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.

The putative RNA‐binding protein SUPPRESSOR OF GENE SILENCING 3 (SGS3) protects RNA from degradation before transformation into dsRNA by the RNA‐dependent RNA polymerase RDR6 during plant post‐transcriptional gene silencing and trans‐acting small interfering (siRNA) pathways. In this study, we show that SGS3 acts as a homodimer, and that the point mutation sgs3‐3 impairs post‐transcriptional gene silencing in a dominant‐negative manner through the formation of SGS3/sgs3‐3 heterodimers. Unlike complete‐loss‐of‐function sgs3 mutants, which are impaired in the accumulation of both micro RNA‐directed TAS cleavage products and mature trans‐acting siRNAs, the sgs3‐3 mutant overaccumulates TAS cleavage products and exhibits slightly reduced trans‐acting siRNA accumulation. Together, these results suggest that sgs3‐3 is a neomorphic allele that shows increased RNA protective activity, resulting in decreased RNA processing by downstream post‐transcriptional gene silencing and trans‐acting siRNA pathway components.