Not all mutations are forever. Epimutations, heritable changes in gene expression without a corresponding change in primary DNA sequence, have been associated with the presence of transposons and DNA methylation in maize. Methylation can alter the expression of at least one endogenous gene in Arabidopsis thaliana: epimutant alleles of SUPERMAN are normal in sequence, but heritably hypermethylated, producing a floral phenotype (which includes an increased stamen number and a distorted fourth whorl) similar to that induced by true mutant alleles. But whether SUPERMAN epialleles are unique or represent a more common phenomenon is not known. We do know, however, that SUPERMAN is no longer alone; Enrico Coen and colleagues have revealed in a recent issue of Nature (vol. 401, 157–161; 1999) that some mutant alleles of Lcyc, a gene controlling floral symmetry in Linaria vulgaris (toadflax), also result from epimutation. They found that, in contrast with the wild-type flower, the mutant plant carries a hypermethylated Lcyc allele that correlates with loss of gene expression.

As pointed out by En Li in September's issue of Nature Genetics (vol. 23, 5–6; 1999), the mechanisms that govern epimutation onset are obscure. Coen and colleagues propose that aberrant activation of a silencing process in the meristem, the pool of stem cells from which plant tissues are derived, might be the cause. Both the Lcyc and SUPERMANepimutations affect genes expressed only in floral development, a relatively late event; it may be that a common aspect of genes that are expressed at a comparatively late stage of development conditions them to epimutation. Curiously, introduction of a demethylating transgene does not decrease methylation of SUPERMAN epialleles, and late onset of SUPERMAN-like floral phenotypes is seen in a demethylated background. It is possible that a compensatory mechanism might initiate localized hypermethylation in response to genomic hypomethylation or deficiency of methyltransferase. So the next time you contemplate flower power, you might ponder whether it's what's in the DNA, or what's on the DNA, that makes the difference.

Wild-type (left) and epimutant flowers of Linaria, which show bilateral and radial symmetry owing to differences in methylation state.