Totipotency: Meaning, Expression and Importance | Plant Tissue Culture

Let us make an in-depth study of the meaning expression and importance of totipotency in plant science.

What is the Meaning of Totipotency?

Totipotency is the genetic potential of a plant cell to produce the entire plant. In other words, totipotency is the cell characteristic in which the potential for forming all the cell types in the adult organism is retained.

Expression of Totipotency in Culture:

The basis of tissue culture is to grow large number of cells in a sterile controlled environ­ment. The cells are obtained from stem, root or other plant parts and are allowed to grow in culture medium containing mineral nutrients, vi­tamins and hormones to encourage cell division and growth. As a result, the cells in culture will produce an unorganised proliferative mass of cells which is known as callus tissue.

The cells that comprise the callus mass are totipo­tent. Thus a callus tissue may be in a broader sense totipotent, i.e., it may be able to regen­erated back to normal plant given certain ma­nipulations of the medium and the cultural en­vironment. Truly speaking, totipotency of the cell is manifested through the process of differ­entiation and the hormones in this process play the major role than any other manipulations.

In the fifties, F. Skoog and C.O. Miller of U.S.A. discovered a new plant growth hormone kinetin from herring sperm DN A. With a correct concen­tration ratio of auxin and cytokinin in tobacco cultures, Skoog was able to demonstrate the role of kinetin in organogenesis. When the ratio of kinetin to auxin was higher, only shoot devel­oped. This is known as caulogenesis. But when the ratio was lower, only roots were formed. This is known as rhizogenesis.

Around the same pe­riod, F. C. Steward and his colleagues at Cornell University of USA, devised a method for growing carrot tissue by excising small disc, from the sec­ondary phloem region of carrot root and placing them in a moving liquid medium under aseptic conditions. In presence of coconut milk in the medium, the phloem tissue began to the grow actively.

In moving liquid medium some single cells and small groups of cells were loosened from the surface of growing tissue. When these iso­lated cells were grown separately it was found that some single cells developed somatic embryos or embryoids by a process that occurs in nor­mal zygotic embryo.

It is also observed in some experiment that 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.

Where the totipotent cells are partially ex­pressed or not expressed, it is obvious that the limitation on its capacity for development must be imposed by the microenvironments. The toti­potency of cells in the callus tissue may be re­tained for a longer period through several sub­cultures.

Practically, it is observed that the ex- plant first forms the callus tissue in the callus inducing medium and such callus tissue is main­tained through some subcultures. After then it is generally transferred to another medium which is expected to be favourable for the expression of totipotent cells. Actually, the regeneration medium is standardized by trial and error method.

In more or less suitable medium, the totipotent cells of the callus tissue give rise to meristematic nodules or meristemoids by repeated cell divi­sion. This may subsequently give rise to vascu­lar differentiation or it may form a primordium capable of giving rise to a shoot or root. Some­times the totipotent cell may produce embryoids through sequential stages of development such as globular stage, heart shaped stage and torpedo stage etc.

After prolonged culture, it has been obser­ved that calluses in some species (e.g. Ntcotiana tabacum, Citrus aurantifolia etc.) maybe- come habituated. This means that they are now able to grow on a standard maintenance medium which is devoid of growth hormones. The cells of habituated callus also remain totipotent and are capable to regenerate a plant without any major manipulation.

A typical crown gall tumour cell has the ca­pacity for unlimited growth independent of ex­ogenous hormones. It shows totally lack of organ genic differentiation. So such tissue is consid­ered to have permanently lost the totipotentiality of the parent cells.

In some plant species, the crown gall bacterium (Agrobactenum tumefaciens) induces a special type of tumour, called teratomas, the cells of which possess the capac­ity to differentiate shoot buds and leaves when they are grown in culture for unlimited periods. Thus it is clear that the mode of expression of totipotency of plant cell in culture varies from plant to plant and also helps us to understand the process of differentiation in vitro.

Importance of Totipotency in Plant Science:

The ultimate objective in plant protoplast, cell and tissue culture is the reconstruction of plants from the totipotent cell. Although the process of differentiation is still mysterious in general, the expression of totipotent cell in cul­ture has provided a lot of information’s.

On the other hand, the totipotentiality of somatic cells has been exploited in vegetative propagation of many economical, medicinal as well as agricul­turally important plant species. Therefore, from fundamental to applied aspect of plant biology, cellular totipotency is highly important.

Recent trends of plant tissue culture include genetic modification of plants, production of homozy­gous diploid plants through haploid cell culture, somatic hybridization, mutation etc. The suc­cess of all these studies depends upon the expres­sion of totipotency. In many cases, successful and exciting results have been obtained.

Plant breeders, horticulturists and commercial plant growers are now more interested in plant tis­sue culture only for the exploitation of totipo­tent cells in culture according to their desirable requirement. Totipotent cells within a bit of cal­lus tissue can be stored in liquid nitrogen for a long period. Therefore, for germplasm preser­vation of endangered plant species, totipotency can be utilized successfully.