a ∣ DICER-LIKE 2 (DCL2)-triggered production of secondary small interfering RNA (siRNA) is crucial for systemic RNA-mediated gene silencing. An inverted repeat transgene encoding a portion of GFP (IR-GF) expressed in the root tip leads to the silencing of GFP in the shoot, which is accompanied by the production of ’P’-specific secondary siRNAs from GFP mRNA through amplification of DCL2-dependent, 22-nucleotide (nt) primary ‘GF’-specific siRNAs. In a dcl2 mutant, GFP silencing does not occur and P-specific siRNAs are not made, although DCL4 still produces 21-nt siRNAs from the GF transgene. b ∣ siRNA amplification in systemic RNA-mediated gene silencing. In grafting experiments, IR-GF expressed in the root triggers GFP silencing in the shoot. Part of the RNA-directed DNA methylation (RdDM) pathway, comprising RNA polymerase IV (Pol IV), RNA-DEPENDENT RNA POLYMERASE 2 (RDR2), DCL3 and ARGONAUTE 4 (AGO4) is required in the recipient tissue for such systemic gene silencing. Dashed lines represent siRNA movement in the plant.