*4.2.4 Metal stress*

Heavy metals occur naturally in soils; however, their increased quantity is undesirable and has become a global concern over the time [150]. Anthropogenic activities like atmospheric pollution, industrial waste disposal, mining, and other practices predominantly contribute to heavy metal toxicity [151]. Heavy metal toxicity leads to inhibition of chlorophyll biosynthesis and proteins required for proper growth of plants and their normal functioning. Plant growth promoting rhizobacteria have the ability to control heavy metal pollution of soils as well as enhancing plant growth in these soils [152]. Bacteria's producing siderophores promote plant growth besides enhancing their nutrient uptake potential under heavy metal stress conditions. Rhizobacteria have been found to release metal-chelating substances (siderophores) in rhizosphere by means of which they affect the bioavailability of toxic heavy metals and their uptake by plants significantly. They transform these compounds into a less toxic form and promote their precipitation, absorption or adsorption. Plant associated rhizobia can be used for bioremediation, as they enhance the phytoextraction and phytostablization potential of plants [153]. By phytoextraction, plants carry the contaminants from the soil with the help of their roots and eventually collect these contaminants in the aboveground parts of the plant [154]. Phyto-stablization on the other hand, immobilizes the soil contaminants. The contaminants either get adsorbed on the root surface or absorbed by the roots or they are transformed into less soluble compounds. Phytoremediation has been accelerated by the application of rhizobacterial species such as *Bacillus*, *Pseudomonas, Azotobacter* [155]. Thus, efficient bioremediation is possible by using rhizobacterial inoculants, still distribution and functioning of microbes in rhizosphere needs to be fully explored.
