Eric Giraud,1 and
1Laboratoire des Symbioses Tropicales et Méditerranéennes, UMR113 IRD, Laboratoire des Symbioses Tropicales et Méditerranéennes, UMR IRD/SupAgro/INRA/UM2/CIRAD, F-34398 Montpellier, France; 2Laboratoire des Interactions plantes/microorganismes LIPM, UMR441/2594 INRA/CNRS 441/2594, 24 chemin de Borde Rouge, 31326 Castanet-Tolosan, France; 3Laboratoire des Interactions Moléculaires et Réactivité Chimique et Photochimique, UMR 5623 CNRS/Université Paul Sabatier, 118 Route de Narbonne, F-31062 Toulouse, France
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Accepted 15 August 2011.
Bradyrhizobium sp. strain ORS285 is a photosynthetic bacterium that forms nitrogen-fixing nodules on the roots and stems of tropical aquatic legumes of the Aeschynomene genus. The symbiotic interaction of Bradyrhizobium sp. strain ORS285 with certain Aeschynomene spp. depends on the presence of nodulation (nod) genes whereas the interaction with other species is nod gene independent. To study the nod gene-dependent molecular dialogue between Bradyrhizobium sp. strain ORS285 and Aeschynomene spp., we used a nodB-lacZ reporter strain to monitor the nod gene expression with various flavonoids. The flavanones liquiritigenin and naringenin were found to be the strongest inducers of nod gene expression. Chemical analysis of the culture supernatant of cells grown in the presence of naringenin showed that the major Nod factor produced by Bradyrhizobium sp. strain ORS285 is a modified chitin pentasaccharide molecule with a terminal N-C18:1-glucosamine and with a 2-O-methyl fucose linked to C-6 of the reducing glucosamine. In this respect, the Bradyrhizobium sp. strain ORS285 Nod factor is the same as the major Nod factor produced by the nonphotosynthetic Bradyrhizobium japonicum USDA110 that nodulates the roots of soybean. This suggests a classic nod gene-dependent molecular dialogue between Bradyrhizobium sp. strain ORS285 and certain Aeschynomene spp. This is supported by the fact that B. japonicum USDA110 is able to form N2-fixing nodules on both the roots and stems of Aeschynomene afraspera.
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, 2011.