title: Salicylic acid treatment and expression of an RNA-dependent RNA polymerase 1 transgene inhibit lethal symptoms and meristem invasion during tobacco mosaic virus infection in Nicotiana benthamiana.
abstract: BACKGROUND
  Host RNA-dependent RNA polymerases (RDRs) 1 and 6 contribute to antiviral RNA silencing in plants. RDR6 is constitutively expressed and was previously shown to limit invasion of Nicotiana benthamiana meristem tissue by potato virus X and thereby inhibit disease development. RDR1 is inducible by salicylic acid (SA) and several other phytohormones. But although it contributes to basal resistance to tobacco mosaic virus (TMV) it is dispensable for SA-induced resistance in inoculated leaves. The laboratory accession of N. benthamiana is a natural rdr1 mutant and highly susceptible to TMV. However, TMV-induced symptoms are ameliorated in transgenic plants expressing Medicago truncatula RDR1.
  
  
  RESULTS
  In MtRDR1-transgenic N. benthamiana plants the spread of TMV expressing the green fluorescent protein (TMV.GFP) into upper, non-inoculated, leaves was not inhibited. However, in these plants exclusion of TMV.GFP from the apical meristem and adjacent stem tissue was greater than in control plants and this exclusion effect was enhanced by SA. TMV normally kills N. benthamiana plants but although MtRDR1-transgenic plants initially displayed virus-induced necrosis they subsequently recovered. Recovery from disease was markedly enhanced by SA treatment in MtRDR1-transgenic plants whereas in control plants SA delayed but did not prevent systemic necrosis and death. Following SA treatment of MtRDR1-transgenic plants, extractable RDR enzyme activity was increased and Western blot analysis of RDR extracts revealed a band cross-reacting with an antibody raised against MtRDR1. Expression of MtRDR1 in the transgenic N. benthamiana plants was driven by a constitutive 35S promoter derived from cauliflower mosaic virus, confirmed to be non-responsive to SA. This suggests that the effects of SA on MtRDR1 are exerted at a post-transcriptional level.
  
  
  CONCLUSIONS
  MtRDR1 inhibits severe symptom development by limiting spread of virus into the growing tips of infected plants. Thus, RDR1 may act in a similar fashion to RDR6. MtRDR1 and SA acted additively to further promote recovery from disease symptoms in MtRDR1-transgenic plants. Thus it is possible that SA promotes MtRDR1 activity and/or stability through post-transcriptional effects.
authors.affiliation: Department of Plant Sciences, University of Cambridge, Downing Street, Cambridge, CB2 3EA, UK. wing-sham.lee@rothamsted.ac.uk.|Rothamsted Research, Harpenden, Hertfordshire, AL5 2JQ, UK. wing-sham.lee@rothamsted.ac.uk.
authors.forename: Wing-Sham
authors.initials: WS
authors.lastname: Lee
authors.affiliation: Department of Plant Sciences, University of Cambridge, Downing Street, Cambridge, CB2 3EA, UK. sffuplant@yahoo.com.tw.|Department of Biology, National Changhua University of Education, 1 Jin-De Road, Changhua City, 500, Taiwan. sffuplant@yahoo.com.tw.
authors.forename: Shih-Feng
authors.initials: SF
authors.lastname: Fu
authors.affiliation: Department of Plant Sciences, University of Cambridge, Downing Street, Cambridge, CB2 3EA, UK. zhengli.ef@gmail.com.
authors.forename: Zheng
authors.initials: Z
authors.lastname: Li
authors.affiliation: Department of Plant Sciences, University of Cambridge, Downing Street, Cambridge, CB2 3EA, UK. amm1013@cam.ac.uk.
authors.forename: Alex M
authors.initials: AM
authors.lastname: Murphy
authors.affiliation: Department of Plant Sciences, University of Cambridge, Downing Street, Cambridge, CB2 3EA, UK. lizziedobson354@gmail.com.
authors.forename: Elizabeth A
authors.initials: EA
authors.lastname: Dobson
authors.affiliation: Department of Plant Sciences, University of Cambridge, Downing Street, Cambridge, CB2 3EA, UK. l.garland.2@googlemail.com.
authors.forename: Laura
authors.initials: L
authors.lastname: Garland
authors.affiliation: Plant Biology Division, Samuel Roberts Noble Foundation, Inc, 2510 Sam Noble Parkway, Ardmore, OK, 73401, USA. srini.chaluvadi@gmail.com.
authors.forename: Srinivasa Rao
authors.initials: SR
authors.lastname: Chaluvadi
authors.affiliation: Department of Plant Sciences, University of Cambridge, Downing Street, Cambridge, CB2 3EA, UK. mlewsey@salk.edu.|Centre for AgriBioscience, Department of Animal, Plant and Soil Science, School of Life Science, La Trobe University, Bundoora, Australia. mlewsey@salk.edu.
authors.forename: Mathew G
authors.initials: MG
authors.lastname: Lewsey
authors.affiliation: Plant Biology Division, Samuel Roberts Noble Foundation, Inc, 2510 Sam Noble Parkway, Ardmore, OK, 73401, USA. rsnelson@noble.org.
authors.forename: Richard S
authors.initials: RS
authors.lastname: Nelson
authors.affiliation: Department of Plant Sciences, University of Cambridge, Downing Street, Cambridge, CB2 3EA, UK. jpc1005@cam.ac.uk.
authors.forename: John P
authors.initials: JP
authors.lastname: Carr
mesh_terms: D004790:Enzyme Induction; D015870:Gene Expression; D046913:Medicago truncatula / Q000201:enzymology* / Q000235:genetics; D018519:Meristem / Q000821:virology; D010935:Plant Diseases / Q000821:virology*; D030821:Plants, Genetically Modified; D012324:RNA-Dependent RNA Polymerase / Q000096:biosynthesis* / Q000378:metabolism; D020156:Salicylic Acid / Q000494:pharmacology*; D014026:Nicotiana / Q000235:genetics / Q000821:virology*; D014027:Tobacco Mosaic Virus / Q000187:drug effects / Q000502:physiology*
chemical_list: D012324:RNA-Dependent RNA Polymerase; D020156:Salicylic Acid