TYMV has dual action on the plant RNA silencing defense through its VSR P69 and the host RNASE THREE LIKE1
Authors: Sehki Hayat , Yu Agnès , Elmayan Taline , Vaucheret Hervé 
Published: March 2022

Post-Transcriptional Gene Silencing (PTGS) is a defense mechanism that targets invading nucleic acids of endogenous (transposons) or exogenous (pathogens, transgenes) origins. During plant infection by viruses, virus-derived primary siRNAs target viral RNAs, resulting in both destruction of single-stranded viral RNAs (execution step) and production of secondary siRNAs (amplification step), which maximize the plant defense. As a counter-defense, viruses express proteins referred to as Viral Suppressor of RNA silencing (VSR). Some viruses express VSRs that totally inhibit PTGS, whereas other viruses express VSRs that have limited effect. Here we show that infection with the Turnip yellow mosaic virus (TYMV) is enhanced in Arabidopsis ago1, ago2 and dcl4 mutants, which are impaired in the execution of PTGS, but not in dcl2, rdr1 and rdr6 mutants, which are impaired in the amplification of PTGS. Consistently, we show that the TYMV VSR P69 localizes in siRNA-bodies, which are the site of production of secondary siRNAs, and limits PTGS amplification. Moreover, TYMV induces the production of the host enzyme RNASE THREE-LIKE 1 (RTL1) to further reduce siRNA accumulation. Together, these results indicate that the inhibition of PTGS amplification through the dual action of P69 and RTL1 let the sole siRNAs produced by the primary plant PTGS defense to limit the infection by TYMV, explaining the mild symptoms Arabidopsis-infected TYMV. This could be considered as an elegant strategy used by viruses to limit the plant PTGS defense and propagate without killing their host.

Author summary

RNA silencing is a conserved defense mechanism directed against viruses in various eukaryotic kingdoms. As a counter-defense, viruses generally express proteins referred to as viral suppressor of RNA silencing (VSR), which promote infection by inhibiting one or the other component of the RNA silencing machinery. So far, most of the work on VSRs has concentrated on those that strongly inhibit RNA silencing, causing severe infections and plant death. However, situations where VSRs only partially inhibit RNA silencing could be considered as advantageous for both partners of the infection because infected plants survive, flower and produce seeds despite virus multiplication. In this study, we show that Turnip yellow mosaic virus (TYMV) encodes a weak VSR, P69, which partially inhibits the amplification but not the execution of RNA silencing. In addition, TYMV induces the expression of the endogenous enzyme RNASE THREE-LIKE 1 (RTL1) to further reduce siRNA accumulation, and this induction is proportional to the amount of virus. This appears a smart strategy for a virus to propagate without killing its host. We propose that the limited effect of P69 on RNA silencing together with the proportional induction of host RTL1 results in a tight balance between virus propagation and plant development. In the light of these results, one could consider the Arabidopsis-TYMV interaction as an elegant model for plant-virus coevolution.