Cytokinins

NCERT Grounding

The cytokinin section appears in NCERT Biology Class XI, Chapter 13 — Plant Growth and Development, Section 13.4.3.3. The NCERT text anchors three testable facts: (1) cytokinins have specific effects on cytokinesis; (2) kinetin was the first cytokinin, isolated from autoclaved herring sperm DNA by Miller et al. (1955) while working in Skoog's laboratory; and (3) zeatin — the first natural cytokinin — was isolated from corn kernels and coconut milk. NIOS Chapter 20 supplements by noting that cytokinins are synthesised in root apices, endosperms of seeds, and young fruits, and lists their key functions: stimulating cell division, preventing leaf ageing, and inhibiting apical dominance.

Discovery of Cytokinins

The discovery of cytokinins emerged from tissue-culture research in the 1950s. F. Skoog and his co-workers at the University of Wisconsin found that callus (undifferentiated cell mass) from tobacco stem internodes would proliferate only when the nutrient medium contained, in addition to auxins, certain supplements — vascular tissue extracts, yeast extract, coconut milk, or DNA. This observation pointed to a cell-division-promoting factor distinct from auxin.

In 1955, Miller and colleagues isolated and crystallised the active substance, naming it kinetin (N6-furfurylaminopurine). Crucially, the source was autoclaved (heat-treated) herring sperm DNA. The heat treatment degraded DNA, producing adenine derivatives that had acquired furfuryl groups — none of which are found intact in living plant tissue. Kinetin therefore does not occur naturally in plants; it is a modified degradation product produced under autoclave conditions.

The search for a naturally occurring cytokinin followed the isolation of kinetin. It led to zeatin, isolated from corn (maize) endosperm (corn kernels) and coconut milk. Zeatin — (E)-2-methyl-4-(1H-purin-6-ylamino)but-2-en-1-ol in full chemical name — is a hydroxylated isopentenyl adenine derivative and remains the most biologically abundant cytokinin in higher plants. Since zeatin's discovery, several other naturally occurring cytokinins and synthetic analogues with cell-division-promoting activity have been characterised.

Chemical Nature

All cytokinins are adenine derivatives — specifically, N6-substituted aminopurines. Their structural core is the purine base adenine with a substituted side chain at the N6 (amino) position.

Synthesis and Transport

Natural cytokinins are synthesised in regions of rapid cell division. NCERT specifies: root apices, developing shoot buds, and young fruits. The endosperm of seeds is an additional site (noted in NIOS). The primary site of biosynthesis in the whole plant is the root apex, from which cytokinins are loaded into the xylem and transported acropetally (upward) to the shoot in the transpiration stream. This basipetal-to-acropetal route through xylem is the canonical transport pathway. Contrast this with auxin, which is transported polarly (basipetally, tip to base) through parenchyma cells via auxin-transport proteins.

Physiological Effects

1. Promotion of Cytokinesis (Cell Division)

The defining function of cytokinins is the promotion of cytokinesis — the division of the cytoplasm following mitosis. Unlike auxins and gibberellins, which are principally associated with cell elongation, cytokinins drive the actual partitioning of the cell. In tissue culture, cells supplied with only auxin enlarge but do not complete division; addition of cytokinin completes the cytokinesis step. The name "cytokinin" reflects this primary role.

2. Shoot Bud Differentiation — the Skoog-Miller Ratio

In tissue culture experiments, Skoog and Miller demonstrated that the relative ratio of cytokinin to auxin, rather than the absolute concentration of either hormone, determines whether a callus differentiates into shoots or roots. This is one of the most frequently tested tissue-culture principles in NEET biology.

The Skoog-Miller Tissue Culture Experiment

3. Delay of Leaf Senescence

Cytokinins delay the yellowing and deterioration of leaves by maintaining protein synthesis and retaining chlorophyll. The Richmond-Lang effect — demonstrated by applying cytokinin to a portion of an excised leaf — shows that cytokinin-treated areas remain green and protein-rich while untreated areas yellow. The mechanism involves directing nutrient mobilisation toward cytokinin-treated regions: amino acids, sugars, and minerals migrate to and accumulate at the treated zone. This property contrasts sharply with abscisic acid (ABA) and ethylene, both of which promote senescence.

4. Override of Apical Dominance

Auxin produced at the shoot apex suppresses the growth of lateral (axillary) buds — a phenomenon called apical dominance. Cytokinins counteract this suppression when applied to lateral buds, stimulating their outgrowth into branches. The mechanism is antagonistic: cytokinins and auxin act in opposition at the axillary bud. In intact plants, the balance between auxin (high at the apex, migrating basipetally) and cytokinins (arriving acropetally from roots through xylem) determines the degree of branching.

5. Stomatal Opening

Cytokinins promote the opening of stomata in some plant species, contrasting with ABA which promotes stomatal closure. The mechanism involves cytokinin stimulating guard cell metabolism and ion uptake, increasing turgor pressure and widening the stomatal aperture. This effect is less prominent in NEET questions than the senescence and apical dominance roles but appears in antagonist-recognition questions.

6. Nutrient Mobilisation

Cytokinins act as strong mobilisers of nutrients, drawing mineral ions, amino acids, and sugars toward treated regions. This underpins the senescence-delay mechanism: by mobilising nutrients into a cytokinin-treated leaf, the hormone maintains the metabolic activity of that tissue. In whole plants, this property may explain the regulatory role of root-derived cytokinins in controlling shoot metabolism in response to soil nitrogen availability.

7. Promotion of New Leaves and Chloroplast Development

Cytokinins promote the formation of new leaves and stimulate differentiation and development of chloroplasts in leaf tissue. NCERT notes: "It helps to produce new leaves, chloroplasts in leaves, lateral shoot growth and adventitious shoot formation." This broadens the cytokinin profile beyond cytokinesis alone.

Worked Examples

Common Confusion & NEET Traps