Identification of viruses capable of breaking transgenic resistance of papaya conferred by the coat protein gene of Papaya ringspot virus

To investigate the divergence of viruses infecting papaya, IS virus isolates were collected from different areas of Taiwan and analyzed by host reactions, serology, and heteroduplex mobility assay. When tested by enzyme-linked immunosorbent assay (ELISA) using the antiserum to Papaya ringspot virus (PRSV), all isolates were strongly positive except the DL1 isolate collected from Dalee, which was negative. When analyzed by reverse transcription-polymerase chain reaction (RT-PCR) using a specific primer pair designed from the coat protein (CP) gene of PRSV, a 0.82 kbp fragment was amplified from all isolates except the DLI isolate. The divergence of the 0.82 fragments amplified from the 17 isolates was further analyzed by heteroduplex mobility assay by reannealing them with that amplified from the strain YK, a prevalent mosaic type of PRSV in Taiwan. The heteroduplexes of 5-19 and TD2 migrated significantly slower than the homoduplexes, indicating that they are PRSV strains diversified from YK. Among the 18 isolates, four (5-19, CY4, TD2, and DL1) were able to breakdown the transgenic resistance of papaya lines carrying the CP gene of PRSV and caused symptoms on the non-transformed papaya plants different from those induced by the strain YK. The DL1 isolate was further identified as Papaya leaf distortion mosaic virus for it reacted strongly with the antiserum to the Okinawa isolate of PLDMV in ELISA. The potential threats of the transgenic resistance-breaking PRSV strains and PLDMV to the application of PRSV CP transgenic papaya lines in Taiwan are discussed.