Cytokinins — The Plant Hormones That Keep Cells Young
Imagine a plant growing. The root tips are busy underground, the shoot tips are reaching for the sky. But what if you want the plant to grow bushier — to produce more side branches instead of just one tall main stem? Or what if you want a cut flower to stay fresh for days longer? That is exactly what cytokinins do.
The Intuition: "Stay Young, Divide, and Branch Out"
Cytokinins are plant growth regulators (PGRs) that promote cell division (cytokinesis — the splitting of one cell into two). Their name comes directly from this job: cyto- (cell) + kinin (movement/division). But their effects go far beyond just making cells multiply.
Think of cytokinins as the "youth hormone" of the plant. They:
- Delay senescence (aging) — they keep leaves green and functional longer, even after they have been detached from the plant.
- Overcome apical dominance — the phenomenon where the main shoot tip suppresses the growth of side (lateral) buds. Cytokinins applied to a lateral bud can wake it up, making the plant branch out.
So if auxin (another PGR) is the "grow tall" hormone, cytokinin is the "grow wide and stay fresh" hormone.
The Precise Statement
Cytokinins are a class of plant growth regulators that promote cytokinesis (cell division) in plant tissues, especially when present along with auxin. They are synthesised primarily in root tips and transported upward through the xylem to the shoots and leaves.
Key effects of cytokinins:
- Stimulate cell division (especially in the presence of auxin)
- Delay leaf senescence (aging) by preventing chlorophyll breakdown
- Promote the growth of lateral buds (overcome apical dominance)
- Promote nutrient mobilisation (attract amino acids and other nutrients to treated areas)
How Cytokinins Work: The Auxin–Cytokinin Balance
Cytokinins do not act alone. Their effects depend critically on the ratio of auxin to cytokinin in a tissue.
| Auxin : Cytokinin Ratio | Effect on Cultured Plant Tissue |
|---|
| High auxin, low cytokinin | Root formation |
| Low auxin, high cytokinin | Shoot formation |
| Equal amounts | Callus (undifferentiated cell mass) continues to grow |
This is a classic experiment in plant tissue culture. If you take a piece of callus and vary the auxin–cytokinin ratio, you can direct it to form roots or shoots at will. This is why cytokinins are indispensable in micropropagation (cloning plants in the lab).
In the context of apical dominance: auxin produced by the shoot tip travels down the stem and suppresses lateral buds. Cytokinins, coming up from the roots, can counteract this suppression. If you remove the shoot tip (decapitation), the lateral buds grow because the auxin source is gone — but you can also achieve the same effect by applying cytokinin directly to a lateral bud.
Natural and Synthetic Cytokinins
The first cytokinin discovered was kinetin (a synthetic compound, not naturally occurring). The most common natural cytokinin is zeatin, first isolated from maize (Zea mays) kernels. Other natural cytokinins include isopentenyl adenine (iP) and dihydrozeatin. …