miR173 specificity in syn-tasiRNA formation. (A) The miR173 target site in 35S:TAS1cPDS-2 was substituted for sites recognized by miR169, miR171, miR390, or miR167. The percentages of transgenic plants (n > 30 primary transformants) with none/weak or modest/strong photobleaching are shown. For plants in the none/weak category, only those containing 35S:TAS1cPDS-167 rarely displayed photobleaching. The proportion of cloned 5′ RACE products corresponding to cleavage at a site is shown above the arrows. Black arrows indicate the canonical miRNA-guided cleavage position. Dashed horizontal arrows indicate the 5′ and 3′ probes used in blot assays in Fig. 3. (B) Mean relative level +/− SEM of PDS mRNA after normalization to ACT2 mRNA, as determined by quantitative RT-PCR (Col-0 vector = 1.0). (C) Mean relative level +/− SEM of syn-tasiRNA, as determined by blot assays (Col-0 35S:TAS1cPDS-2 = 1.0). (D) EtBr-stained 5′ RACE products. 5′ RACE products corresponding to cleavage at the canonical miRNA-guided cleavage site migrate at 135 bp. ACT2 RT-PCR products are shown as a control.