Post-transcriptional gene silencing (PTGS) is graft-transmissible and involves abundant 21 nt siRNAs and low abundance 22 nt siRNAs produced by DCL4 and DCL2, respectively. We developed a reporter line expressing double-stranded RNA (dsRNAs) specifically in the root tip, which then required RNA-dependent RNA polymerase 6 (RDR6) for PTGS to spread from the root tip to throughout the plant. This reporter line was used in a genetic screen to identify mutants impaired in systemic PTGS. As expected, the screen identified new alleles of rdr6 and other genes known to be required for RDR6-dependent PTGS. Surprisingly, however, we also recovered dcl2 but not dcl4 mutants. Thus, even though DCL4 outcompetes DCL2 for siRNA biogenesis in wild-type plants, DCL2 is crucial for systemic RDR6-dependent PTGS. Other genes required for systemic PTGS will also be discussed. Finally, the potential of topical application of double-stranded RNA for inducing systemic RNA silencing in plants will be discussed.