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Spontaneous Transfer of the Ti Plasmid of Agrobacterium tumefaciens and the Nopaline Catabolism Plasmid of A. radiobacter Strain K84 in Crown Gall Tissue. Begonya Vicedo, Instituto Valenciano de Investigaciones Agrarias, Apartado Oficial, Moncada, 46113 Valencia, Spain; María José López, María José Asíns, and María M. López. Instituto Valenciano de Investigaciones Agrarias, Apartado Oficial, Moncada, 46113 Valencia, Spain. Phytopathology 86:528-534. Accepted for publication 7 February 1996. Copyright 1996 The American Phytopathological Society. DOI: 10.1094/Phyto-86-528.

Spontaneous transfer of the Ti plasmid from Agrobacterium tume-faciens to strain K84 of A. radiobacter was observed and studied for the first time in an experiment on biological control of crown gall. This transfer was detected in a tumor from a K84-treated plant grown in soil inoculated with a nopaline strain of A. tumefaciens biovar 1 sensitive to agrocin 84. The transconjugant strain was virulent and produced agrocin 84. Southern blot hybridization analysis with several probes (T-DNA and right adjacent regions and vir genes) showed important changes at the Ti plasmid, suggesting that recombination between Ti plasmid and pAtK84b in K84 could have happened, resulting in a new Ti plasmid. Transfer of both plasmids of strain K84, pAtK84b and pAgK84, responsible for nopaline catabolism and agrocin 84 production, respectively, to A. tume-faciens also was detected in isolates from the same tumor. Southern blot hybridization of plasmids from one of these avirulent isolates with a nopaline plasmid-specific probe of strain K84 indicated there was a replacement of Ti plasmid by pAtK84b in A. tumefaciens, explaining its avirulence. These results show that plasmid exchanges can occur spontaneously between A. tumefaciens and A. radiobacter. This kind of transfer generates genetic diversity in Agrobacterium and may influence the biocontrol efficiency of A. radiobacter.