title: Involvement of the Sinorhizobium meliloti leuA gene in activation of nodulation genes by NodD1 and luteolin.
abstract: The role of leucine biosynthesis by Sinorhizobium meliloti in the establishment of nitrogen-fixing symbiosis with alfalfa ( Medicago sativa) was investigated. The leuA gene from S. meliloti, encoding alpha-isopropylmalate synthase, which catalyses the first specific step in the leucine biosynthetic pathway, was characterized. S. melilotiLeuA(-) mutants were Leu auxotrophs and lacked alpha-isopropylmalate synthase activity. In addition, leuA auxotrophs were unable to nodulate alfalfa. Alfalfa roots did not seem to secrete enough leucine to support growth of leucine auxotrophs in the rhizosphere. Thus, this growth limitation probably imposes the inability to initiate symbiosis. However, in addition to the leucine auxotrophy, leuA strains were impaired in activation of nodulation genes by the transcriptional activator NodD1 in response to the plant flavone luteolin. By contrast, nod gene activation by NodD3, which does not involve plant-derived inducers, was unaffected. Our results suggest that a leucine-related metabolic intermediate may be involved in activation of nodulation genes by NodD1 and luteolin. This kind of control could be of relevance as a way to link bacterial physiological status to the response to plant signals and initiation of symbiosis.
authors.affiliation: Departamento de Microbiología del Suelo y Sistemas Simbióticos, Estación Experimental del Zaidín, CSIC, Profesor Albareda 1, 18008 Granada, Spain.
authors.forename: Julio M
authors.initials: JM
authors.lastname: Sanjuán-Pinilla
authors.forename: Socorro
authors.initials: S
authors.lastname: Muñoz
authors.forename: Joaquina
authors.initials: J
authors.lastname: Nogales
authors.forename: José
authors.initials: J
authors.lastname: Olivares
authors.forename: Juan
authors.initials: J
authors.lastname: Sanjuán
mesh_terms: D001426:Bacterial Proteins / Q000502:physiology*; D004268:DNA-Binding Proteins; D005419:Flavonoids / Q000378:metabolism*; D005786:Gene Expression Regulation; D007535:Isomerases / Q000235:genetics*; D047311:Luteolin; D000455:Medicago sativa / Q000254:growth & development / Q000382:microbiology; D008957:Models, Genetic; D018517:Plant Roots / Q000235:genetics / Q000378:metabolism / Q000382:microbiology; D010957:Plasmids / Q000145:classification; D016962:Sinorhizobium meliloti / Q000235:genetics* / Q000378:metabolism*; D013559:Symbiosis; D015534:Trans-Activators; D015533:Transcriptional Activation
chemical_list: D001426:Bacterial Proteins; D004268:DNA-Binding Proteins; D005419:Flavonoids; D015534:Trans-Activators; C032241:isopropylmalate isomerase; D007535:Isomerases; D047311:Luteolin