Genetics and metabolomics analysis of a non-efficient Medicago-rhizobia symbiosis

GONZALO TORRES TEJERIZO; HANNA BEDNARZ; DANIEL WIBBERG; JULIET NILSSON; LUCAS CASTELLANI; MARIANO PISTORIO; ANTONIO LAGARES; ANDREAS SCHLÜTER; ALFRED PÜHLER; KARSTEN NIEHAUS

Budapest

Conferencia; 12th European Nitrogen Fixation Conference; 2016

Alfalfa (Medicago sativa) is themost widely cultivated forage legume for cattle and animal feeding, occupyingabout 32 million hectares over the world. Therefore, management of the N2-fixing symbiosis isimportant to maximize the production of this crop. Symbiosis is a very complexprocess and, particularly, the Medicago-rhizobia symbiosis is highly specific. Ensifer meliloti is the model ofefficient rhizobia able to infect Medicagothat, after several steps, form nodules where they differentiate intobacteroids. These bacteroids are responsible for a successful nitrogen-fixingstage. Nevertheless, not only Ensifer strainsare able to nodulate Medicago;Oregon-like strains are also able to infect alfalfa. Oregon-like rhizobia (e.g. LPU83) are acid tolerant, verycompetitive for the nodulation of alfalfa in acids soils, but inefficient forbiological nitrogen fixation. The nodulesdeveloped by LPU83 harbored only few bacteroids compared to those of plantsinfected by the reference strain E.meliloti 2011 and the senescence zone develops early. This large senescencezone allows a short nitrogen fixation zone (Wegener et al., 2001). Previously, we havepresented a draft genome of LPU83 (Wibberg et al., 2014). In order toelucidate the deficient phenotype of LPU83, genome analyses and metabolomicswere performed. Also in a previous work, we study nodulations factors and nod gene cluster (Torres Tejerizo et al., 2011).Now, we evaluated others genes also involved in the synthesis of symbiosisdeterminants. Comparison of exopolysaccharide synthesis gene cluster show thatwhile in E. meliloti 2011 only onegene cluster is found,  two clusters werefound in LPU83, one in the chromosome and one in plasmid pLPU83a. Some of thegenes were duplicated or not present in the clusters, but elsewhere in thegenome. Deletion of the exo clusterof the plasmid did not modify the nodules development. Lipopolysaccharidesynthesis gene cluster were also compared and the organization among E. meliloti and LPU83 was found moresimilar than with Rhizobium etli or Rhizobium leguminosarum. Moreover, thegenes for nitrogen metabolism enzymes were present in LPU83. Metabolomicsanalyses of the full nodules of E.meliloti 2011 and LPU83 were performed. Striking differences in metaboliteprofiles were obtained, among them dicarboxylic acids and sugar compounds.Accordingly,Oregon-like rhizobia could be used as key reference strains to performcomparatives studies aiming to detect and identify genes that could be involvedin an efficient or inefficient symbiosis.