Tumor induction by Agrobacterium tumefaciens: specific transfer of bacterial deoxyribonucleic acid to plant tissue.

When Agrobacterium tumefaciens cells grown in the presence of tritiated thymidine to label specifically the bacterial deoxyribonucleic acid (DNA) are incubated with carrot root tissue for short periods of time, an appreciable fraction of the label becomes firmly associated with the root tissue. Such association is not observed in identical experiments when A. tumefaciens cell ribonucleic acid or protein are labeled. The extent of the retention of thymidine-derived label from bacterial cells by the root tissue in experiments with A. radiobacter and poorly tumorigenic strains of A. tumefaciens is significantly less than that afforded by tumorigenic strains of A. tumefaciens but greater than the level afforded by Escherichia coli. Transfer of DNA-specific label from A. tumefaciens to carrot root discs is not enhanced by treatments designed to provoke lysis of the bacterial cells, nor is it decreased by addition of deoxyribonuclease or excess unlabeled thymidine to the incubation medium. Bacterial cell-to-plant cell contact is necessary for transfer. Unlabeled A. radiobacter cells decrease in a competitive manner transfer of label when mixed with labeled A. tumefaciens cells. These findings suggest that transfer of DNA from A. tumefaciens to plant tissue after binding of the bacterial cells to specific plant tissue site(s) is a necessary feature of the mechanism by which A. tumefaciens provokes tumors in plants and provides an experimental technique of potentially great value in study of the early steps in the process of tumor induction by A. tumefaciens.