Proteomics reveals differential expression of proteins related to a variety of metabolic pathways by genistein-induced Bradyrhizobium japonicum strains

The rhizobia legume symbiosis requires a coordinated molecular interaction between the symbionts, initiated by seed and root exudation of several compounds, mainly flavonoids, that trigger the expression of nodulation genes in the bacteria. Since the role of flavonoids seems to be broader than the induction of nodulation genes, we aimed at characterizing genistein-induced proteins of Bradyrhizobium japonicum CPAC 15 (=SEMIA 5079), used in commercial soybean inoculants in Brazil, and of two genetically related strains grown in vitro. Whole-cell proteins were extracted both from induced (1 mu M genistein) and from non-induced cultures of the three strains, and separated by two-dimensional electrophoresis. Spot profiles were compared between the two conditions and selected spots were excised and identified by mass spectrometry. Forty-seven proteins were significantly induced by genistein, including several hypothetical proteins, the cytoplasmic flagellar component FliG, periplasmic ABC transporters, a protein related to biosynthesis of exopolysaccharides (ExoN), and proteins involved in redox-state maintenance. Noteworthy was the induction of the PhyR-sigma(EcfG) regulon, recently demonstrated to be involved in the symbiotic efficiency of, and general stress response in B. japonicum. Our results confirm that the role of flavonoids, such as genistein, can go far beyond the expression of nodulation-related proteins in B. japonicum. (C) 2011 Elsevier B.V. All rights reserved.