Polymyxin resistance in Agrobacterium tumefaciens and its effect on crown gall tumor induction

Polymyxin-resistant (PBLr) mutants of Agrobacterium tumefaciens A6, B6, and B6M were isolated from polymyxin-sensitive (PBLs) parent strains in a defined medium containing 600 microgram of polymyxin B sulfate per millilitre. The weight and number of tumors induced by PBLr mutants on a variety of host plants such as carrot, potato, and pinto bean were 45--75% less than those induced by PBLs wild types. The crude cell envelopes (CCE) prepared from both PBLs and PBLr bacteria were inhibitory for tumor initiation when they were applied before or during the inoculation of viable tumorigenic bacteria, but not when they were applied 30 min after the inoculation of infectious bacteria. The potency to inhibit the tumor initiation by the CCE prepared from PBLs cells was approximately 50% higher than that by the equal amount of the CCE prepared from PBLr cells. The concentration of CCE preparations required to reduce tumor induction 50% in carrot and pinto bean was determined to be 2.6 mg/mL and 4.0--6.2 mg/mL for the CCE derived from PBLs and PBLr cells, respectively. These data suggest that the envelope structure or composition of PBLs and PBLr cells is distinct, and that the acquisition of resistance to polymyxin by agrobacteria modifies envelope structure or components which are essential for tumor initiation.