DEVELOPMENTALLY-REGULATED TRANSGENE SILENCING IN TORENIA

We demonstrated developmental transgene silencing and resetting in tor enia (Torenia fournieri Lind.). The experiments used transformants car rying the neomycin phosphotransferase II gene, the beta-glucuronidase (GUS) gene, and the hygromycin phosphotransferase gene. GUS activity w as measured at various developmental stages in GUS-homozygous and GUS- hemizygous plants derived from a primary transformant, R-0-1. Rapid si lencing of the GUS gene occurred 12 to 19 days after germination in th e homozygous plants, while it occurred at a later developmental stage, 54 days after germination, in the hemizygous plants. The silenced sta te seemed to be stable through vegetative propagation in both the homo zygotes and the hemizygotes. Northern blot analysis showed that mRNA r eduction correlated with a reduction in GUS activity. A single GUS gen e locus was sufficient to trigger the silencing, and initial levels of GUS activity correlated with the timing of silencing. These results s uggested that the GUS gene silencing observed in torenia is not depend ent on allelic interaction of the GUS genes, but is correlated with a dose effect. This type of silencing can be explained by the RNA thresh old model.