Defective viral DNA ameliorates symptoms of geminivirus infection in transgenic plants

Nicotiana benthamiana was transformed with a single copy of a tandem repeat of subgenomic DNA B isolated from plants infected with a Kenyan isolate of the bipartite geminivirus African cassava mosaic virus. Symptoms in transformed plants were less severe than in nontransformed controls when challenged with virus or cloned DNA of Kenyan or Nigerian isolates. Symptom amelioration was associated with the mobilization and amplification of the subgenomic DNA, producing a comparable reduction in the amount of DNA specific to each genomic component. The disproportionate reduction in the levels of full-length components (DNA A, 20%; DNA B, 70%) indicates that the episomally replicating subgenomic DNA has been amplified at the expense of full-length DNA B to three times the level of the latter. Serial infection of transformants resulted in a further decrease in symptom severity and viral DNA levels. No differences were observed in the severity of symptoms or levels of viral DNA when transformants and controls were challenged with the related geminiviruses beet curly top virus and tomato golden mosaic virus, demonstrating the specific nature of the interaction. Analysis of infected tissue showed that tomato golden mosaic virus was unable to amplify the subgenomic DNA. However, since the production of subgenomic DNA is possibly a common feature of the bipartite geminiviruses, this approach might contribute to the production of plants showing increased tolerance to a number of economically important viral diseases.