Flavonoid Inducers of Nodulation Genes Stimulate Rhizobium fredii USDA257 to Export Proteins into the Environment. Hari B. Krishnan. Department of Plant Pathology, University of Missouri, Columbia, MO 65211 U.S.A. Steven G. Pueppke. Department of Plant Pathology, University of Missouri, Columbia, MO 65211 U.S.A. MPMI 6:107-113. Accepted 14 September 1992. Copyright 1993 The American Phytopathological Society.

Genistein, an isoflavone from the roots of soybean (Glycine max), is among the most powerful inducers of the expression of nodulation genes in Rhizobium fredii USDA257, a nitrogen-fixing symbiont of this plant. We show here that concentrations of this compound as low as 100-500 nM also stimulate cells of USDA257 to excrete five major proteins, of molecular masses of 54, 39, 36, 20, and 8.9 kDa. Other known inducers of nodulation genes in R. fredii, including luteolin and naringenin, also elicit accumulation of this set of proteins in USDA257, but the noninducers biochanin A and quercetin are inactive in induction of protein secretion. [³⁵S]Methionine is incorporated into each of the proteins, but none contains detectable polysaccharide. Export of an array of proteins in response to nodulation gene inducers is characteristic of R. fredii and depends on the presence of an intact symbiosis plasmid. R. leguminosarum bv. phaseoli, Brady-rhizobium japonicum, and broad host range Rhizobium sp. strain NGR234 each elaborate a single genistein-induced extracellular protein, but the response is lacking when several other nitrogen-fixing species are exposed to their nodulation gene inducers. The N-terminal sequence of the 36-kDa protein, MetTyrSerSerLysThrGlySerAlaSerGlnSerThr, differs from that of all known nodulation genes, and the conservation of the N-terminal methionine indicates that this protein probably is exported without N-terminal processing.