Sexual Reproduction in Flowering Plants

Sexual Reproduction in Flowering Plants

• Definition: Sexual reproduction is the process of developing new organisms through the formation and fusion of gametes.

• Gamete Production: In flowering plants, gametes are not directly formed by sporophytic structures; instead, plants produce spores, which then develop into gametophytes.

• The Flower: Specialized organs for sexual reproduction in angiosperms. Flowers are modified condensed reproductive shoots.

• Human Significance: Flowers are used for expressing love, affection, happiness, grief, and mourning. They possess aesthetic, ornamental, social, religious, and cultural value. They are sources of dyes, scents, and perfumes.

• Biological Significance: Bees obtain nectar from flowers to convert into honey.

• Floral Induction: Flowers form on mature plants due to hormone-induced structural and physiological changes in shoot apices. Shoot apices transform into an inflorescence where floral primordia develop, which then grow into floral buds and undergo anthesis (opening).

Typical Flower Structure

• Thalamus: The broad base of the flower over which floral whorls are borne.

• Floral Whorls:

  • Calyx (Sepals): Accessory or nonessential organs with a supportive role.

  • Corolla (Petals): Accessory or nonessential organs.

  • Androecium (Stamens): Male reproductive structures (essential).

  • Gynoecium (Carpels): Female reproductive structures (essential).

Stamen: The Male Reproductive Organ

• Microsporophyll: Another term for the stamen.

• Components:

  • Filament: A long and slender stalk. Proximally attached to the thalamus, petal, or tepal. Distally bears the anther.

  • Anther: A broader, knob-like fertile part. Typically bilobed and tetragonal.

• Anther Structure:

  • Connective: A sterile parenchymatous tissue that attaches the two anther lobes on the back side. It contains a vascular strand.

  • Thecae: Each lobe usually has two chambers (microsporangia). A typical anther is tetrasporangiate.

  • Exceptions: Wolffia may have one microsporangium per lobe, and Arceuthobium may have only one microsporangium per anther.

• Anther Development: Microsporangia develop hypodermally from strips of archesporial cells (eusporangiate development). Archesporial cells divide periclinally into outer subepidermal parietal cells and inner sporogenous cells.

Microsporangium and Wall Layers

• Microsporangium: Appear circular in transverse section. Consists of a wall and sporogenous tissue.

• Wall Layers:

  1. Epidermis: Outer common covering. Cells may stretch and shrivel at maturity.

  2. Endothecium: Also called the fibrous layer. Cells develop fibrous thickenings of $\alpha$-cellulose on inner and radial walls. The stomium (line of dehiscence) occurs where hypodermal cells remain thin-walled between microsporangia.

  3. Middle Layers: $1-3$ layers that shrivel in the mature anther.

  4. Tapetum: Innermost layer. Radially enlarged cells with dense protoplasm and nutrients. Often multinucleate or polyploid (endoploidy).

• Tapetum Types:

  • Amoeboid (Invasive/Periplasmodium): Cells fuse to form a plasmodium that passes between sporogenous cells.

  • Secretory (Glandular/Parietal): Cells pass substances over sporogenous cells.

• Tapetum Functions:

  • Nourishment of microspore mother cells and pollen.

  • Production of Ubisch granules (lipid-rich, containing sporopollenin) for exine formation.

  • Production of pollenkitt (oily, sticky lipid/carotenoid covering for insect-pollinated plants).

  • Secretion of special proteins for compatibility and hormone IAA.

  • Secretion of the enzyme callase to degrade the callose wall around the pollen tetrad.

Microsporogenesis

• Microspore Mother Cells (PMC): Diploid cells ($2n$) derived from sporogenous tissue. They develop an internal layer of callose ($\beta-1, 3$ glucan) to isolate themselves.

• Process: PMCs undergo meiosis to produce tetrads of haploid ($n$) microspores.

• Tetrad Arrangements: Tetrahedral (most common), isobilateral, decussate, linear, and T-shaped. Aristolochia elegans shows all five types.

• Special Cases:

  • Cyperaceae: Only one microspore remains functional per meiosis.

  • Compound Pollen: Pollen grains remain attached (e.g., Juncus, Jatropha, Typha).

  • Pollinium: All pollen grains of a lobe unite into a single sac (e.g., Calotropis). Two pollinia form a translator.

  • Polyspory: More than four spores in a tetrad (e.g., up to $11$ in Cuscuta).

• Dehiscence: Mature anthers dry up; endothecium contracts and ruptures at the stomium. Pollen is released as a yellowish powdery mass.

Pollen Grain Structure

• Size/Shape: Globular, diameter $25-50$ $\mu m$.

• Cytoplasm: Rich in starch and unsaturated oils (which protect chromosomes from radiation).

• Sporoderm: The wall of the pollen grain.

  • Intine: Inner pecto-cellulosic layer. Contains enzymatic proteins.

  • Exine: Outer layer made of sporopollenin, a highly resistant fatty substance. Not degraded by any known enzyme, high temperature, or strong acids/alkalis. Differentiated into ektexine (sexine) and endexine (nexine). Ektexine has a foot layer, baculate layer, and tectum.

• Tectum: Provides characteristic surface sculpturing (ridges, spines, tubercles) used in identification (Palynology).

• Germ Pores: Areas where exine is thin or absent for pollen tube emergence. Tricolpate (three pores) in dicots; monocolpate (one furrow) in monocots.

• Pollenkitt: Sticky, yellowish oily layer on insect-pollinated pollen.

Pollen Properties and Human Impact

• Viability: Ability to germinate. $30$ minutes in Rice and Wheat; several months in Rosaceae, Leguminosae, and Solanaceae. Depends on temperature and humidity.

• Storage: Pollen banks use liquid nitrogen at $-196^{\circ}C$ for cryopreservation.

• Pollen Allergy: Major contributor is Parthenium (Carrot Grass). Causes respiratory disorders like rhinitis, asthma, and bronchitis (Hay Fever). Other sources: Chenopodium, Amaranthus, Sorghum, Ricinus, Prosopis, Cynodon.

• Pollen Products: Bee pollen is used as food supplements (tablets/syrups), in cosmetics, and for nature cures. Nutrients: Protein ($7-26\%$), Carbohydrates ($24-48\%$), Fats ($0.9-14.5\%$). Used to enhance performance in athletes and racehorses.

Male Gametophyte Development

• Pre-Pollination (Microgametogenesis): Development is precocious (starts in the anther). Microspore nucleus divides mitotically into a small generative cell and a large vegetative (tube) cell.

  • Vegetative Cell: Rich in food reserves (starch, protein, fat); large, irregular nucleus.

  • Generative Cell: Spindle-shaped with dense cytoplasm. Lies freely in the vegetative cell cytoplasm.

  • Shedding Stage: $60\%$ of angiosperms shed pollen at the $2$-celled stage. Others divide the generative cell into two male gametes before shedding ($3$-celled stage).

• Post-Pollination: Compatible pollen on stigma absorbs water and nutrients. Vegetative cell forms a pollen tube through a germ pore. Pollen tube secretes pectinases and hydrolytic enzymes.

  • Pollen Tube Zones: (i) Growth zone/Cap block at tip; (ii) Nuclear zone (tube nucleus + gametes); (iii) Vacuolization zone (with callose plugs to isolate old cytoplasm).

• Mature Male Gametophyte: A $3$-celled structure (one tube cell + two nonmotile male gametes) including the pollen tube.

Pistil: The Female Reproductive Organ

• Gynoecium: Female part of the flower. Consists of one or more carpels (megasporophylls).

• Classifications:

  • Monocarpellary: One carpel.

  • Multicarpellary: Many carpels.

  • Apocarpous: Carpels are free (e.g., Michelia, Ranunculus).

  • Syncarpous: Carpels are fused (e.g., Hibiscus, Papaver).

• Pistil Components:

  1. Stigma: Terminal landing platform for pollen; determines compatibility.

  2. Style: Narrow stalk connecting stigma and ovary.

  3. Ovary: Basal swollen part containing ovarian cavity (locule) and placenta.

• Ovule Attachment: Placentae bear ovules. Single ovule examples: Wheat, Paddy, Mango. Multiple ovules: Papaya, Watermelon, Orchids.

Structure of the Ovule (Megasporangium)

• Funiculus: The stalk attaching the ovule to the placenta.

• Hilum: Point of attachment between the ovule body and funiculus.

• Raphe: Longitudinal ridge formed by funiculus fusion in anatropous ovules.

• Nucellus: Central parenchymatous mass (equivalent to megasporangium). Crassinucellate (thick) or tenuinucellate (thin).

• Integuments: Protective layers. Unitegmic (one), bitegmic (two), tritegmic (three, e.g., Asphodelus), or ategmic (absent, e.g., Santalum).

• Micropyle: Narrow pore at the tip for pollen tube entry.

• Chalaza: Base of the ovule where integuments originate.

• Ovule Types:

  • Orthotropous (Atropous): Erect.

  • Anatropous: Inverted ($92\%$ of ovules).

  • Hemitropous: Half-inverted.

  • Campylotropous: Body curved.

  • Amphitropous: Body and embryo sac curved.

  • Circinotropous: Funiculus coiled around the ovule.

Megasporogenesis and Megagametogenesis

• Megasporogenesis: Meiotic formation of haploid megaspores from a diploid Megaspore Mother Cell (MMC).

  • Archesporial cell develops in the hypodermal nucellus, becomes MMC.

  • MMC undergoes meiosis producing a linear tetrad of four megaspores.

  • Commonly, only the chalazal megaspore remains functional; the other three degenerate.

• Megagametogenesis: Development of functional megaspore into the embryo sac.

  • Nucleus divides three times (free nuclear divisions) to form eight nuclei.

  • Two nuclei (one from each pole) move to the center as polar nuclei.

  • Polygonum Type (Typical): Monosporic development results in a $7$-celled, $8$-nucleate embryo sac (observed in $80\%$ of flowering plants).

• Embryo Sac (Female Gametophyte) Cells:

  1. Egg Apparatus (Micropylar end): One egg cell (oosphere) and two synergids. Synergids have a filiform apparatus (finger-like wall projections) to guide the pollen tube.

  2. Antipodal Cells (Chalazal end): Three vegetative cells.

  3. Central Cell: Largest cell, containing two polar nuclei (which may fuse into a secondary/definitive nucleus).

Comparison of Gametophytes

Pollination

• Definition: Transfer of pollen grains from anther to stigma.

• Self Pollination: Transfer between same or genetically similar flowers.

  • Autogamy: Same flower. Possible via Homogamy (simultaneous ripening) or Cleistogamy (closed flowers).

  • Geitonogamy: Different flowers on the same plant (genetically self, functionally cross).

• Cross Pollination (Xenogamy/Allogamy): Transfer between genetically different flowers.

• Agencies:

  • Abiotic: Wind (Anemophily), Water (Hydrophily).

  • Biotic: Insects (Entomophily), Birds (Ornithophily), Bats (Chiropterophily), Snails (Malacophily).

Detailed Pollination Methods

• Anemophily (Wind): Flowers small, colorless, nectarless. Anthers versatile, stigmas feathery. High pollen count (e.g., Cannabis produces $500,000$ per flower). Wasteful and non-directional.

• Hydrophily (Water): Limited to $30$ genera (mostly monocots).

  • Hypohydrophily: Under water (e.g., Zostera - ribbon-like pollen).

  • Epihydrophily: On water surface (e.g., Vallisneria - stalks uncoil to bring flowers to surface).

• Entomophily (Insects): Bees ($80\%$ of biotic pollination). Flowers fragrant, colored, with nectar and pollen rewards. Pollen is heavy and sticky (pollenkitt).

• Adaptations:

  • Lever mechanism: Salvia.

  • Trap door mechanism: Ficus (Gall wasp).

  • Symbiosis: Yucca and Pronuba moth.

  • Sexual Deceit: Ophrys and Colpa wasp.

• Ornithophily (Birds): Sunbirds and hummingbirds. Leathery flowers, abundant watery nectar.

• Chiropterophily (Bats): Large, stout flowers with strong fermenting odors (e.g., Adansonia).

Outbreeding Devices (Contrivances to Prevent Selfing)

• Dicliny: Unisexual flowers.

• Dichogamy: Anthers/Stigmas ripen at different times (Protandry vs Protogyny).

• Prepotency: External pollen grows faster.

• Self Sterility (Incompatibility): Genetic inhibition of self pollen ($S$-gene alleles).

• Heterostyly: Different style/stamen heights (e.g., Primula).

• Herkogamy: Mechanical barriers preventing self contact.

Double Fertilization

• Discovery: Nawaschin (1898) in Fritillaria and Lilium.

• Process:

  1. Syngamy (Generative Fertilization): One male gamete ($n$) + Egg cell ($n$) $\rightarrow$ Zygote ($2n$). Formation of embryo.

  2. Triple Fusion (Vegetative Fertilization): One male gamete ($n$) + Secondary Nucleus ($2n$) $\rightarrow$ Primary Endosperm Nucleus (PEN) ($3n$). Formation of endosperm.

• Significance: Ensures nutritive tissue (endosperm) is produced only if fertilization is successful. Triploid endosperm has high physiological activity.

Post-Fertilization: Endosperm and Embryo

• Endosperm Types:

  • Nuclear: PEN divides repeatedly without wall formation (e.g., Coconut water is free nuclear endosperm).

  • Cellular: Every division followed by cytokinesis (e.g., Datura).

  • Helobial: Intermediate; first division unequal, then nuclear development (e.g., Asphodelus).

• Embryogeny (Dicot): Zygote divides into basal (suspensor) and terminal (embryo) cells. Stages: Proembryo $\rightarrow$ Globular $\rightarrow$ Heart-shaped $\rightarrow$ Mature.

• Embryogeny (Monocot): Single cotyledon called Scutellum. Protective sheaths: Coleoptile (plumule) and Coleorhiza (radicle).

Seeds and Fruits

• Seed Parts: Seed coat (Testa/Tegmen), embryo, and sometimes endosperm.

• Seed Classification:

  • Albuminous (Endospermic): Endosperm persists (e.g., Wheat, Maize, Castor, Coconut).

  • Exalbuminous (Non-endospermic): Endosperm consumed; food in cotyledons (e.g., Pea, Groundnut).

• Perisperm: Persistent nucellus in seed (e.g., Black pepper, Beet).

• Dormancy: State of inactivity; moisture drops to $10-15\%$.

• Fruit Types:

  • True Fruit: Develops from ovary only.

  • False Fruit: Other parts like thalamus involved (e.g., Apple, Strawberry, Cashew).

  • Parthenocarpic Fruit: Developed without fertilization; seedless (e.g., Banana).

Apomixis and Polyembryony

• Apomixis: Asexual production of seeds without fertilization (mimics sexual reproduction). Examples: Asteraceae and grasses.

  • Recurrent Agamospermy: Parthenogenesis from diploid egg.

  • Adventive Embryony: Embryo from nucellus or integument.

• Polyembryony: Presence of more than one embryo in a seed. Common in Citrus, Mango, and Onion.

• Significance: Apomictic seeds allow farmers to maintain hybrid characters across generations without segregation of traits.

Additional Quantitative Data and Facts

• Smallest Pollen: Myosotis ($2.5-3.5$ $\mu m$).

• Largest Pollen: Mirabilis ($250$ $\mu m$ diameter); longest is Zostera ($2500$ $\mu m$).

• Largest Flower: Rafflesia ($1$ meter).

• Smallest Flower: Wolffia.

• Oldest Viable Seed: Lupinus arcticus ($10,000$ years from Arctic tundra); Phoenix dactylifera ($2000$ years from King Herod's palace).

• Xenia: Effect of pollen on endosperm. Metaxenia: Effect of pollen on other tissues.

• Artificial Hybridization: Uses emasculation (removal of stamens) and bagging to ensure desired cross-breeding.