A. 2,4-D → III (Herbicide): 2,4-Dichlorophenoxyacetic acid — synthetic auxin at high dose. Kills broad-leaved dicot weeds. Monocot crops (wheat, rice) are unaffected.
B. GA₃ → I (Brewing): Gibberellic acid promotes amylase synthesis in germinating barley → starch → maltose → fermentation → beer/whisky.
C. Kinetin → IV (Nutrient mobilisation): Cytokinin delays senescence. Directs nutrient flow towards areas with high cytokinin → 'Richmond Lang Effect'.
D. ABA → II (Stomatal closure): Abscisic acid triggers stomatal closure during drought. Also called 'stress hormone' or 'dormin'.
Plant growth regulators (PGRs) or phytohormones are organic compounds produced in minute quantities that regulate plant growth and development. They act at a site distant from their point of synthesis (like animal hormones). Five classical PGRs: Auxins (promote elongation, apical dominance), Gibberellins (stem elongation, seed germination), Cytokinins (cell division, anti-senescence), Abscisic acid (stress response, dormancy), Ethylene (fruit ripening, abscission). Modern additions: Brassinosteroids, Polyamines, Salicylic acid, Jasmonates. Each PGR affects multiple processes; the same process may be regulated by multiple PGRs in an integrated network.
First plant hormone discovered. Darwin (1880) demonstrated phototropism (bending toward light) in coleoptiles. Went (1926): isolated 'Wuchsstoff' (growth substance) — named auxin (from Greek 'auxein' = to grow). Chemical identity: IAA (indole-3-acetic acid) — discovered by Kögl and Haagen-Smit (1931). Synthesis: primarily in shoot apex (apical meristem, young leaves), developing seeds. Transport: polar (unidirectional) auxin transport from apex to base (basipetal in stems). Functions: (1) Cell elongation (acid growth — activates H⁺ pumps → wall loosening → elongation). (2) Apical dominance: high IAA from apex inhibits lateral buds. (3) Root initiation: promotes adventitious root formation. (4) Fruit development: auxin from seeds promotes fruit growth. (5) Phototropism: unequal auxin distribution → unequal growth → bending. (6) Geotropism. Synthetic auxins: 2,4-D (herbicide at high dose), NAA (rooting powder), IBA.
Gibberellins were discovered from the fungus Gibberella fujikuroi (causes 'bakanae' disease — foolish seedling disease in rice, characterised by abnormal elongation). Kurosawa (1926): showed fungal extracts caused elongation. Yabuta and Sumiki (1938): isolated gibberellic acid (GA₃). Over 100 gibberellins known (GA₁-GA₁₀₀+). Active forms: GA₁ most active for stem elongation in many plants. Effects: (1) Stem elongation: promotes cell division AND elongation. Dwarf mutants lack GA → GA treatment restores normal height (e.g., 'Rosette' plants). (2) Seed germination: promotes α-amylase synthesis in aleurone layer of barley → starch → sugars for germination. (3) Bolting: promotes elongation of rosette plants before flowering. (4) Fruit elongation: promotes elongation of grapes (Thompson seedless variety). (5) Vernalisation substitution: can replace cold requirement in some biennials. (6) Sex determination: promotes male flower formation in cucumber. Brewing: GA promotes amylase → malting.
Cytokinins promote cytokinesis (cell division). Discovered by Skoog and Miller (1957). First cytokinin isolated: kinetin (from old herring sperm DNA). Natural cytokinin: zeatin (from Zea mays, maize — most active natural cytokinin). Also trans-zeatin, dihydrozeatin, isopentenyl adenine. Synthesis: primarily in root tips and developing seeds/fruits. Transport: from roots to shoots via xylem. Effects: (1) Cell division: essential for cytokinesis. (2) Delay of senescence (anti-aging): keeps leaves green, delays chlorophyll breakdown. 'Richmond-Lang effect': isolated leaf placed in cytokinin solution → stays green while adjacent untreated portion yellows. (3) Nutrient mobilisation: cytokinins attract nutrients to areas of application. (4) Breaking apical dominance: promotes lateral bud growth (antagonistic to auxin). (5) Seed germination: promotes germination in some species. (6) Chloroplast development and greening. IAA:Cytokinin ratio determines organ formation in tissue culture: High IAA/low CK → roots; Low IAA/high CK → shoots; Equal → callus.
ABA (Abscisic acid): discovered independently as 'abscicin II' (promotes leaf abscission) and 'dormin' (induces dormancy). Chemical: sesquiterpenoid. Synthesis: in leaves (mature), roots, seeds. Transport: from leaves to roots via phloem; from roots to leaves via xylem. Effects — ABA is mainly INHIBITORY: (1) Stomatal closure: ABA → K⁺ efflux from guard cells → guard cells lose turgor → stomata close → reduces transpiration during drought. (2) Seed dormancy: high ABA maintains dormancy. GA breaks dormancy by overcoming ABA. ABA:GA ratio determines dormancy. (3) Fruit abscission: involved in abscission zone formation. (4) Response to stress: water stress, salinity, cold → ABA accumulates. ABA is the 'stress signal' hormone. (5) Inhibits cell elongation (opposite of auxin). ABA acts through ABA receptor (PYR/PYL/RCAR proteins) → inhibits PP2C phosphatases → activates SnRK2 kinases → phosphorylates SLAC1 (anion channel) → opens anion channel → K⁺ follows → turgor loss → stomatal closure.
Ethylene (C₂H₄): the only gaseous plant hormone. Discovered when coal gas (containing ethylene) caused premature fruit ripening in stored fruit and leaf epinasty in nearby plants. Gane (1934): showed ripening fruits produce ethylene. Synthesis: from methionine via ACC (1-aminocyclopropane-1-carboxylate) → ethylene (by ACC oxidase/ACC synthase pathway). Synthesis induced by: mechanical damage, wounding, flooding, high temperature, other hormones (IAA at high concentration). Effects: (1) Fruit ripening: climacteric fruits (apple, banana, mango, tomato) show ethylene surge → ripening. Commercial: banana ripening with ethylene gas (calcium carbide → acetylene → similar effect). (2) Abscission: promotes leaf, flower, fruit abscission. (3) Epinasty: leaf petioles curve downward. (4) Hook formation in seedlings (apical hook). (5) Root elongation at low concentration, inhibits at high. (6) Female flower induction in cucumber (opposite to GA). (7) Aerenchyma formation in flooded plants (to survive waterlogging). (8) Senescence acceleration.
Plant hormones rarely act alone — they interact in complex networks. Auxin-Cytokinin antagonism: high IAA/low CK → root formation in tissue culture. Low IAA/high CK → shoot formation. Equal → callus. Auxin-Ethylene: high IAA → induces ACC synthase → ethylene production → ethylene causes some IAA effects (root inhibition, epinasty). ABA-GA antagonism: ABA promotes dormancy; GA promotes germination. The ABA/GA ratio determines whether a seed remains dormant or germinates. Ethylene-ABA: both promote abscission but through different pathways. Brassinosteroids-Auxin: synergistic in cell elongation. Jasmonate-Salicylate: antagonistic in defence responses (JA → herbivore defence; SA → pathogen defence). Crosstalk between signalling pathways allows integration of multiple environmental and developmental signals for coordinated growth responses.
Auxins: IBA (indole butyric acid) — rooting powder for vegetative propagation (cuttings). NAA — prevents pre-harvest fruit drop in apple. 2,4-D — herbicide (kills dicot weeds in monocot crops). 2,4,5-T — defoliant (used in Agent Orange during Vietnam War). Gibberellins: GA₃ in brewing (promotes amylase). Elongation of grapes (Thompson seedless — looser bunches, larger berries). Delay fruit senescence. Bolting of sugarbeet. Cytokinins: delay leaf senescence in stored vegetables. Tissue culture (shoot regeneration). Fruit growth. ABA: stored seeds (maintains dormancy — prevents sprouting). Ethylene: banana ripening (exposure to ethylene gas). Fruit degreening in citrus (removes green chlorophyll). CCC (chlormequat chloride) / Alar (daminozide) — gibberellin biosynthesis inhibitors → dwarfing agents for crop lodging resistance. Paclobutrazol — inhibits GA biosynthesis → compact ornamental plants.