Mitigation of Cu stress by legume-Rhizobium symbiosis in white lupin and soybean plants.

The effect of Bradyrhizobium-legume symbiosis on plant growth, toxicological variables and Cu bioaccumulation was studied in white lupin and soybean plants treated with 1.6, 48, 96 and 192 μM Cu. In both species, those plants grown in the presence of root nodule-forming symbiotic Bradyrhizobium showed less root and shoot growth reduction, plus greater translocation of Cu to the shoot, than those grown without symbiotic Bradyrhizobium. The effective added concentrations of Cu that reduced shoot and root dry weight by 50% (EC50), and the critical toxic concentration that caused a 10% reduction in plant growth (CTC10%), were higher in plants grown with symbiotic Bradyrhizobium, and were in general higher in the roots whether the plants were grown with or without these bacteria. The production of malondialdehyde and total thiols was stimulated by Cu excess in the shoots and roots of white lupin grown with or without symbiotic Bradyrhizobium, but mainly in those without the symbionts. In contrast, in soybean, the increases in malondialdehyde and total thiols associated with rising Cu concentration were a little higher (1.2-5.0 and 1.0-1.6 times respectively) in plants grown with symbiotic Bradyrhizobium than without. Finally, the organ most sensitive to Cu excess was generally the shoot, both in white lupin and soybean grown with or without symbiotic Bradyrhizobium. Further, Bradyrhizobium-legume symbiosis appears to increase the tolerance to Cu excess in both legumes, but mainly in white lupin; plant growth was less reduced and CTC10% and EC50 values increased compared to plants grown without symbiotic Bradyrhizobium. Bradyrhizobium N2 fixation in both legumes would therefore seem to increase the phytoremediation potential of these plants when growing on Cu-contaminated sites.