The following article will guide you about how the callus tissue is formed in plants.
Formation of the callus tissue is the outcome of cell expansion and cell division of the cells of the explants.
During the formation of callus tissue, the explants lose its original characteristic. So, under the influence of exogenously supplied hormone, the explants are triggered off a growth sequence in which cell enlargement and cell division predominate to form an unorganised mass of cells. As a result, the explants undergo an irreversible change of its shape, size, symmetry, structural organisation and cellular integrity.
For the initiation of callus culture, tissues from young seedling or from juvenile part of the mature plant are generally taken. In such tissue, a growth momentum is already present among the cells due to presence of maximum number of physiologically active meristematic cells. As the explants absorb the exogenously supplied hormones along with other nutrients, it makes a continuous nutrient gradient among the different cells of the explants on the basis of their location. As a result, the cells divide asynchronously depending upon the availability of the nutrient and hormones and ultimately the callus tissue is formed.
Depending upon the types of explant viz. leaf, stem segment, root segment etc. either enlargement in size or the swelling followed by rupture of the tissue within few days of inoculation takes place. This change indicates the response of explant for callus formation and is followed by the appearance of little irregular cellular masses around the cut edges or from the ruptured surface.
It is now explained that initial formation of cellular mass particularly at the cut end may be due to injury during excision. Some endogenous growth substance oozes out through the injured tissue at cut end and stimulates the cell division which is simultaneously induced by the exogenously supplied hormones.
There is another explanation that both endogenous product and exogenous hormones make a threshold level and their interaction results the formation of unorganised cellular growth at the cut end. Whatever may be the fact, it is generally accepted that exogenous hormones play the major role for the formation of callus tissue in culture. Both auxin and cytokinin are required for indefinite growth and cell division in callus culture. Sometimes, only 2, 4-D (an auxin) alone is sufficient for callus formation. Regarding the action of auxin and cytokinin, there is a general concept that auxin is required for growth and cytokinin is required for cell division. This concept is unanimously accepted and is equally applicable for callus formation.
After the formation of visible unorganised mass of cells at the cut end, gradually the whole tissue is involved to form callus tissue as mentioned previously. The callus tissue gradually increases in mass as the new cells are added by mitosis. Estimation of callus growth can be made on the basis of changes in fresh or dry weight.
The type of tissue or cell present in the ex- plant is an important factor for the rapid formation of callus tissue. If any meristematic tissue is present in the explant such as vascular cambium, the existing dividing cell of the vascular cambium under in vitro condition continue the rapid growth and proliferate into callus tissue.
The quiescent, vacuolated and highly differentiated cells such as pith cortex, vacuolated and highly differentiated cells are also stimulated to divide and proliferate into irregular mass of cells which spread over most of the surface and inner part of the explant.