Viruses making plants greener

Abstract

The expression levels of transgenes introduced into plants can vary greatly between lines. In some cases, the levels of transgene transcripts that accumulate are below the limit of detection for northern analysis despite the use of strong constitutive promoters. Furthermore, if the transgene contains sequence homology to an endogenous gene, both transgene and endogenous gene transcripts are sometimes greatly reduced. This reduction in both transgene and endogenous gene transcripts is sometimes due to a post-transcriptional reduction of steady-state transcript levels. Termed post-transcriptional gene silencing (PTGS) this mechanism has recently been proposed as a means of detecting and combating viral infections in plants. Support for this idea has come from the finding that viral proteins can suppress PTGS thereby increasing the likelihood of viral infection. To investigate the role of viral proteins in suppressing PTGS, Brigneti et al.1 utilized transgenic lines of Nicotiana benthamiana that had been transformed with a green fluorescent protein cassette. These GFP expressing plants were then inoculated with the same GFP cassette to induce PTGS. When silencing was complete (no green fluorescence), plants were infected with potato virus Y (PVY). After two weeks leaves showed symptoms of PVY infection. Importantly, the regions of the leaf that mottled and curled coincided with GFP fluorescence and increased GFP transcript levels. Thus, the PVY virus was able to overcome the RNA silencing mechanism as monitored by GFP expression. To further define the components of viral-induced suppression of PTGS, chimeric viruses were constructed using a potato virus X (PVX) vector. Because PVX does not suppress PTGS, the effects of putative PTGS-suppressing proteins of PVY or cucumber mosaic virus (CMV) could be tested directly. When plants were infected with either the HCPro protein of PVY or the 2b protein of CMV in a PVX cassette, plants showed severe infection symptoms and green fluorescence. Furthermore, these effects were not observed when frame-shift mutations or premature stop codons were introduced into the HCPro or 2b proteins, respectively, indicating the PTGS silencing was not mediated by the transcripts encoding HCPro or 2b protein. As the authors suggest, although both proteins interfered with PTGS, they are unlikely to affect the same components of the PTGS pathway. HCPro- infected plants suppressed PTGS in old and young leaves, while 2b-infected plants only showed increased GFP fluorescence in young leaves. Thus, HCPro may affect the maintenance of the PTGS pathway whereas the 2b protein may interfere with the entry of a gene silencing signal into older regions of the plant. These findings strongly suggest that PTGS has evolved as a means to control the infection and spreading of viruses within the plant and opens up new doors to the engineering viral resistance in crop plants.