Soil metal contamination is a major concern due to the ever-rising number of areas afflicted worldwide and the detrimental effects of metals to the environment and human health. Due to their non-biodegradability and toxicity, it is paramount to prevent further metal contamination and remediate the thousands of contaminated sites across the planet. Yet, conventional reclamation based on physical and chemical methods is often expensive, impractical, and triggers secondary pollution issues. Hence, microbe-aided phytoremediation has been gaining significant traction due to its environment-friendly character, cost-effectiveness, and the breakthroughs achieved during the past few decades. Microorganisms are an essential part of natural ecosystems and play a crucial role in their restoration. Indeed, plant-microbe associations in metal-polluted soils are pivotal for plants to tolerate metal toxicity and thrive in these harsh environments. Therefore, improving the understanding of this intricate relationship is invaluable for boosting phytoremediation. In this review, we focus on the potential of plant growth promoting bacteria (PGPB) for enhancing phytoremediation of metal-polluted soils. We discuss the mechanisms employed by microbes to promote plant growth and assist the removal or immobilization of metals in soil, thereby enhancing phytoextraction and phytostabilization, respectively. Microbe-mediated metal removal and detoxification through processes entailing adsorption, chelation, transformation, and precipitation, to list but a few, are also critically examined. Moreover, this work covers the direct and indirect mechanisms used by PGPB to facilitate plant acquisition of nutrients like nitrogen and phosphorus, supply and regulate phytohormones, and exert control over antagonistic microorganisms. Lastly, we provide an outlook on the future directions of microbe-aided phytoremediation and phytomining. Clearly, to fully validate and comprehend the potential of PGPB-aided phytoremediation, a considerable shift from bench-scale to field research is necessary. What's more, it is envisaged that recent advancements in genetic engineering may soon help furthering the efficiency of microbe-assisted phytoremediation.

The arrival of sequence-specific endonucleases that allow genome editing has shaken the pillars of basic and applied plant biology. Clustered regularly interspaced palindromic repeats (CRISPR) is a revolutionary genome-engineering tool that enables the enhancement of targeted traits in plants. Numerous plants, including energy crops, known for their potential to tolerate, immobilize, and stabilize inorganic and organic pollutants, have already been edited using different CRISPR systems. Moreover, a large array of genes responsible for increased metal tolerance, metal uptake and hyperaccumulation have already been identified. Thus, the CRISPR-mediated genome reprogramming of plants, including its use in gene expression regulation through transcriptional repression or activation (CRISPRi and CRISPRa), could be of paramount importance for phytoremediation. The simplicity, inexpensiveness, and capabilities of this gene editing technique could soon be used to enhance plants and bacteria involved in phytotechnologies, such as phystabilization, phytoextraction, phytomining, phytovolatilization, and bio-energy generation. In this brief viewpoint piece, we posit some of the potential benefits of CRISPR for phytoremediation.

Bioremediation of mining soils using metal tolerant fungi is widely considered as a promising cost-effective and ecofriendly approach. This study assessed the copper removal efficiency and bioaccumulation ability of the indigenous species Aspergillus hiratsukae LF1 and Aspergillus terreus LF2 isolated from the soils of an abandoned copper mine in Oman. Nutrient medium containing five different Cu (II) levels (0 - control, 100, 200, 300 and 500 mg/L) was employed for assessing both parameters. The removal efficiency from nutrient medium (100-500 mg Cu per L) ranged from 57% to 21% for A. hiratsukae LF1, and from 69% to 24% for A. terreus LF2. A. hiratsukae LF1 and A. terreus LF2 accumulated a maximum of 4.63 and 5.95 mg Cu/g,espectively, at 500 mg/L of Cu (II) concentration. The compositional analysis of extracellular polymeric substances excreted by both species revealed a hormetic response by A. hiratsukae LF1 at 100 mg/L; whereas increasing media Cu levels induced carbohydrates production in A. terreus LF2. These results hint at the involvement of carbohydrates in the Cu-tolerance mechanism of the latter. Copper accumulation in both species was further demonstrated through scanning electron microscopy and energy dispersive spectrometry. In line with the pertaining literature, our results are somewhat inconclusive concerning whether proteins or carbohydrates play a more pivotal role in copper complexation in both species; yet, FTIR analysis showed the participation of different functional groups in Cu sorption. Overall, although additional research is required to advance the knowledge about both Aspergillus species, our findings suggest that A. terreus LF2 presents greater promise for copper bioremediation due to enhanced tolerance and accumulation capacity.

Research highlights: (1) Ecosystem development assessed in an afforested post-mining site. (2) Soil organic carbon (SOC) and total nitrogen (TN) stock reached close to the reference forest site after 25 years of afforestation. (3) Integrated mine soil quality index is developed to assess the reclamation success. Background and Objectives: Estimation of the mine soil quality is one of the most important criterions for evaluating the reclamation success and restoration of novel ecosystems of the post-industrial degraded lands. The aim of this long-term experiment was to investigate the influence of revegetation on Technosol (defined as anthropogenic soil resulted from reclamation of mine spoil materials) as the basic ecosystem development. Materials and Methods: A field study was carried out in the chronosequence afforested post-mining sites (5, 10, 25 years) and compared with natural forest site. We assessed the physicochemical properties and nutrient stock of mine soil and estimated general mine soil quality by using an integrated mine soil quality index (IMSQI). The studies were fully randomized in the chronosequence of afforested post-mining sites. Results: Nutrient dynamics and soil properties (physicochemical and biological) were recovered with the increase age of reclamation. Soil organic carbon (SOC) stock significantly increased from 9.11 Mg C ha⁻¹ in 5 years to 41.37 Mg C ha⁻¹ after 25 years of afforestation. Likewise, total nitrogen (TN) stock significantly increased from 1.06 Mg N ha⁻¹ in 5 years to 4.45 Mg N ha⁻¹ after 25 years of revegetation. Ecosystem carbon pool enhanced at a rate of 6.2 Mg C ha⁻¹ year⁻¹. A Principal Component Analysis (PCA)-based IMSQ index was employed to assess the reclamation success. The most influential properties controlling the health of reclaimed coal mine soil are fine earth fraction, moisture content, SOC and dehydrogenase activity. IMSQ index values are validated with vegetation characteristics. The estimated IMSQI ranged from 0.455 in 5-year-old (RMS5) to 0.746 in 25-year-old reclaimed dump (RMS25). Conclusions: A 25-year-old reclaimed dump having greater IMSQI (0.746) than reference forest soils (0.695) suggested the aptness of revegetation to retrieve soil quality and function in derelict mine land.