Pre Fertilisation Structures and Events

Pre-Fertilisation: Structure and events-

In the flowering plant male and female structures i.e. androecium and gynoecium differentiate and develop.

Androecium:

The androecium is the male reproductive part of a flower and is composed of stamens. The stamens are responsible for producing pollen grains, which contain the male gametes. The structure of the androecium can be described as follows:

  • Filament:

The filament is a slender, elongated stalk-like structure that supports the anther. It is usually thin and flexible, allowing the anther to be positioned for effective pollen dispersal. The filament is typically composed of elongated cells and provides structural support to the anther.

  • Anther

The anther is the terminal part of the stamen and is responsible for the production and release of pollen grains. Generally angiosperms consist of two lobes (bilobed) each lobe has two theca (dithecous) which are attached to the filament by connective tissue. Each lobe of the anther contains pollen sacs or microsporangia, where pollen grains are produced through meiosis.

The transverse section of a young anther reveals distinct layers that play important roles in the development and functioning of the anther. The layers and their functions.

 

 

1. Epidermis:

The outermost layer of the anther is the epidermis. It provides protection to the underlying tissues and helps prevent water loss. The epidermis is typically a single layer of cells.

 

2. Endothecium:

The endothecium is the layer located just beneath the epidermis. It consists of several layers of cells that have thickened walls. The cells of the endothecium play a role in the dehiscence (splitting open) of the anther during pollen release.

 

3. Middle Layer:

The middle layer is located below the endothecium. It consists of several layers of cells that have irregular shapes and varying sizes. The middle layer cells contribute to the nutrition of the developing pollen grains.

 

4.Tapetum:

The tapetum is the innermost layer of the anther. It is a specialised layer of cells that possess dense cytoplasm that surround the developing microspores and pollen grains. The tapetum provides nourishment and metabolic support to the developing pollen grains by supplying nutrients and enzymes required for pollen wall formation, pollen maturation, and pollen viability.

 

 

5. Microspore Mother Cells (MMC):

The microspore mother cells, also known as microsporocytes/ pollen mother cells, are located within the tapetum. They undergo meiosis to produce haploid microspores. These microspores are the precursors to pollen grains.

 

6. Pollen Sac:

The pollen sacs are the cavities or chambers within the anther where the microsporangia (containing the microspore mother cells) are located. Each pollen sac typically contains multiple microsporangia, and collectively they produce a large number of pollen grains.

 

7. Pollen Grain:

Pollen grains are the mature male gametophytes of flowering plants. Each pollen grain consists of a protective outer layer called the exine and an inner layer called the intine. The exine may have distinctive patterns, sculpturing, or spines, which vary among different plant species and can aid in species identification.

 

8.Connective:

The connective is a tissue that connects the two lobes of the anther and attaches them to the filament. It also plays a role in supporting the anther and allowing its movement during pollen dispersal.

 

Microsporogenesis and Pollen Grain Formation in the Anther

 

Microsporogenesis is the process through which the microspores, which are the precursors of pollen grains, are formed within the anther. The anther undergoes specific developmental stages from its early phase to maturity, leading to the production of dehydrated pollen grains. 

 

1. Early Development:

During the early stages of anther development, the anther contains homogeneous cells called sporogenous tissues, which occupy the centre of each microsporangium. These sporogenous cells are responsible for undergoing meiotic divisions to produce microspores.

 

2. Microsporocyte (Microspore Mother Cell) Formation:

Within the sporogenous tissues, certain cells differentiate into microsporocytes or microspore mother cells (MMCs). These microsporocytes are diploid (2n) cells that undergo meiosis to produce haploid (n) microspores. The process of meiosis results in the formation of microspore tetrads.

 

3. Meiosis and Microspore Tetrad Formation:

Meiosis is a type of cell division that involves two rounds of division, resulting in the reduction of chromosome number. In the case of microsporogenesis, the microsporocytes undergo meiotic division, resulting in the formation of four haploid microspores. These microspores often remain connected, forming a structure called a microspore tetrad.

 

 

4. Callose and Pollen Wall Formation:

Following meiosis, the microspores are enclosed within the callose wall, which is a temporary protective covering. The callose wall helps maintain the integrity of the microspore tetrad. Eventually, the callose wall breaks down, and the individual microspores are released.

 

5. Pollen Grain Formation:

As the anther matures and dehydrates, the individual microspores undergo changes and differentiation to form mature pollen grains. 

Pollen grains have a variety of architecture, including sizes, shapes, colours, and designs, which differ among species.

 

 

Structure of Pollen Grains:

  • Pollen grains are generally spherical and measure about 25-50 micrometres in diameter.
  • They possess a prominent two-layered wall.
  • The hard outer layer is called the exine, which is composed of sporopollenin.
  • Sporopollenin is one of the most resistant organic materials known, capable of withstanding high temperatures, strong acids, and alkalis.
  • Sporopollenin is not degraded by any known enzyme, making pollen grains well-preserved as fossils.
  •   The exine of pollen grains exhibits a fascinating array of patterns and designs.
  •  Germ pores are present on the exine, which are areas where sporopollenin is absent.
  •  The inner wall of the pollen grain is called the intine.

 The intine is a thin and continuous layer composed of cellulose and pectin.

 The cytoplasm of the pollen grain is surrounded by a plasma membrane.

 A mature pollen grain contains two cells: the vegetative cell and the generative cell.

○ The vegetative cell is larger, has abundant food reserves, and contains a large, irregularly shaped nucleus.

 The generative cell is smaller and floats in the cytoplasm of the vegetative cell.

  The generative cell is spindle-shaped with dense cytoplasm and a nucleus.

  An over 60% of angiosperms, pollen grains are shed at the 2-celled stage (containing the vegetative and generative cells).

 In the remaining species, the generative cell divides mitotically to produce two male gametes before pollen grains are shed (3-celled stage).

 

Functions of the Exine:

 The exine's hardness provides protection to the delicate internal structures of the pollen grain.

 It acts as a barrier against harsh environmental conditions, ensuring the survival and successful transfer of pollen.

Function of Germ Pores:

  Germ pores are prominent apertures on the exine where sporopollenin is absent.

 They serve as entry points for pollen tubes during pollination.

 Pollen tubes are structures that allow the transport of male gametes (sperm cells) to the female reproductive organs of plants.

 

 

Pollen Grains: Allergies and Nutritional Value

Allergies and Health Effects

     Pollen grains from various species can cause severe allergies and bronchial afflictions in some individuals.

     These allergies can lead to chronic respiratory disorders such as asthma and bronchitis.

     Parthenium or carrot grass, originally introduced to India as a contaminant with imported wheat, has become widespread and causes pollen allergies.

     Cheropodium, amaranthus, parthenium, and sorghum are commonly associated with hay fever.

     Allergic reactions to pollen can vary in severity and may include symptoms like sneezing, coughing, watery eyes, and difficulty breathing.

     Individuals with pollen allergies are advised to take preventive measures during high pollen seasons, such as staying indoors or wearing protective masks. 

Nutritional Value and Supplement Usage

     Pollen grains are rich in nutrients, which has led to the use of pollen tablets as food supplements.

     In recent years, the consumption of pollen products in the form of tablets and syrups has gained popularity, particularly in Western countries.

     Pollen consumption has been claimed to enhance the performance of athletes and racehorses.

     Pollen grains contain proteins, vitamins, minerals, enzymes, amino acids, and antioxidants.

     The specific nutrient composition of pollen can vary depending on the plant species.

     Pollen supplements are often marketed as a source of natural energy and beneficial for overall health.

     However, scientific evidence regarding the effectiveness and specific benefits of pollen supplements is limited.

     It is recommended to consult with a healthcare professional before incorporating pollen products into the diet, especially for individuals with known allergies or medical conditions.

     Viability of Pollen Grains:

     The viability of pollen grains, referring to their ability to germinate and fertilise, varies among different plant species.

     The duration of pollen viability is influenced by factors such as temperature, humidity, and the plant family to which the species belongs.

     In some cereals like rice and wheat, pollen grains lose viability within approximately 30 minutes after their release.

     However, in certain plant families such as Rosaceae, Leguminosae, and Solanaceae, pollen grains can maintain viability for several months.

     Storage of Pollen Grains:

     Pollen grains can be stored for extended periods using cryopreservation techniques.

     Cryopreservation is a method used to store biological materials, including pollen, at extremely low temperatures to maintain their viability.

     Liquid nitrogen (-196°C) is commonly employed to preserve pollen grains for years.

     The temperature and humidity conditions required for optimal pollen storage may vary among species and are typically specific to each particular plant.

     The ability to store pollen grains in liquid nitrogen allows for the establishment of pollen banks, similar to seed banks, in crop breeding programs.

     Pollen banks serve as repositories for diverse pollen samples, enabling researchers to access genetic resources for future breeding efforts.

     Cryopreserved pollen can be thawed and used to cross-pollinate plants, facilitating the transfer of desirable traits and genetic diversity.

     The successful storage and revival of pollen from various species have contributed to advancements in crop improvement and conservation efforts.

6. Pollen Dispersal:

Once the anther matures and dehydrates, the pollen grains are released from the anther. The dehiscence of the anther allows the pollen grains to disperse into the environment, increasing the chances of pollination and successful fertilisation.

 

2.    Gynoecium:

     Gynoecium is the female reproductive part of a flower, also referred to as the pistil.

     It is composed of one or more carpels, which are the structural units of the gynoecium.

     The gynoecium can be further classified into two types based on the number of carpels present:

     Mono-carpellary: In a mono carpellary gynoecium, the pistil consists of a single carpel. The carpel may have a single stigma, a style, and an ovary containing one or more ovules. Examples of flowers with mono carpellary gynoecium include pea (Pisum sativum) and bean (Phaseolus vulgaris).

     Multicarpellary: In a multicarpellary gynoecium, the pistil consists of two or more fused carpels. The carpels can be partially fused or completely fused into a single structure. The number of stigmas, styles, and ovary chambers will depend on the specific arrangement of the carpels. Examples of flowers with multicarpellary gynoecium include rose (Rosa spp.) and apple (Malus domestica).

     Multicarpellary gynoecium can further be classified into the following types:

     Apocarpous: In an apocarpous gynoecium, the carpels are separate and not fused together. Each carpel has its own stigma, style, and ovary chamber. Examples of flowers with apocarpous gynoecium include buttercup (Ranunculus spp.) and strawberry (Fragaria spp.).

     Syncarpous: In a syncarpous gynoecium, the carpels are fused together, forming a single structure. The number of stigma lobes, styles, and ovary chambers can vary depending on the degree of fusion. Examples of flowers with syncarpous gynoecium include tomato (Solanum lycopersicum) and lily (Lilium spp.).

 

     The gynoecium is situated in the centre of the flower, often surrounded by the male reproductive structures, such as stamens.

     The gynoecium typically consists of three main parts:

          -Stigma: The stigma is the receptive surface located at the top of the pistil. Its function is to receive and recognize pollen grains during pollination.

         - Style: The style is a slender, elongated structure that connects the   stigma to the ovary. It provides a pathway for the pollen tubes to grow down towards the ovary for fertilisation.

          - Ovary:

     The ovary is the enlarged basal part of the pistil. It contains one or more ovules, which are structures that contain the female gametes (eggs). The ovary is responsible for protecting the ovules and later developing into a fruit after successful fertilisation.

     Ovarian Cavity (Locule): Inside the ovary, there is a hollow space called the ovarian cavity or locule. The locule provides a protected environment for the development of ovules.

     Placenta: The placenta is a tissue or region within the ovarian cavity. It is responsible for nourishing and supporting the developing ovules.

 

 

     Ovules: Ovules are megasporangia that develop within the ovary. They are responsible for producing and nurturing the female gametes (eggs). The number of ovules present in an ovary can vary.

The number of ovules in an ovary can range from one to many, depending on the plant species. Examples include wheat, paddy, and mango (with one ovule), and papaya, watermelon, and orchids (with many ovules).

The structure of a typical angiosperm ovule consists of several key parts that play important roles in the process of sexual reproduction and seed development. Here is a description of the main components of a typical angiosperm ovule. 

     Integuments: The integuments are protective layers surrounding the central region of the ovule. They provide physical protection to the developing structures inside. Typically, there are two integuments, an inner and an outer integument.

     Micropyle: The micropyle is a small opening or pore present in the integuments near the apex of the ovule. It serves as the entry point for the pollen tube during fertilisation.

     Hilum: The hilum is the point of attachment of the ovule to the ovary wall. It is the region where the funicle is connected to the ovule.

     Funicle: The funicle is a stalk-like structure that connects the ovule to the placenta within the ovary. It provides support and acts as a conduit, supplying nutrients to the developing ovule.

     Chalaza: The chalaza is the region at the base of the ovule where the integuments and the nucellus join together. It serves as an anchor point for the ovule within the ovary.

     Nucellus: The nucellus is the central part of the ovule. It is a mass of tissue that contains several important structures.

Enclosed within the integuments, which are protective layers, is a mass of cells known as the nucellus. The cells of the nucellus contain abundant reserve food materials, which provide nourishment for the developing embryo.

 

 

 

Within the nucellus, the embryo sac, also referred to as the female gametophyte, is located.

     Megaspore Mother Cell: Within the nucellus, the megaspore mother cell undergoes meiosis, resulting in the formation of four megaspores. Typically, three of the megaspores degenerate, and only one functional megaspore survives.

     Megasporogenesis:

     Megasporogenesis refers to the process of formation of megaspores from the megaspore mother cell (MMC).

     In ovules, a single MMC is differentiated in the micropylar region of the nucellus.

     The MMC is a large cell with dense cytoplasm and a prominent nucleus.

     The MMC undergoes meiotic division, which is the process of cell division that reduces the chromosome number by half.

     The importance of MMC undergoing meiosis is the production of four megaspores.

     Female gametophyte:

     In most flowering plants, only one of the four megaspores produced during meiosis is functional, while the other three degenerate.

     The functional megaspore develops into the female gametophyte, also known as the embryo sac.

     This process of embryo sac formation from a single megaspore is called monosporic development.

     The nucleus of the functional megaspore undergoes mitotic division to form two nuclei, which move to opposite poles, resulting in the 2-nucleate embryo sac.

     Sequential mitotic divisions lead to the formation of the 4-nucleate and 8-nucleate stages of the embryo sac.

     These mitotic divisions are free nuclear, meaning they are not immediately followed by cell wall formation.

     After the 8-nucleate stage, cell walls are formed, organising the typical female gametophyte or embryo sac.

     Six of the eight nuclei are surrounded by cell walls and form cells, while the other two nuclei, called polar nuclei, are situated in the large central cell.

     The embryo sac has a characteristic cell distribution: three cells form the egg apparatus at the micropylar end, consisting of two synergids and one egg cell.

     The egg cell is the female gamete responsible for fertilisation. It is located at the base of the embryo sac and is involved in the fusion with the male gamete (sperm cell) during double fertilisation.

     The synergids have cellular thickenings called filiform apparatus, which guide pollen tubes into the synergid.

     At the chalazal end, three cells called antipodals are present.Their function is not fully understood, but they may play a role in nourishment or have a regulatory role during development.

     The large central cell contains two polar nuclei.

     A mature angiosperm embryo sac is 8-nucleate but consists of 7 cells.