Abstract
Pollution of the biosphere by toxic metals is a global threat that has accelerated dramatically since the beginning of the industrial revolution. The primary source of metal pollution includes industrial operations such as mining, smelting, metal forging, combustion of fossil fuels, and sewage sludge application in agronomic practices. The metals released from these sources accumulate in the soil and, in turn, adversely affect the microbial population density and the physico-chemical properties of soils, leading to the loss of soil fertility and crops yields. The heavy metals can not generally be biologically degraded to more or less toxic products, and hence persist in the environment. Conventional methods used for metal detoxification produce large quantities of toxic products and are cost-effective. The advent of bioremediation has provided an alternative to conventional methods for remediating metal-poisoned soils. In metal-contaminated soils, the natural role of metal-tolerant plant growth promoting rhizobacteria to maintain soil fertility is more important than in conventional agriculture, where greater use of agrochemicals minimizes their significance. Besides their role in metal detoxification and removal, rhizobacteria also promotes the growth of plants by other mechanisms, such as the production of growth-promoting substances and siderophores. Phytoremediation is another emerging low-cost, in situ technology employed to remove pollutants from contaminated soils. The efficiency of phytoremediation can be enhanced by the judicious and careful application of appropriate heavy metal-tolerant, plant growth-promoting rhizobacteria, including symbiotic nitrogen-fixing organisms. This review presents the results of studies on the recent developments in the utilization of plant growth-promoting rhizobacteria for direct application in soils contaminated with heavy metals under a wide range of agroecological conditions, with a view to restoring contaminated soils and, consequently, promoting crop productivity in metal-polluted soils across the globe and their significance in phytoremediation.