Plant Growth Regulators

Introduction

Plant growth regulators (PGRs), also known as phytohormones or plant growth hormones, are chemical substances that influence various physiological processes such as growth, development, and movement in plants. These compounds can be synthesized naturally within plants or artificially in laboratories.

Classification

PGRs are broadly categorized into:

  1. Plant Growth Promoters: Auxins, Gibberellins, Cytokinins
  2. Plant Growth Inhibitors: Abscisic Acid (ABA), Ethylene (which can also act as a promoter)

Characteristics

Plant growth regulators exhibit the following characteristics: - Control differentiation and elongation of cells

  • Influence leaf, flower, and stem formation

  • Affect fruit ripening and seed dormancy

  • Regulate leaf wilting and abscission

  • Exist in natural and synthetic forms

1. Plant Growth Promoters

Auxins

Auxins were first discovered by Charles Darwin and Francis Darwin in the late 19th century while studying phototropism in canary grass seedlings. Later, F.W. Went isolated and identified indole-3-acetic acid (IAA) as the main naturally occurring auxin.

Indole-3-acetic acid (IAA)

Source: [Wikipedia](https://en.wikipedia.org/wiki/Indole-3-acetic_acid)

Types

  1. Natural Auxins:

    • Indole-3-acetic acid (IAA) – the most common natural auxin.

  2. Synthetic Auxins:

    • Indole-3-butyric acid (IBA) – used in root formation.

    • Naphthalene acetic acid (NAA) – applied in agriculture for rooting and thinning.

    • 2,4-Dichlorophenoxyacetic acid (2,4-D) – a widely used herbicide for weed control.

Functions

  • Promote flowering and root initiation in stem cuttings
  • Prevent premature leaf and fruit drop
  • Regulate xylem differentiation and cell division
  • Act as herbicides for dicot weeds
  • Facilitate parthenocarpy (fruit development without fertilization)
  • Induce apical dominance (inhibition of lateral buds by apical buds)

Gibberellins

Gibberellins were first discovered in Japan when E. Kurosawa observed that a fungus (Gibberella fujikuroi) caused excessive elongation in rice plants, leading to a disease known as “bakanae” or foolish seedling disease. The active compound responsible for this abnormal growth was later identified as gibberellic acid (GA).

Gibberellins

Source: Wikipedia

Types

There are over 100 different types of gibberellins, denoted as GA₁, GA₂, GA₃, etc. Among them, GA₃ (gibberellic acid) is the most widely studied and used in agriculture and research.

Functions

  • Delay fruit senescence
  • Break bud and seed dormancy
  • Promote leaf expansion and bolting in certain plants
  • Increase sugarcane yield by stem elongation
  • Enhance fruit size and shape in apples and grapes
  • Accelerate seed production in conifers

Cytokinins

Cytokinins were first discovered in the 1950s by F. Skoog and C.O. Miller, who identified kinetin (a degradation product of DNA) as a growth-promoting substance. Later, zeatin was isolated from maize kernels as a naturally occurring cytokinin.

Cytokinins are a class of plant hormones that primarily promote cell division (cytokinesis) and regulate various aspects of plant growth and development. They work in coordination with auxins to control organ formation, leaf senescence, nutrient mobilization, and shoot development.

Produced mainly in roots and sites of active cell division

Cytokinens

Source: Wikipedia

Types

  1. Natural Cytokinins:

    • Zeatin (found in maize)

    • Dihydrozeatin

  2. Synthetic Cytokinins:

    • Kinetin (first discovered cytokinin, used in tissue culture)

    • 6-Benzylaminopurine (BAP) (widely used in agriculture and tissue culture)

    • Thidiazuron (TDZ) (strong cytokinin used in micropropagation)

Functions

  • Promote Cell Division: Stimulate mitosis in meristematic tissues, essential for plant growth.
  • Delay Leaf Senescence: Prevents premature aging by maintaining chlorophyll content.

  • Regulate Shoot and Root Development:

    • High cytokinin-to-auxin ratio → shoot formation

    • Low cytokinin-to-auxin ratio → root formation

  • Enhance Nutrient Mobilization: Promote the movement of nutrients from older leaves to younger tissues.
  • Break Dormancy in Seeds and Buds: Induce germination and growth in dormant seeds.
  • Promote Lateral Bud Growth: Counteract apical dominance imposed by auxins.

2. Plant Growth Inhibitors

Abscisic Acid (ABA)

Abscisic Acid is a plant hormone primarily known for its role in stress response, seed dormancy, and stomatal regulation. Unlike auxins, gibberellins, and cytokinins, which promote growth, ABA inhibits growth and helps plants survive unfavorable conditions such as drought, salinity, and cold stress.

Discovered in the 1960s by F.T. Addicott and colleagues while studying leaf abscission and seed dormancy. Initially thought to regulate leaf fall (abscission) (hence the name), but later found to be more involved in stress responses than abscission.

Abscisic Acid (ABA)

Source: Wikipedia

Types

Abscisic acid (ABA) exists in different forms, primarily based on its chemical structure, active/inactive states, and conjugated derivatives. These forms regulate its biological activity in plants.

  1. Natural Forms of Abscisic Acid

    • (+)-ABA (S-ABA) – The Biologically Active Form

    • (-)-ABA (R-ABA) – The Inactive Form

  2. ABA Conjugates (Storage & Inactivation Forms)

    • ABA-Glucose Ester (ABA-GE) – The Storage Form

    • ABA Methyl Ester – A Translocation Form

    • ABA Conjugates with Proteins or Lipids

  3. ABA Catabolites (Degradation Products – Inactivation Forms)

    • Phaseic Acid (PA) – Primary Degradation Product

    • Dihydrophaseic Acid (DPA) – Final Inactive Product

Summary Table of ABA Types
Type Status Function
(+)-ABA (S-ABA) Active Regulates stress response, dormancy, and stomatal closure
(-)-ABA (R-ABA) Inactive No major biological function
ABA-Glucose Ester (ABA-GE) Inactive (Storage) Stored in vacuoles, converted back to ABA under stress
ABA Methyl Ester Inactive (Transport) Easily transported, reactivated in target cells
Phaseic Acid (PA) Inactive (Degradation) Intermediate product in ABA breakdown
Dihydrophaseic Acid (DPA) Inactive (Degradation) Final breakdown product, permanently inactivated

Functions

  1. Stress Response & Drought Tolerance

    • ABA is often called the “stress hormone” because it helps plants adapt to drought and other environmental stresses.

    • Under drought stress, ABA accumulates in leaves and triggers the closure of stomata to reduce water loss.

    • Increases production of protective proteins and osmolytes to protect cells from dehydration.

  2. Stomatal Regulation

    • ABA binds to guard cells, causing them to lose water and shrink, which closes the stomata and prevents water loss.

    • This regulation is crucial for drought resistance but also limits CO₂ uptake, which can slow photosynthesis.

  3. Seed Dormancy & Germination Inhibition

    • High levels of ABA in seeds prevent premature germination.

    • ABA levels decrease when conditions become favorable, allowing gibberellins (GA) to break dormancy and trigger germination.

  4. Leaf Senescence & Growth Inhibition

    • ABA accelerates senescence in leaves under stress conditions.

    • It also inhibits cell division and elongation, acting as an antagonist to auxins, gibberellins, and cytokinins.

Ethylene

Ethylene (C₂H₄) is a gaseous plant hormone that regulates multiple physiological processes, including fruit ripening, leaf senescence, flower wilting, and stress responses. Unlike other plant hormones, ethylene is a small hydrocarbon gas, making it unique in how it moves and functions in plants.

Source: Wikipedia

Types

Ethylene itself is a single molecule (C₂H₄) and does not have distinct “types” like other plant hormones that exist in multiple forms (e.g., auxins or gibberellins). However, ethylene effects and applications can be categorized into different types or sources based on how it is produced or applied in plants.

  1. Natural Ethylene (Endogenous Ethylene)

    • Developmental Ethylene → Regulates seed germination, root growth, and flowering.

    • Stress-Induced Ethylene → Produced under drought, flooding, wounding, or pathogen attack.

    • Ripening-Associated Ethylene → Increases in climacteric fruits to accelerate ripening.

  2. Synthetic Ethylene (Exogenous Ethylene)

Functions of Ethylene

  1. Fruit Ripening

    • Ethylene is the key hormone responsible for fruit ripening in climacteric fruits (e.g., bananas, tomatoes, apples, mangoes).

    • It stimulates the production of enzymes like pectinases and cellulases, which break down cell walls, softening the fruit.

    • Enhances color change by breaking down chlorophyll and increasing carotenoids and anthocyanins.

    • Increases sugar accumulation by converting starch to simple sugars (e.g., in bananas).

  2. Leaf Senescence & Abscission

    • Ethylene accelerates aging in leaves and flowers, leading to leaf yellowing (chlorophyll degradation).

    • Promotes leaf and flower drop (abscission) by weakening the cell walls in the abscission zone.

    • Plays a crucial role in seasonal leaf fall in deciduous trees.

  3. Response to Mechanical & Environmental Stress

    • Ethylene helps plants survive stress conditions like drought, flooding, high salinity, and mechanical injury.

    • Flooding stress: Induces “hyponastic growth” (upward bending of leaves) and adventitious root formation to help plants cope with oxygen deprivation.

    • Wounding response: Triggers defense mechanisms like lignin deposition and production of secondary metabolites to prevent infections.

  4. Breaking Seed Dormancy & Promoting Germination

    • In some seeds, ethylene breaks dormancy and stimulates radicle (root) emergence.

    • Works with gibberellins (GA) to promote seed germination under stress conditions.

  5. Flowering Induction in Certain Plants

    • In plants like pineapples and mangoes, ethylene induces flowering.

    • Plays a role in sex determination, promoting female flowers in cucurbits (cucumber, pumpkin).

  6. Root Growth & Root Hair Formation

    • Stimulates root hair formation, which increases nutrient and water uptake.

    • Works in symbiosis with auxins to regulate root elongation.

Conclusion

Plant growth and development are controlled by a complex interaction of various phytohormones. Understanding their roles allows for better agricultural practices, including yield improvement, stress management, and plant tissue culture applications.

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