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The Role of the Cucumber mosaic virus 2b Protein in Viral Movement and Symptom Induction

June 2009 , Volume 22 , Number  6
Pages  642 - 654

Mathew Lewsey,1 Monique Surette,1 Fiona C. Robertson,1 Heiko Ziebell,1,2 Sun Hee Choi,3 Ki Hyun Ryu,3 Tomas Canto,4,5 Peter Palukaitis,4 Tina Payne,2 John A. Walsh,2 and John P. Carr1

1Department of Plant Sciences, University of Cambridge, Cambridge, CB2 3EA, U.K.; 2Warwick HRI, University of Warwick, Wellesbourne, Warwick, CV35 9EF, U.K.; 3Plant Virus GenBank, Division of Environmental and Life Sciences, Seoul Women's University, Seoul 139-774, Korea; 4Scottish Crop Research Institute, Invergowrie, Dundee DD2 5DA, U.K.; 5Centro de Investigaciones Biologicas, 28040 Madrid, Spain

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Accepted 1 February 2009.

The Cucumber mosaic virus (CMV) 2b protein is a counter-defense factor and symptom determinant. Conserved domains in the 2b protein sequence were mutated in the 2b gene of strain Fny-CMV. The effects of these mutations were assessed by infection of Nicotiana tabacum, N. benthamiana, and Arabidopsis thaliana (ecotype Col-0) with mutant viruses and by expression of mutant 2b transgenes in A. thaliana. We confirmed that two nuclear localization signals were required for symptom induction and found that the N-terminal domain was essential for symptom induction. The C-terminal domain and two serine residues within a putative phosphorylation domain modulated symptom severity. Further infection studies were conducted using Fny-CMVΔ2b, a mutant that cannot express the 2b protein and that induces no symptoms in N. tabacum, N. benthamiana, or A. thaliana ecotype Col-0. Surprisingly, in plants of A. thaliana ecotype C24, Fny-CMVΔ2b induced severe symptoms similar to those induced by the wild-type virus. However, C24 plants infected with the mutant virus recovered from disease while those infected with the wild-type virus did not. Expression of 2b transgenes from either Fny-CMV or from LS-CMV (a mild strain) in Col-0 plants enhanced systemic movement of Fny-CMVΔ2b and permitted symptom induction by Fny-CMVΔ2b. Taken together, the results indicate that the 2b protein itself is an important symptom determinant in certain hosts. However, they also suggest that the protein may somehow synergize symptom induction by other CMV-encoded factors.

© 2009 The American Phytopathological Society