Integration, expression and inheritance of two linked T-DNA marker genes in transgenic lettuce

T-DNA integration and stability were assessed in Agrobacterium-derived tran sgenic lettuce lines carrying a chimaeric CaMV 35S promoter-driven gus-intr on gene and a chimaeric nos.nptII.nos gene. T-DNA integration was predomina ntly complex in transgenic plants derived from an A. tumefaciens strain car rying the supervirulent plasmid ToK47. Truncation of the right side of the T-DNA was observed in first seed generation R-1 plants from one line. Compl ex T-DNA integration patterns did not always correlate with low transgene e xpression. Despite a high T-DNA copy number, ca. 30% of the lines analysed showed high transgene expression in the R-1 generation. High transgene expr ession was stable at least to the R-4 seed generation in selected high-expr essing lines. Transgene expression was lost in the R-2 generation in a low expressing line, while complete, heritable transgene silencing from the R-0 to R-2 generations was also observed in another line. A 50-fold variation in beta-glucuronidase (GUS) activity and a 16-fold variation in NPTII prote in content were observed between R-1 plants derived from different R-0 pare nts. Reactivation of transgene expression with 5-azacytidine in partially s ilenced lines indicated that low expression was associated with DNA methyla tion.