Detailed characterization of Mirafiori lettuce virus-resistant transgenic lettuce

Lettuce big-vein disease is caused by Mirafiori lettuce virus (MiLV), which is vectored by the soil-borne fungus Olpidium brassicae. A MiLV-resistant transgenic lettuce line was developed through introducing inverted repeats of the MiLV coat protein (CP) gene. Here, a detailed characterization study of this lettuce line was conducted by comparing it with the parental, non-transformed ‘Kaiser’ cultivar. There were no significant differences between transgenic and non-transgenic lettuce in terms of pollen fertility, pollen dispersal, seed production, seed dispersal, dormancy, germination, growth of seedlings under low or high temperature, chromatographic patterns of leaf extracts, or effects of lettuce on the growth of broccoli or soil microflora. A significant difference in pollen size was noted, but the difference was small. The length of the cotyledons of the transgenic lettuce was shorter than that of ‘Kaiser,’ but there were no differences in other morphological characteristics. Agrobacterium tumefaciens used for the production of transgenic lettuce was not detected in transgenic seeds. The transgenic T3, T4, and T5 generations showed higher resistance to MiLV and big-vein symptoms expression than the resistant ‘Pacific’ cultivar, indicating that high resistance to lettuce big-vein disease is stably inherited. PCR analysis showed that segregation of the CP gene was nearly 3:1 in the T1 and T2 generations, and that the transgenic T3 generation was homozygous for the CP gene. Segregation of the neomycin phosphotransferase II (npt II) gene was about 3:1 in the T1 generation, but the full length npt II gene was not detected in the T2 or T3 generation. The segregation pattern of the CP and npt II genes in the T1 generation showed the expected 9:3:3:1 ratio. These results suggest that the fragment including the CP gene and that including the npt II gene have been integrated into two unlinked loci, and that the T1 plant selected in our study did not have the npt II gene. DNA sequences flanking T-DNA insertions in the T2 generation were determined using inverse PCR, and showed that the right side of the T-DNA including the npt II gene had been truncated in the transgenic lettuce.