An Optimized Method for the Construction of a DNA Methylome from Small Quantities of Tissue or Purified DNA from Arabidopsis Embryo

Distinct regulatory pathways contribute to dynamic CHH methylation patterns in transposable elements throughout Arabidopsis embryogenesis

CHH methylation (mCHH) increases gradually during embryogenesis across dicotyledonous plants, indicating conserved mechanisms of targeting and conferral. Although it is suggested that methylation increase during embryogenesis enhances transposable element silencing, the detailed epigenetic pathways underlying this process remain unclear. In Arabidopsis, mCHH is regulated by both small RNA-dependent DNA methylation (RdDM) and RNA-independent Chromomethylase 2 (CMT2) pathways. Here, we conducted DNA methylome profiling at five stages of Arabidopsis embryogenesis, and classified mCHH regions into groups based on their dependency on different methylation pathways. Our analysis revealed that the gradual increase in mCHH in embryos coincided with the expansion of small RNA expression and regional mCHH spreading to nearby sites at numerous loci. We identified distinct methylation dynamics in different groups of mCHH targets, which vary according to transposon length, location, and cytosine frequency. Finally, we highlight the characteristics of transposable element loci that are targeted by different mCHH machinery, showing that short, heterochromatic TEs with lower mCHG levels are enriched in loci that switch from CMT2 regulation in leaves, to RdDM regulation during embryogenesis. Our findings highlight the interplay between the length, location, and cytosine frequency of transposons and the mCHH machinery in modulating mCHH dynamics during embryogenesis.

Contribution of RdDM to the ecotype-specific differential methylation on conserved as well as highly variable regions between Arabidopsis ecotypes

BACKGROUND

Several studies showed genome-wide DNA methylation during Arabidopsis embryogenesis and germination. Although it has been known that the change of DNA methylation mainly occurs at CHH context mediated by small RNA-directed DNA methylation pathway during seed ripening and germination, the causality of the methylation difference exhibited in natural Arabidopsis ecotypes has not been thoroughly studied.

RESULTS

In this study we compared DNA methylation difference using comparative pairwise multi-omics dynamics in Columbia-0 (Col) and Cape Verde Island (Cvi) ecotypes. Arabidopsis genome was divided into two regions, common regions in both ecotypes and Col-specific regions, depending on the reads mapping of whole genome bisulfite sequencing libraries from both ecotypes. Ecotype comparison was conducted within common regions and the levels of DNA methylation on common regions and Col-specific regions were also compared. we confirmed transcriptome were relatively dynamic in stage-wise whereas the DNA methylome and small RNAome were more ecotype-dependent. While the global CG methylation remains steady during maturation and germination, we found genic CG methylation differs the most between the two accessions. We also found that ecotype-specific differentially methylated regions (eDMR) are positively correlated with ecotype-specifically expressed 24-nt small RNA clusters. In addition, we discovered that Col-specific regions enriched with transposable elements (TEs) and structural variants that tend to become hypermethylated, and TEs in Col-specific regions were longer in size, more pericentromeric, and more hypermethylated than those in the common regions. Through the analysis of RdDM machinery mutants, we confirmed methylation on Col-specific region as well as on eDMRs in common region are contributed by RdDM pathway. Lastly, we demonstrated that highly variable sequences between ecotypes (HOT regions) were also affected by RdDM-mediated regulation.

CONCLUSIONS

Through ecotype comparison, we revealed differences and similarities of their transcriptome, methylome and small RNAome both in global and local regions. We validated the contribution of RdDM causing differential methylation of common regions. Hypermethylated ecotype-specific regions contributed by RNA-directed DNA methylation pathway largely depend on the presence of TEs and copy-gain structural variations. These ecotype-specific regions are frequently associated with HOT regions, providing evolutionary insights into the epigenome dynamics within a species.

MET1-Dependent DNA Methylation Represses Light Signaling and Influences Plant Regeneration in Arabidopsis

Plant somatic cells can be reprogrammed into a pluripotent cell mass, called callus, which can be subsequently used for de novo shoot regeneration through a two-step in vitro tissue culture method. MET1-dependent CG methylation has been implicated in plant regeneration in Arabidopsis, because the met1-3 mutant exhibits increased shoot regeneration compared with the wild-type. To understand the role of MET1 in de novo shoot regeneration, we compared the genome-wide DNA methylomes and transcriptomes of wild-type and met1-3 callus and leaf. The CG methylation patterns were largely unchanged during leaf-to-callus transition, suggesting that the altered regeneration phenotype of met1-3 was caused by the constitutively hypomethylated genes, independent of the tissue type. In particular, MET1-dependent CG methylation was observed at the blue light receptor genes, CRYPTOCHROME 1 (CRY1) and CRY2, which reduced their expression. Coexpression network analysis revealed that the CRY1 gene was closely linked to cytokinin signaling genes. Consistently, functional enrichment analysis of differentially expressed genes in met1-3 showed that gene ontology terms related to light and hormone signaling were overrepresented. Overall, our findings indicate that MET1-dependent repression of light and cytokinin signaling influences plant regeneration capacity and shoot identity establishment.