Somaclonal variations are dfined as the genetic variability present among cultures somatic cells. Plants derived from such cells or progeny of such plant is called somaclones. The term somaclonal variation was first used by Larkin and Scowcroft in 1981. The term somaclonal variation is used for genetic variability present among all kinds of cells or obtained from cels cultured in vitro. Plants regenerated from tissue and cell cultures show heritable variations for both qualititative and quantitative traits. Somaclonal variation has been described in apple, sugar cane, potato, tomato etc. some variants are obtained in homozygous conditions in the plants regenerated from cells cultured in vitro R0. But most variants are recovered by progeny of tissue culture regenerated plants R1 generation. Chaleff in 1981 considered variants from tissue culture as R or P plants and selfed progeny of R0 plants as R1 plants. Somaclonal variations considered a novel method of genetic variation and having much interest as a basic genetic process, as it contradicts the theory or clonal uniformity. But it is disadvantageous when the objective is micropropagation of considered genotypes.
Basis for Somaclonal Variation in Plants
â€¢ Number of factors are responsible for somaclonal variation as listed below:
â€¢ Pre-exisitng chromosomal ploidy in the explant cells used for culture initiation.
â€¢ Gene mutations in the nuclear and cytoplasmic genomes that may be translocations, deletions and inversions.
â€¢ Nuclear fragmentation during first divisions at callus induction stage.
â€¢ Endomitosis or endore duplication during culture initiations.
â€¢ Miotic crossing over during culture induction.
â€¢ Activation of transposable elements.
â€¢ Changes in gene expressions and gene amplification.
Procedure for Isolation of Somaclonal Variants
Mutants or variants for several traits can be far more easily isolated from cell cultures than from whole plants populations in the field condition. This is because a large number of cells, as high as 10^6 to 10^9 can be easily and effectively screened for mutant traits. Screening of as many plants would be very difficult to perform.
The mutants can be effectively selected for disease resistance, improvement of nutritional quality, adaptation of plants to stress conditions such as saline soils and low temperature, toxic metals such as aluminum, resistance to herbicides and to increase the biosynthesis of plant products used for medicinal or industrial purposes. The various approaches to the isolation of somaclonal variants can be grouped into to broad categories. They are screening and cell selection.
It involves the observation of a large number of cells or regenerated plants from tissue culture for the detection of variant individuals. This approach is only feasible technique for the isolation of mutants for yield and yield traits in general R1 progeny are scored for the identification of variant plants, and then R2 progeny lines are evaluated for confirmation. This has been employed in number of crop plants.
The screening method has been profitably and widely employed for the isolation of cell clones. That produces higher quantities of certain important biochemical agents. Computer based automated cell sorting devices have also been used to screen as many as 1000 to 2000 cells per second from which desirable variant cells were automatically separated. The variant cells were further regenerated to produce complete plantlets.
In the cell selection an appropriate selection pressure is applied which permits the preferential survival or growth variant cells only during culture. Some examples of cell selection are selection of cells resistant to various pathogens derived toxins, herbicides, acidity; high salt concentration etc. when the selection pressure allows only the mutant cells to survive or divide it is called positive selection. On the other hand in the case of negative selection methods the wild type of cells divide normally and therefore are killed by the counter selection agents used in the medium such as arsenide. The mutant cells are unable to divide as a result of which they are unaffected by the counter selection agents. These cells are subsequently rescued by removal of the counter selection agent from the medium. Negative selection approach is generally employed for the isolation of auxotrophic mutants.
Advantages of Somaclonal Variations:
â€¢ Rather stable and high frequencies
â€¢ New alleles or even novel mutations may be isolated which were not available in the germplasm
â€¢ Can be performed in all types of plants
â€¢ Reduce the time required for the release of new variety by two years as compared to mutation breeding
â€¢ A very effective selection method
â€¢ Only approach are the isolation of biochemical mutants
Disadvantages of Somaclonal Variation
â€¢ Applicable only to those species cell cultures which regenerate into complete plants
â€¢ Selected cell lines often show reduced or no regeneration potential
â€¢ Show undesirable features such as reduced fertility, growth rate and even overall performance of plant
â€¢ Selected clones are unstable not always stable and heritable
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