Plant Growth Regulators (PGRs)
Plant Growth Regulators (PGRs) (phytohormones) are small organic molecules produced in plants in very low concentration, but they control growth + development (cell division, elongation, flowering, dormancy, stress responses).
Key Point to Memorise:
- PGRs act like chemical messengers. Even tiny amounts can cause big changes.
Two broad groups
- Growth promoters: Auxins, Gibberellins, Cytokinins → promote cell division/elongation, flowering, fruit growth.
- Growth inhibitors / stress regulators: Abscisic acid (ABA), Ethylene → dormancy, abscission, stress responses.
Discovery
Auxin Discovery (Darwin → Went)
- Darwin & Francis Darwin: coleoptile bends towards light → tip produces a diffusible growth signal.
- F.W. Went: placed coleoptile tip on agar block → chemical diffused into agar → agar placed on one side of cut coleoptile caused bending.
Memorise Line: Auxin is produced at tip and causes phototropism by unequal distribution.
Gibberellin Discovery (Bakanae disease)
- In rice, infected seedlings grew very tall (“foolish seedling”).
- Kurosawa traced it to a fungus Gibberella fujikuroi producing gibberellin (GA₃).
Memorise Line: Bakanae disease → Gibberellins → excessive stem elongation.
1. Auxins
What are Auxins?
Auxins mainly promote cell elongation and control plant growth patterns like apical dominance.
Types
- Natural: IAA (Indole-3-acetic acid), IBA
- Synthetic: NAA, 2,4-D
Site of synthesis
- Shoot apical buds and young leaves (highest production)
Polar transport
Auxin moves from apex to base (basipetal) in a polar manner.
Major functions of Auxin
1.Apical dominance (TOP CONTROL)
- Auxin from apical bud suppresses lateral buds.
- Decapitation → lateral buds grow → bushy plant.
2. Cell elongation (STEM LENGTH)
- Auxin loosens cell wall → cells elongate.
3. Root initiation (CUTTING ROOTS)
- IAA/IBA/NAA used as rooting hormones in cuttings.
4. Tropisms (BENDING)
- Phototropism + geotropism due to unequal auxin distribution.
5. Parthenocarpy (SEEDLESS FRUIT)
- Fruit formation without fertilisation (e.g., tomato).
6. Abscission control (LEAF/FRUIT DROP CONTROL)
- Prevents premature drop of young leaves/fruits.
- Promotes abscission in older organs (age-dependent response).
7.Weedicide (2,4-D)
- Kills dicot weeds in monocot fields (lawns/cereals).
Important Point: Auxin = Apical dominance + Cell elongation + Rooting + Tropisms + 2,4-D weedicide
Acid Growth Hypothesis
Auxin activates H⁺ pumps → cell wall pH decreases → wall-loosening enzymes work → cell elongation increases.
2.Gibberellins (GA)
What are Gibberellins?
Gibberellins promote stem elongation, bolting, seed germination, and dormancy breaking.
Important GA
- GA₃ = most studied and commonly used.
Site of synthesis
- Young leaves, roots, developing seeds
Major functions of Gibberellins
1.Internode elongation (TALL PLANTS)
- Most striking effect: stem elongation.
2. Bolting (ROSETTE → LONG STEM)
- In cabbage/beet: short internodes suddenly elongate before flowering.
3. Breaking dormancy (WAKE UP SEEDS/BUDS)
- GA helps break seed/bud dormancy.
4. Seed germination (AMYLASE KEYWORD)
- GA from embryo → aleurone layer → α-amylase synthesis → starch → sugars.
5. Malting industry (BARLEY)
- GA speeds up malting by boosting amylase.
6. Fruit size & shape
- Bigger grapes; improves shape of some fruits.
7.Delay senescence
- Keeps leaves/fruits green and usable longer.
Important Point: Gibberellin = Stem elongation + Bolting + Break dormancy + α-amylase in germination + Malting
Auxin vs GA
- Auxin: Apical dominance + rooting + cell elongation + 2,4-D weedicide
- GA: Internode elongation + bolting + seed dormancy break + germination enzymes
Auxin & Gibberellin
AUXIN (A)
A = Apical dominance (top bud suppresses side buds) A = Adventitious roots (rooting hormone) A = Acid growth (H⁺ pump → wall loosens → elongation) A = 2,4-D (dicot weedicide)
GIBBERELLIN (GA)
GA = Grow Above (stem elongation) GA = Bolting (rosette → tall stem) GA = Germination enzyme (α-amylase) GA = Break dormancy GA = Malting (barley)
Discovery Clues
- Auxin: Darwin → Went (agar block)
- GA: Bakanae disease (Gibberella fujikuroi)
💡 Questions and Answers
Q1: What are the physiological functions of Auxins and Gibberellins?
A1:
- Auxins mainly control apical dominance, help in cell elongation, promote roots in cuttings, help in tropisms, can make seedless fruits, and 2,4-D is used to kill dicot weeds.
- Key Points: Apical dominance, elongation, rooting, tropism, parthenocarpy, 2,4-D
- Gibberellins mainly make plants taller by increasing internode length, cause bolting in rosette plants, help in breaking dormancy, and help seeds germinate by making α-amylase. They are also used in malting and fruit enlargement.
- Key Points: Stem elongation, bolting, dormancy break, α-amylase, malting
Q2: Match the PGRs with their functions.
(a) Auxin | (i) Bolting (b) Gibberellin | (ii) Weed-free lawns
A2:
- (a) Auxin → (ii) Weed-free lawns (because 2,4-D kills dicot weeds in lawns)
- (b) Gibberellin → (i) Bolting (causes rapid internode elongation)
Q3: What is apical dominance and how does auxin cause it?
A3: Apical dominance means the top bud controls the plant and does not allow side buds to grow much. This happens because the apical bud makes auxin, and auxin moves downward and suppresses lateral buds. If the top bud is cut, the plant becomes bushy.
- Key Points: Top bud → auxin → suppresses lateral buds → decapitation makes plant bushy
Q4: How did F.W. Went prove the existence of auxin?
A4: Went kept the coleoptile tip on an agar block, so the growth chemical diffused into the agar. When he placed that agar block on one side of a cut coleoptile, it bent. This proved a chemical growth substance (auxin) controls bending.
- Key Points: Tip → agar → chemical diffuses → agar causes bending
Q5: What is bolting caused by gibberellins?
A5: Bolting is when a plant that normally has a short stem (rosette) suddenly shows rapid stem elongation before flowering. Gibberellins cause this by increasing internode length.
- Key Points: Rosette plant → GA → rapid internode elongation → bolting