Plant Growth Regulators — Revision Notes

⚡ 30-Second Revision

Auxins (IAA, IBA, NAA, 2,4-D) — Cell elongation, apical dominance, root initiation, parthenocarpy, herbicide (2,4-D).
Gibberellins (GA3) — Stem elongation (bolting), break seed dormancy, fruit enlargement, malting.
Cytokinins (Kinetin, Zeatin) — Cell division, lateral bud growth, delay senescence, shoot differentiation.
Ethylene ($C_2H_4$) — Gaseous, fruit ripening, senescence, abscission, triple response.
Abscisic Acid (ABA) — Stress hormone, seed dormancy, stomatal closure, abscission, growth inhibitor.
Promoters — Auxins, Gibberellins, Cytokinins.
Inhibitors — ABA, Ethylene (often).
Antagonistic pairs — GA-ABA (dormancy), Auxin-Cytokinin (apical dominance, shoot/root ratio).

2-Minute Revision

Plant Growth Regulators (PGRs) are organic compounds that control plant growth and development. They are broadly categorized into promoters (Auxins, Gibberellins, Cytokinins) and inhibitors (Ethylene, Abscisic Acid).

Auxins, like IAA, promote cell elongation, apical dominance, and root formation, with synthetic forms (2,4-D) used as herbicides. Gibberellins (GA3) are known for stem elongation, breaking seed dormancy, and increasing fruit size.

Cytokinins (e.g., Zeatin) are crucial for cell division, promoting lateral bud growth, and delaying leaf senescence. Ethylene, a unique gaseous hormone, accelerates fruit ripening, senescence, and abscission, and mediates the 'triple response.

' Abscisic Acid (ABA) is a stress hormone, inducing seed dormancy, closing stomata during drought, and promoting abscission. Understanding their individual roles and how they interact (synergistically or antagonistically) is key for NEET, as these interactions fine-tune plant responses to internal cues and environmental changes.

5-Minute Revision

Plant Growth Regulators (PGRs), or phytohormones, are vital chemical messengers in plants, active in minute quantities. They are classified into promoters and inhibitors. Promoters include:

1
Auxins — Discovered by Went, natural form is IAA. Functions include cell elongation, apical dominance, root initiation in cuttings, parthenocarpy (seedless fruits), and xylem differentiation. Synthetic auxins like 2,4-D are selective herbicides for dicot weeds. High auxin inhibits root elongation.
2
Gibberellins (GAs) — Discovered by Kurosawa from *Gibberella fujikuroi*. Over 100 types, GA3 is most common. Effects: dramatic stem elongation (bolting in rosette plants like cabbage), breaking seed dormancy (via $alpha$-amylase synthesis in aleurone layer), increasing fruit size (grapes), and promoting malting.
3
Cytokinins — Discovered by Skoog and Miller (kinetin), natural form is zeatin. Functions: essential for cell division, promoting lateral bud growth (counteracting apical dominance), delaying leaf senescence, and controlling shoot/root differentiation in tissue culture (low auxin:cytokinin ratio for shoot, high for root).

Inhibitors include:

1
Ethylene ($C_2H_4$) — A gaseous hormone. Effects: promotes fruit ripening (climacteric fruits), accelerates senescence and abscission of leaves and flowers, and causes the 'triple response' in dicot seedlings (shortening of hypocotyl, thickening of hypocotyl, horizontal growth).
2
Abscisic Acid (ABA) — The 'stress hormone.' Functions: induces and maintains seed and bud dormancy, promotes stomatal closure during water stress (conserving water), and accelerates abscission of older leaves and fruits. It acts antagonistically to gibberellins in seed dormancy.

Interactions: PGRs rarely act alone. Auxins and cytokinins are antagonistic in apical dominance and synergistic in cell division. Gibberellins and ABA are antagonistic in seed dormancy. Understanding these interactions is crucial for predicting plant responses. NEET questions often focus on specific examples, applications, and the unique properties of each hormone.

Prelims Revision Notes

1
Plant Growth Regulators (PGRs) / Phytohormones — Small, simple molecules, diverse chemical composition, act in low concentrations.
2
Classification — Promoters (Auxins, Gibberellins, Cytokinins) and Inhibitors (Ethylene, Abscisic Acid).
3
Auxins

* Discovery: Darwin (phototropism), Went (isolated IAA from oat coleoptiles). * Natural: Indole-3-acetic acid (IAA), Indole-3-butyric acid (IBA). * Synthetic: Naphthalene acetic acid (NAA), 2,4-Dichlorophenoxyacetic acid (2,4-D). * Functions: Cell elongation, apical dominance, root initiation in cuttings, parthenocarpy (seedless fruits like tomato), prevents premature fruit drop, 2,4-D as selective herbicide (kills dicot weeds).

1
Gibberellins (GAs)

* Discovery: Kurosawa (from *Gibberella fujikuroi* causing 'bakanae' disease in rice). * Type: GA3 (Gibberellic acid) most common. * Functions: Stem elongation (e.g., dwarf peas, maize), bolting (internode elongation in rosette plants like cabbage, beet), breaking seed dormancy, promoting seed germination (induces $alpha$-amylase in aleurone layer), increasing fruit size (grapes, apples), speeding up malting process.

1
Cytokinins

* Discovery: Skoog & Miller (kinetin from degraded DNA), Letham (zeatin from corn kernels). * Natural: Zeatin (corn, coconut milk). * Functions: Cell division (cytokinesis), promoting lateral bud growth (overcoming apical dominance), delaying senescence (Richmond-Lang effect), promoting chloroplast development, shoot differentiation in tissue culture (low auxin:cytokinin ratio).

1
Ethylene ($C_2H_4$)

* Nature: Gaseous hormone. * Discovery: Cousins (volatile substance from oranges ripened bananas). * Functions: Fruit ripening (climacteric fruits like banana, apple, tomato), senescence, abscission (leaves, flowers, fruits), epinasty, root growth and root hair formation, 'triple response' in dicot seedlings (inhibition of stem elongation, radial swelling, horizontal growth).

1
Abscisic Acid (ABA)

* Nature: Carotenoid derivative, 'stress hormone'. * Discovery: Addicott (abscisin II), Wareing (dormin). * Functions: Induces and maintains seed dormancy, promotes stomatal closure during water stress, promotes abscission (older leaves, fruits), inhibits growth and metabolism, induces bud dormancy.

1
Interactions

* GA vs. ABA: Antagonistic in seed dormancy. * Auxin vs. Cytokinin: Antagonistic in apical dominance; synergistic in cell division. * Auxin vs. Ethylene: Auxin can induce ethylene production; ethylene can inhibit auxin transport.

Vyyuha Quick Recall

A G C E A: All Growing Cells Exhibit Activity.

Auxin: Apical dominance, Absence of seeds (parthenocarpy), Allongation.
Gibberellin: Giant growth (stem elongation/bolting), Germination (breaks dormancy), Grape size.
Cytokinin: Cell division, Counteracts apical dominance (lateral buds), Chloroplasts, Celay senescence.
Ethylene: Every fruit Eats Ethylene (ripening), Everything Ends (senescence/abscission), Epinasty.
Abscisic Acid: All Adverse conditions (stress hormone), Arrests growth (dormancy), Absent water (stomatal closure), Abscission.