Salicylic acid‐mediated and RNA‐silencing defense mechanisms cooperate in the restriction of systemic spread of plum pox virus in tobacco

Abstract

SummaryPlum pox virus (PPV) is able to replicate in inoculated leaves of Nicotiana tabacum, but is defective in systemic movement in this host. However, PPV produces a systemic infection in transgenic tobacco expressing the silencing suppressor P1/HC-Pro from tobacco etch virus (TEV). In this work we show that PPV is able to move to upper non-inoculated leaves of tobacco plants expressing bacterial salicylate hydroxylase (NahG) that degrades salicylic acid (SA). Replication and accumulation of PPV is higher in the locally infected leaves of plants deficient in SA or expressing TEV P1/HC-Pro silencing suppressor. Accumulation of viral derived small RNAs was reduced in the NahG transgenic plants, suggesting that SA might act as an enhancer of the RNAsilencing antiviral defense in tobacco. Besides, expression of SA-mediated defense transcripts, such as those of pathogenesis-related (PR) proteins PR-1 and PR-2 or alternative oxidase-1, as well as that of the putative RNAdependent RNA polymerase NtRDR1, is induced in response to PPV infection, and the expression patterns of these defense transcripts are altered in the TEV P1/HC-Pro transgenic plants. Long-distance movement of PPV is highly enhanced in NahG · P1/HC-Pro double-transgenic plants and systemic symptoms in these plants reveal that the expression of an RNA-silencing suppressor and the lack of SA produce additive but distinct effects. Our results suggest that SA might act as an enhancer of the RNA-silencing antiviral defense in tobacco, and that silencing suppressors, such as P1/HC-Pro, also alter the SA-mediated defense. Both an RNA-silencing and an SA-mediated defense mechanism could act together to limit PPV infection.

Modulation of host plant immunity by Tobamovirus proteins

“…Exogenous SA can also inhibit cell to cell movement in epidermal cells (Murphy and Carr, 2002). Moreover, it was found that Plum pox virus (PPV; defective in systemic movement) was able to move to distal tissues in transgenic tobacco plants expressing bacterial salicylate hydroxylase (NahG) which degrades SA (Alamillo et al, 2006). Based on this evidence, it was proposed that SA plays a positive role in antiviral resistance by interfering with different steps of the viral cycle (Chivasa et al, 1997;Murphy and Carr, 2002;Alamillo et al, 2006).…”

“…Orthologous genes of NtRDR1 have also been characterized in other species, such as Nicotiana glutinosa (NgRDR1), Nicotiana benthamiana (NbRDR1m), A. thaliana (AtRDR1), Medicago truncatula (MtRDR1) and rice (OsRDR1), all of them being induced by both viral infection and SA treatment (Yu et al, 2003;Yang et al, 2004;Quilis et al, 2008;Liu et al, 2009). Several findings indicate that RDR1 could be involved in the SA-induced resistance against viruses (Alamillo et al, 2006;Lee et al, 2016). The impact of SA on the modulation of RDR1 action was demonstrated in tobacco NahG transgenic plants infected with PPV.…”

“…The impact of SA on the modulation of RDR1 action was demonstrated in tobacco NahG transgenic plants infected with PPV. These plants accumulated lower levels of vsiRNAs than wild-type tobacco, and this reduced accumulation is associated with the pattern of expression of RDR1 (Alamillo et al, 2006). The involvement of RDR1 in antiviral SA-mediated resistance was also explored in transgenic N. benthamiana plants constitutively expressing MtRDR1 (Lee et al, 2016).…”

“…Both the RNA-silencing and SA-dependent responses were suggested to play key roles in limiting Tomato ringspot virus spread in tobacco (Jovel et al, 2011). SA has already been proposed to enhance antiviral RNA silencing against PPV in tobacco and, accordingly, suppressors of gene silencing such as P1/HC-Pro would interfere with SA-mediated defence (Alamillo et al, 2006). In line with these findings, there are several studies demonstrating that some MPs from certain viruses behave as viral enhancers of RNA silencing (VERs) by facilitating the propagation of vsiRNAs from cell to cell (Vogler et al, 2008;Zhou et al, 2008;Lacombe et al, 2010).…”

“…Moreover, it was found that Plum pox virus (PPV; defective in systemic movement) was able to move to distal tissues in transgenic tobacco plants expressing bacterial salicylate hydroxylase (NahG) which degrades SA (Alamillo et al, 2006). Based on this evidence, it was proposed that SA plays a positive role in antiviral resistance by interfering with different steps of the viral cycle (Chivasa et al, 1997;Murphy and Carr, 2002;Alamillo et al, 2006). In addition, recent findings showed that SA treatment delays systemic movement of Potato virus X (PVX) (SCP1 strain) and ameliorates induced symptoms in tomato plants (Cueto-Ginzo et al, 2016).…”