Instability of Mi-mediated nematode resistance in transgenic tomato plants

The goal of this study was to assess the susceptibility of the nematode resistance gene, Mi-1.2, to transgene inactivation. The stability of Mi-mediated nematode resistance and Mi-1.2 transcripts levels was observed in two independently transformed tomato lines carrying Mi-1.2. In both lines a reduction in resistance was noted in the T2 generation, and was more pronounced in the T3 generation. The decrease in resistance varied among cuttings that were clonally propagated, which suggests that resistance levels were influenced by epigenetic effects. The transgenic plants, however, did not show the reduced transcript levels characteristic of gene silencing or negative position effects. RNase protection assays revealed that Mi-1.2 mRNA levels in the transgenic plants were comparable to levels observed in resistant control plants, and there was no correlation between transcript levels and resistance levels among the transgenic plants. Reductions in nematode resistance could be due to silencing of other endogenous gene (s) involved in resistance. Alternatively, these reductions could be caused by previously uncharacterized mechanisms of transgene inactivation that target the Mi-1.2 protein or block its production.