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.

Important Differences

vs Auxins

Aspect	This Topic	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.