CONSTRUCTION AND ANALYSIS OF INFECTIOUS TRANSCRIPTS FROM A RESISTANCE-BREAKING STRAIN OF TOBACCO VEIN MOTTLING POTYVIRUS

The Burley tobacco (Nicotiana tabacum) cultivar TN 86 is ''resistant'' to most strains of tobacco vein mottling potyvirus (TVMV), the virus being restricted to epidermal cells of inoculated leaves. One strain, designated TVMV-S, overcomes this resistance and infects cv TN 86 syst emically. To begin our investigation of the molecular basis for the re sistance-breaking phenomenon, we have completed the cloning and sequen cing of the TVMV-S RNA genome. The complete cDNA clone, under the cont rol of a T7 RNA polymerase promoter. was used to produce infectious tr anscripts which were tested for their ability to reproduce the charact eristics of TVMV-S RNA on three types of tobacco (N. tabacum cv TN 86, N. tabacum cv KY 14, and N. benthamiana). Timing of symptom appearanc e, symptom type, and titer of virus were identical to those of plants inoculated with TVMV-S RNA. As a step toward mapping the responsible g enetic region(s) that contribute(s) to resistance-breaking by TVMV-S, the nucleotide and deduced amino acid sequences were compared to those of wild-type TVMV, a strain that does not overcome cv TN 86 resistanc e. Variant TVMV-S transcripts containing changes within the VPg cistro n exhibited an altered pattern of infectivity on cv TN 86.