Arabidopsis AGO3 predominantly recruits 24-nt small RNAs to regulate epigenetic silencing.

Argonaute (AGO) proteins recruit 21-24-nucleotide (nt) small RNAs (sRNAs) to constitute RNA-induced silencing complexes (RISCs) to regulate gene expression at transcriptional or posttranscriptional levels(1-3). Arabidopsis encodes nine functional AGO proteins. These proteins are classified into three clusters, AGO1/5/10, AGO2/3/7 and AGO4/6/9, based on their sequence similarity, functional redundancy, as well as species and features of AGO-bound sRNAs(4-7). Although most Arabidopsis AGO proteins have been studied well, AGO3-bound sRNAs and their basic function remain unknown. Here we observed that AGO3 could not complement the signature function of AGO2, the closest genetic paralog of AGO3, in host antiviral defence. We also found, surprisingly, that AGO3 predominantly bound 24-nt sRNAs with 5'-terminal adenine. The spectrum of AGO3-associated sRNAs was different from those bound to AGO2, further indicating their functional divergence. By contrast, approximately 30% of AGO3-bound 24-nt sRNAs overlapped with those bound to AGO4, and over 60% of AGO3-associated 24-nt sRNA-enriched loci were identical to those of AGO4. Moreover, the redundancy of AGO3- and AGO4-bound sRNAs is much more than that of AGO6- and AGO4-recruited sRNAs. In addition, expression of AGO3 driven by the AGO4 promoter partially complemented AGO4 function and rescued a DNA methylation defect in the ago4-1 background. Together, our results indicated that AGO3, similarly to AGO4, is a component in the epigenetic pathway.