Cytokinins, Ethylene and ABA — Revision Notes

⚡ 30-Second Revision

Cytokinins — Adenine derivatives. Promote cell division, break apical dominance, delay senescence, promote lateral bud growth. Used in tissue culture (high cytokinin:auxin for shoots).
Ethylene — Gaseous hormone ($C_2H_4$). Promotes fruit ripening (climacteric), senescence, abscission. Induces 'triple response' in seedlings.
Abscisic Acid (ABA) — Carotenoid derivative. 'Stress hormone'. Induces stomatal closure (drought), promotes seed dormancy, bud dormancy, abscission. Antagonistic to gibberellins.

2-Minute Revision

Cytokinins, Ethylene, and Abscisic Acid (ABA) are vital plant growth regulators. Cytokinins, derived from adenine, are primarily known for promoting cell division (cytokinesis). They effectively break apical dominance by stimulating lateral bud growth, delay leaf senescence (Richmond-Lang effect), and are crucial in plant tissue culture for shoot differentiation when in high ratio to auxin.

Ethylene is unique as a gaseous hormone, playing a central role in fruit ripening, accelerating senescence, and promoting the abscission of leaves and fruits. It also causes the 'triple response' in dark-grown seedlings, aiding their emergence from soil.

Abscisic Acid (ABA), often termed the 'stress hormone,' is a carotenoid derivative. Its main functions include inducing stomatal closure to conserve water during drought, promoting seed and bud dormancy to ensure survival in unfavorable conditions, and contributing to abscission.

These hormones rarely act in isolation, their effects being a result of complex interactions, such as the antagonism between ABA and gibberellins in dormancy, or auxin and cytokinin in apical dominance.

5-Minute Revision

To master Cytokinins, Ethylene, and Abscisic Acid (ABA) for NEET, focus on their distinct characteristics and interactions.

Cytokinins are adenine derivatives, primarily synthesized in roots. Their hallmark function is promoting cell division, making them essential for growth. They counteract auxin's effect by breaking apical dominance, leading to bushier plants, and delay leaf senescence (the Richmond-Lang effect). In tissue culture, a high cytokinin-to-auxin ratio promotes shoot development. Remember, they are growth promoters.

Ethylene is the only gaseous plant hormone, a simple hydrocarbon ($C_2H_4$). It's famous for promoting fruit ripening in climacteric fruits (e.g., bananas, apples), accelerating senescence (aging) of leaves and flowers, and inducing abscission (shedding) of plant parts.

In dark-grown seedlings, it causes the 'triple response': inhibited elongation, hypocotyl swelling, and exaggerated apical hook, which helps the seedling push through soil. Ethylene is generally considered a growth inhibitor in terms of elongation but a promoter of specific developmental processes like ripening.

Abscisic Acid (ABA) is a 15-carbon carotenoid derivative, often called the 'stress hormone.' Its production increases under adverse conditions like drought, salinity, and cold. ABA's key roles include inducing stomatal closure to reduce water loss, promoting seed and bud dormancy to ensure survival during unfavorable periods, and contributing to abscission.

It generally acts as a growth inhibitor, often antagonizing the effects of growth-promoting hormones like gibberellins (in dormancy) and auxins. For example, ABA promotes dormancy, while gibberellins break it.

Key Interactions: Remember the critical balance: Auxin-Cytokinin ratio for apical dominance and tissue culture; ABA-Gibberellin for seed dormancy; Ethylene-Auxin for abscission. Understanding these interactions is crucial for solving complex NEET questions.

Prelims Revision Notes

Cytokinins (CK)

Chemical Nature — Adenine derivatives (N6-substituted adenines). E.g., Zeatin (natural), Kinetin (synthetic).
Synthesis — Primarily in root apices, developing shoot buds, young fruits.
Functions (Growth Promoter)

* Cell Division (Cytokinesis): Primary role, especially in meristematic regions. * Breaking Apical Dominance: Promotes lateral bud growth, counteracting auxin. * Delay Senescence: Richmond-Lang effect; maintains chlorophyll, mobilizes nutrients. * Chloroplast Development: Promotes their formation. * Tissue Culture: High CK:Auxin ratio promotes shoot differentiation. * Nutrient Mobilization: Creates 'sink' regions.

Ethylene (ET)

Chemical Nature — Gaseous hormone ($C_2H_4$), simple hydrocarbon.
Synthesis — Produced by all plant tissues, especially senescing tissues and ripening fruits.
Functions (Growth Inhibitor/Ripening Agent)

* Fruit Ripening: Key role in climacteric fruits (e.g., banana, mango, apple). Triggers respiration burst, softening, color change, aroma. * Senescence & Abscission: Accelerates aging of leaves/flowers, promotes shedding of plant parts.

* Triple Response: In dark-grown dicot seedlings: inhibited hypocotyl elongation, hypocotyl swelling, exaggerated apical hook. * Root Growth: Inhibits elongation, promotes root hair formation.

* Flowering: Promotes in some plants (e.g., pineapple), inhibits in others.

Abscisic Acid (ABA)

Chemical Nature — 15-carbon sesquiterpenoid, derived from carotenoids.
Synthesis — In leaves, roots, stems, seeds, especially under stress.
Functions ('Stress Hormone', Growth Inhibitor)

* Stomatal Closure: Rapidly induces closure during drought stress to conserve water. * Seed Dormancy: Induces and maintains dormancy, preventing premature germination. Antagonistic to gibberellins. * Bud Dormancy: Induces dormancy in perennial plants. * Abscission: Promotes abscission, often by sensitizing tissues to ethylene. * Growth Inhibition: Generally inhibits shoot growth and cell division.

Key Interactions to Remember:

Auxin-Cytokinin — Apical dominance (Auxin promotes, CK breaks); Tissue culture (ratio for root/shoot).
ABA-Gibberellin — Seed dormancy (ABA promotes, GA breaks).
Ethylene-Auxin — Abscission (Auxin can induce ET, which promotes abscission).

Vyyuha Quick Recall

Cell Keeps Always Branching, Delaying Senescence. (Cytokinin: Cell division, Apical dominance breaking, Delay Senescence)

Every Tree Ripens Slowly And Thickens. (Ethylene: Triple response, Ripening, Senescence, Abscission, Thickening)

Always Be Alert, Stress Shuts Down Growth. (ABA: Stress hormone, Stomatal closure, Seed dormancy, Growth inhibition)