Figure 6.

DCL1-Dependent 22-Nucleotide siRNAs Bypass the Loss of DCL4.

(A) In the wild type, tasiR-ARFs are generated via the two-hit TAS3 ta-siRNA pathway. Biogenesis of these small RNAs is confined to the adaxial side of leaf primordia via the localized accumulation of miR390. tasiR-ARFs target arf3 transcripts, limiting expression of these abaxial determinants to the lower side of developing leaves.

(B) In dcl4, targeting of TAS3 transcripts by miR390-AGO7 complexes proceeds on the adaxial side of leaf primordia as in the wild type. However, processing of the double-stranded TAS3 precursors by DCL2/3 leads to the accumulation of 22-nucleotide tasiR-ARF variants. These trigger a unique one-hit mechanism of secondary siRNA biogenesis at the arf3a and arf3d transcripts specifically, leading to the processing of arf3a/d transcripts into 22-nucleotide siRNAs by DCL1. As a result, expression for both ARF3 abaxial determinants remains largely unaffected. See text for further details.