Figure 1.

Pathways for the Biogenesis of PhasiRNAs Modeled on Arabidopsis.

(A) As the first step in secondary siRNA biogenesis, mRNA targets are cleaved by a miRNA. In the one-hit model, exemplified in Arabidopsis by TAS1, TAS2, and TAS4, a 22-nucleotide miRNA targets a single site. In the two-hit model, there are two target sites for a 21-nucleotide miRNA, exemplified in Arabidopsis by TAS3 transcripts cleaved by an AGO7-loaded miR390. Activity of the trigger miRNA recruits RDR6 and SGS3, resulting in production of a second strand of the target mRNA.

(B) The dsRNA is successively processed by DCL4 and other components to generate 21-nucleotide tasiRNAs; the direction of processing depends on the miRNA trigger mechanism. The secondary siRNAs are loaded onto an AGO protein and go on to function against other mRNAs.