2b ORFs encoded by subgroup IB strains of cucumber mosaic virus induce differential virulence on Nicotiana species.

Cucumber mosaic virus (CMV)-encoded 2b protein from subgroup IA or subgroup II was shown to be a determinant of virulence in many solanaceous hosts. In this study, the virulence of 2b proteins from subgroup IB strains was analysed using four intraspecies hybrid viruses, which were generated by precise replacement of the 2b open reading frame (ORF) in subgroup IA strain Fny-CMV with the 2b ORFs of four subgroup IB strains, Cb7-CMV, PGs-CMV, Rad35-CMV and Na-CMV, generating FCb7(2b)-CMV, FPGs(2b)-CMV, FRad35(2b)-CMV and FNa(2b)-CMV, respectively. FCb7(2b)-CMV was more virulent than Fny-CMV, and was similar in phenotype to its parental virus Cb7-CMV on the three Nicotiana species tested. FNa(2b)-CMV also was virulent on these host species, equivalent to Fny-CMV or Na-CMV. However, FRad35(2b)-CMV only caused mild mosaic or undetectable symptoms on all the host species tested, and was less virulent than Fny-CMV or Rad35-CMV. FPGs(2b)-CMV infected all the host species systemically, and induced either mosaic or barely visible symptoms, demonstrating that the inability of PGs-CMV to infect these three Nicotiana species was not due to its 2b protein. The diverse virulence was shown to be mediated by the 2b proteins rather than the C-terminal overlapping parts of the 2a proteins, and was associated with the level of viral progeny RNA accumulation in systemically infected leaves, but not with the rate of long-distance viral movement in host plants. Through analysis of encapsidation of viral RNAs, there was an apparent correlation between the virulence and the high level of encapsidated RNA 2 in virions of Fny-CMV, FCb7(2b)-CMV and FNa(2b)-CMV.