DNA demethylation affects imprinted gene expression in maize endosperm

Background: DNA demethylation occurs in many species and is involved in diverse biological processes. However, the occurrence and role of DNA demethylation in maize remain unknown. Results: We analyze loss-of-function mutants of two major genes encoding DNA demethylases. No significant change in DNA methylation has been detected in these mutants. However, we detect increased DNA methylation levels in the mutants around genes and some transposons. The increase in DNA methylation is accompanied by alteration in gene expression, with a tendency to show downregulation, especially for the genes that are preferentially expressed in endosperm. Imprinted expression of both maternally and paternally expressed genes changes in F₁ hybrid with the mutant as female and the wild-type as male parental line, but not in the reciprocal hybrid. This alteration in gene expression is accompanied by allele-specific DNA methylation differences, suggesting that removal of DNA methylation of the maternal allele is required for the proper expression of these imprinted genes. Finally, we demonstrate that hypermethylation in the double mutant is associated with reduced binding of transcription factor to its target, and altered gene expression. Conclusions: Our results suggest that active removal of DNA methylation is important for transcription factor binding and proper gene expression in maize endosperm.