Plant Growth-Promoting Rhizobacteria: Role in the Detoxification of Heavy Metals in Plants and Bioremediation of Agricultural Soils

Heavy metal contamination of agricultural soils is a critical environmental challenge because of its persistence and the risks it poses to ecosystems and human health. Heavy metals cannot be degraded, and changes in soil conditions can remobilize stable fractions into labile, bioavailable forms. Plant growth-promoting rhizobacteria (PGPR) are increasingly used as bioinoculants that assist phytoremediation rather than as a stand-alone technology. Depending on the strain, host plant and soil conditions, PGPR can either mobilize metals and enhance their uptake and translocation (phytoextraction) or immobilize them in the rhizosphere and roots, reducing their transfer to aboveground tissues (phytostabilization). This critical review, supported by a structured and reproducible literature search, synthesizes the evidence on (i) plant growth-promoting traits under metal stress; (ii) direct PGPR–metal interactions, including biosorption by extracellular polymeric substances, metallothioneins, organic acids, redox transformation and microbially induced carbonate precipitation; (iii) the regulation of plant antioxidant defenses; and (iv) root exudate-driven, endophyte-assisted and formulation-based strategies. The evidence, dominated by short-term pot experiments with single metals, is critically compared, and priorities for field validation, mixed contamination and regulatory approval are identified.