What Is Callus in Plant Tissue Culture? From Undifferentiated Tissue to Plant Regeneration
A callus is a mass of plant tissue that develops when cells from an explant or wounded area are stimulated to resume division under suitable culture conditions.
The starting material may be a leaf, petiole, stem, root, bud, shoot tip, or embryo. When the explant receives an appropriate combination of nutrients, light, temperature, and plant growth regulators, certain groups of cells begin dividing and gradually form a tissue mass that may appear white, cream, yellow, green, or brown.
Callus is not a root, stem, or shoot in itself. It is an intermediate tissue that may later develop into shoots, roots, or somatic embryos. However, callus formation does not automatically mean that a complete plant can be regenerated.
Some callus tissues grow rapidly but are unable to form organs. These are known as non-organogenic callus. Other callus types can produce shoots, roots, or embryos and are therefore more valuable for propagation, breeding, and plant biotechnology.
Callus Is More Organized Than It Appears
Modern research shows that callus is not simply a random mass of functionless cells.
Certain cell populations within callus may follow developmental pathways similar to those involved in lateral root initiation, particularly when auxin-induced callus develops from tissues associated with the plant’s vascular system.
Callus formation occurs when cells receive signals from wounding and plant hormones, causing them to alter gene expression and re-enter the cell cycle. Not every cell within an explant has the same ability to form callus, and the response depends heavily on the original tissue type and its physiological condition.
This means that callus formation is a regulated biological process rather than uncontrolled cell growth.
The Role of Plant Growth Regulators
Plant growth regulators are central to callus induction and subsequent regeneration.
Auxins commonly used to stimulate callus formation include:
2,4-D
NAA
IAA
Picloram
These compounds promote cell division and help cells change their developmental identity.
Cytokinins such as BA, BAP, kinetin, and TDZ also influence cell division and are often important when the objective is to regenerate shoots.
A medium containing a relatively high auxin concentration may stimulate extensive callus formation. When shoot regeneration is required, the callus is commonly transferred to a new medium containing less auxin and a higher cytokinin concentration.
The correct hormone balance depends on the plant species, cultivar, explant, and regeneration pathway.
Different Types of Callus
Callus can be classified according to its texture, structure, and developmental potential.
Friable Callus
Friable callus consists of loosely connected cells and breaks apart easily. It is well suited to:
Cell suspension cultures
Genetic transformation
Gene-editing research
Production of secondary metabolites
Large-scale cell multiplication
Its loose structure allows individual cells or small cell clusters to disperse readily in liquid medium.
Compact Callus
Compact callus forms a firm, tightly connected tissue mass. It may contain organized regions capable of developing into adventitious shoots.
In some species, compact callus is more suitable for shoot regeneration than highly friable tissue.
Embryogenic Callus
Embryogenic callus contains small, actively dividing cells with dense cytoplasm and a high capacity to form somatic embryos.
A somatic embryo differs from an adventitious shoot because it develops both a shoot pole and a root pole within the same structure. Under suitable conditions, it can mature and develop into a complete plant.
Embryogenic callus is particularly important in mass propagation, synthetic seed systems, genetic transformation, and crop improvement.
What Callus Color Can Tell Us
Callus color can provide useful information, but it should not be used as the only measure of quality.
White or cream callus may indicate actively dividing tissue. Green callus contains chlorophyll and may be beginning to develop photosynthetic tissues or shoot structures.
Brown or black callus is often associated with oxidation of phenolic compounds, cellular stress, aging, or tissue death.
However, appearance alone can be misleading. A pale yellow callus may look healthy but still fail to regenerate when transferred to shoot-induction medium. Conversely, callus with small green regions may contain highly active shoot-forming areas.
Callus quality should therefore be evaluated through its response after transfer, not only through color or size.
Plant Species, Cultivar, and Explant Type Affect Callus Formation
Callus response varies greatly among plant species and cultivars.
The age and health of the mother plant, the physiological condition of the explant, and the tissue selected all influence whether callus forms and whether it can regenerate.
Young tissues such as immature leaves, leaf bases, young stems, and embryos often respond more readily than older, lignified tissues. However, there is no single explant type that works equally well for all plants.
One species may form callus effectively from stem tissues while showing almost no response from leaves, even when both explants are cultured on the same medium.
Protocol development must therefore compare explant types and cultivars separately rather than assuming that a formula developed for one tissue will work for another.
Why Callus May Form but Fail to Produce Shoots
One of the most common problems in tissue culture is the production of abundant callus that fails to regenerate into shoots.
Possible causes include:
Prolonged exposure to high auxin concentrations
Callus that has become too old
An unsuitable cytokinin treatment
Loss of regenerative capacity over repeated subcultures
Poor physiological quality
A callus type that is naturally non-organogenic
Genotype-dependent recalcitrance
Another common issue is callus that becomes brown, hyperhydric, or watery. Such tissue may increase rapidly in fresh weight while containing large amounts of water and little regenerative potential.
For example, increasing 2,4-D may cause callus biomass to rise quickly, yet reduce the ability of the tissue to produce shoots or embryos.
Callus quantity should therefore never be used as the sole indicator of successful regeneration.
Commercial Advantages and Risks
Callus-based systems offer several important advantages.
They can be used to:
Multiply large numbers of cells
Produce somatic embryos
Support genetic transformation and gene editing
Generate secondary metabolites
Select stress-tolerant cell lines
Create variation for breeding programs
Conserve or regenerate difficult plant materials
However, prolonged callus culture also increases the risk of somaclonal variation.
Changes may occur at the DNA, chromosome, ploidy, or epigenetic levels. These changes can lead to altered plant height, leaf form, color, fertility, growth rate, or yield.
When the objective is to produce highly uniform, true-to-type commercial plants, direct shoot proliferation from axillary buds or existing shoot meristems is generally safer than regeneration through callus.
If callus must be used, it is advisable to work with young callus, minimize the number of subculture cycles, and evaluate regenerated plants carefully after acclimatization and field establishment.
Callus Formation Is Only the Beginning
Callus is a mass of plant tissue produced when plant cells are stimulated to resume division under controlled culture conditions.
Its formation is an important first step in many tissue culture systems, but it does not prove that the process has succeeded.
A complete regeneration pathway must continue from callus formation to:
Shoot or somatic embryo development
Plantlet regeneration
Root formation
Acclimatization
True-to-type evaluation
For commercial propagation, the time spent in the callus stage should generally be kept as short as practical. Young, responsive callus should be used, and regenerated plants should be monitored systematically for abnormal or off-type characteristics.
The real value of callus does not lie in how large the tissue mass becomes. It lies in whether that tissue can reliably develop into healthy, stable, and commercially usable plants.
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