Exploiting Nitrogen-Fixing Rhizobial Symbionts Genetic Resources for Improving Phytoremediation of Contaminated Soils

Rhizobia are one of the most relevant components of the plant-associated microbiota. They are found both in soil and associated as commensals or symbionts with several plant taxa. In particular, with leguminous plants, they establish a symbiotic association, which allow the bacteria to express the enzyme nitrogenase responsible for the reduction of atmospheric dinitrogen. Consequently, rhizobia allow host plants to colonize marginal lands and nitrogen-deficient soils, for instance, contaminated soils. The use of legume-rhizobial symbiosis for phytoremediation would allow increasing plant coverage (then phytostabilization) of contaminated areas, without the need of expensive nitrogen fertilization of the soil. Moreover, among host legumes, both pioneer plants (of for instance degraded lands) and crops (as alfalfa) are present, which allow an easy implementation of agronomical practices. Finally, the large genomic and phenotypic diversity of rhizobia allows the selection of elite strains resistant to harsh soil conditions and the creation of potentially new strains with the desired features for assisting legume-based phytoremediation.