The Role of theCucumber mosaic virus2b Protein in Viral Movement and Symptom Induction

Abstract

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.

Visual monitoring of Cucumber mosaic virus infection in Nicotiana benthamiana following transmission by the aphid vector Myzus persicae

“…However, a deletion of the 16 C-terminal residues of the 2b protein had little effect on symptoms in Arabidopsis thaliana when compared to the wild-type, but caused an increase in symptom severity in N. benthamiana (Lewsey et al, 2009). A similar slight increase in symptom severity (leaf distortion) was observed in this study for those F-constructs with a truncated 2b (F1, F4 and F5) both in N. benthamiana and N. tabacum.…”

“…It was also engineered to contain a cloning site, this time truncating the 2b ORF, but leaving the 2a ORF intact. The 2b ORF, although essential for normal functioning of the virus, can be truncated at the C terminus and still have a phenotype close to the wild-type (Lewsey et al, 2009); complete deletion of the 2b ORF (and the C-terminal region of the 2a ORF) results in impaired infection or movement and a significant attenuation of symptoms (Ding et al, 1995;Soards et al, 2002).…”

Biological and Molecular Characterization of Cucumber mosaic virus Subgroup II Isolate Causing Severe Mosaic in Cucumber

“…At the amino acid level 1a and MP protein showed close resemblance to Japanese isolate, 2a and 2b to USA isolate, and coat protein to Australian isolate. Changes in the replicase protein and suppressor protein can affect the infectivity of the virus [26,50,54]. Reassortment is a natural mechanism which leads to genetic variation and new strain emergence in multipartite RNA viruses [6].…”

Virus-Induced Necrosis Is a Consequence of Direct Protein-Protein Interaction between a Viral RNA-Silencing Suppressor and a Host Catalase

“…The 2b protein contains two nuclear localization signals (NLSs) in its N terminus (Lucy et al, 2000;Goto et al, 2007) and has been confirmed to be actually localized in the nucleus in infected tissues; the NLSs of the 2b protein are required for viral symptom induction and RSS activity (Lucy et al, 2000;Lewsey et al, 2009). Recently, 2b was found to bind to Argonaute1 (AGO1), necessary for the plant antiviral machinery, and to AGO4, necessary for the DNA methylation process that controls heterochromatin formation and transcription (Zhang et al, 2006;González et al, 2010).…”