Cytokinins, Ethylene and ABA — Core Principles
Core Principles
Plant growth regulators (PGRs) are chemical messengers that control plant development. Cytokinins, Ethylene, and Abscisic Acid (ABA) are three crucial types. Cytokinins, derivatives of adenine, primarily promote cell division, break apical dominance by stimulating lateral bud growth, and delay leaf senescence.
They are vital in tissue culture for shoot differentiation. Ethylene, a gaseous hormone, is renowned for its role in fruit ripening, accelerating senescence, and promoting abscission of leaves and fruits.
It also induces the 'triple response' in dark-grown seedlings, aiding their emergence from soil. Abscisic Acid (ABA), often called the 'stress hormone,' is a carotenoid derivative that helps plants cope with adverse conditions.
Its key functions include inducing stomatal closure to conserve water during drought, promoting seed and bud dormancy, and contributing to abscission. These hormones rarely act alone; their effects are often a result of complex synergistic and antagonistic interactions, forming an intricate regulatory network essential for plant survival and adaptation.
Often confused with
Side-by-side differences the NEET paper likes to test.
| Aspect | Cytokinins, Ethylene and ABA | Auxins |
|---|---|---|
| Primary Site of Production | Root tips, developing seeds, young fruits | Apical meristems of shoots, young leaves, developing seeds |
| Effect on Cell Division | Promotes cell division (cytokinesis) in presence of auxin | Promotes cell elongation; also promotes cell division in cambium and in tissue culture with cytokinins |
| Effect on Apical Dominance | Breaks apical dominance, promotes lateral bud growth | Promotes apical dominance, inhibits lateral bud growth |
| Effect on Senescence | Delays senescence (Richmond-Lang effect) | Generally delays senescence at low concentrations, but high concentrations can induce ethylene production, leading to senescence/abscission |
| Role in Tissue Culture | Promotes shoot differentiation (high cytokinin:auxin ratio) | Promotes root differentiation (high auxin:cytokinin ratio) |
| Chemical Nature | Adenine derivatives (e.g., Zeatin) | Indole derivatives (e.g., IAA - Indole-3-acetic acid) |
Cytokinins and Auxins are two fundamental plant hormones that often interact antagonistically or synergistically to regulate plant growth and development. While auxins, primarily produced in shoot apices, promote cell elongation and maintain apical dominance, cytokinins, largely synthesized in roots, stimulate cell division and break apical dominance by promoting lateral bud growth.
Their balanced ratio is critical in processes like organogenesis in tissue culture, where they determine whether roots or shoots develop. Both play roles in senescence, but cytokinins are known for delaying it, whereas auxins' effect can be complex due to ethylene induction.
Why it is tested: NEET relevance: Understanding the antagonistic and synergistic interactions between different plant hormones is crucial for NEET. Questions frequently test the combined effects of hormones, especially the auxin-cytokinin balance in apical dominance and tissue culture, and their roles in various developmental processes like senescence and organogenesis. This comparison highlights their distinct yet interconnected functions.