Amplification and expression of the β‐glucuronidase gene in maize plants by vectors based on maize streak virus

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Maize streak virus (MSV) is a geminivirus infecting monocotyledonous plants. Its genome consists of one molecule of circular, single‐stranded DNA of 2.7 kb. The viral DNA can be efficiently introduced into maize plants by agroinfection which results in systemic infection. To explore the potential of MSV as a replicative gene vector, a reporter gene coding for β‐glucuronidase (GUS) was inserted into the non‐coding region of the viral genome. The resulting construct (MSV—GUS) of about 5.9 kb was still able to replicate in cells of maize plants although it was unable to induce viral symptoms. This replication led to a five to 10‐fold increase in the mean number of GUS‐positive spots per plant as compared with infections with the GUS gene without the MSV replicon. MSV—D—GUS, which differed from MSV—GUS by the deletion of genes V1 and V2 encoding a putative movement protein and the coat protein, respectively, also replicated and produced even more GUS‐positive spots. In both MSV—GUS‐ and MSV—D—GUS‐infected plants, the GUS‐positive spots were located mainly on the veins of leaves whose primodia had already developed at the time of inoculation and never on the leaves developing later. Thus, neither viral construct was able to move systemically, most probably because the DNAs were too large to be packaged.