Epigenetic variation of cultured somatic cells: evidence for gradual changes in the requirement for factors promoting cell division.

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Abstract
Cells of higher plant species in culture sometimes lose their requirement for an exogenous supply of a cell division factor that, thereafter, they are able to produce. This heritable change, known as cytokinin habituation, appears to be an epigenetic one rather than a classical mutation because it is directed, potentially reversible, leaves the cell totipotent and involves the expression of a latent differentiated function. By using cloned cell lines derived from pith parenchyma of tobacco [Nicotiana tabacum], evidence was obtained that the habituation process is gradual rather than all-or-none and leads to progressively more autotrophic tissues. Cells in culture show reversible shifts among a range of habituated states but remain totipotent and can be induced to regain their requirement for a cell division factor. Thus, it appears that habituation involves epigenetic changes in a quantitative cellular phenotype. The hypothesis that tumor progression in crown gall, a neoplastic disease of higher plants, can be accounted for by heritable changes in the pattern of gene expression was supported.