Fungi-assisted phytoextraction of lead: tolerance, plant growth-promoting activities and phytoavailability

An assessment of nanotechnology-based interventions for cleaning up toxic heavy metal/metalloid-contaminated agroecosystems: Potentials and issues

Heavy metals (HMs) are among the most dangerous environmental variables for a variety of life forms, including crops. Accumulation of HMs in consumables and their subsequent transmission to the food web are serious concerns for scientific communities and policy makers. The function of essential plant cellular macromolecules is substantially hampered by HMs, which eventually have a detrimental effect on agricultural yield. Among these HMs, three were considered, i.e., arsenic, cadmium, and chromium, in this review, from agro-ecosystem perspective. Compared with conventional plant growth regulators, the use of nanoparticles (NPs) is a relatively recent, successful, and promising method among the many methods employed to address or alleviate the toxicity of HMs. The ability of NPs to reduce HM mobility in soil, reduce HM availability, enhance the ability of the apoplastic barrier to prevent HM translocation inside the plant, strengthen the plant's antioxidant system by significantly enhancing the activities of many enzymatic and nonenzymatic antioxidants, and increase the generation of specialized metabolites together support the effectiveness of NPs as stress relievers. In this review article, to assess the efficacy of various NP types in ameliorating HM toxicity in plants, we adopted a 'fusion approach', in which a machine learning-based analysis was used to systematically highlight current research trends based on which an extensive literature survey is planned. A holistic assessment of HMs and NMs was subsequently carried out to highlight the future course of action(s).

Inhibitory activity of bacterial lipopeptides against Fusarium oxysporum f.sp. Strigae

This study investigated the influence of bacterial cyclic lipopeptides (LP; surfactins, iturins, fengycins) on microbial interactions. The objective was to investigate whether the presence of bacteria inhibits fungal growth and whether this inhibition is due to the release of bacterial metabolites, particularly LP. Selected endophytic bacterial strains with known plant-growth promoting potential were cultured in the presence of Fusarium oxysporum f.sp. strigae (Fos), which was applied as model fungal organism. The extracellular metabolome of tested bacteria, with a focus on LP, was characterized, and the inhibitory effect of bacterial LP on fungal growth was investigated. The results showed that Bacillus velezensis GB03 and FZB42, as well as B. subtilis BSn5 exhibited the strongest antagonism against Fos. Paraburkholderia phytofirmans PsJN, on the other hand, tended to have a slight, though non-significant growth promotion effect. Crude LP from strains GB03 and FZB42 had the strongest inhibitory effect on Fos, with a significant inhibition of spore germination and damage of the hyphal structure. Liquid chromatography tandem mass spectrometry revealed the production of several variants of iturin, fengycin, and surfactin LP families from strains GB03, FZB42, and BSn5, with varying intensity. Using plate cultures, bacillomycin D fractions were detected in higher abundance in strains GB03, FZB42, and BSn5 in the presence of Fos. Additionally, the presence of Fos in dual plate culture triggered an increase in bacillomycin D production from the Bacillus strains. The study demonstrated the potent antagonistic effect of certain Bacillus strains (i.e., GB03, FZB42, BSn5) on Fos development. Our findings emphasize the crucial role of microbial interactions in shaping the co-existence of microbial assemblages.

Environmental cadmium inhibits testicular testosterone synthesis via Parkin-dependent MFN1 degradation

Low testosterone (T) levels are associated with many common diseases, such as obesity, male infertility, depression, and cardiovascular disease. It is well known that environmental cadmium (Cd) exposure can induce T decline, but the exact mechanism remains unclear. We established a murine model in which Cd exposure induced testicular T decline. Based on the model, we found Cd caused mitochondrial fusion disorder and Parkin mitochondrial translocation in mouse testes. MFN1 overexpression confirmed that MFN1-dependent mitochondrial fusion disorder mediated the Cd-induced T synthesis suppression in Leydig cells. Further data confirmed Cd induced the decrease of MFN1 protein by increasing ubiquitin degradation. Testicular specific Parkin knockdown confirmed Cd induced the ubiquitin-dependent degradation of MFN1 protein through promoting Parkin mitochondrial translocation in mouse testes. Expectedly, testicular specific Parkin knockdown also mitigated testicular T decline. Mito-TEMPO, a targeted inhibitor for mitochondrial reactive oxygen species (mtROS), alleviated Cd-caused Parkin mitochondrial translocation and mitochondrial fusion disorder. As above, Parkin mitochondrial translocation induced mitochondrial fusion disorder and the following T synthesis repression in Cd-exposed Leydig cells. Collectively, our study elucidates a novel mechanism through which Cd induces T decline and provides a new treatment strategy for patients with androgen disorders.

Ecologically different earthworm species are the driving force of microbial hotspots influencing Pb uptake by the leafy vegetable Brassica campestris

Food chain contamination by soil lead (Pb), beginning with Pb uptake by leafy vegetables, is a threat to food safety and poses a potential risk to human health. This study highlights the importance of two ecologically different earthworm species (the anecic species Amynthas aspergillum and the epigeic species Eisenia fetida) as the driving force of microbial hotspots to enhance Pb accumulation in the leafy vegetable Brassica campestris at different Pb contamination levels (0, 100, 500, and 1,000 mg·kg-1). The fingerprints of phospholipid fatty acids (PLFAs) were employed to reveal the microbial mechanism of Pb accumulation involving earthworm-plant interaction, as PLFAs provide a general profile of soil microbial biomass and community structure. The results showed that Gram-positive (G+) bacteria dominated the microbial community. At 0 mg·kg-1 Pb, the presence of earthworms significantly reduced the total PLFAs. The maximum total of PLFAs was found at 100 mg·kg-1 Pb with E. fetida inoculation. A significant shift in the bacterial community was observed in the treatments with E. fetida inoculation at 500 and 1,000 mg·kg-1 Pb, where the G+/G- bacteria ratio was significantly decreased compared to no earthworm inoculation. Principal component analysis (PCA) showed that E. fetida had a greater effect on soil microbial hotspots than A. aspergillum, thus having a greater effect on the Pb uptake by B. campestris. Redundancy analysis (RDA) showed that soil microbial biomass and structure explained 43.0% (R2 = 0.53) of the total variation in Pb uptake by B. campestris, compared to 9.51% of microbial activity. G- bacteria explained 23.2% of the total variation in the Pb uptake by B. campestris, significantly higher than the other microbes. The Mantel test showed that microbial properties significantly influenced Pb uptake by B. campestris under the driving force of earthworms. E. fetida inoculation was favorable for the G- bacterial community, whereas A. aspergillum inoculation was favorable for the fungal community. Both microbial communities facilitated the entry of Pb into the vegetable food chain system. This study delivers novel evidence and meaningful insights into how earthworms prime the microbial mechanism of Pb uptake by leafy vegetables by influencing soil microbial biomass and community composition. Comprehensive metagenomics analysis can be employed in future studies to identify the microbial strains promoting Pb migration and develop effective strategies to mitigate Pb contamination in food chains.

Warhorses in soil bioremediation: Seed biopriming with PGPF secretome to phytostimulate crop health under heavy metal stress

The fungal symbiosis with the plant root system is importantly recognized as a plant growth promoting fungi (PGPFs), as well as elicitor of plant defence against different biotic and abiotic stress conditions. Thus PGPFs are playing as a key trouper in enhancing agricultural quality and increased crop production and paving a way towards a sustainable agriculture. Due to increased demand of food production, the over and unscientific usage of chemical fertilizers has led to the contamination of soil by organic and inorganic wastes impacting on soil quality, crops quality effecting on export business of agricultural products. The application of microbial based consortium like plant growth promoting fungi is gaining worldwide importance due to their multidimensional activity. These activities are through plant growth promotion, induction of systemic resistance, disease combating and detoxification of organic and inorganic toxic chemicals, a heavy metal tolerance ability. The master key behind these properties exhibited by PGPFs are attributed towards various secretory biomolecules (secondary metabolites or enzymes or metabolites) secreted by the fungi during interaction mechanism. The present review is focused on the multidimensional role PGPFs as elicitors of Induced systemic resistance against phytopathogens as well as heavy metal detoxifier through seed biopriming and biofortification methods. The in-sights on PGPFs and their probable mechanistic nature contributing towards plants to withstand heavy metal stress and stress alleviation by activating of various stress regulatory pathways leading to secretion of low molecular weight compounds like organic compounds, glomalin, hydrophobins, etc,. Thus projecting the importance of PGPFs and further requirement of research in developing PGPFs based molecules and combining with trending Nano technological approaches for enhanced heavy metal stress alleviations in plant and soil as well as establishing a sustainable agriculture.

Aspergillus niger-mediated release of phosphates from fish bone char reduces Pb phytoavailability in Pb-acid batteries polluted soil, and accumulation in fenugreek

Soil receiving discharges from Pb-acid batteries dismantling and restoring units (PBS) can have a high concentration of phytoavailable Pb. Reducing Pb phytoavailability in PBS can decline Pb uptake in food crops and minimize the risks to humans and the environment. This pot study aimed to reduce the concentration of phytoavailable Pb in PBS through Aspergillus niger (A. niger)-mediated release of PO₄^3- from fish bone [Apatite II (APII)] products. The PBS (Pb = 639 mg kg^-1 soil) was amended with APII powder (APII-P), APII char (APII-C), and A. niger inoculum as separate doses, and combining A. niger with APII-P (APII-P + A. niger) and APII-C (APII-C + A. niger). The effects of these treatments on reducing the phytoavailability of Pb in PBS and its uptake in fenugreek were examined. Additionally, enzymatic activities and microbial biomass carbon (MBC) in the PBS and the indices of plant physiology, nutrition, and antioxidant defense machinery were scoped. Results revealed that the APII-C + A. niger treatment was the most efficient one. Compared to the control, it significantly reduced the Pb phytoavailability (DTPA-extractable Pb fraction) in soil and its uptake in plant shoots, roots, and grain, up to 61%, 83%, 74%, and 92%. The grain produced under APII-C + A. niger were safe for human consumption as Pb concentration in grain was 4.01 mg kg^-1 DW, remaining within the permissible limit set by WHO/FAO (2007). The APII-C + A. niger treatment also improved soil pH, EC, CEC, MBC, available P content and enzymatic activities, and the fenugreek quality parameters. A. niger played a significant role in solubilizing PO₄^3- from APII-C, which reacted with Pb and formed insoluble Pb-phosphates, thereby reducing Pb phytoavailability in PBS and its uptake in plants. This study suggests APII-C + A. niger can remediate Pb-polluted soils via reducing Pb phytoavailability in them.

Liquid Organic Fertilizer Amendment Alters Rhizosphere Microbial Community Structure and Co-occurrence Patterns and Improves Sunflower Yield Under Salinity-Alkalinity Stress

Response of rhizosphere microbial community structure and co-occurrence patterns to liquid organic fertilizer in sunflower cropland was investigated. Moderate and severe saline-alkaline soils were treated with liquid organic fertilizer containing mainly small molecular organic compounds (450 g L^-1) at a rate of 4500 L ha^-1 year^-1 over 2 years. Compared with the untreated soils, organic fertilizer treatment increased soil nutrient concentrations by 13.8-137.1% while reducing soil pH and salinity by 5.6% and 54.7%, respectively. Organic fertilizer treatment also improved sunflower yield, plant number, and plant height by 28.6-67.3%. Following organic fertilizer treatment, fungal α-diversity was increased, and the effects of salinity-alkalinity stress on rhizosphere microbial communities were alleviated. The relative abundances of some halotolerant microbes and phytopathogenic fungi were reduced in organic fertilizer-treated soils, in contrast to increases in the relative abundances of plant growth-promoting microbes and organic matter decomposers, such as Nocardioides, Rhizophagus, and Stachybotrys. Network analysis revealed that severe salinity-alkalinity stress stimulated cooperation among bacteria, while organic fertilizer treatment tended to stimulate the ecosystem functions of fungi with higher proportions of fungi-bacteria and fungi-fungi links. More keystone taxa (e.g., Amycolatopsis, Variovorax, and Gemmatimonas) were positively correlated with soil nutrient concentrations and crop yield-related traits in organic fertilizer-treated soils. Overall, liquid organic fertilizer amendment could attenuate the adverse effects of salinity-alkalinity stress on sunflower yield by improving soil quality and optimizing rhizosphere microbial community structure and co-occurrence patterns.

Cadmium Phytotoxicity, Tolerance, and Advanced Remediation Approaches in Agricultural Soils; A Comprehensive Review

Cadmium (Cd) is a major environmental contaminant due to its widespread industrial use. Cd contamination of soil and water is rather classical but has emerged as a recent problem. Cd toxicity causes a range of damages to plants ranging from germination to yield suppression. Plant physiological functions, i.e., water interactions, essential mineral uptake, and photosynthesis, are also harmed by Cd. Plants have also shown metabolic changes because of Cd exposure either as direct impact on enzymes or other metabolites, or because of its propensity to produce reactive oxygen species, which can induce oxidative stress. In recent years, there has been increased interest in the potential of plants with ability to accumulate or stabilize Cd compounds for bioremediation of Cd pollution. Here, we critically review the chemistry of Cd and its dynamics in soil and the rhizosphere, toxic effects on plant growth, and yield formation. To conserve the environment and resources, chemical/biological remediation processes for Cd and their efficacy have been summarized in this review. Modulation of plant growth regulators such as cytokinins, ethylene, gibberellins, auxins, abscisic acid, polyamines, jasmonic acid, brassinosteroids, and nitric oxide has been highlighted. Development of plant genotypes with restricted Cd uptake and reduced accumulation in edible portions by conventional and marker-assisted breeding are also presented. In this regard, use of molecular techniques including identification of QTLs, CRISPR/Cas9, and functional genomics to enhance the adverse impacts of Cd in plants may be quite helpful. The review's results should aid in the development of novel and suitable solutions for limiting Cd bioavailability and toxicity, as well as the long-term management of Cd-polluted soils, therefore reducing environmental and human health hazards.

Challenges in microbially and chelate-assisted phytoextraction of cadmium and lead - A review

Cadmium (Cd) and lead (Pb) are ubiquitously present in surface soils, due to anthropogenic activities, causing threat to ecological and human health because of their carcinogenic nature. They accumulate in large quantities in the environment and affect negatively soil microbiota, plants, animals, and humans. For the cleanup of Cd/Pb polluted soils, different plant species have been studied. Many plants have shown the potential to hyperaccumulate Cd/Pb in their above-ground tissues. These plants decrease soil pH by root exudation or by releasing H⁺ ions, and this, in turn, increases the bioavailability of Cd/Pb for plant uptake. Different environmental processes related to soil organic matter, microorganisms, pH, genetic modifications, and various soil-borne chelating agents affect the potential of phytoremediation technology. Review papers trying to identify a single factor influencing the phytoremediation of heavy metals are available in the literature. However, an integrated approach dealing with different factors involved in the remediation of both metals is scarcely discussed. The main focus of this review is to discuss the phytoextraction technique for Cd/Pb removal from contaminated sites along with detoxification mechanisms. Further, the challenges in the Cd/Pb phytoextraction and different options available to cope with these challenges are also discussed. The update on the relevant findings on the use of microorganisms and amendments in enhancing the Cd/Pb phytoextraction is also provided. Finally, the areas to be explored in future research for the removal of Cd/Pb by integrated strategies have been discussed.

Optimization of integrated phytoremediation system (IPS) for enhanced lead removal and restoration of soil microbial activities

Improving phytoremediation efficiency in lead (Pb) contaminated soil through either bacterial or fungal inoculants have extensively been studied with different successes and limitations. In this study, co-application of bacteria and fungi have been investigated for development of an integrated phytoremediation system (IPS) for efficient Pb removal and restoration of soil microbial and enzymatic activities in degraded soil. For this purpose, Pb tolerant bacterial and fungal strains were firstly analyzed for antifungal and antibacterial activities through disc diffusion method. Afterwards, the co-inoculation studies were performed to investigate the effects on phytoavailability and uptake of Pb by Pelargonium hortorum through soil incubation and pot culture experiments, respectively. Results indicated significant (p < 0.05) antibacterial activity of Mucor spp. against bacterial species (Klebsiella variicola and K. quasipneumoniae). The highest significant increase in extractable Pb fraction (5.0-folds) was observed when soil was co-inoculated with Aspergillus flavus + Microbacterium paraoxydans compared to the control soil (un-inoculated soil) at 2000 mg Pb kg^-1 concentration. Similarly, uptake results also indicated significantly higher Pb uptake in plants inoculated with A. flavus + M. paraoxydans. Soil microbial results indicated significant decrease in microbial health indicators and enzymatic activities with increasing Pb concentration and exposure time, as compared to control soil. A relatively severe decline was observed in soil respiration and dehydrogenase (DEH) activities by 2.8- and 2.5-folds, respectively at 2000 mg Pb kg^-1 of soil. The optimized IPS was effective for restoring enzymatic activities in Pb contaminated soil and could be applied for sustainable restoration of Pb contaminated soil.

Lead availability and phytoextraction in the rhizosphere of Pelargonium species

Availability of lead (Pb) in soil is a major factor controlling the phytoremediation efficiency of plants. This study was focused on investigating the plant-induced changes in rhizosphere and corresponding effect on bioavailable fraction of Pb and accumulation in different plant parts. For rhizosphere study, special cropping device was designed locally. Two Pb accumulator plants Stigmatocarpum criniflorum (L. f.) L. Bolus and Pelargonium × hortorum L.H. Bailey were grown in cropping device setup containing Pb spiked soil (500, 1000, 1500, and 2000 mg kg^-1) for a period of 3 weeks. Further plants were also analyzed for Pb-induced oxidative stress. The results indicated higher ability of soil adjustment for Pb uptake by P. hortorum. The soil pH was (p < 0.05) decreased (ΔpH = - 0.22 pH), and dissolved organic carbon (DOC) content was significantly increased (by 1.7-fold) in rhizosphere of P. hortorum. The bioavailable fraction of Pb was twofold higher in rhizosphere of P. hortorum than S. criniflorum at the same soil Pb concentration (2000 mg kg^-1). Maximum Pb concentration in root and shoot of S. criniflorum was 755 ± 99 and 207 ± 12 mg Pb/kg DW and for P. hortorum was 1281 ± 77 and 275 ± 7 mg Pb/kg DW. P. hortorum uptakes more Pb per plant by threefold compared with S. criniflorum. The oxidative stress results indicated higher Pb tolerance and suitability of P. hortorum for phytoextraction of Pb-contaminated soil.

Securing of an Industrial Soil Using Turfgrass Assisted by Biostimulants and Compost Amendment

This work aimed to study the effects of compost (applied at two rates) and two commercial microbial biostimulants on the mobility and bioavailability of potentially toxic elements (PTEs) in an industrial soil phytostabilized by Dactylis glomerata L. or a mixed stand of grasses (Lolium perenne L., Poa pratensis L. and Festuca arundinacea Shreb.). The soil showed very high pseudototal and bioavailable concentrations of cadmium (Cd) and lead (Pb), due to improper lead-acid batteries storage. Compost amendment in combination with the two biostimulants produced the best outcomes in terms of plant growth and nutrient uptake. The same mix of beneficial microbes improved soil biological fertility enhancing soil nitrogen fixing and ammonia oxidizing bacteria, while reduced the pore water and NH4NO3 extractable concentrations of Cd and at lower extent of Pb in soil. Accordingly, the lower mobility and bioavailability of Cd in soil determined a lower uptake and accumulation of Cd in shoots of different grass species. Our results suggest that a green cap with turfgrass assisted by biostimulants and compost amendment in PTE-contaminated industrial sites could be a reliable and effective practice to protect and restore soil biological fertility and to reduce the risk of PTE dispersion in the surrounding environment.

Lead tolerant endophyte Trametes hirsuta improved the growth and lead accumulation in the vegetative parts of Triticum aestivum L

Rapid industrialization and increasing population are continuously adding contaminants to our environment. Among those, heavy metals are considered to be one of the serious threats to the ecosystem due to their persistent nature. Microbe assisted phytoremediation is an effective tool for metal remediation as microbes enhance the metal availability and uptake to the host plants or reduce it by binding them intracellularly or extracellularly. An endophytic fungus, Trametes hirsuta, was isolated from Chenopodium album L. plant growing in the lead (Pb) contaminated soil of an industrial area. This is the first study citing Trametes hirsuta, as a root endophyte of Chenopodium album L. This endophytic fungus was found to be tolerant to high concentration of Pb i.e., 1500 mg L^-1, when tested in-vitro. Wheat (Triticum aestivum L.) seedlings were infected by Trametes hirsuta and Pb tolerance was observed. With the fungal inoculation plants cumulative growth and total chlorophyll content increased by 24% and 18%, respectively as compared to their respective non-inoculated controls at 1000 mg kg^-1 Pb. Similary, 50% more Pb accumulation was measured in the shoots of fungal inoculated plants at 1500 mg kg^-1 Pb as compared to control. Thus, the results of the present study suggest that mutualism with endophytic fungi can improve the survival of host plants in metal contaminated soils, additionally it can also assist the phytoextraction of heavy metals from polluted sites by increasing their uptake by the host plants.

Lead Toxicity in Cereals: Mechanistic Insight Into Toxicity, Mode of Action, and Management

Cereals are the major contributors to global food supply, accounting for more than half of the total human calorie requirements. Sustainable availability of quality cereal grains is an important step to address the high-priority issue of food security. High concentrations of heavy metals specifically lead (Pb) in the soil negatively affect biochemical and physiological processes regulating grain quality in cereals. The dietary intake of Pb more than desirable quantity via food chain is a major concern for humans, as it can predispose individuals to chronic health issues. In plant systems, high Pb concentrations can disrupt several key metabolic processes such as electron transport chain, cellular organelles integrity, membrane stability index, PSII connectivity, mineral metabolism, oxygen-evolving complex, and enzymatic activity. Plant growth-promoting rhizobacteria (PGPR) has been recommended as an inexpensive strategy for remediating Pb-contaminated soils. A diverse group of Ascomycetes fungi, i.e., dark septate endophytes is successfully used for this purpose. A symbiotic relationship between endophytes and host cereal induces Pb tolerance by immobilizing Pb ions. Molecular and cellular modifications in plants under Pb-stressed environments are explained by transcription factor families such as bZIP, ERF, and GARP as a regulator. The role of metal tolerance protein (MTP), natural resistance-associated macrophage protein (NRAMP), and heavy metal ATPase in decreasing Pb toxicity is well known. In the present review, we provided the contemporary synthesis of existing data regarding the effects of Pb toxicity on morpho-physiological and biochemical responses of major cereal crops. We also highlighted the mechanism/s of Pb uptake and translocation in plants, critically discussed the possible management strategies and way forward to overcome the menace of Pb toxicity in cereals.

Hexavalent chromium reduction ability and bioremediation potential of Aspergillus flavus CR500 isolated from electroplating wastewater

Hexavalent chromium reduction by microbes can mitigate the chromium toxicity to the environment. In the present study Cr_[VI] tolerant fungal isolate (CR500) was isolated from electroplating wastewater, was able to tolerate 800 mg/L of Cr_[VI. Based on the ITS region sequencing, the isolate was identified as Aspergillus flavus CR500, showed multifarious biochemical (reactive oxygen species, antioxidants response and non-protein thiol) and morphological (protrusion less, constriction and swelling/outwards growth in mycelia) response under Cr_[VI] stress. Batch experiment was conducted at different Cr_[VI] concentration (0-200 mg/L) to optimize the Cr_[VI] reduction and removal ability of isolate CR500; results showed 89.1% reduction of Cr_[VI] to Cr_[III] within 24 h and 4.9 ± 0.12 mg of Cr per gram of dried biomass accumulation within 144 h at the concentration of 50 mg/L of Cr_[VI]. However, a maximum of 79.4% removal of Cr was recorded at 5 mg/L within 144 h. Fourier-transform infrared spectroscopy, energy dispersive x-ray spectroscopy and X-ray diffraction analysis revealed that chromium removal also happened via adsorption/precipitation on the mycelia surface. Fungus treated and without treated 100 mg/L of Cr_[VI] solution was subjected to phytotoxicity test using Vigna radiata seeds and result revealed that A. flavus CR500 successfully detoxified the Cr_[VI] via reduction and removal mechanisms. Isolate CR500 also exhibited efficient bioreduction potential at different temperature (20-40 °C), pH (5.0-9.0), heavy metals (As, Cd, Cu, Mn, Ni and Pb), metabolic inhibitors (phenol and EDTA) and in sterilized tannery effluent that make it a potential candidate for Cr_[VI] bioremediation.