Plant cell-directed control of virion sense gene expression in wheat dwarfvirus

We have used particle bombardment (biolistics) to deliver replication-compe tent wheat dwarf virus (WDV)-based constructs, carrying reporter gene seque nces fused to the virion sense promoter (P-v) or the CaMV 35S promoter, to suspension culture cells and immature zygotic embryos of wheat. While the r eplication of WDV double-stranded DNA forms (replicons) was equivalent betw een wheat suspension culture cells and embryos, GUS reporter gene activity was 20-40 times higher in the embryo cultures. Maximum expression of WDV re plicons occurred in the embryonic axis tissue of wheat embryos but their ex pression in suspension cells was compromised, compared with transiently mai ntained input plasmid DNA containing the same sequences. From these studies , we propose that WDV replicons are subject to a host cell-controlled compe tency for virion sense transcription. The term competency is used to distin guish between the phenomenon described here and control of gene expression by specific transcription factors. Control of competency is independent of P-v, the replacement 35S promoter and of the complementary sense control of virion sense expression involving specific sequences in P-v. We propose th at factors controlling the competency for replicon expression may be presen t in cells which, as well as maintaining high rates of DNA synthesis, are t otipotent. Cell type control of active chromatin, methylation of specific s equences in WDV minichromosomes and/or interaction of virus-encoded protein s with specific host factors are considered as possible mechanisms.