Hexavalent Cr, Its Toxicity and Removal Strategy: Revealing PGPB Potential in Its Remediation

Contamination of soil through hexavalent chromium Cr (VI) has increased rapidly during the last few decades. Cr (VI) is a bio-accumulative hazardous metal that can lead to harmful health effects on human being. Several treatment technologies have been used for the treatment of Cr, however, most of them are having some serious limitations. Biological Cr-removal can replace existing physicochemical approaches. The usefulness of phytoremediation in contaminated settings is limited by the sluggish growth rate of plants and low metal absorption. This situation could be mitigated and phytoremediation efficiency can be accelerated by introducing chromium-resistant plant growth-promoting bacterium (PGPB). PGPB inoculation may promote plant growth by producing growth-promoting chemicals and heavy metal remediation by secreting chelating agents, acidification, and redox alterations. Bacterial inoculation improved metal tolerance and absorption through modulating metal transporter, tolerant, and chelator genes. To decrease the harmful impacts brought about by high metal concentrations, PGPB application has shown great potential. The precise molecular mechanism of PGPR-mediated phytoremediation of heavy metals and the stimulation of plant development, however, is little understood. In addition to illuminating the mechanisms underlying plant metal accumulation, this review includes information on the characteristics and mechanisms that PGPB possesses to enhance plant metal tolerance and growth. We evaluated several recent studies of chromium phytoremediation, augmenting the phytoremediation of chromium through PGPB. Examining the possible contribution of bacteria that promote plant growth in microbe-assisted phytoremediation is the goal of this review. The molecular processes by which PGPB strains promote plant development and clean the soil contaminated with chromium are well described in the present review.