Escalating anthropogenic activities have caused heavy metal contamination in the environmental matrices. Due to their recalcitrant and toxic nature, their occurrence in high titers in the environment can threaten survival of biotic components. To take the edge off, remediation of metal-contaminated sites by phytoremediators that exhibit a potential to withstand heavy metal stress and quench harmful metals is considered an eco-sustainable approach. Despite the enormous potential, phytoremediation technique suffers a setback owing to high metal concentrations, occurrence of multiple pollutants, low plant biomass, and soil physicochemical status that affect plants at cellular and molecular levels, inducing morphological, physiological, and genetic alterations. Nevertheless, augmentation of soil with microorganisms can alleviate the challenge. A positive nexus between microbes, particularly plant growth-promoting microorganisms (PGPMs), and phytoremediators can prevent phytotoxicity and augment phytoremediation by employing strategies such as production of secondary metabolites, solubilization of phosphate, and synthesis of 1-aminocyclopropane-1-carboxylic acid (ACC) deaminase and phytohormones. Microbes can mediate tolerance in plants by fortifying their antioxidant machinery, which maintains redox homeostasis and alleviates metal-induced oxidative damage in the plants. Associated microbes can also activate stress-responsive genes in plants and abridge metal-induced toxic effects. An in-depth exploration of the mechanisms employed by plant-associated microbes to trigger tolerance in phytoremediators is crucial for improving their phytoremediation potential and real-world applications. The present article attempts to comprehensively review these mechanisms that eventually facilitate the development of improved/new technology for soil ecosystem restoration.

The waste generated during metal mining activities contains mixtures of heavy metals (HM) that are not biodegradable and can accumulate in the surrounding biota, increasing risk to human and environmental health. Plant species with the capacity to grow and develop on mine tailings can be used as a model system in phytoremediation studies. Dodonaea viscosa (L.) Jacq. is a shrub with wide geographical distribution and the ability to establish itself in mine tailings. The Sierra de Huautla Biosphere Reserve in Mexico contains a metallurgic district where mining activities have generated 780 million kg of waste with large concentrations of toxic heavy metals, mainly cadmium and lead. The present study evaluated the phytoremediation potential of D. viscosa in in situ conditions on soils contaminated with HMs (exposed) and reference sites (non-exposed) for one year. Also, the effects of cadmium (Cd) and lead (Pb) exposure in D. viscosa were analyzed via DNA damage (comet assay) morphological and physiological characters in exposed vs non-exposed individuals. The concentration of Cd and Pb was measured through atomic absorption spectrophotometry in the roots and leaves of plants. In total, 120 D. viscosa individuals were established, 60 growing in exposed and 60 in non-exposed soils. Exposed individuals of D. viscosa hyperaccumulated Cd and Pb in roots and leaves. At the end of the experiment, eight out of twelve characters under evaluation decreased significantly in HM-exposed plants in relation to individuals growing in non-exposed soils, except for stomatal index, stomatal coverage, and fresh leaf biomass. The micro-morphological and physiological traits of D. viscosa were not influenced by Cd and Pb bioaccumulation. In contrast, the bioaccumulation of Cd and Pb significantly influenced the macro-morphological characters and genetic damage; this last biomarker was 3.2 times higher in plants growing in exposed sites. The bioconcentration factor (BCF) of Cd and Pb in root and leaf tissue increased significantly over time. The mean BCF in root and leaf tissue was higher for Pb (877.58 and 798.77) than for Cd (50.86 and 23.02). After 12 months of exposure, D. viscosa individuals growing on mine tailing substrate showed that the total HM phytoextraction capacity was 7.56 kg center dot ha-1 for Pb and 0.307 kg center dot ha-1 for Cd. D. viscosa shows potential for phytoremediation of soils contaminated with Cd and Pb, given its capacity for establishing and developing naturally in contaminated soils with HM. Along with its bioaccumulation, biomass production, abundance, and high levels of bioconcentration factors, but without affecting plant development and not registering associated herbivores, it may incorporate HM into the trophic chain.

The increasing demand for mineral resources has generated mine tailings with heavy metals (HM) that negatively impact human and ecosystem health. Therefore, it is necessary to implement strategies that promote the immobilization or elimination of HM, like phytoremediation. However, the toxic effect of metals may affect plant establishment, growth, and fitness, reducing phytoremediation efficiency. Therefore, adding organic amendments to mine tailings, such as biochar, can favor the establishment of plants, reducing the bioavailability of HM and its subsequent incorporation into the food chain. Here, we evaluated HM bioaccumulation, biomass, morphological characters, chlorophyll content, and genotoxic damage in the herbaceous Crotalaria pumila to assess its potential for phytostabilization of HM in mine tailings. The study was carried out for 100 days on plants developed under greenhouse conditions under two treatments (tailing substrate and 75% tailing/25% coconut fiber biochar substrate); every 25 days, 12 plants were selected per treatment. C. pumila registered the following bioaccumulation patterns: Pb > Zn > Cu > Cd in root and in leaf tissues. Furthermore, the results showed that individuals that grew on mine tailing substrate bioaccumulated many times more metals (Zn: 2.1, Cu: 1.8, Cd: 5.0, Pb: 3.0) and showed higher genetic damage levels (1.5 times higher) compared to individuals grown on mine tailing substrate with biochar. In contrast, individuals grown on mine tailing substrate with biochar documented higher chlorophyll a and b content (1.1 times more, for both), as well as higher biomass (1.5 times more). Therefore, adding coconut fiber biochar to mine tailing has a positive effect on the establishment and development of C. pumila individuals with the potential to phytoextract and phytostabilize HM from polluted soils. Our results suggest that the binomial hyperaccumulator plant in combination with this particular biochar is an excellent system to phytostabilize soils contaminated with HM.

Lead (Pb) is a common toxic heavy metal element that can be absorbed by plant roots and enter the food chain, damaging human health. Alnus cremastogyne has a wide native range, is fast growing, has a wide range of timber uses, and has rhizomatous roots that can improve planted soils. In this study, we evaluated whether Alnus cremastogyne has the potential to remediate Pb-contaminated soils through a 6-month pot experiment in soils with different Pb concentrations (0, 50, 100, 200, and 400 mg/kg). Our results indicate that Alnus cremastogyne plant height, basal diameter, and organ biomass decreased, H2O2 and MDA content increased, and the activities of antioxidant enzymes and osmotic regulators increased and then decreased with increasing lead concentrations. The Pb bioconcentration factor of Alnus cremastogyne was less than 1 at all Pb concentrations, and Pb accumulated mainly in the root system. This indicates that Alnus cremastogyne is not a Pb-enriched plant and does not have outstanding Pb transport capacity. The growth of Alnus cremastogyne was not significantly affected at low Pb concentrations, and its plant height, basal diameter, and biomass were significantly suppressed under high Pb stress. Therefore, Alnus cremastogyne is not a suitable species for the remediation of lead-contaminated soils but can be used as a silvicultural species in environments with low lead levels.