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Early production of rhizopine in nodules induced by Sinorhizobium meliloti strain L5-30

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The rhizopine L-3-O-methyl-scyllo-inosamine (3-O-MSI) is metabolized by approximately 10% of the strains of Rhizobium leguminosarum bv. viciae and Sinorhizobium meliloti. Rhizopine strains enjoy a substantial competitive advantage in nodulation, which is manifest before 14 days post-inoculation, implying that rhizopine is produced before this time. We were able to detect this compound in the roots of alfalfa (Medicago sativum L. cv. Hunter River) four days after germination (six days post-infection) with S. meliloti strain L5-30 by gas chromatography-mass spectrometry (GC-MS). At four days, nodules were not visible, and the concentration of rhizopine was extremely low, estimated at 67 pg/gfw (picograms/gram fresh weight). The amount increased gradually but remained low until 16 days, when there was a 50-fold increase from day four, by which time nodules were well established. This pattern of synthesis is consistent with previous studies indicating that rhizopine synthesis is regulated by nifA/ntrA regulatory genes, which are maximally expressed in bacteroids at the onset of nitrogen fixation. However, the low level of rhizopine synthesis must be responsible for the early effects on competition for nodulation. Production of rhizopine at this time most likely results from micro-aerobic induction of mos genes in free-living bacteria, either in the infection threads or in the rhizosphere.Key words: Medicago sativum, nodulation competitiveness, Rhizobium, rhizopine, Sinorhizobium meliloti.

La rhizopine L-3-O-methyl-scyllo-inoasamine (3-O-MSI) est métabolisée par environ 10% des souches de Rhizobium leguminosarum bv. viciae et Sinorhizobium meliloti. Les souches rhizopine-positives profitent d'un avantage considérable dans la nodulation, qui survient avant 14 jours post-inoculation, ce qui signifie que la rhizopine est produite avant ce moment. Nous avons pu détecter ce composé dans les racines d'alfalfa (Medicago sativum L. cv. Hunter River) avec la souche S. meliloti L5-30, quatre jours après la germination (six jours post-infection), par chromatographie en phase gazeuse-spectrométrie de masse (GS-MS). Les nodules n'étaient pas visibles à quatre jours, et la concentration de rhizopine, estimée à 67 pg/gfw, était extrêmement basse. Le contenu en rhizopine augmenta graduellement mais demeura bas jusqu'au 16e jour, où une hausse d'un facteur de 50 par rapport au jour quatre fut mesurée. À ce moment, les nodules étaient bien implantées. Ce profil de synthèse est conforme à des études antérieures ayant noté que la synthèse de la rhizopine était contrôlée par les gènes régulateurs nifA/ntrA, qui sont le plus intensément exprimés au commencement de la fixation de l'azote dans les bactéroïdes. Cependant, les bas niveaux de synthèse de la rhizopine doivent être responsables des effets précoces sur la compétition pour la nodulation. La production de rhizopine à ce moment entraîne sans doute l'induction micro-aérobique des gènes mos dans les bactéries à l'état libre, soit dans les cordons d'infection ou bien dans la rhizosphère.Mots clés : Medicago sativum, compétition pour la nodulation, Rhizobium, rhizopine, Sinorhizobium meliloti.[Traduit par la Rédaction]

Keywords: Medicago sativum; Rhizobium; Sinorhizobium meliloti; compétition pour la nodulation; nodulation competitiveness; rhizopine

Document Type: Research Article

Publication date: February 1, 2001

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  • Published since 1954, this monthly journal contains new research in the field of microbiology including applied microbiology and biotechnology; microbial structure and function; fungi and other eucaryotic protists; infection and immunity; microbial ecology; physiology, metabolism and enzymology; and virology, genetics, and molecular biology. It also publishes review articles and notes on an occasional basis, contributed by recognized scientists worldwide.
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