Question

Which of the following statements are true with respect to the plant growth regulators used in a growth medium for tissue culture?

I - The specific concentration of the plant growth regulators (PGRs) in the culture medium does not influence the fate of the explant.
II - PGRs play an important role in inducing de-differentiation of the cells of the explant.
III - When parenchyma cells of a callus are cultured in a medium containing high levels of cytokinin, shoot formation is induced.

• A. I and III
• B. I and II
• C. Only I
• D. II and III

Answer 1

The correct option is D II and III

Plant growth hormones/ regulators (PGRs) are chemical compounds produced by plants that regulate their growth and differentiation. Based on their functions in a living plant body, PGRs can be broadly categorised as growth promoters (auxins, gibberellins and cytokinins) and growth inhibitors (ethylene and abscisic acid).

PGRs are added to a culture medium during tissue culture. Tissue culture is the technique of growing explants (plant cells, tissues or organs) in a sterile culture medium under controlled environmental conditions.

Apart from PGRs the culture media also contains inorganic salts (to provide essential micro and macronutrients), carbohydrates (energy source), amino acids (for building proteins) and vitamins (act as cofactors to help enzyme activity in various metabolic reactions). These ingredients help in providing essential nutrients for the healthy growth of the explant.

The specific concentrations of PGRs in the culture media play an important role in determining the fate of the explant. For example, a balanced concentration of auxins and cytokinins is required to induce callus (an unorganised and undifferentiated mass of parenchyma cells) formation from the explant.

The hormones induce the cells of the explant to lose their specialisation (de-differentiate) and become meristematic (actively dividing plant cells responsible for plant growth). These cells actively divide to form a callus.

The cells of the callus are totipotent, that means they can differentiate into any cell type. Hence, when it is placed in a medium containing high concentration of cytokinins, the callus cells differentiate into tissues which form the shoot of a plant. The young shoot is then induced with root formation by placing it in a medium containing high concentration of auxins. Thus, a young plantlet having a young shoot and a young root is produced.

Figure : Effect of PGRs (plant growth regulators) on explant

Alternatively, culturing an individual cell of the callus (indirect somatic embryogenesis) or the explant (direct somatic embryogenesis) in a media containing high concentration of only auxin can induce the formation of embryos from these cells. Such embryos are called somatic embryos as they arise from somatic cells of the plant and not from a zygote (single cell formed due to fusion of male and female gametes).

The development of somatic embryos is comparable to that of zygotic embryos formed due to sexual reproduction. These young embryos can develop into mature embryos either on the same media or a different media. Each embryo can develop into a plantlet which again can develop into an entire plant.