Metal-Legume-Microbe Interactions: Toxicity and Remediation

Heavy metals discharged from various sources accumulate within soils and disrupt ecosystems. The toxic metals are taken up by beneficial soil microbiota and growing plants and cause potential human risks via food chain. Also, heavy metals seriously affect the microbial compositions and their physiological functions. Among plant species, legumes play an important role in human dietary systems and supply nitrogen to legumes through symbiosis with rhizobia. Metals when present in legume habitat act as a devastating stress factor and restrict the growth of rhizobia, legumes, and legume-Rhizobium symbiosis. Several physical and chemical methods have been developed to remediate heavy metal-polluted soils, but these methods are unacceptable due to their high cost, and they are not environmentally friendly. Therefore, the use of metal-tolerant/metal-detoxifying microbes collectively called bioremediation offers a sustainable and low-cost option to clean up polluted soils. Besides remediation, the metal-tolerant microbes also promote plant growth by other direct or indirect means. Owing to the importance of legumes in maintaining soil fertility and human health, there is greater emphasis to identify the metal-resistant/metal-tolerant rhizobia and legume plants. The present chapter gives an in-depth insight into the impact of metals on rhizobia-legume symbiosis. Also, the role of metal-tolerant rhizobia in metal toxicity abatement is highlighted.