This process appears to be part of the natural defense against viral dsRNAs. Small dsRNAs may serve to target nuclear copies of the gene for methylation, resulting in a feedback mechanism for gene silencing. Again, this systemic component of gene silencing is likely part of the natural defense against viruses. They reasoned that in these studies, selection for the reporter gene is accomplished by selection for experssion of a selectable marker. Therefore, results using T-DNA transformants would be biased towards genes that are strongly expressed already .
Biolistic transformation tends to give more transformant lines with multible copies .
A broader distribution of copy numbers makes it easier to evaluate cosuppression at different copy numbers. The authors conclude that the SAR effect is not merely a question of making sure that the GUS genes were packaged into a favorable chromatin configuration. If that were true, it should work regardless of how many copies there are.