Investigation of the potential of Maize streak virus to act as an infectious gene vector in maize plants

There has been a great deal of interest in the possibility that geminiviruses might be used as infectious gene vectors for expression of foreign proteins in plants. However, generic mastreviruses such as Maize streak virus (MSV) have no sequences which are dispensable for systemic infection of plants, and there is a strict limitation on the size of viral DNA which can be moved systemically. We attempted to complement the movement functions deleted from a wild-type-sized, replication-proficient gene replacement vector, by co-infecting plants with it and either wild type MSV, or a replication-deficient but putatively movement-proficient viral construct. While ssDNA formation by the gene replacement vector could be complemented in trans by co-transfected wild type virus, true systemic movement of either the vector, or of co-complementing constructs, did not occur. However, recombination between the two complementing viral constructs frequently occurred to generate wild-type virus genomes. The results therefore suggest that formation of ssDNA and size of the viral replicon are not the sole determinants of whether the MSV movement proteins can mobilise viral sequences and move them systemically in plants.