Mitochondrial DNA and the largest nuclear-mitochondrial DNA in Arabidopsis can be separated by their methylation levels
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Updated: May 27, 2025
Determination of DNA Methylation of Imprinted Genes in Arabidopsis Endosperm
Published on: January 28, 2011
Yuyang Zhong1, Miki Okuno2, Nobuhiro Tsutsumi1
1Laboratory of Plant Molecular Genetics, Graduate School of Agricultural and Life Sciences, The University of Tokyo, Tokyo 113-8657, Japan.
Abstract:
Methylation of cytosines in plant mitochondrial DNA (mtDNA) has been a controversial issue. Results supporting mtDNA methylation may have been subject to contamination due to the presence of nuclear sequences originating from the mitochondrial genome called nuclear mitochondrial insertions (NUMT). In Arabidopsis (Arabidopsis thaliana) Columbia 0 (Col-0), the largest NUMT, located on Chromosome 2, is nearly twice the size of the entire mitochondrial genome and exhibits a sequence almost identical to the mitochondrial genome, albeit with shuffling and repeats. In the presence of such high similarity, it is challenging to eliminate interference when determining mtDNA methylation levels. Here, we applied a methyl-CpG-binding domain (MBD) protein-based affinity assay to separate total DNA, applied next-generation sequencing to the pre- and postseparation DNA samples, and examined the single nucleotide polymorphism (SNP) sites between NUMT and mtDNA. The results revealed successful separation of methylated and non-methylated DNA within the total DNA, with simultaneous separation achieved between NUMT DNA and mtDNA. These results suggest that our method can achieve separation based on the differential methylation levels of the whole lengths of NUMT and mtDNAs. The bisulfite sequencing results for the postseparation DNA samples suggest that mtDNA exhibits not only a lack of CpG methylation but also an absence of CHH and CHG methylation. In contrast, the NUMT shows high levels of methylation across all 3 contexts, at least in the Col-0 accession.
Insights
Plant mitochondrial DNA (mtDNA) methylation is debated due to nuclear mitochondrial insertions (NUMTs). A new method successfully separated NUMTs from mtDNA, revealing mtDNA lacks methylation while NUMTs are heavily methylated.
Area of Science:
- Plant molecular biology
- Epigenetics
- Genomics
Background:
- Cytosine methylation in plant mitochondrial DNA (mtDNA) is controversial, potentially due to contamination from nuclear mitochondrial insertions (NUMTs).
- The largest NUMT in Arabidopsis thaliana (Col-0) is highly similar to mtDNA, complicating methylation analysis.
- Distinguishing between mtDNA and NUMT is crucial for accurate methylation studies.
Purpose of the Study:
- To develop and validate a method for separating methylated and non-methylated DNA, specifically distinguishing between mtDNA and NUMT.
- To accurately assess the methylation status of plant mtDNA and NUMT.
Main Methods:
- Utilized a methyl-CpG-binding domain (MBD) protein-based affinity assay to separate total DNA.
- Applied next-generation sequencing to pre- and post-separation DNA samples.
- Analyzed single nucleotide polymorphism (SNP) sites to differentiate between NUMT and mtDNA sequences.
Main Results:
- The MBD assay successfully separated methylated from non-methylated DNA fractions.
- Simultaneous separation of NUMT DNA and mtDNA was achieved.
- Bisulfite sequencing revealed no CpG, CHH, or CHG methylation in mtDNA.
- NUMT sequences exhibited high methylation across all three contexts in the Col-0 accession.
Conclusions:
- The MBD protein-based assay is effective for separating mtDNA from NUMT based on differential methylation.
- Arabidopsis thaliana mtDNA is unmethylated in CpG, CHH, and CHG contexts.
- NUMTs in Arabidopsis thaliana (Col-0) are highly methylated, suggesting epigenetic regulation differs significantly between organellar and nuclear-integrated sequences.
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