Canalization of genome-wide transcriptional activity in Arabidopsis thaliana accessions by MET1-dependent CG methylation

BACKGROUNDEukaryotes employ epigenetic marks such as DNA methylation at cytosines both for gene regulation and genome defense. In Arabidopsis thaliana, a central role is played by methylation in the CG context, with profound effects on gene expression and transposable element (TE) silencing. Nevertheless, despite its conserved role, genome-wide CG methylation differs substantially between wild A. thaliana accessions.RESULTSWe hypothesized that global reduction of CG methylation would reduce epigenomic, transcriptomic and phenotypic diversity in A. thaliana accessions. To test our hypothesis, we knocked out MET1, which is required for CG methylation, in 18 early-flowering A. thaliana accessions. Homozygous met1 mutants in all accessions suffered from a range of common developmental defects such as dwarfism and delayed flowering, in addition to accession-specific abnormalities in rosette leaf architecture, silique morphology and fertility. Integrated analysis of genome-wide methylation, chromatin accessibility and transcriptomes confirmed that inactivation of MET1 greatly reduces CG methylation and alters chromatin accessibility at thousands of loci. While the effects on TE activation were similarly drastic in all accessions, the quantitative effects on non-TE genes varied greatly. The expression profiles of accessions became considerably more divergent from each other after genome-wide removal of CG methylation, although the expression of genes with diverse expression profiles across wild-type accessions tended to become more similar in mutants.CONCLUSIONSOur systematic analysis of MET1 requirement for genome function in different A. thaliana accessions revealed a dual role for CG methylation: for many genes, CG methylation appears to canalize expression levels, with methylation masking regulatory divergence. However, for a smaller subset of genes, CG methylation increases expression diversity beyond genetically encoded differences.