2. Origin of Somaclonal variation

The somaclonal variations observed in plants regenerated from cultured cells are derived from two sources: (i) some of the variations could be revelation of the inherent cellular heterogeneity of the explant, and (ii) culture conditions may bring about new genetic changes.

 

2.1. Pre-existing variability

Plant development in general involves change in nuclear DNA, such as change in chromosome number, structure (Bennici and D'Amato, 1990). Cells of plant apical meristems like, root-tips and shoot-tips are uniformly diploid in their genome due to DNA synthesis immediately followed by karyokinesis and cytokinesis (normal cell cycle). However, the derivatives of these meristematic cells do not divide by normal mitosis but may undergo DNA duplication and endoreduplication. The varying degree of endoreduplication results in somatic cells with 4C, 8C or higher DNA content or may result in polysomaty. Usually these genetic changes are not noticed as these cells do not divide. However, under culture conditions these cells may divide and undergo redifferentiation and express this change in their genome content as an inheritable character within the whole plant. Another type of pre-existing chromosomal variability which is rarely observed in hybrid plants is aneusomaty. In such plants the apical meristems and, consequently, the mature tissues comprise a mosaic of cells with varying number of aneuploid chromosome numbers. This condition is transferred or enhanced in callus cultures derived from such tissues.

 

2.2. In vitro induced variability

Under the stressful culture conditions, the plant cells undergo genetic and epigenetic changes. This could happen even in the explants from non-polysomatic species. Generally less variations are found in plants than the callus because in mixed population of cells with different ploidy, euploid cells tend to be more regenerative than aneuploid cells. Several factors affect the type and frequency of somaclonal variations, explant source, genotype, culture conditions and age of the culture.

 

i.  Culture medium

Culture media constituents, particularly certain growth regulators, BAP, NAA, 2,4-D, induce mutations in the cultured cells. Sunderland (1977) reported that Haplopappus cells in 2,4-D containing medium is converted from entirely diploid state to a entirely tetraploid state within few months. Torrey (1965) observed that in the cultures of pea root segments on a medium with 2,4-D as the sole hormone, only diploid cells divide but when the medium contained Kinetin and yeast extract in addition to 2,4-D, the tetraploid cells were selectively induced to divide. Most of the literature suggests that growth regulators influence somaclonal variation during the culture phase by affecting cell division, degree of disorganized growth and selective proliferation of specific cell types.