Plant Reproduction: Fertilization & Embryo Development
Prefertilization: Structure and Events
The Flower
Hormonal and structural changes in plants lead to the development of the flower.
The androecium consists of a whorl of stamens representing the male sex organ.
The gynoecium represents the female reproductive organ.
Stamen, Microsporangium, and Pollen Grain
A typical stamen consists of two parts:
- A long and slender stalk called the filament
- A terminal bilobed structure called the anther
A typical angiosperm anther is bilobed.
Each lobe has two thecae (dithecous).
Each anther contains four microsporangia located at the corners, two in each lobe.
Microsporangia become pollen sacs and are packed with pollen grains.
Structure of Microsporangium
Each microsporangium is surrounded by four wall layers:
- Epidermis
- Endothecium
- Middle layer
- Tapetum
The innermost layer is the tapetum, which is multinucleated with dense cytoplasm; it nourishes the developing pollen grain.
The centers of each microsporangium contain homogenous cells called sporogenous tissue.
Microsporogenesis
The process of formation of microspores from pollen mother cells through meiosis is called microsporogenesis.
The sporogenous tissue of the microsporangium differentiates into microspore mother cells (or pollen mother cells).
Each microspore mother cell undergoes meiosis and gives rise to a haploid microspore tetrad.
On dehydration, the microspore tetrad dissociates to form four microspores.
Each microspore develops into a pollen grain.
Pollen Grain
The pollen grain represents the male gametophytes.
It is spherical and measures about 25-50 micrometers in diameter.
It is covered by two layers:
- The hard outer layer called the exine is made up of sporopollenin, which is one of the most resistant organic materials known. It can withstand high temperatures and strong acids and alkali. No enzyme has been found that can degrade sporopollenin so far.
- The exine has prominent apertures called germ pores where sporopollenin is absent.
- The inner wall of the pollen grain is called the intine. It is a thin and continuous layer made of cellulose and pectin.
On maturity, the pollen grain contains two cells:
- The vegetative cell: Larger, has abundant food reserves, and has a large, irregularly shaped nucleus.
- The generative cell: Small and floats in the cytoplasm of the vegetative cell.
In 60% of angiosperms, pollen grains are shed at this 2-celled stage.
In others, the generative cell divides mitotically to form two male gametes before the pollen grains are shed (3-celled stage).
Economic Importance of Pollen Grains
- Pollen grains may cause severe allergies and bronchial afflictions.
- They may cause chronic respiratory disorders such as asthma, bronchitis, etc.
- Pollen grains of Parthenium (or carrot grass) cause pollen allergies.
- Pollen grains are rich in nutrients and are used as pollen tablets for food supplements.
- Pollen consumption increases the performance of athletes and racehorses.
- After shedding, the viability of pollen depends on temperature and humidity.
- In wheat and rice, pollen grains lose viability within 30 minutes of their release.
- In Rosaceae, Leguminosae, and Solanaceae, they remain viable for months.
- Pollen grains can be preserved for years in liquid nitrogen (-196°C).
The Pistil, Megasporangium (Ovule), and Embryo Sac
The gynoecium represents the female reproductive part of the flower.
The gynoecium may contain a single pistil (monocarpellary) or may have more than one pistil (multicarpellary).
Fused pistils are called syncarpous, and free pistils are called apocarpous.
Each pistil has three parts:
- The stigma
- The style
- The ovary
Inside the ovary is the ovarian cavity (locule).
The placenta is located inside the ovarian cavity.
Megasporangia (or ovules) arise from the placenta.
The number of ovules inside the ovary may be single or many.
The Megasporangium (Ovule)
The ovule is a small structure attached to the placenta of the locule with a stalk called the funicle.
The body of the ovule is fused with the funicle in a region called the hilum.
The hilum is the junction between the funicle and the ovule.
Each ovule has one or two protective envelopes called integuments.
The integuments cover the ovule except for an opening at the top called the micropyle.
Opposite the micropylar end is the chalaza, representing the basal part of the ovule.
Megasporogenesis
The process of formation of megaspores from the megaspore mother cell is called megasporogenesis.
In the center of the ovule, there is a mass of tissue called the nucellus.
Cells of the nucellus have abundant reserve food materials.
One cell of the nucellus towards the micropylar end differentiates into the megaspore mother cell (MMC).
It is a large diploid cell with dense cytoplasm and a prominent nucleus.
The MMC undergoes meiotic division, resulting in four haploid megaspores.
Female Gametophyte
Out of the four megaspores, one megaspore is functional, and the other three degenerate.
The functional megaspore develops into the female gametophyte.
The female gametophyte is known as the embryo sac.
Development of the embryo sac from a single megaspore is called the monosporic type of embryo sac.
The nucleus of the functional megaspore divides by mitotic division to form two nuclei, which move to opposite poles, creating a 2-nucleated embryo sac.
Two successive mitotic divisions lead to the formation of 4-nucleate and later 8-nucleate stages of the embryo sac.
All mitotic divisions are of the free nuclear type; karyokinesis is not followed by cytokinesis.
Six of the eight nuclei are surrounded by cell walls and organized into cells.
Three cells are grouped together at the micropylar end, constituting the egg apparatus.
The egg apparatus, in turn, consists of two synergids and one egg cell.
Synergids have special filiform apparatus, which play an important role in guiding the entry of the pollen tube into the synergids.
Three cells arranged towards the chalazal end are called antipodal cells.
The large central cell has two polar nuclei.
A typical angiosperm embryo sac at maturity is 8-nucleated and 7-celled.
Pollination
Transfer of pollen grains from the anther to the stigma of a pistil is termed as pollination.
Both male and female gametes are non-motile.
Kinds of Pollination
Autogamy
Pollination within the same flower.
In open and exposed anthers and stigma, autogamy is rare.
Viola, Oxalis, and Commelina produce two types of flowers:
- Chasmogamous: Exposed anther and stigma
- Cleistogamous: Closed anther and stigma
Cleistogamous flowers are invariably autogamous and ensure seed set even in the absence of a pollinator.
Geitonogamy
Pollination between two flowers of the same plant.
- Pollination by a pollinating agent.
- Genetically similar to autogamy.
Xenogamy
Transfer of pollen grains from the anther to the stigma of a different plant.
- It is commonly called cross-pollination.
- It brings genetically different types of pollen grains to the stigma.
Agents of Pollination
Plants use two abiotic agents (wind and water) for pollination.
One biotic agent for pollination is animals.
The majority of plants use biotic agents for pollination.
Few plants use abiotic pollinating agents.
Anemophily
Pollinating agent is wind.
Plants produce an enormous amount of pollen compared to the number of ovules available for pollination to compensate for the uncertainties of pollination.
Flowers have well-exposed stamens.
Large, feathery stigma to trap air-borne pollen grains.
Most wind-pollinated flowers contain a single ovule in one ovary, and numerous flowers are packed into an inflorescence (e.g., corn cob).
Pollen grains are light and non-sticky.
Hydrophily
Pollination by abiotic agents like water.
This type of pollination is very rare, occurring in about 30 genera, mostly monocots.
Vallisneria, Hydrilla, and Zostera are common examples of hydrophily.
Not all aquatic plants use hydrophily.
Pollen grains are released onto the surface of the water and carried to the stigma by air currents, as in Vallisneria.
In seagrass, the flowers remain submerged.
Pollen grains are long, ribbon-like, and carried passively inside the water.
Pollen grains are protected from wetting by a mucilaginous covering.
Pollination by Biotic Agents
The majority of flowering plants use a range of animals as pollinating agents.
Among animals, insects, particularly bees, are the dominant biotic agents for pollination.
Insect-pollinated flowers are large, colorful, fragrant, and rich in nectar.
Small flowers are present in clusters to make them conspicuous.
Flowers pollinated by flies and beetles secrete foul odors.
Nectar and pollen grains are the usual floral rewards for insects.
In some species, floral rewards provide safe places to lay eggs (e.g., Amorphophallus).
A species of moth and the Yucca plant cannot complete their lifecycle without each other.
The moth deposits its eggs in the locule of the ovary, and the flower, in turn, gets pollinated by the moth.
Many insects may consume pollen or nectar without bringing about pollination. Such floral visitors are referred to as pollen/nectar robbers.
Outbreeding Devices
The majority of flowering plants produce hermaphrodite flowers and undergo autogamy.
Continuous autogamy (or self-pollination) results in inbreeding depression.
Flowering plants have developed many devices to avoid self-pollination and encourage cross-pollination. These devices are called outbreeding devices.
- Pollen release and stigma receptivity are not synchronized.
- Spatial separation of anthers and stigma.
- Anther and stigma are placed in different positions.
- Self-incompatibility.
- Production of unisexual flowers.
Pollen-Pistil Interaction
All the events—from pollen deposition on the stigma until the pollen tube enters the ovule—are together referred to as pollen-pistil interaction.
Pollination does not guarantee the transfer of the right type of pollen grain to the right type of stigma.
The pistil has the ability to recognize the pollen, whether it is compatible or incompatible.
If it is the right type, the stigma allows the pollen to germinate.
If it is the wrong type, the stigma rejects the pollen, preventing germination.
The ability of the pistil to recognize the pollen is mediated by a continuous dialogue facilitated by chemicals like boron, inositol, and sucrose levels.
Following compatible pollination, the pollen grain produces a pollen tube through one of the germ pores.
The contents of the pollen grain move into the pollen tube.
The pollen tube grows through the tissues of the stigma and style and reaches the ovary.
Artificial Hybridization
Artificial hybridization is a technique used for crop improvement by ensuring only desired pollen is used.
It involves emasculation (removal of anthers) and bagging to prevent contamination.
Double Fertilization
Double fertilization is a unique characteristic of angiosperms.
It involves two fertilization events:
- One male gamete fuses with the egg cell (syngamy).
- The other male gamete fuses with the secondary nucleus (triple fusion).
These events lead to the formation of the zygote (2n) and the endosperm (3n).
Post-Fertilization Events
Post-fertilization events involve the development of the endosperm, embryo, and maturation of the ovule into a seed and the ovary into a fruit.
Endosperm
The endosperm is a triploid tissue that provides nutrition for the developing embryo.
It may be consumed during embryo development or be persistent in the mature seed and utilized during seed germination.
Embryo Development
Embryos in dicots and monocots develop differently.
Dicots
Dicot embryos have two cotyledons, an epicotyl (plumule), and a hypocotyl (radicle).
Monocots
Monocot embryos have one cotyledon (scutellum), with specialized structures like coleorhiza and coleoptile.
Seed
The seed is the final product of sexual reproduction in angiosperms.
It consists of a seed coat, cotyledon(s), and the embryo axis.
Seeds may be albuminous (retain endosperm) or non-albuminous (endosperm is consumed during embryo development).
Apomixis and Polyembryony
Apomixis
Apomixis is a form of asexual reproduction that mimics sexual reproduction, where seeds are formed without fertilization.
Polyembryony
Polyembryony is the occurrence of multiple embryos in one ovule.
It is common in Citrus and Mango.