Chromium(VI) Toxicity in Legume Plants: Modulation Effects of Rhizobial Symbiosis

Characteristics and numerical simulation of chromium transportation, migration and transformation in soil-groundwater system

Understanding chromium (Cr) migration and dispersion patterns in the soil-groundwater system is critical for the control and remediation of subsurface Cr contamination. In this study, a typical Cr-contaminated site from the Pearl River Delta (PRD) in China was simulated with a three-dimensional (3D) sandbox experiment to investigate the migration and transformation behavior of Cr. Results revealed that under the combined influence of rainfall and groundwater flow, a complex flow field favorable for 3D migration and solute dispersion was formed. The flow field characteristics were influenced by water-table depth, which in turn affected Cr behavior in the system. Moreover, downward flow field expansion under low water-table conditions led to Cr vertical migration range expansion, causing greater contamination in the deep soil. The migration process was accompanied with Cr(VI) reduction, during which approximately 75 % of the total Cr was immobilized in soils. The reactive transport model achieved a good fit for Cr retention and morphological distribution in the solid phase. The model indicates that Cr is more readily transported and dispersed with groundwater, and Cr migrated and spread downstream by 15 m during the eighth year. Therefore, managing water-table depth could be a strategy to minimize the Cr vertical migration and contamination.

Rhizobium symbiosis improves amino acid and secondary metabolite biosynthesis of tungsten-stressed soybean (Glycine max)

The industrially important transition metal tungsten (W) shares certain chemical properties with the essential plant micronutrient molybdenum and inhibits the activity of molybdoenzymes such as nitrate reductase, impacting plant growth. Furthermore, tungsten appears to interfere with metabolic processes on a much wider scale and to trigger common heavy metal stress response mechanisms. We have previously found evidence that the tungsten stress response of soybeans (Glycine max) grown with symbiotically associated N2-fixing rhizobia (Bradyrhizobium japonicum) differs from that observed in nitrogen-fertilized soy plants. This study aimed to investigate how association with symbiotic rhizobia affects the primary and secondary metabolite profiles of tungsten-stressed soybean and whether changes in metabolite composition enhance the plant's resilience to tungsten. This comprehensive metabolomic and proteomic study presents further evidence that the tungsten-stress response of soybean plants is shaped by associated rhizobia. Symbiotically grown plants (N fix) were able to significantly increase the synthesis of an array of protective compounds such as phenols, polyamines, gluconic acid, and amino acids such as proline. This resulted in a higher antioxidant capacity, reduced root-to-shoot translocation of tungsten, and, potentially, also enhanced resilience of N fix plants compared to non-symbiotic counterparts (N fed). Taken together, our study revealed a symbiosis-specific metabolic readjustment in tungsten-stressed soybean plants and contributed to a deeper understanding of the mechanisms involved in the rhizobium-induced systemic resistance in response to heavy metals.

Distribution and migration of uranium, chromium, and accompanying metal(loid)s in soil-plants system around a uranium hydrometallurgical area

The extraction and utilization of uranium (U) ores have led to the release of significant amounts of potentially toxic metal(loid)s (PTMs) into the environment, constituting a grave threat to the ecosystem. However, research on the distribution and migration mechanism of U, chromium (Cr), and their accompanying PTMs in soil-plant system around U hydrometallurgical area remains insufficient and poorly understood. Herein, the distribution, migration, and risk level of PTMs were evaluated in soil and plant samples around U hydrometallurgical area, Northern Guangdong, China. The results demonstrated that the maximum content of U and Cr found in the analyzed soils were up to 84.2 and 238.9 mg/kg, respectively. These values far exceed the soil background values in China and other countries. The highest content of U (53.6 mg/kg) was detected in Colocasia antiquorum Schott, and the highest content of Cr (349.5 mg/kg) was observed in Pteridium aquilinum, both of which were enriched in their roots. The risk assessment of PTMs demonstrated that the study area suffered from severe pollution (PN > 3), especially from U, Cr, Th, and As, suggesting the non-negligible anthropogenic impacts. Hence, in light of the significant ecological hazard posed by the U hydrometallurgical area, it is imperative to implement appropriate restoration measures to ensure the human health and maintain the stability of the ecosystem.

Effect of riboflavin on redox balance, osmolyte accumulation, methylglyoxal generation and nutrient acquisition in indian squash (Praecitrullus fistulosus L.) under chromium toxicity

The presence of high chromium (Cr) levels induces the buildup of reactive oxygen species (ROS), resulting in hindered plant development. Riboflavin (vitamin B2) is produced by plants, fungi, and microbes. It serves as a precursor to the coenzymes flavin adenine dinucleotide (FAD) and flavin mononucleotide (FMN), which play a crucial role in cellular metabolism. The objective of this work was to clarify the underlying mechanisms by which riboflavin alleviates Cr stress in Praecitrullus fistulosus L. Further, the role of riboflavin in growth, ions homeostasis, methylglyoxal detoxification, and antioxidant defense mechanism are not well documented in plants under Cr toxicity. We found greater biomass and minimal production of ROS in plants pretreated with riboflavin under Cr stress. Results manifested a clear abridge in growth, chlorophyll content, and nutrient uptake in Indian squash plants exposed to Cr stress. Findings displayed that Cr stress visibly enhanced oxidative injury reflected as higher malondialdehyde (MDA), hydrogen peroxide (H2O2), superoxide radical (O2•‒), methylglyoxal (MG) levels alongside vivid lipoxygenase activity. Riboflavin strengthened antioxidant system, enhanced osmolyte production and improved membrane integrity. Riboflavin diminished Cr accumulation in aerial parts that led to improved nutrient acquisition. Taken together, riboflavin abridged Cr phytotoxic effects by improving redox balance because plants treated with riboflavin had strong antioxidant system that carried out effective ROS detoxification. Riboflavin protected membrane integrity that, in turn, improved nutrient uptake in plants.

ZnO nanoparticles mediated by Azadirachta indica as nano fertilizer: Improvement in physiological and biochemical indices of Zea mays grown in Cr-contaminated soil

The current investigation aimed at evaluating the impact of Azadirachta indica-mediated zinc oxide nanoparticles (Ai-ZnONPs) on the growth and biochemical characteristics of maize (sweet glutinous 3000) under exposure to 50 mg kg-1Ai-ZnONPs with Cr (VI) concentrations of 50 and 100 mg kg-1. The results indicate that plants exposed to Cr (VI) only experienced a decline in growth parameters. Conversely, the inclusion of Ai-ZnONPs caused a noteworthy increase in physiological traits. Specifically, shoot and root fresh weight increased by 28.02% and 16.51%, and 63.11% and 97.91%, respectively, when compared to Cr-50 and 100 treatments. Additionally, the SPAD chlorophyll of the shoot increased by 91.08% and 15.38% compared to Cr-50 and 100 treatments, respectively. Moreover, the antioxidant enzyme traits of plant shoot and root, such as superoxide dismutase (SOD 7.44% and 2.70%, and 4.45% and 3.53%), catalase (CAT 1.18% and 3.20%, and 5.03% and 5.78%), and peroxidase (POD 0.31% and 5.55%, and 4.72% and 3.61%), exhibited significant increases in Cr 50 and 100 treatments, respectively. The addition of Ai-ZnONPs to the soil also enhanced soil nutrient status and reduced Cr (VI) concentrations by 40.69% and 19.82% compared to Cr-50 and 100 treated soils. These findings suggest that Ai-ZnONPs can trigger the activation of biochemical pathways that enable biomass accumulation in meristematic cells. Further investigations are required to elucidate the mechanisms involved in growth promotion.

Ascorbic acid and selenium nanoparticles synergistically interplay in chromium stress mitigation in rice seedlings by regulating oxidative stress indicators and antioxidant defense mechanism

Ascorbic acid (AsA) and selenium nanoparticles (SeNPs) were versatile plant growth regulators, playing multiple roles in promoting plant growth under heavy metal stresses. This study aimed to evaluate the beneficial role of individual and combined effects of AsA and SeNPs on morpho-physio-biochemical traits of rice with or without chromium (Cr) amendment. The results indicated that Cr negatively affected plant biomass, gas exchange parameters, total soluble sugar, proline, relative water contents, and antioxidant-related gene expression via increasing reactive oxygen species (MDA, H2O2, O2•-) formation, resulting in plant growth reduction. The application of AsA and SeNPs, individually or in combination, decreased the uptake and translocation of Cr in rice seedlings, increased seedlings with tolerance to Cr toxicity, and significantly improved the rice seedling growth. Most notably, AsA + SeNP treatment strengthened the antioxidative defense system through ROS quenching and Cr detoxification. The results collectively suggested that the application of AsA and SeNPs alone or in combination had the potential to alleviate Cr toxicity in rice and possibly other crop species.

A comprehensive review on environmental pollutants and osteoporosis: Insights into molecular pathways

A significant problem that has an impact on community wellbeing is environmental pollution. Environmental pollution due to air, water, or soil pollutants might pose a severe risk to global health, necessitating intense scientific effort. Osteoporosis is a common chronic condition with substantial clinical implications on mortality, morbidity, and quality of life. It is closely linked to bone fractures. Worldwide, osteoporosis affects around 200 million people, and every year, there are almost 9 million fractures. There is evidence that certain environmental factors may increase the risk of osteoporosis in addition to traditional risk factors. It is crucial to understand the molecular mechanisms at play because there is a connection between osteoporosis and exposure to environmental pollutants such as heavy metals, air pollutants, endocrine disruptors, metal ions and trace elements. Hence, in this scoping review, we explore potential explanations for the link between pollutants and bone deterioration through deep insights into molecular pathways. Understanding and recognizing these pollutants as modifiable risk factors for osteoporosis would possibly help to enhance environmental policy thereby aiding in the improvement of bone health and improving patient quality of life.

Characterization of a biofilm-forming, amylase-producing, and heavy-metal-bioremediating strain Micrococcus sp. BirBP01 isolated from oligotrophic subsurface lateritic soil

Lateritic soil is the reddish to brown-colored soil composed mainly of iron or aluminium oxides, hydroxides, or oxyhydroxides. Information on bacteria that inhabit this soil type, their ecological role, and metabolic potential are scarce. We have isolated and partially characterized a bacterial strain BirBP01 from a lead, calcium, and magnesium-rich, oligotrophic subsurface lateritic soil-sample collected from 12-feet deep horizon of a laterite mining pit in Birbhum district, India. The isolate is a biofilm-forming, Gram-positive bacterium having a sarcinae arrangement, mesophilic, slightly alkaliphilic, able to produce amylase, and resistant against multiple heavy-metals. BirBP01 has the ability to bioremediate 51% of Pb, 30% of Zn, and 22% of Cu through biosorption, possibly into the biofilm matrix. The bioremediating ability of the bacterium alleviated the inhibitory effect of heavy-metals on the germination of chickpea (Cicer arietinum L.) seeds. 16S rRNA gene-based phylogenetic analysis revealed that BirBP01 is a member of the genus Micrococcus. It showed more than 99% identity of the 16S rRNA gene sequence, and clustered within the same branch of the phylogenetic tree, with strains of M. yunnanensis, M. endophyticus, and M. luteus. The ability to produce amylase, and bioremediate heavy-metals signify that Micrococcus sp. BirBP01 could be potentially a good candidate for industrial applications, and to clean up heavy-metal contaminated sites.

Contamination, ecological, and human health risks of heavy metals in water from a Pb-Zn-F mining area, North Eastern Nigeria

In Nigeria, artisanal mining has become a serious issue. In the Nigerian mining region of Arufu Pb-Zn-F, this study assessed the level of pollution, ecological hazards, and health risks related to the presence of metals in the water. In the dry and rainy seasons, 36 water samples (20 from the ground, 10 from the surface, and six from the mine) were gathered. Samples were examined for the presence of heavy metals such as Cr, Co, Ni, Cu, Zn, As, Cd, and Pb. Other than Cu, Zn, As, Cd, Sb, and Cd (surface water, dry season), which were below the acceptable norm, all water samples had metals over the suggested limits. Heavy metals from nearby mining activities polluted the water, according to contamination evaluations utilizing the contamination factor (CF). Metals in the water may pose very significant ecological dangers, according to ecological risk assessments. The evaluation of human health risks revealed that both adults and children in the region are susceptible to carcinogenic and non-carcinogenic health hazards since the hazard index (HI) values for both indices were above 1 × 10-5 and above 1, respectively. This report emphasizes the need for monitoring mining operations in the nation to safeguard public health.

A review on chromium health hazards and molecular mechanism of chromium bioremediation

Living beings have been devastated by environmental pollution, which has reached its peak. The disastrous pollution of the environment is in large part due to industrial wastes containing toxic pollutants. The widespread use of chromium (Cr (III)/Cr (VI)) in industries, especially tanneries, makes it one of the most dangerous environmental pollutants. Chromium pollution is widespread due to ineffective treatment methods. Bioremediation of chromium (Cr) using bacteria is very thoughtful due to its eco-friendly and cost-effective outcome. In order to counter chromium toxicity, bacteria have numerous mechanisms, such as the ability to absorb, reduce, efflux, or accumulate the metal. In this review article, we focused on chromium toxicity on human and environmental health as well as its bioremediation mechanism.

Nano zero-valent iron enhances the absorption and transport of chromium in rice (Oryza sativa L.): Implication for Cr risks management in paddy fields

Chromium (Cr) accumulating in soil caused serious pollution to cultivated land. At present, nano zero-valent iron (nZVI) is considered to be a promising remediation material for Cr-contaminated soil. However, the nZVI impact on the behavior of Cr in the soil-rice system under high natural geological background value remains unknown. We studied the effects of nZVI on the migration and transformation of Cr in paddy soil-rice by pot experiment. Three different doses of nZVI (0, 0.001 % and 0.1 % (w/w)) treatments and one dose of 0.1 % (w/w) nZVI treatment without plant rice were set up. Under continuous flooding conditions, nZVI significantly increased rice biomass compared with the control. At the same time, nZVI significantly promoted the reduction of Fe in the soil, increased the concentration of oxalate Fe and bioavailable Cr, then facilitated the absorption of Cr in rice roots and the transportation to the aboveground part. In addition, the enrichment of Fe(III)-reducing bacteria and sulfate-reducing bacteria in soil provided electron donors for Cr oxidation, which helps to form bioavailable Cr that is easily absorbed by plants. The results of this study can provide scientific basis and technical support for the remediation of Cr -polluted paddy soil with high geological background.

Responses of rhizosphere microbial community structure and metabolic function to heavy metal coinhibition

Metal mineral mining results in releases of large amounts of heavy metals into the environment, and it is necessary to better understand the response of rhizosphere microbial communities to simultaneous stress from multiple heavy metals (HMs), which directly impacts plant growth and human health. In this study, by adding different concentrations of cadmium (Cd) to a soil with high background concentrations of vanadium (V) and chromium (Cr), the growth of maize during the jointing stage was explored under limiting conditions. High-throughput sequencing was used to explore the response and survival strategies of rhizosphere soil microbial communities to complex HM stress. The results showed that complex HMs inhibited the growth of maize at the jointing stage, and the diversity and abundance of maize rhizosphere soil microorganisms were significantly different at different metal enrichment levels. In addition, according to the different stress levels, the maize rhizosphere attracted many tolerant colonizing bacteria, and cooccurrence network analysis showed that these bacteria interacted very closely. The effects of residual heavy metals on beneficial microorganisms (such as Xanthomonas, Sphingomonas, and lysozyme) were significantly stronger than those of bioavailable metals and soil physical and chemical properties. PICRUSt analysis revealed that the different forms of V and Cd had significantly greater effects on microbial metabolic pathways than all forms of Cr. Cr mainly affected the two major metabolic pathways: microbial cell growth and division and environmental information transmission. In addition, significant differences in rhizosphere microbial metabolism under different concentrations were found, and this can serve as a reference for subsequent metagenomic analysis. This study is helpful for exploring the threshold for the growth of crops in toxic HM soils in mining areas and achieving further biological remediation.

Nanomaterials in biochar: Review of their effectiveness in remediating heavy metal-contaminated soils

Biochar can be used for soil remediation in environmentally beneficial manner, especially when combined with nanomaterials. After a decade of research, still, no comprehensive review was conducted on the effectiveness of biochar-based nanocomposites in controlling heavy metal immobilization at soil interfaces. In this paper, the recent progress in immobilizing heavy metals using biochar-based nanocomposite materials were reviewed and compared their efficacy against that of biochar alone. In details, an overview of results on the immobilization of Pb, Cd, Cu, Zn, Cr, and As was presented by different nanocomposites made by various biochars derived from kenaf bar, green tea, residual bark, cornstalk, wheat straw, sawdust, palm fiber, and bagasse. Biochar nanocomposite was found to be most effective when combined with metallic nanoparticles (Fe₃O₄ and FeS) and carbonaceous nanomaterials (graphene oxide and chitosan). This study also devoted special consideration to different remediation mechanisms by which the nanomaterials affect the effectiveness of the immobilization process. The effects of nanocomposites on soil characteristics related to pollution migration, phytotoxicity, and soil microbial composition were assessed. A future perspective on nanocomposites' use in contaminated soils was presented.

Aquaculture sediments amended with biochar improved soil health and plant growth in a degraded soil

Sustainable and safe management of aquaculture sediments is of great concern. Biochar (BC) and fishpond sediments (FPS) are rich in organic carbon and nutrients and thus can be used as soil amendments; however, it is not fully explored how the biochar amended fishpond sediments can affect soil properties/fertility and modulate plant physiological and biochemical changes, particularly under contamination stress. Therefore, a comprehensive investigation was carried out to explore the effects of FPS and BC-treated FPS (BFPS) on soil and on spinach (Spinacia oleracea L.) grown in chromium (Cr) contaminated soils. Addition of FPS and BFPS to soil caused an increase in nutrients content and reduced Cr levels in soil, which consequently resulted in a significant increase in plant biomass, chlorophyll pigments, and photosynthesis, over the control treatment. The most beneficial effect was observed with the BFPS applied at 35 %, which further increased the antioxidant enzymes (by 2.75-fold, at minimum), soluble sugars by 24.9 %, and upregulated the gene expression activities. However, the same treatment significantly decreased proline content by 74.9 %, Malondialdehyde by 65.6 %, H₂O₂ by 65.1 %, and Cr concentration in spinach root and shoot tissues. Moreover, the average daily intake analysis showed that BFPS (at 35 %) could effectively reduce human health risks associated with Cr consumption of leafy vegetables. In conclusion, these findings are necessary to provide guidelines for the reutilization of aquaculture sediments as an organic fertilizer and a soil amendment for polluted soils. However, more future field studies are necessary to provide guidelines and codes on aquaculture sediments reutilization as organic fertilizer and soil amendment for polluted soils, aiming for a more sustainable food system in China and globally, with extended benefits to the ecosystem and human.

Exogenous hydrogen sulfide alleviates chromium toxicity by modulating chromium, nutrients and reactive oxygen species accumulation, and antioxidant defence system in mungbean (Vigna radiata L.) seedlings

Chromium (Cr), a highly toxic redox-active metal cation in soil, seriously threatens global agriculture by affecting nutrient uptake and disturbing various physio-biochemical processes in plants, thereby reducing yields. Here, we examined the effects of different concentrations of Cr alone and in combination with hydrogen sulfide (H₂S) application on the growth and physio-biochemical performance of two mungbeans (Vigna radiata L.) varieties, viz. Pusa Vishal (PV; Cr tolerant) and Pusa Ratna (PR; Cr sensitive), growing in a pot in hydroponics. Plants were grown in the pot experiment to examine their growth, enzymatic and non-enzymatic antioxidant levels, electrolyte balance, and plasma membrane (PM) H⁺-ATPase activity. Furthermore, root anatomy and cell death were analysed 15 days after sowing both varieties in hydroponic systems. The Cr-induced accumulation of reactive oxygen species caused cell death and affected the root anatomy and growth of both varieties. However, the extent of alteration in anatomical features was less in PV than in PR. Exogenous application of H₂S promoted plant growth, thereby improving plant antioxidant activities and reducing cell death by suppressing Cr accumulation and translocation. Seedlings of both cultivars treated with H₂S exhibited enhanced photosynthesis, ion uptake, glutathione, and proline levels and reduced oxidative stress. Interestingly, H₂S restricted the translocation of Cr to aerial parts of plants by improving the nutrient profile and viability of root cells, thereby relieving plants from oxidative bursts by activating the antioxidant machinery through triggering the ascorbate-glutathione cycle. Overall, H₂S application improved the nutrient profile and ionic homeostasis of Cr-stressed mungbean plants. These results highlight the importance of H₂S application in protecting crops against Cr toxicity. Our findings can be utilised to develop management strategies to improve heavy metal tolerance among crops.

Biogeochemical behaviour and toxicology of chromium in the soil-water-human nexus: A review

Chromium (Cr) affects human health if it accumulates in organs to elevated concentrations. The toxicity risk of Cr in the ecosphere depends upon the dominant Cr species and their bioavailability in the lithosphere, hydrosphere, and biosphere. However, the soil-water-human nexus that controls the biogeochemical behaviour of Cr and its potential toxicity is not fully understood. This paper synthesizes information on different dimensions of Cr ecotoxicological hazards in the soil and water and their subsequent effects on human health. The various routes of environmental exposure of Cr to humans and other organisms are also discussed. Human exposure to Cr(VI) causes both carcinogenic and non-carcinogenic health effects via complicated reactions that include oxidative stress, chromosomal and DNA damage, and mutagenesis. Chromium (VI) inhalation can cause lung cancer; however, incidences of other types of cancer following Cr(VI) exposure are low but probable. The non-carcinogenic health consequences of Cr(VI) exposure are primarily respiratory and cutaneous. Research on the biogeochemical behaviour of Cr and its toxicological hazards on human and other biological routes is therefore urgently needed to develop a holistic approach to understanding the soil-water-human nexus that controls the toxicological hazards of Cr and its detoxification.

Regulation mechanism of plant response to heavy metal stress mediated by endophytic fungi

Endophytic fungi exist widely in plants and play an important role in the growth and adaptation of plants. They could be used in phytoremediation techniques against heavy metal contaminated soil since beneficial microbial symbionts can endow plants with resistance to external heavy metal stresses. This review summarized the regulation mechanism of plant response to heavy metal stress mediated by endophytic fungi. Potential endophytic fungi in enhancing plant's adaption to heavy metal stresses include arbuscular mycorrhizal fungi, dark septate endophytic fungi, plant growth promoting endophytic fungi. The mechanisms involve coevolution strategy, immune regulation and detoxification transport to improve the ability of plants to adapt to heavy metal stress. They can increase the synthesis of host hormones and maintaining the balance of endogenous hormones, strengthen osmotic regulation, regulate carbon and nitrogen metabolism, and increase immune activity, antioxidant enzyme and glutathione activity. They also help to improve the detoxification transport and heavy metal emission capacity of the host by significantly producing iron carrier, metallothionein and 1-aminocyclopropane-1-carboxylic acid deaminase. The combination of endophytic fungi and hyperaccumulation plants provides a promising technology for the ecological restoration of heavy metal contaminated soil. Endophytic fungi reserves further development on enhancing host plant's adaptability to heavy metal stresses.

Bioinspired Cobalt Oxide Nanoball Synthesis, Characterization, and Their Potential as Metal Stress Absorbants

Massive accumulation of heavy metals in agricultural land as a result of enhanced levels of toxicity in the soil is an emerging global concern. Among various metals, zinc contamination has severe effects on plant and human health through the food chain. To remove such toxicity, a nanotechnological neutralizer, cobalt oxide nanoballs (Co₃O₄ Nbs) were synthesized by using the extract of Cordia myxa. The Co₃O₄ Nbs were well characterized via UV-vis spectrophotometry, scanning electron microscopy, and X-ray diffraction techniques. Green-synthesized Co₃O₄ Nbs were exposed over Acacia jacquemontii and Acacia nilotica at different concentrations (25, 50, 75, and 100 ppm). Highly significant results were observed for plant growth by the application of Co₃O₄ Nbs at 100 ppm, thereby increasing the root length (35%), shoot length (48%), fresh weight (44%), and dry weight (40%) of the Acacia species with respect to the control. Furthermore, physiological parameters including chlorophyll contents, relative water contents, and osmolyte contents like proline and sugar showed a prominent increase. The antioxidant activity and atomic absorption supported and justified the positive response to using Co₃O₄ Nbs that mitigated the heavy-metal zinc stress by improving the plant growth. Hence, the biocompatible Co₃O₄ Nbs counteract the zinc toxicity for governing and maintaining plant growth. Such nanotechnological tools can therefore step up the cropping system and overcome toxicity to meet the productivity demand along with the development of agricultural management strategies.

Chromium toxicity, speciation, and remediation strategies in soil-plant interface: A critical review

In recent decades, environmental pollution with chromium (Cr) has gained significant attention. Although chromium (Cr) can exist in a variety of different oxidation states and is a polyvalent element, only trivalent chromium [Cr(III)] and hexavalent chromium [Cr(VI)] are found frequently in the natural environment. In the current review, we summarize the biogeochemical procedures that regulate Cr(VI) mobilization, accumulation, bioavailability, toxicity in soils, and probable risks to ecosystem are also highlighted. Plants growing in Cr(VI)-contaminated soils show reduced growth and development with lower agricultural production and quality. Furthermore, Cr(VI) exposure causes oxidative stress due to the production of free radicals which modifies plant morpho-physiological and biochemical processes at tissue and cellular levels. However, plants may develop extensive cellular and physiological defensive mechanisms in response to Cr(VI) toxicity to ensure their survival. To cope with Cr(VI) toxicity, plants either avoid absorbing Cr(VI) from the soil or turn on the detoxifying mechanism, which involves producing antioxidants (both enzymatic and non-enzymatic) for scavenging of reactive oxygen species (ROS). Moreover, this review also highlights recent knowledge of remediation approaches i.e., bioremediation/phytoremediation, or remediation by using microbes exogenous use of organic amendments (biochar, manure, and compost), and nano-remediation supplements, which significantly remediate Cr(VI)-contaminated soil/water and lessen possible health and environmental challenges. Future research needs and knowledge gaps are also covered. The review's observations should aid in the development of creative and useful methods for limiting Cr(VI) bioavailability, toxicity and sustainably managing Cr(VI)-polluted soils/water, by clear understanding of mechanistic basis of Cr(VI) toxicity, signaling pathways, and tolerance mechanisms; hence reducing its hazards to the environment.

Sensitivity of Zea mays and Soil Microorganisms to the Toxic Effect of Chromium (VI)

Chromium is used in many settings, and hence, it can easily enter the natural environment. It exists in several oxidation states. In soil, depending on its oxidation-reduction potential, it can occur in bivalent, trivalent or hexavalent forms. Hexavalent chromium compounds are cancerogenic to humans. The aim of this study was to determine the effect of Cr(VI) on the structure of bacteria and fungi in soil, to find out how this effect is modified by humic acids and to determine the response of Zea mays to this form of chromium. A pot experiment was conducted to answer the above questions. Zea mays was sown in natural soil and soil polluted with Cr(VI) in an amount of 60 mg kg^-1 d.m. Both soils were treated with humic acids in the form of HumiAgra preparation. The ecophysiological and genetic diversity of bacteria and fungi was assayed in soil under maize (not sown with Zea mays). In addition, the following were determined: yield of maize, greenness index, index of tolerance to chromium, translocation index and accumulation of chromium in the plant. It has been determined that Cr(VI) significantly distorts the growth and development of Zea mays, while humic acids completely neutralize its toxic effect on the plant. This element had an adverse effect on the development of bacteria of the genera Cellulosimicrobium, Kaistobacter, Rhodanobacter, Rhodoplanes and Nocardioides and fungi of the genera Chaetomium and Humicola. Soil contamination with Cr(VI) significantly diminished the genetic diversity and richness of bacteria and the ecophysiological diversity of fungi. The negative impact of Cr(VI) on the diversity of bacteria and fungi was mollified by Zea mays and the application of humic acids.

Cloncurry buffel grass mitigated Cr(III) and Cr(VI) toxicity in tomato plant

Contamination of agricultural soil with chromium (Cr) ions has threatened global crop, human and ecosystem health. Its two oxidation states viz. Cr(III) and Cr(VI) are most stable and readily available to the plants. The study explored the impact of increasing exposure (up to 500 ppm) of Cr(III) and Cr(VI) on bio-physical traits of 15-day-old seedlings (in vitro) as well as 60-day-old tomato plant (in vivo), and highlighted the importance of buffel grass (Cenchrus pennisetiformis) in mitigating Cr levels in the tomato plants. In vitro, Petri plate bioassays with 13 different concentrations (20-500 ppm) of Cr(III) and Cr(VI) depicted the highly toxic effect of metal ions ≥ 200 ppm on all bio-physical traits of tomato seedlings. In vivo, soil spiked with Cr(III) and Cr(VI) (200, 300, and 400 mg/kg) was amended with 1% and 2% dry biomass of buffel grass. Phytotoxicity was higher in Cr(VI)-spiked soil compared with Cr(III)-spiked soil. Cr was mainly accumulated in tomato roots, and more Cr was translocated from roots to shoots from Cr(VI)-spiked soil than Cr(III)-spiked soil. Soil amendments with 2% weed biomass reduced metal toxicity in plants, particularly at 200 and 300 mg/kg of Cr. Protein profiles through SDS-PAGE revealed 12-50 kDa (mainly PR proteins) as an important region in tomato leaf, where many new bands were expressed under different treatments, particularly in the treatments provided with buffel grass. PCA-based biplot clearly separated Cr tolerance treatments from highly sensitive treatments. For the cultivation of tomato plants in Cr(III) and Cr(VI) contaminated soil (200 and 300 mg/kg), the biomass of Cloncurry buffel grass should be considered an effective and easily available phyto-management option.

Mitigation of Negative Effects of Chromium (VI) Toxicity in Faba Bean (Vicia faba) Plants through the Supplementation of Kinetin (KN) and Gibberellic Acid (GA3)

The present study was carried out to explore the possible role of kinetin and gibberellic acid (GA3) on faba bean under chromium (Cr) stress. Cr treatment negatively affected growth and biomass production, reduced photosynthetic pigments, and inhibited photosynthesis, gas exchange parameters, antioxidant enzymes, and the glyoxylase cycle. Moreover, Cr stress enhanced the production of malondialdehyde (MDA, 216.11%) and hydrogen peroxide (H₂O₂, 230.16%), electrolyte leakage (EL, 293.30%), and the accumulation of proline and glycine betaine. Exogenous application of kinetin and GA3 increased growth and biomass, improved pigment contents and photosynthesis, as well as up-regulated the antioxidant system by improving the antioxidant enzyme activities and the content of nonenzymatic components, and the glyoxylase cycle. Additionally, kinetin and GA3 application displayed a considerable enhancement in proline (602.61%) and glycine betaine (423.72), which help the plants to maintain water balance under stress. Furthermore, a decline in Cr uptake was also observed due to kinetin and GA3 application. Exogenous application of kinetin and GA3 ameliorated the toxic effects of Cr in faba bean plants, up-shooting the tolerance mechanisms, including osmolyte metabolism and the antioxidant system.

Health Risk Assessment during In Situ Remediation of Cr(VI)-Contaminated Groundwater by Permeable Reactive Barriers: A Field-Scale Study

The presence of residual Cr(VI) in soils causes groundwater contamination in aquifers, affecting the health of exposed populations. Initially, permeable reactive barriers(PRB) effectively removed Cr(VI) from groundwater. However, as PRB clogging increased and Cr(VI) was released from upstream soils, the contamination plume continued to spread downstream. By 2020, the level of contamination in the downstream was nearly identical to that in the upstream. The study results show that during normal operation, the PRB can successfully remove Cr(VI) from contaminated groundwater and reduce the carcinogenic and non-carcinogenic risks to humans from the downstream side of groundwater. However, the remediated groundwater still poses an unacceptable risk to human health. The sensitivity analysis revealed that the concentration of the pollutant was the most sensitive parameter and interacted significantly with other factors. Ultimately, it was determined that the residual Cr(VI) in the soil of the study region continues to contaminate the groundwater and constitutes a serious health danger to residents in the vicinity. As remediated groundwater still poses a severe threat to human health, PRB may not be as effective as people believe.

The Anatomical Basis of Heavy Metal Responses in Legumes and Their Impact on Plant-Rhizosphere Interactions

Rapid industrialization, urbanization, and mine tailings runoff are the main sources of heavy metal contamination of agricultural land, which has become one of the major constraints to crop growth and productivity. Finding appropriate solutions to protect plants and agricultural land from heavy metal pollution/harmful effects is important for sustainable development. Phytoremediation and plant growth-promoting rhizobacteria (PGPR) are promising methods for this purpose, which both heavily rely on an appropriate understanding of the anatomical structure of plants. Specialized anatomical features, such as those of epidermis and endodermis and changes in the root vascular tissue, are often associated with heavy metal tolerance in legumes. This review emphasizes the uptake and transport of heavy metals by legume plants that can be used to enhance soil detoxification by phytoremediation processes. Moreover, the review also focuses on the role of rhizospheric organisms in the facilitation of heavy metal uptake, the various mechanisms of enhancing the availability of heavy metals in the rhizosphere, the genetic diversity, and the microbial genera involved in these processes. The information presented here can be exploited for improving the growth and productivity of legume plants in metal-prone soils.

Chromium toxicity and its remediation by using endophytic bacteria and nanomaterials: A review

Chromium (Cr) is a crucial element for all life forms. Various anthropogenic activities have been responsible for environmental contamination with Cr (VI) in recent years. For this review, articles were collected using electronic databases such as Web of Science, Pubmed, ProQuest, and Google Scholar as per the guidelines of PRISMA-2015, applying the Boolean search methods. Chromium can cause severe health complications in humans and animals and threatens the surrounding environment, with negative impacts on crop yield, development, and quality. Hence, monitoring Cr contamination is essential, and various remediation technologies have emerged in the past 50 years to reduce the amount of Cr in the environment. This review focuses on chromium exposure and the associated environmental health risks. We also reviewed sustainable remediation processes, with emphasis on nanoparticle and endophytic remediation processes.

Biochemical Analysis and Toxicity Assessment of Utilization of Argon Oxygen Decarbonization Slag as a Mineral Fertilizer for Tall Fescue (Festuca arundinacea Schreb) Planting

Argon oxygen decarbonization (AOD) slag refers to a byproduct of stainless steel (SS) production, which has caused considerable environmental stress. Finding an effective approach for recycling AOD slag is essential to environmental safety. In this work, batch leaching tests were carried out to explore the leaching behavior of AOD slag and soil. Pot experiments was conducted to analyze the fertilization effect of AOD slag for tall fescue (Festuca arundinacea Schreb) planting. The plant height, biomass, total root length (TRL), root surface area (RSA), root tips (RT), root hairs (RH)), chlorophyll content, malondialdehyde (MDA) content, and antioxidant enzyme activities of the tall fescue seedlings were measured. As indicated from the results, adding AOD slag into soil increased soil pH. The leaching concentration of Ca, Si, Al, Cr of the AOD slag was higher than the original soil, while that of Mg, Mn, and Fe was lower. Low addition rate (≤1%) of AOD slag fertilization was good for plant height, biomass, root growth, and chlorophyll synthesis, whereas high addition rate (≥2%) exerted an opposite effect. Elevating the rate of AOD slag fertilization increased the Cr accumulation in the tall fescue seedling that aggravated damage of reactive oxygen species (ROS). When the AOD slag fertilization was at a low rate (≤1%), ROS scavenging was attributed to the synergistic effects of superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT) defense systems, while at a high rate (≥2%) of AOD slag fertilization, scavenging of excessive ROS could be mainly due to the CAT defense system.

Coronilla juncea, a native candidate for phytostabilization of potentially toxic elements and restoration of Mediterranean soils

Soil contamination pattern due to industrial activities often leads to high concentrations of potentially toxic elements (PTE) decreasing with depth. This spatial heterogeneity of the soil contamination may have significant consequences on the soil properties and soil living communities. We evaluated the effects of both surface and solum soil contamination heterogeneity on Coronilla juncea L. (Fabaceae) functional traits in field conditions and the phytostabilization potential of this species. Plant and soil samples were collected on 3 sites along a PTE contamination gradient. The correlations between PTE concentration in plant and soil samples at 2 depths, physico-chemical properties of soil, plant biomass and soil microbial activity were tested. Field measurements highlight a decreasing PTE concentration with soil depth in addition to an important surface heterogeneity of As, Cu, Pb, Sb and Zn soil concentrations. Root PTE concentrations in C. juncea did not follow soil PTE concentrations. Concentrations of PTE in the root parts were higher than those of the aerial parts. Low PTE translocation and root symbioses with microorganisms suggest that this native plant species may play a role as engineer species with positive implications for the phytostabilization of Mediterranean PTE contaminated soils and their ecological restoration.

Accumulation of chromium in plants and its repercussion in animals and humans

The untreated effluents released from industrial operations have adverse impacts on human health, environment and socio-economic aspects. Environmental pollution due to chromium is adversely affecting our natural resources and ecosystem. Chromium is hazardous carcinogenic element released from spontaneous activities and industrial procedures. Chromium toxicity, mobility and bioavailability depend mainly on its speciation. Chromium mainly exists in two forms, first as an immobile, less soluble trivalent chromium [Cr(III)] species under reducing conditions whereas hexavalent chromium [Cr(VI)] as a mobile, toxic and bioavailable species under oxidizing conditions. Hexavalent chromium is more pernicious in comparison to trivalent form. Chromium negatively affects crop growth, total yield and grain quality. Exposure of chromium even at low concentration enhances its accretion in cells of human-beings and animals which may show detrimental health effects. Many techniques have been utilized for the elimination of chromium. The selection of the green and cost-efficient technology for treatment of industrial effluent is an arduous task. The present review highlights the problems associated with chromium pollution and need of its immediate elimination by suitable remediation strategies. Further, investigations are required to fill the gaps to overcome the problem of chromium contamination and implementation of sustainable remediation strategies with their real-time applicability on the contaminated sites.

Utilization of Legume-Nodule Bacterial Symbiosis in Phytoremediation of Heavy Metal-Contaminated Soils

Simple Summary

The legume–rhizobium symbiosis is one of the most beneficial interactions with high importance in agriculture, as it delivers nitrogen to plants and soil, thereby enhancing plant growth. Currently, this symbiosis is increasingly being exploited in phytoremediation of metal contaminated soil to improve soil fertility and simultaneously metal extraction or stabilization. Rhizobia increase phytoremediation directly by nitrogen fixation, protection of plants from pathogens, and production of plant growth-promoting factors and phytohormones.

Abstract

With the increasing industrial activity of the growing human population, the accumulation of various contaminants in soil, including heavy metals, has increased rapidly. Heavy metals as non-biodegradable elements persist in the soil environment and may pollute crop plants, further accumulating in the human body causing serious conditions. Hence, phytoremediation of land contamination as an environmental restoration technology is desirable for both human health and broad-sense ecology. Legumes (Fabaceae), which play a special role in nitrogen cycling, are dominant plants in contaminated areas. Therefore, the use of legumes and associated nitrogen-fixing rhizobia to reduce the concentrations or toxic effects of contaminants in the soil is environmentally friendly and becomes a promising strategy for phytoremediation and phytostabilization. Rhizobia, which have such plant growth-promoting (PGP) features as phosphorus solubilization, phytohormone synthesis, siderophore release, production of beneficial compounds for plants, and most of all nitrogen fixation, may promote legume growth while diminishing metal toxicity. The aim of the present review is to provide a comprehensive description of the main effects of metal contaminants in nitrogen-fixing leguminous plants and the benefits of using the legume–rhizobium symbiosis with both wild-type and genetically modified plants and bacteria to enhance an efficient recovery of contaminated lands.

Proteomic changes in various plant tissues associated with chromium stress in sunflower

Heavy metal stress is one of the major abiotic stresses that cause environmental pollution in recent decades. An elevated concentration of these heavy metals is highly toxic to plant. Chromium (Cr) is one of the heavy metals whose concentration in the environment is still increasing alarmingly. It is harmful for plant growth and achene yield. To check out the growth and protein alternation towards pollutants, two sunflower varieties (RA-713 and AHO-33) were subjected to different chromium concentrations (control, 200 ppm, 400 ppm) by soil application. This study has elaborated that 400 ppm Cr resulted in a reduction of various growth parameters. Sodium dodecyl sulphate polyacrylamide gel electrophoresis (SDS-PAGE) was used to enhance the understanding of plant proteomic modulation under Cr stress. Different protein bands like 48 and 49, 26 kDa have newly appeared, and three 60, 47, and 42 kDa, and two protein bands 49 and 13 kDa were up-regulated in seeds of RA-713 and AHO-33, respectively. Some proteins (52, 16 kDa) are down-regulated in leaf tissues of both varieties. Only 6 and 81 kDa protein showed up-regulation and 154 kDa down-regulation behavior in the shoot in response to stress. The finding s of study might support the selection of tolerant genotype under Cr contamination and the discovery of new protein biomarkers that can use as monitoring tools in heavy metal stress biology.

Carcinogenic Risk of Pb, Cd, Ni, and Cr and Critical Ecological Risk of Cd and Cu in Soil and Groundwater around the Municipal Solid Waste Open Dump in Central Thailand

Several consequences of health effects from municipal solid waste caused by carcinogenic and noncarcinogenic metals have been recognized. The water quality index ( $\text{WQI}$ ) in the groundwater around this landfill is 2945.58, which is unacceptable for consumption. The contaminated groundwater mainly appears within a 1 km radius around the landfill. The metal pollution levels in the soil in descending order were Cu > Cd > Zn=Cr > Pb > Ni. The pollution degree (ER) of Cd was 2898.88, and the potential ecological risk index (RI) was 2945.58, indicating that the risk level was very high. Surprisingly, the hazard index (HI) of Pb (2.05) and Fe (1.59) in children was higher than 1. This indicated that the chronic risk and cancer risk caused by Pb and Fe for children were at a medium level. Carcinogenic risk by oral (CR oral) consumption of Ni, Cd, and Cr in children was 1.4E − 04, 2.5E − 04, and 1.8E − 04, respectively, while the lifetime carcinogenic risk (LCR) of Ni, Cd, and Cr in children was 1.5E − 04, 2.8E − 04, and 2.0E − 04, respectively. In adults, CR oral of Ni and Cr were 1.6E − 03 and 3.0E − 04, respectively, while LCR of Ni and Cr were 1.6E − 03 and 3.4E − 04, respectively, which exceeded the carcinogenic risks limits. Our study indicated a lifetime carcinogenic risk to humans. Environmental surveillance should focus on reducing health risks such as continuous monitoring of the groundwater, soil, and leachate treatment process.

Health hazards of hexavalent chromium (Cr (VI)) and its microbial reduction

Industrial effluents/wastewater are the main sources of hexavalent chromium (Cr (VI)) pollutants in the environment. Cr (VI) pollution has become one of the world’s most serious environmental concerns due to its long persistence in the environment and highly deadly nature in living organisms. To its widespread use in industries Cr (VI) is highly toxic and one of the most common environmental contaminants. Cr (VI) is frequently non-biodegradable in nature, which means it stays in the environment for a long time, pollutes the soil and water, and poses substantial health risks to humans and wildlife. In living things, the hexavalent form of Cr is carcinogenic, genotoxic, and mutagenic. Physico-chemical techniques currently used for Cr (VI) removal are not environmentally friendly and use a large number of chemicals. Microbes have many natural or acquired mechanisms to combat chromium toxicity, such as biosorption, reduction, subsequent efflux, or bioaccumulation. This review focuses on microbial responses to chromium toxicity and the potential for their use in environmental remediation. Moreover, the research problem and prospects for the future are discussed in order to fill these gaps and overcome the problem associated with bacterial bioremediation’s real-time applicability.

Chromium biogeochemical behaviour in soil-plant systems and remediation strategies: A critical review

Chromium (Cr) is a toxic heavy metal that is heavily discharged into the soil environment due to its widespread use and mining. High Cr levels may pose toxic hazards to plants, animals and humans, and thus have attracted global attention. Recently, much progress has been made in elucidating the mechanisms of Cr uptake, transport and accumulation in soil-plant systems, aiming to reduce the toxicity and ecological risk of Cr in soil; however, these topics have not been critically reviewed and summarised to date. Accordingly, based on available data-especially from the last five years (2017-2021)-this review traces a plausible link among Cr sources, levels, chemical forms, and phytoavailability in soil; Cr accumulation and translocation in plants; and Cr phytotoxicity and detoxification in plants. Additionally, given the toxicity and hazard posed by Cr(VI) in soils and the application of reductant materials to reduce Cr(VI) to Cr(III) for the remediation of Cr(VI)-contaminated soils, the reduction and immobilisation mechanisms by organic and inorganic reductants are summarised. Finally, some priority research challenges concerning the biogeochemical behaviour of Cr in soil-plant systems are highlighted, as well as the environmental impacts resulting from the application of reductive materials and potential research prospects.

Chromium-resistant Staphylococcus aureus alleviates chromium toxicity by developing synergistic relationships with zinc oxide nanoparticles in wheat

Chromium (Cr) is a toxic heavy metal that contaminates soil and water resources after its discharge from different industries. It can act as carcinogen and mutagen for biological systems. Microbe-assisted phytoremediation is one of the most emergent and environment friendly technique used for detoxification of Cr from Cr-contaminated soils. In this study, wheat as a test crop was grown under varying stress levels (0, 50, 100 and 200 mg/kg) of Cr in a pot experiment under a complete randomized design. Alleviative role of Staphylococcus aureus strain K1 was assessed by applying as a treatment in different combinations of zinc oxide nanoparticles (0, 50, 100 mg/L). Growth and yield attributes data presented nurturing impact of bacterial inoculation and ZnO NPs in improvement of wheat defense system by decreasing Cr toxicity. Increase in chlorophyll and carotenoids contents, antioxidant enzymes (SOD, POD, APX, CAT) activities and nutrient uptake also confirmed the mitigative potential of bacterial inoculation when applied solely or in combination with ZnO NPs. The Cr accumulation in different parts of plant was significantly reduced with the application of NPs and S. aureus strain K1. Taken together, the results showed that combined application of Staphylococcus aureus strain K1 and ZnO NPs detoxifies the effects of Cr on wheat plants and boosts its growth, physiology and defense system.

Natural, low-cost adsorbents for toxic Pb(II) ion sequestration from (waste)water: A state-of-the-art review

Pb(II) ions is an inorganic pollutant that is present in the environment. Its presence affects both human health and ecosystem. Economically, amongst many wastewater treatment approaches, adsorption is both cheap and environmentally friendly for removing Pb(II) ion from contaminated water. In this state of the art review, about 227 research and review based publications on adsorption-based studies between 1989 and 2021, which have used various materials as adsorbents of Pb (II) ions, were selected and reviewed for more evaluation. A number of adsorbents which have been reported in these literatures for the adsorption of Pb(II) ion are agrobased, modified agrobased, clay minerals, modified/nanocomposite clay minerals, silica-based, zeolite-based and chitosan-based adsorbents, respectively. The adsorption potential of the adsorbents is exhibited under optimum experimental conditions. The unmodified and modified agro based adsorbents were shown to exhibit the greatest Pb(II) adsorption capacity, with great potential for further exploration, compared to the others afore-listed. The effects of operating parameters such as pH, initial metal ion concentration, adsorbent dose and reaction time are discussed. Furthermore, in order to comprehend the nature of adsorption process between the adsorbent and contaminant (Pb(II)), thermodynamic analyses of adsorption systems are intensively described. All these discussions revealed the applicability of adsorption process for toxic Pb(II) ions removal with respect to wastewater treatment techniques. The review concludes by commenting on the various adsorbents' adsorption capacity and proposes some studies that should also be considered in future works.

Amelioration potential of β-pinene on Cr(VI)-induced toxicity on morphology, physiology and ultrastructure of maize

Heavy metals' amassment in the soil environment is a threat to crop and agricultural sustainability and consequentially the global food security. For achieving enhancement of crop productivity in parallel to reducing chromium (Cr) load onto food chain demands continuous investigation and efforts to develop cost-effective strategies for maximizing crop yield and quality. In this context, we investigated the amelioration of Cr(VI) toxicity through β-pinene in experimental dome simulating natural field conditions. The protective role of β-pinene was determined on physiology, morphology and ultrastructure in Zea mays under Cr(VI) stress (250 and 500 μM). Results exhibited a marked reduction in the overall growth (shoot and root length and dry matter) of Z. mays plants subjected to Cr(VI) stress. Photosynthetic pigments (chlorophyll and carotenoids) were evidently reduced, and there was a loss of membrane integrity. Supplementation of β-pinene (100 μM), however, declined the toxicity induced by Cr(VI). Interestingly, Cr-tolerant abilities were improved in relation to plant growth, photosynthetic pigments and membrane integrity with the combined treatment of Cr(VI) and β-pinene. β-Pinene also reduced the root-mediated uptake of Cr(VI) and translocation to shoots. Moreover, significant ultrastructural damages recorded in roots and shoots under Cr(VI) stress were partially reverted upon addition of β-pinene. Our analyses revealed that β-pinene mitigates Cr(VI) toxicity in Z. mays, either by membrane stabilization or serving as a barrier to the uptake of Cr from soil. Thus, exogenous supply of β-pinene can be an effective alternative to mitigate Cr toxicity in soil. However, it is deemed essential to investigate further the responses throughout the life cycle of the plant on β-pinene supplementation under natural conditions.

Analysis of Cr(VI) Bioremediation by Citrobacter freundii Using Synchrotron Soft X-ray Scanning Transmission X-ray Microscopy

Scanning transmission X-ray microscopy (STXM) was utilized for analysing the bioremediation of Cr(VI) by Citrobacter freundii, a species of gram-negative bacteria. The biosorption and bioreduction processes were analysed by the chemical mapping of cells biosorbed at different concentrations of Cr(VI). STXM spectromicroscopy images were recorded at O K-edge and Cr L-edge. A thorough analysis of the X-ray absorption features corresponding to different oxidation states of Cr in the biosorbed cell indicated the coexistence of Cr(III) and Cr(VI) at higher concentrations. This signifies the presence of partially reduced Cr(VI) in addition to biosorbed Cr(VI). In addition, the Cr(III) signal is intense compared with Cr(VI) at different regions of the cell indicating excess of reduced Cr. Speciation of adsorbed Cr was analysed for the spectral features of biosorbed cell and comparison with Cr standards. Analysis of absorption onset, L3/L2 ratio and absorption fine structure concludes that adsorbed Cr is predominantly present as Cr(III) hydroxide or oxyhydroxide. The evolution of absorption features in the duration of biosorption process was also studied. These time lapse studies depict the gradual decrement in Cr(VI) signal as biosorption proceeds. A strong evidence of interaction of Cr with the cell material was also observed. The obtained results provide insights into the biosorption process and chemical speciation of Cr on the cells.

Heavy Metals Induced Modulations in Growth, Physiology, Cellular Viability, and Biofilm Formation of an Identified Bacterial Isolate

The release of untreated tannery effluents comprising biotoxic heavy metal (HM) compounds into the ecosystem is one of our society's most serious environmental and health issues. After discharge, HM-containing industrial effluents reach agricultural soils and thus negatively affect the soil microbial diversity. Considering these, we assessed the effect of HMs on identified soil beneficial bacteria. Here, the effects of four heavy metals (HMs), viz., chromium (Cr), cadmium (Cd), nickel (Ni), and lead (Pb), on cellular growth, physiology, cell permeability, and biofilm formation of Enterobacter cloacae MC9 (accession no.: MT672587) were evaluated. HMs in a concentration range of 25-200 μg mL-1 were used throughout the study. Among HMs, Cd in general had the maximum detrimental effect on bacterial physiology. With increasing concentrations of HMs, bacterial activities consistently decreased. For instance, 200 μgCr mL-1 concentration greatly and significantly (p ≤ 0.05) reduced the synthesis of indole-3-acetic acid (IAA) by 70% over control. Furthermore, 200 μg mL-1 Cd maximally and significantly (p ≤ 0.05) reduced the synthesis of 2,3-dihydroxybenzoic acid (2,3-DHBA), salicylic acid (SA), 1-aminocyclopropane 1-carboxylate (ACC) deaminase, and extra polymeric substances (EPSs) of E. cloacae MC9 by 80, 81, 77, and 59%, respectively, over control. While assessing the toxic effect of HMs on the P-solubilizing activity of E. cloacae, the toxicity pattern followed the order Cr (mean value = 94.6 μg mL-1) > Cd (mean value = 127.2 μg mL-1) > Pb (mean value = 132.4 μg mL-1) > Ni (mean value = 140.4 μg mL-1). Furthermore, the colony-forming unit (CFU) count (Log10) of strain MC9 was completely inhibited at 150, 175, and 200 μg mL-1 concentrations of Cr and Cd. The confocal laser scanning microscopic (CLSM) analysis of HM-treated bacterial cells showed an increased number of red-colored dead cells as the concentration of HMs increased from 25 to 200 μg mL-1. Likewise, the biofilm formation ability of strain MC9 was maximally (p ≤ 0.05) inhibited at higher concentrations of Cd. In summary, the present investigation undoubtedly suggests that E. cloacae strain MC9 recovered from the HM-contaminated rhizosphere endowed with multiple activities could play an important role in agricultural practices to augment crop productivity in soils contaminated with HMs. Also, there is an urgent need to control the direct discharge of industrial waste into running water to minimize heavy metal pollution. Furthermore, before the application of HMs in agricultural fields, their appropriate field dosages must be carefully monitored.

Performance of Zea mays L. cultivars in tannery polluted soils: Management of chromium phytotoxicity through the application of biochar and compost

Soil contamination with heavy metals caused by various industrial activities is a threatening global environmental issue of the current era. Chromium (Cr) is the most toxic heavy metal used in leather industry and disposal of untreated wastewater into natural water bodies leads to contamination of natural soil and water resources. We studied the combined effect of biochar and compost on improving the tolerance to Cr toxicity by enhancing the morpho-physiological and biochemical attributes of two maize cultivars (P-1543 and NK-8441) grown in tannery waste polluted soils. The results of this study reveal that Cr toxicity reduced the plant growth by affecting physiological and biochemical attributes. Here, compost and biochar application significantly increased the plant biomass (fresh and dry), height, photosynthesis, chlorophyll content, water relation, starch, and protein content over treatment set as control. However, significant decline in electrolyte leakage (EL), proline, lipid peroxidation, soluble sugars, and antioxidant enzymes (APX, GPX, GR, GST, GSH, SOD, and CAT) was observed by combined application of compost and biochar. Hexavalent chromium concentration was maximum decreased to 4.1 μg g^-1 in soil after post-harvesting of maize cultivar NK-8441, while in roots and shoots to 22.6 and 19.2 μg g^-1 of maize cultivar P-1543, respectively, by combined application of compost and biochar. Moreover, these both amendments in combination showed considerably better results than their sole application and cultivar P-1543 comparatively performed better than NK 8441, in both K and S soils. Correlation and principal component analysis (PCA) revealed mostly highly positive associations among all the studied morpho, physio, and biochemical attributes of maize plant with the few exceptions, particularly concentration of Cr(III) and Cr(VI) in soil. The present work concluded that combined use of biochar and compost has great potential to decrease Cr toxicity and improve plant growth in tannery polluted soils.

Influence of foliar glutathione and putrescine on metabolism and mineral status of genetically diverse rapeseed cultivars under hexavalent chromium stress

We studied the physio-biochemical involvement of exogenous signaling compounds, glutathione and putrescine (alone and in combination), on three contrasting genotypes (cvs. Shiralee, Rainbow, and Dunkled) of canola (Brassica napus L.) of plants exposed to chromium stress. Seeds were germinated in Cr-contaminated soil (0 and 50 μg/g Cr⁶⁺), and both signaling compounds were applied as a foliar spray to 20-day-old plants. Changes in root, stem, and leaf nitro-oxidative metabolism, endogenous GSH level, secondary metabolites, and mineral nutrients were investigated from 60-day-old plants. Exposure to Cr⁶⁺ increased stem GSH and NO concentrations in all cultivars. Maximum root Cr⁶⁺ bioaccumulation was recorded in cv. Rainbow and the least in cv. Shiralee. Also, Cr⁶⁺ stress decreased number and weight of seeds and pod length. Disturbances in root and shoot mineral profile were evident; however, its magnitude varied in all cultivars. The exogenous GSH improved root and shoot P, Fe, S, and Zn concentrations; however, the effect was cultivar specific. Leaf endogenous GSH was increased by exogenous GSH while NO levels remained unaffected. The GSH application also promoted shoot Cr⁶⁺ bioaccumulation while PUT application caused a recovery in seed number and seed weight. Both PUT and GSH differentially affected tissue-specific secondary metabolite profile. Overall, the exogenous GSH was much more effective in alleviating the Cr⁺⁶ toxicity in canola.

High throughput screening of native species for tailings eco-restoration using novel computer visualization for plant phenotyping

Historical hard-rock mine activities have resulted in nearly half a million mining-impacted sites scattered around the US. Compared to conventional remediation, (aided) phytostabilization is generally cost-effective and ecologically productive approach, particularly for large-scale sites. Native species act to maintain higher local biodiversity, providing a foundation for natural ecological succession. Due to heterogeneity of mine waste, revegetation strategies are inconsistent in approach, and to avoid failure scenarios, greenhouse screening studies can identify candidate plants and amendment strategies before scaling up. This greenhouse study aimed to concurrently screen a variety of native species for their potential to revegetate Cu/Pb/Zn mine tailings and develop a high throughput and non-destructive approach utilizing computer vision and image-based phenotyping technologies to quantify plant responses. A total number of 34 species were screened in this study, which included: 5 trees, 8 grasses, and 21 forbs and legumes. Most of the species tested were Missouri native and prairie species. Plants were non-destructively imaged, and 15 shape and color phenotypic attributes were extracted utilizing computer vision techniques of PlantCV. Compared to reference soil, all species tested were negatively impacted by the tailings' characteristics, with lowest tolerance generally observed in tree species. However, significant improvement in plant growth and tolerance generally observed with biosolids addition with biomass surpassing reference soil for most legumes. Accumulation of Cu, Pb, and Zn was below Domestic Animal Toxicity Limits in most species. Statistically robust differences in species responses were observed using phenotypic data, such as area, height, width, color, and 9 other morphological attributes. Correlations with destructive data indicated that area displayed the greatest positive correlation with biomass and color the greatest negative correlation with shoot metals. Computer visualization greatly increased the phenotypic data and offers a breakthrough in rapid, high throughput data collection to project site-specific phytostabilization strategies to efficiently restore mine-impacted sites.

Foliar application of seed water extract of Nigella sativa improved maize growth in cadmium-contaminated soil

Cadmium (Cd) is a widespread heavy metal, which commonly exert negative impacts on agricultural soils and living organisms. Foliar application of seed water extract of black cumin (Nigella sativa L.) can mitigate the adverse impacts of Cd-toxicity in plants through its rich antioxidants. This study examined the role of seed water extracts of N. sativa (NSE) in mitigating the adverse impacts of Cd-toxicity on maize growth. Two maize genotypes (synthetic ‘Neelum’ and hybrid ‘P1543’) were grown under 0, 4, 8 and 12 mg Cd kg^-1 soil. The NSE was applied at three different concentrations (i.e., 0, 10 and 20%) as foliar spray at 25 and 45 days after sowing. All Cd concentrations had no effect on germination percentage of both genotypes. Increasing Cd concentration linearly decreased root and allometric attributes, gas exchange traits and relative water contents of hybrid genotype. However, gas exchange traits of synthetic genotype remained unaffected by Cd-toxicity. Overall, hybrid genotype showed better tolerance to Cd-toxicity than synthetic genotype with better germination and allometric attributes and less Cd accumulation. Foliar application of NSE lowered negative effects of Cd-toxicity on all studied traits, except relative water contents. In conclusion, foliar application of NSE seemed a viable option to improve maize growth in Cd-contaminated soil.

Menadione sodium bisulfite alleviated chromium effects on wheat by regulating oxidative defense, chromium speciation, and ion homeostasis

Menadione sodium bisulfite (MSB) is a crucial growth regulator mediating plant defense response. MSB-mediated regulation of defense mechanisms in wheat under chromium (Cr) toxicity has not been reported in the literature. Therefore, the present study was undertaken to appraise the efficacy of exogenous MSB on circumventing Cr phytotoxic effects on wheat. We also compared the effects of water-soluble MSB with that of water-insoluble menadiol diacetate (MD). The levels used in the present investigation for MSB and MD were 100 and 200 mg L-1. Wheat plants grown in soil contaminated with 25 mg kg-1 Cr in the form of K2Cr2O7 showed a notable reduction in growth, chlorophyll molecules, relative water contents, grain yield, total soluble sugars, phenolics, flavonoids, ascorbic acid, activities of antioxidant enzymes (SOD, POD, CAT), and uptake of essential nutrients (K, P, and Ca). Cr toxicity caused a noticeable accretion in total free amino acids, proline, malondialdehyde, H2O2, O2•-, relative membrane permeability, methylglyoxal contents, activities of enzymes (lipoxygenase, glutathione-S-transferase, and ascorbate peroxidase), nitric oxide and H2S contents, glutathione and oxidized glutathione contents, total Cr contents, and Cr6+ and Cr3+ accumulation. MSB application significantly reduced lipid peroxidation, ROS overproduction, methylglyoxal levels, total Cr contents, and maintained higher Cr3+:Cr6+ ratio in aerial parts. Besides, Cr-mediated inhibition in essential nutrient uptake was significantly circumvented by exogenous MSB. Consequently, MSB enhanced wheat growth by lessening oxidative damage, total Cr contents in aerial parts, and strengthening antioxidant enzyme activities. MD was not effective in mediating defense responses in wheat under Cr toxicity.

Bioremediation of Chromium by Microorganisms and Its Mechanisms Related to Functional Groups

Heavy metals generated mainly through many anthropogenic processes, and some natural processes have been a great environmental challenge and continued to be the concern of many researchers and environmental scientists. This is mainly due to their highest toxicity even at a minimum concentration as they are nonbiodegradable and can persist in the aquatic and terrestrial environments for long periods. Chromium ions, especially hexavalent ions (Cr(VI)) generated through the different industrial process such as tanneries, metallurgical, petroleum, refractory, oil well drilling, electroplating, mining, textile, pulp and paper industries, are among toxic heavy metal ions, which pose toxic effects to human, plants, microorganisms, and aquatic lives. This review work is aimed at biosorption of hexavalent chromium (Cr(VI)) through microbial biomass, mainly bacteria, fungi, and microalgae, factors influencing the biosorption of chromium by microorganisms and the mechanism involved in the remediation process and the functional groups participated in the uptake of toxic Cr(VI) from contaminated environments by biosorbents. The biosorption process is relatively more advantageous over conventional remediation technique as it is rapid, economical, requires minimal preparatory steps, efficient, needs no toxic chemicals, and allows regeneration of biosorbent at the end of the process. Also, the presence of multiple functional groups in microbial cell surfaces and more active binding sites allow easy uptake and binding of a greater number of toxic heavy metal ions from polluted samples. This could be useful in creating new insights into the development and advancement of future technologies for future research on the bioremediation of toxic heavy metals at the industrial scale.

Rhizobial-Host Interactions and Symbiotic Nitrogen Fixation in Legume Crops Toward Agriculture Sustainability

Symbiotic nitrogen fixation (SNF) process makes legume crops self-sufficient in nitrogen (N) in sharp contrast to cereal crops that require an external input by N-fertilizers. Since the latter process in cereal crops results in a huge quantity of greenhouse gas emission, the legume production systems are considered efficient and important for sustainable agriculture and climate preservation. Despite benefits of SNF, and the fact that chemical N-fertilizers cause N-pollution of the ecosystems, the focus on improving SNF efficiency in legumes did not become a breeder’s priority. The size and stability of heritable effects under different environment conditions weigh significantly on any trait useful in breeding strategies. Here we review the challenges and progress made toward decoding the heritable components of SNF, which is considerably more complex than other crop allelic traits since the process involves genetic elements of both the host and the symbiotic rhizobial species. SNF-efficient rhizobial species designed based on the genetics of the host and its symbiotic partner face the test of a unique microbiome for its success and productivity. The progress made thus far in commercial legume crops with relevance to the dynamics of host–rhizobia interaction, environmental impact on rhizobial performance challenges, and what collectively determines the SNF efficiency under field conditions are also reviewed here.

Alleviating Cr(VI) stress in horse gram (Macrotyloma uniflorum Var. Madhu) by native Cr-tolerant nodule endophytes isolated from contaminated site of Sukinda

Sukinda chromite mine of Odisha is a heavily polluted site, generating huge overburden dumps. The present experiment was designed to evaluate the potential of two native nodule endophytic bacterial strains, viz. Bacillus aryabhattai AS03 (MT645244) and Rhizobium pusense AS05 (MT645243), isolated from contaminated sites to be considered remediation tool to minimize the effect of Cr toxicity on Macrotyloma uniflorum var. Madhu. The two nodule endophytic bacterial strains AS03 and AS05 exhibited tolerance to 1800 and 3000 ppm of Cr(VI) respectively in vitro when cultured alone. AAS analysis confirmed higher accumulation of Cr(VI) in roots and less accumulation in shoots which is dose-specific (bio-inoculant) either treated alone or combined. Complete absence of Cr accumulation approximately 99% in shoots of Macrotyloma was observed owing to synergistic effect of both the strains (biochar-based formulation). This study also suggests increased shoot and root length, nodule nos., and leghemoglobin content of the plant at 60 days indicating the plant growth-promoting effects of both the strains. ROS and antioxidant enzymes of the plant recorded decreasing trend in inoculated plants. However, a significant increment in transpiration rate, total photosynthetic rate, intracellular CO2 conc., and stomatal conductance in leaves was observed owing to dual inoculation. Our findings corroborate the supremacy of synergistic effect of both the strains applied in the form of biochar-based biofertilizer in enhancing growth and tolerance index of M. uniflorum cultivated in Cr(VI)-stressed soil. This investigation depicts the efficiency of the two nodule bacteria as a mixed inoculant to alleviate Cr toxicity and making the seeds safe for consumption.

Chromium contamination and effect on environmental health and its remediation: A sustainable approaches

Chromium (Cr) is a trace element critical to human health and well-being. In the last few decades, its contamination, especially hexavalent chromium [Cr(VI)] form in both terrestrial and aquatic ecosystems, has amplified as a result of various anthropogenic activities. Chromium pollution is a significant environmental threat, severely impacting our environment and natural resources, especially water and soil. Excessive exposure could lead to higher levels of accumulation in human and animal tissues, leading to toxic and detrimental health effects. Several studies have shown that chromium is a toxic element that negatively affects plant metabolic activities, hampering crop growth and yield and reducing vegetable and grain quality. Thus, it must be monitored in water, soil, and crop production system. Various useful and practical remediation technologies have been emerging in regulating chromium in water, soil, and other resources. A sustainable remediation approach must be adopted to balance the environment and nature.

Metabolic response of bok choy leaves under chromium pollution stress

Chromium (Cr) pollution in farmlands is a common environmental issue, that can seriously inhibit plant growth, damage plant cells, and even cause plant death. In this study, bok choy (Brassica campestris L. ssp. chinensis Makino (var. communis Tsen et Lee)) was selected as a model plant to investigate the metabolic response to Cr stress at concentrations of 2.0 mg/L and 8.0 mg/L. Metabolites were identified using gas chromatography-mass spectrometry. Principal component analysis and orthogonal projections to latent structure discriminant analysis revealed the notable effect of Cr stress on the metabolites of bok choy. Under Cr stress, 145 metabolites were identified in the bok choy leaves. At 2.0 mg/L Cr stress, 10 and 26 metabolites changed compared to the control after 7 d and 14 d, respectively. At 8.0 mg/L Cr stress, 24 and 24 metabolites changed significantly after 7 and 14 d, respectively. The data showed that metabolism was affected by the Cr stress concentration and exposure time. Specifically, under the Cr stress, the tricarboxylic acid cycle, glutamine synthetase/glutamate synthase cycle, and partial amino acid metabolic pathways were blocked, inhibiting the normal growth and development of bok choy. The change of citric acid content was the most significant, and the accumulation of citric acid indicated the degree of plant Cr toxicity and resistance. These results would facilitate further dissection of the mechanisms of heavy metal accumulation/tolerance in plants and the effective management of such contamination in vegetable crops by genetic manipulation.

Exogenous application of black cumin (Nigella sativa) seed extract improves maize growth under chromium (Cr) stress

Accumulation of non-essential heavy metals like chromium (Cr) is among major abiotic stresses, which adversely affect crop growth. Hexavalent chromium [Cr(VI)] is the most dangerous form negatively affecting the growth and productivity of crops. This study evaluated the role of black cumin extracts (BCE) in improving growth and productivity of maize genotypes under different concentrations of Cr(VI). Two maize genotypes ("Neelum" and "P1543") were grown under 0, 4, 8 and 12 mg Cr(VI) kg^-1 concentrations. The BCE was applied as foliar spray at three concentrations (0, 10 and 20%) at 25 and 45 days after sowing. Increasing Cr(VI) concentration significantly (p < 0.05) reduced seed germination, root and allometric traits, gas exchange attributes and relative water contents of tested genotypes. Hybrid maize genotype better tolerated tested Cr(VI) concentrations than synthetic genotype with lower Cr accumulation and better allometric and gas exchange traits. Exogenous application of 20% BCE proved effective in lowering the adverse effects of Cr(VI) toxicity on maize genotypes. It is concluded that 20% BCE could be used to improve maize performance through better allometric and gas exchange traits under different Cr(VI) concentrations. Nonetheless, actual mechanisms involved in improved Cr(VI)-tolerance of maize with BCE application must be explored. Novelty statement Black cumin has been widely used to reduce Cr toxicity in animals. However, the role of black cumin in reducing Cr toxicity in plants has never been studied. The present study was conducted to infer the role of different concentrations of black cumin extract in improving the growth of synthetic and hybrid maize genotypes under different levels of Cr stress. It is concluded that black cumin extract could be used to lower Cr toxicity in maize grown under Cr-contaminated soils.

Chromium stress induced oxidative burst in Vigna mungo (L.) Hepper: physio-molecular and antioxidative enzymes regulation in cellular homeostasis

Vigna mungo (L.) Hepper commonly known as blackgram is an important legume crop with good quality dietary proteins and vitamins. Low production of blackgram in the chromium rich soil of Odisha is a serious concern against its demand. Chromium (VI) was tested on V. mungo var. B3-8-8 at 100, 150, 200, 250 and 300 µM concentration on growth, anti-oxidative enzymes and chromium content at 15, 30 and 45 d of treatments. Seed germination and growth decreased with increase dose and duration. Cr uptake induced oxidative burst with significant increase of osmolytes was observed in cell at lower doses but failed to adjust homeostasis at higher dose. Increase of GPX and SOD and decrease of CAT was observed as dose dependent. Increased protein content was detected in < 200 µM Cr concentration whereas, significant decrease of protein was noted thereafter. Down regulation of proteins (29.2 kDa and 32.6 kDa) was observed at > 250 µM of Cr. Total Cr uptake was greater in root than in shoot which might be due to poor translocation of heavy metal or detoxification. Thus, blackgram was able to maintain homeostasis at lower concentrations of Cr by activating the cascade of enzymes following cellular detoxification mechanism.

SUPPLEMENTARY INFORMATION

The online version of this article contains supplementary material available at (10.1007/s12298-021-00941-3).