Stability of recombinant plant viruses containing genes of unrelated plant viruses

The stability of hybrid plant viruses that might arise by recombination in transgenic plants was examined using hybrid viruses derived from the viral expression vectors potato virus X (PVX) and tobacco rattle virus (TIRV). The potato virus Y (PVY) NIb and HCPro open reading frames (ORFs) were introduced into PVX to generate PVX-NIb and PVX-HCPro, while the PVY NIb ORF was introduced into a vector derived from TRV RNA2 to generate TRV-NIb. All three viruses were unstable and most of the progeny viruses had lost the inserted sequences between 2 and 4 weeks post-inoculation. There was some variation in the rate of loss of part or all of the inserted sequence and the number of plants containing the deleted viruses, depending on the sequence, the host (Nicotiana tabacum vs Nicotiana benthamiana) or the vector, although none of these factors was associated consistently with the preferential loss of the inserted sequences. PVX-NIb was unable to accumulate in NIb-transgenic tobacco resistant to infection by PVY and also showed loss of the NIb insert from PVX-NIb in some NIb-transgenic tobacco plants susceptible to infection by PVY. These data indicate that such hybrid viruses, formed in resistant transgenic plants from a transgene and an unrelated virus, would be at a selective disadvantage, first by being targeted by the resistance mechanism and second by not being competitive with the parental virus.