Reconnoitering the Efficacy of Plant Growth Promoting Rhizobacteria in Expediting Phytoremediation Potential of Heavy Metals

The abrupt release of heavy metals (HM) within environment by anthropogenic sources affect the living populations. Plants face many intricacies to survive under such conditions, therefore remediation of HM-contaminated soils is need of the hour. However, chemical processes are very costly and non-sustainable, henceforth HM-microbe associations play an integral role in reducing HM-stress from plants. Plants and microbes within rhizosphere are well adapted to metalliferous environment, thereby prove to be a best assistant for phytoremediation. Root exudates act as nutrients for microbes for establishment of inter-communication systems. Plant–microbe associations is a conducive dimension for phytoremediation, a low input, highly productive, and sustainable technology for maintaining HM-toxicity in soils. It is an emerging technology and is recommended for cleaning up the polluted sites, since plant growth promoting microbes (PGPM) have shown their effectively toward metal toxicity through their detoxification and resistance mechanisms along with growth promoting traits. Microbe-assisted phytoremediation is mainly facilitated either through direct or indirect manner. They synthesize various nutritional and phytohormonal substances namely, growth regulators, siderophores, enzymes, transformation of mineral nutrients (phosphate, potassium, nitrogen etc.). Further, they also modulate the metal detoxification, accumulation, and sequestration abilities of plants through secreting extracellular components, organic acids, biosurfactants, chelators etc. Besides, the metal bioavailability within soil is also modulated via different mechanisms like acidification, precipitation, complexation, or redox reactions. Apart from this, genetic engineering is a progressive approach that is combined with microbe-assisted phytoremediation to attain excellent results. The cumulative knowledge of transgenics, engineering designs, ecological knowledge is an essential element for phytoremediation using genetically engineered microbes. In this review, we have presented the advancement made hitherto for effective understanding of molecular, biochemical, and physiological mechanisms associated with plant–microbe interactions during phytoremediation. Along with this, we have shed light on the mechanisms involved in phytoremediation, therefore, this sustainable technology is widely accepted in reclaiming HM-toxicity to induce the yield and quality of soils and crops.