Abstract
Plant RNA-dependent RNA Polymerase 1 (RDR1) is an important element of the RNA silencing pathway in the plant defense against viruses. RDR1 expression can be elicited by viral infection and salicylic acid (SA), but the mechanisms of signaling during this process remains undefined. The involvement of hydrogen peroxide (H2O2) and nitric oxide (NO) in RDR1 induction in the compatible interactions between Tobacco mosaic tobamovirus (TMV) and Nicotiana tabacum, Nicotiana benthamiana, and Arabidopsis thaliana was examined. TMV inoculation onto the lower leaves of N. tabacum induced the rapid accumulation of H2O2 and NO followed by the increased accumulation of RDR1 transcripts in the non-inoculated upper leaves. Pretreatment with exogenous H2O2 and NO on upper leaf led to increased RDR1 expression and systemic TMV resistance. Conversely, dimethylthiourea (an H2O2 scavenger) and 2-(4-carboxyphenyl)- 4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (an NO scavenger) partly blocked TMV- and SA-induced RDR1 expression and increased TMV susceptibility, whereas pretreatment with exogenous H2O2 and NO failed to diminish TMV infection in N. benthamiana plants with naturally occurring RDR1 loss-of-function. Furthermore, in N. tabacum and A. thaliana, TMV-induced H2O2 accumulation was NO-dependent, whereas NO generation was not affected by H2O2. These results suggest that, in response to TMV infection, H2O2 acts downstream of NO to mediate induction of RDR1, which plays a critical role in strengthening RNA silencing to restrict systemic viral infection.
Highlights
Most eukaryotes possess an RNA silencing system as a gene regulation and host defense mechanism
We found that RNA-dependent RNA Polymerase 1 (RDR1) expression was elevated by H2O2 and nitric oxide (NO) donors, whereas TMV- and salicylic acid (SA)-induced RDR1 transcript levels were compromised by H2O2 and NO scavengers in all three plant species
Employing Arabidopsis mutants that were defective in H2O2 or NO synthesis and transgenic N. benthamiana plants transformed with SAinducible MtRDR1 from M. truncatula [6], we showed that H2O2 may function downstream of NO and may mediate the induction of RDR1 in response to TMV challenge, thereby limiting the systemic infection and accumulation of the virus
Summary
Most eukaryotes possess an RNA silencing system as a gene regulation and host defense mechanism. Employing Arabidopsis mutants that were defective in H2O2 or NO synthesis and transgenic N. benthamiana plants transformed with SAinducible MtRDR1 from M. truncatula [6], we showed that H2O2 may function downstream of NO and may mediate the induction of RDR1 in response to TMV challenge, thereby limiting the systemic infection and accumulation of the virus. These results provide initial insights into the signaling mechanisms underpinning virus- and SA-induced RDR1 activation in host plants. Dpi compared to untreated plants that received TMV inoculation alone
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