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Agrobacterium tumefaciens harboring pTiA6 incite unorganized tumors on Nicotiana rustica, sunflowers, carrots, and tomatoes, whereas isogenic strains of agrobacteria harboring pTiAg63 form "rooty" tumors on N. rustica and are essentially avirulent on sunflowers, carrots, and tomatoes. In this report we show that the different host range characteristics of these two plasmids were due, in part, to differences in the T-DNA oncogene complements of the plasmids. Specifically, we constructed derivatives of pTiAg63 that contained pTiA6 oncogenes 4, 6a, and 6b inserted into the TB-DNA region and found that agrobacteria harboring these plasmids could incite unorganized tumors on N. rustica, tomatoes, carrots, and the inbred sunflower line HA202R. Undefined host factors, however, also appeared to be involved in determining A. tumefaciens host range since three inbred sunflower lines, HA303B, HA89B, and HA290B, were susceptible to tumor formation by agrobacteria harboring pTiA6 but not by strains harboring pTiAg63 or the modified pTiAg63 plasmids.

The T-DNA oncogene complements of the limited-host-range tumor-inducing plasmid pTiAg63 and the wide-host-range plasmid pTiA6 were compared. The resulting data indicate that pTiAg63 has DNA sequences related to most of the genes encoded by the oncogene region, the TL-DNA, of pTiA6 and that these sequences are divided between two T-DNA regions, the TA-DNA, which encoded sequences related to pTiA6 genes 4 (the cytokinin independence gene) and 6a, as well as to a pTiA6 TL-DNA fragment that encoded gene 6b and a portion of gene 3, and the TB-DNA, which encoded sequences related to genes 1 and 2 (the auxin independence genes). Tumor tissues of Nicotiana rustica incited by Agrobacterium tumefaciens harboring either pTiA6 or pTiAg63 grew axenically in vitro on phytohormone-free medium. The morphologies of the tissues, however, differed; whereas those incited with pTiA6 grew as loose, friable, unorganized callus, the tumors incited by pTiAg63 grew as clumps of rootlike structures. Thus, the T-DNA oncogene complements of these plasmids were not equivalent. The results are discussed in relation to the A. tumefaciens host range.