Fine-tuning expression of begomoviral movement and nuclear shuttle proteins confers cell-to-cell movement to mastreviral replicons in nicotiana benthamiana leaves

Geminiviruses are a group of small plant viruses responsible for devastating crop damage worldwide. The emergence of agricultural diseases caused by geminiviruses is attributed in part to their high rates of recombination, leading to complementary function between viral components across species and genera. We have developed a mastreviral reporter system based on bean yellow dwarf virus (BeYDV) that replicates to high levels in the plant nucleus, expressing very high levels of GFP. To investigate the potential for complementation of movement function by other geminivirus genera, the movement protein (MP) and nuclear shuttle protein (NSP) from the bipartite begomovirus Bean dwarf mosaic virus (BDMV) were produced and characterized in Nicotiana benthamiana leaves. While overexpression of MP and NSP strongly inhibited GFP expression from the mastreviral reporter and caused adverse plant symptoms, optimizing the expression levels of MP and NSP allowed functional cell-to-cell movement. Hybrid virus vectors were created that express BDMV MP and NSP from mastreviral replicons, allowing efficient cell-to-cell movement comparable to native BDMV replicons. We find that the expression levels of MP and NSP must be fine-tuned to provide sufficient MP/NSP for movement without eliciting the plant hypersensitive response or adversely impacting gene expression from viral replicons. The ability to confer cell-to-cell movement to mastrevirus replicons depended strongly on replicon size: 2.1-2.7 kb replicons were efficiently moved, while 3 kb replicons were inhibited, and 3.9 kb replicons were very strongly inhibited. Optimized expression of MP/NSP from the normally phloem-limited Abutilon mosaic virus (AbMV) allows efficient movement in non-phloem cells.