1. Introduction to Plant Hormones

  • Definition: Plant hormones (also called phytohormones) are chemical messengers that regulate plant growth, development, and responses to stimuli.
  • Main Types: Auxins, Gibberellins, Cytokinins, Abscisic Acid, Ethylene, Brassinosteroids, Jasmonates, Salicylic Acid, Strigolactones.

2. Historical Background

  • Early Observations: In the late 19th century, scientists noticed plants bending toward light (phototropism) and growing toward gravity (gravitropism).
  • Charles Darwin (1880): Studied phototropism in grass seedlings, suggesting a “transmissible influence” from the tip.
  • Boysen-Jensen Experiment (1910): Demonstrated that a chemical signal moves from the tip to the growing region.
  • Frits Went (1926): Isolated the first plant hormone (auxin) using oat coleoptiles, showing that it promotes cell elongation.

3. Key Experiments

a. Auxin and Phototropism

  • Darwin’s Experiment: Covered the tips of seedlings; those with covered tips did not bend toward light.
  • Went’s Agar Block Experiment: Placed agar with auxin on decapitated coleoptiles; bending occurred, proving auxin’s role.

b. Gibberellins and Stem Growth

  • Rice Seedlings (1926): Japanese scientists discovered “bakanae” disease (foolish seedling disease) caused by excessive gibberellin from a fungus, leading to tall, spindly plants.
  • Gibberellin Isolation (1935): Purified from fungus Gibberella fujikuroi, later found in plants.

c. Ethylene and Fruit Ripening

  • Gas Lamp Experiment (1901): Noticed that street lights caused nearby plants to drop leaves; ethylene gas was the cause.
  • Commercial Use: Ethylene now used to ripen fruits like bananas and tomatoes.

d. Cytokinins and Cell Division

  • Discovery (1955): Found in coconut milk, cytokinins stimulate cell division and shoot formation.

4. Major Plant Hormones and Their Functions

Hormone Main Functions
Auxins Cell elongation, root initiation, tropisms
Gibberellins Stem elongation, seed germination, flowering
Cytokinins Cell division, shoot growth, delay leaf aging
Abscisic Acid Stress response, seed dormancy, stomatal closure
Ethylene Fruit ripening, leaf drop, flower opening
Brassinosteroids Cell expansion, vascular differentiation
Jasmonates Defense against insects, wound response
Salicylic Acid Disease resistance, systemic acquired resistance
Strigolactones Root development, control of shoot branching

5. Modern Applications

  • Agriculture: Hormones used to control fruit ripening, seed germination, and crop yield.
  • Tissue Culture: Auxins and cytokinins used to grow new plants from cells.
  • Weed Control: Synthetic auxins (e.g., 2,4-D) used as herbicides.
  • Genetic Engineering: Manipulating hormone pathways to create stress-resistant crops.

6. Recent Breakthroughs

a. Hormone Crosstalk

  • Discovery: Hormones interact in complex networks, not just single pathways.
  • Example: Auxin and cytokinin balance controls root and shoot growth.

b. Hormones and Environmental Stress

  • 2022 Study: Zhang et al. (“Plant Hormone Signaling Under Environmental Stress,” Frontiers in Plant Science) showed how abscisic acid and jasmonates help plants survive drought and salinity by closing stomata and activating defense genes.

c. Synthetic Biology

  • New Tools: Scientists use CRISPR to edit hormone-related genes, creating plants with improved growth or resistance.

d. Non-traditional Hormones

  • Strigolactones: Recent research (2021, Nature Plants) revealed their role in communication between plant roots and soil microbes, helping plants absorb nutrients.

7. Debunking a Myth

Myth: “Plant hormones only affect growth, not defense.”

Fact: Many hormones (like jasmonates and salicylic acid) play key roles in plant defense against pests and diseases. Plants use these hormones to activate protective genes and produce chemicals that deter attackers.

8. Latest Discoveries

  • Root Communication: Strigolactones help plants “talk” to beneficial fungi, improving nutrient uptake.
  • Hormone Sensors: In 2023, scientists developed biosensors that visualize hormone movement in living plants, helping us understand how plants respond to stress in real time.
  • Brassinosteroids and Climate Change: Recent studies show brassinosteroids can help plants withstand heat and drought, offering hope for future food security.

Cited Study:
Zhang, Y., et al. (2022). “Plant Hormone Signaling Under Environmental Stress.” Frontiers in Plant Science. Link

9. Summary

  • Plant hormones are vital chemical messengers that control growth, development, and defense.
  • Key experiments from the 19th and 20th centuries revealed their roles in tropisms, cell division, and fruit ripening.
  • Modern agriculture and biotechnology rely on hormones for crop improvement and protection.
  • Recent breakthroughs include hormone crosstalk, advanced gene editing, and real-time hormone tracking.
  • Latest discoveries show hormones help plants survive stress and communicate with soil microbes.
  • Myth: Hormones only affect growth—debunked! They also protect plants from threats.
  • Understanding plant hormones helps us develop resilient crops for a changing world.