Callus Culture in Plant Tissue Culture

Callus Culture in Plant Tissue Culture

• Callus is a tissue that forms when plant cells multiply in a disorganized way.
• It can be induced naturally in plants by wounding, insects, microorganisms, or stress.
• Callus can be initiated in a lab setting (in vitro) by placing small pieces of a plant (explant) on a growth-supporting medium under sterile conditions.
• Cell differentiation and specialization that occurred in the intact plant are reversed during the process, and the explant produces new tissue composed of meristematic and unspecialized cells.
• The process of callus induction and organ formation is significant.
• Applications and challenges of callus culture are discussed.
• During dedifferentiation, the storage products found in resting cells tend to disappear.
• Callus cultures can be generated from gymnosperms, ferns, and mosses.
• Callus cultures are more easily established from some organs than others; young meristematic tissues are best, but even areas in older parts of a plant, like the cambium, can form callus.
• The choice of tissues to start a callus culture is most effective in dicotyledonous species.
• In cereals, callus growth is only achieved from specific parts of the plant like embryos, germinating seeds, seed endosperm and seedling mesocotyl, and very young leaves; the mature leaf tissue cannot.
• Primary callus is the callus tissue formed from an original explant.
• Secondary callus cultures arise from parts of the primary callus tissue.
• Callus culture is best performed in the dark; light encourages differentiation of the callus.

Types of Callus

• Callus tissues come in various types.
• They differ in appearance, color, compaction level, and morphogenetic potential.
• The type of callus, its differentiation level, and plant regeneration ability depend on the origin and age of the explant tissue, which is selected.
• Callus variability is higher when the selected explant contains multiple cell types.

Types of Callus

• Compact callus shows densely aggregated cells.
• Friable callus shows loosely associated cells that can easily break apart.
• Friable callus is often used in cell suspension cultures.

Stages of Callus Culture

• Induction phase: Cells in the explant dedifferentiate and start dividing.
• Division phase: Rapid cell division. Long-term culture requires reduced auxin and cytokinin requirements.
• Differentiation phase: Differentiation and formation of organized structures (including organogenesis and somatic embryogenesis).

Notes on Callus Culture

• Explants cultivated on an agar medium with appropriate nutrition and auxin/cytokinin ratios show callus formation, starting from cut ends and expanding to the whole tissue.
• Callus cultures need regular subculturing (every 3-5 weeks).
• Repeated subculturing improves callus friability.
• Subculturing is transferring a culture to a fresh culture medium in a new vessel; this is also known as passage.
• Callus tissue may lose its ability to form organs as subculturing continues.
• Callus cultures may show chromosomal changes (e.g., aneuploidy, polyploidy).

Current Applications of Callus Culture

• Manipulating auxin/cytokinin ratios in the culture medium can produce shoots, roots, or somatic embryos, leading to whole plant regeneration.
• Callus cultures can be used to establish cell suspensions.
• Callus cultures are used for studying plant nutrition requirements.
• Callus cultures are used to examine cell and organ differentiation, morphogenesis, somaclonal variations, genetic transformation, and production of secondary metabolites.