SEXUAL REPRODUCTION IN FLOWERING PLANTS

SEXUAL REPRODUCTION IN FLOWERING PLANTS

Ø Stamen/Androecium: (male part) – long slendered stalk (Filament) attached to thalamus & terminal bilobed structure ANTHER (Bilobed; each lobe Dithecous). Two Microsporangia present in each lobe (total 4 Pollen sacs).

(a) A Stamen (b) Section of Anther showing lobes & Microsporangia

Ø STRUCTURE OF MICROSPORANGIUM: central mass of SPOROGENOUS TISSUE is surrounded by four layers (EPIDERMIS, ENDOTHECIUM, MIDDLE LAYER and TAPETUM) outer three protective while Tapetum (multinucleated with dense cytoplasm) nourishes (Food) the developing pollen grains/sporogenous tissue.

(a) Section of Anther showing Microsporangia  (b) Section of a Microsporangium

 

Ø MICROSPOROGENESIS (PROCESS OF FORMATION OF POLLEN GRAIN FROM Pollen Mother Cell): 

Each sporogenous tissue acts as Pollen Mother Cell (PMC) or Microspore Mother Cell (mmc). Through Meiosis Pollen Tetrad is formed. As anther mature and dehydrate, pollen grains mature and are released with dehiscence of anther.

(Microsporigenesis up to Pollination)

Ø POLLEN GRAIN (MALE GAMETOPHYTE):

Spherical structures made of outer EXINE (thick, warty & made of SPOROPOLLENIN-one of the most resistant organic materials known/ no enzyme can degrade it) and inner INTINE (made of cellulose & pectin) & plasma membrane surrounding the protoplasm. Mature pollen grain contains two cells (a) VEGETATIVE CELL: bigger, with abundant food reserve & large irregularly shaped nucleus. (b) Small GENERATIVE CELL- with spindle shaped nucleus and dense cytoplasm, floats in the cytoplasm of Vegetative cell.

      (Electron Microscopic view of pollen grains of some ferns)

Ø In Over 60% angiosperms pollen grains are shed at 2 – Cell stage, in others Generative cell again divides to produce Two male Gametes (3- Cell Stage) before dispersal.

(Stages of Development of Male Gametophyte)

Ø Pollen Allergy leading to severe respiratory disorders (Asthma, Bronchitis) eg.. Parthenium (carrot Grass) imported with Wheat has became an important weed in India causing pollen allergy. Pollen Grains as rich nutrient & protein Supplements for athletes & race horses. Pollen Banks store varieties of pollen grains in liquid Nitrogen (-196⁰C).

    (CRYOGENIC using liquid nitrogen)

Ø Gynoecium (female part) may contain single Pistil (Monocarpellary) or more (Multicarpellary). They may be free (Apocarpous) or Fused (Syncarpous). Each Pistil has Three parts –

(a) STIGMA: serve as landing platform for pollen grains

 (b) STYLE: elongated slendered part projecting Stigma 

(c) OVARY: bulging base containing ovarian cavities/Locules demarked by walls called Placenta, from which arises OVULES (MEGASPORANGIUM).

(Pistil / Carpel)

Ø Number of Ovules: ONE (wheat, rice, mango, litchi) or MANY (papaya, orchids, watermelon, guava).

Ø OVULE (MEGASPORANGIUM): Small structure attached to placenta by a stalk/Hilum. Each ovule has 1-2 protective INTEGUMENTS covering it except at the tip called MICROPYLE; opposite side is called CHALAZA (base of ovule). Inside is a mass of cells/ NUCELLUS with food reserves. Central part contains EMBRYO SAC / female gametophyte. Most common Ovule is a typical ANATROPOUS ovule.

(Anatropous Ovule)

Ø MEGASPOROGENESIS: (formation of megaspore/Ovule form megaspore mother cells or MMC) Ovule generally develops a single megaspore mother cell (MMC) in micropylar  region   Ovule generally develops a single large megaspore mother cell (MMC)with dense cytoplasm& a nucleus, in micropylar  region in Nucellus. MMC divides meiotically to produce four Megaspores. The numbers of functional megaspore vary: Mono-/ Bisporic Embryo sac.

(Basic Types of Embryo Sacs)

Ø EMBRYO SAC (FEMALE GAMETOPHYTE): MOSTLY only one functional Megaspore (generally basal) develops into Female Gametophyte (Embryo sac)-Monosporic Development. Nucleus of Functional Megaspore undergo three (free nuclear) mitotic divisions forming 8 – nucleated cell. Later cell walls formed. A typical Embryo Sac is an 8-nucleated & 7- celled structure {POLYGONUM TYPE of Embryo Sac}.

(Female Gametophyte)

(Compared)

Ø POLLINATION (Transfer of a mature pollen grain to a receptive stigma)

(A) Autogamy :- pollination between anther and stigma of same flower Cleistogamous flowers like Pisum sativum, some like Oxalis & Commelina produce two types of flowers –Chasmogamous & Cleistogamous . Cleistogamy is essentially Autogamy and hence produce Clones with assured set of seeds.

 (B) Geitonogamy: - pollination between different flowers of same plant.             

  (C) Xenogamy:-  pollination between two different plants of same species.

(Types of Pollination)

Ø Agents of Pollination: Abiotic (wind & water) and Biotic (insects, birds, animals, bat etc..) Agents. Majority of plants use BIOTIC AGENTS. Only few use abiotic agents due to Uncertainty of Pollination. Hence to increase chances of pollination such plants produce enormous amount of pollen grain.

(Pollinating Agents)

Ø Wind Pollination (ANEMOPHYLLY):- Most common in abiotic types. Pollen Grains are small, light & non-sticky. Stamen are well exposed & large, often Feathery. Ovary bear single ovule & flowers packed into Inflorescence. Eg.. Tassels of Corn Cob are stigma & style of the flower. (method is most common in Grasses)

Ø  Water Pollination (HYDROPHYLLY):- Common in Monocots. Eg..Freshwater plants (Vallisneria & Hydrilla) & marine plants (Zostera)                    

       [[water Lily & Water Hyacinth pollination by Wind or Insect?]]

Ø Vallisneria:- Female flower reach water surface by long stalk & male flower/pollen grain released on surface of water. They reach the stigma by positive Water Current. In Sea Grasses, same happen inside the water. In most water pollinated plants, pollen grains are protected from water by mucilaginous coverings.

Ø INSECT POLLINATION (Entomophylly) :- Most common type of pollination. Bees are the most important Pollinators. Flowers are large, colorful, fragrant & rich in Nectar. When flowers are small, then clustered as Inflorescence. In flowers pollinated by Beetles & flies, the flower secrets foul odors to attract animals. Nectar & pollen grains are usual rewards. Often floral rewards are in providing safe place to lay Eggs. Eg.. Tallest flower Amorphophallus (6 ft. high flower).  Similarly, between a species of moth & plant Yucca. Moth deposits its eggs in the locule of ovary & flower gets pollinated by moth. The larva of moth comes out of eggs as the seeds develop.

(Bees are the most important pollinators for Mankind)

(Comparison)

Ø OUTBREEDING DEVICES (Contrivances for Cross Pollination):- [ continued Self pollination results in Inbreeding Depression & loss of Genetic Vigour]

(A) Bisexual Flowers: Pollen release & stigma receptivity is not synchronized (Protandry/Protogyny), Difference in position of anther & stigma, Self-Incompatibility (genetic mechanism)

(B) Unisexual Flowers on same plant (may not prevent Geitonogamy) eg. Castor & Maize.

(C) Dioecy:- male & female plants separate eg. Papaya, prevent both Auto-/ Geitono-gamy.

Ø POLLEN PISTIL INTERACTION: Pollen grains are shed at 2-celled stage. Pistil recognizes compatible pollen grain and allows the pollen tube to grow. Other incompatible pollen grains are stopped through chemicals regulated by a genetic mechanism of Incompatibility. Pollen tube contains two male gametes & tube nucleus. In ovary pollen tube enter through Micropyle & enters one of the Synergids with the help of Filiform Apparatus.

Ø ARTIFICIAL HYBRIDIZATION:- Cross pollination by Human beings to produce superior breeds. Steps are:     

          (A) EMASCULATION:-removing immature anther from a bisexual flower to prevent self pollination. Eg. Mechanical (forceps), Chemical (nitrous oxide), Thermal (Heat/Cold) or Hormonal.

(Emasculation)

(B) BAGGING:- covering emasculated flower with a suitable transparent bag.

(Bagging)

 (C) TAGGING:- attaching a tag of details to recognize the events of breeding. For a Unisexual flower , No emasculation is needed, only Tagging & Bagging is done.

(Tagging)

Ø DOUBLE FERTILIZATION: On entering  one of the Synergids, either of the male gamete move towards the egg cell and fuse with its nucleus (Syngamy) to form ZYGOTE (2n). Other male gamete move towards the central cell (2n) and fuse to form a Triploid (3n) PEN (Primary Endospermic Nucleus) [TRIPLE FUSION].  Triple fusion & Syngamy together is referred to as DOUBLE FERTILIZATION. PEN develops into ENDOSPERM, Zygote into EMBRYO.

Ø ENDOSPERM (3n):- PEC (Primary Endospermic Cell) divides into a Triploid endospermic tissue filled with reserve food for developing embryo. Free Nuclear Endosperm (most common):- PEN divides into many nuclei in a single cell (later becomes CELLULAR ENDOSPERM by developing walls) eg. Coconut Water later becoming solid White Kernel. Endosperm may be totally consumed by developing embryo (Non Endospermic eg. Pea, Beans Groundnut,) or may persist in mature seed (Endospermic eg. Castor & Coconut) & used up during seed germination.

 

(Types of Endosperm)

Ø EMBRYO:- develops at Micropylar end of embryo sac. Most zygote divides only after some amount of endosperm formation to assure Nutrition. Early stages of embryo development (EMBRYOGENY) are similar in both Dicots & Monocots. Zygote form PROEMBRYO which form Globular, Heart shaped and Mature embryo. A typical Dicot embryo has an EMBRYONIC AXIS & two COTYLEDONS (fleshy leaves). Axis above cotyledon level is EPICOTYL (covered in COLEOPTILE) & forms Plumule (Shoot Tip) while below cotyledon level, the axis is HYPOCOTYL (Covered in COLEORRHIZA) forming RADICLE (Root Tip).  Root tip is covered by a sterile layer for protection (Root Cap). MONOCOT EMBRYO:- Only one Cotyledon. In grass family the cotyledon is present on lateral side of embryonic axis & called SCUTELLUM.

Ø SEED (fertilized ovule):- [ Seed Coats + Cotyledons + Embryonic Axis]   may be Albuminous & Non Albuminous. PERISPERM is the remnant persistent Nucellus in some seeds (beet, Black Pepper).

Ø DORMANCY:- A mature seed has tough & hard seed coats(integuments of ovule) . Micropyle remains as a small pore in the seed coat through which oxygen & water enters in it during germination. Seed becomes relatively dry (10-15 % water by mass) & metabolism slows down, embryo becomes inactive (DORMANT). It germinates only under favorable conditions (moisture, oxygen, suitable temperature).

Ø PERICARP:- Wall of ovary develops into the wall of Fruit (PERICARP). Fruit may be FLESHY (Guava, Orange, Mango) or DRY (groundnut, mustard). Most fruit develops only from ovary (TRUE eg. Mango); sometimes thalamus or some other part of flower also contribute to fruit formation (FALSE FRUIT eg. Apple, strawberry, cashew); in some plants fruit develops without fertilization (no seeds) they are PARTHENOCARPIC FRUITS eg. Banana.

Ø ADVANTAGES OF SEEDS:-more dependable without water, dispersal strategies, food reserves, protective coat.

Ø Dehydration & Dormancy of mature seeds is crucial for storage & subsequent germination. Oldest available record is that of Lupine (Lupinus arcticus) from arctic Tundra, with a record of 10,000 years of Dormancy.

Ø Recently 2000 year old seeds of Date Palm (Phoenix dactylifera) discovered from King Herod’s palace found viable.

Ø APOMIXIS:- [Production of seeds without fertilization eg. Asteraceae [Grass family]    “A form of asexual reproduction that mimics sexual reproduction”. Methods to develop apomictic seeds:

 (a) diploid egg (2n) formed without meiosis develops into embryo without fertilization

 (b) Nucellar cells start dividing, protrude into embryo sac & develop into embryo (2n) eg. Citrus, Mango. Here ovule contains many embryo (POLYEMBRYONY).

Ø IMPORTANCE OF APOMIXIX IN HYBRID SEED INDUSTRY:- Hybrid Varieties seeds are produced every year as once sown the characters segregate during progeny formation and cannot be maintained in subsequent generations. Production of Hybrid variety is too expensive for farmers. ( if hybrids are made into APOMICTS, no characters will segregate in progeny).

  VIDEOS TO WATCH (Click on the link)

Formation of Male Gametes

Development of male gametophyte in Angiosperms

Ovule & Female Gametophyte

Structure and formation of Embryo Sac
Double Fertilization
Development of Embryo in Angiosperms

















(Courtesy Google images and you tube video links)