Microbial-assisted alleviation of chromium toxicity in plants: a critical review

Soil contamination with chromium (Cr) is a serious and burgeoning environmental problem. The infiltration of excess Cr into the food chain causes a number of human health issues, including respiratory disorders, cardiovascular diseases, renal failure, and several types of cancer. The Cr pollution can be contained by different physical, chemical, and biological remediation approaches. Physical and chemical methods are costly and hazardous to the environment as they cause secondary pollution. Biological approaches such as bioremediation that employ plants (phytoremediation) and microbes are eco-friendly, efficient, and cost-effective. Nonetheless, conventional phytoremediation encounters limitations in large-scale applications due to a restricted pool of hyperaccumulator plant species, slow growth rate, limited biomass production, plant-contaminant specificity, and contaminant-mediated oxidative stress in plants. Interestingly, microbes such as bacteria and fungi have the potential to survive and thrive under extreme environmental conditions. Plant growth-promoting bacteria (PGPB) utilize siderophores, organic acids, biosurfactants, redox mechanisms, and biomethylation to convert metals into soluble and bioavailable forms. Further, these bacteria are involved in synthesizing phytohormones and 1-aminocyclopropane-1-carboxylic acid (ACC) deaminase, acquisition of iron, nitrogen fixation, and phosphorus solubilization, which improve plant growth and strengthening eco-physiological resilience, thereby aiding in phytoremediation. This literature review encompasses a breadth of research conducted over the preceding decade, underscoring the contemporary remedial approaches with a primary focus on the crucial role of microbes in facilitating the phytoremediation of Cr. Moreover, this article revealed the underlying and plausible mechanisms involved in the microbe-assisted phytoremediation potential of plants grown under Cr-contaminated soils.