DCL3

In plants, the RNA-directed DNA methylation (RdDM) pathway mediates transcriptional silencing of repetitive elements and transposable elements through small RNA–guided epigenetic modification. DCL3 plays a central role in this pathway by generating 24-nucleotide siRNAs from endogenous dsRNA substrates.^[3]

These dsRNA precursors are derived primarily from transcripts produced by RNA polymerase IV at heterochromatic loci. The single-stranded RNA products of Pol IV are converted into double-stranded RNA by RNA-dependent RNA polymerase 2 (RDR2). DCL3 then cleaves these dsRNA molecules into 24-nt siRNAs.^[3]

The resulting siRNAs are incorporated into the Argonaute 4 (AGO4) protein. AGO4-associated siRNAs base-pair with scaffold transcripts generated by RNA polymerase V, facilitating recruitment of domains-rearranged methylase 2 (DRM2). This process results in cytosine DNA methylation and transcriptional repression of the target locus.^[3]

DCL3 arose as a specialized paralog within the plant Dicer-like (DCL) gene family during early land plant evolution.^[1][2][4] Phylogenetic analyses indicate that DCL3 is present in bryophytes and vascular plants, consistent with the early establishment of RNA-directed DNA methylation (RdDM) pathways in terrestrial plant lineages.^[2][4] In contrast to animal Dicer proteins, which primarily generate microRNAs and siRNAs involved in post-transcriptional regulation, plant DCL3 became functionally specialized for the production of 24-nucleotide siRNAs associated with transcriptional gene silencing and heterochromatin formation.^[1]

The emergence of DCL3 coincided with the evolution of plant-specific RNA polymerases, including RNA polymerase IV and RNA polymerase V, which are central to the RdDM pathway.^[4] This coordinated expansion of silencing machinery enabled plants to establish robust epigenetic regulation of transposable elements and repetitive DNA. Comparative genomic studies suggest that the 24-nucleotide siRNA pathway is broadly conserved across land plants, reflecting an early and stable integration of DCL3-mediated silencing into plant genome defense and epigenetic regulation systems.^[1]