Bioremediation Potential of Rhizobacteria associated with Plants Under Abiotic Metal Stress

Abstract

Biological remediation is considered as the most effective method of toxic metal removal as these are natural, environmentally friendly, low-cost, and high societal acceptance technologies. One such technology is the use of plant growth promoting microbes for bioremediation of heavy metal polluted soil and is quite important in the context of the global climate change and excessive fertilizer use in agricultural soils. This is feasible because microorganisms have developed many resistance mechanisms for survival in the presence of toxic heavy metals in their environment. Microbes also enhance bioavailability of metals from soil by chelation, acidification, and precipitation as, for example, organic acids released by microbes and plant roots lower the soil pH and help in the sequestration of metal ions. Plant growth promoting rhizobacteria (PGPR) are the microorganism basically present in the rhizosphere, which includes the bacterial species Alcaligenes, Azospirillum, Arthrobacter, Acinetobacter, Azoarcus, Bradyrhizobium, Bacillus, Burkholderia, Enterobacter, Erwinia, Flavobacterium, Pseudomonas, Pantoea, Paenibacillus, Rhizobium Azorhizobium, Bradyrhizobium, Allorhizobium, Sinorhizobium, Methylobacterium, Frankia, and Mesorhizobium. Genetically transformed bacteria possess one or more genes to increase the remediation of heavy metals. Against this background, genes for metal chelators, metal homeostasis, transporters, biodegradative enzymes, metal uptake regulators, and biotic and abiotic stress tolerance are important candidates for making recombinant bacteria. The heavy metals Hg, Cr, As, Zn, Cd, Pb, Ur, Se, Ag, Au, and Ni are hazardous heavy metals. Among heavy metals, four heavy metals As, Pb, Cd, Zn, and Hg are considered as the most toxic metals by the US Agency for Toxic Substances and Disease Registry, based on their toxicity, frequency of occurrence, and most importantly, their exposure potential to flora and fauna. The effectiveness of bioremediation depends on several factors such as the nature of the organisms utilized, the prevailing environmental factors at the contaminated site, as well as the degree of the pollutants in that environment. © 2021 John Wiley & Sons Ltd. All rights reserved.