High-Affinity Sulfate Transporter Sultr1;2 Is a Major Transporter for Cr(VI) Uptake in Plants

Selenium alleviates chromium stress and promotes chromium uptake in As-hyperaccumulator Pteris vittata: Cr reduction and cellar distribution

Arsenic-hyperaccumulator Pteris vittata exhibits remarkable absorption ability for chromium (Cr) while beneficial element selenium (Se) helps to reduce Cr-induced stress in plants. However, the effects of Se on the Cr uptake and the associated mechanisms in P. vittata are unclear, which were investigated in this study. P. vittata plants were grown for 14 days in 0.2-strength Hoagland solution containing 10 (Cr10) or 100 μM (Cr100) chromate (CrVI) and 1 μM selenate (Se1). The plant biomass, malondialdehyde contents, total Cr and Se contents, Cr speciation, expression of genes associated with Cr uptake, and Cr subcellular distribution in P. vittata were determined. P. vittata effectively accumulated Cr by concentrating 96-99% in the roots under Cr100 treatment. Further, Se substantially increased its Cr contents by 98% to 11,596 mg kg-1 in the roots, which may result from Se's role in reducing its oxidative stress as supported by 27-62% reduction in the malondialdehyde contents. Though supplied with CrVI, up to 98% of the Cr in the roots was reduced to insoluble chromite (CrIII), with 83-89% being distributed on root cell walls. Neither Cr nor Se upregulated the expression of sulfate transporters PvSultr1;1-1;2 or phosphate transporter PvPht1;4, indicating their limited role in Cr uptake. P. vittata effectively accumulates Cr in the roots mainly as CrIII on cell walls and Se effectively enhances its Cr uptake by reducing its oxidative stress. Our study suggests that Se can be used to enhance P. vittata Cr uptake and reduce its oxidative stress, which may have application in phytostabilization of Cr-contaminated soils.

Ecotoxicity of hexavalent chromium [Cr (VI)] in soil presents predominate threats to agricultural production with the increase of soil Cr contamination

The relative severity between chromium (Cr)-mediated ecotoxicity and its bioaccumulation has rarely been compared and evaluated. This study employed pot incubation experiments to simulate the soil environment with increased Cr pollution and study their effects on the growth of crops, including pepper, lettuce, wheat, and rice. Results showed that increasing total Cr presented ascendant ecotoxicity in upland soils when pH > 7.5, and significantly reduced the yield of pepper, lettuce and wheat grain by 0.3-100 %, whereas, this effect was weakened even reversed as the pH decreased. Surprisingly, a series of soils with Cr concentration of 22.7-623.5 mg kg-1 did not cause Cr accumulation in four crops over the Chinese permissible limit. The toxicity of Cr was highly associated with extractable Cr, where Cr (VI) made the greater contributions than Cr (III). Conclusively, the ecotoxicity of Cr poses a greater environmental issue as compared to the bioaccumulation of Cr in crops in upland soils, while extractable Cr (VI) makes the predominant contributions to the ecotoxicity of Cr as the total Cr increased. Our study proposes a synchronous consideration involving total Cr and Cr (VI) as the theoretical basis to establish a more reliable soil quality standard for safe production in China.

Hexavalent chromium uptake in rice (Oryza sativa L.) mediated by sulfate and phosphate transporters OsSultr1;2 and OsPht1;1

Chromium (Cr) soil contamination is a critical global environmental concern, with hexavalent chromium (Cr[VI]) being especially perilous due to its high mobility, bioavailability, and phytotoxicity. This poses a significant threat to the cultivation of crops, particularly rice, where the mechanisms of Cr(VI) absorption remain largely unexplored. This study uncovered a competitive interaction between Cr(VI) and essential nutrients-sulfate and phosphate during the uptake process. Notably, deficiencies in sulfate and phosphate were associated with a marked increase in Cr(VI) accumulation in rice, reaching up to 76.5 % and 77.7 %, respectively. Employing q-PCR, this study identified significant up-regulation of the sulfate transporter gene, OsSultr1;2, and the phosphate transporter gene, OsPht1;1, in response to Cr(VI) stress. Genetic knockout studies have confirmed the crucial role of OsSultr1;2 in Cr(VI) uptake, with its deletion leading to a 36.1 % to 69.6 % decrease in Cr uptake by rice roots. Similarly, the knockout of OsPht1;1 resulted in an 18.1 % to 25.7 % decrease in root Cr accumulation. These findings highlight the key role of the sulfate transporter OsSultr1;2 in Cr(VI) uptake, with phosphate transporters also contributing significantly to the process. These insights are valuable for developing rice varieties with reduced Cr(VI) accumulation, ensuring the safety of rice grain production.

Cr(VI) behaves differently than Cr(III) in the uptake, translocation and detoxification in rice roots

Excessive accumulation of chromium (Cr) causes severe damage to both physiological and biochemical processes and consequently growth repression in plants. Hexavalent chromium [Cr(VI)]-elicited alterations in plants have been widely elucidated at either physiological or molecular level, whereas little is known about trivalent chromium [Cr(III)]. Here, we found that both Cr(III) and Cr(VI) significantly inhibited root growth in rice plants. However, rice plants under Cr(VI) showed significantly less inhibition in root growth than those under Cr(III) at low levels, which might be attributed to the different hormetic effects of Cr(III) and Cr(VI) on rice plants. It was unexpected that Cr(III) could be actively taken up by rice roots similarly to Cr(VI); whereas they exhibited different kinetic uptake patterns. Furthermore, root-to-shoot Cr translocation under Cr(VI) was much lower than that under Cr(III). These results indicate that the uptake, translocation, and toxicity of Cr(III) differed greatly from those of Cr(VI). Transcriptome profiling of rice roots revealed that a series of gene families involved in detoxification, including ATP-binding cassette (ABC) transporters, multidrug and toxic compound extrusion proteins (MATEs), and Tau class glutathione S-transferases (GSTUs), were significantly associated with Cr accumulation and detoxification in rice roots. In addition, much more members of these gene families were upregulated by Cr(VI) compared to Cr(III), suggesting their vital roles in Cr uptake, translocation, and detoxification, especially under Cr(VI) stress. Further comparison of gstu9 and gstu10/50 mutants with their wild type confirmed that GSTUs play complex roles in the intracellular Cr transport and redox homeostasis during Cr(III) or Cr(VI) stress. Taken together, our findings provides new insights into the differential behaviors of Cr(III) and Cr(VI) in rice roots, as well as new candidate genes such as OsABCs and OsGSTUs, to further elucidate the mechanisms of the uptake, translocation, and detoxification of Cr(III) and Cr(VI).

Fe(III) transporter OsYSL15 may play a key role in the uptake of Cr(III) in rice (Oryza sativa L.)

Due to the widely discharge of chromium (Cr) by mining and smelting industries, etc., contamination of paddy soils and rice has become serious problems. Therefore it is crucial to explore how rice takes up Cr. Cr(III) is the most common Cr form in the long-term water flooding paddy soils. Here, we demonstrate that OsYSL15, a key gene for Fe(III) uptake, is equally applicable for Cr(III) uptake in rice. Firstly, the antagonistic effect of Cr(III) and Fe(III) in the uptake process was found. Rice could accumulate more Cr(III) under Fe-deficient conditions. And the Fe(III) content in the protoplasts of rice root cells gradually decreased with the increase exposure of Cr(III). Knockdown of OsYSL15 in rice significantly reduced the Cr(III) uptake rate. Compared with wild type rice, the accumulation of Cr(III) in OsYSL15 mutant was decreased by 40.7%- 70.6% after gene editing. These results indicate that OsYSL15 is a key gene responsible for Cr(III) uptake in rice, which can guide the screening or genetic modification for low-Cr-accumulation rice varieties.

Applications and opportunities of click chemistry in plant science

The Nobel Prize in Chemistry for 2022 was awarded to the pioneers of Lego-like 'click chemistry': combinatorial chemistry with remarkable modularity and diversity. It has been applied to a wide variety of biological systems, from microorganisms to plants and animals, including humans. Although click chemistry is a powerful chemical biology tool, comparatively few studies have examined its potential in plant science. Here, we review click chemistry reactions and their applications in plant systems, highlighting the activity-based probes and metabolic labeling strategies combined with bioorthogonal click chemistry to visualize plant biological processes. These applications offer new opportunities to explore and understand the underlying molecular mechanisms regulating plant composition, growth, metabolism, defense, and immune responses.

Selenium Nanoparticles Mitigate Cadmium Stress in Tomato through Enhanced Accumulation and Transport of Sulfate/Selenite and Polyamines

Accumulation of cadmium (Cd) ions in soil is an increasingly acute ecological problem in agriculture production. Selenium nanoparticles (SeNPs) can mediate Cd tolerance in plants; however, the underlying mechanisms remain unclear. Herein, we show that the foliar application of SeNPs improved the adaptive capacity of tomato plants to decrease Cd-induced damage. SeNPs induced more Cd in roots but not in shoots despite greater accumulation of selenium and sulfur in both tissues and high selenate influx. Additionally, SeNPs significantly increased thiol compounds, including glutathione, cysteine, and phytochelatins, contributing to enhanced Cd detoxification. Importantly, SeNPs induced the expression of sulfate transporters 1:3, S-adenosylmethionine 1 and polyamine transporter 3. Then, experiments with mutants of these genes showed that SeNP-reduced Cd stress largely relies on the levels and shoot-to-root transport of selenium/sulfur and polyamines. These findings highlight the potential of SeNPs to improve crop production and phytoremediation in heavy metal-contaminated soils.

Physio-anatomical modifications and element allocation pattern in Alternanthera tenella Colla. associated with phytoextraction of chromium

Intensive industrial activities have elevated chromium (Cr) concentrations in the environment, particularly in soil and water, posing a significant threat due to its cytotoxic and carcinogenic properties. Phytoremediation has emerged as a sustainable and economical alternative for detoxifying pollutants. In this context, an attempt has been made to assess the efficacy of Cr remediation by the invasive plant Alternanthera tenella Colla. The study investigated morphological, anatomical, and physiological adaptations in plant tissues in response to 240 µM of K2Cr2O7, considering elemental distribution patterns and bioaccumulation potential. Growth parameter assessments revealed a notable 50% reduction in root elongation and biomass content; however, the plant exhibited a comparatively higher tolerance index (47%) under Cr stress. Chromium significantly influenced macro and micro-elemental distribution in plant tissues, particularly in roots and leaves. Structural modifications, including changes in the thickness and diameter of xylem walls in the root, stem, and leaf tissues of Cr-treated A. tenella, were observed. Distinct cell structural distortions and Cr deposit inclusions in the xylem wall and inner parenchyma cells were distinct. Under Cr stress, there was a reduction in pigment content and metabolites such as proteins and soluble sugars, while proline, phenol, and malondialdehyde showed a twofold increase. The concentration of Cr was higher in the shoots of A. tenella (185.7 mg/kg DW) than in the roots (179.625 mg/kg DW). With a high BCFroot value (16.23) and TF > 1, coupled with effective mechanisms to cope with metal stress, A. tenella emerges as an ideal candidate for chromium phytoextraction.

Progress in phytoremediation of chromium from the environment

As chromium (Cr) in ecosystems affects human health through food chain exposure, phytoremediation is an environmentally friendly and efficient way to reduce chromium pollution in the environment. Here, we review the mechanism of absorption, translocation, storage, detoxification, and regulation of Cr in plants. The Cr(VI) form is more soluble, mobile, and toxic than Cr(III), reflecting how various valence states of Cr affect environmental risk characteristics, physicochemical properties, toxicity, and plant uptake. Plant root's response to Cr exposure leads to reactive oxygen species (ROS) generation and apoptosis. Cell wall immobilization, vacuole compartmentation, interaction of defense proteins and organic ligand with Cr, and removal of reactive oxygen species by antioxidants continue plant life. In addition, the combined application of microorganisms, genetic engineering, and the addition of organic acids, nanoparticles, fertilization, soil amendments, and other metals could accelerate the phytoremediation process. This review provides efficient methods to investigate and understand the complex changes of Cr metabolism in plants. Preferably, fast-growing, abundantly available biomass species should be modified to mitigate Cr pollution in the environment as these green and efficient remediation technologies are necessary for the protection of soil and water ecology.

Elucidating growth and biochemical characteristics of rice seedlings under stress from chromium VI salt and nanoparticles

Plants are usually provoked by a variety of heavy metal (HM) stressors that have adverse effects on their growth and other biochemical characterizations. Among the HMs, chromium has been considered the most toxic for both plants and animals. The present study was conducted to compare the phytotoxic effects of increasing chromium (VI) salt and nanoparticles (NPs) concentrations on various growth indexes of rice (Oryza sativa L. var. swat 1) seedlings grown in a hydroponic system. The 7-day rice seedlings were exposed to Cr (VI) salt and NPs hydroponic suspensions which were adjusted to the concentration of 0, 50, 100, 150, 200 and 250 mg/L. Both the Cr (VI) salt and NPs with lower concentrations (up to 100mg/L) exerted minimum inhibitory effects on the growth performance of rice seedlings. However, a significant decrease in shoot and root length and their fresh and dry weight was recorded at higher doses of Cr (VI) salt (200 mg/L) and NPs (250 mg/L). The stress induced by Cr (VI) salt has drastically affected the roots, whereas, Cr (VI) NPs significantly affected the shoot tissues. Photosynthetic pigments decreased significantly in a dose-dependent manner, and the reduction was more pronounced in rice seedlings exposed to Cr (VI) NPs compared to Cr (VI) salt. Cr (VI) NPs enhanced the membrane permeability in shoots and roots as compared to that of Cr (VI) salt, which resulted in higher concentration of reactive oxygen species (ROS) and increased lipid peroxidation. The activities of antioxidant enzymes superoxide dismutase (SOD), peroxidase (POD), catalase (CAT) and ascorbate peroxidase (APX) increased significantly in shoot/root tissue following exposure to higher doses of Cr (VI) salt (200 mg/L) and NPs stress (250 mg/L), while minor changes in CAT and APX activities were observed in root and shoot tissues after exposure to higher concentration of Cr (VI) NP. Furthermore, the increasing concentrations of Cr (VI) NPs increased the length of stomatal guard cells. Conclusively, Cr (VI) salt and NPs in higher concentrations have higher potential to damage the growth and induce oxidative stress in rice plants.

Evaluation and Assessment of Trivalent and Hexavalent Chromium on Avena sativa and Soil Enzymes

Chromium (Cr) can exist in several oxidation states, but the two most stable forms-Cr(III) and Cr(VI)-have completely different biochemical characteristics. The aim of the present study was to evaluate how soil contamination with Cr(III) and Cr(VI) in the presence of Na2EDTA affects Avena sativa L. biomass; assess the remediation capacity of Avena sativa L. based on its tolerance index, translocation factor, and chromium accumulation; and investigate how these chromium species affect the soil enzyme activity and physicochemical properties of soil. This study consisted of a pot experiment divided into two groups: non-amended and amended with Na2EDTA. The Cr(III)- and Cr(VI)-contaminated soil samples were prepared in doses of 0, 5, 10, 20, and 40 mg Cr kg-1 d.m. soil. The negative effect of chromium manifested as a decreased biomass of Avena sativa L. (aboveground parts and roots). Cr(VI) proved to be more toxic than Cr(III). The tolerance indices (TI) showed that Avena sativa L. tolerates Cr(III) contamination better than Cr(VI) contamination. The translocation values for Cr(III) were much lower than for Cr(VI). Avena sativa L. proved to be of little use for the phytoextraction of chromium from soil. Dehydrogenases were the enzymes which were the most sensitive to soil contamination with Cr(III) and Cr(VI). Conversely, the catalase level was observed to be the least sensitive. Na2EDTA exacerbated the negative effects of Cr(III) and Cr(VI) on the growth and development of Avena sativa L. and soil enzyme activity.

A preliminary study to explain how Streptomyces pactum (Act12) works on phytoextraction: soil heavy metal extraction, seed germination, and plant growth

Streptomyces pactum (Act12) can both promote plant growth and strengthen heavy metal mobilization. Nevertheless, the mechanisms of how Act12 works during the phytoextraction process are still unknown. The present work investigated whether the metabolites produced by Act12 could influence the seed germination and the growth of potherb mustard and explored its mobilizing effect on soil cadmium (Cd) and zinc (Zn). The results showed that the germination potential and rate of potherb mustard seed treated with Act12 fermentation broth were 1.0- and 0.32-folds higher than those of control, probably due to the interruption of seed dormant stage. We also found that Act12 inoculation not only promoted the dry biomass (6.82%) of potherb mustard, but also increased the leaf chlorophyll (11.8%) and soluble protein (0.35%) production. The boosted seed germination rate under Act12 treatment (up to 63.3%) indicated that Act12 enhanced the resistance of potherb mustard seeds to Cd and Zn and alleviated their physiological toxicity. The generated metabolites during the Act12 fermentation posed positive impact on the availability of soil Cd and Zn. These findings bring new insight into the Act12-assisted phytoextraction of Cd and Zn from contaminated soils.

Physiological and transcriptional studies reveal Cr(VI) reduction mechanisms in the exoelectrogen Cellulomonas fimi Clb-11

A facultative exoelectrogen, Cellulomonas fimi strain Clb-11, was isolated from polluted river water. This strain could generate electricity in microbial fuel cells (MFCs) with carboxymethyl cellulose (CMC) as the carbon source, and the maximum output power density was 12.17 ± 2.74 mW·m^-2. In addition, Clb-11 could secrete extracellular chromate reductase or extracellular electron mediator to reduce Cr(VI) to Cr(III). When the Cr(VI) concentration was less than 0.5 mM in Luria-Bertani (LB) medium, Cr(VI) could be completely reduced by Clb-11. However, the Clb-11 cells swelled significantly in the presence of Cr(VI). We employed transcriptome sequencing analysis to identify genes involved in different Cr(VI) stress responses in Clb-11. The results indicate that 99 genes were continuously upregulated while 78 genes were continuously downregulated as the Cr(VI) concentration increased in the growth medium. These genes were mostly associated with DNA replication and repair, biosynthesis of secondary metabolites, ABC transporters, amino sugar and nucleotide sugar metabolism, and carbon metabolism. The swelling of Clb-11 cells might have been related to the upregulation of the genes atoB, INO1, dhaM, dhal, dhak, and bccA, which encode acetyl-CoA C-acetyltransferase, myo-inositol-1-phosphate synthase, phosphoenolpyruvate-glycerone phosphotransferase, and acetyl-CoA/propionyl-CoA carboxylase, respectively. Interestingly, the genes cydA and cydB related to electron transport were continuously downregulated as the Cr(VI) concentration increased. Our results provide clues to the molecular mechanism of Cr(VI) reduction by microorganisms in MFCs systems.

Water Management Impacts on Chromium Behavior and Uptake by Rice in Paddy Soil with High Geological Background Values

Chromium (Cr) is an expression toxic metal and is seriously released into the soil environment due to its extensive use and mining. Basalt is an important Cr reservoir in the terrestrial environment. Cr in paddy soil can be enriched by chemical weathering. Therefore, basalt-derived paddy soils contain extremely high concentrations of Cr and can enter the human body through the food chain. However, the water management conditions' effect on the transformation of Cr in basalt-derived paddy soil with high geological background values was less recognized. In this study, a pot experiment was conducted to investigate the effects of different water management treatments on the migration and transformation of Cr in a soil-rice system at different rice growth stages. Two water management treatments of continuous flooding (CF) and alternative wet and dry (AWD) and four different rice growth stages were set up. The results showed that AWD treatment significantly reduced the biomass of rice and promoted the absorption of Cr in rice plants. During the four growth periods, the root, stem and leaf of rice increased from 11.24-16.11 mg kg^-1, 0.66-1.56 mg kg^-1 and 0.48-2.29 mg kg^-1 to 12.43-22.60 mg kg^-1, 0.98-3.31 mg kg^-1 and 0.58-2.86 mg kg^-1, respectively. The Cr concentration in roots, stems and leaves of AWD treatment was 40%, 89% and 25% higher than CF treatment in the filling stage, respectively. The AWD treatment also facilitated the potential bioactive fractions conversion to the bioavailable fraction, compared with the CF treatment. In addition, the enrichment of iron-reducing bacteria and sulfate-reducing bacteria with AWD treatment also provided electron iron for the mobilization of Cr, thus affecting the migration and transformation of Cr in the soil. We speculated that the reason for this phenomenon may be the bioavailability of Cr was affected by the biogeochemical cycle of iron under the influence of alternating redox. This indicates that AWD treatment may bring certain environmental risks in contaminated paddy soil with high geological background, and it is necessary to be aware of this risk when using water-saving irrigation to plant rice.

Staphylococcus edaphicus KCB02A11 incorporated with natural adsorbents: first report on its tolerance and removal of hexavalent chromium [Cr(VI)]

Deteriorating the quality of different parts of the ecosystem due to toxic metals is a serious global issue. Hexavalent chromium is a metal that can cause adverse effects on all living beings, including plants, animals, and microorganisms, on exposure to high concentrations for prolonged periods. Removing hexavalent chromium from various types of wastes is challenging; hence the present study investigated the use of bacteria incorporated with selected natural substrates in removing hexavalent chromium from water. Isolated Staphylococcus edaphicus KCB02A11 has shown higher removal efficiency with a wide hexavalent chromium range (0.025-8.5 mg/L) within 96 h. Incorporating the isolated strain with natural substrates commonly found in the environment (hay and wood husk) showed high removal potential [100% removal with 8.5 mg/L of Cr(VI)], even within less than 72 h, with the formation of biofilms on the used substrates applied for metal removal on a large scale for prolonged periods. This study is the first report investigating hexavalent chromium tolerance and removal by Staphylococcus edaphicus KCB02A11.

Chromium transformation driven by iron redox cycling in basalt-derived paddy soil with high geological background values

The flooding and drainage of paddy fields has great effects on the transformation of heavy metals, however, the transformation of Cr in basalt-derived paddy soil with high geological background values was less recognized. The typical basalt-derived paddy soil was incubated under alternating redox conditions. The Cr fractions and the dynamics of Fe/N/S/C were examined. The HCl-extractable Cr increased under anaerobic condition and then decreased during aerobic stage. The UV-vis spectra of the supernatant showed that amounts of colloids were released under anaerobic condition, and then re-aggregated during aerobic phase. The scanning transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) revealed that Fe oxides were reduced and became dispersed during anaerobic stage, whereas Fe(II) was oxidized and recrystallized under aerobic condition. Based on these results, a kinetic model was established to further distinguish the relationship between the transformation of Cr and Fe. During anaerobic phase, the reduction of Fe(III) oxides not only directly released the structurally bound Cr, but also enhanced the breakdown of soil aggregation and dissolution of organic matter causing indirect mobilization of Cr. During aerobic phase, the oxidation of Fe(II) and further recrystallization of newly formed Fe(III) oxides might induce the re-aggregation of soil colloids and further incorporation of Cr. In addition, the kinetic model of Cr and Fe transformation was further verified in the pot experiment. The model-based findings demonstrated that the Cr transformation in the basalt-derived paddy soil with high geological background values was highly driven by redox sensitive iron cycling.

Brassica juncea and the Se-hyperaccumulator Stanleya pinnata exhibit a different pattern of chromium and selenium accumulation and distribution while activating distinct oxidative stress-response signatures

Soils high in chromium and selenium exist in some countries, like China, India and the US. In the forms of chromate and selenate, these elements can compete during uptake by plants and lead to secondary effects on the absorption of the essential nutrient sulfur. In this study, we evaluated the potential of Brassica juncea and the Se-hyperaccumulator Stanleya pinnata to take-up and store chromium and selenium when applied individually or jointly, the effect on sulfur content, and the plant antioxidant responses. The aim is to advise the best use of these species in phytotechnologies. Plants were grown hydroponically with 50 μM chromate, 50 μM selenate and equimolar concentrations of both elements (50 μM chromate + 50 μM selenate). Our results suggest that B. juncea and S. pinnata possess transport systems with different affinity for chromate and selenate. The joint application of chromate and selenate restricted the accumulation of both elements, but the reduction of selenate uptake by chromate was more evident in B. juncea. On the other hand, selenate decreased chromium accumulation in B. juncea, whereas in S. pinnata such effect was evident only in roots. B. juncea plants stored more chromium and selenium than S. pinnata due to the higher biomass produced, but less selenium when treated with both elements. Chromate and selenate decreased sulfur accumulation in both species, but B. juncea was more sensitive to their toxicity when applied individually, as revealed by increased lipid peroxidation, hydrogen peroxide content in roots and antioxidant enzyme activity. This species can still be efficient for chromium and selenium phytoextraction as these elements in soil are less available than in hydroponics. In soils high in both elements, or low in selenium, S. pinnata is preferred for selenium phytoextraction and the biomass could be used for crop biofortification due its negligible chromium content.

Regulation of chromium translocation to shoot and physiological, metabolomic, and ionomic adjustments confer chromium stress tolerance in the halophyte Suaeda maritima

Chromium (Cr) is a highly toxic element adversely affecting the environment, cultivable lands, and human populations. The present study investigated the effects of Cr (VI) (100-400 μM) on plant morphology and growth, photosynthetic pigments, organic osmolytes, ionomics, and metabolomic dynamics of the halophyte Suaeda maritima to decipher the Cr tolerance mechanisms. Cr exposure reduced the growth and biomass in S. maritima. The photosynthetic pigments content significantly declined at higher Cr concentrations (400 μM). However, at lower Cr concentrations (100-300 μM), the photosynthetic pigments remained unaffected or increased. The results suggest that a high concentration of Cr exposure might have adverse effects on PS II in S. maritima. The enhanced uptake of Na⁺ in S. maritima imposed to Cr stress indicates that Na⁺ might have a pivotal role in osmotic adjustment, thereby maintaining water status under Cr stress. The proline content was significantly upregulated in Cr-treated plants suggesting its role in maintaining osmotic balance and scavenging ROS. The metabolomic analysis of control and 400 μM Cr treated plants led to the identification of 62 metabolites. The fold chain analysis indicated the upregulation of several metabolites, including phytohormones (SA and GA₃), polyphenols (cinnamic acid, sinapic acid, coumaric acid, vanillic acid, and syringic acid), and amino acids (alanine, leucine, proline, methionine, and cysteine) under Cr stress. The upregulation of these metabolites suggests the enhanced metal chelation and sequestration in vacuoles, reducing oxidative stress by scavenging ROS and promoting photosynthesis by maintaining the chloroplast membrane structure and photosynthetic pigments. Furthermore, in S. maritima, Cr tolerance index (T_i) was more than 60% in all the treatments, and Cr bio-concentration factor (BCF) and translocation factor (T_f) values were all greater than 1.0, which clearly indicates the Cr-hyperaccumulator characteristics of this halophyte.

Physiological and transcriptomic evidence of antioxidative system and ion transport in chromium detoxification in germinating seedlings of soybean

Chromium (Cr) toxicity impairs the productivity of crops and is a major threat to food security worldwide. However, the effect of Cr toxicity on seed germination and transcriptome of germinating seedlings of soybean crop has not been fully explored. In this study, two Cr-tolerant lines (J82, S125) and two Cr-sensitive ones (LD1, RL) were screened out of twenty-one soybean (Glycine max L.) genotypes based on seed germination rate, seed germinative energy, seed germination index, and growth of germinating seedlings under 50 mg L^-1 Cr treatment. We found that Cr stress inhibits the growth of soybean seed germinating seedlings due to the Cr-induced overaccumulation of reactive oxygen species (ROS). Significantly different levels of element contents, antioxidant enzyme activities, malondialdehyde content were observed in the four soybean genotypes with contrasting Cr tolerance. Further, a total of 13,777 differentially expressed genes (DEGs) were identified in transcriptomic sequencing and 1298 DEGs in six gene modules were found highly correlated with the physiological traits by weighted correlation network analysis (WGCNA) analysis. The DEGs encoding antioxidant enzymes, transcription factors, and ion transporters are proposed to confer Cr tolerance in soybean germinating seedlings by reducing the uptake and translocation of Cr, decreasing the level of ROS, and keeping the osmotic balance in soybean germinating seedings. In conclusion, our study provided a molecular regulation network on soybean Cr tolerance at seed germinating stage and identified candidate genes for molecular breeding of low Cr accumulation soybean cultivars.

Overexpression of the sulfate transporter-encoding SULTR2 increases chromium accumulation in Chlamydomonas reinhardtii

Hexavalent chromium [Cr(Ⅵ)] is a highly toxic contaminant in aquatic systems, and microalgae represent promising bioremediators of metal-containing wastewater. However, the metal-binding capacity of algal cells is limited. Therefore, we improved the cellular Cr(Ⅵ) biosorption capacity of Chlamydomonas reinhardtii by overexpressing the sulfate transporter gene SULTR2. SULTR2 was predominantly located in the cytoplasm of the cell, and few proteins mobilized to the cell membrane as a Cr transporter under Cr stress conditions. Intracellular Cr accumulation was almost doubled in SULTR2-overexpressing transgenic strains after exposure to 30 μM K₂ Cr₂ O₇ for 4 d. Alginate-based immobilization increased the rate of Cr removal from 43.81% to 88.15% for SULTR2-overexpressing transgenic strains after exposure to 10 μM K₂ Cr₂ O₇ for 6 d. The immobilized cells also displayed a significant increase in nutrient removal efficiency compared to that of free-swimming cells. Therefore, SULTR2 overexpression in algae has a great potential for the bioremediation of Cr(Ⅵ)-containing wastewater.

Molecular mechanisms underlying the toxicity and detoxification of trace metals and metalloids in plants

Plants take up a wide range of trace metals/metalloids (hereinafter referred to as trace metals) from the soil, some of which are essential but become toxic at high concentrations (e.g., Cu, Zn, Ni, Co), while others are non-essential and toxic even at relatively low concentrations (e.g., As, Cd, Cr, Pb, and Hg). Soil contamination of trace metals is an increasing problem worldwide due to intensifying human activities. Trace metal contamination can cause toxicity and growth inhibition in plants, as well as accumulation in the edible parts to levels that threatens food safety and human health. Understanding the mechanisms of trace metal toxicity and how plants respond to trace metal stress is important for improving plant growth and food safety in contaminated soils. The accumulation of excess trace metals in plants can cause oxidative stress, genotoxicity, programmed cell death, and disturbance in multiple physiological processes. Plants have evolved various strategies to detoxify trace metals through cell-wall binding, complexation, vacuolar sequestration, efflux, and translocation. Multiple signal transduction pathways and regulatory responses are involved in plants challenged with trace metal stresses. In this review, we discuss the recent progress in understanding the molecular mechanisms involved in trace metal toxicity, detoxification, and regulation, as well as strategies to enhance plant resistance to trace metal stresses and reduce toxic metal accumulation in food crops.

Effect of potassium intake on cadmium transporters and root cell wall biosynthesis in sweet potato

Large areas of farmland soil in southern China are deficient in potassium (K) and are contaminated with cadmium (Cd). Previously, we suggested that the K supplementation could reduce Cd accumulation in sweet potatoes (Ipomoea batatas (L.) Lam). In the present study, we investigated the underlying physiological and molecular mechanisms. A hydroponic experiment with different K and Cd treatments was performed to compare the transcriptome profile and the cell wall structure in the roots of sweet potato using RNA sequencing, Fourier transform infrared spectroscopy (FTIR) and transmission electron microscopy (TEM). The results showed that K supply inhibits the expressions of IRT1 and YSL3, which are responsible for root Cd uptake under Cd exposure. Furthermore, the expressions of COPT5 and Nramp3 were downregulated by K, which increased Cd retention in the root vacuoles. The upregulation of POD, CAD, INT1 and SUS by K contributed to lignin and cellulose biosynthesis and thickening of root xylem cell wall, which further reduced Cd translocation to the shoot. In addition, K affected the expressions of LHT, ACS, TPS and TPP associated with the production of ethylene and trehalose, which involved in plant resistance to Cd toxicity. In general, K application could decrease the uptake and translocation of Cd in sweet potatoes by regulating the expression of genes associated with Cd transporters and root cell wall components.

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.

Biomass production and uptake of sulfur, chromium and micronutrients by Tanzania guinea grass grown with sulfur and chromium

Soil contamination caused by improper waste disposal can affect plant growth. Tropical forage plants have rapid growth, high biomass production and vigorous regrowth. Chromium (Cr) is one of the most common contaminants in the environment. Sulfur (S) is a nutrient involved in various cell detoxification processes. The objective was to assess the effects of excess Cr on biomass production and concentrations of this metal, S and cationic micronutrients, under conditions of varying supply of S for Tanzania guinea grass. The experiment was conducted in randomized complete blocks in a 3 × 4 factorial scheme, corresponding to three levels of S (0.1, 1.9 and 3.7 mmol L^-1) and four levels of Cr (0.0, 0.5, 1.0 and 2.0 mmol L^-1) in three replicates. Root surface area, shoot and root biomass production, concentrations of total S, sulfate-S, Cr and cationic micronutrients, and Cr transport factor were determined. S level of 1.9 mmol L^-1 reduced the toxicity by Cr in Tanzania guinea grass, expressed by the higher shoot and root dry weight production, greater stability in the cellular concentration of sulfate-S and higher concentrations of micronutrients. It was concluded that the high availability of S reduced the toxicity by Cr(VI) in Tanzania guinea grass, expressed by the higher shoot and root dry weight production, greater stability in the cellular concentration of sulfate-S and higher concentration of micronutrients in the grass shoots.

Chromium Contamination and Health Risk Assessment of Soil and Agricultural Products in a Rural Area in Southern China

With the rapid development of industry, chromium (Cr) pollutants accumulate constantly in the soil, causing severe soil Cr pollution problems. Farmland Cr pollution hurts the safety of agricultural production and indirectly affects human health and safety. However, the current situation of Cr pollution in farmland soil and crops has not been detailed enough. In this study, the evaluation of Cr potential risk in soil-crop systems was conducted in a rural area that was affected by industry and historic sewage irrigation. Ten different crops and rhizosphere soils were sampled from four fields. The results showed that Cr contents in farmland soil exceeded the national standard threshold in China (>21.85%), and the Cr content in edible parts of some agricultural products exceeded that too. According to the PCA and relation analysis, the Cr accumulation in edible parts showed a significant correlation with soil Cr contents and available potassium contents. Except for water spinach, the target hazard quotient (THQ) of the other crops was lower than 1.0 but the carcinogenic health risks all exceeded the limits. The carcinogenic risks (CR) of different types of crops are food crops > legume crops > leafy vegetable crops and root-tuber crops. A comprehensive assessment revealed that planting water spinach in this area had the highest potential risk of Cr pollution. This study provided a scientific and reliable approach by integrating soil environmental quality and agricultural product security, which helps evaluate the potential risk of Cr in arable land more efficiently and lays technical guidelines for local agricultural production safety.

Characterization of the simultaneous degradation of pyrene and removal of Cr(VI) by a bacteria consortium YH

Microorganisms that can simultaneously remediate organic pollutants and heavy metal contamination are great significance in bioremediation. Nevertheless, reports of such microorganisms are still scarce. Here, Pseudomonas sp. YH-1 and Rhodococcus sp. YH-3 were isolated and identified, and they showed greater tolerance to hexavalent (VI) (750 and 800 mg·L^-1). The constructed bacteria consortium YH (YH-1:YH-3 = 1:1) could simultaneously degrade 41.69% of pyrene (50 mg·L^-1) and remove 76.67% of Cr(VI) (30 mg·L^-1) within 5 days. The potential mechanism of Cr(VI) tolerance of YH was further explored by genomic and microscopic analysis. The results showed that YH responded to Cr(VI) stress mainly through efflux of Cr(VI) by chrA and copZ, chromate reduction, DNA-repaired proteases reduces ROS damage, and biosorption by carboxyl, hydroxyl, amino functional groups. Strains YH-1 and YH-3 also contained a variety of genes associated with resistance to other heavy metals, such as cadmium (czcBD), mercury (merAPTR), manganese (mntABC) and copper (copAC, cusABRF and pcoBD). Based on GC-MS and genomic analysis, pyrene was degraded via salicylic acid and phthalic acid pathways. Moreover, a great number of genes related to aromatic hydrocarbon catabolism were identified in the genomes of YH-1 and YH-3. These results confirmed the potential application of the bacteria consortium YH in the bioremediation of water and soil co-contaminated with PAHs-heavy metals.

Efficacy of metallic nanoparticles in attenuating the accumulation and toxicity of chromium in plants: Current knowledge and future perspectives

Nanoparticles have emerged as cutting-edge technology for the improvement of crops yield and safe cultivation of agricultural plants, especially in peripheral areas impaired with toxic heavy metals including chromium (Cr). The uncontrolled release of Cr mainly from anthropogenic factors is substantially polluting the surrounding environment, thereby extensively accumulated in soil-plant system. The excessive Cr-accretion in plant tissues disturbed the morph-physiological, biochemical, cellular, metabolic and molecular processes, and impaired the plants functionality. Therefore, it is obligatory to restrict the accumulation and toxic effects of Cr in plant organs. Recent studies on metallic nanoparticles (MNPs) such as iron oxide, silicon dioxide, copper oxide and zinc oxide have approved their efficacy as potent pool to curb the Cr-induced phytotoxicities and improved the plant tolerance. MNPs attenuated the bioaccumulation and phytotoxicity of Cr by utilizing key mechanisms such as improved photosynthetic machinery, regulation of cellular metabolites, greater chelation capacity to bind with Cr, release of corresponding metallic ions, upsurge in the uptake of essential nutrients, activation of antioxidants (enzymatic and non-enzymatic), reduction in oxidative stress, and cellular injuries, thus improvement in plant growth performances. We have briefly discussed the current knowledge and research gaps in existing literature along with possible recommendations for future research. Overall, Cr-detoxification by MNPs may depends upon the target plant species, Cr speciation, plant growth stages (seedling, vegetative and ripening etc.), treatment methods (foliar spray, seed priming and nutrient solution etc.), type, size, dose and coating of applied MNPs, and conditions (hydroponic and soil environment etc.). This review would help plant scientists to develop MNPs based strategies such as nano-fertilizers to alleviate the Cr-accumulation and its toxic impacts. This may leads to safe and healthy food production. The review outcomes can provide new horizons for research in the applications of MNPs for the sustainable agriculture.

Chromium in plant-soil nexus: Speciation, uptake, transport and sustainable remediation techniques

Heavy metal (HM) pollution has become a serious global problem due to the non-biodegradable nature of the HMs and their persistence in the environment. Agricultural soil is a non-renewable resource that requires careful management so that it can fulfill the increasing demand for agricultural food production. However, different anthropogenic activities have resulted in a large-scale accumulation of HMs in soil which is detrimental to soil and plant health. Due to their ubiquity, increased bioavailability, toxicity, and non-biodegradable nature, HM contamination has formed a roadblock in the way of achieving food security, safety, and sustainability in the future. Chromium (Cr), specifically Cr(VI) is a highly bioavailable HM with no proven role in the physiology of plants. Chromium has been found to be highly toxic to plants, with its toxicity also influenced by chemical speciation, which is in turn controlled by different factors, such as soil pH, redox potential, organic matter, and microbial population. In this review, the different factors that influence Cr speciation were analyzed and the relationship between biogeochemical transformations of Cr and its bioavailability which may be beneficial for devising different Cr remediation strategies has been discussed. Also, the uptake and transport mechanism of Cr in plants, with particular reference to sulfate and phosphate transporters has been presented. The biological solutions for the remediation of Cr contaminated sites which offer safe and viable alternatives to old-style physical and chemical remediation strategies have been discussed in detail. This review provides theoretical guidance in developing suitable approaches for the better management of these remediation strategies.

Immobilization of hexavalent chromium in soil-plant environment using calcium silicate hydrate synthesized from coal gangue

The presence of excessive hexavalent chromium (Cr(VI)) in the contaminated soils and plants has become a global environmental issue due to its toxicity and carcinogenicity. This work investigated the feasibility of immobilizing Cr(VI) in the soil-plant environment using calcium silicate hydrate (C-S-H) synthesized from coal gangue. The results revealed that the C-S-H amendment increased soil pH and organic matter (OM), which further promoted Cr(VI) immobilization. Results also revealed that exchangeable and carbonate bound fractions of Cr were either converted into Fe/Mn oxide and OM bound fractions of Cr or hardly released residual fraction of Cr due to C-S-H treatment. The C-S-H accelerated conversion of Cr(VI) into Cr(III) promoting plant growth and alleviating the toxic effect of Cr(VI). Cr(VI) was mainly immobilized and accumulated in the plant roots which resulted in comparatively lower Cr(VI) content in the edible part of plants. The exchangeable fraction of Cr in soil could be used as a bioavailability evaluation index of Cr(VI) in plants. In short, C-S-H was proved to be a practical and environmentally friendly amendment for in-situ immobilization of Cr(VI) contaminated soil.

Asymmetrical alternating current electrochemically-mediated washing method for sustainable remediation of Cr(VI)-contaminated soil

The demands for genuine remediation of heavy metal contaminated soil have triggered extensive studies in the soil washing method. However, numerous soil washing methods show poor sustainability for target soil, due to the tremendous cost, hidden secondary pollution and severe soil deterioration. Here, an asymmetrical alternating current electrochemically-mediated remediation platform (ACRP) is developed by fabricating an amidoxime-functionalized electrode (Ami-electrode). The real soil contaminated with 1200 mg/kg Cr(VI) is remediated efficiently to less than safety level (30 mg/kg), meanwhile no exorbitant soil nutrient loss is observed and no secondary pollution occurs. Furthermore, the consumption of washing effluents for the ACRP method is 24 times lower than the traditional washing method. Ami-electrode with asymmetrical alternating current promote the electrocatalytic efficiency by inhibiting the Coulomb repulsion between Cr(VI) species and cathode. With the aid of Ami-electrode and positive bias, Cr(VI) species in effluents are adsorbed on chelating site. By subsequent negative bias, Cr element is reduced and recycled in the less hazardous form of amorphous Cr(III) hydroxide, and effluents are regenerate concurrently in a cyclic system. Durability experiment and cost calculation verify the exceptional sustainability and feasibility for remediation practices. This work provides a sustainable remediation method for Cr(VI)-contaminated soil, and then paves the way to develop electrochemically soil remediation platform for practical applications.

Important Roles of Thiols in Methylmercury Uptake and Translocation by Rice Plants

The bioaccumulation of the neurotoxin methylmercury (MeHg) in rice is a significant concern due to its potential risk to humans. Thiols have been known to affect MeHg bioavailability in microorganisms, but how thiols influence MeHg accumulation in rice plants remains unknown. Here, we investigated effects of common low-molecular-weight thiols, including cysteine (Cys), glutathione (GSH), and penicillamine (PEN), on MeHg uptake and translocation by rice plants. Results show that rice roots can rapidly take up MeHg, and this process is influenced by the types and concentrations of thiols in the system. The presence of Cys facilitated MeHg uptake by roots and translocation to shoots, while GSH could only promote MeHg uptake, but not translocation, by roots. Conversely, PEN significantly inhibited MeHg uptake and translocation to shoots. Using labeled ¹³Cys assays, we also found that MeHg uptake was coupled with Cys accumulation in rice roots. Moreover, analyses of comparative transcriptomics revealed that key genes associated with metallothionein and SULTR transporter families may be involved in MeHg uptake. These findings provide new insights into the uptake and translocation of MeHg in rice plants and suggest potential roles of thiol attributes in affecting MeHg bioavailability and bioaccumulation in rice.

Identification and quantification of the combined phytotoxicity of one element with various valences: Cr(III) and Cr(VI) for barley root elongation as an example

There is uncertainty in quantifying the toxic effects of total chromium (Cr) in the environment by modeling the toxicity of individual Cr(III) or Cr(VI). In the present study, the effects of Cr(III) and Cr(VI) on barley root elongation were investigated in a hydroponic system where Cr(III) and Cr(VI) combination dose-response experiments under monotoxicity concentration, single-dose addition, and fixed concentration ratios were designed to identify and quantify their combined phytotoxicity of one element with various valences. The results show that the calculated mixed toxicity unit values for 50% inhibition (TU_mix50) ranged from 1.06 to 1.45, indicating the weak antagonism effects of Cr(III) and Cr(VI) on barley root toxicity. Also, the single-dose group experiment has proved that the EC₅₀ of Cr(VI) was increased from 71.2 μM to 119.9 μM with Cr(III) addition, which suggested that Cr(III) has antagonism on the toxicity of Cr(VI). While EC₅₀ of Cr(III) was not affected by Cr(VI) addition. After introducing the expansion coefficient of Cr(III) on Cr(VI) toxicity, both the extended concentration addition model (e-CA) based on the log-logistic and Weibull equations and the extended independent action model (e-IA) could more accurately predict the barley root elongation under Cr(III) and Cr(VI) interaction. The e-CA model based on the Weibull equation had almost the best correlation coefficient (R²) and lowest root mean square error (RMSE) between the measured and predicted values. Finally, the combined toxicity and antagonism of the same element with co-existing different valences simultaneously were successfully and firstly identified and quantified in the present study.

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.

A review on plant-microbial interactions, functions, mechanisms and emerging trends in bioretention system to improve multi-contaminated stormwater treatment

Management and treatment of multi-polluted stormwater in bioretention system have gained significant attraction recently. Besides nutrients, recent source appointment studies found elevated levels of Potentially toxic metal(loid)s (PTMs) and contaminants of emerging concern (CECs) in stormwater that highlighted many limitations in conventional media adsorption-based pollutant removal bioretention strategies. The substantial new studies include biological treatment approaches to strengthen pollutants degradation and adsorption capacity of bioretention. The knowledge on characteristics of plants and their corresponding mechanisms in various functions, e.g., rainwater interception, retention, infiltration, media clogging prevention, evapotranspiration and phytoremediation, is scattered. The microorganisms' role in facilitating vegetation and media, plant-microorganism interactions and relative performance over different functions in bioretention is still unreviewed. To uncover the underneath, it was summarised plant and microbial studies and their functionality in hydrogeochemical cycles in the bioretention system in this review, contributing to finding their interconnections and developing a more efficient bioretention system. Additionally, source characteristics of stormwater and fate of associated pollutants in the environment, the potential of genetical engineered plants, algae and fungi in bioretention system as well as performance assessment of plants and microorganisms in non-bioretention studies to propose the possible solution of un-addressed problems in bioretention system have been put forward in this review. The present review can be used as an imperative reference to enlighten the advantages of adopting multidisciplinary approaches for the environment sustainability and pollution control.

Chromium Stress in Plants: Toxicity, Tolerance and Phytoremediation

Extensive industrial activities resulted in an increase in chromium (Cr) contamination in the environment. The toxicity of Cr severely affects plant growth and development. Cr is also recognized as a human carcinogen that enters the human body via inhalation or by consuming Cr-contaminated food products. Taking consideration of Cr enrichment in the environment and its toxic effects, US Environmental Protection Agency and Agency for Toxic Substances and Disease Registry listed Cr as a priority pollutant. In nature, Cr exists in various valence states, including Cr(III) and Cr(VI). Cr(VI) is the most toxic and persistent form in soil. Plants uptake Cr through various transporters such as phosphate and sulfate transporters. Cr exerts its effect by generating reactive oxygen species (ROS) and hampering various metabolic and physiological pathways. Studies on genetic and transcriptional regulation of plants have shown the various detoxification genes get up-regulated and confer tolerance in plants under Cr stress. In recent years, the ability of the plant to withstand Cr toxicity by accumulating Cr inside the plant has been recognized as one of the promising bioremediation methods for the Cr contaminated region. This review summarized the Cr occurrence and toxicity in plants, role of detoxification genes in Cr stress response, and various plants utilized for phytoremediation in Cr-contaminated regions.

MicroRNA-Mediated Responses to Chromium Stress Provide Insight Into Tolerance Characteristics of Miscanthus sinensis

Chromium (Cr) is a heavy metal in nature, which poses a potential risk to toxicity to both animals and plants when releasing into the environment. However, the regulation of microRNA (miRNA)-mediated response to heavy metal Cr has not been studied in Miscanthus sinensis. In this study, based on high-throughput miRNA sequencing, a total of 104 conserved miRNAs and 158 nonconserved miRNAs were identified. Among them, there were 45 differentially expressed miRNAs in roots and 13 differentially expressed miRNAs in leaves. The hierarchical clustering analysis showed that these miRNAs were preferentially expressed in a certain tissue. There were 833 differentially expressed target genes of 45 miRNAs in roots and 280 differentially expressed target genes of 13 miRNA in leaves. After expression trend analysis, five significantly enriched modules were obtained in roots, and three significantly enriched trend blocks in leaves. Based on the candidate gene annotation and gene ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) function analysis, miR167a, novel_miR15, and novel_miR22 and their targets were potentially involved in Cr transportation and chelation. Besides, miR156a, miR164, miR396d, and novel_miR155 were identified as participating in the physiological and biochemical metabolisms and the detoxification of Cr of plants. The results demonstrated the critical role of miRNA-mediated responses to Cr treatment in M. sinensis, which involves ion uptake, transport, accumulation, and tolerance characteristics.