Fig. 4

DNA methylation and demethylation in plants. Methylation in plants can occur through two processes: DNA methylation maintenance [1] and de novo DNA methylation [2]. Methyltransferase 1 (MET1) maintains symmetric CG site methylation. Chromomethylase 2 and 3(CMT 2, CMT 3) maintains symmetrical CHG site methylation. De novo CHH methylation is performed by domain-rearranged methyltransferase 2 (DRM2) or CMT2. DRM2 induces CHH methylation via the RNA-directed DNA methylation (RdDM) pathway, relying on the presence of 24 nt small interfering RNA (siRNA). This siRNA is loaded onto ARGONAUTE proteins (AGO), primarily AGO4 and AGO6, which then interact with DRM2. Demethylation of DNA encompasses passive demethylation [3] and active demethylation [4]. Replacement of 5mC with unmethylated cytosine during passive demethylation involves the binding of nuclear factor (NF) to 5mC during DNA replication. This binding makes it difficult to maintain DNA methylation, causing loss of DNA methylation on the newly synthesized strand. Active demethylation relies on the removal of 5mC by DNA glycosylases: repressor of silencing 1 (ROS1), Demeter (DME), Demeter-like 2 and 3 (DML2, DML3). These DNA glycosylases can remove 5-mC from any sequence context