Forms a complex with IDN2 and FDM2/IDNL2 that is required for RNA-directed DNA methylation (RdDM) and that functions at a downstream step of the RdDM pathway (PubMed:22302148, PubMed:22757778, PubMed:22570638, PubMed:22592791). Required for de novo DNA methylation and 24 nucleotide small interfering RNA (siRNA) accumulation (PubMed:22570638). Binds unmethylated but not methylated DNAs through its coiled-coil domain (PubMed:22757778). May bind double-stranded RNAs (dsRNAs) with 5'-overhangs through its XS domain (PubMed:22302148, PubMed:22757778). However, according to (PubMed:22570638), FMD1 does not bind dsRNAs. Homodimer (PubMed:22757778). Interacts with IDN2 (PubMed:22757778, PubMed:22592791) and AGO4 (PubMed:22302148). Forms a complex with IDN2 and FMD2/INDL2 (PubMed:22570638, PubMed:22592791). Highly expressed in flowers and at lower levels in roots, leaves and stems. No visible phenotype under normal growth conditions. The double mutants idnl1-1 and idnl2-1 show a late-flowering phenotype and reduced level of DNA methylation (PubMed:22592791). The double mutants fdm1-1 and fdm2-1 show reduced level of DNA methylation and repeat-associated small interfering RNAs (ra-siRNAs) (PubMed:22302148).