Fig. 6.

Model depicting the role of histone H1 and DNA methylation in selective repression of GYPSY and COPIA78/ONSEN retrotransposons in Arabidopsis. Upregulated GYPSY transposable elements (TEs) in heat‐treated h1 mutants are located in pericentromeric regions, whereas upregulated COPIA78/ONSEN elements are dispersed in the chromosome arms. In h1 mutants, chromatin structure is opened and the two groups of TEs gain DNA methylation and remain silenced. After heat stress of wild‐type plants (WT + heat), DNA methylation level remains unchanged on upregulated GYPSY TEs, but decrease on upregulated COPIA78/ONSEN elements, thus likely contributing to their activation. Opened chromatin structure is likely sufficient to induce expression of GYPSY TEs. In heat‐treated h1 mutants (h1 + heat), GYPSY TEs gain DNA methylation, which, however, is not sufficient for repression and GYPSY TEs become more strongly expressed compared to heat‐treated WT. By contrast, COPIA78/ONSEN TEs lose DNA methylation in heat‐treated h1 mutants, which contributes to their upregulation, because COPIA78/ONSEN TEs, but not GYPSY TEs, are strongly activated in heat‐treated chromomethylase 2 (cmt2) mutans (cmt2 + heat).