Assessment of the effectiveness of a nuclear-launched TMV-based replicon as a tool for foreign gene expression in plants in comparison to direct gene expression from a nuclear promoter

An environmentally safe Tobacco Mosaic Virus (TMV)-based expression replicon was constructed that lacks movement protein (MP) and coat protein (CP), and which expresses the green fluorescent protein (GFP) gene from a full CP subgenomic promoter. The TMV replicon, whose cDNA was positioned between an enhanced Cauliflower Mosaic Virus 35S promoter (CaMV) and a self-cleaving hammerhead ribozyme with a downstream nopaline synthase gene polyadenylation signal [nos-poly(A)], was assessed for its effectiveness to accumulate GFP upon agroinfiltration into plant leaves compared to a control construct in which GFP was directly expressed from the enhanced CaMV 35S promoter. It was determined that individually expressing cells produced ca. 9-fold more GFP from the TMV-based replicon than from the enhanced 35S promoter. In contrast, GFP measurements from total leaf extracts determined that leaves infiltrated with the TMV-based replicon produced ca. 7-fold less GFP than the control construct. These apparently contradictory results can be explained by the low infectivity of the TMV-based replicon as it was found that the number of foci expressing GFP produced in leaves agroinfiltrated with the TMV-based replicon was ca. 66-fold lower than produced by the control.