The tobacco element, Tnt1, is one of the few active retrotransposons in plants. Its transposition is activated during protoplast culture in tobacco and tissue culture in the heterologous host Arabidopsis thaliana. Here, we report its transposition in the R108 line of Medicago truncatula during the early steps of the in vitro transformation±regeneration process. Two hundred and twenty-®ve primary transformants containing Tnt1 were obtained. Among them, 11.2% contained only transposed copies of the element, indicating that Tnt1 transposed very early and ef®ciently during the in vitro transformation process, possibly even before the T-DNA integration. The average number of insertions per transgenic line was estimated to be about 15. These insertions were stable in the progeny and could be separated by segregation. Inspection of the sequences ¯anking the insertion sites revealed that Tnt1 had no insertion site speci®city and often inserted in genes (one out of three insertions). Thus, our work demonstrates the functioning of an ef®cient transposable element in leguminous plants. These results indicate that Tnt1 can be used as a powerful tool for insertion mutagenesis in M. truncatula.
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