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

T-DNA integration and stability were assessed in Agrobacterium-derived transgenic lettuce lines carrying a chimaeric CaMV 35S promoter-driven gus-intron gene and a chimaeric nos.nptII.nos gene. T-DNA integration was predominantly complex in transgenic plants derived from an A. tumefaciens strain carrying the supervirulent plasmid ToK47. Truncation of the right side of the T-DNA was observed in first seed generation R₁ plants from one line. Complex T-DNA integration patterns did not always correlate with low transgene expression. Despite a high T-DNA copy number, ca. 30% of the lines analysed showed high transgene expression in the R₁ generation. High transgene expression was stable at least to the R₄ seed generation in selected high-expressing lines. Transgene expression was lost in the R₂ generation in a low expressing line, while complete, heritable transgene silencing from the R₀ to R₂ generations was also observed in another line. A 50-fold variation in -glucuronidase (GUS) activity and a 16-fold variation in NPTII protein content were observed between R₁ plants derived from different R₀ parents. Reactivation of transgene expression with 5-azacytidine in partially silenced lines indicated that low expression was associated with DNA methylation.