RNA-Directed DNA Methylation Links Viral Disease and Plant Architecture in Rice

Cytosine methylation is an epigenetic modification of DNA that can affect gene expression and silence transposable element(TE)activities.In plants,cytosine methylation occurs in three distinct sequence contexts:CG,CHG,and CHH(where H=C,A,or T)(Law and Jacobsen,2010).The RNA-directed DNA methylation(RdDM)pathway catalyzes all types of de novo cytosine methylation,especially within small TEs or at the edges of large TEs.In brief,two plant-specific RNA polymerases,Pol IV and Pol V,independently synthesize relatively short single-stranded RNAs at RdDM target regions(Law and Jacobsen,2010).Pol IV transcripts are converted to double-stranded RNA by RNA-DEPENDENT RNA POLYMERASE 2.These double-stranded RNAs are processed into 24-nt small interfering RNAs(siRNAs)by DICER-LIKE 3.ARGONAUTE 4 binds these siRNAs and is targeted to RdDM target loci,with Pol V transcripts as scaffolds.Finally,DOMAINS REARRANGED METHYLTRANSFERASE 2 is recruited to RdDM target regions through an indirect interaction with ARGONAUTE 4 and catalyzes cytosine methylation.