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PLANTS INTERACTING WITH OTHER ORGANISMS

The DMI1 and DMI2 Early Symbiotic Genes of Medicago truncatula Are Required for a High-Affinity Nodulation Factor-Binding Site Associated to a Particulate Fraction of Roots¹

Surfaces Cellulaires et Signalisation chez les Végétaux, Unité Mixte de Recherche 5546 Centre National de la Recherche Scientifique-Université Paul Sabatier, Toulouse III, Pôle de Biotechnologie Végétale, 31326 Castanet-Tolosan, France (B.V.H., R.R., J.-J.B.); and Laboratoire des Interactions Plantes-Microorganismes, Unité Mixte de Recherche Institut National de la Recherche Agronomique-Centre National de la Recherche Scientifique (441/2594), 31326 Castanet-Tolosan, France (B.V.H., J.V.C., J.-J.B.)

The establishment of the legume-rhizobia symbiosis between Medicago spp. and Sinorhizobium meliloti is dependent on the production of sulfated lipo-chitooligosaccharidic nodulation (Nod) factors by the bacterial partner. In this article, using a biochemical approach to characterize putative Nod factor receptors in the plant host, we describe a high-affinity binding site (K_d = 0.45 nM) for the major Nod factor produced by S. meliloti. This site is termed Nod factor-binding site 3 (NFBS3). NFBS3 is associated to a high-density fraction prepared from roots of Medicago truncatula and shows binding specificity for lipo-chitooligosaccharidic structures. As for the previously characterized binding sites (NFBS1 and NFBS2), NFBS3 does not recognize the sulfate group on the S. meliloti Nod factor. Studies of Nod factor binding in root extracts of early symbiotic mutants of M. truncatula reveals that the new site is present in Nod factor perception and does not make infections 3 (dmi3) mutants but is absent in dmi1 and dmi2 mutants. Roots and cell cultures of all these mutants still contain sites similar to NFBS1 and NFBS2, respectively. These results suggest that NFBS3 is different from NFBS2 and NFBS1 and is dependent on the common symbiotic genes DMI1 and DMI2 required for establishment of symbioses with both rhizobia and arbuscular mycorrhizal fungi. The potential role of this site in the establishment of root endosymbioses is discussed.

² Present address: Institute of Agroecology and Bioengineering, Department of Biology, NUI Maynooth, Kilcock Road, Co. Kildare, Ireland.

The author responsible for distribution of materials integral to the findings presented in this article in accordance with the policy described in the Instructions for Authors (www.plantphysiol.org) is: Jean-Jacques Bono (bono{at}scsv.ups-tlse.fr).

Article, publication date, and citation information can be found at www.plantphysiol.org/cgi/doi/10.1104/pp.105.068981.

^* Corresponding author; e-mail bono{at}scsv.ups-tlse.fr; fax 33–562–19–35–02.

Received July 27, 2005; returned for revision November 14, 2005; accepted November 15, 2005.

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