5 Induced Mutations

are new sudden stable inheritable discontinuous variations which appeat in organism due to permanent change in their genotypes. Mutations can occur naturally and automatically in organisms without apparent reasons. They are termed as spontaneous mutations or spots. Useful spontaneous mutation occurring in osmotic cells of vegetatively propagated plants can be easily picked up and multiplied, e.g. colour sports in apple varieties, seedless grapes, navel orange etc. in others somatic variations vanish with the death of the organisms. Mutations which occur in the germ cells and can be transferred to progeny are germinal mutations. They may not express their effect immediately because most of the mutations are recessive.
Recessive mutations would produce their effect only in the homozygous state. This is mainly in self pollinated and vegetatively propagated plants. In many crop plants, genetic improvement is made through sexual reproduction which is then maintained through cloning or vegetative multiplication by means of tubers (potato), cutting (apple, sugarcane), runners or stolons (strawberry) etc. An important mutations occurring in the sexually reproducing plants is the stiff ears (nonshattering quality) in wheat.
The frequency of spontaneous mutation is very low. Therefore they cannot be relied upon for rapid improvement of crop plants. Plants breeders employ induced mutations are those mutations which develop in response specific factors or chemicals called mutagens.
6 Tissue culture and Genetic Engineering – Plant tissue culture is the technique of maintaining and growing plant cells tissues or organs on artificial medium in suitable containers under controlled environment conditions. The part which is cultured is called explants. It has to be first disinfected with clorax water, dilute hypochlorite or methiolate. The explants may be root, stem shoot tip, leaf petiole, embryo etc. It may be grown directly or sectioned into this discs, plates etc. The culture medium can be liquid, semi solid or solid. The nutrition solution contains source of energy (2-4% sucrose), vitamins, amino acid, minerals etc. The growth regulators can be replaced by organic complex like coconut water or milk, yeast extract, banana pulp etc.
The explants can produce the whole plant or specific organ like fruit from a pollinated pistil or ovary. Tissues, rections or cells usually produce first an irregular, undifferentiated, unorganized but actively dividing mass called callus. Darkness favours callus formation. The callus can divided and subcultured. Differentiation of organogenesis occurs when callus is exposed to light or provided with a proper dosage of auxin and cytokinins. Therefore, conditions in the culture room and composition of culture medium. Tissue culture can help improvement of crop plants by the following techniques:
1. Mircopropagation
2. Production of diseases free plants
3. Haploid
4. Embryo rescue
5. Induced mutation etc.
Genetic engineering or recombinant DNA is the most recent mechanism of providing superior heredity in crop plants. The technique is useful in deleting undesirable genes and introduction of useful or desirable genes. The most different job in genetic engineering is to locate and isolate fragment of DNA having the desirable gene or genes. For this the chromosome mapping or genome study of all crop pant and their wild relatives would genes in its genome. After obtaining the desirable DNA segment, the same is introduced in the cells through vector (virus, plasmid etc.) microinjection electroporation etc. The transformed cells are then allowed to multiply and form a whole plant.