Cytodifferentiation

Cytodifferentiation

The cells of some callus mass fre­quently differentiate into vascular elements such as xylem and phloem without forming any plant organs or embryoids. This process is known as histogenesis or Cyto-differentiation. Thus the totipotent cells may express themselves in dif­ferent way on the basis of differentiation process and manipulation.

The potential of a plant cell to grow and develop into a whole new multicellular plant is described as cellular totipotency. In other words, the property of a single cell for differentiating into many other cell types is called as totipotency. This is the property which is found only in living plant cells and not in animal cells (exception being stem cells in animals). The term totipotency was coined in 1901 by Morgan. During culture practice, an explant is taken from a differentiated, mature tissue. It means, the cells in explants are generally non-dividing and quiescent in nature.

To show totipotency, such mature, non-dividing cells undergo changes which revert them into a meristematic state (usually a callus state). This phenomenon of reverting back of mature tells to dividing state is called dedifferentiation. Now, these dedifferentiated cells have the ability to form a whole plant or plant organ. This phenomenon is termed as re-differentiation.

Dedifferentiation and re-differentiation are the two inherent phenomena involved in the cellular totipotency. Regarding this, it is clear that the cell differentiation is the basic event for development of plants and it is also referred to as cyto-differentiation.

To express its totipotency, a differentiated cell first undergoes the phenomenon of dedifferentiation and then undergoes the re-differentiation phenomenon (Fig. 3). Usually the dedifferentiation of the explant leads to the formation of a callus. However, the embryonic explants, sometimes, result in the differentiation of roots or shoots without an intermediary callus state.

Thus, from the above account it is clear that unlike animals (in which differentiation is irreversible usually), the plants have such a quality that even highly mature and differentiated cells have an ability to revert back to meristematic state. The property of totipotency of plant cells indicate that even the undifferentiated cells of a callus carry the essential genetic information required for regeneration of a whole plant.

It is also clear that all the genes responsible for dedifferentiation or re-differentiation are present within the individual cells and they become active for expression under adequate culture conditions. As totipotent cells are the basis of whole plant tissue culture techniques, so, by the exploitation of this potential of plant cells, biotechnologists are trying to improve the crop plants and other commercially important plants.