Speaker: Damon Lisch, Purdue University associate professor

Title: Loss of DNA methylation in maize results in heritable transposon activation and the appearance of novel epialleles

Abstract: The maize genome is composed largely of transposable elements (TEs), nearly all of which are epigenetically silenced. The bulk of these TEs are located either in heterochromatin. These TEs are kept in a deeply silenced state largely via symmetric (CG and CHG) DNA methylation and H3K9 dimethylation. In contrast, silencing of TEs immediately adjacent to genes is associated with RNA-directed DNA methylation. Surprisingly, mutations that largely eliminate this methylation, such as mediator of paramutation1 (mop1), have little effect on the phenotype of maize, nor do they result in reactivation of either of the two genes carried by silenced MuDR transposons, mudrA and mudrB. Although all methylation of the mudrA gene promoter is immediately lost in the mop1 mutant, reactivation of this gene takes multiple generations. Instead, in early generations, maintenance of the silenced state of mudrA in the mop1 mutant is associated with an increase in H3K9me2, suggesting that DNA and H3K9 methylation can balance each other to maintain silencing. In contrast to mudrA, mudrB is silenced via H3K27me3, a histone modification normally associated with gene, not TE silencing. Remarkably, a brief heat treatment rapidly and transgenerationally reactivates both mudrA and mudrB, but only in mop1 mutant plants, suggesting that RdDM is required to buffer the effects of heat in maize. Interestingly, although mop1, like rdr2 in Arabidopsis, has a minimal effect on plant phenotype, multiple generations in a mop1 mutant background can result in the sudden appearance of multiple new phenotypes. These phenotypes behave as stable dominant epialleles in a mop1 mutant background, but disappear in wild type plants. Analysis of one such epimutant, Shad, reveals that the phenotype is due to a dramatic reduction in microRNA mi166 in somatic sectors that exhibit reversed polarity, suggesting a connection between miRNA-mediated gene repression and siRNA mediated TE repression. Together, these observations suggest that although RdDM in maize is dispensable in the short term it can be essential for long term repression of TE silencing and for normal expression of maize genes.

Host: Sarah Anderson, genetics, development and cell biology assistant professor