LAUSR

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. 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. 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. 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.