Interplay and Transition Between Small RNA-directed Posttranscriptional and Transcriptional Gene Silencing in Plants

RNA silencing refers to small RNA (sRNA)-directed, sequence-specific regulatory mechanisms operating in diverse eukaryotic organisms. Plants possess several genetically distinct sRNA-directed regulatory pathways that are known to operate either posttranscriptionally or transcriptionally. Over the years, the sRNA-directed posttranscriptional gene silencing (PTGS) and sRNA-directed DNA methylation (RdDM), which can lead to epigenetic transcriptional gene silencing (TGS), were considered distinct pathways with little or no interactions. Recent studies have uncovered an expression-dependent pathway termed RDR6-RdDM which involves the nuclear RNA polymerase II (Pol II), RNA-dependent RNA polymerase 6 (RDR6), and Pol V, but not Pol IV and RDR2, key components of the canonical Pol IV-, RDR2-, and DCL3-dependent RdDM (Pol IV-RdDM). The RDR6-RdDM pathway provides a mechanistic link between sRNA-directed PTGS and TGS, revealing previously unappreciated dynamic features of sRNA-directed regulation in plants. This 21- and 22-nucleotide (nt) sRNA-directed RdDM may represent a general transition step during de novo establishment of TGS through the canonical Pol IV-RdDM in plants.