Successful development of a plant regeneration protocol is essential in that it has a cascading effect. The success of many other advanced in vitro techniques such as in vitro mutant selection, protoplast fusion, and genetic transformation depends directly upon the ability to regenerate plants from the derived tissue. In addition, polyploidy, aneu-ploidy, and chromosome structural changes, which commonly occur in fast-dividing cells, along with many other factors, result in accumulating heritable variation in the regenerating plants, giving rise to new variability in the gene pool (somaclonal variation) that can be used for crop improvement.
Callus can be initiated in vitro by culturing small explants on growth-supporting medium supplemented with exogenous growth-regulating factors. During this process, cell differentiation and specialization, which might be occurring in the intact plant tissue, are reversed and the explant gives rise to new tissue that is composed of mer-istematic and unspecialized cells. During dedifferentiation, storage products typically found in resting cells tend to disappear. New meristems are formed in the tissue and these give rise to undifferentiated parenchymal cells without any of the structural order that was characteristic of the organ or tissue from which they were derived. Although callus remains unorganized, as growth proceeds, some kind of specialized cells may be again formed. Such differentiation can take place at random, but may be associated with centers of morphogenesis, which can give rise to organs. Thus, callus culture is usually made up of two types of tissues, differentiated and nondifferentiated.
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