The potential use of the legume-rhizobium symbiosis for the remediation of arsenic contaminated sites

The sustainable remediation of arsenic (As) contaminated sites requires an understanding of how As alters the biogeochemical processes in soil. Leguminous species are often used in the remediation of contaminated sites because of their capacity to fix nitrogen and enhance site fertility. While excess As is known to reduce the formation of root nodules in legumes, currently, little is known about how the legume-rhizobium symbiosis is affected by high As concentrations. Soybean (Glycine max) cv. Curringa and its rhizobial symbiont, Bradyrhizobium japonicum strain CB1809, were studied in dilute solution culture at As concentrations of 0, 1, 5 and 10 micromolar. As the As concentration of the nutrient solution increased, greater time was required for inoculated plants to produce root nodules (P=0.001) and the number of root nodules per plant at harvest decreased (P=0.007). Inspection of the soybean roots showed the number of root hairs decreased as the As concentration in the solution increased. The dry weight of soybean roots and shoots decreased significantly as the As concentration of the nutrient solution increased (P<0.05). Inoculated plants had significantly larger dry weights than noninoculated plants (P<0.05) including a 38% greater biomass for inoculated vs. noninoculated plants in the 10 micromolar As treatment. The increased biomass in inoculated plants could not be explained by improved N nutrition nor decreased As absorption and it is hypothesised that B. japonicum stimulated the growth of soybean via the production of growth-promoting hormones. This is the first reported evidence of rhizobial bacteria promoting the growth of plants at elevated concentrations of a heavy metal via a mechanism other than improved nitrogen nutrition. The potential use of rhizobia as growth-promoting bacteria for the remediation of heavy metal contaminated sites is an exciting new area of research.