Metal detoxification in plants is a complex process that involves different mechanisms, such as the retention of metals to the cell wall and their chelation and subsequent compartmentalization in plant vacuoles. In order to identify the mechanisms involved in metal accumulation and tolerance in Betula pubescens, as well as the role of mycorrhization in these processes, mycorrhizal and non-mycorrhizal plants were grown in two industrial soils with contrasting concentrations of heavy metals.Mycorrhization increased metal uptake at low metal concentrations in the soil and reduced it at high metal concentrations, which led to an enhanced growth and biomass production of the host when growing in the most polluted soil. Our results suggest that the sequestration on the cell wall is the main detoxification mechanism in white birch exposed to acute chronic metal-stress, while phytochelatins play a role mitigating metal toxicity inside the cells. Given its high Mn and Zn root-to-shoot translocation rate, Betula pubescens is a very promising species for the phytoremediation of soils polluted with these metals.

The effects of two Pteris vittata L. accessions and a polycyclic aromatic hydrocarbon (PAH)-degrading bacterium (Alcaligenes sp.) on arsenic (As) uptake and phenanthrene dissipation were studied. The Alcaligenes sp. survived in the rhizosphere and improved soil As bioavailability with co-exposure. However, bacterial inoculation altered Pteris vittata L. stress tolerance, and substantially affected the As distribution in the rhizosphere of the two P. vittata accessions. Bacterial inoculation was beneficial to protect the Guangxi accession against the toxic effects, and significantly increased plant As and phenanthrene removal ratios by 27.8% and 2.89%, respectively. In contrast, As removal was reduced by 29.8% in the Hunan accession, when compared with corresponding non-inoculated treatments. We conclude that plant genotype selection is critically important for successful microorganism-assisted phytoremediation of soil co-contaminated with As and PAHs, and appropriate genotype selection may enhance remediation efficiency.

Phytoremediation of realistic environmental concentrations (10 μg L−1) of the chiral pesticides tebuconazole and imazalil by Phragmites australis was investigated. This study focussed on removal dynamics, enantioselective mechanisms and transformation products (TPs) in both hydroponic growth solutions and plant tissues. For the first time, we documented uptake, translocation and metabolisation of these pesticides inside wetland plants, using enantioselective analysis. Tebuconazole and imazalil removal efficiencies from water reached 96.1% and 99.8%, respectively, by the end of the experiment (day 24). Removal from the solutions could be described by first-order removal kinetics with removal rate constants of 0.14 d−1 for tebuconazole and 0.31 d−1 for imazalil. Removal of the pesticides from the hydroponic solution, plant uptake, within plant translocation and degradation occurred simultaneously. Tebuconazole and imazalil concentrations inside Phragmites peaked at day 10 and 5d, respectively, and decreased thereafter. TPs of tebuconazole i.e., (5-(4-Chlorophenyl)-2,2-dimethyl-3-(1H-1,2,4-triazol-1-ylmethyl)-1,3-pentanediol and 5-(3-((1H-1,2,4-Triazol-1-yl)methyl)-3-hydroxy-4,4-dimethylpentyl)-2-chlorophenol) were quantified in solution, while the imazalil TPs (α-(2,4-Dichlorophenyl)-1H-imidazole-1-ethanol and 3-[1-(2,4-Dichlorophenyl)-2-(1H-imidazol-1-yl)ethoxy]-1,2-propanediol) were quantified in both solution and plant tissue. Pesticide uptake by Phragmites was positively correlated with evapotranspiration. Pesticide removal from the hydroponic solution was not enantioselective. However, tebuconazole was degraded enantioselectively both in the roots and shoots. Imazalil translocation and degradation inside Phragmites were also enantioselective: R-imazalil translocated faster than S-imazalil.

Arbuscular mycorrhizal (AM) fungi inoculation is considered a potential biotechnological tool for an eco-friendly remediation of hazardous contaminants. However, the mechanisms explaining how AM fungi attenuate the phytotoxicity of metal(oid)s, in particular arsenic (As), are still not fully understood. The influence of As on plant growth and the antioxidant system was studied in Leucaena leucocephala plants inoculated with different isolates of AM fungi and exposed to increasing concentrations of As (0, 35, and 75 mg dm⁻³) in a Typic Quartzipsamment soil. The study was conducted under greenhouse conditions using isolates of AM fungi selected from uncontaminated soils (Acaulospora morrowiae, Rhizophagus clarus, Gigaspora albida; and a mixed inoculum derived from combining these isolates, named AMF Mix) as well as a mix of three isolates from an As-contaminated soil (A. morrowiae, R. clarus, and Paraglomus occultum). After 21 weeks, the activities of superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APX), and glutathione reductase (GR) were determined in the shoots in addition to measuring plant height and mineral contents. In general, AM fungi have shown multiple beneficial effects on L. leucocephala growth. Although the activity of most of the stress-related enzymes increased in plants associated with AM fungi, the percentage increase caused by adding As to the soil was even greater for non-mycorrhizal plants when compared to AM-fungi inoculated ones, which highlights the phytoprotective effect provided by the AM symbiosis. The highest P/As ratio observed in AM-fungi plants, compared to non-mycorrhizal ones, can be considered a good indicator that the AM fungi alter the pattern of As(V) uptake from As-contaminated soil. Our results underline the role of AM fungi in increasing the tolerance of L. leucocephala to As stress and emphasize the potential of the symbiosis L. leucocephala-R. clarus for As-phytostabilization at moderately As-contaminated soils.

Understanding the processes that regulate contaminant impacts in nature is an increasingly important challenge. For insecticides in surface waters, the ability of aquatic plants to sorb, or bind, hydrophobic compounds has been identified as a primary mechanism by which toxicity can be mitigated (i.e. the sorption-based model). However, recent research shows that submerged plants can also rapidly mitigate the toxicity of the less hydrophobic insecticide malathion via alkaline hydrolysis (i.e. the hydrolysis-based model) driven by increased water pH resulting from photosynthesis. However, it is still unknown how generalizable these mitigation mechanisms are across the wide variety of insecticides applied today, and whether any general rules can be ascertained about which types of chemicals may be mitigated by each mechanism. We quantified the degree to which the submerged plant Elodea canadensis mitigated acute (48-h) toxicity to Daphnia magna using nine commonly applied insecticides spanning three chemical classes (carbamates: aldicarb, carbaryl, carbofuran; organophosphates: malathion, diazinon, chlorpyrifos; pyrethroids: permethrin, bifenthrin, lambda-cyhalothrin). We found that insecticides possessing either high octanol-water partition coefficients (log Kow) values (i.e. pyrethroids) or high susceptibility to alkaline hydrolysis (i.e. carbamates and malathion) were all mitigated to some degree by E. canadensis, while the plant had no effect on insecticides possessing intermediate log Kow values and low susceptibility to hydrolysis (i.e. chlorpyrifos and diazinon). Our results provide the first general insights into which types of insecticides are likely to be mitigated by different mechanisms based on known chemical properties. We suggest that current models and mitigation strategies would be improved by the consideration of both mitigation models.

Phytate is abundant in soils, which is stable and unavailable for plant uptake. However, it occurs in root exudates of As-hyperaccumulator Pteris vittata (PV). To elucidate its effect on As uptake and growth, P. vittata was examined on agar media (63 μM P) containing 50 μM As and/or 50 or 500 μM phytate with non As-hyperaccumulator Pteris ensiformis (PE) as a congeneric control. Phytate induced efficient As and P uptake, and enhanced growth in PV, but had little effects on PE. The As concentrations in PV fronds and roots were 157 and 31 mg kg−1 in As50+phytate50, 2.2- and 3.1-fold that of As50 treatment. Phosphorus uptake by PV was reduced by 27% in As treatment than the control (P vs. P + As) but increased by 73% comparing phytate500 to phytate500+As, indicating that PV effectively took up P from phytate. Neither As nor phytate affected Fe accumulation in PV, but phytate reduced root Fe concentration in PE (46–56%). As such, the increased As and P and the unsuppressed Fe uptake in PV probably promoted PV growth. Thus, supplying phytate to As-contaminated soils may promote As uptake and growth in PV and its phytoremediation ability.

Selenium (Se) speciation in soil is critically important for understanding the solubility, mobility, bioavailability, and toxicity of Se in the environment. In this study, Se fractionation and chemical speciation in agricultural soils from seleniferous areas were investigated using the elaborate sequential extraction and X-ray absorption near-edge structure (XANES) spectroscopy. The speciation results quantified by XANES technique generally agreed with those obtained by sequential extraction, and the combination of both approaches can reliably characterize Se speciation in soils. Results showed that dominant organic Se (56–81%) and lesser Se(IV) (19–44%) were observed in seleniferous agricultural soils. A significant decrease in the proportion of organic Se to the total Se was found in different types of soil, i.e., paddy soil (81%) > uncultivated soil (69–73%) > upland soil (56–63%), while that of Se(IV) presented an inverse tendency. This suggests that Se speciation in agricultural soils can be significantly influenced by different cropping systems. Organic Se in seleniferous agricultural soils was probably derived from plant litter, which provides a significant insight for phytoremediation in Se-laden ecosystems and biofortification in Se-deficient areas. Furthermore, elevated organic Se in soils could result in higher Se accumulation in crops and further potential chronic Se toxicity to local residents in seleniferous areas.

Environmental pollution by organic pollutants (OPs) has become a global concern due to its detrimental effects on the environment and human health. As plants are used to remediate contaminated sites, understanding the responses of plants to various OPs and fortification of plant tolerance are of great significance. In this work, we studied the biochemical and molecular responses of cucumber plants to three well-known OPs, 2,4,6-trichlorophenol, chlorpyrifos and oxytetracycline in the absence or presence of 24-epibrassinolide (EBR), a potent regulator of plant growth and stress tolerance. The results showed that the selected three OPs retarded root elongation; however, the phytotoxic effects of OPs were attenuated by exogenous EBR. OPs induced accumulations of both hydrogen peroxide (H2O2) and nitric oxide (NO) in root tips and resulted in an increased malondialdehyde (MDA) content, an indicator of membrane lipid peroxidation. Exogenous EBR reduced accumulations of H2O2, NO and MDA in the roots by increasing the expression of antioxidant and detoxification genes and the activities of the corresponding enzymes. Intriguingly, EBR not only promoted the activities of glutathione S-transferase and glutathione reductase, but also increased the content of reduced glutathione without altering the content of oxidized glutathione, which resulted in a reduced redox state under OPs stress. Furthermore, EBR increased the free radical scavenging capacity, flavonoid content and the activity and transcription of secondary metabolism related enzymes. Our results suggest that EBR treatment may fortify secondary metabolism to enhance antioxidant capacity in response to OPs treatment, which might have potential implication in phytoremediation of OPs.

Derelict mines pose potential risks to environmental health. Several factors such as soil structure, organic matter, and nutrient content are the greatly affected qualities in mined soils. Soil microbial communities are an important element for successful reclamation because of their major role in nutrient cycling, plant establishment, geochemical transformations, and soil formation. Yet, microorganisms generally remain an undervalued asset in mined sites. The microbial diversity in derelict mine sites consists of diverse species belonging to four key phyla: Proteobacteria, Acidobacteria, Firmicutes, and Bacteroidetes. The activity of plant symbiotic microorganisms including root-colonizing rhizobacteria and ectomycorrhizal fungi of existing vegetation in the mined sites is very high since most of these microbes are extremophiles. This review outlines the importance of microorganisms to soil health and the rehabilitation of derelict mines and how microbial activity and diversity can be exploited to better plan the soil rehabilitation. Besides highlighting the major breakthroughs in the application of microorganisms for mined site reclamation, we provide a critical view on plant−microbiome interactions to improve revegetation at the mined sites. Also, the need has been emphasized for deciphering the molecular mechanisms of adaptation and resistance of rhizosphere and non-rhizosphere microbes in abandoned mine sites, understanding their role in remediation, and subsequent harnessing of their potential to pave the way in future rehabilitation strategies for mined sites.

Phytoremediation, a technique used to reclaim heavy metal-contaminated soils, requires an understanding of plant physiological responses to heavy metals. However, the majority of studies documenting heavy metal impact on plant functioning have been performed in laboratory or greenhouse settings. We predicted that increased soil heavy metal concentrations reduce photosynthesis and biomass production in trees growing in metal contaminated soil in a naturally re-vegetated urban brownfield. Leaf gas exchange, leaf carbon and nitrogen concentration, and tree biomass were recorded and compared for Populus deltoides and Populus tremuloides growing in an urban brownfield. The CO2 compensation point (CCP) differed significantly between soil metal concentrations and species, with P. deltoides displaying a greater CCP and P. tremuloides displaying a lower CCP as soil metal concentration increased, despite no changes in dark respiration for either species. In terms of biomass, only total branch weight (TBW) and leaf area (LA) differed significantly between soil metal concentrations, though the difference was largely attributable to variation in diameter at breast height (DBH). Furthermore, TBW and LA values for P. deltoides did not decrease with increasing soil metal concentration. Soil metal concentration, thus, had minimal effect on the relationship between tree age and DBH, and no effect on relationships of tree age and height or LA, respectively. Significant differences between soil metal concentrations and species were found for δ15N (isotopic nitrogen ratio) while leaf nitrogen content (% N) also differed significantly between species. Long-term water use efficiency derived from carbon isotope analysis (iWUEisotope) differed significantly between trees grown on different soil metal concentrations and a significant species-metal concentration interaction was detected indicating that the two study species responded differentially to the soil metal concentrations. Specifically, P. tremuloides enhanced while P. deltoides reduced long-term iWUEisotope as soil metal concentration increased, further emphasizing the importance of species and possible genotype selection for phytoremediation.