Foxtail mosaic virus: A viral vector for protein expression in cereals

Rapid and cost-effective virus-derived transient expression systems for plants are invaluable in elucidating gene function and are particularly useful in plant species for which transformation-based methods are unavailable or are too time- and labor-demanding, such as wheat (Triticum aestivum) and maize (Zea mays). The virus-mediated overexpression (VOX) vectors based on Barley stripe mosaic virus (BSMV) and Wheat streak mosaic virus (WSMV) previously described for these species are incapable of expressing free recombinant proteins of g150-250 amino acids (aa), are not suited for high-throughput screens, and have other limitations. In this study, we report the development of a VOX vector based on a monopartite single-stranded positive sense RNA virus, Foxtail mosaic virus (FoMV, genus Potexvirus). In this vector, PV101, the gene of interest was inserted downstream of the duplicated sub-genomic promoter of the viral coat protein gene, and the corresponding protein was expressed in its free form. The vector 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 the host-specific toxin ToxA of Parastagonospora nodorum. Therefore, this VOX vector opens possibilities for functional genomics studies in two important cereal crops.