T-DNA of the Agrobacterium Ti and Ri plasmids

The study of T-DNA transmission from Agrobacterium Ti or Ri plasmid into the genomes of higher plant cells has revealed much about the consequences of transformation. It is now clear that the transformed phenotype is caused by hormonal changes produced directly or indirectly by T-DNA genes. The opine synthases are enzymes encoded in T-DNA that function in the plant cell. Our level of understanding of T-DNA-encoded functions is already sufficient to reveal clear and feasible ways to exploit T-DNA as a gene vector. What remains to challenge the crown gall investigator are many questions of fundamental importance: What is the mechanism of the seemingly illegitimate recombination between T-DNA and plant DNA, and is this process catalyzed by bacterial or host plant enzymes, or both? Do T-DNA genes encode enzymes that catalyze biosynthesis of auxin- and cytokinin-active substances? What gene in T-DNA confers immunity to A. tumefaciens, and what is its mode of action? Does T-DNA insert into random or specific sites in the host plant genome? Did T-DNA derive from plant genetic information or has prokaryotic DNA arrived at functional eukaryotic gene structure by convergent evolution? Although there is keen interest in T-DNA as a vector for genetic engineering, it holds equal interest as a unique interface between the biology of prokaryotes and eukaryotes.