Figure 6.

Variability of Transgene Expression between Different Leaves and among Isogenic Plants.

(A) Difference in the onset of silencing among plants harboring four copies of the GUS gene under the control of the CaMV 35S promoter. Colorimetric GUS activity assays were performed for transgenic plants that were either hemizygous or homozygous for two different GUS 1x T-DNA loci. Two leaf discs per plant were analyzed, and the results for a particular plant at the age of 5 and 9 weeks are shown underneath each other. The intensity of the blue staining reflected the GUS expression level.

(B) Systemic spread of GFP gene silencing in a 6-week-old plant homozygous for three different GFP 1x T-DNA loci. Fluorescence was detected using different filter sets. Red corresponds to chlorophyll fluorescence, whereas the green signal is specific for GFP fluorescence. The leaf marked by an arrow did not show any GFP fluorescence.