilisation: Structures and Events
Following double fertilization, events including endosperm and embryo development, maturation of ovules into seeds, and ovary into fruit are termed post-fertilization events.
1.4.1 Endosperm
Endosperm development precedes embryo development to provide nutrition to the developing embryo. The primary endosperm cell divides repeatedly, forming a triploid endosperm tissue filled with reserve food materials. In the most common type of endosperm development, the PEN undergoes successive nuclear divisions to give rise to free nuclei (free-nuclear endosperm). Subsequently, cell wall formation occurs, and the endosperm becomes cellular. The number of free nuclei varies greatly before cellularization. Coconut water is free-nuclear endosperm, while the surrounding white kernel is cellular endosperm. Endosperm may be completely consumed by the developing embryo (e.g., pea, groundnut, beans) or persist in the mature seed (e.g., castor, coconut).
1.4.2 Embryo
Embryo develops at the micropylar end of the embryo sac where the zygote is situated. The zygote divides only after some endosperm forms, ensuring nutrition for the developing embryo. Early stages of embryo development (embryogeny) are similar in monocotyledons and dicotyledons. The zygote gives rise to the proembryo and subsequently to the globular, heart-shaped, and mature embryo. A typical dicotyledonous embryo consists of an embryonal axis and two cotyledons. The portion of the embryonal axis above the level of cotyledons is the epicotyl, which terminates with the plumule (stem tip). The portion below the level of cotyledons is the hypocotyl, terminating in the radicle (root tip), covered by the root cap. Monocotyledons possess only one cotyledon, called scutellum in the grass family, situated laterally on the embryonal axis. The embryonal axis has the radicle and root cap enclosed in the coleorrhiza. The portion of the embryonal axis above the level of attachment of scutellum is the epicotyl, which has a shoot apex and a few leaf primordia enclosed in a hollow foliar structure, the coleoptile.
1.4.3 Seed
The seed is the final product of sexual reproduction in angiosperms, formed inside fruits. A seed consists of seed coat(s), cotyledon(s), and an embryo axis. Cotyledons are simple, thick structures swollen with food reserves (legumes). Mature seeds may be non-albuminous or ex-albuminous (no residual endosperm) or albuminous (retain part of the endosperm). In some seeds (black pepper, beet), remnants of the nucellus persist as perisperm. Integuments of ovules harden into tough seed coats. The micropyle remains as a small pore, facilitating entry of oxygen and water during germination. As the seed matures, its water content is reduced, and the embryo's metabolic activity slows. The embryo may enter dormancy or germinate under favorable conditions. Ovules mature into seeds, and the ovary develops into a fruit. The ovary wall develops into the pericarp. Fruits may be fleshy (guava, orange, mango) or dry (groundnut, mustard). Many fruits have evolved mechanisms for seed dispersal. Fruits are the results of fertilization, but some species develop fruits without fertilization (parthenocarpic fruits), like banana. Parthenocarpy can be induced with growth hormones, producing seedless fruits.
Advantages of Seeds to Angiosperms
Reproductive processes are independent of water.
Better adaptive strategies for dispersal to new habitats.
Sufficient food reserves nourish young seedlings.
Hard seed coat protects the young embryo.
Generate new genetic combinations leading to variations.
Basis of agriculture, allowing for food storage and crop raising.
Dehydration and dormancy of mature seeds are crucial for storage. Seed viability varies. Some lose viability within months, while others remain alive for years.Lupinus arcticus: excavated from Arctic Tundra germinated and flowered after an estimated record of 10,000 years of dormancy.
Phoenix dactylifera: (date palm), discovered during the archeological excavation at King Herod’s palace near the Dead Sea had a recent record of 2000 years old viable seed.
Other Comprehension Factors:How many eggs are present in an embryo sac?
How many embryo sacs are present in an ovule?
How many ovules are present in an ovary?
How many ovaries are present in a typical flower?
How many flowers are present on a tree?
Orchid fruits contain thousands of tiny seeds. Fruits of parasitic species such as Orobanche and Striga contain many seeds. Ficus tree is very large and developed from that tiny seed and produces billions of seeds.
1.5 Apomixis and Polyembryony
Apomixis is a form of asexual reproduction that mimics sexual reproduction, producing seeds without fertilization in some flowering plants (Asteraceae, grasses). In some species, the diploid egg cell is formed without reduction division and develops into the embryo without fertilization. More often, some nucellar cells surrounding the embryo sac divide, protrude into the embryo sac, and develop into embryos. Polyembryony is the occurrence of more than one embryo in a seed. Examples include Citrus and Mango varieties. Hybrid varieties have increased productivity. If the seeds collected from hybrids are sown, the plants in the progeny will segregate and do not maintain hybrid characters. Production of hybrid seeds is costly and hence the cost of hybrid seeds become too expensive for the farmers.
If these hybrids are made into apomicts, there is no segregation of characters in the hybrid progeny. Then the farmers can keep on using the hybrid seeds to raise new crop year after year and he does not have to buy hybrid seeds every year. Active research is being conducted to understand and transfer apomictic genes into hybrid varieties.