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Inserts bearing the coding sequences of NPTll and P-glucuronidase (GUS) were placed between the nuclear inclusion b (Nlb) and coat protein (CP) domains of the wheat streak mosaic virus (WSMV) polyprotein ORF. The WSMV Nlb-CP junction containing the nuclear inclusion a (Nla) protease cleavage site was duplicated, permitting excision of foreign protein domains from the viral polyprotein. Wheat, barley, oat and maize seedlings supported systemic infection of WSMV bearing NPTII. The NPTII insert was stable for at least 18-30 days post-inoculation and had little effect on WSMV CP accumulation. Histochemical assays indicated the presence of functional GUS protein in systemically infected wheat and barley plants inoculated with WSMV bearing GUS. The GUS constructs had greatly reduced virulence on both oat and maize. RT-PCR indicated that the GUS insert was subject to deletion, particularly when expressed as a GUS-Nlb protein fusion. Both reporter genes were expressed in wheat roots at levels comparable to those observed in leaves. These results clearly demonstrate the utility of WSMV as a transient gene expression vector for grass species, including two important grain crops, wheat and maize. The results further indicate that both host species and the nature of inserted sequences affect the stability and expression of foreign genes delivered by engineered virus genomes.