RESUMEN
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.
Asunto(s)
Proteínas Bacterianas/fisiología , Proteínas de Unión al ADN , Flavonoides/metabolismo , Isomerasas/genética , Sinorhizobium meliloti/genética , Sinorhizobium meliloti/metabolismo , Transactivadores , Regulación de la Expresión Génica , Luteolina , Medicago sativa/crecimiento & desarrollo , Medicago sativa/microbiología , Modelos Genéticos , Raíces de Plantas/genética , Raíces de Plantas/metabolismo , Raíces de Plantas/microbiología , Plásmidos/clasificación , Simbiosis , Activación TranscripcionalRESUMEN
Rhizobium etli type strain CFN42 contains six plasmids. We analyzed the distribution of genetic markers from some of these plasmids in bean-nodulating strains belonging to different species (Rhizobium etli, Rhizobium gallicum, Rhizobium giardinii, Rhizobium leguminosarum, and Sinorhizobium fredii). Our results indicate that independent of geographic origin, R. etli strains usually share not only the pSym plasmid but also other plasmids containing symbiosis-related genes, with a similar organization. In contrast, strains belonging to other bean-nodulating species seem to have acquired only the pSym plasmid from R. etli.