Question

Describe the development of the female gametophyte in a flowering plant. How does double fertilization occur?

Short answer

The development of the female gametophyte in a flowering plant begins with the differentiation of a haploid megaspore, which undergoes mitotic divisions to form an eight-nucleate, seven-celled embryo sac. Double fertilization is a unique feature of flowering plants where one sperm nucleus fuses with the egg cell to form a diploid zygote, and another sperm nucleus fuses with the two polar nuclei to form a triploid primary endosperm nucleus. This process contributes to the formation of zygote, endosperm, seed development, and genetic diversity of the species.

Step-by-step solution

Development of the Female Gametophyte in Flowering Plants

The development of the female gametophyte, also known as the embryo sac or megagametophyte, begins with the differentiation of a haploid cell called the megaspore. The megaspore undergoes three mitotic divisions, resulting in an eight-nucleate, seven-celled structure. One large central cell with two polar nuclei and six other smaller cells, which include the egg cell and two synergids (the egg apparatus) at the micropylar end, and three antipodal cells at the chalazal end, are present inside the embryo sac.

Process of Double Fertilization

Double fertilization is a unique feature of flowering plants. It refers to the fusion of one sperm nucleus with the egg cell to form a diploid zygote (2n) and another sperm nucleus with the two polar nuclei in the central cell to form a triploid (3n) primary endosperm nucleus. The process involves the following steps: 1. Pollination: Pollen grains from an anther (male reproductive organ) are transferred to the stigma of the pistil (female reproductive organ) in a process called pollination. 2. Pollen Germination: The pollen grain germinates and forms a pollen tube that grows through the style and passes through the micropyle to reach the embryo sac. 3. Release of Sperm Cells: Once the pollen tube reaches the embryo sac, the tube bursts and releases the two sperm cells inside the embryo sac. 4. Fertilization: One sperm cell fuses with the egg cell to form the diploid zygote (2n), and the other sperm cell fuses with the two polar nuclei to form the triploid primary endosperm nucleus (3n).

Significance of Double Fertilization

The double fertilization process has several advantages and significant roles in flowering plants: 1. Formation of Zygote: Fusion of a sperm nucleus with the egg cell forms a zygote, which eventually develops into an embryo and gives rise to a new plant. 2. Formation of Endosperm: Fusion of a sperm nucleus with the polar nuclei forms the primary endosperm nucleus, which later develops into the endosperm to nourish the growing embryo. 3. Seed Development: After double fertilization, the ovule develops into a seed, and the surrounding ovary develops into a fruit, providing protection and aiding in seed dispersal. 4. Genetic Diversity: Double fertilization enables genetic recombination, which contributes to the genetic diversity of the species and enhances the adaptation ability of plants in their environment.

Key concepts

Double Fertilization

Double fertilization is a remarkable process specific to flowering plants. It involves two separate fusion events that are crucial to the plant's reproductive cycle.
First, let's explore how it happens:

• Pollination initiates the process, where pollen grains land on the stigma of a flower's pistil.
• The pollen grain germinates and a pollen tube grows through the style down towards the ovule.
• As the pollen tube reaches the ovule, it ruptures to release two sperm cells into the embryo sac.
• The first sperm cell fuses with the egg cell, producing a diploid zygote (2n).
• The second sperm cell unites with two polar nuclei to form a triploid primary endosperm nucleus (3n).

This double fertilization process is vital as it gives rise to a zygote capable of developing into a new plant, and a nutritive endosperm that supports embryo growth. This efficient use of resources enhances the success rate of seed development and germination.

Flowering Plants

Flowering plants, also known as angiosperms, are the most diverse group of land plants. These are characterized by having flowers and producing seeds enclosed within a fruit.
Flowering plants exhibit unique reproductive features:

• Flowers are the reproductive structures that attract pollinators or facilitate wind pollination.
• They contain both male (stamens) and female (pistils) reproductive organs.
• Post-fertilization, the ovule develops into a seed, while the ovary often matures into a fruit.

The evolution of flowering plants revolutionized ecological systems by providing a greater diversity of habitable environments and food sources for a myriad of life forms. Their intricate reproductive processes, such as double fertilization, allowed them to adapt efficiently to diverse environments.

Embryo Sac

The embryo sac, also known as the female gametophyte or megagametophyte, plays a central role in the reproduction of flowering plants. It originates from a single megaspore within the ovule.
Through successive mitotic divisions, the megaspore gives rise to an eight-nucleate structure arranged into seven cells:

• One central cell with two polar nuclei.
• Three antipodal cells located at the chalazal end.
• An egg apparatus at the micropylar end, consisting of one egg cell and two synergids.

The embryo sac serves as the site where fertilization and subsequent zygote development occur. It supports the arrival and function of sperm cells and is crucial in forming both the future plant (through the zygote) and its initial nutrient supply (the endosperm).

Pollination Process

Pollination is the initial step in the reproduction of flowering plants, setting the stage for double fertilization. Successful pollination ensures the transfer of pollen from the male anther to the female stigma.
There are various means by which pollination occurs:

• Biotic Pollination: Pollinators like bees, birds, and butterflies carry pollen between flowers, enabling genetic diversity.
• Abiotic Pollination: Wind and water can also transfer pollen, especially in plants with less showy flowers.

Once pollen lands on a compatible stigma, it germinates, with each pollen grain forming a tube that travels down the style and penetrates the ovule's micropyle. This is crucial for delivering sperm cells to the embryo sac, thus initiating the fertilization processes that follow. Pollination is not just about plant reproduction but holds ecological importance for the biodiversity and sustainability of many ecosystems.