Heavy Metal Toxicity to Symbiotic Nitrogen-Fixing Microorganism and Host Legumes

Legume species of the flowering family Fabaceae are well known for their ability to fix atmospheric nitrogen and enhance nitrogen pool of soil, leading to increase in crop especially legumes both in conventional or derelict soils. The interaction between Rhizobia and legumes provides nutrients to plants, increases soil fertility, facilitates plant growth and restores deranged/damaged ecosystem. These characteristics together make legume extremely interesting crop for evaluating the effect of heavy metals. Environmental pollutants like heavy metals at lower concentrations are required for various metabolic activities of microbes including Rhizobia and legume crops. The excessive metal concentrations on the other hand cause undeniable damage to Rhizobia, legumes and their symbiosis. Currently, little is, however, known about how free-living Rhizobia or the legume–Rhizobium symbiosis is affected by varying metal concentration. We focus here that how the nitrogen-fixing root nodule bacteria, the “rhizobia,” increase plant growth and highlight gaps in existing knowledge to understand the mechanistic basis of how different metals affect rhizobia–legume symbiosis which is likely to help to manage legume cultivation in metal contaminated locations.