- Genetic variability is an essential component of any breeding program designed to improve the characteristics of crop plant.
- Tissue culture is a process that involves the establishment of a differentiated cell or tissue culture under defined condition proliferation for a number of generations and subsequent regeneration of plants.
- In other words one impose a period of callus proliferation on between an explant the regeneration of plant.
- The first report of morphological variation in plant regenerated from cell culture concerned dose of sugarcane by heize and mee in 1971.
- Occurance of genetic variation is essential for organic evolution.
- It may be expected that organic evolution would conserve the mechanism that allow for critical level of de novo genetic variations.
- The initiating explant for a tissue culture cycle make com virtually from any plant organ or cell type including embryo, microspores, root, leaves and protoplast.
- All plants arising from tissue culture should be exact copies of parental plant.
- However phenotypic variant where frequent observed among regenerated plant.
- These were usually dismissed earlier as tissue culture artefacts due to recent exposure of exogenous phytohormones and sometime they were level as epigenetic events.
- However evidence have now showing this variance are not artefacts but variation arising due two culture of self and this has been termed as somaclonal variation.
- The cause of variation is attributed to change in the chromosome number and structure.
- Genetic heterogeneity in culture arise mainly due to:-
- The expression of chromosomal mechanism for genetic disorder in cells of explant.
- New irregularities brought about by culture condition through spontaneous mutation.
- The genetic variability present among cultured cell plant derived from such cells or progeny of such plants is called somaclonal variation.
- This term was introduced by larkin and scowcroft in 1981.
Schemes for obtaining somaclonal variation
Two schemes with and without in vitro selection have been generally followed for getting somaclonal variation in crop plants.
Without in vitro selection
- An explant is cultured on a sweet able medium as examples small shoot segments, protoplasts, leaves, embryos.
- The basil medium is supplemental with growth regulators which support differentiated stage as example callus.
- Normally this culture are subcultured and then transferred to shoot induction medium for plant regeneration.
- The plants so regenerated are transferred to pita, grown to maturity and analysed to variance.
- The approach is to find somaclonal variants among the region rated plant for various character.
- Here no directed approach is used and appearance of desirable variant is a chance event.
- In such method both dominant and recessive traits can be directly selected.
- If the regenerates are heterozygous then the recessive traits can be selected in the progeny of re generates.
- Epigenetic variation will also we avoid when progeny are used.
- Unfortunately a disadvantage of this approach is that it is time consuming due to the fertilization step and requires screening of many plants.
Within vitro selection
- In vitro culture of higher plants can be used for selection of mutants.
- Protoplast cell suspension and callus culture are handled like microorganisms to search for biochemical mutants.
- Selection for resistance is the most straightforward method for mutant selection wear resistant cells in a large population can be selected by their ability to grow in the presence of an inhibitor while the sensitive cell do not.
- Here’s the differentiated culture is subjected to selection against inhibitors like antibiotics analogues.
- This compound are put in the medium at a concentration such that some sale population survives and can be further grown on a selective medium.
- Different selection cycle are performed to get tolerant cell callus culture that are subsequently regenerated into plant.
- If the plant are resistant to the inhibitor than stable transmission of that character is analysed in subsequent generations.
Factors influencing somaclonal variation
When attempting to produce somaclones for a new crop plant some factors that are valid for both schemes must be considered.
- The genotype of plant used for somaclonal variation is an important variable.
- Genotype can influence both frequency of regeneration and the frequency of somaclones.
- The source of explant has been considered a critical variable for somaclonal variation.
- Similarly in sugarcane which is a chromosome mosaic propagation by stem cutting can result in plants with variable chromosome number.
Duration of cell culture
- Chromosome variation has been reported for many established plant cell culture
- It is widely accepted that most long-term culture are chromosomally variable.
- Polyploid and anaploid plant have been frequently recovered in large number of species.
- It has been state that variation increased with increasing duration of culture.
- It has been known that growth regulator composition of the culture medium can influence frequently of karyotypic attention in cultured cells.
- The growth regulators have been most frequently considered to be responsible for chromosome variability.
- All about the variable are important in the recovery of somaclones among region related plant.
Application of somaclonal variation
- To have an impact on a plant breeding program and in vitro scheme must either produce plant useful for breeding or useful directly.
- Implication of somaclonal variation in breeding is that novel variant can arise and this can be agronomically used.
- A number of breeding line have been developed by somaclonal variation.
- Some of the variant developed as varieties in rubus, torenia, alpia.
Somaclonal variation and disease resistant
- The greatest contribution of somaclonal variation towards plant improvement is in the development of disease resistant genotype in various crop species.
- Resistant was first reported in sugarcane for spot disease.
- Variability from protoplast derived plant was also examined in the tetraploid russet burbank variety of potato.
- Selection of somaclonal variant with disease resistant has resulted in the release of few cultivars or genotype.
- Through in vitro selection several cell lines resistant to herbicides have been isolated and a few have been reached and rated into complete plants.
- Among the important achievements are tobacco ,soybean ,wheat etc.
- Zemetra et al used in vitro selection technique for generation of somaclonal variant for Russian wheat.
- Recently a variety bio L212 has been identified for cultivation in Central India which has been developed through somaclonal variation and has low ODAP neurotoxin indicating the potential of somaclonal variation for the development of varieties with improved seed quality.
Basis of somaclonal variation
- Variant plant with altered chromosome number have been reported by several workers.
- Polyploidy is the most frequently observed chromosome abnormality.
- The change in chromosome number in a variant plant is commonly associated with reduced fertility and with altered generation in the progeny of self fertilized plant.
Change in chromosome structure
- In contrast to grass changes in chromosome number more cryptic chromosome rearrangement may be responsible for genetic variation.
- Translocation have been reported in potato, oats etc.
- Published work also suggest that chromosome deletion duplication invention and other minor reciprocal and non reciprocal rearrangement ocular among regenerated plant.
Single gene mutation
- Single gene mutation have been decteted by several workers.
- Recessive single gene mutation are suspected if the variant does not appear in generation but the self fertilized are R1 or SC2 progeny reach hundred in 3: 1 Mendelian ratio for the rate of interest.
- This type of analysis has been reported for rice ,wheat etc.
- Uncountable and unpredictable nature of variation and most of the variations are of no apparent use.
- The variation is cultivar dependent.
- The variation obtained is not always stable and heritable the changes of occur at variable frequency.
- Not all the changes obtained are novel. In majority of cases improved variant have not been selected for breeding purpose.