🌱 Plant Growth Regulators

- by Prof. Siddharth Sanghvi Sir(BSI SIR)

🌿 AUXIN

Discovery Experiments

  • Darwin & Darwin: Canary grass (Phalaris canariensis) coleoptile experiment - showed unilateral light causes positive curvature toward light source
  • Boysen-Jensen: Proved curvature is due to differential growth of different parts - high auxin causes more cell elongation
    • Key Point: Gelatin allows substance to pass (water soluble), but Mica slab blocks it. Butter paper shows it's not electrical impulse, not fat-soluble.
  • Paal: Cut tip placed eccentrically showed curvature without light, proving substance is chemical, not electrical impulse
  • Went: Isolated AUXIN using Agar blocks on Avena coleoptile - more tips = more curvature (Avena curvature test/Bioassay)
  • KΓΆgl, Haagen-Smit: Isolated auxin from human urine

Chemical Nature

  • Natural Auxin: Indole-3-acetic acid (IAA), Indole-3-butyric acid (IBA)
  • Precursor: Tryptophan β†’ Zn²⁺ β†’ IAA
  • Structure: Contains indole ring (benzene + pyrrole)
  • Synthetic Auxins: 2,4-D (2,4-Dichlorophenoxyacetic acid), 2,4,5-T (Agent Orange), NAA (Naphthalene acetic acid)

Transport & Formation

  • Transport: Generally Basipetal (tip to base), polar transport
  • Formation site: Shoot tip (maximum), Root tip (also)
  • Charge: Auxin has negative charge

Functions

  • Root Initiation: Promotes root formation
  • Apical Dominance: Stops growth of lateral branches - used in hedge making in tea plantations
  • Cell Elongation: High concentration in shoot elongation; cone shape in conifers
  • Feminism: In monoecious/unisexual flowers, auxin favors female flower development (e.g., cucumber, pineapple)
  • Parthenocarpy: Induces seedless fruit formation in tomato
  • Vascular Differentiation: Auxin + 2-3% sugar β†’ xylem; Auxin + 4% sugar β†’ phloem
  • Abscission: Delays abscission in young plants (leaf, flowers); Can induce abscission in mature/old plants
  • Weedicide: 2,4-D and 2,4,5-T used as herbicide/weedicide on broadleaf plants, safe for monocot crops

🌾 GIBBERELLIN (GA₃)

Discovery

  • Source: Japan - Rice "Foolish Seedling" (Bakanae disease)
  • Causative Agent: Fungus - Gibberella fujikuroi (Fusarium moniliforme)
  • Kurosawa: Infected plant extract on healthy plant β†’ Bakanae disease symptoms, proving presence of substance that increases plant height
  • Nature: Acidic, terpenoid (type of lipid) - Over 100 forms (GA₁, GAβ‚‚, GA₃...GA₁₀₀), Most common is GA₃

Functions

  • Internodal Elongation: Main function - fast stem elongation
  • Bolting: Converts biennial plants to annuals by causing internodal elongation just prior to flowering
  • Seed Germination: GA₃ induces DNA transcription β†’ mRNA β†’ Ξ±-amylase enzyme β†’ hydrolyzes starch β†’ maltose β†’ glucose
  • Delay Senescence: Delays overall plant aging
  • Hastens Maturity: In young conifers - leads to early seed production
  • Rosette Breakdown: In cabbage, beet - causes internodal elongation

Applications

  • Grapes: Increases stalk length
  • Sugarcane: Internodal elongation increases production by 20 tonnes/acre
  • Apple: Improves shape and size
  • Cabbage & Beet: Promotes bolting for early flowering
  • Young Conifers: Hastens maturity and seed production
  • Brewing Industry: GA₃ speeds up malting process in barley for alcohol production
  • Extend Market Period: Delays senescence, allowing fruits to stay on tree longer
Remember: GA₃ is primarily a growth promoter focusing on stem elongation and breaking dormancy patterns.

πŸ”¬ CYTOKININ (CK)

Discovery

  • Skoog: Tobacco internodal explant experiment
  • Experiment: Auxin + Yeast extract/Coconut milk/DNA extract β†’ Cells separated (Callus formation)
  • Callus: Mass of undifferentiated cells
  • Isolation: Skoog & Miller - Autoclaved herring sperm DNA β†’ 6-furfurylaminopurine β†’ Named KINETIN
  • Natural CK: Zeatin (isolated by Letham from Zea mays/Corn)
Note: Kinetin is not normally found in plants naturally.

Chemical Nature

  • Type: Adenine derivatives (purine base - contains imidazole ring + benzene)
  • Natural CK: Zeatin from corn
  • Synthetic: Kinetin

Functions

  • Cytokinesis: Induces cell division
  • Richmond-Lang Effect: Delays leaf senescence by:
    • Increasing chlorophyll/chloroplast production
    • Inducing cell division and protein production in old leaves
    • Nutrient retention and mobilization
  • Breaks Apical Dominance: CK favors lateral shoot growth
  • Organogenesis in Tissue Culture: Critical for shoot/root differentiation

Tissue Culture Ratios

  • High Auxin + Low CK β†’ Root initiation and development
  • Low Auxin + High CK β†’ Shoot differentiation
  • High Auxin + Intermediate CK β†’ Callus formation
  • No Auxin + CK only β†’ No growth

Production Sites

  • Root apex
  • Shoot bud (young)
  • Young fruits

πŸ’¨ ETHYLENE

General Characteristics

  • Only Gaseous Hormone
  • Formula: Cβ‚‚Hβ‚„
  • Synthetic form: Ethephon (aqueous solution, readily absorbed and transported, releases ethylene slowly)

Functions

  • Fruit Ripening (Climacteric Fruits): Ethylene induces ripening in:
    • Banana
    • Mango
    • Apple
    No effect in Non-Climacteric fruits like grapes
  • Climacteric: Rate of respiration increases in harvested fruit
  • Triple Response (Dicot Seedling Germination):
    • Swelling of shoot apex
    • Horizontal growth of shoot
    • Apical hook formation (protection of shoot tip/plumule)
  • Breaks Dormancy: Seed dormancy and bud dormancy
  • Germination: Helps in germination of peanuts and sprouting of potato
  • Fruit Thinning: In walnut, cherry, cotton by inducing abscission in extra flowers
  • Induces Abscission: Promotes leaf, flower, and fruit drop
  • Flowering: Helps in flower and fruit set formation in pineapple; Induces flowering in mango
  • Feminism: Increases female flowers in cucumber
  • Deep Water Rice: Ethylene promotes rapid internode elongation in submerged rice
  • Root Development: Root hair formation increases surface area for water and mineral absorption

Applications

  • Ethephon hastens: Fruit ripening in tomatoes and apples
  • Ethephon accelerates: Abscission in flowers and fruits
Key Distinction: Climacteric fruits show increased respiration during ripening and respond to ethylene. Non-climacteric fruits do not.

πŸ›‘ ABSCISIC ACID (ABA)

General Characteristics

  • Derived from: Carotenoids
  • Type: Sesquiterpene (15-carbon compound)
  • Isolation: Separately isolated in 3 different forms - Abscisin II, Dormin, and ABA (all are basically same)
  • Nature: Growth inhibitor / Stress hormone

Functions of ABA

  • Induces Abscission: Leaf, flower, and fruit drop
  • Dormancy: Induces seed dormancy and bud dormancy
  • Inhibition of Growth: Especially during biological stress conditions
  • Stomatal Closing: Helps in stress conditions (water stress/drought) by closing stomata to reduce transpiration
  • Seed Development: Helps in seed maturation and seed desiccation tolerance
  • Stress Hormone: Overall protection during adverse environmental conditions
Remember: ABA is the primary stress hormone and growth inhibitor. It helps plants survive unfavorable conditions by reducing water loss and metabolic activity.

Photoperiodism & Phytochrome (Related Concept)

  • Photoperiodism: Effect of light duration on flowering
  • SDP (Short Day Plant / LNP): Light period less than critical period induces flowering; Dark period MORE than critical period required
  • LDP (Long Day Plant / SNP): Light period more than critical period induces flowering; Dark period LESS than critical period required
  • Phytochrome: Light receptor - Pr (660nm red light) ↔ Pfr (730nm far-red light)
  • Flash of Light: Can interrupt dark period in SDP and prevent flowering

πŸ“ Interactive Quiz - Plant Hormones