An overview of heavy metal challenge in plants: from roots to shoots

Ectopic overexpression of Eleusine coracana CAX3 confers tolerance to metal and ion stress in yeast and Arabidopsis

Ionic and metal toxicity in plants is still a global problem for the environment, agricultural productivity and ultimately poses human health threats when these metal ions accumulate in edible organs of plants. Metal and ion transport from cytosol to the vacuole is considered an important component of metal and ion tolerance and a plant's potential utility in phytoremediation. Finger millet (Eleusine coracana) is an orphan crop but has prominent nutritional value in comparison to other cereals. Previous transcriptomic studies suggested that one of the calcium/proton exchanger (EcCAX3) is strongly upregulated during different developmental stages of spikes development in plant. This finding led us to speculate that high calcium accumulation in the grain might be because of CAX3 function. Moreover, phylogenetic analysis shows that EcCAX3 is more closely related to foxtail millet, sorghum and rice CAX3 protein. To decipher the functional role of EcCAX3, we have adopted complementation of yeast triple mutant K677 (Δpmc1Δvcx1Δcnb1), which has defective calcium transport machinery. Furthermore, metal tolerance assay shows that EcCAX3 expression conferred tolerance to different metal stresses in yeast. The gain-of-function study suggests that EcCAX3 overexpressing Arabidopsis plants shows better tolerance to higher concentration of different metal ions as compared to wild type Col-0 plants. EcCAX3-overexpression transgenic lines exhibits abundance of metal transporters and cation exchanger transporter transcripts under metal stress conditions. Furthermore, EcCAX3-overexpression lines have higher accumulation of macro- and micro-elements under different metal stress. Overall, this finding highlights the functional role of EcCAX3 in the regulation of metal and ion homeostasis and this could be potentially utilized to engineer metal fortification and generation of stress tolerant crops in near future.

Multicontamination Toxicity Evaluation in the Model Plant Lactuca sativa L

Many contaminated soils contain several toxic elements (TEs) in elevated contents, and plant-TE interactions can differ from single TE contamination. Therefore, this study investigated the impact of combined contamination (As, Cd, Pb, Zn) on the physiological and metabolic processes of lettuce. After 45 days of exposure, TE excess in soil resulted in the inhibition of root and leaf biomass by 40 and 48%, respectively. Oxidative stress by TE accumulation was indicated by markers-malondialdehyde and 5-methylcytosine-and visible symptoms of toxicity (leaf chlorosis, root browning) and morpho-anatomical changes, which were related to the change in water regime (water potential decrease). An analysis of free amino acids (AAs) indicated that TEs disturbed N and C metabolism, especially in leaves, increasing the total content of free AAs and their families. Stress-induced senescence by TEs suggested changes in gas exchange parameters (increase in transpiration rate, stomatal conductance, and intercellular CO2 concentration), photosynthetic pigments (decrease in chlorophylls and carotenoids), a decrease in water use efficiency, and the maximum quantum yield of photosystem II. These results confirmed that the toxicity of combined contamination significantly affected the processes of lettuce by damaging the antioxidant system and expressing higher leaf sensitivity to TE multicontamination.

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.

Advances in atmospheric pressure plasma-based optical emission spectrometry for the analysis of heavy metals

Over the past decade, miniaturized optical emission spectrometry (OES) systems utilizing atmospheric pressure plasmas (APPs) as radiation sources have exhibited impressive capabilities in trace heavy metal analysis. As the core of the analytical system, APPs sources possess unique properties such as compact size, light weight, low energy requirement, ease of fabrication, and relatively low manufacturing cost. This critical review focuses on recent progress of APP-based OES systems employed for the determination of heavy metals. Influences of technical details including the sample introduction manner, the sampling volume, the sample flow rate, the pH of the solutions on the plasma stability and the intensity of analytical signals are comprehensively discussed. Furthermore, the review emphasizes the analytical challenges faced by these techniques and highlights the opportunities for further development in the field of heavy metal detection.

Futuristic advancements in phytoremediation of endocrine disruptor Bisphenol A: A step towards sustainable pollutant degradation for rehabilitated environment

Bisphenol A (BPA) accumulates in the environment at lethal concentrations because of its high production rate and utilization. BPA, originating from industrial effluent, plastic production, and consumer products, poses serious risks to both the environment and human health. The widespread aggregation of BPA leads to endocrine disruption, reactive oxygen species-mediated DNA damage, epigenetic modifications and carcinogenicity, which can disturb the normal homeostasis of the body. The living being in a population is subjected to BPA exposure via air, water and food. Globally, urinary analysis reports have shown higher BPA concentrations in all age groups, with children being particularly susceptible due to its occurrence in items such as milk bottles. The conventional methods are costly with a low removal rate. Since there is no proper eco-friendly and cost-effective degradation of BPA reported so far. The phytoremediation, green-biotechnology based method which is a cost-effective and renewable resource can be used to sequestrate BPA. Phytoremediation is observed in numerous plant species with different mechanisms to remove harmful contaminants. Plants normally undergo several improvements in genetic and molecular levels to withstand stress and lower levels of toxicants. But such natural adaptation requires more time and also higher concentration of contaminants may disrupt the normal growth, survival and yield of the plants. Therefore, natural or synthetic amendments and genetic modifications can improve the xenobiotics removal rate by the plants. Also, constructed wetlands technique utilizes the plant's phytoremediation mechanisms to remove industrial effluents and medical residues. In this review, we have discussed the limitations and futuristic advancement strategies for degrading BPA using phytoremediation-associated mechanisms.

Modulation of plant photosynthetic processes during metal and metalloid stress, and strategies for manipulating photosynthesis-related traits

Metals constitute vital elements for plant metabolism and survival, acting as essential co-factors in cellular processes which are indispensable for plant growth and survival. Excess or deficient provision of metal/metalloids puts plant's life and survival at risk, thus considered a potent stress for plants. Chloroplasts as an organelle with a high metal demand form a pivotal site within the metal homeostasis network. Therefore, the metal-mediated electron transport chain (ETC) in chloroplasts is a primary target site of metal/metalloid-induced stresses. Both excess and deficient availability of metal/metalloids threatens plant's photosynthesis in several ways. Energy demands from the photosynthetic carbon reactions should be in balance with energy output of ETC. Malfunctioning of ETC components as a result of metal/metalloid stress initiates photoinhiition. A feedback inhibition from carbon fixation process also impedes the ETC. Metal stress impairs antioxidant enzyme activity, pigment biosynthesis, and stomatal function. However, genetic manipulations, nutrient management, keeping photostasis, and application of phytohormones are among strategies for coping with metal stress. Consequently, a comprehensive understanding of the underlying mechanisms of metal/metalloid stress, as well as the exploration of potential strategies to mitigate its impact on plants are imperative. This review offers a mechanistic insight into the disruption of photosynthesis regulation by metal/metalloids and highlights adaptive approaches to ameliorate their effects on plants. Focus was made on photostasis, nutrient interactions, phytohormones, and genetic interventions for mitigating metal/metalloid stresses.

Unveiling the Dual Nature of Heavy Metals: Stressors and Promoters of Phenolic Compound Biosynthesis in Basilicum polystachyon (L.) Moench In Vitro

The global industrial revolution has led to a substantial rise in heavy metal levels in the environment, posing a serious threat to nature. Plants synthesize phenolic compounds under stressful conditions, which serve as protective agents against oxidative stress. Basilicum polystachyon (L.) Moench is an herbaceous plant of the Lamiaceae family. Some species within this family are recognized for their capacity to remediate sites contaminated with heavy metals. In this study, the effects of mercury (II) chloride and lead (II) nitrate on the in vitro propagation of B. polystachyon were investigated. Shoot tips from in vitro plantlets were cultured in Murashige and Skoog's (MS) media with heavy metals ranging from 1 to 200 µM to induce abiotic stress and enhance the accumulation of phenolic compounds. After three weeks, MS medium with 1 µM of lead (II) supported the highest shoot multiplication, and the maximum number of roots per explant was found in 100 µM of lead (II), whereas a higher concentration of heavy metals inhibited shoot multiplication and root development. The plantlets were hardened in a greenhouse with a 96% field survival rate. Flame atomic absorption spectroscopy (FAAS) was used to detect heavy metal contents in plant biomass. At both 200 µM and 50 µM concentrations, the greatest accumulation of mercury (II) was observed in the roots (16.94 ± 0.44 µg/g) and shoots (17.71 ± 0.66 µg/g), respectively. Similarly, lead (II) showed the highest accumulation in roots (17.10 ± 0.54 µg/g) and shoots (7.78 ± 0.26 µg/g) at 200 µM and 50 µM exposures, respectively. Reverse-phase high-performance liquid chromatography (RP-HPLC) identified and quantified various phenolic compounds in B. polystachyon leaves, including gallic acid, caffeic acid, vanillic acid, p-coumaric acid, ellagic acid, rosmarinic acid, and trans-cinnamic acid. These compounds were found in different forms, such as free, esterified, and glycosylated. Mercury (II)-exposed plants exhibited elevated levels of vanillic acid (1959.1 ± 3.66 µg/g DW), ellagic acid (213.55 ± 2.11 µg/g DW), and rosmarinic acid (187.72 ± 1.22 µg/g DW). Conversely, lead (II)-exposed plants accumulated higher levels of caffeic acid (42.53±0.61 µg/g DW) and p-coumaric acid (8.04 ± 0.31 µg/g DW). Trans-cinnamic acid was the predominant phenolic compound in control plants, with a concentration of 207.74 ± 1.45 µg/g DW. These results suggest that sublethal doses of heavy metals can act as abiotic elicitors, enhancing the production of phenolic compounds in B. polystachyon. The present work has the potential to open up new commercial opportunities in the pharmaceutical industry.

The damage caused by Cd toxicity to photosynthesis, cellular ultrastructure, antioxidant metabolism, and gene expression in young cacao plants are mitigated by high Mn doses in soil

Manganese (Mn) is one of the essential mineral micronutrients most demanded by cacao. Cadmium (Cd) is highly toxic to plants and other living beings. There are indications that Mn can interact with Cd and mitigate its toxicity. The objective of this study was to evaluate the action of Mn on the toxic effect of Cd in young plants of the CCN 51 cacao genotype, subjected to different doses of Mn, Cd, and Mn+Cd in soil, through physiological, biochemical, molecular, and micromorphological and ultrastructural changes. High soil Mn doses favored the maintenance and performance of adequate photosynthetic processes in cacao. However, high doses of Cd and Mn+Cd in soil promoted damage to photosynthesis, alterations in oxidative metabolism, and the uptake, transport, and accumulation of Cd in roots and leaves. In addition, high Cd concentrations in roots and leaf tissues caused irreversible damage to the cell ultrastructure, compromising cell function and leading to programmed cell death. However, there was a mitigation of Cd toxicity when cacao was grown in soils with low Cd doses and in the presence of Mn. Thus, damage to the root and leaf tissues of cacao caused by Cd uptake from contaminated soils can be attenuated or mitigated by the presence of high Mn doses in soil.

Acetylcholine (ACh) enhances Cd tolerance through transporting ACh in vesicles and modifying Cd absorption in duckweed (Lemna turionifera 5511)

Acetylcholine (ACh), an important neurotransmitter, plays a role in resistance to abiotic stress. However, the role of ACh during cadmium (Cd) resistance in duckweed (Lemna turionifera 5511) remains uncharacterized. In this study, the changes of endogenous ACh in duckweed under Cd stress has been investigated. Also, how exogenous ACh affects duckweed's ability to withstand Cd stress was studied. The ACh sensor transgenic duckweed (ACh 3.0) showed the ACh signal response under Cd stress. And ACh was wrapped and released in vesicles. Cd stress promoted ACh content in duckweed. The gene expression analysis showed an improved fatty acid metabolism and choline transport. Moreover, exogenous ACh addition enhanced Cd tolerance and decreased Cd accumulation in duckweed. ACh supplement reduced the root abscission rate, alleviated leaf etiolation, and improved chlorophyll fluorescence parameters under Cd stress. A modified calcium (Ca2+) flux and improved Cd2+ absorption were present in conjunction with it. Thus, we speculate that ACh could improve Cd resistance by promoting the uptake and accumulation of Cd, as well as the response of the Ca2+ signaling pathway. Also, plant-derived extracellular vesicles (PDEVs) were extracted during Cd stress. Therefore, these results provide new insights into the response of ACh during Cd stress.

Plant cadmium resistance 10 enhances tolerance to toxic heavy metals in poplar

Toxic heavy metals originating from human activities have caused irreversible harm to the environment. Toxic heavy metal ions absorbed by crop plants can seriously threaten human health. Therefore, decreasing heavy metal contents in crop plants is an urgent need. The plant cadmium resistance protein (PCR) is a heavy metal ion transporter. In this study, PePCR10 was cloned from Populus euphratica. Bioinformatics analyses revealed its transmembrane structure and gene sequence motifs. The transcript profile of PePCR10 was analyzed by RT-qPCR, and its transcript levels increased under toxic heavy metal (cadmium, lead, aluminum) treatments. Subcellular localization analyses in tobacco cells revealed that PePCR10 localizes at the plasma membrane. Compared with wild type (WT), PePCR10-overexpressing lines showed significantly higher values for plant height, root length, fresh weight, and dry weight under heavy metal stress. Electrolyte leakage, nitroblue tetrazolium staining, and chlorophyll fluorescence analyses indicated that Cd/Al tolerance in PePCR10-overexpressing lines was stronger than that in WT. The Cd/Al contents were lower in the PePCR10-overexpressing lines than in WT under Cd/Al stress. Our results show that PePCR10 can reduce the heavy metal content in poplar and enhance its Cd/Al tolerance. Hence, PePCR10 is a candidate genetic resource for effectively reducing heavy metal accumulation in crops.

Elevated CO2 reduced antimony toxicity in wheat plants by improving photosynthesis, soil microbial content, minerals, and redox status

Introduction

Antimony (Sb), a common rare heavy metal, is naturally present in soils at low concentrations. However, it is increasingly used in industrial applications, which in turn, leads to an increased release into the environment, exerting a detrimental impact on plant growth. Thus, it is important to study Sb effects on plants under the current and future CO2 (eCO2).

Methods

To this end, high Sb concentrations (1500 mg/kg soil) effects under ambient (420 ppm) and eCO2 (710 ppm) on wheat growth, physiology (photosynthesis reactions) and biochemistry (minerals contents, redox state), were studied and soil microbial were evaluated.

Results and discussion

Our results showed that Sb uptake significantly decreased wheat growth by 42%. This reduction could be explained by the inhibition in photosynthesis rate, Rubisco activity, and photosynthetic pigments (Cha and Chb), by 35%, 44%, and 51%, respectively. Sb significantly reduced total bacterial and fungal count and increased phenolic and organic acids levels in the soil to decrease Sb uptake. Moreover, it induced oxidative markers, as indicated by the increased levels of H2O2 and MDA (1.96 and 2.8-fold compared to the control condition, respectively). To reduce this damage, antioxidant capacity (TAC), CAT, POX, and SOD enzymes activity were increased by 1.61, 2.2, 2.87, and 1.86-fold, respectively. In contrast, eCO2 mitigated growth inhibition in Sb-treated wheat. eCO2 and Sb coapplication mitigated the Sb harmful effect on growth by reducing Sb uptake and improving photosynthesis and Rubisco enzyme activity by 0.58, 1.57, and 1.4-fold compared to the corresponding Sb treatments, respectively. To reduce Sb uptake and improve mineral availability for plants, a high accumulation of phenolics level and organic acids in the soil was observed. eCO2 reduces Sb-induced oxidative damage by improving redox status. In conclusion, our study has provided valuable insights into the physiological and biochemical bases underlie the Sb-stress mitigating of eCO2 conditions. Furthermore, this is important step to define strategies to prevent its adverse effects of Sb on plants in the future.

Heavy metal induced resistance to herbivore of invasive plant: implications from inter- and intraspecific comparisons

Introduction

Heavy metals can affect the content of secondary metabolites in plants, which are one of the important defenses of plants against herbivores. However, studies on the effects of heavy metals on secondary metabolites of invasive plants are scarce. Phytolacca americana is an invasive plant in China, which can hyperaccumulate the heavy metal Mn.

Methods

This study used two Mn treatments (control and treatment group) and four species from Phytolacca (including the native and introduced populations of P. americana, its native and exotic congeners in China) to investigate the impact of heavy metal Mn on the invasive ability of P. americana.

Results

The results show that heavy metal Mn can enhance the inhibitory effect of the introduced populations of P. americana on the growth of herbivore (the weight of herbivore has decreased by 66%), and altered the feeding preferences of herbivore. We also found that heavy metal Mn can significantly increase the content of quantitative resistance in the leaves of the introduced populations of P. americana and is higher than its native populations, native and exotic congeners. In addition, heavy metal Mn caused the quantitative resistance of the exotic congener significantly higher than that of the native congeners.

Discussion

In summary, the heavy metal Mn can increase the content of secondary metabolites in leaves to enhance the interspecific competitive advantage of P. americana and promote its invasion, and also increase the invasion risk of exotic species.

Functional characterization of a DNA-damage repair/tolerance 100 (DRT100) gene in Sedum alfredii Hance for genome stability maintenance and Cd hypertolerance

Cd contamination is a world-wild concern for its toxicity and accumulation in food chain. Sedum alfredii Hance (Crassulaceae) is a zinc (Zn) and cadmium (Cd) hyperaccumulator native to China and widely applied for the phytoremediation at Zn or Cd contaminated sites. Although many studies report the uptake, translocation and storage of Cd in S. alfredii Hance, limited information is known about the genes and underlying mechanisms of genome stability maintenance under Cd stress. In this study, a gene resembling DNA-damage repair/toleration 100 (DRT100) was Cd inducible and designated as SaDRT100. Heterologous expression of SaDRT100 gene in yeasts and Arabidopsis thaliana enhanced Cd tolerance capability. Under Cd stress, transgenic Arabidopsis with SaDRT100 gene exhibited lower levels of reactive oxygen species (ROS), fewer Cd uptake in roots and less Cd-induced DNA damage. Evidenced by the subcellular location in cellular nucleus and expression in aerial parts, we suggested the involvement of SaDRT100 in combating Cd-induced DNA damage. Our findings firstly uncovered the roles of SaDRT100 gene in Cd hypertolerance and genome stability maintenance in S. alfredii Hance. The potential functions of DNA protection make SaDRT100 gene a candidate in genetic engineering for phytoremediation at multi-component contaminated sites.

The Potential of CRISPR/Cas Technology to Enhance Crop Performance on Adverse Soil Conditions

Worldwide food security is under threat in the actual scenery of global climate change because the major staple food crops are not adapted to hostile climatic and soil conditions. Significant efforts have been performed to maintain the actual yield of crops, using traditional breeding and innovative molecular techniques to assist them. However, additional strategies are necessary to achieve the future food demand. Clustered regularly interspaced short palindromic repeat/CRISPR-associated protein (CRISPR/Cas) technology, as well as its variants, have emerged as alternatives to transgenic plant breeding. This novelty has helped to accelerate the necessary modifications in major crops to confront the impact of abiotic stress on agriculture systems. This review summarizes the current advances in CRISPR/Cas applications in crops to deal with the main hostile soil conditions, such as drought, flooding and waterlogging, salinity, heavy metals, and nutrient deficiencies. In addition, the potential of extremophytes as a reservoir of new molecular mechanisms for abiotic stress tolerance, as well as their orthologue identification and edition in crops, is shown. Moreover, the future challenges and prospects related to CRISPR/Cas technology issues, legal regulations, and customer acceptance will be discussed.

Heavy metal distribution in wheat plant components following foliar Cd application

Atmospheric deposition of Cd, from anthropogenic activities, can be directly deposited onto and absorbed into wheat plants, yet, how foliar absorbed Cd is translocated in wheat plants is not well understood. A pot experiment investigated foliar Cd application on the accumulation and distribution of heavy metals in various wheat parts. Wheat was grown in a Cd/heavy metal contaminated soil, and from grain heading to the filling stage, 0, 10, 20, 30 and 40 mg kg^-1 Cd solution was sprayed repeatedly on leaves (grain heads were covered). Foliar Cd application had no effect on grain yield and Cd concentration (3.01-3.51 mg kg^-1 for all treatments), while increased flag leaf blade and sheath Cd concentrations by 1.06-2.77 and 0.00-0.66 times, respectively. Cadmium concentration in the center of the peduncle, from the 40 mg kg^-1 Cd solution treatment, was 1.41 times that of the control (10.3 vs 7.30 mg kg^-1). Foliar Cd application also increased Cd accumulation (concentration × mass) of the flag leaf blade and sheath. Rachis and grain Pb concentrations were reduced, while stem Pb concentration was increased by Cd application. Cadmium application negatively affected whole plant Ni accumulation and concentration of certain wheat parts; Ni absorption inhibition may have occurred in roots via the downward transport of Cd. Overall results implied that the predominant portion of foliar applied Cd was retained in leaves, while lesser portions migrated to peduncle or root and affected the absorption/distribution of other metals in wheat plants. These results are important for further discerning the mechanism of wheat grain Cd accumulation, especially when grain is raised in areas where atmospheric deposition of Cd (e.g., near smelting facilities) is an issue from an environmental and human health perspective.

Insight into the physiological and biochemical mechanisms of biostimulating effect of Ascophyllum nodosum and Moringa oleifera extracts to minimize cadmium-induced oxidative stress in rice

Cadmium (Cd) is a serious threat for environmental sustainability as it can be taken up quickly by plants and transported to the food chain of living organisms. It alters plants' metabolic and physiological activities and causes yield loss, thereby, enhancing plant tolerance to Cd stress is of utmost essential. Therefore, an experiment was executed to investigate the potential role of Ascophyllum nodosum extract (ANE) and moringa (Moringa oleifera) leaf extract (MLE) to confer Cd tolerance in rice (Oryza sativa cv. BRRI dhan89). Thirty-five-day-old seedling was subjected to Cd stress (50 mg kg-1 CdCl2) alone and in a combination of ANE (0.25%) or MLE (0.5%) in a semi-controlled net house. Exposure to Cd resulted in accelerated production of reactive oxygen species, enhanced lipid peroxidation, and disrupted antioxidant defense and glyoxalase system, thus retarded plant growth, biomass production, and yield attributes of rice. On the contrary, the supplementation of ANE or MLE enhanced the contents of ascorbate and glutathione, and the activities of antioxidant enzymes such as ascorbate peroxidase, dehydroascorbate reductase, monodehydroascorbate reductase, glutathione reductase, glutathione peroxidase, and catalase. Moreover, supplementation of ANE and MLE enhanced the activities of glyoxalase I and glyoxalase II which prevented the overgeneration of methylglyoxal in Cd stressed rice plants. Thus, because of ANE and MLE addition Cd-induced rice plants showed a noticeable declination in membrane lipid peroxidation, hydrogen peroxide generation, and electrolyte leakage, whereas improved water balance. Furthermore, the growth and yield attributes of Cd-affected rice plants were improved with the supplementation of ANE and MLE. All the studied parameters indicates the potential role of ANE and MLE in mitigating Cd stress in rice plants through improving the physiological attributes, modulating antioxidant defense and glyoxalase system.

Exogenous melatonin enhances Cd stress tolerance in Platycladus orientalis seedlings by improving mineral nutrient uptake and oxidative stress

The development of agriculture and industry has led to a gradual increase in the levels of cadmium (Cd) in the soil, which, due to its high mobility in soil, makes Cd deposition in plants a serious threat to the health of animals and humans. The important role of melatonin (MT) in regulating plant growth and adaptation to environmental stress has become a pertinent research topic, but the mechanisms of action of MT in Cd-stressed Platycladus orientalis seedlings are unclear. Here, we investigated the mitigation mechanism of exogenous MT application on P. orientalis seedlings under Cd stress. Cd stress significantly inhibited the growth of P. orientalis seedlings by disrupting photosynthetic pigments, mineral balance, osmotic balance, and oxidative balance. In contrast, the application of exogenous MT significantly increased the growth parameters of P. orientalis seedlings, reduced Cd accumulation and transfer in the seedlings, increased the content of iron, manganese, zinc, copper, chlorophyll, soluble protein, soluble sugar, and proline, reduced the content of glutathione, increased the activities of superoxide dismutase and peroxidase, and significantly enhanced the expression of antioxidant-related genes (POD, GST, and APX). It also effectively reduced the content of hydrogen peroxide and malondialdehyde to inhibit the production of reactive oxygen species, thus alleviating Cd-induced oxidative stress. In addition, MT significantly upregulated the expression of the ethanol dehydrogenase (ADH) gene, which is effective in removing the acetaldehyde produced by anaerobic respiration in seedlings under stress, thereby reducing the toxic effects on P. orientalis. The results showed that exogenous MT enhanced the tolerance of P. orientalis seedlings to Cd stress by regulating photosynthesis, mineral balance, osmotic balance, and the antioxidant system and that the optimal concentration of MT was 200 μmol·L^-1.

Phytoremediation of Potentially Toxic Elements: Role, Status and Concerns

Environmental contamination with a myriad of potentially toxic elements (PTEs) is triggered by various natural and anthropogenic activities. However, the industrial revolution has increased the intensity of these hazardous elements and their concentration in the environment, which, in turn, could provoke potential ecological risks. Additionally, most PTEs pose a considerable nuisance to human beings and affect soil, aquatic organisms, and even nematodes and microbes. This comprehensive review aims to: (i) introduce potentially toxic elements; (ii) overview the major sources of PTEs in the major environmental compartments; (iii) briefly highlight the major impacts of PTEs on humans, plants, aquatic life, and the health of soil; (iv) appraise the major methods for tackling PTE-caused pollution; (v) discuss the concept and applications of the major eco-technological/green approaches (comprising phytoextraction, rhizofiltration, phytostabilization, phytovolatilization, and phytorestoration); (vi) highlight the role of microbes in phytoremediation under PTE stress; and (vii) enlighten the major role of genetic engineering in advancing the phytoremediation of varied PTEs. Overall, appropriate strategies must be developed in order to stop gene flow into wild species, and biosafety issues must be properly addressed. Additionally, consistent efforts should be undertaken to tackle the major issues (e.g., risk estimation, understanding, acceptance and feasibility) in order to guarantee the successful implementation of phytoremediation programs, raise awareness of this green technology among laymen, and to strengthen networking among scientists, stakeholders, industrialists, governments and non-government organizations.

Dual tolerance of ageratum (Ageratum conyzoides L.) to combined pollution of acid and cadmium: Field survey and pot experiment

A field survey and pot experiment were carried out to screen tolerant plants growing in cadmium (Cd)-polluted mining areas which were co-polluted with acid in soil, and the related physiological and biochemical mechanisms were also analyzed. Thirty-seven species of wild plants and their corresponding soil were collected from a farmland around the mining areas. Ageratum (Ageratum conyzoides L.) with strong Cd-accumulative ability was selected, and its tolerance experiment for acid and Cd with different levels were carried out separately or orthogonally, respectively. Furthermore, the activities of superoxide dismutase (SOD), peroxidase (POD) and catalase (CAT), and the contents of malondialdehyde (MDA), photosynthetic pigments, soluble sugar and proline in its leaves were determined. The results showed that the Cd accumulation in ageratum and sticktight (Bidens pilosa L.) was relatively high, but the latter has been well documented, so we focused on ageratum in the present work. In pot experiment, ageratum grew normally at 100 mg kg^-1 Cd in soil, and the Cd concentrations in its roots, stems and leaves were 75.37 ± 7.37, 31.01 ± 3.76 and 53.92 ± 10.05 mg kg^-1, respectively. In the case of acid tolerance experiment, all plant individuals of ageratum grew normally when soil pH was over 3.5. In the orthogonal experiment, the Cd accumulation in this plant increased with the decrease of soil pH under the same Cd treatment. Under strong acid conditions, the activity of SOD in leaves of ageratum was increased significantly. When the Cd concentration was 10 mg kg^-1 and the soil pH was 5.5 or 3.5, the activities of POD and CAT were significantly increased. In addition, based on stepwise regression analysis, the leaf Cd concentration was significantly positive correlated with the activities of SOD and POD in leaves of ageratum. Therefore, ageratum not only had a strong tolerance for Cd and acid pollution in soil, but also had a strong ability to accumulate Cd. As a common plant in the mining area, it has a great potential for the phytoremediation of Cd and acid co-contaminated soil.

Combined cadmium and fluorine inhibit lettuce growth through reducing root elongation, photosynthesis, and nutrient absorption

Cadmium (Cd) and fluorine (F) often coexist in environment and are toxic to organisms; however, their combined effects on plants are still not well documented. In this study, the co-effects of Cd and F on germination, biomass, photosynthesis, and nutrients uptake of lettuce were carried out in hydroponic culture. The results showed that the seed germination and seedling biomass decreased with an increase in Cd and F supplementation. The root morphology verified these effects as excess combined Cd and F diminished the root tips and surface area of lettuce, while single Cd and F inhibited the growth by decreasing root length and average diameter, respectively. These effects were also consistence with a reduction in photosynthesis which was mainly regulated by reducing the quantum yield of PS II, electron transport activity, stomatal conductance, intercellular CO2 concentration, and transpiration rate in response to the pollutants. Moreover, when lettuce exposed to Cd and F stress, the accumulation of several essential elements in shoot decreased. In a sum, the synergistic negative effects of Cd and F on the seed germination and seedling growth of lettuce were observed, and these might be owed to nutrient absorption and translocation in the plant. These findings aid in understanding the harmful effects and specific mechanisms of action of Cd and F on plants.

Mercury contamination in the riparian ecosystem during the reservoir discharging regulated by a mega dam

Mercury (Hg) is extremely poisonous and can be absorbed through touch, inhalation, or consumption. In the living environment, Hg in contaminated sediment can be transferred into grass by the direct absorption through the roots or shoots. The intake of Hg due to Hg emissions may pose a threat to living bodies especially to human beings. The present study aims to provide a novel insight about total mercury (THg) and methyl mercury (MeHg) in a riparian grass (Cynodon dactylon (L).Pers) and sediments during the discharging phase (summertime at 145 m water level) in Three Gorges Reservoir (TGR-China); where C. dactylon is a dominant perennial herb in the riparian zone. Yet, the potential risk of Hg contamination in the riparian ecosystem is not thoroughly assessed in the dam regulated reservoir. This study was conducted in the riparian zones of the reservoir formed by a mega dam (Three Gorge Dam) which regulates the water levels during the summer and winter period in the TGR. Our results showed that riparian sediments were acting as a sink for THg and MeHg. Insignificant correlation of THg and MeHg was found between the amphiphyte C. dactylon and its surrounding sediments in the TGR. Bioconcentration factors values for MeHg were found higher than 1 in all study locations in the riparian zones in TGR, which could be due to action of certain bacteria/purely chemical-based methylation on inorganic form of Hg. Additionally, translocation factor indices also highlighted that the amphiphyte C. dactylon was MeHg accumulator in riparian zones. These results suggested that since riparian sediment was found acting as the sink for THg and MeHg during discharging phase, MeHg contamination in the amphiphyte C. dactylon in riparian zones was not caused by the riparian sediments but by other factors, for instance, the anthropogenic activities in the TGR. Finally, this study leads to conclude that amphiphyte C. dactylon can be used as biomonitoring agent for Hg pollution in the TGR.

ERF subfamily transcription factors and their function in plant responses to abiotic stresses

Ethylene Responsive Factor (ERF) subfamily comprise the largest number of proteins in the plant AP2/ERF superfamily, and have been most extensively studied on the biological functions. Members of this subfamily have been proven to regulate plant resistances to various abiotic stresses, such as drought, salinity, chilling and some other adversities. Under these stresses, ERFs are usually activated by mitogen-activated protein kinase induced phosphorylation or escape from ubiquitin-ligase enzymes, and then form complex with nucleic proteins before binding to cis-element in promoter regions of stress responsive genes. In this review, we will discuss the phylogenetic relationships among the ERF subfamily proteins, summarize molecular mechanism how the transcriptional activity of ERFs been regulated and how ERFs of different subgroup regulate the transcription of stress responsive genes, such as high-affinity K⁺ transporter gene PalHKT1;2, reactive oxygen species related genes LcLTP, LcPrx, and LcRP, flavonoids synthesis related genes FtF3H and LhMYBSPLATTER, etc. Though increasing researches demonstrate that ERFs are involved in various abiotic stresses, very few interact proteins and target genes of them have been comprehensively annotated. Hence, future research prospects are described on the mechanisms of how stress signals been transited to ERFs and how ERFs regulate the transcriptional expression of stress responsive genes.

Foliar selenium fertilization alters the content of dietary phytochemicals in two rocket species

Biofortification is the process that aims to enrich crops in micronutrients and valuable compounds. Selenium (Se) biofortification has particularly attracted increasing interest in recent times due to the growing number of individuals suffering from Se deficiency. Selenate and selenite are the Se forms most frequently administered to crops. In this study, Se was applied foliarly as selenate at 2.5, 5, or 10 mg per plant to two rocket species, Diplotaxis tenuifolia and Eruca sativa, grown in soil and the effects in terms of Se enrichment and content of primary and secondary metabolites were comparatively analyzed. We also compared our results with those obtained previously when selenate was supplied to the same species in hydroponics by addition to the nutrient solution. In most cases, the results were the opposite. In E. sativa, foliar Se treatment was more effective in promoting Se accumulation, sulfur (S), cysteine, and glucosinolates. No significant effect of Se was evident on total phenolic content, but there were individual phenols. Among amino acids, the content of proline was increased by Se, perhaps to counteract osmotic stress due to high Se accumulation. In D. tenuifolia, the content of S and cysteine decreased under Se treatment, but the amount of glutathione was steady, suggesting a preferred assimilation of cysteine toward the synthesis of this antioxidant. Consistent, the content of methionine and glucosinolates was reduced. The content of total phenolics was enhanced only by the low Se dosage. In both species, selenocysteine (SeCys) was identified, the content of which was higher compared to plants grown hydroponically. Concluding, most metabolic differences between rocket species were observed at high Se supplementation. Low Se foliar fertilization was effective in an enriching rocket in Se without affecting other phytochemicals. However, the Se dosages sufficient for biofortification could be even lower, as the Se concentration in rocket treated with 2.5 mg Se per plant was still very high and the edible part should not be eaten undiluted. Also, a single method of Se supplementation does not appear to be optimal for all plant species or the same species, as the metabolic responses could be very different.

Medicinal Plant Growth in Heavy Metals Contaminated Soils: Responses to Metal Stress and Induced Risks to Human Health

Accelerating heavy metal pollution is a hot issue due to a continuous growth in consumerism and increased activities in various global industries. Soil contamination with heavy metals has resulted in their incorporation into the human food web via plant components. Accumulation and amplification of heavy metals in human tissues through the consumption of medicinal plants can have hazardous health outcomes. Therefore, in this critical review we aim to bring together published information on this subject, with a special highlight on the knowledge gaps related to heavy metal stress in medicinal plants, their responses, and human health related risks. In this respect, this review outlines the key contamination sources of heavy metals in plants, as well as the absorption, mobilization and translocation of metal ions in plant compartments, while considering their respective mechanisms of detoxification. In addition, this literature review attempts to highlight how stress and defensive strategies operate in plants, pointing out the main stressors, either biotic or abiotic (e.g., heavy metals), and the role of reactive oxygen species (ROS) in stress answers. Finally, in our research, we further aim to capture the risks caused by heavy metals in medicinal plants to human health through the assessment of both a hazard quotient (HQ) and hazard index (HI).

microRNAs: Key Players in Plant Response to Metal Toxicity

Environmental metal pollution is a common problem threatening sustainable and safe crop production. Heavy metals (HMs) cause toxicity by targeting key molecules and life processes in plant cells. Plants counteract excess metals in the environment by enhancing defense responses, such as metal chelation, isolation to vacuoles, regulating metal intake through transporters, and strengthening antioxidant mechanisms. In recent years, microRNAs (miRNAs), as a small non-coding RNA, have become the central regulator of a variety of abiotic stresses, including HMs. With the introduction of the latest technologies such as next-generation sequencing (NGS), more and more miRNAs have been widely recognized in several plants due to their diverse roles. Metal-regulated miRNAs and their target genes are part of a complex regulatory network. Known miRNAs coordinate plant responses to metal stress through antioxidant functions, root growth, hormone signals, transcription factors (TF), and metal transporters. This article reviews the research progress of miRNAs in the stress response of plants to the accumulation of HMs, such as Cu, Cd, Hg, Cr, and Al, and the toxicity of heavy metal ions.

Effects of vermiculite on the growth process of submerged macrophyte Vallisneria spiralis and sediment microecological environment

Ecological restoration is one of the hot technologies for the reconstruction of eutrophic lake ecosystems in which the restoration and propagation of submerged plants is the key and difficult step. In this paper, the effect of vermiculite on the growth process of Vallisneria spiralis and sediment microenvironment were investigated, aiming to provide a theoretical basis for the application of vermiculite in aquatic ecological restoration. Results of growth indexes demonstrated that 5% and 10% vermiculite treatment groups statistically promote the growth of Vallisneria spiralis compared to the control. Meanwhile, the results of ecophysiological indexes showed that photosynthetic pigment, soluble sugar content, superoxide dismutase (SOD), and catalase (CAT) activity of 5% and 10% group were increased compared with the control while the malondialdehyde (MDA) content exhibited the opposite result (p < 0.05), which illustrated that vermiculite can improve the resistance of plants and delay the aging process of Vallisneria spiralis. In addition, result of PCA (Principal Component Analysis) demonstrated 5% and 10% group has improved the sediment physical conditions and create more ecological niche for microorganisms directly, and then promoted the growth of plants. The dissolution results showed that vermiculite can dissolve the constant and trace elements needed for plant growth. Furthermore, the addition of vermiculite increased the diversity of microorganisms in the sediments, and promoted the increase of plant growth-promoting bacteria and phosphorus-degrading bacteria. This study could provide a technique reference for the further application of vermiculite in the field of ecological restoration.

Global leaf and root transcriptome in response to cadmium reveals tolerance mechanisms in Arundo donax L

The expected increase of sustainable energy demand has shifted the attention towards bioenergy crops. Due to their know tolerance against abiotic stress and relatively low nutritional requirements, they have been proposed as election crops to be cultivated in marginal lands without disturbing the part of lands employed for agricultural purposes. Arundo donax L. is a promising bioenergy crop whose behaviour under water and salt stress has been recently studied at transcriptomic levels. As the anthropogenic activities produced in the last years a worrying increase of cadmium contamination worldwide, the aim of our work was to decipher the global transcriptomic response of A. donax leaf and root in the perspective of its cultivation in contaminated soil. In our study, RNA-seq libraries yielded a total of 416 million clean reads and 10.4 Gb per sample. De novo assembly of clean reads resulted in 378,521 transcripts and 126,668 unigenes with N50 length of 1812 bp and 1555 bp, respectively. Differential gene expression analysis revealed 5,303 deregulated transcripts (3,206 up- and 2,097 down regulated) specifically observed in the Cd-treated roots compared to Cd-treated leaves. Among them, we identified genes related to “Protein biosynthesis”, “Phytohormone action”, “Nutrient uptake”, “Cell wall organisation”, “Polyamine metabolism”, “Reactive oxygen species metabolism” and “Ion membrane transport”. Globally, our results indicate that ethylene biosynthesis and the downstream signal cascade are strongly induced by cadmium stress. In accordance to ethylene role in the interaction with the ROS generation and scavenging machinery, the transcription of several genes (NADPH oxidase 1, superoxide dismutase, ascorbate peroxidase, different glutathione S-transferases and catalase) devoted to cope the oxidative stress is strongly activated. Several small signal peptides belonging to ROTUNDIFOLIA, CLAVATA3, and C-TERMINALLY ENCODED PEPTIDE 1 (CEP) are also among the up-regulated genes in Cd-treated roots functioning as messenger molecules from root to shoot in order to communicate the stressful status to the upper part of the plants. Finally, the main finding of our work is that genes involved in cell wall remodelling and lignification are decisively up-regulated in giant reed roots. This probably represents a mechanism to avoid cadmium uptake which strongly supports the possibility to cultivate giant cane in contaminated soils in the perspective to reserve agricultural soil for food and feed crops.

Comparison of metal bioaccumulation in crop types and consumable parts between two growth periods

A high proportion of populations in most developing countries live below the poverty line and those near refuse grounds resort to dumpsite farming to grow food. Consequently, high levels of waste-derived contaminants are found in crops consumed by these people. This study investigates the extent to which crops cultivated on the Mbale dumpsite (Uganda) were contaminated by 11 metals and 2 non-metals: iron (Fe), aluminum (Al), zinc (Zn), manganese (Mn), copper (Cu), mercury (Hg), lead (Pb), nickel (Ni), cobalt (Co), cadmium (Cd), selenium (Se), chromium (Cr), and arsenic (As). We investigated how element bioaccumulation in crops was influenced by the growth period (short- and long-term crop maturity). The short-term crops were Zea mays and Amaranthus cruentus, whereas the long-term crops were Manihot esculenta, Colocasia esculenta, Musa acuminata, Carica papaya, Coffea arabica, and Saccharum officinarum. Results showed that nine metals were present at concentrations above World Health Organization/Food and Agriculture Organization (WHO/FAO) food safety recommendations and hence may pose health risks to consumers. In this study, leaves contained higher metal concentrations than other analyzed consumable parts. Pb and Co were found at higher concentrations in leaves of short-term crops than in long-term crops. Among short-term crops, only Z. mays seeds contained permissible metal concentrations by WHO/FAO standards. The growth period was also found to influence metal bioaccumulation in crop types. Pb, Co, Fe, Al, and Cu concentrations were significantly higher in the short-term crops than in long-term crops, while Mn, Ni, and Cr concentrations were higher in long-term crops than in short-term crops. Overall, public awareness about the health risks associated with consuming short-term leafy crops grown on dumpsites should be improved to reduce toxic metal exposure. While implementing such a campaign, the food supply of individuals whose survival depends on such crops should not be jeopardized. Therefore, farmers need alternative farming areas outside dumpsites. Integr Environ Assess Manag 2022;18:1056-1071. © 2021 The Authors. Integrated Environmental Assessment and Management published by Wiley Periodicals LLC on behalf of Society of Environmental Toxicology & Chemistry (SETAC).

A review on bioremediation approach for heavy metal detoxification and accumulation in plants

Nowadays, the accumulation of toxic heavy metals in soil and water streams is considered a serious environmental problem that causes various harmful effects on plants and animals. Phytoremediation is an effective, green, and economical bioremediation approach by which the harmful heavy metals in the contaminated ecosystem can be detoxified and accumulated in the plant. Hyperaccumulators exude molecules called transporters that carry and translocate the heavy metals present in the soil to different plant parts. The hyperaccumulator plant genes can confine higher concentrations of toxic heavy metals in their tissues. The efficiency of phytoremediation relies on various parameters such as soil properties (pH and soil type), organic matters in soil, heavy metal type, nature of rhizosphere, characteristics of rhizosphere microflora, etc. The present review comprehensively discusses the toxicity effect of heavy metals on the environment and different phytoremediation mechanisms for the transport and accumulation of heavy metals from polluted soil. This review gave comprehensive insights into plants tolerance for the higher heavy metal concentration their responses for heavy metal accumulation and the different mechanisms involved for heavy metal tolerance. The current status and the characteristic features that need to be improved in the phytoremediation process are also reviewed in detail.

GABA-mediated inhibition of cadmium uptake and accumulation in apples

GABA, a four-carbon non-protein amino acid, plays an important role in animals and plants. We previously found GABA could alleviate alkali stress in apple seedlings. However, its physiological mechanism under heavy metal cadmium (Cd) stress need to be further studied. Thus, we explored its biological role in response to Cd stress. It was verified that 0.5 mM GABA could effectively alleviate Cd toxicity. Using NMT technique, we found that exogenous GABA could significantly reduce the net Cd²⁺ fluxes in apple roots, and Cd content was significantly lower than that in roots under Cd stress. Further analysis indicated exogenous GABA could significantly reduce the expression of genes related to the uptake and transport of Cd in apples under Cd stress. In addition, exogenous GABA could significantly increase the content of amino acids in apple roots under Cd stress. GAD is a key enzyme in GABA synthesis, we obtained transgenic apple roots of overexpression MdGAD1. Compared with the control, transgenic roots accumulated less Cd, maintained lower Cd uptake by roots, and lower expression of related transport genes. These results showed that GABA could effectively alleviate Cd toxicity in apple seedlings and provide a new perspective of GABA to alleviate Cd stress.

Genome-Wide Identification and Expressional Profiling of the Metal Tolerance Protein Gene Family in Brassica napus

The Cation Diffusion Facilitator (CDF) family, also named Metal Tolerance Protein (MTP), is one of the gene families involved in heavy metal transport in plants. However, a comprehensive study of MTPs in Brassica napus has not been reported yet. In the present study, we identified 33 BnMTP genes from the rapeseed genome using bioinformatic analyses. Subsequently, we analyzed the phylogenetic relationship, gene structure, chromosome distribution, conserved domains, and motifs of the BnMTP gene family. The 33 BnMTPs were phylogenetically divided into three major clusters (Zn-CDFs, Fe/Zn-CDFs, and Mn-CDFs) and seven groups (group 1, 5, 6, 7, 8, 9, and 12). The structural characteristics of the BnMTP members were similar in the same group, but different among groups. Evolutionary analysis indicated that the BnMTP gene family mainly expanded through whole-genome duplication (WGD) and segmental duplication events. Moreover, the prediction of cis-acting elements and microRNA target sites suggested that BnMTPs might be involved in plant growth, development, and stress responses. In addition, we found the expression of 24 BnMTPs in rapeseed leaves or roots could respond to heavy metal ion treatments. These results provided an important basis for clarifying the biological functions of BnMTPs, especially in heavy metal detoxification, and will be helpful in the phytoremediation of heavy metal pollution in soil.

A Glucuronic Acid-Producing Endophyte Pseudomonas sp. MCS15 Reduces Cadmium Uptake in Rice by Inhibition of Ethylene Biosynthesis

Dynamic regulation of phytohormone levels is pivotal for plant adaptation to harmful conditions. It is increasingly evidenced that endophytic bacteria can regulate plant hormone levels to help their hosts counteract adverse effects imposed by abiotic and biotic stresses, but the mechanisms underlying the endophyte-induced stress resistance of plants remain largely elusive. In this study, a glucuronic acid-producing endophyte Pseudomonas sp. MCS15 alleviated cadmium (Cd) toxicity in rice plants. Inoculation with MCS15 significantly inhibited the expression of ethylene biosynthetic genes including OsACO3, OsACO4, OsACO5, OsACS2, and OsACS5 and thus reduced the content of ethylene in rice roots. In addition, the expression of iron uptake-related genes including OsIRT1, OsIRT2, OsNAS1, OsNAS2 and OsYSL15 was significantly downregulated in the MCS15-inoculated roots under Cd stress. Similarly, glucuronic acid treatment also remarkably inhibited root uptake of Cd and reduced the production of ethylene. However, treatment with 1-aminocyclopropyl carboxylic acid (ACC), a precursor of ethylene, almost abolished the MCS15 or glucuronic acid-induced inhibition of Cd accumulation in rice plants. Conversely, treatment with aminoethoxyvinyl glycine (AVG), an inhibitor of ethylene biosynthesis, markedly reduced the Cd accumulation in plants. Taken together, our results revealed that the endophytic bacteria MCS15-secreted glucuronic acid inhibited the biosynthesis of ethylene and thus weakened iron uptake-related systems in rice roots, which contributed to preventing the Cd accumulation.

Impact of arbuscular mycorrhiza on maize P1B-ATPases gene expression and ionome in copper-contaminated soils

Arbuscular mycorrhizal (AM) fungi, symbionts of most land plants, increase plant fitness in metal contaminated soils. To further understand the mechanisms of metal tolerance in the AM symbiosis, the expression patterns of the maize Heavy Metal ATPase (HMA) family members and the ionomes of non-mycorrhizal and mycorrhizal plants grown under different Cu supplies were examined. Expression of ZmHMA5a and ZmHMA5b, whose encoded proteins were predicted to be localized at the plasma membrane, was up-regulated by Cu in non-mycorrhizal roots and to a lower extent in mycorrhizal roots. Gene expression of the tonoplast ZmHMA3a and ZmHMA4 isoforms was up-regulated by Cu-toxicity in shoots and roots of mycorrhizal plants. AM mitigates the changes induced by Cu toxicity on the maize ionome, specially at the highest Cu soil concentration. Altogether these data suggest that in Cu-contaminated soils, AM increases expression of the HMA genes putatively encoding proteins involved in Cu detoxification and balances mineral nutrient uptake improving the nutritional status of the maize plants.

Tomato (Solanum lycopersicum L.) accumulation and allergenicity in response to nickel stress

Vegetables represent a major source of Ni exposure. Environmental contamination and cultural practices can increase Ni amount in tomato posing significant risk for human health. This work assesses the tomato (Solanum lycopersicum L.) response to Ni on the agronomic yield of fruits and the related production of allergens. Two cultivars were grown in pots amended with Ni 0, 30, 60, 120, and 300 mg kg⁻¹, respectively. XRF and ICP-MS analyses highlighted the direct increase of fruit Ni content compared to soil Ni, maintaining a stable biomass. Leaf water content increased at Ni 300 mg kg⁻¹. Total protein content and individual allergenic components were investigated using biochemical (RP-HPLC and N-terminal amino acid sequencing) and immunological (inhibition tests of IgE binding by SPHIAa assay on the FABER testing system) methodologies. Ni affected the fruit tissue concentration of pathogenesis-related proteins and relevant allergens (LTP, profilin, Bet v 1-like protein and TLP). This study elucidates for the first time that tomato reacts to exogenous Ni, uptaking the metal while changing its allergenic profiles, with potential double increasing of exposure risks for consumers. This evidence highlighted the importance of adequate choice of low-Ni tomato cultivars and practices to reduce Ni uptake by potentially contaminated matrices.

A comprehensive review on sources, analysis and toxicity of environmental pollutants and its removal methods from water environment

Natural and human anthropogenic activities increase the concentration of the toxic pollutant in the water environment; they could cause harmful effects even in their lower concentration. In humans, toxic pollutants damage the structural and functional properties of essential organs including the heart, liver, kidneys, reproductive systems and pancreas. To avoid the toxicity of the pollutant, they should be removed from the water environment. Since various conventional water/wastewater treatment technologies including precipitation, ion exchange, flocculation, filtration, electrodialysis and membrane separation are employed to reduce the concentration of the pollutant, they have various difficulties in implementation, efficiency and ecological perspective. Therefore, several researchers are now focusing on alternative and eco-friendly approach called biosorption to remove toxic contaminants from the water environment. The biosorption innovation is one of the acclaimed systems for water treatment. The noteworthy endeavours have been made throughout the years to grow profoundly particular and effective biosorbent materials that are more effective, abundantly available, and cost-effective. Biosorption is effectively executed by utilizing both living and dead biomasses of bacteria, fungi and algae. Moreover, agro-waste materials are also utilized as biosorbents due to their excellent surface properties, abundant availability and cost-effectiveness. A variety of physical and chemical treatments enhances the biosorption capabilities of biosorbents via modifying their surface properties. In this review, biosorption mechanism, influencing parameters and application of biosorbent materials towards the removal of toxic pollutants are discussed. The future research opportunities for sustainable wastewater treatment were also explained.

Cell cycle checkpoint control in response to DNA damage by environmental stresses

Being sessile organisms, plants are ubiquitously exposed to stresses that can affect the DNA replication process or cause DNA damage. To cope with these problems, plants utilize DNA damage response (DDR) pathways, consisting of both highly conserved and plant-specific elements. As a part of this DDR, cell cycle checkpoint control mechanisms either pause the cell cycle, to allow DNA repair, or lead cells into differentiation or programmed cell death, to prevent the transmission of DNA errors in the organism through mitosis or to its offspring via meiosis. The two major DDR cell cycle checkpoints control either the replication process or the G2/M transition. The latter is largely overseen by the plant-specific SOG1 transcription factor, which drives the activity of cyclin-dependent kinase inhibitors and MYB3R proteins, which are rate limiting for the G2/M transition. By contrast, the replication checkpoint is controlled by different players, including the conserved kinase WEE1 and likely the transcriptional repressor RBR1. These checkpoint mechanisms are called upon during developmental processes, in retrograde signaling pathways, and in response to biotic and abiotic stresses, including metal toxicity, cold, salinity, and phosphate deficiency. Additionally, the recent expansion of research from Arabidopsis to other model plants has revealed species-specific aspects of the DDR. Overall, it is becoming evidently clear that the DNA damage checkpoint mechanisms represent an important aspect of the adaptation of plants to a changing environment, hence gaining more knowledge about this topic might be helpful to increase the resilience of plants to climate change.

Metal Detoxification in Land Plants: From Bryophytes to Vascular Plants. STATE of the Art and Opportunities

Potentially toxic elements are a widespread concern due to their increasing diffusion into the environment. To counteract this problem, the relationship between plants and metal(loid)s has been investigated in the last 30 years. In this field, research has mainly dealt with angiosperms, whereas plant clades that are lower in the evolutive scale have been somewhat overlooked. However, recent studies have revealed the potential of bryophytes, pteridophytes and gymnosperms in environmental sciences, either as suitable indicators of habitat health and elemental pollution or as efficient tools for the reclamation of degraded soils and waters. In this review, we summarize recent research on the interaction between plants and potentially toxic elements, considering all land plant clades. The focus is on plant applicability in the identification and restoration of polluted environments, as well as on the characterization of molecular mechanisms with a potential outlet in the engineering of element tolerance and accumulation.

Ectomycorrhizal fungi enhance the tolerance of phytotoxicity and cadmium accumulation in oak (Quercus acutissima Carruth.) seedlings: modulation of growth properties and the antioxidant defense responses

Ectomycorrhizal fungi (EMF), which form symbiotic ectomycorrhiza with tree roots, mediate heavy metal tolerance of host plants. To investigate the roles of EMF in the growth, modulation of oxidative stress, and cadmium (Cd) accumulation and translocation in Quercus acutissima seedlings, ectomycorrhizal seedlings inoculated with Suillus luteus were treated with different Cd concentrations (0.1, and 5 mg kg^-1) for 14 days. EMF accelerated seedling growth and Cd accumulation in roots under the highest Cd concentration of 5 mg kg^-1. Catalase (CAT), ascorbate peroxidase (APX), and glutathione reductase (GR) activities increased in the leaves of ectomycorrhizal seedlings under the highest Cd concentration. Superoxide dismutase (SOD) trended to increase under both Cd concentrations. Although reduced glutathione (GSH) increased after inoculation of EMF under both Cd concentrations, the release of malondialdehyde increased in the leaves and roots under the highest Cd concentration, indicating that the defense role of EMF in Q. acutissima depends on the Cd concentration. These results indicate that EMF mitigate Cd stress by promoting plant growth and nutrient uptake while modulating the antioxidant system to reduce oxidative stress.

Potassium reduces oxidative stress in tanzania guinea grass under cadmium toxicity

Plants used for phytoextraction of metals need to tolerate toxicity conditions. Potassium (K) participates in physiological and biochemical processes that can alleviate toxicity by heavy metals, including cadmium (Cd). This study aimed to evaluate the effect of K on photosynthesis and on the changes in the antioxidant system of tanzania guinea grass [Panicum maximum Jacq. cv. Tanzania (syn. Megathyrsus maximus (Jacq,) B.K. Simon & S.W.L. Jacobs)] under Cd toxicity. Plants were grown in a greenhouse, in nutrient solution, in a randomized complete block design, arranged in a 3 × 4 factorial, with three replications. Plants were supplied with three K levels (0.4 [K deficiency], 6.0, and 11.6 mmol L^-1) and exposed to four Cd levels (0.0, 0.5, 1.0, and 1.5 mmol L^-1). Two plant growth periods were evaluated. High Cd level (1.5 mmol L^-1) led to a reduction in net photosynthesis (76%) by causing low stomatal conductance and losses in quantum efficiency of photosystem II. However, high K supply (11.6 mmol L^-1) increased the net photosynthesis by 15% in plants exposed to 1.0 mmol L^-1 Cd, due to upregulation of proline synthesis. Cd toxicity resulted in increases in lipid peroxidation and hydrogen peroxide concentration (35 and 50%; 25 and 30%, at first and second harvest, respectively) and reduction by 80-100% in activity of the antioxidant enzymes: superoxide dismutase, catalase, ascorbate peroxidase, guaiacol peroxidase, and glutathione reductase in the shoots of the grass. However, the high K supply (11.6 mmol L^-1) increased the activity of these enzymes (about 50-75%) and reduced lipid peroxidation (36%), restoring cellular homeostasis. Moreover, high K supply promoted a 25% increase in spermidine and spermine concentrations in the shoots. Therefore, K reduced the Cd-induced oxidative stress and increased the net photosynthesis in tanzania guinea grass by increasing the activity of antioxidant enzymes and proline and polyamines synthesis, which enhances the tolerance of this grass to Cd.

Melatonin Confers Plant Cadmium Tolerance: An Update

Cadmium (Cd) is one of the most injurious heavy metals, affecting plant growth and development. Melatonin (N-acetyl-5-methoxytryptamine) was discovered in plants in 1995, and it is since known to act as a multifunctional molecule to alleviate abiotic and biotic stresses, especially Cd stress. Endogenously triggered or exogenously applied melatonin re-establishes the redox homeostasis by the improvement of the antioxidant defense system. It can also affect the Cd transportation and sequestration by regulating the transcripts of genes related to the major metal transport system, as well as the increase in glutathione (GSH) and phytochelatins (PCs). Melatonin activates several downstream signals, such as nitric oxide (NO), hydrogen peroxide (H₂O₂), and salicylic acid (SA), which are required for plant Cd tolerance. Similar to the physiological functions of NO, hydrogen sulfide (H₂S) is also involved in the abiotic stress-related processes in plants. Moreover, exogenous melatonin induces H₂S generation in plants under salinity or heat stress. However, the involvement of H₂S action in melatonin-induced Cd tolerance is still largely unknown. In this review, we summarize the progresses in various physiological and molecular mechanisms regulated by melatonin in plants under Cd stress. The complex interactions between melatonin and H₂S in acquisition of Cd stress tolerance are also discussed.

Biochemical and Metabolic Plant Responses toward Polycyclic Aromatic Hydrocarbons and Heavy Metals Present in Atmospheric Pollution

Heavy metals (HMs) and polycyclic aromatic hydrocarbons (PAHs) are toxic components of atmospheric particles. These pollutants induce a wide variety of responses in plants, leading to tolerance or toxicity. Their effects on plants depend on many different environmental conditions, not only the type and concentration of contaminant, temperature or soil pH, but also on the physiological or genetic status of the plant. The main detoxification process in plants is the accumulation of the contaminant in vacuoles or cell walls. PAHs are normally transformed by enzymatic plant machinery prior to conjugation and immobilization; heavy metals are frequently chelated by some molecules, with glutathione, phytochelatins and metallothioneins being the main players in heavy metal detoxification. Besides these detoxification mechanisms, the presence of contaminants leads to the production of the reactive oxygen species (ROS) and the dynamic of ROS production and detoxification renders different outcomes in different scenarios, from cellular death to the induction of stress resistances. ROS responses have been extensively studied; the complexity of the ROS response and the subsequent cascade of effects on phytohormones and metabolic changes, which depend on local concentrations in different organelles and on the lifetime of each ROS species, allow the plant to modulate its responses to different environmental clues. Basic knowledge of plant responses toward pollutants is key to improving phytoremediation technologies.

Biochemical Responses of Medicinal Plant Tussilago farfara L. to Elevated Heavy Metal Concentrations in Soils of Urban Areas

This study was conducted in Tyumen (Russian Federation) to establish the effects of heavy metals' (Cu, Zn, Fe, Mn, Pb, and Cd) accumulation in soil and coltsfoot, as well as plants' biochemical responses to such an accumulation. The mobile and acid-soluble heavy metal fractions in soils, and the heavy metal contents in plants, were determined by atomic absorption spectrophotometry. The Cu, Zn, Fe, Mn, and Pb concentrations in soils exceeded background values. Pb content at the battery manufacturing plant was above the maximum permitted concentration. The percentages of the mobile heavy metal fractions decreased in the following order: Mn > Zn > Cu > Fe. The greatest heavy metal accumulation in soils and plants was found at the battery manufacturing and metallurgical plants examined in our study. Heavy metals' accumulation in the aboveground part of Tussilago farfara decreased in the following order: Fe > Zn > Cu > Mn > Pb > Cd. The accumulation of heavy metals stimulated the synthesis of photosynthetic pigments by 6-30%. Heavy metals provoked oxidative stress in cells, increasing the concentration of lipid peroxidation in products by up to 80%. Plant phenolics and flavonoids in the urban area of our study decreased compared to those in the control by 1.05, reaching up to 6.5 times. The change in coltsfoot catalase activity both increased and declined. Biochemical responses and heavy metal accumulation in coltsfoot from urban areas limit its use for medicinal purposes.

Contribution of strigolactone in plant physiology, hormonal interaction and abiotic stresses

Strigolactones (SLs) are carotenoid-derived molecules, which regulate various developmental and adaptation processes in plants. These are engaged in different aspects of growth such as development of root, leaf senescence, shoot branching, etc. Plants grown under nutrient-deficient conditions enhance SL production that facilitates root architecture and symbiosis of arbuscular mycorrhizal fungi, as a result increases nutrient uptake. The crosstalk of SLs with other phytohormones such as auxin, abscisic acid, cytokinin and gibberellins, in response to abiotic stresses indicates that SLs actively contribute to the regulatory systems of plant stress adaptation. In response to different environmental circumstances such as salinity, drought, heat, cold, heavy metals and nutrient deprivation, these SLs get accumulated in plant tissues. Strigolactones regulate multiple hormonal responsive pathways, which aids plants to surmount stressful environmental constraints as well as reduce negative impact on overall productivity of crops. The external application of SL analog GR24 for its higher bioaccumulation can be one of the possible approaches for establishing various abiotic stress tolerances in plants.

PGPR in Agriculture: A Sustainable Approach to Increasing Climate Change Resilience

Growing environmental concerns are potentially narrowing global yield capacity of agricultural systems. Climate change is the most significant problem the world is currently facing. To meet global food demand, food production must be doubled by 2050; over exploitation of arable lands using unsustainable techniques might resolve food demand issues, but they have negative environmental effects. Current crop production systems are a major reason for changing global climate through diminishing biodiversity, physical and chemical soil degradation, and water pollution. The over application of fertilizers and pesticides contribute to climate change through greenhouse gas emissions (GHG) and toxic soil depositions. At this crucial time, there is a pressing need to transition to more sustainable crop production practices, ones that concentrate more on promoting sustainable mechanisms, which enable crops to grow well in resource limited and environmentally challenging environments, and also develop crops with greater resource use efficiency that have optimum sustainable yields across a wider array of environmental conditions. The phytomicrobiome is considered as one of the best strategies; a better alternative for sustainable agriculture, and a viable solution to meet the twin challenges of global food security and environmental stability. Use of the phytomicrobiome, due to its sustainable and environmentally friendly mechanisms of plant growth promotion, is becoming more widespread in the agricultural industry. Therefore, in this review, we emphasize the contribution of beneficial phytomicrobiome members, particularly plant growth promoting rhizobacteria (PGPR), as a strategy to sustainable improvement of plant growth and production in the face of climate change. Also, the roles of soil dwelling microbes in stress amelioration, nutrient supply (nitrogen fixation, phosphorus solubilization), and phytohormone production along with the factors that could potentially affect their efficiency have been discussed extensively. Lastly, limitations to expansion and use of biobased techniques, for instance, the perspective of crop producers, indigenous microbial competition and regulatory approval are discussed. This review largely focusses on the importance and need of sustainable and environmentally friendly approaches such as biobased/PGPR-based techniques in our agricultural systems, especially in the context of current climate change conditions, which are almost certain to worsen in near future.

Robinsonecio gerberifolius as a sentinel organism for atmospheric pollution by heavy metals in several sites of Mexico city and its metropolitan area

Evaluate the effect of heavy metals (HM) on sentinel organisms such as vascular plants represent a model to estimate toxic hazard due to environmental pollution. In the present study, the plant Robinsonecio gerberifolius was used to evaluate the toxic effects of the HM contained in the leaves of plants that were exposed to 4 different sites in Mexico city and its metropolitan area, during the rainy and dry seasons in the period 2017-2019. The comet assay to evaluate genotoxicity revealed an increase with respect to control (p < 0.05), in 2nd and 8th week of exposure, in all 4 study sites and in both seasons, more significant in the rainy period. An increase in the induction of oxidative stress was also observed in the exposed leaves from the 4 study sites when compared with the control; in some cases, the increases were significant (p < 0.05). In general, α- and ß-carotenoids were increased at 8th week of exposure, in all plants exposed in both seasons, while miR398 increased in plants exposed in 2 study sites (p < 0.05). Finally, toxic HM like aluminum, vanadium, and cadmium, increased significantly in the rainy season, while lead increased in the dry season. We conclude that R. gerberifolius can be considered a sentinel plant for evaluating the presence and general toxic effects caused by the presence of toxic HM that have been documented in the atmosphere of Mexico City and its metropolitan area.

Potentials of Miscanthus x giganteus for phytostabilization of trace element-contaminated soils: Ex situ experiment

Phytomanagement is proposed as a cost-effective and environmentally-friendly suggestion for sustainable use of large metal-contaminated areas. In the current work, the energy crop miscanthus (Miscanthus × giganteus) was grown in ex situ conditions on agricultural soils presenting a Cd, Pb and Zn contamination gradient. After 93 days of culture, shoot and root growth parameters were measured. Soils and plants were sampled as well to study the TE accumulation in miscanthus and the effects of this plant on TE mobility in soils. Results demonstrated that miscanthus growth depended more on the soils silt content rather than TE-contamination level. Moreover, soil organic carbon at T93 increased in the soils after miscanthus cultivation by 25.5-45.3%, whereas CaCl₂-extractible TEs decreased due to complex rhizosphere processes driving plant mineral uptake, and organic carbon inputs into the rhizosphere. In the contaminated soils, miscanthus accumulated Cd, Pb and Zn mainly in roots (BCF in roots: Cd " Zn > Pb), while strongly reducing the transfer of these elements from soil to all organs and from roots to rhizomes, stems and leaves (average TFs: 0.01-0.06, 0.11-1.15 and 0.09-0.79 corresponding to Cd, Pb and Zn respectively). Therefore, miscanthus could be considered a TE-excluder, hence a potential candidate crop for coupling phytostabilization and biomass production on the studied Metaleurop TE-contaminated soils.

Heavy metal accumulation of urban Scots pine (Pinus sylvestris L.) plantation

In soil and plant tissues, the concentrations and reserves of manganese (Mn), zinc (Zn), nickel (Ni), copper (Cu), and lead (Pb) were calculated to measure the heavy metal quantities of the Scots pine afforestation. In the soil and plant tissues, heavy metal concentrations varied. The highest concentration of heavy metal was found in Mn and the lowest concentration in Pb in the soil. Heavy metal stocks followed the order of Mn > Zn > Ni > Cu > Pb in soil, and the order of Zn > Cu > Mn > Ni > Pb in the plant. In the Scots pine parts, Pb and Zn concentrations and BCFs follow Needle > Branch > Bark > Root > Trunk order, while Cu concentrations follow Root > Needle > Branch > Bark > Trunk order, Mn concentrations follow Needle > Branch > Bark > Trunk > Root order and Ni concentrations follow Needle > Branch > Root > Bark > Trunk order. In particular, Scots pine needles are considered to be ideal to be used as biomonitoring plants, especially in monitoring increases in heavy metal concentrations in urban areas. Besides, in order to be more effective in the retaining of heavy metals, it is proposed that mixtures of hyperaccumulator plants be added which are appropriate for climatic conditions.