Deciphering the synergistic and redundant roles of CG and non-CG DNA methylation in plant development and transposable element silencing.

DNA methylation plays key roles in transposable element (TE) silencing and gene expression regulation. DNA methylation occurs at CG, CHG and CHH sequence contexts in plants. However, the synergistic and redundant roles of CG and non-CG methylation are poorly understood. By introducing CRISPR/Cas9-induced met1 mutation into the ddcc (drm1 drm2 cmt2 cmt3) mutant, we attempted to knock out all five DNA methyltransferases in Arabidopsis and then investigate the synergistic and redundant roles of CG and non-CG DNA methylation. We found that the homozygous ddcc met1 quintuple mutants are embryonically lethal, although met1 and ddcc mutants only display some developmental abnormalities. Unexpectedly, the ddcc met1 quintuple mutations only reduce transmission through the male gametophytes. The ddcc met1 mutants show apparent size divergence, which is not associated with difference in DNA methylation patterns, but associated with the difference in the levels of DNA damage. Finally, we show that a group of TEs are specifically activated in the ddcc met1 mutants. This work reveals that CG and non-CG DNA methylation synergistically and redundantly regulate plant reproductive development, vegetative development and TE silencing in Arabidopsis. Our findings provide insights into the roles of DNA methylation in plant development.