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

The rhizopine L-3-O-methyl-scyllo-inosamine (3-O-MSI) is metabolized by app roximately 10% of the strains of Rhizobium leguminosarum bv. viciae and Sin orhizobium meliloti. Rhizopine strains enjoy a substantial competitive adva ntage in nodulation, which is manifest before 14 days post-inoculation, imp lying that rhizopine is produced before this time. We were able to detect t his compound in the roots of alfalfa (Medicago sativum L. cv. Hunter River) four days after germination (six days post-infection) with S. meliloti str ain L5-30 by gas chromatography-mass spectrometry (GC-MS). At four days, no dules were not visible, and the concentration of rhizopine was extremely lo w, estimated at 67 pg/gfw (picograms/gram fresh weight). The amount increas ed gradually but remained low until 16 days, when there was a 50-fold incre ase from day four, by which time nodules were well established. This patter n of synthesis is consistent with previous studies indicating that rhizopin e synthesis is regulated by nifA/ntrA regulatory genes, which are maximally expressed in bacteroids at the onset of nitrogen fixation. However, the lo w level of rhizopine synthesis must be responsible for the early effects on competition for nodulation. Production of rhizopine at this time most like ly results from micro-aerobic induction of mos genes in free-living bacteri a, either in the infection threads or in the rhizosphere.