An RNA-Dependent RNA Polymerase Prevents Meristem Invasion by Potato Virus X and Is Required for the Activity But Not the Production of a Systemic Silencing Signal

TLDR

RNA silencing in plants functions as antiviral defense and spreads systemically, yet the nature of the systemic silencing signal and the proteins involved in its production, transport, and reception remain poorly understood. We propose that RDR6 uses incoming silencing signals to generate double‑stranded RNA precursors of secondary siRNAs, which then mediate immediate RNA silencing to slow systemic viral spread. RDR6 is required for defense against potato virus X by preventing systemic spread and exclusion from the apical meristem, acting through secondary siRNAs rather than primary replication or movement, and is necessary for cells to respond to the systemic silencing signal but not for its production or transport.

Abstract

One of the functions of RNA silencing in plants is antiviral defense. A hallmark of RNA silencing is spreading of the silenced state through the plant. Little is known about the nature of the systemic silencing signal and the proteins required for its production, transport, and reception in plant tissues. Here, we show that the RNA-dependent RNA polymerase RDR6 in Nicotiana benthamiana is involved in defense against potato virus X at the level of systemic spreading and in exclusion of the virus from the apical growing point. It has no effect on primary replication and cell-to-cell movement of the virus and does not contribute significantly to the formation of virus-derived small interfering (si) RNA in a fully established potato virus X infection. In grafting experiments, the RDR6 homolog was required for the ability of a cell to respond to, but not to produce or translocate, the systemic silencing signal. Taking these findings together, we suggest a model of virus defense in which RDR6 uses incoming silencing signal to generate double-stranded RNA precursors of secondary siRNA. According to this idea, the secondary siRNAs mediate RNA silencing as an immediate response that slows down the systemic spreading of the virus into the growing point and newly emerging leaves.