Study Notes: Reproduction in Lower and Higher Plants

REPRODUCTION OVERVIEW

• Definition of Reproduction: Reproduction is the production of young ones like parents. It is an essential process as it leads to the continuation of species and maintains the continuity of life.

• Methodologies: Each organism has its own particular method of reproduction, generally falling into two categories:     * Asexual reproduction: Does not involve the fusion of two compatible gametes.     * Sexual reproduction: Involves the fusion of two compatible gametes.

1.1 ASEXUAL REPRODUCTION

• Core Characteristics: This process results in the production of genetically identical progeny from a single organism, which inherits the genes of that parent. These morphologically and genetically identical individuals are termed clones.

• Modes of Asexual Reproduction:     * Fragmentation: Multicellular organisms break into fragments (e.g., Spirogyra), and these fragments grow into new individuals.     * Budding: The most common method in unicellular Yeast. It occurs during favourable conditions by producing one or more outgrowths called buds. Once separated, these buds develop into new individuals.     * Spore formation: In Chlamydomonas, reproduction occurs via flagellated, motile zoospores that grow independently into new individuals.

• Other Methods:     * Binary fission: Occurs in Amoeba and Paramoecium.     * Conidia formation: Occurs in Penicillium.     * Gemmules formation: Occurs in Sponges.

• Natural Vegetative Propagation: Plants reproduce asexually through their vegetative parts:     * Root: Examples include Sweet potato, Asparagus, and Dahlia.     * Leaf: Examples include Bryophyllum, Kalanchoe, and Begonia.     * Stem: Examples include rhizomes (turmeric), tubers (potato), and bulbs (onion).

VEGETATIVE REPRODUCTION AND ARTIFICIAL METHODS

• Nature of Vegetative Reproduction: Plants formed are genetically identical to their parents. Agriculture and horticulture exploit these methods to multiply fresh stocks and propagate desired varieties.

• Artificial Methods:     * Cutting: Using a small piece of a vegetative part containing one or more buds.         * Stem cutting: Rose, Bougainvillea.         * Leaf cutting: Sansevieria.         * Root cutting: Blackberry.     * Grafting: Joining parts of two plants so they grow as one.         * Scion: The part of the stem containing more than one bud.         * Stock: The rooted plant onto which the scion is joined.         * Budding (Bud Grafting): A variation where only one bud is joined onto the stock (e.g., Apple, Pear, Rose).     * Tissue Culture: A method where a small amount of plant tissue is grown to produce many plantlets, also known as micropropagation.

1.2 SEXUAL REPRODUCTION

• Requirement for Maturity: All organisms must reach maturity before reproducing sexually. In plants, the end of the juvenile or vegetative phase marks the beginning of the reproductive phase, visible at the time of flowering.

• The Flower: A specialized reproductive structure. Its function is to produce haploid gametes and ensure fertilization.     * Floral Whorls: A typical flower consists of four whorls: calyx, corolla, androecium, and gynoecium.

• Major Events:     1. Meiosis.     2. Fusion of gametes to form a diploid zygote ($2n$).

• Variations: Sexual reproduction produces genetically dissimilar offspring, which are vital for survival and the evolution of species over time.

• Sequential Stages: Classified into Pre-fertilization, Fertilization, and Post-fertilization (including embryogenesis).

THE MALE REPRODUCTIVE UNIT: ANDROECIUM AND POLLEN

• Androecium: The male reproductive whorl of a flower. Its individual members are called stamens.     * Stamen Components: Filament, connective, and anther.

• Structure of Anther:     * Immature anthers consist of parenchymatous tissue surrounded by an epidermis.     * Anther is generally dithecous (two lobes) and tetrasporangiate (containing four pollen sacs).     * Heterogeneity: Arises when hypodermal cells transform into archesporial cells.

• T.S. of Anther (Wall Layers):     1. Epidermis: Outermost protective layer made of flattened (tabular) cells.     2. Endothecium: Sub-epidermal layer with radially elongated cells containing fibrous thickenings.     3. Middle Layer: $1$-$2$ layers of thin-walled cells that may disintegrate in a mature anther.     4. Tapetum: Innermost nutritive layer enclosing the sporogenous tissue (microspore mother cells).

• Microsporogenesis: Each microspore mother cell (MMC) divides meiotically to form a tetrad of haploid microspores (pollen grains).

• Structure of Microspore (Pollen Grain):     * Non-motile, haploid, unicellular, with a single nucleus.     * Sporoderm: The two-layered wall:         * Exine: Thick outer layer made of sporopollenin (complex, non-biodegradable, chemical-resistant). It contains germ-pores for the pollen tube to emerge.         * Intine: Inner wall layer consisting of cellulose and pectin.

• Development of Male Gametophyte:     * First Mitotic Division: Produces a large, naked vegetative cell (rich in food with an irregular nucleus) and a small, thin-walled generative cell.     * Second Mitotic Division: The generative cell divides to produce two non-motile male gametes. This happens in the pollen grain or the pollen tube.     * Most angiosperms shed pollen at the two-celled stage.

• Pollen Viability: The ability to germinate. It lasts $30\,\text{minutes}$ in rice and wheat, but can last for months in Solanaceae, Rosaceae, and Leguminosae.

THE FEMALE REPRODUCTIVE UNIT: GYNOECIUM AND OVULE

• Gynoecium (Pistil): The female reproductive whorl. Individual members are called carpels (megasporophylls).     * Apocarpous: Flower with many free carpels (e.g., Michelia).     * Syncarpous: Flower with fused carpels (e.g., Brinjal).     * Carpel Parts: Ovary, style, and stigma.     * Ovule quantity: Uniovulate (paddy, wheat, mango) or multiovulate (tomato, lady's finger).

• Structure of Anatropous Ovule:     * The most common type in angiosperms; the micropyle is directed downwards and is adjacent to the funiculus.     * Funiculus (Funicle): Stalk attaching the ovule to the placenta.     * Hilum: Attachment point of funiculus to the ovule body.     * Nucellus: Central parenchymatous tissue.     * Integuments: Protective coverings (outer and inner).     * Micropyle: Narrow opening at the apex.     * Chalaza: Base of the ovule opposite the micropyle.     * Embryo Sac: The multicellular female gametophyte embedded in the nucellus.

• Megasporogenesis: The formation of haploid megaspores from a diploid megaspore mother cell (MMC) via meiosis. A linear tetrad of four haploid megaspores is formed.

• Development of Female Gametophyte:     * The lowest megaspore (towards the center) remains functional; the upper three abort.     * The functional megaspore undergoes three successive free nuclear mitotic divisions, forming $8$ nuclei.     * Structure of Mature Embryo Sac:         * 7-celled, 8-nucleated structure.         * Egg Apparatus (at micropylar end): One central haploid egg cell and two haploid synergid cells. Synergids have filiform apparatus (hair-like projections) to guide the pollen tube.         * Antipodal Cells: Three cells at the chalazal end.         * Secondary Nucleus: Formed by the fusion of two haploid polar nuclei in the large central cell just prior to fertilization.     * Monosporic Development: Development of the embryo sac from a single megaspore.     * Endosporous: Development occurs within the megaspore.

1.6 POLLINATION AND ITS AGENTS

• Definition: The transfer of pollen grains from the anther to the stigma. It is essential because gametes are non-motile and produced at different sites.

• Types Based on Source:     * Autogamy (Self-pollination): Same flower pollination. Offspring are genetically identical (e.g., pea).         * Chasmogamous: Flowers that open to expose sex organs.         * Cleistogamous: Flowers that never open (e.g., underground flowers of Commelina benghalensis).         * Homogamy: Simultaneous maturation of anther and stigma.     * Geitonogamy: Transfer to a different flower on the same plant. Form of self-breeding but involves agents (e.g., Cucurbita maxima).     * Xenogamy (Cross-pollination/Out-breeding): Transfer to a flower on a different plant of the same species. Generates genetic variation.

• Abiotic Agents:     * Anemophily (Wind): Small, inconspicuous, odourless flowers. Pollen is light, dry, and numerous. Stigma is feathery. Anthers are versatile (e.g., wheat, rice, corn, palms, maize).     * Hydrophily (Water): Found in approximately $30$ genera of aquatic monocots (e.g., Vallisneria, Zostera).         * Hypohydrophily: Pollination below the water level; pollen is heavier than water (e.g., Zostera).         * Epihydrophily: Pollen floats on the surface to reach the stigma (e.g., Vallisneria).

• Biotic Agents:     * Entomophily (Insects): Large, showy, brightly coloured, scented flowers with nectar. Pollen is spiny with a sticky pollen-kit. Specific adaptations include the lever mechanism or turn-pipe mechanism in Salvia.     * Ornithophily (Birds): Brightly coloured, showy flowers with profuse, dilute nectar and no fragrance (e.g., Bombax, Callistemon). Specialized birds include Sun birds and Hummingbirds.     * Chiropteryphily (Bats): Dull coloured flowers with strong fragrance and abundant nectar/pollen (e.g., Anthocephalus, Adansonia, Kigelia).

1.7 OUTBREEDING DEVICES

• Purpose: To discourage self-pollination and prevent inbreeding depression.

• Mechanisms (Contrivances):     * Unisexuality (Dioecism): Plant bears only male or female flowers (e.g., Mulberry, Papaya).     * Dichogamy: Anthers and stigmas mature at different times.         1. Protandry: Androecium matures first (e.g., Sunflower disc florets).         2. Protogyny: Gynoecium matures first (e.g., Gloriosa).     * Prepotency: Pollen from other flowers germinates faster than pollen from the same flower (e.g., Apple).     * Heterostyly (Heteromorphy): Stigmas and anthers are at different levels (e.g., Primula).     * Herkogamy: A physical mechanical barrier prevents contact between sex organs (e.g., Calotropis with its pentangular stigma above anthers).     * Self-incompatibility (Self-sterility): Genetic mechanism inhibiting pollen germination from the same flower (e.g., Tobacco, Thea).

1.8 POLLEN PISTIL INTERACTION AND ARTIFICIAL HYBRIDIZATION

• Interaction: All events from pollen deposition on the stigma to the entry of the pollen tube into the ovule. The pistil uses special proteins to recognize compatible pollen.

• Process: Compatible pollen absorbs water/nutrients, germinates, and grows through the style guided by chemicals. The pollen tube tip entering a synergid ruptures to release gametes.

• Artificial Hybridization: Major crop improvement approach. Only desired pollen is used.     * Emasculation: Removal of anthers from a bisexual flower.     * Bagging: Covering flowers to prevent unwanted pollination.

1.9 DOUBLE FERTILIZATION

• Discovery: Discovered by Nawaschin in Lilium and Fritillaria.

• Pollen Tube Entry:     * Porogamy: Through the micropyle.     * Chalazogamy: Through the chalaza.     * Mesogamy: Through the integuments.

• Two Fertilization Events:     * Syngamy (Generative Fertilization): Haploid male gamete ($n$) + haploid egg ($n$) → diploid zygote ($2n$).     * Triple Fusion (Vegetative Fertilization): Second haploid male gamete ($n$) + diploid secondary nucleus ($2n$) → triploid primary endosperm nucleus (PEN, $3n$).

• Siphonogamy: The process where non-motile male gametes are carried through a hollow pollen tube.

• Significance: Restores diploidy, initiates endosperm formation, and avoids polyembryony.

1.10 DEVELOPMENT OF ENDOSPERM

• Types of Endosperm:     1. Nuclear Type: Most common ($161\,\text{families}$). PEN divides repeatedly without wall formation (e.g., wheat, sunflower, coconut liquid).     2. Cellular Type: Wall formation follows division immediately ($72\, \text{dicot families}$, e.g., Balsam, Petunia).     3. Helobial Type: First division forms a large micropylar and small chalazal cell; subsequent divisions are nuclear (e.g., Asphodelus).

• Mosaic Endosperm: Contains tissues of two different types/colours (e.g., corn).

1.11 DEVELOPMENT OF EMBRYO (EMBRYOGENESIS)

• Initial Division: Zygote divides into a large basal/suspensor initial cell (micropylar) and a small terminal/embryonal initial cell (chalazal).

• Suspensor: A filament of $6$-$10$ cells. The first cell is the haustorium; the lowermost is the hypophysis.

• Embryo Growth Stages: Proembryo → Octant ($8\,\text{cells}$) → Heart-shaped → Horse-shoe shaped.

• Monocot Structure: Single shield-shaped cotyledon is called the scutellum. Plumule sheath is coleoptile, and radicle sheath is coleorhiza.

1.12 SEED AND FRUIT DEVELOPMENT

• Transformation: Ovule becomes the seed; ovary becomes the fruit.

• Seed Coats: Testa (outer) and Tegmen (inner).

• Perisperm: Persisting nucellus (e.g., black pepper, beet).

• Types of Seeds:     * Endospermic (Albuminous): Endosperm remains conspicuous (e.g., Castor, Coconut, Maize).     * Non-endospermic (Ex-albuminous): Endosperm is consumed during development (e.g., Pea, bean).

• Dormancy: A state of metabolic arrest for survival during adverse conditions.     * Lupinus arcticus seeds can stay viable for $10,000\,\text{years}$.     * Phoenix dactylifera seeds can stay viable for $2000\,\text{years}$.

1.13 APOMIXIS, PARTHENOCARPY, AND POLYEMBRYONY

• Apomixis: Formation of embryo without fertilization/meiosis.     * Apogamy: Gametophytic cell produces embryo.     * Apospory: Diploid sporophyte cell produces gametophyte without meiosis.     * Recurrent: Embryo sac from archesporial cell. Includes diplospory (from diploid MMC).     * Non-recurrent: Haploid embryo from egg (parthenogenesis) or other gametophytic cell (apogamy). Resulting plants are sterile (e.g., Nicotiana).     * Adventive Embryony: Embryos from somatic nucellus or integuments (e.g., Mango, Orange).

• Parthenocarpy: Fruit development without fertilization, resulting in seedless fruit (coined by Noll, $1902$). Driven by auxin IAA (Indole-3 Acetic Acid).

• Polyembryony: Development of more than one embryo inside a seed (noticed by Leeuwenhoek, $1719$).     * Cleavage Polyembryony: Zygote/proembryo divides into units.

QUESTIONS, DISCUSSION, AND ACTIVITIES

• Activity (Yeast): Sprinkle yeast over warm water and sugar; it becomes bubbly in $10\,\text{minutes}$, proving activity.

• Activity (Flower): Label the whorls: calyx, corolla, androecium, and gynoecium.

• Recall Question: Why does a gardener choose to propagate plants asexually? (Answer: To multiply fresh stocks and maintain desired genetically identical varieties).

• Do you know?: Why is pollen well preserved? (Answer: Due to the presence of sporopollenin in the exine).

• Do you know?: What is the kernel in tender coconut? (Answer: It is the multicellular endosperm).

• Question: How does vegetative propagation occur in nature? (Answer: Via roots, leaves, and stems like rhizomes and tubers).

• Question: What is the difference between true and false fruit? (Answer: True fruit develops from the ovary; false fruit involves other floral parts).