Expression of disease symptoms in cauliflower mosaic virus genomic hybrids

In an effort to determine if particular regions of the cauliflower mosaic virus (CaMV) genome could be associated with particular phenotypic characters, strains of CaMV differing markedly in biological properties were recombined to produce hybrids. DNA from pairs of (infectious) genomic clones was cleaved with restriction endonucleases, then mixed and ligated. Recombinants were found by screening transformants in E. coli, or by selection in vivo for infectious hybrids. Recombinants in infected turnip plants were characterized by restriction endonuclease mapping of their DNA to confirm the hybrid genotype. New hybrid strains that induced less severe disease, or conversely, more severe disease than either parent were observed. The experiments revealed that typical disease expression, consisting of leaf chlorosis and mottling, mapped to a genome segment containing open reading frame VI (ORF VI) and the full-length promoter. This basic disease symptom was found to be influenced by other regions of the genome. Insect transmissibility mapped to ORF II. The ability to develop generalized infections in solanaceous plants was tested in hybrids between CaMV CM1841 and a variant that infects Datura stramonium systemically. In this case the systemic mobilization of virus appeared to be controlled by ORF VI, suggesting that this gene may function in cell-to-cell movement of virus.