SUMMARY

Plants expressing viral sequences may show homolo-gy-dependent virus resistance (HDR), which is charac-terized by specific degradation of transgenic RNA and viral RNAs homologous to the transgene. This resis-tance mechanism is a form of post-transcriptional gene silencing (PTGS), which can be suppressed by virus-en-coded proteins. We analysed the effect of Potato virus Y, Cucumber mosaic virus and Potato virus X infections on HDR to Cymbidium ringspot virus (CymRSV) shown by Nicotiana benthamiana transgenic line 92KA1 ex-pressing the full-length replicase gene of CymRSV. We observed breaking of resistance in transgenic plants doubly infected with PTGS suppressor viruses and CymRSV. The need of considering the effect of PTGS suppressor viruses on resistance or PTGS-dependent phenotype in transgenic crops is discussed. Key words: Post-transcriptional gene silencing, trans-genic resistance, PVY, PVX, CMV CymRSV. Post-transcriptional gene silencing (PTGS) in plants involves specific RNA degradation of invasive RNAs (for review see Meins, 2000; Waterhouse et al., 2001). Phenomena related to PTGS are ‘quelling ’ observed in fungi (Cogoni and Macino, 1997) and ‘RNA interfer-ence ’ observed in nematodes, insects and mammals (Bosher and Labouesse, 2000). In PTGS-related sys-tems no change in transgene transcription rate is ob-served, whereas a specific degradation mechanism is targeted against transgenic RNA and all RNAs contain-ing sequences homologous or complementary to the transgene. Several lines of evidence suggest links be-tween PTGS and natural plant defence mechanisms against invading RNAs such as those deriving from plant viruses and transposons (reviewed by Waterhous