Argentinian potato leafroll virus P0 protein: Novel activities for a previously known suppressor

Abstract

The potato leafroll virus (PLRV) P0 protein (P0PL) is a suppressor of RNA silencing. In this study, we showed that P0 protein from an Argentinian isolate of PLRV (P0PL‐Ar) has an additional activity not described for other PLRV or P0 proteins from poleroviruses. Besides reporting that P0PL‐Ar displays both local and systemic silencing suppressor activity, we demonstrated, for the first time, that P0PL‐Ar impedes accumulation of dsRNA‐derived siRNAs. We also showed that P0PL‐Ar interacts with Solanum tuberosum SKP1 orthologue (StSKP1) and triggers destabilization of ARGONAUTE 1 (AGO1) and that these actions are mediated by the F‐box‐like domain. A mutant in the GW/WG motif within the P0PL‐Ar F‐box‐like motif lost the suppression activity, the interaction with StSKP1 and abolished AGO1 decay. Interestingly, a mutant in the L76/P77 residues within the P0PL‐Ar F‐box‐like motif, which lost the suppression activity and the interaction with StSKP1, retained the capacity to enable AGO1 decay. Thus, unlike other P0 proteins of previously characterized poleroviruses, P0PL‐Ar seems to have a dual activity, according to the findings of this study. This protein would act at both an upstream and a downstream step of the RNA silencing pathway: upstream of Dicer‐like enzyme (DCL)‐mediated primary siRNA production and downstream at the RNA‐induced silencing complex (RISC) complex level. Our results contribute to the understanding of the different ways PLRV P0 proteins function as silencing suppressors.

What we know about poleroviruses: Advances in understanding the functions of polerovirus proteins

“…The Argentinian PLRV isolate P0 protein (P0 PL‐Ar ), which has F‐box and WG domains, displays intermediate silencing suppressor activity both locally and systemically and, according to an inverted repeat assay, inhibits the production of siRNAs from dsRNA. These results indicate that, unlike other P0 proteins of previously characterized poleroviruses, P0 PL‐Ar would act at more than one step of the silencing path, upstream of dicer‐like protein (DCL)‐mediated primary siRNA production and at the RISC complex level, by decreasing AGO1 accumulation (Barrios Barón et al, 2021).…”

“…This was evidenced by western blot analyses in which the studied P0 mutants unable to interact with SKP exhibited reduced or no accumulation in N. benthamiana leaves; similar results were obtained for P0 CLRDV F-box mutants (authors' unpublished data) in contrast to some F-box mutants of the Argentinian isolates of PLRV (P0 PL-Ar ). In the case of P0 PL-Ar , studies showed that not only residues L76P77, but also the GW/WG motif, both within the F-box domain, may be involved in this difference (Barrios Barón et al, 2021).…”

“…Most of the studied P0 proteins have two domains, the F‐box domain (LPXXL/IX (10–13) P), which is located near the amino terminal of the protein, and the carboxyl terminus conserved sequence (K/R)IYGEDGX 3 FWR. Also, some species like PLRV, cereal yellow dwarf virus‐RPV (CYDV‐RPV) and CYDV‐RPS have a WG‐like motif in these proteins (Almasi et al, 2015; Barrios Barón et al, 2021; Zhuo et al, 2014).…”

“…Several species have isolates with varying levels or null suppression activity (Cascardo et al, 2015; Delfosse et al, 2014; Filardo & Sharman, 2019; Kozlowska‐Makulska et al, 2010). Most of the P0 proteins suppress silencing locally, although some, like PLRV (Barrios Barón et al, 2021), sugarcane yellow leaf virus (ScYLV; Mangwende et al, 2009), CYDV (Fusaro et al, 2012), wheat yellow dwarf virus (WYDV‐GPV; Liu et al, 2012), brassica yellows virus (BrYV; Li et al, 2019), and maize yellow dwarf virus (MYDV‐RMV2; Wang et al, 2018), act systemically as well (Table 1).…”

Argonaute 5-mediated antiviral defense and viral counter-defense in Nicotiana benthamiana

“…However, some viruses have evolved to exploit these host mechanisms to degrade proteins involved in the RNA silencing pathway. For example, P0 proteins of viruses in the genera Polerovirus and Enamovirus may target and degrade AGO1 through the autophagy pathway ( Derrien et al., 2012 ; Fusaro et al., 2012 ; Michaeli et al., 2019 ; Barrios Barón et al., 2021 ), while PVX P25 destabilizes AGO1 via the 26S proteasome pathway ( Chiu et al., 2010 ). TuMV P1/HC-Pro degrades AGO1 through autophagy ( Hu et al., 2020 ; Hong et al., 2023 ), and TuMV VPg interacts with, and induces the degradation of SGS3 through both proteasome and autophagy pathways ( Cheng and Wang, 2017 ).…”