An Isoflavone from Soybean Roots Induces Sinorhizobium Fredii Strain USDA257 to Produce Pilus-Like Surface Appendages

Abstract

Sinorhizobium fredii [Rhizobium fredii] strain USDA257 nodulates the soyabean cultivar Peking, but it fails to infect the cultivar McCall. Disruption of nolXWBTUV, a symbiosis plasmid locus, allows the bacterium to form nitrogen-fixing nodules on McCall. The flavonoid genistein induces expression of nolX and nolBTUV and the secretion of at least proteins from USDA257. It was shown that the flavonoid signal leads to the production of fibrillar, pilus-like appendages on the surface of the bacterial cell. The appendages were purified and immunocytochemistry was used to show that three proteins, including NolX, are physically associated with them. These results were confirmed by Western blots with purified pilus proteins. The S. fredii proteins are immunologically distinct from HrpA, a structural protein from the Hrp pilus of Pseudomonas syringae pv. tomato.