Transfer of pAgK84 from the Biocontrol Agent Agrobacterium radiobacter K84 to A. tumefaciens Under Field Conditions

Abstract

Agrobacterium radiobacter strain K84 effects biological control of crown gall disease caused by A. tumefaciens in part by the plasmid-conferred production of agrocin 84, an antibiotic with specific activity against certain strains of the pathogen. A transmissible plasmid (pAgK84) resident in the biocontrol bacterium contains genes for agrocin 84 biosynthesis and immunity. The frequency of pAgK84 transfer from K84 to A. tumefaciens strain B49c, a naturally occurring strain that is insensitive to agrocin 84, in gall tissue of cherry seedlings was evaluated under field conditions. Putative transconjugants were detected by colony hybridization in 4 of 13 galls evaluated. The identity of transconjugants recovered from one of those galls was confirmed by serology, agrocin production, pathogenicity, and Southern blot analysis using agrocin 84 biosynthesis, T-DNA, and nopaline synthase gene probes. In the four galls in which transconjugants were detected, the estimated frequency of plasmid transfer from K84 to B49c was approximately 10 -4 transconjugants per recipient. A transconjugant strain and B49c did not differ in their capacities to colonize the rhizosphere of cherry or to cause crown gall under field conditions. The transconjugant retained pAgK84 for up to 7 months in the rhizosphere of field-grown plants. These results confirm that Agrobacterium strains harboring both a tumor-inducing plasmid and pAgK84 can develop from plasmid transfer in the field and that such newly derived strains can persist under field conditions.