Expression of single copies of a strongly expressed 35S transgene can be silenced post‐transcriptionally

Abstract

Summary: The bacterial UidA gene cloned between the 35S promoter with a double enhancer and the terminator sequences of the pea rbcS 9C gene was introduced into tobacco plants. All 11 transformants carrying the transgene at a single locus showed silencing at each generation, but the timing of silencing occurred at different rates in the different transformants. Two plants showed high levels of UidA mRNA accumulation and GUS activity in young seedlings and a rapid decline of these levels during the first month of growth irrespective of the allelic state of the T‐DNA. The other plants showed high levels of UidA mRNA accumulation and GUS activity in young seedlings and a slow decline of these levels during the first 4 months of growth in homozygous plants, whereas these levels decline only after 1 year of growth in hemizygous plants. Haploids showed the same kinetics of silencing as the homozygous plants from which they derived. Silencing correlated with a strong decrease of the steady‐state level of UidA mRNA, while the transcription of the transgene in the nucleus was not affected. Since haploids and hemizygous plants derived from transformants carrying a single copy of the T‐DNA show silencing, and since all the plants express the transgene at a very high level before the triggering of silencing, the results suggest that post‐transcriptional silencing occurs through a dose effect and not through DNA‐DNA interaction.