Deciphering the cell-specific role of Arabidopsis Argonaute 3 [smallRNA-Seq]

ARGONAUTE (AGO) proteins are the major effectors of RNA silencing. In Arabidopsis, loading of 21-22-nt long small RNAs into AGOs results in post-transcriptional gene silencing (PTGS) by mRNA cleavage and/or translational repression. On the other hand, loading of 24-nt sRNAs results in transcriptional gene silencing (TGS) by RNA directed DNA methylation (RdDM). The Arabidopsis genome encodes 10 AGOs, which are known, for most, to belong to either pathway. Here we characterized the cell specific role of Arabidopsis AGO3. It is specifically expressed in tissue close to vascular termination and strongly accumulates in chalazal seed integuments. AGO3 encodes a functional AGO able to bind sRNAs with a preference for 24nt in length with a 5’ nucleotide bias for Adenosine. Down-regulation of AGO3 affects gene expression in siliques and its expression is strongly induced in the plant vasculature upon proteasome inhibition. Our results suggest that AGO3 might act to regulate gene expression by a novel RNA silencing pathway involving 24nt sRNA-directed PTGS. small RNAs co-Immunoprecipitating with FLAG AGO3 protein from siliques and, after a SyrginolinA treatment, from plantlets were sequenced using Illumina platform.