Cytokinins
சைட்டோகைனின்கள்
Stop confusing Auxin and Cytokinin. Auxin promotes apical dominance; Cytokinin actively fights it. If you cannot distinguish between these two hormonal antagonisms, you will lose marks on the NEET paper.
The idea
Cytokinins are plant growth regulators that specifically promote cytokinesis. They are synthesized primarily in root apices and developing shoot buds where rapid cell division occurs. Beyond division, they are essential for overcoming apical dominance and delaying leaf senescence. They also promote the growth of lateral shoots and adventitious shoot formation.
Why it matters
They exist to stimulate tissue growth and maintain metabolic activity in leaves, ensuring the plant remains productive for longer periods.
NEET lens
NEET focuses on the functional antagonism between Auxin and Cytokinin. Memorize that Cytokinin delays senescence, while Ethylene promotes it. Expect 'Match the Following' questions linking these hormones to their specific physiological effects. Do not confuse the site of synthesis; root apices are the primary source.
Clinical significance
In humans, the equivalent of uncontrolled cytokinesis is neoplasia. While plants use Cytokinins to regulate growth, humans rely on complex cell cycle checkpoints like p53 to prevent similar runaway division. Failure in these human regulatory mechanisms leads to malignancy.
Introduction
Cytokinins are plant hormones (adenine derivatives) that promote cell division (cytokinesis), delay senescence, and promote lateral bud growth. The natural cytokinin is zeatin; kinetin was the first synthetic one discovered.
Core concept
Discovery: Miller, Skoog et al. (1955) found that autoclaved herring sperm DNA promoted cell division in tobacco tissue culture. Active compound = kinetin (6-furfurylaminopurine). Natural cytokinins: zeatin (from maize kernels, most common). Produced in: root apical meristem (primarily), also in young fruits and seeds. Transport: upward (acropetal) via xylem. Key effects: Cell division — synergy with auxin. Delay of leaf senescence — keep leaves green longer (applied to cut flowers). Lateral bud promotion — antagonizes apical dominance. Chloroplast maturation — promotes greening. Nutrient mobilization — cytokinins attract nutrients to young leaves.
Go deeper
Senescence delay: senescence = yellowing/aging of leaves due to chlorophyll breakdown. Cytokinins delay senescence by: maintaining protein synthesis, inhibiting chlorophyll degradation, preventing RNA/protein breakdown. Applied to lettuce to extend shelf life (post-harvest treatment).
From Class 10
Cell division is promoted by a class of plant hormones called cytokinins. They work together with auxins to control when and where cells divide.