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Application
of
Somoclonal
Variant - Disease Resistant Variants
Somaclonal variants resistant to diseases can be isolated by screening a large number of plants regenerated through tissue culture of which some may show useful resistance. Alternatively, they may be isolated by selecting cells, protoplasts, calli, embryos or meristems for resistance to the concerned toxins by placing them on a medium containing a lethal concentration of the toxin. Calli, cells, embryos or plants, which survive and grow on this medium, are expected to be resistant to the toxin and, in many cases, to the concerned pathogen as well. The toxins may be in the form of culture filtrate, a crude preparation or in the purified form.
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Many pathogenic bacteria and fungi produce toxins that are toxic to plant cells. These toxins may be either specific or nonspecific. Specific toxins exhibit a very high specificity for the host species and the host varieties, which very closely parallels the susceptibility of the latter to the pathogen strain that produced the toxin, e.g., oat varieties and the toxin produced by Helminthosporium victoriae (victoria blight), maize varieties and the toxin produced by Helminthosporium maydis race T, etc.
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But the toxins produced by many pathogens are toxic to plant species and varieties of crops in general without any reference to their susceptibility to the races of the pathogen, which produced these toxins; such toxins are called nonspecific toxins, e.g., toxins produced by many Alternaria spp., many pathogenic soil bacteria, etc.
Plant cell cultures may be exposed to lethal concentrations of these toxins and resistant clones isolated. Plants regenerated from the toxin resistant clones would be resistant to the disease producing pathogen as well provided the toxin is involved in disease development.
For example, maize (2. mays) lines having the Texas male sterility cytoplasm are susceptible to Southern leaf blight caused by Helminthosporium maydis, which produces a toxin that binds to the mitochondria. Maize cells resistant to this toxin have been selected, and plants regenerated from them were resistant to leaf blight caused by H. maydis. There is a long list of successful reports on the isolation of disease resistant lines through selection of cells resistant to the toxin produced by pathogens.
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It may be emphasized that this approach is successful only in the case of those pathogens, which produce toxins that are involved in disease development. But many of the pathogens either do not produce a toxin or produce a toxin with an uncertain role in disease development. The cell selection approach can not be successfully applied in such cases.
In the case of nonspecific toxins, the toxin resistant lines may be susceptible to the pathogen, e.g., in case of bacterial wilt of tomato. In all such cases, somaclones may be obtained from un selected cell cultures, and the RI progeny of these somaclones may be screened for resistance to the concerned pathogen.
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This approach was successful in the isolation of a bacterial wilt resistant tomato line, and of a Fiji disease resistant sugarcane line (from variety 'Pindar'), which was subsequently released as a new variety (variety 'Ono').
Stress Resistant and Other Mutants
Plant cells resistant to 4-5 times the normally toxic salt (NaCl) concentration have been isolated. In many cases, the plants regenerated from them were also tolerant to saline conditions. For example, tobacco plants regenerated from high salt (0.88%) tolerant, cell lines were also salt tolerant, and this feature was passel onto two successive sexual generations.
But in some cases, salt tolerant cell lines become salt dependent in that they required the presence of high salt levels for optimum growth, e.g., alfalfa and citrus cell lines. This approach may permit the development of crop varieties suitable for cultivation in saline soils.
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Low temperature is another important environmental factor affecting survival and performance of crop plants. Cell lines resistant to chilling have been isolated in several cases, e.g., chillies, Nicotiana sylvestris, etc. In some soils, an excess of metal ions, e.g., aluminium, may adversely affect the performance of crop varieties. Varieties suitable for cultivation on such problem soils may be developed by selecting cell lines resistant to the concerned ion.
The feasibility of this approach has been amply demonstrated by the selection of aluminium resistant cell lines of N. plumbaginifolia and regeneration of resistant plants from these lines.
Mutants for Agronomic Characters
In vitro regenerated plants often show variation in quantitative characters. Such variants have been described in a number of crops including tobacco, com, barley, tomato, sorghum, wheat, carrot, Brassica, celery, sugarcane, potato, etc. There is at least one example of a commercial variety being developed using such variants. The alfalfa variety 'Sigma' was developed as a polycross among superior somaclones derived from certain commercial varieties. Sigma shows improved yield as well disease resistance.
Mutants for Efficient Nutrient Utilization
In vitro selection may be a useful method for developing phosphate ultra efficient crop plants. Isolation of somaclonal variant tomato cell lines, which are able to grow normally under phosphate deficient condition due to a high secretion of acid phosphatase, which greatly increased the rate of phosphate uptake, has been reported.
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