Tombusvirus P19-Mediated Suppression of Virus-Induced Gene Silencing Is Controlled by Genetic and Dosage Features That Influence Pathogenicity

Abstract

The p19 protein (P19) of Tomato bushy stunt virus (TBSV) is a pathogenicity determinant with host-dependent effects on virus spread and symptom induction. In addition, results in this study confirm that Potato virus X-mediated delivery of P19 suppresses posttranscriptional gene silencing (PTGS). To study the relevance of this activity for TBSV biology, we evaluated whether TBSV activates virus-induced gene silencing (VIGS) and if this process is suppressed by P19. TBSV vectors with the green fluorescent protein (GFP) gene, either active or inactive for P19 expression, were inoculated onto GFP-transgenic Nicotiana benthamiana plants. In the absence of P19 expression, VIGS was activated, as evidenced by the disappearance of GFP mRNA and green fluorescence. Coexpression of GFP and P19 from the TBSV vector suppressed VIGS, except in the newly emerging leaves. The suppressor activity required a central P19 region that is also known to be essential for host-dependent virus spread and symptom induction. Defective interfering RNAs (DIs) that contained the 3' end of the GFP gene induced silencing very effectively. The concomitant DI-instigated reduction in P19 accumulation failed to suppress this process, analogous to the known P19 dosage effects for other biological activities. In conclusion, (i) TBSV and its DIs are very effective inducers of VIGS, (ii) P19 is a strong suppressor of PTGS, (iii) P19 is a moderate suppressor of VIGS, and (iv) the suppressor activity is influenced by genetic and dosage features that are also important for P19-associated pathogenesis.

Suppressors of RNA silencing encoded by plant viruses and their role in viral infections

“…It was initially recognized as a suppressor of RNA silencing based on its ability to reactivate expression of a silenced GFP transgene in the systemic leaves of plants infected with either TBSV or PVX carrying a p19 insert [29]. Subsequently, several groups have independently demonstrated the potent silencing suppressor activity of p19 from a number of different tombusviruses using the agro-infiltration assay [30][31][32].…”

RNAi-associated ssRNA-specific ribonucleases in Tombusvirus P19 mutant-infected plants and evidence for a discrete siRNA-containing effector complex

“…One of the best characterized suppressors is the Tombusvirusencoded p19 protein (P19); it interferes with RNA silencing in plants and RNAi in other model systems (2,3,8). In the context of Tombusvirus infections, P19 prevents RNAi-mediated viral RNA degradation, which correlates with a dramatic exacerbation of the disease phenotype compared with infections with virus mutants not expressing P19 (9)(10)(11)(12)(13)(14)(15)(16).…”

Modulation of host plant immunity by Tobamovirus proteins

“…The action of VSRs is based on different strategies: inhibiting DCL activity, binding to the viral dsRNAs and protecting them from the subsequent DCL processing, sequestering and/or degrading vsiRNAs, inactivating RISCs and also inhibiting the amplification and/or spread of the silencing signal (Mallory et al, 2002;Qiu et al, 2002;Vargason et al, 2003;Soitamo et al, 2011;Varallyay et al, 2014). In particular, the TMV replicase suppressor of silencing activity is mediated by the protection of viral transcripts from enzymes of the RNA silencing pathway rather than defeating them (Ding et al, 2004;Kurihara et al, 2007).…”