Loss of mCHH islands in maize chromomethylase and DDM1-type nucleosome remodeler mutants

Plants make use of three types of DNA methylation, each characterized by distinct DNA methyltransferases. One type, RNA-directed DNA methylation (RdDM), is guided by siRNAs to the edges of transposons that are close to genes, areas called mCHH islands in maize. Another type, chromomethylation, is guided by histone H3 lysine 9 methylation to heterochromatin across the genome. We examined DNA methylation and small RNA expression in plant tissues that were mutant for both copies of the genes encoding chromomethylases as well as mutants for both copies of the genes encoding DDM1-type nucleosome remodelers, which facilitate chromomethylation. Both sets of double mutants were nonviable but produced embryos and endosperm. RdDM was severely compromised in the double mutant embryos, both in terms of DNA methylation and siRNAs. Loss of 24nt siRNA from mCHH islands was coupled with a gain of 21, 22, and 24nt siRNAs in heterochromatin. These results reveal a requirement for both chromomethylation and DDM1-type nucleosome remodeling for RdDM in mCHH islands, which we hypothesize is due to dilution of RdDM components across the genome when heterochromatin is compromised.