trans-acting siRNAs (ta-siRNAs) are endogenous RNAs that direct the cleavage of complementary mRNA targets . TAS gene transcripts are cleaved by miRNAs; the cleavage products are protected against degradation by SGS3, copied into dsRNA by RDR6, and diced into ta-siRNAs by DCL4 . We describe hypomorphic rdr6 and sgs3 Arabidopsis mutants, which do not exhibit the leaf developmental defects observed in null mutants and which, like null alleles, are impaired in sense-transgene-induced posttranscriptional gene silencing and virus resistance. Null rdr6 and sgs3 mutants lack TAS1, TAS2, and TAS3 ta-siRNAs and overaccumulate ARF3/ETTIN and ARF4 mRNAs, which are TAS3 ta-siRNA targets. A hypomorphic rdr6 mutant accumulates wild-type TAS3 ta-siRNA levels but not TAS1 and TAS2 ta-siRNAs, suggesting that TAS3 is required for proper leaf development. Consistently, tas3 but not tas1 or tas2 mutants exhibits leaf morphology defects, and ago7/zip and drb4 mutants, which exhibit leaf morphology defects, lack TAS3 but not TAS1 and TAS2 ta-siRNAs in leaves. These results indicate that the dsRNA binding protein DRB4 is required for proper ta-siRNA production, presumably by interacting with DCL4, an interaction analogous to that of HYL1 with DCL1 during miRNA production , and that TAS3 ta-siRNAs are required for proper leaf development through the action of AGO7/ZIPPY.