Somaclonal variation

This technique has been described in detail by Larkin and Scowcroft (1981) and specifically in fruit crops by Hammerschlag (1992). It arises when plant explants are subjected to a tissue culture cycle. The cycle includes establishment of a dedifferentiated cell or tissue culture under defined conditions and the subsequent regeneration of plants. Variation at cellular level occurs either in cells before explant excision or during the tissue culture cycle (Skirvin 1978; Jain 2001; Remotti 1998; Rami and Raina 2000). The degree of variation depends on many factors, including:
  • the origin of the explant used (organ, age, genotype) (Murashige 1974; D’Amato 1975; Barbier and Dulieu 1980)
  • the time that cells or tissues are maintained in vitro (Barbier and Dulieu 1980)
  • the time and intensity of the mutagenic agents used (Burk and Matzinger 1976)

Reduction of somaclonal variation is achieved by using appropriate culture media and by shortening subculture intervals. Somaclonal variation can be 10,000 times higher than spontaneous mutation rates in whole plants (Larkin and Scowcroft 1981). Many phenotypic variations reported in the regenerated fruit crop plants were extensively reviewed by Hammerschlag (1992). Important changes include growth rate and reproductive apparatus modification (sterility, precocious flowering and flower abnormalities, internodal length), and leaf (variegation, albino, chlorotic, etc.), thornlessness, isoenzymatic activity changes, and increased salt resistance, fruit colour, etc. An increased ploidy level has been reported in kiwi subcultures (Rugini et al. 2000b) and in grape (Kuksova et al. 1997). Some changes are not hereditable, since they have epigenetic origin. These changes include:
  • cytokinin and auxin habituation (Meins and Binns 1977)
  • chilling resistance (Dix and Street 1976)
  • changing susceptibility to fungal attack (Potter 1980)
  • susceptibility to certain pathogens, due maybe to virus elimination during regeneration, which can also alter plant habit.