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The Suppressor of Transgene RNA Silencing Encoded by Cucumber mosaic virus Interferes with Salicylic Acid-Mediated Virus Resistance

June 2001 , Volume 14 , Number  6
Pages  715 - 724

Liang-Hui Ji and Shou-Wei Ding

Molecular Virology Laboratory, Institute of Molecular Agrobiology, National University of Singapore, 1 Research Link, 117604 Singapore

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Accepted 16 February 2001.

The Cucumber mosaic virus (CMV)-encoded 2b protein (Cmv2b) is a nuclear protein that suppresses transgene RNA silencing in Nicotiana benthamiana. Cmv2b is an important virulence determinant but nonessential for systemic spread in N. glutinosa, in contrast to its indispensable role for systemic infections in cucumber. Here, we report that Cmv2b became essential for systemic infections in older N. glutinosa plants or in young seedlings pre-treated 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 correlation is demonstrated between Cmv2b expression and a reduced SA-dependent induction of the alternative oxidase gene, a component of the recently proposed 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 degrades 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 RNA-silencing antiviral defense that is targeted by Cmv2b.

Additional keywords: disease resistance ; pathogenesis-related proteins ; posttranscriptional gene silencing ; systemic acquired resistance.

© 2001 The American Phytopathological Society