An infectious clone pTEV-7DA [35] (GeneBank DQ986288), kindly provided by Prof. J.C. Carrington (Oregon State Univ.) was used as ancestor virus. This infectious clone contains a full-length cDNA of TEV and a 44 nt long poly-T tail followed by a BglII restriction site cloned into the pGEM-4 vector downstream of the SP6 promoter. 59 capped infectious RNA was obtained upon transcription of BglII-digested pTEV-7DA using SP6 mMES-SAGE mMACHINE kit (Ambion). A stock of ancestral TEV viral particles was generated as follows. Five mg of RNA transcripts were rub-inoculated into the third true leaf of four-week old Nicotiana tabacum var Xanthi plants. Afterwards, plants were maintained in the green house at 25uC and 16 h light photoperiod. Seven dpi, virions were purified as described elsewhere [36] , aliquoted and stored at 280uC.