Reproduction - Sexual Reproduction in Flowering Plants - Entry of Pollen Tube into Ovule

• Sexual reproduction involves the fusion of male and female reproductive structures.
• In flowering plants, the male reproductive structure is the stamen, which consists of anther and filament.
• The female reproductive structure is the pistil, which consists of stigma, style, and ovary.
• Pollination is the transfer of pollen grains from anther to stigma.
• After pollination, the pollen grain germinates to form a pollen tube.

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Pollen Tube Growth and Entry into Ovule

• After landing on the stigma, the pollen grain hydrates and germinates.
• The pollen tube grows out from the pollen grain through the style towards the ovary.
• The tip of the pollen tube is known as the pollen tube apex.
• The pollen tube apex secretes enzymes that help in the penetration of ovule.
• The growth of the pollen tube is guided by chemical signals produced by the ovule.

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Growth of Pollen Tube through the Style

• The elongation of the pollen tube is driven by the growth of its tip.
• The pollen tube is composed of a thin wall and a dense cytoplasmic contents.
• As the pollen tube grows, it passes through the transmitting tissue in the style.
• The transmitting tissue provides nourishment to the pollen tube.

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Guidance of Pollen Tube by Chemical Signals

• Chemical signals guide the pollen tube towards the ovule.
• These signals are secreted by the synergids, located in the embryo sac of the ovule.
• The pollen tube follows the concentration gradient of these signals.
• The guidance of the pollen tube ensures its entry into the ovule.

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Entry of Pollen Tube into Ovule

• Once the pollen tube reaches the micropyle, it enters the ovule.
• The micropyle is a small opening in the integuments, the outer covering of the ovule.
• The pollen tube releases the male gametes into the embryo sac through the micropyle.
• The male gametes fertilize the egg cell and the central cell to form the zygote and endosperm, respectively.

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Summary

• Sexual reproduction in flowering plants involves the fusion of male and female reproductive structures.
• The pollen tube grows from the pollen grain after pollination.
• The pollen tube grows through the style with the help of chemical signals.
• The pollen tube ultimately enters the ovule through the micropyle.
• The male gametes are released into the embryo sac and fertilize the egg and central cell.

Factors Influencing Pollen Tube Growth

• Several factors can influence the growth and entry of pollen tubes into the ovule.
• Temperature: Optimal temperature conditions are required for successful pollen tube growth.
• Moisture: Proper moisture levels are necessary for hydration and germination of pollen grains.
• pH Level: The pH of the pistil affects the growth and guidance of the pollen tube.
• Nutrients: The availability of nutrients in the pistil nourishes the pollen tube.
• Pollen-Specific Factors: Different plants may have specific requirements for successful pollen tube growth.

Pollen Tube Competition

• In plants with multiple pistils, pollen tubes from different pollen grains may compete for ovule entry.
• Competition can occur due to limited space or limited resources within the pistil.
• The pollen tube that grows faster or is more effective in overcoming barriers will have an advantage.
• Competition between pollen tubes can lead to selective fertilization and ensure the survival of the fittest.

Self-Incompatibility Mechanism

• Some plants have evolved self-incompatibility mechanisms to prevent self-fertilization.
• This mechanism ensures that the plant only receives pollen from other individuals, promoting genetic diversity.
• Self-incompatibility can be determined by specific proteins present in the stigma.
• These proteins recognize and interact with pollen grains carrying matching alleles, hindering their growth and entry into the ovule.

Fertilization in the Embryo Sac

• Inside the embryo sac, the pollen tube releases the male gametes.
• One male gamete fuses with the egg cell to form a zygote, which develops into the embryo.
• The other male gamete fuses with the two polar nuclei of the central cell to form the endosperm.
• This double fertilization is a unique characteristic of flowering plants.

Double Fertilization

• Double fertilization involves the fusion of two gametes with different nuclei in the embryo sac.
• One sperm nucleus fuses with the egg cell to form the zygote (2n), which develops into the embryo.
• The other sperm nucleus fuses with the two polar nuclei to form the primary endosperm nucleus (3n).
• The primary endosperm nucleus undergoes further divisions to develop into the endosperm, providing nourishment to the growing embryo.

Significance of Double Fertilization

• Double fertilization ensures that the developing embryo has a source of nutrition.
• The endosperm, derived from the fusion of the male gamete with the polar nuclei, provides nourishment to the developing embryo.
• This efficient use of resources increases the chances of successful seed germination and plant growth.
• Double fertilization also promotes genetic diversity through the combination of genetic material from both parents.

Post-Fertilization Events

• After fertilization, the ovule develops into a seed.
• The zygote undergoes further divisions and differentiations to form the embryo.
• The endosperm also develops further and stores reserves of nutrients.
• The ovary wall develops into the fruit, protecting and facilitating the dispersal of the seeds.
• Meanwhile, other parts, such as the stigma and anther, wither and may eventually fall off.

Study Material: Pollination and Fertilization

• To understand the concepts of pollination and fertilization better, refer to the recommended textbook chapter on the topic.
• Review the diagrams and illustrations provided in the textbook to visualize the process.
• Make sure to clarify any doubts or questions you may have with your teacher or classmates.
• Practice solving related questions and diagrams from previous board examination papers.
• Additionally, refer to online resources or video lectures for additional study material and interactive learning.

Example: Cross Pollination in Corn (Maize)

• Corn plants rely on wind for pollination, making them an example of cross-pollination.
• The tassels at the top of the plant produce pollen grains, while the silks on the ears of corn are stigmas.
• The pollen grains are released into the air and carried by wind to nearby silks.
• The pollen tubes grow down the stylus, enter the ovule, and fertilize the eggs.
• This process ensures genetic diversity by preventing self-fertilization.

Equation: Fertilization in Flowers

• Fertilization in flowers can be represented by the equation:

Zygote (2n) + Primary Endosperm Nucleus (3n) → Embryo (2n) + Endosperm (3n)

• This equation highlights the fusion of male and female gametes during double fertilization.
• The resulting embryo (2n) develops into a new plant, while the endosperm (3n) provides nourishment for the growing embryo.
• Double fertilization ensures the survival and growth of the next generation of flowering plants.

Factors Affecting Pollen Tube Growth

• Temperature: Optimal temperature conditions are required for pollen tube growth and development.
• Moisture: Proper hydration and availability of water are crucial for successful pollen tube growth.
• Oxygen: Sufficient oxygen levels are necessary for aerobic respiration in the growing pollen tube.
• Nutrients: Availability of essential nutrients, such as sugars and minerals, supports pollen tube growth.
• pH Level: The pH of the pistil affects enzyme activities and pollen tube growth.

Microscopic Structure of Pollen Tube

• Pollen tubes are thin, elongated structures that grow from the exine of the pollen grain.
• The primary wall of the pollen tube comprises cellulose and pectin.
• Callose, a β-1,3-glucan, reinforces the tube wall during growth.
• Pollen tubes have a dense cytoplasmic content with abundant organelles, including mitochondria and endoplasmic reticulum.
• The tube apex, responsible for growth and guidance, contains a specialized cytoplasm and secretory vesicles.

Growth Mechanism of Pollen Tube

• The growth of pollen tubes occurs through a process called tip growth.
• Tip growth involves continuous deposition of new plasma membrane and cell wall material at the apex.
• The deposition of new material pushes the cytoplasm forward, causing the tube to elongate.
• The cytoplasmic streaming and the dynamic arrangement of actin filaments are also involved in tip growth.
• Calcium ions play a crucial role in regulating pollen tube growth and guidance.

Role of Chemicals in Pollen Tube Guidance

• The ovule produces guidance signals that guide the pollen tube towards the embryo sac.
• Various chemical signals, such as peptides and small molecules, are involved in this process.
• These signals establish concentration gradients that guide the growth of the pollen tube.
• Chemotropism is the directional growth response of the pollen tube towards these chemical signals.
• The precise mechanism of pollen tube guidance is still an area of active research.

Self-Incompatibility Mechanisms

• Self-incompatibility prevents self-fertilization in flowering plants.
• It ensures the exchange of genetic material between different individuals, promoting genetic diversity.
• Plants have evolved multiple mechanisms for self-incompatibility, including gametophytic and sporophytic self-incompatibility.
• These mechanisms involve the recognition and rejection of self-pollen through genetic and biochemical interactions.
• Self-incompatibility mechanisms vary among different plant species and can be controlled by multiple genes.

Mechanisms of Pollen Tube Attraction

• In certain plant species, the female can attract and guide specific pollen tubes for fertilization.
• The female reproductive structures release attractant molecules known as LUREs.
• LUREs attract pollen tubes with matching receptor proteins, ensuring successful fertilization.
• This mechanism ensures that only compatible pollen tubes are attracted and allowed to enter the ovule.
• The specific interactions between LUREs and receptor proteins are essential for successful reproduction.

Role of Pollen Tube in Plant Breeding

• Pollen tube growth and entry into the ovule are crucial in plant breeding programs.
• Plant breeders use controlled pollination techniques to transfer desirable traits from one plant to another.
• They control the fertilization process by selectively transferring pollen from a desired parent to the stigma of another parent.
• By manipulating the pollination process, breeders can create new plant varieties with improved traits.
• The successful growth and guidance of pollen tubes ensure the transfer of genetic material to produce desired offspring.

Importance of Reproduction in Flowering Plants

• Sexual reproduction in flowering plants is significant for several reasons.
• It promotes genetic diversity, which is essential for the adaptation and survival of plant populations.
• Sexual reproduction allows for the recombination and exchange of genetic material, leading to variations in traits.
• It ensures the formation of seeds and fruits, which contribute to the dispersal and propagation of plants.
• Sexual reproduction enables evolution and the development of new plant species through natural selection and genetic changes.

Overview: Sexual Reproduction in Flowering Plants

• Sexual reproduction in flowering plants involves the fusion of pollen and ovule.
• Pollination is the transfer of pollen from the anther to the stigma.
• The pollen tube grows from the pollen grain towards the ovule, guided by chemical signals.
• The successful entry of the pollen tube into the ovule leads to double fertilization.
• Double fertilization results in the formation of the zygote and endosperm, ensuring the development of the embryo and nourishment of the growing plant.

Review Questions

1. What is the role of the pollen tube apex in the growth and guidance of the pollen tube?
2. Explain the process of double fertilization in flowering plants.
3. How do chemical signals guide the pollen tube towards the ovule?
4. Describe the factors that can influence the growth of pollen tubes.
5. What are the mechanisms of self-incompatibility in flowering plants?
6. How can pollen tube growth be manipulated in plant breeding programs?
7. Why is sexual reproduction important in flowering plants?
8. What are the advantages of double fertilization in flowering plants?
9. How does pollen tube competition occur, and what are its implications?
10. Explain the process of tip growth and the components involved in pollen tube growth.