DNA cytosine methylationis an important epigenetic process that is correlated with transgene silencing, transposon
suppression, and gene imprinting. In plants, small interfering RNAs (siRNAs) can trigger DNA methylation at loci
containing their homolog sequences through a process called RNA-directed DNA methylation (RdDM). In canonical
RdDM, 24 nucleotide (nt) siRNAs (ra-siRNAs) will be loaded into their effector protein called ARGONAUTE 4 (AGO4)
and subsequently targeted to RdDM loci through base-pairing with the non-coding transcripts produced by DNA-directed
RNA Polymerase V. Then, the AGO4-ra-siRNA will recruit the DNA methyltransferase to catalyze de novo DNA methylation.
Recent studies also identified non-canonical RdDM pathways that involve microRNAs or 21 nt siRNAs. These
RdDM pathways are biologically important since they control responses biotic and abiotic stresses, maintain genome stability
and regulate development. Here, we summarize recent progresses of mechanisms governing canonical and noncanonical
Keywords: Plants, siRNA, ARGONAUTE, DNA methylation.
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