TRANSGENE INACTIVATION IN PETUNIA-HYBRIDA IS INFLUENCED BY THE PROPERTIES OF THE FOREIGN GENE

Petunia mutant RL01 was transformed with maize A1 and gerbera gdfr cDN As, which both encode dihydroflavonol-4-reductase (DFR) activity. The same Agrobacterium vector and the same version of the CaMV 35S promote r were used in both experiments. Transformation with the cDNAs resulte d in production of pelargonidin pigments in the transformants. However , the A1 and gdfr transformants showed deafly different phenotypes. Th e flowers of the primary A1 transformants were pale and showed variabi lity in pigmentation during their growth, while the flowers of the gdf r transformants showed intense and highly stable coloration. The color difference in the primary transformants was reflected in the expressi on levels of the transgenes as well as in the levels of anthocyanin pi gment. As previously reported by others, the instability in pigmentati on in the Al transformants was more often detected in clones with mult iple copies of the transgene and was associated with methylation of th e 35S promoter and of the transgene cDNA itself. In the gdfr transform ants, the most intense pigmentation was observed in plants with multip le transgenes in their genome. Only rarely was partial methylation of the 35S promoter detected, while the gdfr cDNA always remained in an u nmethylated state. We conclude that the properties of the transgene it self strongly influence the inactivation process. The dicotyledonous g dfr cDNA with a lower GC content and fewer possible methylation sites is more 'compatible' the genomic organization of petunia and this prev ents it being recognized as a foreign gene and hence silenced by methy lation.