Plant Growth Regulators (PGRs)

Plants grow and develop using special chemicals called Plant Growth Regulators (PGRs) 🌿. These are tiny molecules that control everything from cell division to fruit ripening! They work with internal (like genes) and external factors (like light or temperature) to help plants thrive.

Key Concepts for NEET

  1. 🍃 Classification of PGRs: Growth promoters (auxins, gibberellins, cytokinins) vs. growth inhibitors (abscisic acid). Ethylene is a “wildcard” gas!
  2. ⚡ Physiological effects: Auxins cause apical dominance, gibberellins boost stem length, cytokinins delay leaf aging, ethylene ripens fruits, ABA handles stress.
  3. 🔍 Discovery: All major PGRs were found accidentally! (e.g., ethylene from ripe oranges, auxins from oat tips).
  4. 🌾 Agricultural uses: PGRs are farmers’ friends! (e.g., auxins for weed control, gibberellins for bigger grapes).

Characteristics of PGRs

  • Small, simple molecules with diverse chemical structures.
  • Also called plant hormones or phytohormones.
  • Two main groups:
    • ✅ Growth promoters: Help cell division, flowering, fruiting (e.g., auxins, gibberellins, cytokinins).
    • ⛔ Growth inhibitors: Control stress response, dormancy, leaf drop (e.g., abscisic acid).
    • 🌫️ Ethylene: A gaseous PGR that mostly inhibits growth but has unique roles.

Discovery of PGRs

  • 🌾 Auxins: Found after Charles Darwin saw grass tips bending toward light! Isolated from oat seedlings.
  • 🍚 Gibberellins: Discovered when a fungus (Gibberella fujikuroi) made rice seedlings grow too tall.
  • 🧬 Cytokinins: Kinetin was found in herring sperm DNA! Later, natural zeatin was found in corn.
  • 🍊 Ethylene: Ripe oranges released a gas that ripened bananas faster.
  • 🥀 Abscisic acid (ABA): Identified from three “inhibitors” that caused dormancy and leaf fall.

Physiological Effects of PGRs

1. Auxins (e.g., IAA, NAA, 2,4-D) 📏

  • Made in shoot/root tips and transported to other parts.
  • Uses:
    • Stem cuttings grow roots → great for plant cloning! 🌱
    • Promote flowering (e.g., pineapples).
    • Prevent early fruit/leaf drop but help older leaves fall.
    • Cause apical dominance: Top bud suppresses side buds. Removing the top bud makes bushes bushier! (Used in tea gardens ☕).
    • Induce seedless fruits (e.g., tomatoes 🍅).
    • Herbicides: 2,4-D kills weeds but not grass.

2. Gibberellins (e.g., GA3) 📈

  • Over 100 types! Mostly acidic.
  • Uses:
    • Make grape stalks longer 🍇.
    • Improve apple shape.
    • Delay aging → fruits stay on trees longer.
    • Boost sugarcane yield by 20 tons/acre by lengthening stems.
    • Speed up beer malting 🍺.
    • Help cabbages flower faster.

3. Cytokinins (e.g., kinetin, zeatin) 🔄

  • Made where cells divide fast (root tips, young fruits).
  • Uses:
    • Create new leaves and shoots.
    • Overcome apical dominance.
    • Delay leaf aging by moving nutrients.

4. Ethylene 🎈

  • A gas! Released by ripe fruits and aging tissues.
  • Uses:
    • Ripens fruits (e.g., tomatoes 🍅, apples 🍏).
    • Promotes leaf/flower drop.
    • Breaks seed/bud dormancy (e.g., peanuts 🌰).
    • Helps rice stems grow taller in water.
    • Induces flowering (e.g., pineapples, mangoes 🥭).
    • Ethephon (liquid ethylene) is used in farming.

5. Abscisic Acid (ABA) 🛡️

  • Known as the “stress hormone”.
  • Uses:
    • Stops seed germination.
    • Closes leaf pores (stomata) during drought.
    • Helps seeds survive harsh conditions via dormancy.
    • Generally opposes gibberellins!

Summary & Interactions

Every stage of plant growth needs PGRs! They can work together (synergistic) or against each other (antagonistic). For example:

  • 🌱 Apical dominance: Controlled by auxins + cytokinins.
  • 🍂 Leaf drop: Involves ethylene + auxins.
  • 😴 Seed dormancy: ABA promotes it, gibberellins break it!

PGRs are just one part of plant control. Genes and environment (light, temperature) also shape growth. For instance, light/temperature affect flowering via PGRs!