26 Variation in Tissue Culture

As discussed in Chapter 19 (“Genetic Engineering Technologies”), plant cell culture is an important technique for plant genetic improvement. Historically, and as implied throughout this book, plant cell culture has been viewed by most to be a method for rapid cloning. In essence, it was seen as a method of sophisticated asexual propagation, rather than a technique to add new variability to the existing population. For example, it was believed that all plants arising from such tissue culture were exact clones of the parent, such that terms like “calliclone,” “mericlone,” and “protoclone” were used to describe the regenerants from callus, meristems and protoplasts, respectively. Although phenotypic variants were observed among these regenerants, often they were considered artifacts of tissue culture. Such variation was thought to be due to “epigenetic” factors such as exposure to plant growth regulators (PGRs) and prolonged culture time. As more and more species were subjected to tissue culture, however, reports of variation among regenerants increased. In a historically significant review, Larkin and Scowcroft (1981) proposed the more general term “somaclones” for the regenerants coming out of tissue culture, irrespective of the explant used. Variation displayed by such regenerants from tissue culture would then be somaclonal variation. Tissue culture studies in the 1970s and early 1980s started to focus their attention on this type of variation, and it was soon recognized that somaclonal variation exists for almost all the phenotypic characters. To a plant scientist, somaclonal variation is perhaps the best route for studying somatic cell genetics. In contrast to the earlier view of “true to type” regeneration among plants derived from tissue culture, the frequency of genetic variation may actually be quite high. In some species, such as oil palm and banana, variation among tissue culture derived progenies is higher than one would expect to occur in vivo . In perennial crops that are asexually propagated, somaclonal variation offers an excellent opportunity to add new genotypes to the gene pool. In such cases it is important to understand and identify the causal mechanism behind the variations, so that we can effectively control them to our advantage.