Post-transcriptional gene silencing (PTGS) of involves processing of double-stranded RNA (dsRNA) into abundant 21 nt siRNAs and low abundance 22 nt siRNAs by DCL4 and DCL2, respectively. It has long been considered that DCL2 plays a subordinate and redundant role to DCL4 in siRNA biogenesis in plants. However, we have identified dcl2 but not dcl4 mutants in a genetic screen for mutants defective in systemic PTGS, indicating that DCL2 is crucial for efficient transmission of PTGS throughout the plant. Moreover, grafting experiments showed that DCL2 is required in both the source rootstock and recipient shoot tissue for efficient systemic PTGS. Furthermore, dcl4 roots produced more 22 nt siRNAs than wild type, and showed enhanced systemic PTGS. Our data indicate that DCL2-dependent 22 nt siRNAs enhance systemic PTGS by facilitating the recruitment of RDR6 to produce dsRNA from target RNA. This dsRNA is predominantly processed into 21 nt siRNAs by DCL4 in both source and recipient tissues. While both 21 and 22 nt siRNAs act as mobile silencing signals for systemic PTGS, 22 nt siRNAs are more effective in inducing PTGS in recipient tissue. Our genetic screen for defects in systemic PTGS has uncovered a unique and crucial role for DCL2 in systemic PTGS in plants, and defines a primary evolutionary role for DCL2 in systemic PTGS.