1. What is Callus?
Callus is a mass of dedifferentiated cells formed when plant tissue is exposed to stress, wounding, or high concentrations of auxins.
In tissue culture, we exploit this natural wound response to produce:
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Regenerable callus for plant propagation
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Experimental callus for transformation, mutation, or metabolite research
2. Hormonal Induction of Callus
Callus is induced by applying plant growth regulators (PGRs), especially:
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Auxins: 2,4-D, NAA
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Cytokinins: BAP, Kinetin
The exact hormone combination depends on:
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Plant species
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Explant type
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Desired callus type
| Ratio | Result |
|---|---|
| High auxin + low cytokinin | Callus induction |
| Balanced ratio | Organogenesis potential |
| Low auxin + high cytokinin | Shoot induction (less callus) |
📌 Hormonal Control of Callus Development
3. Types of Callus
| Callus Type | Use |
|---|---|
| Compact – Dense and smooth; used for shoot regeneration |  |
| Friable – Loose and granular; ideal for cell suspensions or protoplast extraction |  |
| Embryogenic – Contains somatic embryos; used in synthetic seed production |  |
| Non-morphogenic – Can’t regenerate; used in drug production and stress testing |  |
Morphogenic (regenerable) callus is typically yellowish to white and soft, while aged or necrotic callus becomes brown or hard.
4. Applications of Callus Culture
Callus is a versatile platform in plant science:
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Transformation platform: Target for Agrobacterium tumefaciens or gene gun delivery
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Metabolite production: Used to produce chemicals like taxol, vincristine, or rosmarinic acid
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Mutation screening: Mutant lines generated and selected via callus
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Stress tolerance testing: Callus exposed to salt, drought, or pH extremes to assess genetic resilience
📌 Callus as a Tool in Genetic Engineering
5. Culture Conditions and Maintenance
| Parameter | Typical Setting |
|---|---|
| Medium | MS or modified MS with PGRs |
| Light | 16-hour light OR complete dark (species-dependent) |
| Temperature | 24–26°C |
| Subculture | Every 2–4 weeks |
| Vessel | Petri dish or small flask with vented lid |
Regular transfer is critical—old callus becomes necrotic and loses totipotency.