History of Plant Growth Regulators (PGRs)

• Early Observations: The concept of plant growth regulators began in the late 19th and early 20th centuries. Early studies focused on the effects of naturally occurring substances, like auxins, on plant growth.
• Discovery of Auxins: The discovery of auxins (specifically indoleacetic acid or IAA) in the early 20th century by scientists like Frits Went (1928) led to the recognition of chemicals influencing plant growth.
• Progress in the 1930s and 1940s: Following the discovery of auxins, more growth regulators were identified, such as gibberellins (discovered from rice seedlings) and cytokinins (discovered in coconut milk and other plant tissues).
• Modern Era: In the mid-20th century, scientists began synthesizing synthetic PGRs (e.g., synthetic auxins, gibberellins) for agricultural applications, improving the ability to manipulate plant growth and development.

Types of Plant Growth Regulators

Plant growth regulators are classified into two categories based on their origin:

1. Natural PGRs: Naturally occurring chemicals produced by plants.
   • Auxins
   • Cytokinins
   • Gibberellins
   • Ethylene
   • Abscisic Acid (ABA)
   • Brassinosteroids
   • Salicylic Acid
   • Jasmonates

2. Synthetic PGRs: Manufactured chemicals that mimic the effects of natural PGRs.

• • 2,4-D (2,4-dichlorophenoxyacetic acid): A synthetic auxin.
  • NAA (Naphthalene Acetic Acid): A synthetic auxin.
  • GA3 (Gibberellic Acid): A synthetic gibberellin.
  • Benzylaminopurine (BAP): A synthetic cytokinin.
  • Paclobutrazol: A plant growth retardant (inhibits gibberellin biosynthesis).

Physiological Roles of Plant Growth Regulators

• Auxins
  • Cell Elongation: Promote elongation of cells, especially in stems and roots.
  • Root Initiation: Stimulate adventitious root formation.
  • Tropisms: Regulate phototropism (growth toward light) and gravitropism (growth in response to gravity).
  • Vascular Development: Influence the development of vascular tissues (xylem and phloem).

• Cytokinins
  • Cell Division: Promote mitosis, leading to cell division and shoot development.
  • Delayed Senescence: Slow down the aging of leaves and other plant tissues.
  • Lateral Bud Growth: Stimulate the growth of lateral buds, leading to branching.
  • Nutrient Mobilization: Aid in the transport of nutrients within the plant.

• Gibberellins
  • Seed Germination: Promote the germination of seeds by stimulating the production of hydrolytic enzymes.
  • Stem Elongation: Enhance stem elongation by promoting cell division and elongation.
  • Flowering: Stimulate flowering in certain plants.
  • Fruit Set: Enhance fruit development, especially in seedless varieties.

• Ethylene
  • Fruit Ripening: Regulate the ripening of fruits, making them ready for harvest.
  • Flower Senescence: Promote the aging and shedding of flowers and leaves.
  • Stress Response: Regulate responses to environmental stress, such as wounding or flooding.

• Abscisic Acid (ABA)
  • Stress Response: Plays a role in the plant’s response to drought and other environmental stresses.
  • Stomatal Closure: Regulates the closing of stomata to conserve water.
  • Seed Dormancy: Induces dormancy in seeds, preventing premature germination.

• Brassinosteroids
  • Cell Elongation and Division: Promote cell division and elongation, especially in tissues like the stem.
  • Stress Tolerance: Enhance tolerance to abiotic stress factors like drought, heat, and salinity.
  • Vascular Development: Regulate vascular differentiation and development.

• Salicylic Acid (SA)
  • Defense Mechanism: Regulates plant immune responses to pathogens.
  • Stress Tolerance: Enhances plant tolerance to abiotic stresses, such as drought and high temperatures.
  • Leaf Senescence: Delays leaf senescence and aging.

• Jasmonates (JAs)
  • Defense Response: Play a role in the plant’s defense mechanism against herbivores and pathogens.
  • Flowering Regulation: Influence the timing of flowering.
  • Seed Germination: Promote seed germination and root growth.

Agricultural Uses of Plant Growth Regulators

• Auxins:
  • Rooting Hormones: Used for root initiation in cuttings and grafting.
  • Weed Control: Synthetic auxins like 2,4-D are used as herbicides to control broadleaf weeds.
  • Fruit Thinning: Auxins are applied to reduce fruit set, ensuring better fruit size and quality.

• Cytokinins:
  • Cell Division: Used to promote shoot proliferation in tissue culture.
  • Delay Senescence: Used in post-harvest treatments to extend the shelf life of fruits and vegetables.
  • Enhance Branching: Applied in ornamental plants to improve branching and form.

• Gibberellins:
  • Fruit Size Enhancement: Gibberellins are used to increase the size of fruits, particularly in grapes and apples.
  • Seedless Fruit Production: Used in the production of seedless varieties of fruits like grapes, cucumbers, and watermelons.
  • Flower Induction: Applied to induce flowering in certain crops like pineapples and strawberries.
  • Malting of Barley: Used in malting to break dormancy and promote enzyme production.

• Ethylene:
  • Fruit Ripening: Widely used in the commercial ripening of bananas, tomatoes, and other fruits.
  • Flowering Synchronization: Used to synchronize flowering in certain crops (e.g., pineapples).
  • Stress Response: Helps in the response to mechanical damage or environmental stress.

• Abscisic Acid (ABA):
  • Drought Tolerance: Applied to crops to enhance drought tolerance by regulating water loss through stomatal closure.
  • Seed Dormancy: Used in agriculture to control seed dormancy and synchronize seed germination.
  • Stress Management: Applied to crops to increase resistance to various abiotic stresses.

• Brassinosteroids:
  • Crop Yield: Used to enhance crop productivity, especially in cereals and legumes.
  • Stress Tolerance: Used to improve resistance to abiotic stresses like heat, cold, and salinity.
  • Vegetative Growth: Applied to improve overall plant health and growth, leading to better yields.

• Salicylic Acid (SA):
  • Disease Resistance: Used to enhance plant disease resistance, particularly to fungal pathogens.
  • Stress Tolerance: Applied to improve tolerance to drought, heat, and other stress factors.
  • Post-Harvest: Used to delay senescence and extend the shelf life of cut flowers.

• Jasmonates (JAs):
  • Pest Resistance: Applied to induce the production of defense chemicals, enhancing resistance to herbivory and pathogens.
  • Flowering Regulation: Used in controlled environments to regulate flowering and fruiting times.