Isolation of Variation, Toxic, Somaclonal Variation, Wheat, Geranium, Lucerne, Sugarcane, Potato, Toxic compound

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	Isolation of Variation - Somaclonal variants can be isolated either without selection pressure or with selection pressure. In the program without selection pressure unorganized callus and cells are grown in cultures for various periods on a medium that contains no selective agent (toxic or inhibitory substance); such calli are induced to differentiate whole plants. The regenerated plants are ultimately transferred to the field and screened for somaclonal variation. Somaclonal variants of sugarcane, potato, tomato, geranium, wheat, maize, lucerne, and many other plants have been obtained through this approach.
In the program with selection presssure, variant cell lines are screened from cultures by their ability to survive in the presence of a substance in the medium that may be toxic or inhibitory or may survive under the conditions of environmental stress. Variants may be obtained by direct selection, indirect selection, or by individually testing single cell derived colonies. In the case of direct selection the new variant cell types have a selective advantage over the rest of the population because of their tolerance to a specific toxic compound. As for indirect selection, the wild type cells are selectively killed and only the mutant recovered by transferring the surviving cells to a supplemented (enriched) medium. Isolation is carried out in most cases in suspension cultures or by plating single cells or protoplasts.