Cytokinins: Plant Hormones for Cell Division and Growth

Cytokinins are plant hormones that promote cell division, called cytokinesis, especially when auxin is also present. They are mainly produced in root tips and developing seeds and fruits. The first cytokinin discovered was kinetin, and a common natural one is zeatin.

Cytokinins matter because they delay ageing of leaves, help buds grow, and allow tissue culture scientists to produce shoots in the laboratory. Without cytokinins, plants could not coordinate root and shoot growth properly.

Key Concepts

Cytokinin: A hormone that promotes cell division and shoot formation.

Zeatin: A natural cytokinin first isolated from corn kernels.

Kinetin: A synthetic cytokinin like compound first identified from herring sperm DNA.

Senescence: Ageing of leaves involving breakdown of chlorophyll. Cytokinins delay senescence.

Auxin to cytokinin ratio: High cytokinin relative to auxin promotes shoots, high auxin relative to cytokinin promotes roots in tissue culture.

Apical dominance release: Cytokinins help lateral buds grow when the shoot tip is removed.

How It Works

1. Cytokinins are synthesised in root tips where cells divide actively.
2. They are transported upward through xylem to shoots and leaves.
3. In dividing tissues, they stimulate cells to enter mitosis and complete cytokinesis.
4. In leaves, they promote chloroplast development and keep proteins and chlorophyll from breaking down.
5. When cytokinin reaches lateral buds, it counteracts auxin inhibition and buds start growing.
6. In tissue culture, adjusting the ratio of cytokinin to auxin directs callus to form shoots or roots.

Worked Example

In a tobacco tissue culture, 100 callus pieces are placed on medium with high cytokinin to auxin ratio. After 4 weeks, 80 pieces form shoots. Shoot formation percentage is 80 divided by 100 multiplied by 100 equals 80 percent. On medium with high auxin to cytokinin ratio, only 10 pieces form shoots but 75 form roots. Root formation is 75 percent. The example shows the ratio, not just the presence of one hormone, decides the developmental outcome.

Common Misconceptions

• Cytokinins work alone to cause division. In most tissues they need auxin for complete cell division.
• Cytokinins are only made in shoots. Major production is in roots and developing seeds.
• Delaying leaf ageing means the leaf lives forever. Cytokinins slow senescence but do not stop it permanently.

Real-World Applications

Cytokinins are used in micropropagation to multiply shoots rapidly and produce many plantlets. They help keep cut flowers and leafy vegetables green for longer during storage. In agriculture, they can increase branching and improve fruit set in some crops. Tissue culture labs depend on cytokinin balance for clonal propagation.

Cytokinins drive cell division, support shoot growth, and slow leaf ageing. Their interaction with auxin gives growers a precise tool to shape plant development in fields and laboratories.