Bradyrhizobium japonicum Rhizobitoxine Mutants with Altered Host-Range on Rj4 Soybeans. Thomas J. W. Stokkermans. The Ohio State Biotechnology Center, Molecular Cellular and Developmental Biology Program, The Ohio State University, Columbus 43210 U.S.A. Juan Sanjuan(4), Xiaoan Ruan(1,2), Gary Stacey(4), and N. Kent Peters(1-3). (1)The Ohio State Biotechnology Center, (2)Department of Agronomy, and (3)Molecular Cellular and Developmental Biology Program, The Ohio State University, Columbus 43210 U.S.A.; (4)The Center for Legume Research and Department of Microbiology, University of Tennessee, Knoxville 37996 U.S.A. MPMI 5:504-512. Accepted 28 July 1992. 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, 1992.

Genotype-specific nodulation (GSN) of Rj4 soybean is defined by limited nodulation with Bradyrhizobium japonicum strain USDA 61, but normal nodulation with other strains, e.g., USDA 110. Strain USDA 61 is a rhizobitoxine-producing Bradyrhizobium for which we had previously isolated rhizobitoxine null and in planta overproducing strains. The null mutant nodulated Rj4 plants at a rate similar to strain USDA 61. Surprisingly, the in planta overproducing mutants nodulated Rj4 plants at a rate similar to nonrestricted strains of Bradyrhizobium. These results suggested that rhizobitoxine was involved in the ability of USDA 61 derivatives to overcome the Rj4 nodulation restriction. Rhizobitoxine is an inhibitor of ethylene biosynthesis, and ethylene has been shown to be detrimental to nodulation. Therefore, experiments were performed to test whether ethylene was involved in mediating the GSN response of Rj4 soybeans either directly or indirectly via the induction of a plant defense response. The data indicate that Rj4 soybeans do not respond to USDA 61 by induction of ethylene, chalcone synthase, or extensin, which would be indicative of a defenselike response. Moreover, the exogenous addition of inhibitors of ethylene biosynthesis did not restore nodulation to USDA 61 inoculated Rj4 soybeans. Therefore, ethylene does not appear to be the primary mediator of GSN in Rj4 soybean. Because nodulation host range has been shown to be affected by the structure of Nod factors produced by rhizobia, we analyzed by thin-layer chromatography the Nod factors produced by strain USDA 61 and rhizobitoxine-mutant derivatives. In those mutants that nodulate Rj4 plants, minor Nod factors appeared different and the distribution of major Nod factors was shifted towards a compound that comigrates with the primary compound produced by the unrestricted strain, USDA 110. This suggests that mutations leading to overproduction of rhizobitoxine in USDA 61 have the pleiotropic effect of modifying the profile of Nod metabolites produced. This latter effect may explain the nodulation of Rj4 soybean by USDA 61 derivatives.