Nodulation of Lotus spp. by Rhizobia

ESTRELLA, M.J., LORITE M.J., OLIVARES J., SOTO M.J., RUIZ O.A., & SANJUÁN J.

Universidad de Aarhnus. Dinamarca

Workshop; Satellite meeting on The Bean-Lotus Genomic Interface. 7th European Nitrogen Fixation Meeting.; 2006

The genus Lotus may include up to 200 species. However, few studies on the genetic diversity of rhizobia nodulating Lotus spp. have been conducted. Indeed it is assumed that most Lotus spp. are nodulated by either Mesorhizobium (medium to fast growers) or  Bradyrhizobium (slow growers) strains, as deduced from studies with agriculturally (forage species) or science-relevant species (model species as L. japonicus). The information about the Bradyrhizobia is scarce and limited to few reports on their isolation and symbiotic efficiency. More is known about the Mesorhizobia, including the complete genome sequence of strain MAFF303099 and of the symbiosis island from strain R7A. The fact that these two strains contain very different symbiosis islands is an indication of the great genetic diversity that must exist among Lotus-nodulating rhizobia. The studies on these bacteria, however, lag behind the developments on the model species L. japonicus. There is an incompatibility between certain species of Lotus and certain rhizobia, so Mesorhizobia usually can form Nod+Fix+ symbiosis with species like L. corniculatus, L. glaber, L. japonicus, whereas the Bradyrhizobia do so only with species of the L. pedunculatus/L. subbiflorus group. Few Mesorhizobium strains can effectively nodulate both groups of Lotus spp., a trait that has been associated with the capacity to resist growth inhibition by flavolans (condensed tannins) produced in the plant roots. No genetic data exist, however, to support this assumption.