Transgenic banana plants expressing small interfering RNAs targeted against viral replication initiation gene display high-level resistance to banana bunchy top virus infection

The banana aphid-transmitted Banana bunchy top virus (BBTV) is the most destructive viral pathogen of bananas and plantains worldwide. Lack of natural sources of resistance to BBTV has necessitated the exploitation of proven transgenic technologies for obtaining BBTV-resistant banana cultivars. In this study, we have explored the concept of using intron-hairpin-RNA (ihpRNA) transcripts corresponding to viral master replication initiation protein (Rep) to generate BBTV-resistant transgenic banana plants. Two ihpRNA constructs namely ihpRNA-Rep and ihpRNA-ProRep generated using Rep full coding sequence or Rep partial coding sequence together with its 5' upstream regulatory region, respectively, and castor bean catalase intron were successfully transformed into banana embryogenic cells. ihpRNA-Rep- and ihpRNA-ProRep-derived transgenic banana plants, selected based on preliminary screening for efficient reporter gene expression, were completely resistant to BBTV infection as indicated by the absence of disease symptoms after 6 months of viruliferous aphid inoculation. The resistance to BBTV infection was also evident by the inability to detect cDNAs coding for viral coat protein, movement protein and Rep protein by RT-PCR from inoculated transgenic leaf extracts. Southern analysis of the two groups of transgenics showed that ihpRNA transgene was stably integrated into the banana genome. The detection of small interfering RNAs (siRNAs) derived from the ihpRNA transgene sequence in transformed BBTV-resistant plants positively established RNA interference as the mechanism underlying the observed resistance to BBTV. Efficient screening of optimal transformants in this vegetatively propagated non-segregating fruit crop ensured that all the transgenic plants assayed were resistant to BBTV infection.