Plant-associated Microbe System in Treatment of Heavy Metals–contaminated Soil: Mechanisms and Applications

Owing to the ecological applicability, phytoremediation has attracted extensive attention in the remediation of heavy metals–contaminated soil, but the slow plant growth and low remediation efficiency limit its application. Recently, the developed plant-associated microbes has opened up promising areas of research in the field of phytoremediation technology. Various plant growth–promoting rhizobacterias (PGPR) are suggested to be involved in the phytoremediation of heavy metals–contaminated soils, thereby significantly enhancing the removal efficiency of heavy metals. Currently, the published reviews focus on the resistance mechanism of plants and microorganisms to heavy metals, but the function and regulatory machinery of PGPR on phytoremediation have been overlooked. This paper will give a critical review on the processes of PGPR in phytoremediation, including both direct and indirect mechanisms such as (i) the secretion of siderophores, organic acids, biosurfactants, and redox processes and (ii) stimulating plant growth or enhancing their resistance via N fixation, P solubilization, and phytohormone and aminocyclopropane-1-carboxylic acid deaminase synthesis. Moreover, the development of PGPR in phytoremediation is prospected. This work would help readers and researchers better understand the principle and application of PGPR in promoting phytoremediation heavy metals–contaminated soil.