Rhizobial strains isolated from nodules of Medicago marina in southwest Spain are abiotic-stress tolerant and symbiotically diverse

The isolation and characterisation of nitrogen-fixing root nodule bacteria from Medicago marina, a tolerant legume species, were studied in two areas from southwest Spain. A total of 30 out of 82 isolates with distinct ERIC-PCR fingerprints were analysed on the basis of molecular (PCR-RFLP of the 16S-23S rDNA intergenic spacer region (IGS) with two endonucleases, analysis of the 16S rDNA and symbiotic nodC gene sequences, plasmid profiles and SDS-PAGE of LPS, including the partial sequence of the housekeeping gene glnII and the symbiotic gene nodA of some representatives), physiological (utilisation of sole carbon sources, tolerance to antibiotics, NaCl, heavy metals, temperature and pH) and symbiotic parameters (efficacy on M. marina, M. minima, M. murex, M. orbicularis, M. polymorpha, M. sativa and M. truncatula). All the bacteria isolated from M. marina nodules belonged to Ensifer meliloti, except for one strain that belonged to E. medicae. To determine the nodulation range of M. marina, 10 different Ensifer species were tested for their ability to nodulate on this plant. E. kummerowiae CCBAU 71714 and the E. medicae control strain M19.1 were the only Ensifer species tested that developed nitrogen-fixing nodules on this plant. Most of the M. marina-nodulating strains showed tolerance to stress factors and all of them shared the presence of a gene similar to cadA, a gene that encodes for a PIB-type ATPase, which is a transporter belonging to the large superfamily of ATP-driven pumps involved in the transport of metals across cell membranes.