title: Plant-activated bacterial receptor adenylate cyclases modulate epidermal infection in the Sinorhizobium meliloti-Medicago symbiosis.
abstract: Legumes and soil bacteria called rhizobia have coevolved a facultative nitrogen-fixing symbiosis. Establishment of the symbiosis requires bacterial entry via root hair infection threads and, in parallel, organogenesis of nodules that subsequently are invaded by bacteria. Tight control of nodulation and infection is required to maintain the mutualistic character of the interaction. Available evidence supports a passive bacterial role in nodulation and infection after the microsymbiont has triggered the symbiotic plant developmental program. Here we identify in Sinorhizobium meliloti, the Medicago symbiont, a cAMP-signaling regulatory cascade consisting of three receptor-like adenylate cyclases, a Crp-like regulator, and a target gene of unknown function. The cascade is activated specifically by a plant signal during nodule organogenesis. Cascade inactivation results in a hyperinfection phenotype consisting of abortive epidermal infection events uncoupled from nodulation. These findings show that, in response to a plant signal, rhizobia play an active role in the control of infection. We suggest that rhizobia may modulate the plant's susceptibility to infection. This regulatory loop likely aims at optimizing legume infection.
authors.affiliation: Laboratoire des Interactions Plantes-Microorganismes,  Institut National de la Recherche Agronomique, Unité Mixte de Recherche 441, F-31326 Castanet-Tolosan,  France.
authors.forename: Chang Fu
authors.initials: CF
authors.lastname: Tian
authors.forename: Anne-Marie
authors.initials: AM
authors.lastname: Garnerone
authors.forename: Céline
authors.initials: C
authors.lastname: Mathieu-Demazière
authors.forename: Catherine
authors.initials: C
authors.lastname: Masson-Boivin
authors.forename: Jacques
authors.initials: J
authors.lastname: Batut
mesh_terms: D000262:Adenylyl Cyclases / Q000378:metabolism*; D000242:Cyclic AMP / Q000378:metabolism; D029902:Medicago / Q000469:parasitology*; D010944:Plants / Q000378:metabolism*; D015398:Signal Transduction; D016962:Sinorhizobium meliloti / Q000472:pathogenicity*; D013559:Symbiosis
chemical_list: D000242:Cyclic AMP; D000262:Adenylyl Cyclases