Differential requirements for Tombusvirus coat protein and P19 in plants following leaf versus root inoculation

Traditional virus inoculation of plants involves mechanical rubbing of leaves, whereas in nature viruses like Tomato bushy stunt virus (TBSV) are often infected via the roots. A method was adapted to compare leaf versus root inoculation of Nicotiana benthamiana and tomato with transcripts of wild-type TBSV (wtTBSV), a capsid (Tcp) replacement construct expressing GFP (T-GFP), or mutants not expressing the silencing suppressor P19 (TBSVΔp19). In leaves, T-GFP remained restricted to the cells immediately adjacent to the site of inoculation, unless Tcp was expressed in trans from a Potato virus X vector; while T-GFP inoculation of roots gave green fluorescence in upper tissues in the absence of Tcp. Conversely, leaf inoculation with wtTBSV or TBSVΔp19 transcripts initiated systemic infections, while upon root inoculation this only occurred with wtTBSV, not with TBSVΔp19. Evidently the contribution of Tcp or P19 in establishing systemic infections depends on the point-of-entry of TBSV in the plants.