Foxtail mosaic virus: a new viral vector for protein expression in cereals

Rapid and cost-effective virus-derived transient expression systems for plants are invaluable in elucidating gene function. These are particularly useful in the case of plant species for which transformation-based methods are either not yet developed, or are too time- and labor-demanding, such as wheat and maize. The Virus-mediated overexpression (VOX) vectors based on Barley stripe mosaic virus (BSMV) or Wheat streak mosaic virus (WSMV) previously described for these species are incapable of expressing free recombinant proteins >150-250 amino-acids (aa), not suited for high throughput screens, and have other limitations. In this study, we report the development of a new VOX vector based on a monopartite single-stranded positive sense RNA virus, Foxtail mosaic virus (FoMV, genus Potexvirus). The gene of interest is inserted downstream of a duplicated sub-genomic promoter of the viral coat protein gene and the corresponding protein is expressed in its free form. This new vector, PV101, allowed expression of a 239 aa-long green fluorescent protein (GFP) in both virus inoculated and upper uninoculated (systemic) leaves of wheat and maize, and directed systemic expression of a larger ca. 600 aa protein GUSPlus in maize. Moreover, we demonstrated that PV101 can be used for in planta expression and functional analysis of apoplastic pathogen effector proteins such as host-specific toxin ToxA of Parastagonospora nodorum. Therefore, this new VOX vector opens new possibilities for functional genomics studies in two of the most important cereal crops.