Somaclonal variation is defined as genetic and phenotypic variation among clonally propagated plants from a single donor clone. Somaclonal variation caused by the process of tissue culture is also called tissue culture induced variation to more specifically define the inducing environment. Somaclonal variation can be manifested as either somatically or meiotically stable events. Somatically stable variation includes phenotypes such as habituation of cultures and physiologically induced variation observed among primary regenerants. This type of variation is not transmitted to next generations and is of most impact in situations where primary regenerant is the end products like the amplification of ornamental plants or trees for direct use. Meiotically heritable variation occurs and is most important in situations where the end product, the tissue culture is propagated and sold as seed. The loss of culture health with time is a major detriment to the efficiency of transgenic plant production. Epigenetic control of gene expression can be defined as a somatically or meiotically heritable alteration in gene expression that is potentially reversible and is not due to sequence modification. Epigenetic aspects of somaclonal variation would therefore involve mechanisms of gene silencing or gene activation that were not due to chromosomal aberrations or sequence change.

The term 'somaclone' was coined to refer to plants derived from any form of cell culture, and the term 'somaclonal variation' was coined to refer to the genetic variation among such plants. The growth of plant cells in vitro and its regeneration into whole plants is an asexual process that involves only mitotic division of the cells. The occurrence of uncontrolled and random spontaneous variation when culturing plant tissue is a major problem. In vitro conditions the culture can be mutagenic and regenerated plants derived from organ cultures, calli, protoplasts and somatic embryos sometimes can show phenotypic and genotypic variation. Some somaclones may be physically different from the stock donor plants. Commonly variability occurs spontaneously and can result of temporary changes or permanent genetic changes in cells or tissue during in vitro culture. Temporary changes result from epigenetic or physiological effects and are nonheritable and reversible. As compared to permanent changes are heritable and often represent expression of pre existing variation in the source plant or is a result of denovo variation. Somaclonal variation can range in scope from specific trait to the whole plant genome. Somaclonal variation provides a valuable source of genetic variation for the improvement of crops through the selection of novel variants, which may show resistance to disease, improved quality, and higher yield.

Somaclonal variation was first detected by the high frequency of qualitatively segregating phenotypes observed among progeny of plants which expected to be genetically identical. This was true in diploid species such as maize where mutations can be easily observed. For example, a study of maize grown for 8 months in culture found that, on average every regenerated plant contained 1.32 mutants that produced a visible phenotype. These qualitative mutants were stably inherited for several subsequent seed derived generations. Then a detailed phenotypic analyses in later studies showed that quantitative variation is also frequently found among regenerants derived progeny. Quantitative variation has been described for many phenotypes including plant height, plant biomass, grain yield, and agronomic performance. A generalization of studies that have assessed quantitative variation is that quantitative variation is frequent and inheritance studies indicate alteration of numerous loci.

Plants regenerated from tissue and cell cultures show heritable variation for both qualitative and quantitative traits; such variations are also known as Somaclonal variation. Somaclonal variation has been described in sugarcane, potato, tomato etc. Some variants are obtained in homozygous condition in the plants regenerated from the cells in vitro, but most variants are recovered in the self progeny of the tissue culture-regenerated plants. Somaclonal variation most likely arises as a result of chromosome a structural change that is small deletions and duplications, gene mutations, plasma gene mutations, mitotic crossing over and possibly, transposons. Somaclonal variation may be profitably utilized in crop improvement since it reduces the time required for releasing the new variety by at least two years as compared to mutation breeding and by three years in comparison to back cross method of gene transfer. Some of the variants so far is considered as boon to the crop improvement and some of the systems are:-

Sugarcane: by tissue culture methods, variants which are resistance to eye spot disease (Helminthosporium sacchari) Fiji disease (Virus) and downy mildew (Sclerospora sacchari) were isolated. These variants showed higher resistance to Fiji disease and downy mildew than their parent clones. Most of the resistant lines exhibited a shift towards higher resistance.

Potato: The protoplast culture in potato cultivar Russet Burbank, an important cultivar excluded from potato improvement because of its sterility, produced total of 1,700 somaclones. From this 15 stable somaclones were identified, thus providing enough variability for potato improvement. In many samples somaclone having resistance to late blight (Phytophthora infestans) and early blight (Alternaria solani) were identified.

Maize: In maize, the plant with T cytoplasm is male sterile and Drechslera maydis T which are toxin susceptible. When this plant was subject to in vitro culture, somaclones were produced with the characters of male fertility and toxin resistance. The result was due to alterations in mtDNA which is responsible for toxin tolerance.

Wheat: The embryo culture technique adopted in wheat has thrown out some 200 plants from a single immature embryo. The initial somaclonal regenerants displayed phenotypic variations. The analysis of regenerants obtained from the cultivar Yaqui 50E showed variations for the characters like plant height, maturity, tiller number, presence of awns, glume colour, grain colour, etc. The existence of somaclonal variation was also supported by the appearance and disappearance of some specific bands of gliadin protein.

Tomato: somaclones were isolated with variants phenotypes such as recessive mutation for male sterility resistance to fusaruim oxysporium joint less pedicel, tangerine pedicel, tangerine veriscent leaf, flower and fruit colour.

Reference
Ahloowalia, B.S. 1986. Limitations to the use of somaclonal variation in crop improvement. In: J. Semal (Ed.) Somaclonal Variation and Crop Improvement, Martinus Nijhoff, Boston.

Bregitzer, P., Halbert, S.E. and Lemaux, P.G. 1998. Somaclonal variation in the progeny of transgenic barley. Theor. Appl. Genet.

Larkin, P. J., and S. C. Scowcroft, 1981: Somaclonal variation-a novel source of variability from cell culture for plant improvement. Theor. Appl. Genet. 60





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