Biology presented on August 26, 2008. Title: Small RNA Pathways in Plants

miRNA-guided cleavage initiates entry of primary transcripts into the trans-acting siRNA (tasiRNA) biogenesis pathway involving RNA-DEPENDENT RNA POLYMERASE6 (RDR6), DICER-LIKE 4 (DCL4), and SUPPRESSOR OF GENE SILENCING3 (SGS3). Arabidopsis thaliana TAS1 and TAS2 families yield tasiRNA that form through miR173-guided initiation cleavage on the 5 ’ side of the siRNA-generating regions. The TAS3 family yields tasiRNA that form through miR390-guided initiation cleavage on the 3’ side of the siRNA-generating regions. miR390 also functions in a non-cleavage mode on the 5 ’ side of the siRNA-generating regions. TAS1 and TAS2 tasiRNA target several transcripts encoding pentatricopeptide repeat (PPR) proteins and proteins of unknown function. TAS3 tasiRNA regulate mRNAs encoding several AUXIN RESPONSE FACTORs. The TAS1c and TAS3a loci were modified to produce synthetic (syn) tasiRNA to target an endogenous transcript encoding PHYTOENE DESATURASE (PDS). TAS1c-and TAS3a-based syn-tasiRNA were used to test the unique requirement for miR173 and miR390 in routing TAS1c and TAS3a transcripts, respectively, through th