The suppressor of transgene RNA silencing encoded by Cucumber mosaic virusinterferes with salicylic acid-mediated virus resistance

The Cucumber mosaic virus (CMV)-encoded 2b protein (Cmv2b) is a nuclear pro tein that suppresses transgene RNA silencing in Nicotiana benthamiana. Cmv2 b is an important virulence determinant but nonessential for systemic sprea d ill N. glutinosa, in contrast to its indispensable role for systemic infe ctions in cucumber, Here, we report that Cmv2b became essential for systemi c infections in older N. glutinosa plants or in young seedlings pretreated with salicylic acid (SA). Expression of Cmv2b from the genome of either CMV or Tobacco mosaic virus significantly reduced the inhibitory effect of SA on virus accumulation in inoculated leaves and systemic leaves. A close cor relation is demonstrated between Cmv2b expression and a reduced SA-dependen t induction of the alternative oxidase gene, a component of the recently pr oposed SA-regulated antiviral defense, These results collectively reveal a novel activity of Cmv2b in the inhibition of SA-mediated virus resistance. We used a N. tabacum line expressing a bacterial nahG transgene that degrad es SA to provide evidence for a Cmv2b-sensitive antiviral defense mechanism in tobacco in which SA acts as a positive modifier but not as an essential component. We propose that SA induces virus resistance by potentiating a R NA-silencing antiviral defense that is targeted by Cmv2b.