POSITION EFFECTS AND EPIGENETIC SILENCING OF PLANT TRANSGENES

Nuclear processes that silence plant transgenes are being revealed by analyses of natural triggers of epigenetic modifications, particularly cytosine methylation, and by comparisons of the genomic environments of differentially expressed transgene loci. It is increasingly apparen t that plant genomes can sense and respond to the presence of foreign DNA in certain sequence contexts and at multiple dispersed sites. Dete rmining the basis of this sensitivity and how nuclear defense systems are activated poses major challenges for the future.