Effector of RNA-directed DNA methylation (RdDM) triggered by small interfering RNAs (siRNAs, 24-nt RNAs). Functions as an adapter protein that binds scaffold transcripts generated by polymerase V and recruits AGO4 and AGO4-bound siRNAs to form an RdDM effector complex (PubMed:19410546, PubMed:19343051). Promotes the expression of 24-nt RNAs (PubMed:19343051). Required for the initial establishment of DNA methylation (PubMed:21150311). Together with AGO4, required for transcriptional gene silencing (TGS) by DNA methylation and repressive histone modifications (H3K9me2) of several chromatin loci (PubMed:21738482). Interacts with AGO4 via its C-terminal region and with RNA transcripts (PubMed:19410546, PubMed:19343051). Binds chromatin at loci subject to transcriptional silencing downstream of RNA Polymerase V, but independently from the presence of 24-nt siRNA (PubMed:21738482). Localized at punctate nuclear foci. Colocalizes with AGO4 and polymerase V in the nucleoplasm. Decrease in the accumulation of several 24-nt RNAs (PubMed:19343051). Reduced DNA methylation and released silencing of RNA-directed DNA methylation (RdDM) target loci promoters (e.g. LTI78/RD29A, YKT61/AtGP1 and AtMU1) without abolishing the siRNA triggers (PubMed:19410546, PubMed:19343051, PubMed:21738482). Suppression of gene silencing mediated by ROS1 disruption in the double mutants ros1 rdm3-1, ros1 rdm3-2 and ros1 rdm3-3 (PubMed:19410546). Loss in non-CG methylation at DRM2-dependent sites (PubMed:21150311). Reduced H3K9me2 at IGN5 and IGN26 loci (PubMed:21738482).