Dark green islands in plant virus infection are the result of posttranscriptional gene silencing

Dark green islands (DGIs) are a common symptom of plants systemically infected with a mosaic virus. DGIs are dusters of green leaf cells that are free of virus but surrounded by yellow, virus-infected tissue. We report here on two lines of evidence showing that DGIs are caused by posttranscriptional gene silencing (PTGS), First, transcripts of a transgene derived from the coat protein of Tamarillo mosaic potyvirus (TaMV) were reduced in DGIs relative to adjacent yellow tissues when the plants were infected with TaMV. Second, nontransgenic plants coinfected with TaMV and a heterologous virus vector carrying TaMV sequences showed reduced titers of the vector in DGIs compared with surrounding tissues. DGIs also were compared with recovered tissue at the top of transgenic plants because recovery has been shown previously to involve PTGS, Cytological analysis of the cells at the junction between recovered and infected tissue was undertaken. The interface between recovered and infected cells had very similar features to that surrounding DGIs, We conclude that DGIs and recovery are related phenomena, differing in their ability to amplify or transport the silencing signal.