Cucumber mosaic virus mutants with altered physical properties and defective in aphid vector transmission

Two mutant strains of cucumber mosaic virus (CMV) were investigated with respect to virion stability and molecular determinants of aphid vector transmission. The mutant 2A1-MT-60x, derived from the mechanically passaged wild type 2A1-AT, is poorly transmissible by the aphid Aphis gossypii and not transmissible by the aphid Myzus persicae, whereas the wild type virus is transmissible by both aphid species. The mutant phenotype was shown to be conferred by a single encoded amino acid change of alanine to threonine at position 162 of the coat protein (CP). Modifying the mutant CP gene to encode the wild type sequence (alanine) at position 162 restored aphid transmission. To test for a correspondence between changes in the physical stability of virions and defects in aphid transmission, a urea disruption assay was developed. Virions of aphid-transmissible strains 2A1-AT and CMV-Fny were stable with treatments of up to between 3 and 4 M urea. In this assay mutant viruses 2A1-MT-60x and CMV-M were less stable, as they were completely disrupted at urea concentrations of 2 and 1 M urea, respectively. The mutant 2A1-MT-60x also accumulated at a reduced level in infected squash relative to the wild type virus. These studies suggest that a primary factor in the loss of aphid transmissibility of some strains of CMV is a reduction in virion stability.