The legume tribe Genisteae comprises 618, predominantly temperate species, showing an amphi-Atlantic distribution that was caused by several long-distance dispersal events. Seven out of the 16 authenticated rhizobial genera can nodulate particular Genisteae species. Bradyrhizobium predominates among rhizobia nodulating Genisteae legumes. Bradyrhizobium strains that infect Genisteae species belong to both the Bradyrhizobium japonicum and Bradyrhizobium elkanii superclades. In symbiotic gene phylogenies, Genisteae bradyrhizobia are scattered among several distinct clades, comprising strains that originate from phylogenetically distant legumes. This indicates that the capacity for nodulation of Genisteae spp. has evolved independently in various symbiotic gene clades, and that it has not been a long-multi-step process. The exception is Bradyrhizobium Clade II, which unlike other clades comprises strains that are specialized in nodulation of Genisteae, but also Loteae spp. Presumably, Clade II represents an example of long-lasting co-evolution of bradyrhizobial symbionts with their legume hosts.

Aims Ultramafic/serpentine soils constitute a stressful environment with many plant growth constrains such as a lack of macronutrients and high levels of potentially toxic metals. We considered the adaptive strategy of Lotus corniculatus L.-rhizobia symbiosis to Ni, Co and Cr stress conditions. Methods L. corniculatus nodulating rhizobia from ultramafic soil were isolated, identified and tested for nitrogen fixation, metal tolerance and plant growth promoting abilities. The structural and immunocytochemical analyses of root nodules were also performed. Results The isolates effective in nitrogen fixation were identified as Rhizobium and Mesorhizobium tolerant to Ni, Co, and Cr. Some strains directly promoted root growth of L. corniculatus and non-legume Arabidopsis thaliana under metal stress. The metal treated nodules showed structural alternations, i.e. enhanced accumulation of phenols and wall thickening with higher cellulose, hemicellulose, pectins, glycoproteins and callose content. Conclusions Our results revealed that metal tolerant, growth promoting rhizobacteria inhabiting L. corniculatus root nodules may improve plant growth in the ultramafic environment. Accumulation of phenols and reorganization of nodule apoplast can counteract harmful effects of Ni, Co and Cr on the symbiosis. These findings imply that L. corniculatus-rhizobia symbiosis is an important element of plant adaptation to metal stress occurring on the ultramafic soils.

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