Epimutation in plant culture

Cultured plant cells show phenotypic variation, some of which results from epigenetic changes – mitotically heritable but reversible alterations in gene expression that do not result from permanent genetic modifications. Some epigenetic changes are also transmitted during meiosis; these are called epimutations, and Meins and Thomas now report that epimutations can arise frequently in cultured tobacco plant cells (see p. 6201). Leaf cells normally require the cell-division factor cytokinin for continuous growth in culture – they have a C^- phenotype. However, when cultured in media containing cytokinin, some of these cells rapidly alternate between a C^- and a C⁺ (cytokinin-independent) state, a phenomenon called pseudodirected variation. Meins and Thomas show that in plants regenerated from most C⁺ clones, leaf tissues retained the C⁺ phenotype. This trait was meiotically transmitted but rapidly reverted to the C^- phenotype during successive sexual generations,leading the researchers to conclude that pseudodirected variation is a new form of epimutation.