Plant Hormones And Their Functions Detailed Point-wise Notes

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

Plant Hormones And Their Functions

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.
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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 senescence (aging) 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.

6 major types of plant hormones
6 major types of plant hormones

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 to respond to the environment and to 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.

Types of Plant Hormones

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.

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

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

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

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

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

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

main functions of plant hormones
main functions of plant hormones

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.

▪ 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.

▪ 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, 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.

▪ However, excessive use of synthetic auxins can have negative effects on the environment, including contamination of soil and water sources.

▪ 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.

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.

Auxin promotes flowering in some plants, such as pineapple.

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, which are known to be acidic in nature, are a group of more than 100 plant hormones. They are found in higher plants and fungi and play a significant role in various plant growth and development processes, such as stem elongation, seed germination, and flowering.

▪ The first gibberellin, GA1, was discovered in Japan in the 1930s when scientists were studying a rice disease called “foolish seedling disease.” They noticed that the infected plants grew taller and thinner than healthy plants, and further investigation led to the discovery of gibberellins.

▪ Since then, researchers have identified and characterized numerous other gibberellins, with GA3 being the most widely studied and commercially used. GA3 is commonly used in agriculture to increase plant height and fruit size, improve seed germination, and accelerate the flowering process.

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:

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 aging 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 group of plant hormones that play a vital role in the cytokinesis process, which is the division of the cell cytoplasm and nuclei during cell division.

▪ These hormones are naturally synthesized in plants where rapid cell division occurs, such as root apices, shoot buds, and young fruits.

▪ The movement of cytokinins in plants is basipetal, meaning they move from the apical meristem towards the roots, and polar, meaning they move from the shoot towards the root.

Zeatin and isopentenyl adenine are two naturally occurring cytokinins found in plants.

▪ Zeatin is found in corn kernels and coconut milk, while isopentenyl adenine is found in various plant tissues.

▪ Synthetic cytokinins include kinetin, benzyladenine, diphenylurea, and thidiazuron, which are commonly used in agriculture to improve crop yield and quality.

▪ Cytokinins not only stimulate cell division but also affect other plant processes, including shoot and root growth, chloroplast development, delay of senescence, and stress response.

▪ Cytokinins also interact with other plant hormones, such as auxins and gibberellins, to regulate various plant processes, including seed germination, apical dominance, and flowering. In addition, researchers are exploring the potential of cytokinins in various applications, such as improving plant tolerance to environmental stress and increasing 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 to 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 to increase the production of proteins that are involved in plant defense.

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. Unlike other hormones like gibberellins, ABA acts as a growth-inhibiting hormone, countering the effects of growth-promoting hormones. It acts as an antagonist to gibberellins, inhibiting plant metabolism and regulating processes such as abscission and dormancy.

One of the key functions of ABA is its role as a stress hormone. When plants are exposed to stressors such as drought, heat, or cold, they produce higher levels of ABA, which helps increase their tolerance to these stressors. ABA can help regulate the plant’s response to stress by reducing water loss through transpiration, closing stomata to prevent dehydration, and promoting the synthesis of protective proteins and enzymes.

In addition to stress response, ABA also plays a role in seed development and germination. During seed development, ABA accumulates in the embryo and is responsible for maintaining seed dormancy until environmental conditions are favorable for germination. Once conditions such as sufficient water and light are present, the ABA levels in the seed decrease, and germination can occur.

Recent research has also shown that ABA may have potential applications in agriculture, particularly in crop yield and stress tolerance. By increasing the plant’s tolerance to stressors such as drought or heat, ABA can help increase crop yield and quality, particularly in regions affected by climate change.

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 favorable 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 defense: ABA helps plants to defend themselves against pests and diseases.

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 Plant Hormone

▪ 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 aging 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.

▪ It also helps to regulate plant growth and development by promoting cell division and elongation in certain tissues, such as roots and stems.

▪ 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 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, color 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 aging 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.

▪ 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 seedling emerge from the soil. It helps to protect the delicate shoot tip from damage and allows the seedling to grow toward the light.

Frequently Asked Questions (FAQs)

Q1. Which plant hormone promotes cell division?

Cytokinin is the plant hormone that promotes cell division in plants. It plays a crucial role in stimulating cell division and lateral growth, contributing to the overall growth and development of the plant.

Q2. What are plant hormones? Can you give some examples?

Plant hormones are natural chemicals that regulate various growth and development processes in plants. Here are some examples of plant hormones: auxin, gibberellin, cytokinin, ethylene, and abscisic acid.

Q3. What are the functions of different plant hormones?

Auxins: They promote cell growth and differentiation, especially at the tips of plants.
Cytokinins: These hormones stimulate cell division and lateral growth in plants.
Gibberellins: They help break dormancy in seeds and buds, promoting their growth.
Abscisic acid: This hormone induces dormancy in seeds and buds, preventing premature growth.
Ethylene: It plays a role in fruit ripening and regulates various physiological processes in plants.

Q4. What does cytokinin do in plants?

Cytokinins are plant growth regulators that primarily promote cell division in roots and shoots. They help in the growth, development, and differentiation of cells, and also influence apical dominance and delay leaf aging.

Q5. What is the function of abscisic acid in plants?

Abscisic acid (ABA) is a plant hormone that is involved in several plant developmental processes. It helps in seed and bud dormancy, regulates organ size, and controls stomatal closure.

Q6. What are the functions of ethylene and abscisic acid in plants?

Ethylene mainly works in expanding and dividing leaf cells, while abscisic acid acts in mature cells.

Q7. What role does ethylene play in plants?

Ethylene serves as both a growth promoter and an inhibitor. It is a gaseous hormone that plays a significant role in ripening fruits and aging tissues. Additionally, it regulates various physiological processes and finds wide use in agriculture.

Q8. What does auxin do in plants?

Auxin is responsible for promoting cell growth and elongation in plants. It makes the plant’s cell walls more flexible, allowing the plant to grow upwards more easily.

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