Growth regulators of plant

Growth regulators of plant

Plant growth regulators (PGRs), often referred to as phytohormones or plant hormones, are synthetic or natural compounds that control a number of physiological functions in plants. These regulators are essential to the growth, development, and reaction of plants to their surroundings. There are five primary categories of growth regulators for plants:


Auxins are a class of plant hormones that play a crucial role in regulating various aspects of plant growth and development. Here are key aspects of auxins:

  1. Cell Elongation:

    • Promotion: Auxins promote cell elongation, leading to increased length of plant cells. This is particularly important in the elongation of stems and roots.
  2. Apical Dominance:

    • Inhibition of Lateral Buds: Auxins inhibit the growth of lateral buds in favor of the terminal (apical) bud. This is known as apical dominance and helps in the upward growth of the plant.
  3. Root Initiation:

    • Root Growth: Auxins stimulate the initiation and development of roots. This property is often utilized in the propagation of plants from cuttings.
  4. Phototropism:

    • Bending Toward Light: Auxins are involved in phototropism, where a plant bends towards a light source. They accumulate on the shaded side of the stem, promoting elongation on that side.
  5. Gravitropism:

    • Root Orientation: In roots, auxins help in gravitropism, influencing the orientation of roots in response to gravity.
  6. Fruit Development:

    • Seedless Fruit Growth: Auxins are sometimes used in agriculture to induce parthenocarpy, the development of seedless fruits, by promoting fruit growth without fertilization.
  7. Leaf Abscission:

    • Control of Leaf Drop: Auxins are involved in controlling the abscission (shedding) of leaves. Abscisic acid, another plant hormone, also plays a role in this process.
  8. Tissue Culture:

    • Rooting of Cuttings: In horticulture and tissue culture, synthetic auxins like indole-3-butyric acid (IBA) are often used to stimulate root development in cuttings.
  9. Senescence:

    • Delaying Aging: Auxins can delay the senescence (aging) of plant tissues, influencing the longevity of leaves and other plant parts.
  10. Synthetic Auxins:

    • Indole-3-butyric acid (IBA) and Naphthaleneacetic acid (NAA): These are examples of synthetic auxins used in various applications, such as rooting hormones in plant propagation.

Understanding the role of auxins is essential in manipulating plant growth and development for agricultural and horticultural purposes. The interplay between auxins and other plant hormones contributes to the complex regulatory network governing plant physiology.


Cytokinins are a class of plant hormones that play a crucial role in regulating cell division and differentiation, particularly in meristematic tissues. Here are key aspects of cytokinins:

  1. Cell Division and Differentiation:

    • Stimulating Cell Division: Cytokinins promote cell division, leading to the formation of new cells. They are particularly important in meristematic tissues like the shoot and root apical meristems.
  2. Shoot Growth:

    • Promoting Shoot Development: Cytokinins work in conjunction with auxins to regulate shoot growth. The balance between cytokinins and auxins influences whether a bud will develop into a shoot or remain dormant.
  3. Root Growth:

    • Inhibiting Root Senescence: Cytokinins help prevent root senescence and promote lateral root growth, contributing to overall root development.
  4. Delaying Leaf Senescence:

    • Anti-Senescence Effect: Cytokinins have anti-senescence effects, helping to delay the aging of leaves and other plant organs. They contribute to the maintenance of green and functional tissues.
  5. Delaying Abscission:

    • Inhibiting Leaf Drop: Cytokinins can delay the abscission (shedding) of leaves by influencing the balance between abscisic acid (ABA) and ethylene, two other plant hormones involved in this process.
  6. Seed Development:

    • Promoting Seed Development: Cytokinins play a role in seed development, influencing the growth and development of seeds.
  7. Callus Formation:

    • Inducing Callus Formation: Cytokinins, often in combination with auxins, are used in tissue culture to induce the formation of callus, a mass of undifferentiated cells, which can later be regenerated into whole plants.
  8. Delaying Chlorophyll Degradation:

    • Preserving Chlorophyll: Cytokinins help in delaying the degradation of chlorophyll in leaves, contributing to the maintenance of photosynthetic activity.
  9. Senescence Regulation:

    • Interplay with Ethylene: Cytokinins and ethylene have antagonistic effects in regulating senescence. While cytokinins delay senescence, ethylene often promotes it.
  10. Natural Cytokinins:

    • Zeatin: Zeatin is a natural cytokinin that is found in various plant tissues and is involved in promoting cell division and shoot development.
  11. Synthetic Cytokinins:

    • Kinetin and 6-Benzylaminopurine (BAP): These are examples of synthetic cytokinins that are often used in plant tissue culture and agriculture to promote cell division, shoot formation, and overall plant growth.

Understanding the role of cytokinins is crucial for manipulating plant growth and development, and the balance between cytokinins and other hormones influences various physiological processes in plants.


Gibberellins (GAs) are a class of plant hormones that play a crucial role in regulating various physiological processes in plants. These processes include stem elongation, germination, flowering, and fruit development. Gibberellins were first discovered in Japan in the early 20th century, when researchers observed the abnormal elongation of rice plants infected with a fungus (Gibberella fujikuroi) that produced these compounds.

Here are key aspects of Gibberellins:

  1. Stem Elongation:

    • One of the primary functions of gibberellins is to stimulate stem elongation. In the absence of these hormones, plants may exhibit a dwarfed phenotype with shortened internodes.
    • Gibberellins promote cell elongation by enhancing the plasticity of cell walls, allowing cells to expand more easily.
  2. Germination:

    • Gibberellins are essential for the germination of seeds. They break seed dormancy by promoting the synthesis of enzymes like amylase, which hydrolyzes starch into sugars, providing energy for seedling growth.
  3. Flowering:

    • Gibberellins are involved in the regulation of flowering, particularly in some long-day plants. They influence the transition from the vegetative to the reproductive phase.
  4. Fruit Development:

    • Gibberellins contribute to fruit development by promoting cell division and enlargement. They are often used in agriculture to enhance fruit size in certain crops.
  5. Bolting in Biennials:

    • In biennial plants (plants with a two-year life cycle), gibberellins induce bolting, the rapid elongation of the flowering stem, in the second year.
  6. Seedless Fruit Production:

    • Gibberellins are sometimes applied to induce parthenocarpy, the development of seedless fruits. This is particularly useful in crops like grapes and tomatoes.
  7. Interaction with Other Hormones:

    • Gibberellins often interact with other plant hormones, such as auxins and cytokinins, to orchestrate various aspects of plant growth and development.
  8. Commercial Applications:

    • Synthetic gibberellins, such as gibberellic acid (GA3), are used in agriculture to manipulate plant growth. They are applied to crops to increase fruit size, promote uniform germination, and induce flowering.

It’s important to note that different species and even different cultivars within a species may respond differently to gibberellins. The effects of these hormones are highly dependent on the specific plant species, the stage of development, and the environmental conditions.

Abscisic Acid (ABA):

Abscisic Acid (ABA) is a plant hormone that plays a crucial role in several physiological processes, particularly in response to environmental stress conditions. Here are some key aspects of Abscisic Acid:

  1. Role in Stress Response:

    • ABA is often referred to as the “stress hormone” because its levels increase in response to various environmental stresses, such as drought, high salinity, and cold temperatures.
    • It helps plants adapt to and survive adverse conditions by promoting mechanisms that reduce water loss, such as stomatal closure and the synthesis of protective proteins.
  2. Seed Dormancy and Germination:

    • ABA is involved in the regulation of seed dormancy, preventing premature germination under unfavorable conditions.
    • During seed development, ABA accumulates in the embryo and inhibits germination until conditions are suitable for seedling establishment.
  3. Stomatal Regulation:

    • ABA regulates the opening and closing of stomata, the tiny pores on plant surfaces. Stomatal closure reduces water loss through transpiration.
    • In response to water deficiency or other stress factors, ABA signals the closure of stomata to conserve water.
  4. Root Growth Inhibition:

    • ABA can inhibit root growth, particularly under water deficit conditions. This helps the plant allocate resources to essential functions and survive in arid environments.
  5. Role in Leaf Senescence:

    • ABA is involved in the regulation of leaf senescence, the aging process of leaves. It promotes the breakdown of chlorophyll and other cellular components during senescence.
  6. Regulation of Flowering:

    • ABA can influence flowering time and floral development, interacting with other hormonal signals to regulate the timing of reproductive processes.
  7. Interaction with Other Hormones:

    • ABA often interacts with other plant hormones, such as auxins, gibberellins, and cytokinins, to coordinate various aspects of plant growth and development.
  8. Synthetic ABA Analogs:

    • Some synthetic analogs of ABA, such as abscisic acid agonists, have been developed for agricultural use. These compounds mimic the effects of ABA and can be applied to enhance stress tolerance in crops.

Understanding the role of ABA in plant physiology is essential for developing strategies to enhance stress tolerance in crops, improve water use efficiency, and optimize plant growth under challenging environmental conditions.


Ethylene is a gaseous plant hormone that plays a crucial role in various aspects of plant growth and development. It is involved in several physiological processes and responses, including:

  1. Fruit Ripening: Ethylene is well-known for its role in the ripening of fruits. It triggers the breakdown of cell walls, softening the fruit, and promoting color changes. This process is important for the development of flavors, aromas, and other characteristics associated with ripe fruits.

  2. Senescence: Ethylene promotes senescence, the aging and eventual death of plant tissues. It is involved in the regulation of leaf and flower senescence, leading to the shedding of older leaves and petals.

  3. Abscission: Ethylene plays a role in the abscission (shedding) of leaves, flowers, and fruits. It induces the formation of abscission layers, facilitating the detachment of plant organs.

  4. Root Growth: Ethylene inhibits root growth and promotes root hair development. It can also stimulate adventitious root formation in certain conditions.

  5. Stress Responses: Plants produce ethylene in response to various stress factors, such as drought, flooding, and mechanical damage. Ethylene helps plants cope with stress by triggering adaptive responses.

  6. Seed Germination: Ethylene is involved in the regulation of seed germination, influencing processes such as seed dormancy and breaking dormancy during the germination process.

  7. Regulation of Other Hormones: Ethylene interacts with other plant hormones, such as auxins and abscisic acid, to coordinate various growth and developmental processes.

Ethylene is unique among plant hormones because it is a gas, allowing it to diffuse through the air and influence nearby plants. Its production is tightly regulated by various internal and external factors, including developmental stages, environmental stresses, and the presence of other hormones.

In agriculture, ethylene is sometimes used intentionally to regulate fruit ripening, for example, by exposing fruits to ethylene gas in controlled environments. Additionally, ethylene inhibitors or synthetic compounds that release ethylene, like ethephon, are used for specific applications in plant management.


Brassinosteroids (BRs) are a class of plant hormones that play crucial roles in various physiological processes, including cell elongation, division, differentiation, and responses to environmental stress. They were first identified for their ability to promote stem elongation and cell division in bean plants. Since then, research has revealed their involvement in a wide range of plant growth and development processes.

Key features and functions of brassinosteroids include:

  1. Cell Elongation:

    • Brassinosteroids promote cell elongation, leading to increased stem and petiole length. This effect is particularly important in regulating plant height.
  2. Cell Division:

    • Brassinosteroids stimulate cell division in the root and shoot apical meristems, contributing to overall plant growth.
  3. Seed Germination:

    • Brassinosteroids play a role in seed germination by promoting cell expansion in the embryo and endosperm.
  4. Vascular Differentiation:

    • They are involved in vascular tissue differentiation, influencing the development of xylem and phloem.
  5. Drought and Stress Responses:

    • Brassinosteroids contribute to plant stress tolerance, including drought resistance. They help plants cope with environmental stresses by regulating various physiological and biochemical processes.
  6. Senescence Delay:

    • Brassinosteroids can delay senescence (aging) in leaves and other plant organs, contributing to extended plant productivity.
  7. Pathogen Resistance:

    • There is evidence that brassinosteroids play a role in plant defense mechanisms against pathogens.
  8. Interaction with Other Hormones:

    • Brassinosteroids interact with other plant hormones, such as auxins, cytokinins, and abscisic acid, to coordinate complex growth and developmental processes.
  9. Receptor Signaling:

    • The brassinosteroid signaling pathway involves a receptor-mediated process. The brassinosteroid receptor, BRI1 (Brassinosteroid Insensitive 1), is a membrane-bound receptor kinase that initiates the signal transduction cascade upon brassinosteroid binding.
  10. Synthetic Applications:

    • Some synthetic brassinosteroid analogs have been developed for agricultural use to enhance crop growth, yield, and stress tolerance.

Understanding the functions of brassinosteroids has practical implications in agriculture, where they can be manipulated to improve crop performance and stress resistance. However, the practical applications are still being researched and developed.


To sum up, plant growth regulators are essential for controlling the complex processes involved in a plant’s growth, development, and reaction to external stimuli. Hormone classes: auxins, cytokinins, gibberellins, abscisic acid, ethylene, and brassinosteroids are among those that modulate a variety of physiological processes in plants. These regulators are essential for controlling many aspects of the plant life cycle, including stress responses, blooming, and seed germination. They also facilitate cell elongation and division. Understanding and modifying these growth regulators can help maximize crop output, improve stress tolerance, and better comprehend the intricate workings of plant biology. These developments have significant ramifications for horticulture, science, and agriculture. The secrets of plant development continue to be revealed as we learn more about the complexities of these signaling molecules.

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