A callus is a mass of undifferentiated, actively dividing cells that forms when a small piece of plant tissue (explant) is grown on a nutrient medium with auxin. This happens because mature cells undergo dedifferentiation (they regain the power to divide), which proves totipotency, the power of any plant cell to make a whole plant. Memory hook: "Explant to callus to plant" and remember "Auxin makes roots, Cytokinin makes shoots, together they make both."
The tissue culture pathway: a mature explant dedifferentiates with auxin to form a callus, then the auxin-to-cytokinin ratio drives redifferentiation into shoots and roots that grow into a whole plant, showing totipotency.
Your doubts, answered
Is callus formation an example of differentiation or dedifferentiation?
It is dedifferentiation. The mesophyll or parenchyma cells taken as the explant are mature, differentiated cells that have already lost the power to divide. On the culture medium they regain the power to divide and form a callus, a mass of undifferentiated cells. Regaining the power to divide is exactly what dedifferentiation means. NEET 2023 tested this directly.
Which two hormones must you add to get both shoots and roots from a callus?
Auxin and cytokinin. The auxin-to-cytokinin ratio decides which organ forms. A high auxin ratio favours roots, a high cytokinin ratio favours shoots, and a balanced combination of both gives shoots as well as roots. Gibberellin and abscisic acid are not used for this; ABA is an inhibitor. This was the exact answer to NEET 2016.
What is the difference between callus and totipotency?
Totipotency is the property or capacity of a single plant cell to grow into a whole plant. Callus is the actual mass of undifferentiated cells that you grow in culture on the way to that whole plant. So totipotency is the power, and callus is one visible step in using that power.
What is an explant and why does callus need a nutrient medium?
An explant is the small piece of plant part (like a bit of leaf, stem or root) that you cut and place on the medium. The medium is artificial and sterile and supplies sugar, minerals and plant growth regulators like auxin. Without these nutrients and auxin the explant cells cannot divide to form a callus.
Does callus formation prove that plant cells are totipotent?
Yes. The fact that a mature, already-specialised cell can dedifferentiate, divide to form a callus, and then redifferentiate into shoots and roots to give a full plant is the direct proof of totipotency. This is the basis of tissue culture and micropropagation.
⚠️ The NEET trap ✗ Students write that forming a callus is 'differentiation' because new cells are being made. ✓ Forming a callus is dedifferentiation. Mature cells that had stopped dividing regain the power to divide to make undifferentiated callus. Differentiation is the opposite, when callus later matures into shoots and roots (that step is redifferentiation). 🧠 Callus is made of UNdifferentiated cells, so making it cannot be differentiation. Regaining division power = deDIFFERENTIATION.
Real NEET questions
NEET 2023
In tissue experiments, leaf mesophyll cells are put in a culture medium to form callus. This phenomenon may be called as:
A · Differentiation
B · Dedifferentiation ✓
C · Development
D · Senescence
Solution: Mature leaf mesophyll cells are differentiated cells that have lost the capacity to divide. When cultured to form a callus (a mass of undifferentiated dividing cells) they regain the capacity to divide, which is dedifferentiation. Differentiation is the opposite (maturation into specialised cells), while development and senescence are unrelated to this regaining of division.
NEET 2016
You are given a tissue with its potential for differentiation in an artificial culture. Which of the following pairs of hormones would you add to the medium to secure shoots as well as roots?
A · IAA and gibberellin
B · Auxin and cytokinin ✓
C · Auxin and abscisic acid
D · Gibberellin and abscisic acid
Solution: In plant tissue culture the auxin-to-cytokinin ratio decides organ formation. A balanced combination of auxin and cytokinin induces both shoot and root differentiation from the callus. Gibberellin and abscisic acid are not used for this; ABA is an inhibitor.
NEET 2024
The capacity to generate a whole plant from any cell of the plant is called
A · Micropropagation
B · Differentiation
C · Somatic hybridization
D · Totipotency ✓
Solution: The ability of any plant cell to regenerate a whole plant is totipotency, the property underlying tissue culture. Micropropagation and somatic hybridization are techniques, not the inherent capacity, and differentiation is the maturation of cells into specialised types.
Solved Plant Growth and Development NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
A callus is an unshaped mass of undifferentiated, dividing cells that grows from a small plant piece placed on a nutrient medium in the lab.
What hormone starts callus formation?
Auxin. When an explant is placed on the medium with auxin, its mature cells dedifferentiate and start dividing to form the callus.
How do you get a full plant from callus?
By adjusting the auxin and cytokinin balance. The right ratio makes the callus form shoots and roots (redifferentiation), which then grow into a complete plantlet.
Why is callus formation important for NEET?
It links three key ideas the exam repeats: dedifferentiation, totipotency and the auxin-cytokinin ratio. NEET 2016, 2023 and 2024 all asked questions built on this single topic.
