Plant Hormones And Their Functions Detailed Point-wise Notes

“Plant Hormones and Their Functions” is an important topic in General Science Biology. In this article, we will discuss the details of major plant hormones and their functions.

Many competitive exams frequently ask about the uses of hormones, their full forms, and other important characteristics. To help you prepare for this topic, take a look at this post where I provide detailed information about plant hormones.

Plant Hormones: The Chemical Messengers of the Plant Kingdom

• Plants, like humans, have a complex internal communication system that regulates their growth, development, and response to environmental stimuli. This intricate network relies on a group of specialized chemical messengers known as plant hormones.

• These hormones, also termed phytohormones, are organic substances produced in minute quantities within various plant tissues.

• Despite their low concentrations, they exert profound effects on plant physiology, influencing virtually every aspect of their life cycle.

What are Plant Hormones?

• Plant hormones, also known as phytohormones, are naturally occurring chemical messengers that regulate various physiological processes in plants, including growth, development, and responses to environmental stimuli.

Characteristics of Plant Hormones

• Plant Hormones are produced in small amounts by specific plant cells and transported to target cells, where they bind to specific receptors and trigger a response.

• Plant hormones are chemical compounds found in low concentrations in plants.

• Hormones are produced in various parts of the plant and can act synergistically or individually.

• They play roles in vernalization, phototropism, seed germination, and dormancy, among others, in conjunction with external factors.

• Synthetic plant hormones can be applied to crops to achieve controlled production.

• They are transported through the plant in a variety of ways.

• They can have both positive and negative effects on plant growth and development.

• They can interact with each other to produce complex effects.

Charles Darwin first observed phototropism in canary grass coleoptiles, while F.W. Went isolated auxin from oat seedling coleoptiles.

Read Also: Modification of Stem PDF:General Science Notes

Read Also:  Economic Importance of Fungi

Examples of Common Plant Hormones

There are six major types of plant hormones:

• Auxins promote cell growth and differentiation, especially on the tips of plants. They also play a role in apical dominance, phototropism, and gravitropism.

• Cytokinins promote cell division and lateral growth in plants. They also help to delay the senescence (ageing) of leaves.

• Gibberellins promote stem elongation, seed germination, and flowering. They also help to break dormancy in seeds and buds.

• Abscisic acid (ABA) promotes seed dormancy, bud dormancy, and leaf abscission. It also helps plants to tolerate stress, such as drought and cold.

• Ethylene promotes fruit ripening, leaf abscission, and flower senescence. It also plays a role in wound healing and responses to pathogens.

• Brassinosteroids promote cell division, cell elongation, and flowering. They also help to improve plant stress tolerance.

How Plant Hormones Worked

• Plant hormones are often classified as either growth promoters or growth inhibitors.

• Growth promoters, such as auxins and cytokinins, stimulate cell growth and division.

• Growth inhibitors, such as ABA and ethylene, slow down or stop cell growth and division.

• Plant hormones work by binding to receptors on the surface of plant cells.

• Once bound, the hormone receptor activates a signal transduction pathway that leads to changes in gene expression.

• These changes in gene expression then lead to the characteristic effects of the plant hormone.

• Plant hormones are essential for plant growth and development. They help plants respond to the environment and cope with stress.

• By understanding how plant hormones work, we can better understand how plants grow and develop, and we can develop new ways to improve plant productivity and resilience.

What are the main functions of plant hormones?

Plant hormones, also known as phytohormones, have various functions in plants, including:

Plant Growth Regulator

• Plant hormones regulate cell division, elongation, and differentiation, controlling plant growth and development.

Tropism

• Plant hormones mediate the response of plants to external stimuli, such as light, gravity, and touch, leading to tropic growth.

Reproduction

• Plant hormones play a crucial role in regulating flowering, fruiting, and seed development.

Stress Response

• Plant hormones help plants respond to various biotic and abiotic stresses, such as drought, cold, heat, and pathogen attacks.

Dormancy

• Plant hormones regulate the dormancy and germination of seeds and buds, controlling the timing of plant growth.

Senescence

• Plant hormones regulate the ageing process, controlling the timing of leaf shedding and plant death.

Defence

• Some plant hormones, such as jasmonates, play a role in defence against herbivores and pathogens.

List of Plant Hormones and Their Functions

AUXIN HORMONE

Auxins, which means “to grow,” are a class of plant hormones widely used in agricultural and horticultural practices to promote plant growth and development.

Source of Natural Auxins

• These hormones are primarily synthesized in the growing apices of roots and stems, and then migrate to other parts of the plant where they act.

Most Common Natural Auxins

• There are both natural and synthetic forms of auxins.
• The most common natural auxin is Indole-3-acetic acid (IAA), which is produced in the tips of the plant’s shoots and roots.

Another natural auxin is Indole butyric acid (IBA), which is found in the seeds and fruits of many plants.

Synthetic Auxins

• Synthetic auxins, such as 2,4-D (2,4-Dichlorophenoxyacetic acid) and NAA (Naphthalene acetic acid), are widely used in agriculture and horticulture to control weed growth, stimulate rooting in cuttings, and regulate plant growth.

Environmental Impact

• Excessive use of synthetic auxins can have negative effects on the environment, including contamination of soil and water sources.

Regulation of Auxin Levels

• It’s worth noting that while auxins are essential for plant growth and development, their levels and distribution must be carefully regulated to ensure optimal plant health. Too much or too little auxin can lead to developmental abnormalities or stunted growth.

Auxin Hormone Functions

Auxins are a class of plant hormones that play crucial roles in various aspects of plant growth, development, and survival. Here are some of the key functions of auxins:

Cell Elongation

• Auxins promote the elongation of plant cells, especially in the stems and roots.

Apical dominance

• The presence of auxin in the apical bud suppresses the growth of lateral buds, thus promoting upward growth and maintaining the plant’s vertical orientation.

Parthenocarpy

• Auxin induces parthenocarpy, which is the development of fruit without fertilization. This process is commonly used in the production of seedless fruits, such as tomatoes.

Prevents premature fall

• Auxin helps to prevent the premature fall of leaves, flowers, and fruits by promoting their attachment to the stem.

Rooting and grafting

• Auxin is useful in stem cuttings and grafting as it initiates rooting and promotes the fusion of plant tissues.

Flowering

• Auxin promotes flowering in some plants, such as pineapple.

Herbicide

• Synthetic auxins like 2,4-D are widely used as herbicides to kill undesirable weeds of dicot plants without affecting monocot plants.

Cell division and differentiation

• Auxin is involved in cell division and xylem differentiation, which is essential for the growth and development of plant tissues.

GIBBERELLINS HORMONE

• Gibberellins are a group of more than 100 plant hormones.

• They are acidic in nature and are found in higher plants and fungi.

• Gibberellins play a crucial role in various plant growth and development processes.

• Functions include stem elongation, seed germination, and flowering.

• The discovery of the first gibberellin, GA1, occurred in Japan in the 1930s during the study of “foolish seedling disease” in rice.

• Infected plants exhibited taller and thinner growth, leading to the identification of gibberellins.

• Since then, researchers have identified and characterized numerous gibberellins.

• GA3 is the most widely studied and commercially used gibberellin.

• GA3 is commonly employed in agriculture for purposes such as increasing plant height, enhancing fruit size, improving seed germination, and accelerating flowering.

• The use of gibberellins, particularly GA3, has practical applications in modern agricultural practices.

Functions of Gibberellins Hormone

Gibberellins are a group of plant hormones that play a crucial role in various plant growth and development processes. Some of their functions include:

Bolting

• Gibberellins promote bolting, which is the sudden elongation of internodes just before flowering in rosette plants like cabbage and beet. This process is essential for the plants to produce a stem that can hold the flowers and seeds.

Delaying senescence

• Gibberellins delay senescence, which is the natural ageing and death of plant tissues. This delay allows the plant to continue photosynthesizing and producing food for a more extended period, increasing its yield.

Inducing parthenocarpy

• Gibberellins induce parthenocarpy, which is the development of fruit without fertilization. This process is beneficial in fruit production and can increase crop yield.

Elongation of the stem and reversing dwarfism

• Gibberellins promote the elongation of stems and can reverse dwarfism in some plants, making them taller and more robust.

Inducing maleness in certain plants

• Gibberellins can induce maleness in certain plants, such as cannabis, which is essential for their reproduction.

Inducing the formation of hydrolytic enzymes

• Gibberellins induce the formation of hydrolytic enzymes, such as lipase and amylase, in the endosperm of germinating cereal grains and barley seeds. This process provides the energy and nutrients necessary for seedling growth.

Breaking seed dormancy

• Gibberellins play a crucial role in breaking seed dormancy, allowing the seeds to germinate and grow into healthy plants.

CYTOKININS HORMONE

• Cytokinins are a class of plant hormones that primarily regulate cell division, particularly during cytokinesis.

• Cytokinins are naturally synthesized and actively divide plant tissues like root apices, shoot buds, and young fruits.

• Cytokinin movement in plants is basipetal (downward) and polar (from shoot to root).

• Naturally occurring cytokinins include zeatin (found in corn kernels and coconut milk) and isopentenyl adenine (present in various plant tissues).

• Synthetic cytokinins like kinetin, benzyl adenine, diphenylurea, and thidiazuron are widely used in agriculture to enhance crop yield and quality.

Cytokinins’ influence extends beyond cell division, impacting various plant processes:

• Shoot and root growth stimulation
• Chloroplast development enhancement
• Senescence delay
• Stress response modulation

Cytokinins interact with other plant hormones, such as auxins and gibberellins, to regulate various plant processes:

• Seed germination
• Apical dominance
• Flowering

Research is underway to explore the potential of cytokinins in various applications:

• Enhancing plant tolerance to environmental stress.
• Boosting the production of plant secondary metabolites with medicinal properties.

Functions of Cytokinins Hormone

Cell division

• Cytokinins are the most potent promoters of cell division in plants. They do this by stimulating the production of proteins that are necessary for mitosis.

Apical dominance  

• Cytokinins can overcome apical dominance, which is the tendency for the apical bud of a plant to inhibit the growth of lateral buds. This is because cytokinins promote the growth of lateral buds by increasing the production of auxin, another plant hormone that promotes cell division.

Axil bud growth

• Cytokinins also promote the growth of axillary buds, which are the buds that grow in the axils of leaves. This is because cytokinins increase the production of auxin, which promotes cell division in the axillary buds.

Leaf senescence

• Cytokinins can delay leaf senescence, which is the process by which leaves die and fall off of a plant. This is because cytokinins help to maintain the levels of chlorophyll and other proteins in leaves.

Seed germination

• Cytokinins can also promote seed germination. This is because cytokinins help to break down the seed coat and stimulate the growth of the embryo.

Stress tolerance

• Cytokinins can help plants tolerate stress, such as drought, flooding, and exposure to high temperatures. This is because cytokinins help to protect the DNA of plant cells from damage.

Plant defense

• Cytokinins can also help plants to defend themselves against pests and diseases. This is because cytokinins help increase the production of proteins involved in plant defence.

Production of cytokinins

• Cytokinins are produced in the roots of plants, and they are transported to the shoots.

• The levels of cytokinins in a plant can be affected by a number of factors, including the amount of light, the temperature, and the availability of nutrients.

Applications of cytokinins

Cytokinins are used in a variety of applications, including:

Horticulture

• Cytokinins are used to promote the rooting of cuttings, to stimulate the growth of flowers and fruits, and to delay leaf senescence.

Agriculture

• Cytokinins are used to increase the yield of crops, to improve the quality of crops, and to protect crops from stress.

Biotechnology

• Cytokinins are used in biotechnology to produce transgenic plants with desirable traits, such as resistance to pests and diseases.

ABSCISIC ACID

• Abscisic acid (ABA) is a plant hormone that plays a crucial role in plant growth and development.

• ABA acts as a growth-inhibiting hormone, countering the effects of growth-promoting hormones.

• ABA plays a key role as a stress hormone, helping plants cope with drought, heat, and cold stress.

• ABA regulates water loss through transpiration, promotes protective protein synthesis, and helps maintain seed dormancy.

• ABA has potential applications in agriculture, particularly in improving crop yield and stress tolerance under challenging conditions.

Functions of Abscisic Acid

Abscisic acid (ABA) is a plant hormone that plays a key role in plant growth and development. It is involved in a wide range of processes, including:

• Seed dormancy: ABA helps to keep seeds dormant until conditions are favourable for germination.

• Leaf abscission: ABA causes leaves to fall off of plants in the fall.

• Stomatal closure: ABA causes stomata to close, which helps to conserve water during times of drought.

• Stress tolerance: ABA helps plants to tolerate a variety of stresses, including drought, heat, cold, and salt.

• Plant defence: ABA helps plants to defend themselves against pests and diseases.

Key Points

• ABA is synthesized in the plastids of plant cells, and it is transported to other parts of the plant through the vascular system.

• The name “abscisic acid” comes from the fact that it was first discovered in leaves that were abscising, or falling off.

• ABA is sometimes called the “stress hormone” because it helps plants to tolerate a variety of stresses.

• ABA can be used as a plant growth regulator to promote seed germination, delay leaf abscission, and increase the tolerance of plants to stress.

ETHYLENE

• Ethylene is a unique plant hormone that can act both as a growth promoter and inhibitor, depending on the plant’s needs.

• It is different from other plant hormones because it occurs in gaseous form.

• Ethylene is naturally produced in ripening fruits and tissues that are undergoing senescence, which is the natural ageing process of cells.

• Ethylene plays a crucial role in regulating many physiological processes in plants, such as fruit ripening, leaf and flower senescence, and abscission.

• Ethylene is widely used in agriculture to regulate plant growth, increase fruit yield, and ripen fruits after harvest.

• It is also used to control the abscission of flowers and fruits, as well as to induce flowering in certain plants.

• Additionally, ethylene is involved in plant responses to environmental stresses, such as drought, flooding, and high temperatures.

Functions of Ethylene Plant Hormone

Ripening

• Ethylene is well known for its role in hastening the ripening of fruits, such as bananas, apples, and tomatoes.

• It triggers a series of physiological and biochemical changes that lead to fruit softening, colour change, and aroma development.

Epinasty

• Ethylene controls the epinasty or downward bending of leaves, which is a response to environmental stresses such as high humidity, wind, or insect attack. This helps to protect the plant from damage and promote air circulation.

Dormancy

• Ethylene helps to break seed and bud dormancy, which is a state of suspended growth and development. It stimulates the release of enzymes that break down the seed coat and promote germination.

Elongation

• Ethylene stimulates the rapid elongation of petioles and internodes, which are the parts of the plant stem that connect leaves and branches. This allows the plant to reach for light and other resources.

Senescence and Abscission

• Ethylene promotes the senescence and abscission of leaves and flowers, which is the natural ageing process of plant cells. This helps to conserve resources and prepare the plant for new growth.

Root Growth

• Ethylene induces root growth and root hair formation, which increases the surface area for water and nutrient absorption. This helps the plant to better withstand drought and other environmental stresses.

Note:- (How Ethylene induces root growth?)
-----------------------------------------------------------------

Ethylene triggers a cascade of events that lead to increased production of auxin within the plant. 

This newly synthesized auxin then embarks on a journey towards the root tip, guided by specialized transport mechanisms.

At the root tip, auxin accumulates and exerts its influence on cell growth and development. 

It promotes cell elongation, leading to longer roots that can effectively anchor the plant and absorb water and nutrients from the soil.

Femaleness

• Ethylene stimulates femaleness in monoecious plants, which have separate male and female flowers on the same plant. It promotes the development of female flowers, which are essential for fruit production.

Apical Hook

• Ethylene is involved in the formation of the apical hook in dicot seedlings, which is a protective structure that helps the seedlings emerge from the soil. It helps to protect the delicate shoot tip from damage and allows the seedling to grow toward the light.

What is monoecious plants?

Used of Plant Hormones in Agriculture and Horticulture

• Plant hormones are used in agriculture and horticulture to improve crop yields, quality, and resilience.

• They can be used to promote growth, delay senescence, and regulate plant responses to stress.

Improving crop yields

• Plant hormones can be used to increase crop yields in a number of ways. For example, auxins can be used to promote root growth, which can help plants to absorb more nutrients and water.

• Gibberellins can be used to promote stem elongation and fruit development.

• Cytokinins can be used to increase leaf size and number, which can lead to higher photosynthetic rates and biomass production.

Improving crop quality

• Plant hormones can also be used to improve the quality of crops. For example, auxins can be used to increase fruit size and firmness.

• Gibberellins can be used to delay fruit ripening, which can extend the shelf life of fresh produce.

• Cytokinins can be used to improve the appearance of flowers and foliage.

Improving Crop resilience

• Plant hormones can also be used to improve the resilience of crops to stress.

• For example, abscisic acid can be used to induce stomatal closure, which can help plants to conserve water during drought.

• Ethylene can be used to promote fruit ripening, which can help plants to reproduce before they are killed by frost.

Specific examples of plant hormone use in agriculture and horticulture

Auxins

• Auxins are used to promote root growth in cuttings, to increase fruit size in tomatoes and other fruits, and to prevent premature fruit drop in apples and other fruits.

Gibberellins

• Gibberellins are used to increase stem elongation in grapes and other fruits, to promote seed germination in barley and other grains, and to delay fruit ripening in citrus fruits.

Cytokinins

• Cytokinins are used to improve the appearance of flowers in roses and other ornamentals, to increase leaf size in tobacco and other crops, and to delay leaf senescence in lettuce and other leafy vegetables.

Abscisic acid

• Abscisic acid is used to induce stomatal closure in potatoes and other crops during drought and to promote seed dormancy in wheat and other grains.

Ethylene

• Ethylene is used to promote fruit ripening in bananas and other fruits, and to promote leaf abscission in cotton and other crops.

Plant hormones are a powerful tool that can be used to improve crop yields, quality, and resilience. By understanding how plant hormones work, farmers and horticulturists can develop strategies to produce better crops with fewer inputs.

Plant Hormones and their Functions Table