Enhanced Tumor Initiation by Mixtures of Tumorigenic and Nontumorigenic Strains of Agrobacterium

Mixtures of two strains of Agrobacterium from the species A. tumefaciens (11 strains), A. rhizogenes (6 strains), and A. radiobacter (7 strains) were tested for their ability to initiate tumors on primary pinto bean leaves. Intra- and interspecific mixtures of A. tumefaciens and A. rhizogenes strains induced 10- to 20-fold increases in the number of tumors initiated. This complementation was observed only in mixtures in which at least one of the strains was tumorigenic when inoculated separately. Tests of various combinations of strains that formed complementary pairs showed two groups described as tumorigenic “helper” strains (donors) and conditionally tumorigenic strains (receptors). Donor, receptor, and noncomplementing strains were found among the strains of A. rhizogenes and A. tumefaciens . Donor or receptor A. tumefaciens strains substituted respectively for A. rhizogenes donor or receptor strains to form complementary pairs. A. radiobacter strains mixed with strains of the same or other Agrobacterium species did not initiate tumors and failed to promote tumor initiation in combination with a tumorigenic strain. Washing the complementing strains prior to mixing had no effect on the response, and the centrifuge supernatants from either donor or receptor strains when inoculated with cells of the opposite type did not increase tumor initiation. Neither indole acetic acid, kinetin, nor gibberellic acid could substitute for either the donor or receptor strain when tested separately on each member of a representative pair of complementing organisms. It is proposed that a substance is either produced or induced in the host by the donor strain after its inoculation on the leaf which permits tumor initiation by a receptor strain at a separate site.