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In plants, RNA silencing can be induced by highly transcribed sense transgenes (S-PTGS) [1, 2] or by transgene loci producing double-stranded RNA (dsRNA) due to the presence of inverted repeats (IR-PTGS) [3, 4, 5, 6]. Both phenomena correlate with accumulation of 21-25 nt sense and anti-sense RNA homologous to the silent gene [7] and with methylation of the coding sequence [3, 4, 6, 8]. We have challenged IR-PTGS with four viruses known to inhibit S-PTGS: CMV, TuMV, TVCV, and TCV ([9, 10, 11] this work) and in sgs2, sgs3, and ago1 mutants impaired in S-PTGS [8, 11, 12, 13, 14]. Surprisingly, whereas the four viruses inhibit IR-PTGS, IR-PTGS and methylation of a GUS trangene and IR-PTGS of three endogeneous genes occur in the sgs2, sgs3, and ago1 mutations. Based on these results, we propose a branched pathway for RNA silencing in plants. RNA silencing would occur via the action of dsRNA …