The polerovirus F box protein PO targets ARGONAUTE1 to suppress RNA silencing

Plants employ post-transcriptional gene silencing (PTGS) as an antiviral defense response [1]. In this mechanism, viral-derived small RNAs are incorporated into the RNA-induced silencing complex (RISC) to guide degradation of the corresponding viral RNAs [2]. ARGONAUTE1 (AGO1) is a key component of RISC: it carries the RNA slicer activity [3]. As a counter-defense, viruses have evolved various proteins that suppress PTGS [4]. Recently, we showed that the Pole-rovirus PO protein carries an F box motif required to form an SCF-like complex, which is also essential for P0's silencing suppressor function [5]. Here, we investigate the molecular mechanism by which PO impairs PTGS. First we show that Po's expression does not affect the biogenesis of primary siRNAs in an inverted repeat-PTGS assay, but it does affect their activity. Moreover, P0's expression in transformed Arabidopsis plants leads to various developmental abnormalities reminiscent of mutants affected in miRNA pathways, which is accompanied by enhanced levels of several miRNA-target transcripts, suggesting that PO acts at the level of RISC. Interestingly, ectopic expression of PO triggered AGO1 protein decay in planta. Finally, we provide evidence that PO physically interacts with AGO1. Based on these results, we propose that PO hijacks the host SCF machinery to modulate gene silencing by destabilizing AGO1.