Figure 4.

Silencing by Exceeding a Certain Copy Number of a Particular Transgene under the Control of the Strong CaMV 35S Promoter in a Transformant.

RNA gel blot analyses of 8-week-old plants harboring different copy numbers of the SPT (A), GFP (B), or GUS genes (C). Transgenic lines homozygous for a particular T-DNA locus or progeny plants derived from crosses among homozygous transgenic lines or with wild-type plants were analyzed. For each line, numbers of transgene copies present in the genome and at the different T-DNA loci are given. Forward slashes separate copies of a particular locus that are present on the homologous chromosomes, and dashes divide copies of different loci. Single plants were analyzed for GFP reporter gene expression, whereas leaves of several plants were pooled for the analysis of SPT and GUS transcript levels. The resulting blots were hybridized with probes corresponding to the different reporter genes and with a probe derived from Arabidopsis actin genes 2 and 8 (An et al., 1996).