Pathway conversion enables a double-lock mechanism to maintain DNA methylation and genome stability

Significance In plants, several DNA methylation pathways help maintain DNA methylation patterns to ensure that transposons remain in a silenced state and cell-specific DNA methylation is preserved after cell division. Here, we demonstrate that loss of function of the chromatin remodeler DDM1 induces a pathway conversion from CMT2- to RdDM-dependency to ensure the maintenance of CHH methylation. In plants defective in both DDM1 and RdDM, there is strong reactivation of TEs and a burst of TE transposition. Thus, our work not only identifies a variant of the RdDM pathway, but also shows that DNA methylation maintenance at a given locus is an adaptable process, revealing the existence of a double-insurance mechanism for the regulation of DNA methylation to guarantee genome integrity.