Multiple pathogenic factors influence aphid transmission of cauliflower mosaic virus from infected plants

Aphid transmission of cauliflower mosaic virus (CaMV) is mediated by a polypeptide (P18) encoded by the viral gene II. We have investigated the factors which influence acquisition by aphids of CaMV variants from infected plants. Aphid non-transmissible (AT^-) CaMV isolates with a full-length gene II sequence share two amino acid changes, gly to arg at position 94 and ile to val at 105, relative to wild type transmissible (AT⁺) isolates. We have mutated the gly to arg at position 94 in the AT⁺ isolate Cabb B-JI which then exhibited the AT^- phenotype as predicted. However, replacement of a DNA fragment in Cabb B-JI with one containing the gly to arg change from the AT^- isolate Campbell to produce hybrid pBJIC1 resulted in a change in symptom phenotype as well as in aphid transmissibility. pBJIC1 also showed characteristics of partial transmissibility related to the stage of infection when it was tested. The level of P18 was measured in plants and showed that recombinants based upon the Campbell (AT^-) genome accumulated P18 later than those based upon the Cabb B-JI genome (AT⁺). However, the Campbell P18 or recombinant proteins like it, were still not able to mediate transmission even when the P18 level in plants was relatively high and by employing large numbers of aphids. We conclude that aphid transmissibility of CaMV is influenced by multiple factors including P18 levels, inherent functionality of the protein, pathogenic characters of the infecting strain, and the number of aphids used to test transmissibility.