Abstract
The resistance of transgenic papaya breeding lines to Papaya ringspot virus (PRSV) was examined. Resistance was conferred by non-translatable transgenes derived from the coat protein (CP) gene of a PRSV isolate (H1K) from Florida. To render the CP gene non-translatable, either a stop-codon (D6 lines) or frame-shift (X17-2 lines) mutation had been introduced into the CP gene. Non-transgenic and transgenic papaya lines (R[sub3] generation) were mechanically inoculated with three isolates (H1A, H1C, and H1K) of PRSV representing the genetic diversity of the virus in Florida. The mean severity of symptoms evaluated weekly for 8 weeks post-inoculation was consistently lower in the transgenic lines regardless of the PRSV isolate, and transgenic resistance to the different virus isolates did not differ noticeably. Ten or more plants each of 12 transgenic papaya lines and 23 non-transgenic accessions, including named varieties and selections, were planted in a field in May 2003 and evaluated for the incidence and severity of PRSV following natural infections. Within 8 months, all of the non-transgenic papaya plants became infected by PRSV and exhibited moderate to high levels of disease severity. In contrast, only a few plants of four of the 12 transgenic lines developed mild symptoms of PRSV. Thus, although not immune to PRSV infection, especially when mechanically inoculated, transgenic lines exhibited a high level of resistance to natural infection in the field.
