Résumé
The Methylobacterium nodulans / Crotalaria podocarpa interaction is an original symbiosis. First, major metabolic modifications in the nodule will take place due to bacterial methylotrophic property located at the nodule apex: a significant apical degradation was observed after cell wall digestion, with free methanol emission, directly available for the bacterium. The bacteroid (size and form unusually large) provides the useful energy to nitrogenase activity, allowing the plant to accumulate organic compounds inside infected cells in amyloplasts. In addition, the bacterial methylotrophic property contributes to plant biomass benefit (40% higher when compared to non methylotrophic mutants). However, any Crotalaria species take advantage in terms of nitrogen fixation to be combined with either M. nodulans or Bradyrhizobium sp., the latter bacterial genus being commonly associated with Crotalaria sp. The feature of the symbiosis is also original due to molecular interactions: M. nodulans nod genes are insensitive to Bradyrhizobium flavonoid inducers (isoflavones) but are inducible by flavanones and flavones. The phylogenetic analysis of identified nod genes shows that M. nodulans belongs to the same branch as Methylobacterium sp. 4-46 and Burkholderia tuberum STM678, isolated from others species belonging to Crotalarieae tribe. A new Nod factor was identified as a bacterial signal as follows MnV(C18:1, S). Thus, if the specific M. nodulans – plant interaction was only reported for three Crotalaria species, the bacterium capacity to form pseudonodules with different Crotalaria points out the major key role of other chemical key signals during the infection process and suggests various molecular mechanisms supporting the symbiotic specificity.