Tailoring the agrobacterium Ti plasmid as a vector for plant genetic engineering : (site specific insertions, chimeric gene construction, regeneration of plants containing T- DNA)
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Agrobacterium tumefaciens and its tumor-inducing (Ti) plasmidi constitute a promising vector system for the introduction of desirable genes into higher plant cells. This bacterialpathogen has an unusually wide host range, inciting galls on many kinds of dicotyledonous plants. By a mechanism as yet unknown, during tumor induction a specific part of the Ti plasmid becomes covalently joined to plant nuclear DNA. This foreign DNA, called T-DNA, is stably maintained and expressed in the transformed plant cell. There are several technical barriers to be overcome if we are to exploit the Ti plasmid as a vector for introducing new genes into plants. First is the problem of inserting specific desired genes into the T-DNA portion of the Ti plasmid. Second is how to induce or regulate the expression of foreign genes in the new host plant. Third is the challenge of regenerating healthy plants containing T-DNA. Progress in each of these areas is encouraging. The most difficult problem remaining will be the choice and isolation of genes (as genomic or cDNA clones) whose introduction into plants will produce agronomically desirable changes.