Cucumber Mosaic Virus Suppresses PVY~NCP Gene Silencing and Breaks Transgenic Resistance to PVY~N in Tobacco

The RNA-mediated virus resistant transgenic tobacco lines, in which the resistant mechanism was similar to post-transcriptional gene silencing (PTGS),were attained in our previous study.The genetics and resistance analyses of T1 and T2 progeny transgenic tabacco lines were also studied. Here, using T3 progeny transgenic tobacco in which the expression of untranslatable PVYN CP gene is silenced, we demonstrated that cucumber mosaic virus (CMV) was able to suppress PVYN CP gene silencing and the associated potato virus Y(PVY) resistance in tobacco. Northern blot analysis indicated that CMV infection led to reversal of PVYN CF gene silencing in the renascent upper leaves of the transgenic lines, as revealed by the elevation of transgene mRNA accumulation , but transgene mRNA levels in inoculated and down non-inoculated leaves were unaffected, with very low steady-state levels. ELISA assay at 35 days post inoculation showed that the PVYN was not detected in the inoculated leaf and the down non-inoculated leaves of the transgenic lines after inoculated with PVYN + CMV, while the renascent upper leaves of the transgenic lines became susceptible to infection by PVYN(OD405 values showed no difference from that of non-transgenic plants inoculated with PVYN + CMV). In this study, we demonstrated that the RNA-mediated virus resistance can be overcome via infection by a heterologous virus that suppresses gene silencing,making the plant susceptible to the virus that it was engineered to resist.