Plant Growth Regulators — All Key Points for NEET

Auxins:

• Chemical nature: Indole compounds
• Natural: IAA; Synthetic: 2,4-D, NAA, IBA
• Discovered by: F.W. Went (Avena coleoptile experiment)
• Functions: Cell elongation, apical dominance, phototropism, gravitropism, parthenocarpy
• Dose-response: Low = promotes; High = inhibits
• 2,4-D = selective herbicide: kills dicots, not monocots
• NAA = prevents fruit drop; promotes adventitious rooting
• Produced at: shoot apex, young leaves, developing seeds

Gibberellins:

• Chemical nature: Terpenoids
• Most studied: $GA_{3}$
• Discovered by: Kurosawa from Gibberella fujikuroi (bakanae disease of rice)
• Functions: Stem elongation/bolting in rosette plants, breaks seed and bud dormancy, parthenocarpy, delays senescence, induces alpha-amylase in aleurone layer of barley
• Agricultural uses: Grape stalk elongation, malting acceleration
• Antagonist to: ABA (in dormancy/germination)
• Both GA and ABA are terpenoids

Cytokinins:

• Chemical nature: Adenine derivatives (substituted purines)
• Examples: Kinetin, Zeatin, BAP
• Discovered by: Skoog and Miller (from herring sperm DNA)
• Functions: Promotes cell division (cytokinesis), delays leaf senescence (Richmond-Lang effect), promotes lateral bud growth (counteracts auxin), nutrient mobilization
• Richmond-Lang effect: Kinetin delays chlorophyll degradation and protein breakdown
• Used in: Tissue culture, post-harvest treatment

Ethylene:

• Chemical nature: Gaseous hydrocarbon ($C_{2}H_{4}$) — ONLY gaseous hormone
• Discovered by: Cousins (ripening near gas burners)
• Functions: Promotes fruit ripening (banana, mango), leaf and flower abscission, triple response in etiolated seedlings, feminization in cucumber, breaks seed dormancy
• Triple response: Inhibited elongation + radial swelling + horizontal growth
• Commercial use: Ethephon for ripening; pineapple flowering; cucumber feminization
• Inhibited by: Low $O_{2}$ (blocks ACC oxidase)

ABA (Abscisic Acid):

• Chemical nature: Terpenoid (sesquiterpenoid)
• Common name: Stress hormone
• Discovered by: Addicott and Carns
• Functions: Stomatal closure ($K^{+}$ efflux from guard cells), maintains seed dormancy, inhibits growth, promotes senescence
• Antagonist to: Gibberellin (GA breaks dormancy; ABA maintains it)
• Agricultural use: Antitranspirant
• Signal pathway: ABA → PYR/PYL → SnRK2 → $K^{+}$ efflux → stomata close