Legumes Symbioses: Absence of Nod Genes in Photosynthetic Bradyrhizobia

En el presente trabajo se describe la secuenciación completa de dos cepas de Bradyrhizobium y una comparación funcional entre ambas. Además, se enfoca un análisis de los genes que gatillan el proceso de nodulación y la infección de la bacteria en las leguminosas, mostrando (en este caso en particular) un sistema diferente de los genes nod requeridos comúnmente en estos sistemas simbiontes.

Science 316, 1307-1312 (2007) 
Eric Giraud, Lionel Moulin, David Vallenet, Valérie Barbe, Eddie Cytryn, Jean-Christophe Avarre, Marianne Jaubert, Damien Simon, Fabienne Cartieaux, Yves Prin, Gilles Bena, Laure Hannibal, Joel Fardoux, Mila Kojadinovic, Laurie Vuillet, Aurélie Lajus, Stéphane Cruveiller, Zoe Rouy, Sophie Mangenot, Béatrice Segurens, Carole Dossat, William L. Franck, Woo-Suk Chang, Elizabeth Saunders, David Bruce, Paul Richardson, Philippe Normand, Bernard Dreyfus, David Pignol, Gary Stacey, David Emerich, André Verméglio, Claudine Médigue, Michael Sadowsky

Leguminous plants (such as peas and soybeans) and rhizobial soil bacteria are symbiotic partners that communicate through molecular signaling pathways, resulting in the formation of nodules on legume roots and occasionally stems that house nitrogen-fixing bacteria. Nodule formation has been assumed to be exclusively initiated by the binding of bacterial, host-specific lipochito-oligosaccharidic Nod factors, encoded by the nodABC genes, to kinase-like receptors of the plant. Here we show by complete genome sequencing of two symbiotic, photosynthetic, Bradyrhizobium strains, BTAi1 and ORS278, that canonical nodABC genes and typical lipochitooligosaccharidic Nod factors are not required for symbiosis in some legumes. Mutational analyses indicated that these unique rhizobia use an alternative pathway to initiate symbioses, where a purine derivative may play a key role in triggering nodule formation.