The Chromosomal Background of Agrobacterium tumefaciens Chry5 Conditions High Virulence on Soybean. László G. Kovács. Department of Plant Pathology, University of Missouri, Columbia 65211, U.S.A. Steven G. Pueppke. Department of Plant Pathology, University of Missouri, Columbia 65211, U.S.A. MPMI 6:601-608. Accepted 11 June 1993. This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. The American Phytopathological Society, 1993.

Agrobacterium tumefaciens Chry5, a wild-type strain from chrysanthemum, is highly virulent on soybean stems (Bush and Pueppke, Appl. Environ. Microbiol. 57:2468-2472, 1991). Using soybean cotyledons, we have developed a quantitative tumorigenesis assay and show here that only strain A281, the so-called "supervirulent" strain, matches Chry5 in tumor-inducing ability. Six of the remaining 19 test strains failed to induce tumors, and the other 13 produced no more than 60% of the tumor mass elicited by Chry5 and A281. The Chry5 Ti plasmid has been genetically dissected from the remainder of the bacterial genome to independently study the relative contributions of the Ti plasmid and the chromosomal background to virulence. This was achieved by curing Chry5 of its Ti plasmid and then independently mobilizing three heterologous Ti plasmids, pTiAch5, pTiT37, and pTiBo542, into the Ti plasmidless Chry5 derivative. The tumor-inducing abilities of the resulting constructions and of the wild-type Ti plasmid donors were quantified and compared. We document that the chromosome and the cryptic plasmid of Chry5 provide a genetic background that potentiates the tumorigenic abilities of all three Ti plasmids in comparison to their normal genetic backgrounds.