Abstract
Upon tobacco mosaic virus (TMV) infection, mosaic symptoms develop on the systemic leaves of tobacco plants. Mosaic symptoms are characterized by intermingled yellow and dark green tissues (YTs and DGTs). In YTs, TMV accumulates to high levels, whereas DGTs contain no or little TMV and are, furthermore, immune to the same or closely related strains. It has been proposed that the resistant nature of DGTs may be related to PTGS, a homology-dependent RNA degradation system with a role to defend plants against viruses. In mosaic leaves, TMV-targeting RNA silencing operates in DGTs, at least in the border regions adjacent to virus-accumulating YTs, where silencing is suppressed by the action of the TMV-encoded replication protein. It appears that the battle between PTGS and its suppression determines the DGT-YT junctions at an early time of infection and subsequently maintains the outline of the mosaic patterns.
