The miRNA pathway limits AGO1 availability during siRNA-mediated PTGS defense against exogenous RNA

In plants, most microRNAs (miRNAs) and several endogenous small interfering RNAs (siRNAs) bind to ARGONAUTE1 (AGO1) to regulate the expression of endogenous genes through post-transcriptional gene silencing (PTGS). AGO1 also participates in a siRNA-mediated PTGS defense response that thwarts exogenous RNA deriving from viruses and transgenes. Here, we reveal that plants supporting transgene PTGS exhibit increased levels of AGO1 protein. Moreover, increasing AGO1 levels either by mutating miRNA pathway components or, more specifically, by impairing miR168-directed regulation of AGO1 mRNA leads to increased PTGS efficiency, indicating that the miRNA pathway dampens the efficiency of PTGS, likely by limiting the availability of AGO1. We propose that during the transgene PTGS initiation phase, transgene siRNAs and endogenous siRNAs and miRNA compete to bind to AGO1, leading to a transient reduction in AGO1-miR168 complexes and a decline in AGO1 mRNA cleavage. The concomitant increase in AGO1 protein levels would facilitate the formation of AGO1-transgene siRNA complexes and the entry into the PTGS amplification phase. We suggest that the miRNA pathway imposes an important limitation on PTGS efficiency, which could help protect endogenous mRNAs from being routinely targeted by PTGS.