The Role of Hydrogen Peroxide and Nitric Oxide in the Induction of Plant-Encoded RNA-Dependent RNA Polymerase 1 in the Basal Defense against Tobacco Mosaic Virus

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 (H(2)O(2)) 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 H(2)O(2) and NO followed by the increased accumulation of RDR1 transcripts in the non-inoculated upper leaves. Pretreatment with exogenous H(2)O(2) and NO on upper leaf led to increased RDR1 expression and systemic TMV resistance. Conversely, dimethylthiourea (an H(2)O(2) 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 H(2)O(2) 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 H(2)O(2) accumulation was NO-dependent, whereas NO generation was not affected by H(2)O(2). These results suggest that, in response to TMV infection, H(2)O(2) acts downstream of NO to mediate induction of RDR1, which plays a critical role in strengthening RNA silencing to restrict systemic viral infection.