title: Extreme resistance as a host counter-counter defense against viral suppression of RNA silencing.
abstract: RNA silencing mediated by small RNAs (sRNAs) is a conserved regulatory process with key antiviral and antimicrobial roles in eukaryotes. A widespread counter-defensive strategy of viruses against RNA silencing is to deploy viral suppressors of RNA silencing (VSRs), epitomized by the P19 protein of tombusviruses, which sequesters sRNAs and compromises their downstream action. Here, we provide evidence that specific Nicotiana species are able to sense and, in turn, antagonize the effects of P19 by activating a highly potent immune response that protects tissues against Tomato bushy stunt virus infection. This immunity is salicylate- and ethylene-dependent, and occurs without microscopic cell death, providing an example of "extreme resistance" (ER). We show that the capacity of P19 to bind sRNA, which is mandatory for its VSR function, is also necessary to induce ER, and that effects downstream of P19-sRNA complex formation are the likely determinants of the induced resistance. Accordingly, VSRs unrelated to P19 that also bind sRNA compromise the onset of P19-elicited defense, but do not alter a resistance phenotype conferred by a viral protein without VSR activity. These results show that plants have evolved specific responses against the damages incurred by VSRs to the cellular silencing machinery, a likely necessary step in the never-ending molecular arms race opposing pathogens to their hosts.
authors.affiliation: Centre SEVE, Département de Biologie, Faculté des Sciences, Université de Sherbrooke, Sherbrooke, Quebec, Canada.
authors.forename: Raphaël
authors.initials: R
authors.lastname: Sansregret
authors.forename: Vanessa
authors.initials: V
authors.lastname: Dufour
authors.forename: Mathieu
authors.initials: M
authors.lastname: Langlois
authors.forename: Fouad
authors.initials: F
authors.lastname: Daayf
authors.forename: Patrice
authors.initials: P
authors.lastname: Dunoyer
authors.forename: Olivier
authors.initials: O
authors.lastname: Voinnet
authors.forename: Kamal
authors.initials: K
authors.lastname: Bouarab
mesh_terms: D016923:Cell Death / Q000235:genetics; D060467:Disease Resistance / Q000502:physiology*; D030821:Plants, Genetically Modified / Q000235:genetics / Q000378:metabolism / Q000821:virology; D011485:Protein Binding; D034622:RNA Interference / Q000502:physiology*; D018749:RNA, Plant / Q000235:genetics / Q000378:metabolism*; D014026:Nicotiana / Q000235:genetics / Q000378:metabolism* / Q000821:virology; D017864:Tombusvirus / Q000235:genetics / Q000378:metabolism*; D014764:Viral Proteins / Q000235:genetics / Q000378:metabolism*
chemical_list: C493885:P19 protein, tomato bushy stunt virus; D018749:RNA, Plant; D014764:Viral Proteins