DNA methylation is a conserved epigenetic mark in plants and many animals. How parental alleles interact in progeny to influence the epigenome is poorly understood. We analyzed the DNA methylomes of Arabidopsis Col and C24 ecotypes, and their hybrid progeny. Hybrids displayed nonadditive DNA methylation levels, termed methylation interactions, throughout the genome. Approximately 2,500 methylation interactions occurred at regions where parental DNA methylation levels are similar, whereas almost 1,000 were at differentially methylated regions in parents. Methylation interactions were characterized by an abundance of 24-nt small interfering RNAs. Furthermore, dysfunction of the RNA-directed DNA methylation pathway abolished methylation interactions but did not affect the increased biomass observed in hybrid progeny. Methylation interactions correlated with altered genetic variation within the genome, suggesting that they may play a role in genome evolution.D NA methylation is a conserved epigenetic mark in many eukaryotes (1-6). In plants and mammals, DNA methylation plays important roles in genome stability, genomic imprinting, paramutation, and gene regulation during development and diseases (1-6). Parental genetic alleles interact in the filial 1 (F1) progeny in a Mendelian manner. DNA methylation may affect this interaction such that methylated epialleles may show nonadditive interactions. This notion is supported by recent observations that hybridization results in nonadditive changes in the F1 plant DNA methylome (7,8). It has been proposed that genome-wide interactions between parental epialleles in F1 hybrids are critical for hybrid vigor, the superior performance of hybrids compared with their parents (8-13). Epigenome interactions may confer nonadditive transcriptional and epigenetic activities in hybrid offspring compared with the parental lines. In contrast to additive regulation, which leads to midparent value (MPV) levels equal to the average of the two parental lines, nonadditive regulation results in a deviation from the MPV, such that the F1 hybrid resembles the high-or low-expression parent. In addition, nonadditive regulation can be transgressive, which is beyond the range of the parental levels (14).Studies in rice and Arabidopsis have identified nonadditive changes in DNA methylation levels at loci where parental methylation levels are different [differentially methylated regions (DMRs)] (7,8,11,12,15,16). These "methylation interactions" were attributed to two mechanisms, transchromosomal methylation (TCM) and transchromosomal demethylation (TCdM), whereby the methylation level of one parental allele is altered to resemble the methylation level of the other parental allele (7,8,17). At some genomic loci, the transmethylation events in F1 were shown to be inherited to the next generation, as observed for paramutation (17). Nonetheless, a thorough profile of DNA methylation interactions across the whole Arabidopsis genome has yet to be determined. In particular, it remains unclear whether DNA m...
A contribution of DNA methylation to defense against invading nucleic acids and maintenance of genome integrity is uncontested; however, our understanding of the extent of involvement of this epigenetic mark in genome-wide gene regulation and plant developmental control is incomplete. Here, we knock out all five known DNA methyltransferases in Arabidopsis, generating DNA methylation-free plants. This quintuple mutant exhibits a suite of developmental defects, unequivocally demonstrating that DNA methylation is essential for multiple aspects of plant development. We show that CG methylation and non-CG methylation are required for a plethora of biological processes, including pavement cell shape, endoreduplication, cell death, flowering, trichome morphology, vasculature and meristem development, and root cell fate determination. Moreover, we find that DNA methylation has a strong dose-dependent effect on gene expression and repression of transposable elements. Taken together, our results demonstrate that DNA methylation is dispensable for Arabidopsis survival but essential for the proper regulation of multiple biological processes.
Epigenetic variation has been proposed to facilitate adaptation to changing environments, but evidence that natural epialleles contribute to adaptive evolution has been lacking. Here we identify a retrotransposon, named “NMR19” (naturally occurring DNA methylation variation region 19), whose methylation and genomic location vary among Arabidopsis thaliana accessions. We classify NMR19 as NMR19-4 and NMR19-16 based on its location, and uncover NMR19-4 as an epiallele that controls leaf senescence by regulating the expression of PHEOPHYTIN PHEOPHORBIDE HYDROLASE (PPH). We find that the DNA methylation status of NMR19-4 is stably inherited and independent of genetic variation. In addition, further analysis indicates that DNA methylation of NMR19-4 correlates with local climates, implying that NMR19-4 is an environmentally associated epiallele. In summary, we discover a novel epiallele, and provide mechanistic insights into its origin and potential function in local climate adaptation.
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