Stable Epigenetic States in Differentiated Plant Cells: Implications for Somaclonal Variation and Gene Silencing in Transgenic Plants

Abstract

Specialized cells that retain the ability to develop into a complete adult organism are considered totipotent. The totipotency of many plant cell types enables the regeneration of entire fertile plants from cultured somatic or gametic cells of a number of species (see Meins, this volume). However, the ready availability of virtually unlimited numbers of asexually propagated plants has in turn unveiled an unexpected yet ubiquitous phenomenon, which can be described as follows: Because epigenetic modifications of genomes (in the form of varying patterns of methylation or chromatin structure) and not nucleotide changes or structural alterations of genes are believed to underlie cell diffentiation, genetic uniformity is expected in a population of plants regenerated from any given explant source; i.e., regenerants obtained from somatic cells of a single plant should be clones and have a similar appearance. Instead of a monotonous collection of identical plants, however, an astonishing spectrum of phenotypic variability is often observed in sibling regenerants, particularly if an undifferentiated callus phase precedes the regeneration process. Although initially this variation was often slighted or considered artifactual (Skirvin et al. 1993), it was legitimized in 1981 when Larkin and Scowcroft consolidated the scattered observations in a review and proposed the term “somaclonal variation” to refer to the phenotypic variability observed among plants regenerated from cell or tissue culture (Larkin and Scowcroft 1981). In the intervening years, somaclonal variation has had an impact on both applied and basic plant science. On the positive side, it has captured the interest of breeders,...