Heavy Metal Stress and Tolerance in Plants Mediated by Rhizospheric Microbes

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The environmental contamination has become a serious issue in recent times due to human engagements, such as application of pesticides, chemical preservatives, mining, coal combustion, etc. These anthropogenic activities have imposed escalated heavy metal concentrations in water, soil, and air. Specifically, heavy metal pollution of soils causes numerous environmental complications and imparts detrimental effect on living organisms including microbes, plants, and animals. In order to adapt, tolerate, and survive in these adverse situations, plants have evolved with multifaceted molecular and biological mechanisms. Though plants possess many defensive mechanisms to overcome heavy metal intoxication, these strategies of tolerance may not be effective beyond certain limit. Hence, plants will be at the risk of survival. Some of the methods used for removing heavy metals from soil include soil washing with physical or chemical methods; excavation, i.e., the physical elimination from polluted sites; and in situ fixation, the addition of chemicals to stabilize and alter heavy metals to a state that cannot be absorbed by plants. Still, these chemical and physical techniques are not very efficient and the process is expensive. Alternatively, the biological ways of cleaning the contaminated areas have gained more importance in recent times. These approaches include the phytoremediation and the application of rhizospheric microorganisms to clean up the soil. Particularly, rhizospheric microorganisms have the ability to shield the plant from heavy metal stress. To be specific, microbes have the molecular machinery to adopt and can survive even in the existence of toxic levels of heavy metals. In the present chapter, the knowledge of heavy metal toxicity and its remediation using microbes is discussed and the utilization of soil microbes for combating the heavy metal stress in plants is also highlighted.