Site-specific mutagenesis of Agrobacterium Ti plasmids and transfer of genes to plant cells.

A general method is described for the use of tumor-inducing (Ti) plasmids as experimental gene-vectors for plant cells. Intermediate vectors, containing specific Ti plasmid sequences and capable of replication in both E. coli and Agrobacterium strains, were constructed and used for the in vitro introduction of isolated DNA fragments into predetermined sites of the T-region derived fragments. Site-specific inserts and/or deletions-substitutions in Ti plasmids were produced by exchange of the modified T-region sequences for the wild-type sequence by in vivo recombination between intermediate vectors and resident Ti plasmids. This method was applied for the isolation of mutant Ti plasmids affecting either morphogenetic properties or opine synthesis in crown gall tumors. Foreign DNA, inserted at different sites in the T-region of both octopine and nopaline Ti plasmids, was shown to be cotransferred with the T-DNA and to be stably maintained in the transformed plant cells.