Silencing of antibody genes in plants with single-copy transgene inserts as a result of gene dosage effects

The stability of Fab antibody fragment expression during plant development was studied using two homozygous Arabidopsis thaliana lines that contain si ngle copies of the transgenes. These lines exhibited expression characteris tics that are typical for homology-based post-transcriptional gene silencin g. Their developmental silencing profiles differed markedly, presumably due to the influence of the genomic context on the T-DNAs. In both lines, a cl ear gene dosage effect could be observed: in contrast to the homozygous lin es, derived hemizygous plants accumulated high levels of Fab fragments thro ughout development. Interestingly, silencing also occurred in double-hemizy gous plants, which resulted from a cross between the two homozygous lines a nd had two copies of each T-DNA at non-allelic positions in their genome. I n all cases, down-regulation of the Fab levels was strictly correlated with methylation of cytosine residues in the transcribed regions of the transge nes. Remarkably, this methylation was also found in regions in which the tr ansgenes were non-homologous regions. Finally, the time point of down-regul ation depended on the culture conditions and differed for leaves and roots of the same transgenic plant.