We have used particle bombardment (biolistics) to deliver replication-competent wheat dwarf virus (WDV)-based constructs, carrying reporter gene sequences fused to the virion sense promoter (Pv) or the CaMV 35S promoter, to suspension culture cells and immature zygotic embryos of wheat. While the replication of WDV double-stranded DNA forms (replicons) was equivalent between wheat suspension culture cells and embryos, GUS reporter gene activity was 20-40 times higher in the embryo cultures. Maximum expression of WDV replicons occurred in the embryonic axis tissue of wheat embryos but their expression in suspension cells was compromised, compared with transiently maintained input plasmid DNA containing the same sequences. From these studies, we propose that WDV replicons are subject to a host cell-controlled competency for virion sense transcription. The term competency is used to distinguish between the phenomenon described here and control of gene expression by specific transcription factors. Control of competency is independent of Pv, the replacement 35S promoter and of the complementary sense control of virion sense expression involving specific sequences in Pv. We propose that factors controlling the competency for replicon expression may be present in cells which, as well as maintaining high rates of DNA synthesis, are totipotent. Cell type control of active chromatin, methylation of specific sequences in WDV minichromosomes and/or interaction of virus-encoded proteins with specific host factors are considered as possible mechanisms.
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