Evidence for intrastrand complementation in cowpea mosaic virus infection

In supplementation tests both middle component (M)-RNA and bottom component (B)-RNA of cowpea mosaic virus mutant N142 were shown to carry mutations affecting local symptoms in bean and cowpea. The M- and B-RNA mutations were separately incorporated into the two reciprocal hybrids of N142 and wild-type Sb, designated as M142BSb and MSbV142. Mixtures of the two hybrids induced normal local symptoms in bean and cowpea as a result of reassortment of components. This was also the case with a mixture of MSbV142 and mutant N123. The latter mutant was earlier shown to contain a M-RNA mutation for defective symptom expression in bean and cowpea. Although both M142BSb and N123 carried M-RNA mutations, mixtures of these isolates induced normal local symptoms. Subculturing showed no wild-type virus to be present in the inoculated leaves. Thus, intrastrand complementation was assumed to result in restoration of normal symptom production. We conclude that the results of genetic mixing experiments with multicomponent plant viruses must be interpreted cautiously. Subculturing experiments should be performed to determine whether restoration of wild-type symptoms results from exchange of nonmutated components or from intrastrand complementation.