The mechanism of T-DNA transfer has many similarities with bacterial conjugation systems, and indeed it has been shown that broad host range bacterial plasmids will transfer from Agrobacterium to the plant genome under the control of their own mobilization genes (35). Another system with close similarities to the A. tumefaciens Ti plasmid is the Ri plasmid of Agrobacterium rhizogenes. This causes hairy root disease though the integration of its own T-DNA, carrying genes that activate endogenous plant hormones. Ri plasmids have also been exploited as gene transfer vectors and have the advantage that they do not induce cell proliferation in the host plant (i.e., they are naturally disarmed) (Table 3). Agrobacterium strains containing both Ti and Ri plasmids often transfer both T-DNAs to the host plant (36). The Ri T-DNA induces hairy root disease and acts as a marker for transformation, while the disarmed Ti plasmid carries the transgenes of interest. In this system, dominant markers are not required and regeneration is not prevented by the Ri T-DNA.
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