The RNA-binding protein RBP45D of Arabidopis plays a role in epigenetic control of flowering time and DCL3-independent RNA-directed DNA methylation

RNA-directed DNA methylation (RdDM) helps to defend plants against invasive nucleic acids. In the canonical form of RdDM, 24-nt small interfering RNAs (siRNAs) are produced by DICER-LIKE 3 (DCL3). Here, we describe the Arabidopsis thaliana prors1 (LUC) transgenic system, in which transcriptional gene silencing (TGS) is independent of DLC3. A forward genetics screen performed with this system identified both known components of RdDM, and the RNA-binding protein RBP45D. RBP45D promotes DNA methylation, and its loss delays flowering, especially at high temperature, presumably mediated by elevated FLC levels. RBP45D is localized to the nucleus, where it is associated with snRNAs and snoRNAs. RBP45D maintains siRNA production originating from the LUC transgene, but does not alter mRNA levels or affect processing of transcripts of known RdDM genes. We suggest that RBPD45 facilitates DCL3-independent siRNA production by stabilising either the precursor RNA or the - as yet unidentified - slicer protein.