The use of somaclonal variation as a source of genetic variability for plant improvement has been examined extensively (Larkin and Scowcroft, 1981; Reisch, 1983). Although most somaclonal variants described are of limited value for crop improvement, some variants have been obtained with resistance to diseases or abiotic stress. For tomato successful selection of somaclonal variants resistant to Fusarium oxysporum f.sp. lycopersici (Miller et al., 1985; Shahin and Spivey, 1986) and TMV/ToMV (Barden et al., 1986; Smith and Murakishi, 1987) has been reported.
In this report we evaluate the screening of tomato somaclones for ToMV resistance. 1050 R1 plants were obtained by regeneration of leaf, cotyl and hypocotyl explants of Lycopersicon esculentum cv. Moneymaker. All plants were self pollinated and the R2-lines were screened for ToMV resistance. Ten plants per R2-line were sown. After 10-13 days cotyledons were dusted with carborundum and 'rub'-inoculated with ToMV strain 0 (kindly provided by Dr A.Th.B. Rast, Glasshouse Crops Research and Experiment Station, Naaldwijk, The Netherlands). The susceptible cv. Moneymaker and the resistant hybrid cv. Sonatine were used as controls. Most plants showed symptoms after 10-20 days and were discarded. Plants which didn't show symptoms 2-3 weeks after inoculation were analyzed for the presence of virus by bioassay on detached leaves of 7 weeks old Nicotiana glutinosa plants. Leaf extracts of the susceptible 'Moneymaker' and the resistant 'Sonatine' scored respectively over 50 and 0 lesions in this bioassay. If less than 10 lesions were detected those plants were inoculated once again and scored once more for symptoms and the presence of virus.
Variations in symptom expression and amounts of virus were observed. Such variation is not necessarily genetically determined, and might be caused by environmental factors. However, we were only interested in absolute resistance (immunity, hypersensitivity). In our screening no absolute resistant plants were found.
Somaclonal mutants of tomato with specific traits have been found by others with high frequencies. Evans and Sharp (1983) identified 13 putative nuclear mutations arising from 230 regenerated plants. Out of 370 somaclones Barden et al. (1986) found 6 plants with resistance to TMV/ToMV. In contrast to these high frequencies no resistant plants were found in our screening of a large number of tomato somaclones. This is in agreement with the result obtained by Balazs who also didn't succeed in obtaining ToMV resistant tomato somaclones regenerated from a susceptible variety (personal communication). Probably genotype, tissue culture methods or other factors effect the frequency and nature of induced mutations. This implicates that use of somaclonal variation for the plant breeder is not generally applicable, but should be investigated for each program separately.
Literature cited:
Barden, K.A., S.L.S. Smith and H.H. Murakishi. 1986. Regeneration and screening of tomato somaclones for resistance to tobacco mosaic virus. Plant Sci. 45:209-213.
Evans, D.A. and W.R. Sharp. 1983. Single gene mutations in tomato plants regenerated from tissue culture. Science 22-1:949-951.
Larkin, P.J. and W.R. Scowcroft. 1981. Somaclonal variation - a novel source of variability from cell cultures for plant improvement. Theor. Appl. Genet. 60:197-214.
Miller, S.A., G.R. Williams, H. Medina-Filho and D.A. Evans. 1985. A somaclonal variant of tomato resistant to race 2 of Fusarium oxysporum f.sp. lycopersici. Phytopathology 75:1354.
Reisch, B. 1983. Genetic variability in regenerated plants. In: Evans, D.A., W.R. Sharp, P.V. Ammirato and Y. Yamada (Eds). Handbook of Plant Cell Culture Vol. 1. Macmillan Publ. Co, New York, pp. 748-769.
Shahin, E.A. and R. Spivey. 1986. A single dominant gene for Fusarium wilt resistance in protoplast-derived tomato plants. Theor. Appl. Genet. 73:164-169.
Smith, S.L.S. and H.H. Murakishi. 1987. Inheritance of resistance to tomato mosaic virus (ToMV-0) in tomato somaclones. TGC Report 37:65-66.