Root anatomy and element distribution vary between two Salix caprea isolates with different Cd accumulation capacities

Physiology and transcriptome analysis of Artemisia argyi adaptation and accumulation to soil cadmium

The soil pollution caused by cadmium (Cd) poses a significant threat to the environment. Therefore, identifying plants that can effectively remediate Cd-contaminated soils is urgently needed. In this study, physiological, cytological, and transcriptome analyses were performed to comprehensively understand the changes in Artemisia argyi under Cd stress. Physiological and cytological analyses indicated that A. argyi maintained normal growth with intact cell structure under Cd stress levels up to 10 mg/kg. Cytological analysis showed that Cd precipitation in leaf cells occurred in the cytoplasm and intercellular spaces. As the levels of Cd stress increased, proline accumulation in leaves increased, whereas soluble protein and soluble sugar initially increased, followed by a subsequent decline. The translocation factor was above 1 under 0.6 mg/kg Cd stress but decreased when it exceeded this concentration. Transcriptome analyses revealed several crucial Cd-influenced pathways, including amino acid, terpenoid, flavonoid, and sugar metabolisms. This study not only proved that A. argyi could enrich Cd in soil but also revealed the response of A. argyi to Cd and its resistance mechanisms, which provided insight into the cleaner production of A. argyi and the remediation of Cd-contaminated soil.

Growth, gas exchange, and leaf anatomy of Echinodorus grandiflorus (Cham. & Schltr.) Micheli under Pb contamination

Lead (Pb) can be deposited in aquatic environments that are especially subject to pollution due to wastewater and sewage disposal. This study aimed to evaluate the tolerance of Echinodorus grandiflorus (Cham. & Schltr.) Micheli to Pb and changes in growth, gas exchange, and leaf anatomy. Experiments were conducted with E. grandiflorus plants exposed to the following Pb concentrations in nutrient solution: [0; 0.75; 1.5; 3.0 and 9.0 μM Pb (NO 3)2] in a greenhouse for 60 days. At the end of the experiment, the lead concentration, growth, leaf gas exchange, and changes in leaf anatomy were evaluated. There was no mortality of E. grandiflorus plants, and they accumulated higher concentrations of Pb proportional to the concentration of the pollutant in the solution. Pb did not cause significant changes in growth, stomatal conductance, transpiration, and Ci/Ca rate but reduced the photosynthesis in E. grandiflorus. The leaf anatomy showed significant changes in the presence of Pb, reducing the epidermis and chlorophyll parenchyma. E. grandiflorus demonstrated tolerance to Pb, surviving and growing under contamination; however, it negatively modified its leaf anatomy and photosynthesis in the presence of the metal.

Unveiling the variability in cadmium accumulation and tolerance characteristics: a comparative study of Basma and Yunyan 87 tobacco varieties

Tobacco (Nicotiana tabacum L.) shows promise for remediating Cd-contaminated soil due to its significant Cd accumulation capabilities. Although various tobacco varieties exhibit distinct Cd bioaccumulation capacities, a comprehensive understanding of the underlying mechanisms is lacking. This study, conducted using hydroponics, explores differences in Cd accumulation and tolerance mechanisms between two tobacco varieties, Basma and Yunyan 87. The results showed that Cd stress reduced the dry weight, tolerance index, and root morphology for both varieties. Basma exhibited a relatively smaller decline in these indices compared to Yunyan 87. Moreover, Basma demonstrated a higher Cd bioconcentration factor (BCF), concentration, and accumulated content, signifying its superior tolerance and bioaccumulation capacity to Cd compared to Yunyan 87. The Carbonyl Cyanide3-ChloroPhenylhydrazone (CCCP) addition resulted in reduced Cd accumulation and BCFs in both tobacco species. This effect was more pronounced in Basma, suggesting that Basma relies more on an active transport process than Yunyan 87. This could potentially explain its enhanced bioaccumulation ability. Subcellular Cd distribution analysis revealed Basma's preference for distributing Cd in soluble fractions, while Yunyan 87 favoured the cell wall fractions. Transmission electron microscope showed that Basma's organelles were less damaged than Yunyan 87's under Cd stress, possibly contributing to the superior tolerance of Basma. Therefore, these results provided a theoretical foundation for development of Cd-contaminated soil tobacco remediation technology.

The combined contamination of nano-polystyrene and nanoAg: Uptake, translocation and ecotoxicity effects on willow saplings

Nanoplastics (NPLs) and nanoAg (AgNPs) are emerging contaminants commonly detected in aquatic and terrestrial environments due to their widespread use in various domains. However, their uptake, translocation, and toxic effects on plants in cooccurrence environments remain largely unexplored. Therefore, a hydroponic experiment was conducted using 100 nm NPLs (1 mg/L and 10 mg/L), AgNPs (100 μg/L and 1000 μg/L) and saplings of willow (Salix matsudana 'J172') to investigate absorption, translocation and the physio-biochemical responses of the plants. The results indicated that NPLs and AgNPs were agglomerated with each other in solutions. NPLs not only penetrated the roots of the saplings but also translocated to the branches and leaves through xylem ducts. However, AgNPs was only detected in the roots, suggesting that the internalization of nanoparticles in plants depends on the properties and types of particles themselves. The combined exposure to NPLs and AgNPs selectively affected the absorption and distribution of K, Ca, Mg and Fe, resulting in inhibited saplings growth and photosynthesis. Furthermore, the presence of NPLs and AgNPs induced oxidative damage and stimulated the antioxidant stress system in the plants. This study provides novel insights into the internalization and ecotoxicological mechanisms of NPLs and AgNPs in woody vascular plants.

Zinc and nano zinc mediated alleviation of heavy metals and metalloids in plants: an overview

Heavy metals and metalloids (HMs) contamination in the environment has heightened recently due to increasing global concern for food safety and human livability. Zinc (Zn2+ ) is an important nutrient required for the normal development of plants. It is an essential cofactor for the vital enzymes involved in various biological mechanisms of plants. Interestingly, Zn2+ has an additional role in the detoxification of HMs in plants due to its unique biochemical-mediating role in several soil and plant processes. During any exposure to high levels of HMs, the application of Zn2+ would confer greater plant resilience by decreasing oxidative stress, maintaining uptake of nutrients, photosynthesis productivity and optimising osmolytes concentration. Zn2+ also has an important role in ameliorating HMs toxicity by regulating metal uptake through the expression of certain metal transporter genes, targeted chelation and translocation from roots to shoots. This review examined the vital roles of Zn2+ and nano Zn in plants and described their involvement in alleviating HMs toxicity in plants. Moving forward, a broad understanding of uptake, transport, signalling and tolerance mechanisms of Zn2+ /zinc and its nanoparticles in alleviating HMs toxicity of plants will be the first step towards a wider incorporation of Zn2+ into agricultural practices.

Differences and similarities in selenium biopathways in Astragalus, Neptunia (Fabaceae) and Stanleya (Brassicaceae) hyperaccumulators

BACKGROUND AND AIMS

Selenium hyperaccumulator species are of primary interest for studying the evolution of hyperaccumulation and for use in biofortification because selenium is an essential element in human nutrition. In this study, we aimed to determine whether the distributions of selenium in the three most studied hyperaccumulating taxa (Astragalus bisulcatus, Stanleya pinnata and Neptunia amplexicaulis) are similar or contrasting, in order to infer the underlying physiological mechanisms.

METHODS

This study used synchrotron-based micro-X-ray fluorescence (µXRF) techniques to visualize the distribution of selenium and other elements in fresh hydrated plant tissues of A. racemosus, S. pinnata and N. amplexicaulis.

KEY RESULTS

Selenium distribution differed widely in the three species: in the leaves of A. racemosus and N. amplexicaulis selenium was mainly concentrated in the pulvini, whereas in S. pinnata it was primarilylocalized in the leaf margins. In the roots and stems of all three species, selenium was absent in xylem cells, whereas it was particularly concentrated in the pith rays of S. pinnata and in the phloem cells of A. racemosus and N. amplexicaulis.

CONCLUSIONS

This study shows that Astragalus, Stanleya and Neptunia have different selenium-handling physiologies, with different mechanisms for translocation and storage of excess selenium. Important dissimilarities among the three analysed species suggest that selenium hyperaccumulation has probably evolved multiple times over under similar environmental pressures in the US and Australia.

Root radial apoplastic transport contributes to shoot cadmium accumulation in a high cadmium-accumulating rice line

Radial transport of cadmium (Cd) in roots governs the amount of Cd loaded into xylem vessels, where Cd ions are translocated upward into shoots, while the mechanism of differential Cd radial transport between the high Cd-accumulating rice line Lu527-8 and the normal rice line Lu527-4 remains ambiguous. A higher Cd distribution in cross sections and root apoplast and higher bypass flow of Cd were found in Lu527-8, explaining a greater Cd translocation through the apoplastic pathway. The lower relative area of the epidermis and the constant relative area of the cortex in Lu527-8 opened-up root radial transport for Cd. Deposition of apoplastic barriers (Casparian strips and suberin lamellae) was stimulated by Cd, which effectively prevented Cd from entering the stele through the apoplastic pathway. In Lu527-8, apoplastic barriers were further from the root apex with lower expression of genes responsible for biosynthesis of Casparian strips and suberin lamellae, enhancing radial transport of Cd. Our data revealed that the higher radial apoplastic transport of Cd played an integral role in Cd translocation, contributed to a better understanding of the mechanism involved in high Cd accumulation in Lu527-8 and helped achieve the practical application of phytoextraction.

Assessment of the capability of cadmium accumulation and translocation among 31 willows: four patterns of willow biomass variation response to cadmium

Cadmium (Cd) pollution threatens food security and the environment. Willow species (Salix, Salicaceae) exhibit a remarkable potential to restore Cd-polluted sites due to their high biomass production and high Cd accumulation capacities. This study examined the Cd accumulation and tolerance in 31 genotypes of shrub willow in hydroponic conditions at varying Cd levels (0 μM Cd, 5 μM Cd, and 20 μM Cd). The root, stem, and leaf biomass of 31 shrub willow genotypes showed significant differences to Cd exposure. Among 31 willow genotypes, four patterns of biomass variation response to Cd were identified: insensitive to Cd; growth inhibition due to excessive Cd supply (high Cd inhibition); low Cd causing inhibited growth, whereas high Cd leading to increased biomass (U-shape); and growth increment with excessive Cd exposure (high Cd induction). The genotypes belonging to the "insensitive to Cd" and/or "high Cd induction" were candidates for the utilization of phytoremediation. Based on the analysis of Cd accumulation of 31 shrub willow genotypes at high and low Cd levels, genotypes 2372, 51-3, and 1052 obtained from a cross between S. albertii and S. argyracea grew well and accumulated relatively more Cd levels than other genotypes. In addition, for Cd-treated seedlings, root Cd accumulation was positively correlated with shoot Cd accumulation and total Cd uptake, demonstrating that Cd accumulation in roots could serve as a biomarker for evaluating the Cd extraction capacity of willows, especially in hydroponics screening. The results of this study screened out willow genotypes with high Cd uptake and translocation capacities, which will provide valuable approaches for restoring Cd-contaminated soils with willows.

Applications of Metabolomics for the Elucidation of Abiotic Stress Tolerance in Plants: A Special Focus on Osmotic Stress and Heavy Metal Toxicity

Plants undergo metabolic perturbations under various abiotic stress conditions; due to their sessile nature, the metabolic network of plants requires continuous reconfigurations in response to environmental stimuli to maintain homeostasis and combat stress. The comprehensive analysis of these metabolic features will thus give an overview of plant metabolic responses and strategies applied to mitigate the deleterious effects of stress conditions at a biochemical level. In recent years, the adoption of metabolomics studies has gained significant attention due to the growing technological advances in analytical biochemistry (plant metabolomics). The complexity of the plant biochemical landscape requires sophisticated, advanced analytical methods. As such, technological advancements in the field of metabolomics have been realized, aided much by the development and refinement of separatory techniques, including liquid and gas chromatography (LC and GC), often hyphenated to state-of-the-art detection instruments such as mass spectrometry (MS) or nuclear resonance magnetic (NMR) spectroscopy. Significant advances and developments in these techniques are briefly highlighted in this review. The enormous progress made thus far also comes with the dawn of the Internet of Things (IoT) and technology housed in machine learning (ML)-based computational tools for data acquisition, mining, and analysis in the 4IR era allowing for broader metabolic coverage and biological interpretation of the cellular status of plants under varying environmental conditions. Thus, scientists can paint a holistic and comprehensive roadmap and predictive models for metabolite-guided crop improvement. The current review outlines the application of metabolomics and related technological advances in elucidating plant responses to abiotic stress, mainly focusing on heavy metal toxicity and subsequent osmotic stress tolerance.

Lead Responses and Tolerance Mechanisms of Koelreuteria paniculata: A Newly Potential Plant for Sustainable Phytoremediation of Pb-Contaminated Soil

Phytoremediation could be an alternative strategy for lead (Pb) contamination. K. paniculata has been reported as a newly potential plant for sustainable phytoremediation of Pb-contaminated soil. Physiological indexes, enrichment accumulation characteristics, Pb subcellular distribution and microstructure of K. paniculata were carefully studied at different levels of Pb stress (0-1200 mg/L). The results showed that plant growth increased up to 123.8% and 112.7%, relative to the control group when Pb stress was 200 mg/L and 400 mg/L, respectively. However, the average height and biomass of K. paniculata decrease when the Pb stress continues to increase. In all treatment groups, the accumulation of Pb in plant organs showed a trend of root > stem > leaf, and Pb accumulation reached 81.31%~86.69% in the root. Chlorophyll content and chlorophyll a/b showed a rising trend and then fell with increasing Pb stress. Catalase (CAT) and peroxidase (POD) activity showed a positive trend followed by a negative decline, while superoxide dismutase (SOD) activity significantly increased with increasing levels of Pb exposure stress. Transmission electron microscopy (TEM) showed that Pb accumulates in the inactive metabolic regions (cell walls and vesicles) in roots and stems, which may be the main mechanism for plants to reduce Pb biotoxicity. Fourier transform infrared spectroscopy (FTIR) showed that Pb stress increased the content of intracellular -OH and -COOH functional groups. Through organic acids, polysaccharides, proteins and other compounds bound to Pb, the adaptation and tolerance of K. paniculata to Pb were enhanced. K. paniculata showed good phytoremediation potential and has broad application prospects for heavy metal-contaminated soil.

Enhanced cadmium phytoremediation capacity of poplar is associated with increased biomass and Cd accumulation under nitrogen deposition conditions

Nitrogen (N) deposition plays a significant role in soil cadmium (Cd) phytoremediation, and poplar has been considered for the remediation of contaminated soil because of its enormous biomass and strong Cd resistance. To reveal the underlying physiological and root phenotypic mechanisms of N deposition affecting Cd phytoextraction in poplar, we assessed root phenotypic characteristics, Cd absorption and translocation, chlorophyll fluorescence performance, and antioxidant enzyme activities of a clone of Populus deltoides × P. nigra through combined greenhouse Cd and N experiments. Our results showed that Cd significantly changed the root phenotype by reducing root length, tip number, and diameter. Cd also caused the peroxidation of lipids, damaged the photosystem II (PSII) reaction centre, and reduced photosynthetic capacity, resulting in a decrease in biomass accumulation in poplar. The N60 (60 kg N·ha^-1·yr^-1) and N90 (90 kg N·ha^-1·yr^-1) treatments promoted the net photosynthetic rate of poplar by increasing the activity of antioxidant enzymes and proline content and repairing the PSII reaction centre, thus increasing the biomass accumulation of poplar exposed to Cd stress. Simultaneously, the N60 and N90 treatments might have increased Cd uptake from the soil by upregulating total root length, root tips, and fine root length. Cd mainly accumulated in roots and stems but not in leaves. The N30 (30 kg N·ha^-1·yr^-1) treatment had no obvious effects on these parameters compared with the single Cd treatment. Consequently, our study suggested that adequate N can improve biomass and Cd accumulation to enhance the phytoremediation capacity of poplar for Cd, which might be related to the improvement of leaf physiological defence and the change in root phenotypic characteristics.

The regulatory role of abscisic acid on cadmium uptake, accumulation and translocation in plants

To date, Cd contamination of cropland and crops is receiving more and more attention around the world. As a plant hormone, abscisic acid (ABA) plays an important role in Cd stress response, but its effect on plant Cd uptake and translocation varies among plant species. In some species, such as Arabidopsis thaliana, Oryza sativa, Brassica chinensis, Populus euphratica, Lactuca sativa, and Solanum lycopersicum, ABA inhibits Cd uptake and translocation, while in other species, such as Solanum photeinocarpum and Boehmeria nivea, ABA severs the opposite effect. Interestingly, differences in the methods and concentrations of ABA addition also triggered the opposite result of Cd uptake and translocation in Sedum alfredii. The regulatory mechanism of ABA involved in Cd uptake and accumulation in plants is still not well-established. Therefore, we summarized the latest studies on the ABA synthesis pathway and comparatively analyzed the physiological and molecular mechanisms related to ABA uptake, translocation, and detoxification of Cd in plants at different ABA concentrations or among different species. We believe that the control of Cd uptake and accumulation in plant tissues can be achieved by the appropriate ABA application methods and concentrations in plants.

Ethylene positively regulates Cd tolerance via reactive oxygen species scavenging and apoplastic transport barrier formation in rice

Ethylene regulates plant root growth and resistance to environment stress. However, the role and mechanism of ethylene signaling in response to Cd stress in rice remains unclear. Here, we revealed that ethylene signaling plays a positive role in the resistance of rice to Cd toxicity. Blocking the ethylene signal facilitated root elongation under normal conditions, but resulted in severe oxidative damage and inhibition of root growth under Cd stress. Conversely, ethylene signal enhancement by EIN2 overexpression caused root bending, similar to the response of roots to Cd stress, and displayed higher Cd tolerance than the wildtype (WT) plants. Comparative transcriptome analysis indicated EIN2-mediated upregulation of genes involved in flavonoid biosynthesis and peroxidase activity under Cd stress. The synthesis of phenolic acids and flavonoids were positively regulated by ethylene. Thus, the ein2 (ethylene insensitive 2) mutants displayed lower ROS scavenging capacity than the WT. Moreover, a significant increase in Cd accumulation and relatively increased apoplastic flow were observed in the root apex of the ein2 mutant compared with the WT plants. Overall, EIN2-mediated Cd resistance in rice is mediated by the upregulation of flavonoid biosynthesis and peroxidase activity to induce ROS scavenging, and apoplastic transport barrier formation reduces Cd uptake.

Integration of manganese accumulation, subcellular distribution, chemical forms, and physiological responses to understand manganese tolerance in Macleaya cordata

Macleaya cordata (Willd.) R. Br. are proposed for the application in phytoremediation of heavy metal-contaminated soil. In this paper, the physiological response, subcellular distribution, chemical form, ultrastructure, and manganese (Mn) absorption characteristics of M. cordata under the stress of 0, 3, 6, 9, 12, and 15 mmol/L manganese concentration were studied by sand culture experiment. The results showed that M. cordata seedlings show high tolerance to Mn stress with a concentration of less than 6 mmol/L, while higher Mn concentration showed a significant toxic effect. A low concentration of Mn (≤ 6 mmol/L) can promote the synthesis of chlorophyll and soluble protein; furthermore, superoxide dismutase and peroxidase activities responded positively. The accumulation of Mn in the inactive metabolic part (cell wall and vacuole) of M. cordata leaves might be one of the main Mn detoxification mechanism. According to the ultrastructure of M. cordata, high-concentration Mn²⁺ (≥ 12 mmol/L) stress can cause M. cordata cells to be distorted and deformed, black precipitates appeared in the intercellular space, mitochondria decrease, chloroplasts shrink, hungry particles increased, and starch granules decrease. The uptake ability of different tissues for Mn is leaf > root > stem, and transport coefficient decreases with the increase of Mn concentration. Clearly, M. cordata has a certain tolerance to manganese, which has the ecological application potential in Mn-polluted areas.

Phytotoxic effects on chloroplast and UHPLC-HRMS based untargeted metabolomic responses in Allium tuberosum Rottler ex Sprengel (Chinese leek) exposed to antibiotics

Introduction of antibiotics into agricultural fields poses serious health risks to humans. This study investigated the uptake of antibiotics, their effects on metabolic pathways, and chloroplast structure changes of Allium tuberosum exposed to norfloxacin (NFL), oxytetracycline (OTC), and tetracycline (TC). Among all the antibiotic treatments, the highest accumulation of antibiotics in roots (4.15 mg/kg) and leaves (0.29 mg/kg) was TC, while in bulbs it was NFL (5.94 mg/kg). OTC was with the lowest accumulation in roots: 0.19 mg/kg, bulbs: 0.18 mg/kg, and leaves: 0.11 mg/kg. The number of mitochondira and the number of plastoglobulli increased. The chloroplast structure was disturbed under the stress of NFL, OTC, and TC. Disturbance in the chloroplast ultrastructure leads to altered chlorophyll fluorescence variables. Simultaneously, metabolomic profiling of leaves demonstrated that NFL stress regulated more of metabolic pathways than OTC and TC. Differences in metabolic pathways among the antibiotic treatments showed that each antibiotic has different impact even under the same experimental conditions. TC and NFL have more toxic effects than OTC antibiotic. Metabolic variations induced by antibiotics stress highlighted pools of metabolites that affect the metabolic activities, chlorophyll fluorescence, ultrastructural adjustments, and stimulate defensive impact in A. tubersoum. These findings provide an insight of metabolic destabilization as well as metabolic changes in defensive mechanism and stress response of A. tuberosum to different antibiotics.

Advances in "Omics" Approaches for Improving Toxic Metals/Metalloids Tolerance in Plants

Food safety has emerged as a high-urgency matter for sustainable agricultural production. Toxic metal contamination of soil and water significantly affects agricultural productivity, which is further aggravated by extreme anthropogenic activities and modern agricultural practices, leaving food safety and human health at risk. In addition to reducing crop production, increased metals/metalloids toxicity also disturbs plants' demand and supply equilibrium. Counterbalancing toxic metals/metalloids toxicity demands a better understanding of the complex mechanisms at physiological, biochemical, molecular, cellular, and plant level that may result in increased crop productivity. Consequently, plants have established different internal defense mechanisms to cope with the adverse effects of toxic metals/metalloids. Nevertheless, these internal defense mechanisms are not adequate to overwhelm the metals/metalloids toxicity. Plants produce several secondary messengers to trigger cell signaling, activating the numerous transcriptional responses correlated with plant defense. Therefore, the recent advances in omics approaches such as genomics, transcriptomics, proteomics, metabolomics, ionomics, miRNAomics, and phenomics have enabled the characterization of molecular regulators associated with toxic metal tolerance, which can be deployed for developing toxic metal tolerant plants. This review highlights various response strategies adopted by plants to tolerate toxic metals/metalloids toxicity, including physiological, biochemical, and molecular responses. A seven-(omics)-based design is summarized with scientific clues to reveal the stress-responsive genes, proteins, metabolites, miRNAs, trace elements, stress-inducible phenotypes, and metabolic pathways that could potentially help plants to cope up with metals/metalloids toxicity in the face of fluctuating environmental conditions. Finally, some bottlenecks and future directions have also been highlighted, which could enable sustainable agricultural production.

Assessment of differences in anatomical and hydraulic properties of the root and xylem of three willow (Salix L.) clones during phytostabilization after exposure to elevated cadmium

An anatomical study of adventitious roots of three Salix clones, B-44, SV068 and SM4041, treated with 3 and 6 mg Cd kg-1 dry weight in soil in a greenhouse experiment. The aim was to analyze the anatomical characteristics of roots in response to pollution by cadmium and to assess the potential application of anatomical and hydraulic characteristics in the selection of the most suitable Salix clones for phytostabilization of pollutants in soils. Anatomical parameters measured in this study included root crosssectional area, root diameter, the proportion of periderm, secondary phloem (cortex) and wood (secondary and primary xylem), and parameters of the vessels (lumen area, diameter and frequency). Based on the measurements of individual vessel lumens and the number of vessels, the theoretical hydraulic conductivity (kh) of roots was calculated. The effects of applied Cd concentrations on root traits were studied in clones and control plants. Following treatments with both Cd concentrations, plants of clone B-44 had the highest values of most parameters and significantly higher kh in comparison with control samples due to the significantly larger root cross-sectional area and lumen of vessels. It was concluded that these characteristics can serve for effective evaluation and selection of clones for remediation of sites contaminated with cadmium.

Insights into decontamination of soils by phytoremediation: A detailed account on heavy metal toxicity and mitigation strategies

In the current era of rapid industrialization, the foremost challenge is the management of industrial wastes. Activities such as mining and industrialization spill over a large quantity of toxic waste that pollutes soil, water, and air. This poses a major environmental and health challenge. The toxic heavy metals present in the soil and water are entering the food chain, which in turn causes severe health hazards. Environmental clean-up and reclamation of heavy metal contaminated soil and water are very important, and it necessitates efforts of environmentalists, industrialists, scientists, and policymakers. Phytoremediation is a plant-based approach to remediate heavy metal/organic pollutant contaminated soil and water in an eco-friendly, cost-effective, and permanent way. This review covers the effect of heavy metal toxicity on plant growth and physiological process, the concept of heavy metal accumulation, detoxification, and the mechanisms of tolerance in plants. Based on plants' ability to uptake heavy metals and metabolize them within tissues, phytoremediation techniques have been classified into six types: phytoextraction, phytoimmobilization, phytovolatilization, phytodegradation, rhizofiltration, and rhizodegradation. The development of research in this area led to the identification of metal hyper-accumulators, which could be utilized for reclamation of contaminated soil through phytomining. Concurrently, breeding and biotechnological approaches can enhance the remediation efficiency. Phytoremediation technology, combined with other reclamation technologies/practices, can provide clean soil and water to the ecosystem.

Targeting aquaporins to alleviate hazardous metal(loid)s imposed stress in plants

Uptake of hazardous metal(loid)s adversely affects plants and imposes a threat to the entire food chain. Here, the role of aquaporins (AQPs) providing tolerance against hazardous metal(loid)s in plants is discussed to provide a perspective on the present understanding, knowledge gaps, and opportunities. Plants adopt complex molecular and physiological mechanisms for better tolerance, adaptability, and survival under metal(loid)s stress. Water conservation in plants is one such primary strategies regulated by AQPs, a family of channel-forming proteins facilitating the transport of water and many other solutes. The strategy is more evident with reports suggesting differential expression of AQPs adopted by plants to cope with the heavy metal stress. In this regard, numerous studies showing enhanced tolerance against hazardous elements in plants due to AQPs activity are discussed. Consequently, present understanding of various aspects of AQPs, such as tertiary-structure, transport activity, solute-specificity, differential expression, gating mechanism, and subcellular localization, are reviewed. Similarly, various tools and techniques are discussed in detail aiming at efficient utilization of resources and knowledge to combat metal(loid)s stress. The scope of AQP transgenesis focusing on heavy metal stresses is also highlighted. The information provided here will be helpful to design efficient strategies for the development of metal(loid)s stress-tolerant crops.

Anatomical changes, osmolytes accumulation and distribution in the native plants growing on Pb-contaminated sites

Native plants growing on heavy-metal-contaminated sites are the potential candidates for phytoremediation of contaminated sites due to their tolerance and adaptation to toxic environment. For better application of the technology, in-depth knowledge and understanding of the anatomy and physiology of these plant species are essential. In this study, anatomical changes in roots and leaves of various plant species growing on Pb-contaminated sites as well as osmolytes (proline, PR; glycine betaine, GB; and phenolics, PH) production and distribution in different plant parts were investigated. The soil and plant samples were collected in triplicates from two different Pb-contaminated sites, while control plants were collected from the University of Ibadan. The plants were washed and partitioned into different parts for anatomical and biochemical studies, and soil adhering to the roots of different plant species was collected for the determination of Pb contents. Sporobolus pyramidalis, Cynodon dactylon, Imperata cylindrica, Eleusine indica, Gomphrena celosioides, Rhinconspora corymbosa and Echinochloa colona are the plant species common to these contaminated sites. Though with variations based on the site, thick epidermis and sclerenchyma, pronounced and numerous vascular bundles as well as trichomes were some of the anatomical characteristics of these metallophytes from contaminated sites compared to their counterparts from uncontaminated site. These plants also produced PR, GB and PH, with PH being the highest followed by PR and GB. Accumulation in different parts of the plants also varied depending on the plant species and the osmolyte. They were more in the leaf than other plant parts. Among the plant species, Sporobolus pyramidalis had the highest value of proline (3.26 and 2.63 µg/g FW) and glycine betaine (3.44 and 2.23 µg/FW) in the leaf and stem, respectively, followed by Cynodon dactylon having 2.20 and 0.89 µg/g FW proline and 2.02 and 0.94 µg/g FW glycine betaine. On the Pb accumulation in plant, more lead (Pb) was found in the roots than other plant parts. Gomphrena celosioides accumulated the highest Pb (4537.50 and 1081.50 mg/kg) in the root and shoot, respectively, followed by Eleusine indica (3988 and 699.00 mg/kg), while the lowest values were recorded for Echinochloa colona (36.87 and 5.97 mg/kg). In rhizospheric soils, G. celosioides had the highest Pb content (34,405.00 mg/kg) which was higher than other rhizospheric soils. In conclusion, phenolics were produced more in these metallophytes and all the osmolytes were more in leaf than root. Anatomical modifications in response to heavy metal exposure differed between plant species and level of contamination. Lead accumulation in plant also varied depending on plant species.

Cadmium toxicity in plants: Impacts and remediation strategies

Cadmium (Cd) is an unessential trace element in plants that is ubiquitous in the environment. Anthropogenic activities such as disposal of urban refuse, smelting, mining, metal manufacturing, and application of synthetic phosphate fertilizers enhance the concentration of Cd in the environment and are carcinogenic to human health. In this manuscript, we reviewed the sources of Cd contamination to the environment, soil factors affecting the Cd uptake, the dynamics of Cd in the soil rhizosphere, uptake mechanisms, translocation, and toxicity of Cd in plants. In crop plants, the toxicity of Cd reduces uptake and translocation of nutrients and water, increases oxidative damage, disrupts plant metabolism, and inhibits plant morphology and physiology. In addition, the defense mechanism in plants against Cd toxicity and potential remediation strategies, including the use of biochar, minerals nutrients, compost, organic manure, growth regulators, and hormones, and application of phytoremediation, bioremediation, and chemical methods are also highlighted in this review. This manuscript may help to determine the ecological importance of Cd stress in interdisciplinary studies and essential remediation strategies to overcome the contamination of Cd in agricultural soils.

Ethylene-mediated apoplastic barriers development involved in cadmium accumulation in root of hyperaccumulator Sedum alfredii

Ethylene is an important phytohormone for plant adaptation to heavy metal stress. However, the effects of ethylene on radial apoplastic transport of Cd remain elusive. This study investigated the role of ethylene on apoplastic barriers development and consequences for Cd uptake in Sedum alfredii. In response to Cd, endogenous ethylene production in hyperaccumulating ecotype (HE) roots was decreased due to the down-regulated expressions of ethylene biosynthesis genes, while the opposite result was observed in non-hyperaccumulating ecotype (NHE). Interestingly, the ethylene emission in HE was always higher than that in NHE, regardless of Cd concentrations. Results of exogenous application of ethylene biosynthesis precursor/inhibitor indicate that ethylene with high level would delay the formation of apoplastic barriers in HE through restraining phenylalanine ammonia lyase activity and gene expressions related to lignin/suberin biosynthesis. Simultaneously, correlation analyses suggest that Cd-induced apoplastic barriers formation may be also regulated by ethylene signaling. By using an apoplastic bypass tracer and scanning ion-selected electrode, we observed that the delayed deposition of apoplastic barriers significantly promoted Cd influx in roots. Taken together, high endogenous ethylene in HE postponed the formation of apoplastic barriers and thus promoted the Cd accumulation in the apoplast of roots.

Structural modifications of plant organs and tissues by metals and metalloids in the environment: A review

At the dawn of the industrial revolution, the exorbitant use of heavy metals and toxic elements by mankind unfurls a powerful and complex web of hazard all around the world that significantly contributed to unprecedented trends in environmental degradation. Plants as sessile organisms, that cannot escape from the stress directly, have adapted to this environment via concurrent configurations of several traits. Among them the anatomy has been identified as much more advanced field of research that brought the explosion of interest among the expertise and its prodigious importance in stress physiology is unavoidable. In conjunction with various other disciplines, like physiology, biochemistry, genomics and metabolomics, the plant anatomy provides a large data sets that are paving the way towards a comprehensive and holistic understanding of plant growth, development, defense and productivity under heavy metal and toxic element stress. Present paper advances our recent knowledge about structural alterations of plant tissues induced by metals and metalloids, like antimony (Sb), arsenic (As), aluminium (Al), copper (Cu), cadmium (Cd), chromium (Cr), lead (Pb), manganese (Mn), mercury (Hg), nickel (Ni) and zinc (Zn) and points on essential role of plant anatomy and its understanding for plant growth and development in changing environment. Understanding of anatomical adaptations of various plant organs and tissues to heavy metals and metalloids could greatly contribute to integral and modern approach for investigation of plants in changing environmental conditions. These findings are necessary for understanding of the whole spectra of physiological and biochemical reactions in plants and to maintain the crop productivity worldwide. Moreover, our holistic perception regarding the processes underlying the plant responses to metal(loids) at anatomical level are needed for improving crop management and breeding techniques.

A comparative study of root cadmium radial transport in seedlings of two wheat (Triticum aestivum L.) genotypes differing in grain cadmium accumulation

The radial transport of cadmium (Cd) is essential for Cd influx in roots. The role of radial transport pathway on the Cd translocation from root to shoot among wheat genotypes are still poorly understood. This study explored the role of apoplastic and symplastic pathway on root Cd uptake and root-to-shoot translocation in Zhenmai 10 (ZM10, high Cd in grains) and Aikang 58 (AK58, low Cd in grains). Under Cd treatment, the deposition of Casparian strips (CSs) and suberin lamellae (SL) initiated closer to the root apex in ZM10 than that in AK58, which resulted in the lower Cd concentration in apoplastic fluid of ZM10. Simultaneously, Cd-induced expression levels of genes related to Cd uptake in roots were significantly higher in AK58 by contrast with ZM10, contributing to the symplastic Cd accumulation in AK58 root. Moreover, the addition of metabolic inhibitor CCCP noticeably decreased the Cd accumulation in root of both genotypes. Intriguingly, compared to ZM10, greater amounts of Cd were sequestrated in the cell walls and vacuoles in roots of AK58, limiting the translocation of Cd from root to shoot. Furthermore, the elevated TaHMA2 expression in ZM10 indicates that ZM10 had a higher capacity of xylem loading Cd than AK58. All of these results herein suggest that the radial transport is significant for Cd accumulation in roots, but it cannot explain the difference in root-to-shoot translocation of Cd in wheat genotypes with contrast Cd accumulation in grains.

The cadmium tolerance development of poplar callus is influenced by silicon

Cadmium-tolerant plants were studied for their possible usage in phytoremediation techniques. However, their response to cadmium cations at a cellular level has not been properly studied. Silicon is a beneficial element that seems to change the plant's response to the Cd²⁺ presence. The aim of the present study was to investigate the Cd²⁺ tolerance patterns of poplar callus cells exposed to Cd⁺² and/or Si over short and long cultivation periods. We determined the growth parameters of the callus, the growth dynamics, cell vitality, photosynthetic pigment concentrations and the activity of antioxidant enzymes. The effects were studied over short (21 days) and long (63 days) cultivation periods. The most important result proved that the poplar callus tissue is able to build up a tolerance to Cd²⁺ after a longer cultivation period. On the 63^rd day of the cultivation, Cd²⁺ stressed calli showed improvement in studied parameters and the callus cells accumulated Cd²⁺ more efficiently than on the 21^st day. Supplementation with Si in higher concentrations (2.5 mM and 5 mM) heightened the Cd-tolerance potential of the tissue. The treatment of Cd²⁺, and Si in a 2.5 mM concentration was the most efficient variant for Cd²⁺ removal from medium. The activity of antioxidant enzymes showed that poplar callus cells effectively develop tolerance against Cd²⁺ after a longer cultivation period.

The role of root apoplastic barriers in cadmium translocation and accumulation in cultivars of rice (Oryza sativa L.) with different Cd-accumulating characteristics

The radial translocation of cadmium (Cd) from the root to the shoot is one of the major processes affecting Cd accumulation in rice (Oryza sativa L.) grains, but few studies have focused on Cd apoplastic transport in rice. The aim of this study was to determine how apoplastic barriers affect Cd translocation via the apoplastic pathway, Cd accumulation levels in upper parts (shoot and grains) of rice cultivars, and the possible mechanism involved. Hydroponic and soil pot trials were conducted to study the development and chemical constituents of apoplastic barriers and their permeability to bypass flow, and to determine Cd localization in the roots of rice cultivars with different Cd-accumulating characteristics. The Cd accumulation in upper parts was positively correlated with bypass flow in the root and the apparent Cd concentration in the xylem, indicating that the apoplastic pathway may play an important role in Cd root-shoot translocation in rice. Apoplastic barriers were deposited closer to the root tip and were thicker in low Cd-accumulating cultivars than in high Cd-accumulating cultivars. The amounts and rates of increase in lignin and suberin were significantly higher in ZD14 (a low Cd-accumulating cultivar) than in FYXZ (a high Cd-accumulating cultivar) under Cd stress, indicating that stronger barriers were induced by Cd in ZD14. The stronger and earlier formation of barriers in the low Cd-accumulating cultivar decreased bypass flow more efficiently, so that more Cd was retained in the root during apoplastic translocation. This was confirmed by localization analyses of Cd in root transverse sections. These results suggest that apoplastic barriers reduce Cd root-to-shoot translocation via the apoplastic pathway, leading to lower Cd accumulation in the upper parts of rice plants. Bypass flow may have the potential to be used as a rapid screening indicator for low Cd-accumulating rice cultivars.

The plant-growth promoting bacteria promote cadmium uptake by inducing a hormonal crosstalk and lateral root formation in a hyperaccumulator plant Sedum alfredii

Plant growth-promoting bacteria (PGPB) that inhabit hyperaccumulating plants assist cadmium (Cd) absorption, but the underlying mechanism has not been comprehensively studied. For this reason, we combined the fluorescence imaging, and transcriptomic and metabolomic methods in a Cd hyperaccumulator, Sedum alfredii, inoculated or not with PGPB Pseudomonas fluorescens. The results showed that the newly emerged lateral roots, that were heavily colonized by P. fluorescens, are the main entry for Cd influx in S. alfredii. Inoculation with P. fluorescens promoted a lateral root formation of its host plant, leading to a higher Cd phytoremediation efficiency. Furthermore, the plant transcriptome revealed that 146 plant hormone related genes were significantly up-regulated by the bacterial inoculation, with 119 of them showing a complex interaction, which suggests that a hormonal crosstalk participated root development. The targeted metabolomics analysis showed that P. fluorescens inoculation significantly increased indole acetic acid concentration and significantly decreased concentrations of abscisic acid, brassinolide, trans-zeatin, ethylene and jasmonic acid in S. alfredii roots, thereby inducing lateral root emergence. Altogether, our results highlight the importance of PGPB-induced lateral root formation for the increased Cd uptake in a hyperaccumulating plant.

Mushroom residue modification enhances phytoremediation potential of Paulownia fortunei to lead-zinc slag

Phytoremediation is an effective strategy for the remediation of lead-zinc slag, while the response of plant on lead and zinc was less concerned. In this study, mushroom residue was adding in lead-zinc slag to enhance the phytoremediation potential of P. fortunei, the effects of three treatments (lead-zinc slag, red soil, lead-zinc slag + 10% (m/m) mushroom residue) on the growth, physiology and microstructure of P. fortunei were determined. The results showed that the addition of mushroom residue increased the biomass, plant height and chlorophyll concentration of P. fortunei, indicating that the addition of mushroom residue can facilitate the growth of P. fortunei. Moreover, the proportions of oxalate-Pb forms and phosphate-Zn were dominant in leaves and stems of P. fortunei. With the addition of mushroom residue, Pb and Zn were transformed to the extraction state with weak migration activity, which can reduce the damage level of Pb and Zn to P. fortunei. The results from scanning transmission electron microscopy (STEM) showed that, the mushroom residue amendment could maintain the integrity of the cell structural of P. fortunei. The results from fourier transform infrared spectrometer (FTIR) analysis showed that the mushroom residue amendment could increase the contents of proteins and polysaccharides in P. fortunei, which can combine with the metals. Clearly, the mushroom residue amendment could promote the growth ability of P. fortunei in lead and zinc slag and strengthen the phytoremediation potential.

Assessment of phytostabilization potential of two Salix L. clones based on the effects of heavy metals on the root anatomical traits

Willow species (Salix L.) are a useful tool for assessing phytostabilization of the sites polluted by heavy metals. Phytostabilization potential of two willow genotypes (Salix alba L. clone '68/53/1' and Salix nigra Marshall clone '0408') has been evaluated in a 45-day hydroponic experiment, using stem cuttings (diameter 12 to 14 mm, length 20 cm) exposed to two concentrations (10^-4 M and 10^-5 M) of individually applied Cd, Ni, and Pb. Metals were diluted in 25% Hoagland's solution, in forms of CdCl₂·H₂O, NiSO₄·6H₂O, and Pb-EDTA. The control group of cuttings was grown in 25% Hoagland's solution without heavy metals. High Cd concentrations in willow roots, 8637 mg/kg (clone '68/53/1') and 6728 mg/kg of dry weight (clone '0408'), have indicated a high phytostabilization potential. However, detailed analyses of cross-sectional area of the root cortex and the central cylinder revealed that the excess concentration of Cd led to a significant reduction of measured anatomical root's traits of clone '68/53/1' in comparison with the control samples. Excessive concentration of Ni and Pb in nutrient solution increased the values of quantitatively measured root's traits of clone '0408', implying stimulatory effects of the applied concentrations. Concentration of 10^-4 M of each metal had more negative effects on the roots' anatomical traits, notably on parenchymal and exodermal cells and vessels. Deposits of metals were observed in root tissues. Clone '0408' demonstrated an increased tolerance to heavy metals, which could potentially make this clone useful in phytostabilization.

The barrier function of plant roots: biological bases for selective uptake and avoidance of soil compounds

The root is the main organ through which water and mineral nutrients enter the plant organism. In addition, root fulfils several other functions. Here, we propose that the root also performs the barrier function, which is essential not only for plant survival but for plant acclimation and adaptation to a constantly changing and heterogeneous soil environment. This function is related to selective uptake and avoidance of some soil compounds at the whole plant level. We review the toolkit of morpho-anatomical, structural, and other components that support this view. The components of the root structure involved in selectivity, permeability or barrier at a cellular, tissue, and organ level and their properties are discussed. In consideration of the arguments supporting barrier function of plant roots, evolutionary aspects of this function are also reviewed. Additionally, natural variation in selective root permeability is discussed which suggests that the barrier function is constantly evolving and is subject of natural selection.

Cd-induced difference in root characteristics along root apex contributes to variation in Cd uptake and accumulation between two contrasting ecotypes of Sedum alfredii

The root apex is the most active part for water and ions uptake, however, longitudinal alterations in root characteristics along root apex and consequences for metal uptake in hyperaccumulator are poorly understood. Here, we compared cadmium (Cd)-induced longitudinal alterations in root apex of two ecotypes of Sedum alfredii and assess their effects on Cd uptake. Under Cd treatment, cell death began from epidermis to the stele in non-hyperaccumulating ecotype (NHE) over time, and the number of dead cells was significantly higher than that in hyperaccumulating ecotype (HE). Cd-induced the presence of border-like cells (BLCs) surrounding the root tip of NHE prevented Cd from entering roots, however, almost no BLCs were observed in the root tip of in HE. Besides, Cd-treated NHE exhibited 76% and 52% decrease in the proportions of meristematic and elongation zone, respectively, resulting in lower Cd influx and less intensive Cd-fluorescence in these zones, as compared with HE. In the differentiation zone, Cd induced earlier initiation of root hairs (RHs), lower RHs-density, shorter RHs-length, thicker RHs-radius and less trichoblasts in NHE than those in HE. These remarkable variations led to less Cd influx and lower intensity of Cd-fluorescence in RHs of NHE than those of HE. Furthermore, decline in cell wall thickness under Cd exposure resulted in less cell-wall-bond Cd in the cell wall of HE. Therefore, Cd-induced alterations in root characteristics alongside root apex contributed to the difference in Cd uptake and accumulation between two ecotypes of S. alfredii.

Distinct redox signalling and nickel tolerance in Brassica juncea and Arabidopsis thaliana

Despite of its essentiality, nickel (Ni) in excess is toxic for plants partly due to the overproduction of reactive oxygen species (ROS) and the consequent increase in oxidative stress signalling. However, in Ni-stressed plants little is known about the signal transduction of reactive nitrogen species (RNS) and protein tyrosine nitration as the protein-level consequence of increased RNS formation. Our experiments compared the nickel accumulation and tolerance, the redox signalling and the protein nitration in the agar-grown Arabidopsis thaliana and Brassica juncea exposed to Ni (50 μM nickel chloride). Studying GUS-tagged Arabidopsis lines (ARR5::GUS, ACS8::GUS and DR5::GUS) revealed that Ni-increased lateral root (LR) emergence, and concomitantly reduced LR initiation were accompanied by elevated levels of auxin, cytokinin, and ethylene in the LRs or in upper root parts, whereas Ni-induced primary root shortening is related to decreased auxin, and increased cytokinin and ethylene levels. These suggest the Ni-induced disturbance of hormonal balance in the root system. Results of the comparative study showed that weaker Ni tolerance of A. thaliana was coupled with a Ni-induced increase in RNS, ROS, and hydrogen sulfide levels, as well as with an increase in redox signalling and consequent increment of protein nitration. However, in relative Ni tolerant B. juncea, redox signalling (except for peroxynitrite) was not modified, and Ni-induced intensification of protein tyrosine nitration was less pronounced. Data collectively show that the better Ni tolerance of Brassica juncea may be related to the capability of preventing the induction of redox signalling and consequently to the slighter increase in protein nitration.

Resveratrol ameliorates the physiological, biochemical, cytogenetic, and anatomical toxicities induced by copper(II) chloride exposure in Allium cepa L

Excessive copper (Cu) exposure ultimately results in toxicosis in all organisms. The protective potential of resveratrol compound against the CuCl₂ toxicity was evaluated in Allium cepa L. root tip cells. For this aim, A. cepa bulbs were divided into six groups and the groups were treated with tap water, 400 mg/L resveratrol, 800 mg/L resveratrol, 20 μM CuCl₂, 400 mg/L resveratrol + 20 μM CuCl₂, and 800 mg/L resveratrol + 20 μM CuCl₂ solutions, respectively for 72 h. The first group irrigated with tap water was accepted as control. All groups were screened for the germination percentage, root elongation, total bulb weight gain, micronucleus (MN) frequency, mitotic index (MI), chromosomal aberrations, and anatomical changes. Furthermore, superoxide dismutase (SOD) and catalase (CAT) activities as well as malondialdehyde (MDA) level as an indicator of lipid peroxidation were investigated. It was found that CuCl₂ exposure alone triggered a definite negative effects on all parameters examined. On the other hand, the groups treated with resveratrol did not have statistically different values compared to the control group. Resveratrol doses induced a remarkable recovery in growth parameters including germination percentage, root elongation, and total bulb weight gain when applied with CuCl₂. In these groups, MN frequency, chromosomal aberrations, and anatomical abnormalities were alleviated, whereas the MI levels increased significantly. Moreover, lipid peroxidation level and antioxidant enzyme activities showed a marked amelioration when resveratrol and CuCl₂ were applied together. Copper compounds have become common pollutants due to their direct uses as pesticides in agricultural areas as well as their spreading to natural areas from industrial fields. The study clearly demonstrated the therapeutic potential of resveratrol against the harmful effects of CuCl₂ exposure in Allium roots. So, resveratrol could be considered as a plant-derived restorative agent for the reduction of the risks from the other copper compounds.

In vivo removal of profenofos in agricultural soil and plant growth promoting activity on Vigna radiata by efficient bacterial formulation

This study evaluated the plant growth and profenofos (PF) removal efficiency of Acinetobacter sp.33F and Comamonas sp. 51 F bacteria as individual strains and in combination F1. Plant growth-promoting activities such as indole 3 acetic acid (IAA) production, 1-aminocyclopropane-1-carboxylate (ACC) deaminase, phosphate solubilization, ammonia production, and exopolysaccharide (EPS) production were observed in Acinetobacter sp. 33 F and Comamonas sp. 51 F. However, PGP properties observed were higher in Acinetobacter sp. 33 F as compared to the Comamonas sp. 51 F. In pot sand and pot soil studies, the physiological parameters such as sprout length, shoot length, root length, chlorophyll a, chlorophyll b, and carotenoids were higher for combination F1. PF degradation in pot sand and pot soil resulted in highest degradation by combination F1. In pot soil study, soil enzyme activities such as cellulase, dehydrogenase, urease, protease, and phosphate activities and root cross-section area, total stele area and xylem vessel area were recorded higher for the formulation F1. The study demonstrated that the together Acinetobacter sp. 33 F and Comamonas sp. 51 F as formulation has higher plant growth-promoting activities as compared to the individual bacteria.

Abscisic acid-mediated modifications of radial apoplastic transport pathway play a key role in cadmium uptake in hyperaccumulator Sedum alfredii

Abscisic acid (ABA) is a key phytohormone underlying plant resistance to toxic metals. However, regulatory effects of ABA on apoplastic transport in roots and consequences for uptake of metal ions are poorly understood. Here, we demonstrate how ABA regulates development of apoplastic barriers in roots of two ecotypes of Sedum alfredii and assess effects on cadmium (Cd) uptake. Under Cd treatment, increased endogenous ABA level was detected in roots of nonhyperaccumulating ecotype (NHE) due to up-regulated expressions of ABA biosynthesis genes (SaABA2, SaNCED), but no change was observed in hyperaccumulating ecotype (HE). Simultaneously, endodermal Casparian strips (CSs) and suberin lamellae (SL) were deposited closer to root tips of NHE compared with HE. Interestingly, the vessel-to-CSs overlap was identified as an ABA-driven anatomical trait. Results of correlation analyses and exogenous applications of ABA/Abamine indicate that ABA regulates development of both types of apoplastic barriers through promoting activities of phenylalanine ammonialyase, peroxidase, and expressions of suberin-related genes (SaCYP86A1, SaGPAT5, and SaKCS20). Using scanning ion-selected electrode technique and PTS tracer confirmed that ABA-promoted deposition of CSs and SL significantly reduced Cd entrance into root stele. Therefore, maintenance of low ABA levels in HE minimized deposition of apoplastic barriers and allowed maximization of Cd uptake via apoplastic pathway.

Distribution and chemical forms of cadmium in Coptis chinensis Franch. determined by laser ablation ICP-MS, cell fractionation, and sequential extraction

Coptis chinensis Franch., is a widely used medicinal plant in China. This plant is often contaminated by cadmium (Cd) and render health risk to human consumers. Understanding distribution of Cd and its chemical forms is important to evaluate accumulation of the metal and its detoxification mechanisms in this plant. Since few studies have focused on this aspect, we used laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) to spatially locate Cd in rhizome cross-sections, and ICP-MS to analyze the Cd subcellular distribution and the chemical forms of Cd in different tissues. Rhizome bioimaging results showed that Cd was distributed predominantly within the periderm, cortex, pith, and root trace vascular bundle. The LA-ICP-MS results suggested that Ca²⁺ channels might be a pathway for Cd entry into the plant. Subcellular distribution data indicated that most of Cd was associated with the cell wall (41.8-77.1%) and the soluble fraction (14.4-52.7%) in all tissues. Analysis of chemical forms revealed that majority Cd existed in less mobile and less toxic forms in all tissues, and P could convert to insoluble phosphate with Cd to moderate Cd toxicity. The new understanding of Cd accumulation and detoxification might provide novel strategies for reducing the levels of Cd in C. chinensis Franch., thereby mitigating its potential transfer to humans and providing a theoretical basis for evaluating the Cd status in other medicinal plants. Further, our findings might provide a basis for establishing a reasonable Cd limit level of traditional Chinese medicinal materials.

Scavenging effect of oxidized biochar against the phytotoxicity of lead ions on hydroponically grown chicory: An anatomical and ultrastructural investigation

To evaluate the scavenging effect of functionalized biochar against the phytotoxicity of Pb2+, original biochar (O-B) was chemically oxidized with either HNO3 or KMnO4 to serve as biofilters (O-BF, HNO3-BF and KMnO4-BF) to hydroponically grown chicory (Cichorium intybus L. var. intybus). Plants subjected to Pb-stress showed various deteriorations in cell organelles including visible alterations in chloroplasts, malformations in plant cells, abnormalities in the mitochondrial system, inward invagination of cell walls, distortions in the plasma membrane, oversized vacuoles and irregular increase in plastoglobuli formation. In addition, disorganization in xylem and phloem tissues and numerous variations in the stomatal number, density and dimensions as well as stomata movement were noticeable in the abaxial leaf surface. Pb-stressed plants showed increments in root diameter, vascular cylinder and metaxylem vessels as well as an obvious increase in the thickness of cortex, intercellular aerenchyma and endodermis layer. Furthermore, a noticeable disturbance in macro-and micronutrient concentrations was recorded in Pb-stressed plants due to the defect in their water status. O-BF showed a limited scavenging effect against the phytotoxicity of Pb2+. However, oxidized biochar filters (particularly KMnO4-BF) recorded a noticeable safeguard effect due to their high affinity to Pb2+ ions. The higher sorption capacity of KMnO4-BF reduced the concentration of Pb in leaf tissues compared to the unequipped filtration treatment (117 vs. 19 µg g-1). In conclusion, data of this hydroponic study provides baseline information regarding the detoxification mechanisms of functionalized biochar against the phytotoxicity of trace elements.

In-situ preparation of plant samples in ESEM for energy dispersive x-ray microanalysis and repetitive observation in SEM and ESEM

The Extended Low Temperature Method (ELTM) for the in-situ preparation of plant samples in an environmental scanning electron microscope enables carrying out repetitive topographical and material analysis at a higher resolution in the vacuum conditions of a scanning electron microscope or in the low gas pressure conditions of an environmental scanning electron microscope. The method does not require any chemical intervention and is thus suitable for imaging delicate structures rarely observable with common treatment methods. The method enables both sample stabilization as close to their native state as possible, as well as the transfer of the same sample from a low vacuum to an atmospheric condition for sample storage or later study. It is impossible for wet samples in the environmental scanning electron microscope. Our studies illustrate the high applicability of the ELTM for different types of plant tissue, from imaging of plant waxes at higher resolution, the morphological study of highly susceptible early somatic embryos to the elemental microanalysis of root cells. The method established here provides a very fast, universal and inexpensive solution for plant sample treatment usable in a commercial environmental scanning electron microscope equipped with a cooling Peltier stage.

Exogenous spermidine elevating cadmium tolerance in Salix matsudana involves cadmium detoxification and antioxidant defense

In this study, exogenous spermidine role on Salix matsudana tolerance to cadmium was evaluated. Spermidine and cadmium presented antagonistic effects on the biomass, copper and zinc concentrations in S. matsudana. cadmium mainly distributed in the cell wall of subcellular fraction; 46.97%-60.43% of cadmium existed in a sodium chloride-extracted form. Cadmium contents in roots, leaves, and twigs ranged from 2002.67 to 3961.00, 111.59 to 229.72, and 102.56 to 221.27 mg/kg, respectively. Spermidine application elevated cadmium concentrations in the roots, cuttings, and cell wall and the ratio of deionized water-extracted cadmium, but decreased cadmium levels in the twigs and leaves and the fractions of cadmium extracted by ethanol and sodium chloride, respectively. Putrescine and malondialdehyde were important indicators of cadmium-induced oxidative damage. Exogenous spermidine alleviated the accumulation of superoxide anion, hydrogen peroxide, malondialdehyde via promoting the levels of spermidine, soluble protein, superoxide dismutase, reductive ascorbate, glutathione reductase, and glutathione peroxidase in S. matsudana leaves under the corresponding cadmium stress. The results indicated that S. matsudana was a candidate for cadmium rhizoremediation and extraction in leaves; the spermidine application enhanced the cadmium tolerance of S. matsudana through promoting cadmium accumulation in roots, cell wall, and less bioactive chemical forms and the antioxidative ability.

Effects of root morphology and anatomy on cadmium uptake and translocation in rice (Oryza sativa L.)

A clear description of the certain mechanisms of cadmium (Cd) uptake and translocation in rice (Oryza sativa L.) may help to reduce Cd accumulation in rice grain. Hydroponic experiments were carried out to determine the effects of cultivation conditions (aerated and stagnant) on the uptake, translocation and subcellular distribution of Cd in relation to the morphology and anatomy of roots in two rice genotypes with different Cd accumulations in grains. Marked differences in morphology and anatomy were observed between these two genotypes under different cultivation conditions. Genotypes with low Cd accumulation in grains tended to develop fewer root tips per root surface area, larger root porosity and more mature apoplastic barriers. The stagnant cultivation condition decreased the number of root tips per root surface area but increased root porosity and accelerated apoplastic barrier formation in root tissues. Correlative Cd uptake studies revealed that rice plants with fewer number of root tips per root surface area reduced root Cd uptake ability, while mature apoplastic barriers increased root Cd retention in cell walls and the symplast. Thus, the fewer number of root tips per root surface area and the earlier formation of mature apoplastic barriers led to lower Cd uptake and translocation. The results indicated that the morphology and anatomy of roots could play important roles in Cd uptake and translocation in rice, and could be influenced by both genotype and cultivation conditions. The present results would be useful in screening and planting rice plants with low Cd accumulation.

Formation of a subero-lignified apical deposit in root tip of radish (Raphanus sativus) as a response to copper stress

BACKGROUND AND AIMS

Heavy metals induce changes in root metabolism and physiology, which can lead to a complex remodelling of the root system. The final morphological responses of radish (Raphanus sativus) roots exposed to toxic concentrations of the heavy metal (Cu) include root growth inhibition, differentiation of xylem vessels close to the root tip, enhanced suberin lamellae deposition and enhanced lateral root production. Recently, we have found that such changes in root morphology and anatomy are coupled to the formation of a subero-lignified apical deposit (SLAD) very close to the root tip.

METHODS

To clarify the details of the formation of a SLAD in the root tip, we conducted experiments with radish roots exposed to a high Cu concentration (60 µm). Histochemical analysis of lignin and suberin as well as analysis of spatial-temporal characteristics of SLAD formation were performed by bright-field, fluorescence and confocal microscopy.

KEY RESULTS

This unique structure, not longer than 100 µm, consists of modified cell walls of the central cylinder that are encircled by a short cylinder of prematurely suberized endodermal cells. A SLAD starts to form, in both primary and lateral roots, after cessation of root elongation, and it is coupled with xylem differentiation and root branching close to the root apex. We noticed that deposition of phenolic substances into a SLAD, mainly suberin in the endodermis, is spatially separated from suberization or lignification in basally located endodermis.

CONCLUSIONS

Although the main reason for formation of a SLAD is elusive, we suggest that it is a part of stress-induced responses which relate to decreased root growth or permeability in heavy metal stress.

Nitro-Oxidative Stress Correlates with Se Tolerance of Astragalus Species

At high concentrations, selenium (Se) exerts phytotoxic effects in non-tolerant plant species partly due to the induction of nitro-oxidative stress; however, these processes are not fully understood. In order to obtain a more accurate view of the involvement of nitro-oxidative processes in plant Se sensitivity, this study aims to characterize and compare Se-triggered changes in reactive oxygen (ROS) and nitrogen species (RNS) metabolism and the consequent protein tyrosine nitration as a marker of nitrosative stress in the non-accumulator Astragalus membranaceus and the Se hyperaccumulator Astragalus bisulcatus. The observed parameters (Se accumulation, microelement homeostasis, tissue-level changes in the roots, germination, biomass production, root growth and cell viability) supported that A. membranaceus is Se sensitive while the hyperaccumulator A. bisulcatus tolerates high Se doses. We first revealed that in A. membranaceus, Se sensitivity coincides with the Se-induced disturbance of superoxide metabolism, leading to its accumulation. Furthermore, Se increased the production or disturbed the metabolism of RNS (nitric oxide, peroxynitrite and S-nitrosoglutathione), consequently resulting in intensified protein tyrosine nitration in sensitive A. membranaceus. In the (hyper)tolerant and hyperaccumulator A. bisulcatus, Se-induced ROS/RNS accumulation and tyrosine nitration proved to be negligible, suggesting that this species is able to prevent Se-induced nitro-oxidative stress.

Exogenous Glycinebetaine Promotes Soil Cadmium Uptake by Edible Amaranth Grown during Subtropical Hot Season

Exogenous glycinebetaine treatment is an effective measure for preventing crops from being exposed to drought and high temperature; however, the effects of this approach on the soil Cd uptake and accumulation by crops remain unclear. Pot experiments were conducted in this study to analyze the effect of glycinebetaine on the soil Cd uptake and accumulation by edible amaranth cultivated in Cd-contaminated soil. Results revealed that after exogenous glycinebetaine treatment on amaranth leaves during the vigorous growth period, the plant biomass, the Cd concentrations in the roots and shoots, and the Cd translocation factor (TF) were significantly higher than those of the control group. The highest Cd concentrations in the roots and shoots and the TF were higher by 91%, 96% and 23.8%, respectively, than the corresponding values in the control group. In addition, exogenous glycinebetaine treatment significantly increased leaf chlorophyll content and promoted the photosynthesis of edible amaranth. Consequently, the contents of soluble sugar, dissolved organic carbon, and low-molecular-weight organic acids significantly increased in the rhizosphere, resulting in Cd mobilization. Significant positive correlations were observed among the contents of leaf chlorophyll, Mg, Fe, pectin and Ca. Given that Cd shares absorption and translocation channels with these elements, we speculated that the increased leaf chlorophyll and pectin contents promoted the absorption and accumulation of Mg, Fe and Ca, which further promoted the absorption and translocation of Cd. These results indicated that exogenous glycinebetaine treatment during hot season would aggravate the health risks of crops grown in Cd-contaminated soils.

Intraspecific variability of ciprofloxacin accumulation, tolerance, and metabolism in Chinese flowering cabbage (Brassica parachinensis)

To investigate the mechanism of genotype differences in ciprofloxacin (CIP) accumulation, this study was designed to compare the tolerance and metabolic responses to CIP exposure between low (Cutai) and high (Sijiu) CIP-accumulation cultivars of Brassica parachinensis. Decreases in biomass and chlorophyll content were significantly greater (p < 0.05) and toxicities were more severe within cell ultrastructures of Cutai compared to Sijiu. A sequential growth test also revealed that Sijiu was more tolerant to CIP stress compared to Cutai. Meanwhile, significantly higher (p < 0.05) root parameters and higher areas of the stele and xylem may be responsible for the increased uptake and transport of CIP in Sijiu. Ultra performance liquid chromatography-electrospray ionization tandem mass spectrometry (UPLC-ESI-MS/MS) analysis revealed that CIP was metabolized to three major metabolites by the hydroxylation and breakdown of the piperazinyl substituent in the CIP molecule. The enhanced metabolic transformation of CIP in Sijiu indicated a more efficient capacity to detoxify, which in turn favored an increased accumulation of CIP in this cultivar. Thus, the present study demonstrated that the stronger tolerance and metabolism of Sijiu to CIP were responsible for its high CIP accumulation, suggesting an evolutionary mechanism for adaptation to environmental stress.

Aquatic plants for phytostabilization of cadmium and zinc in hydroponic experiments

Cadmium (Cd) may be toxic to aquatic plants even at modest concentrations, and excessive quantities of zinc (Zn) decrease plant performance. The Cd and Zn phytoremediation potential of several aquatic plant species (Thalia geniculate, Cyperus alternifolius, Canna indica, Eichhornia crassipes, Pistia stratiotes) and one grass species (Vetiveria zizanioides) was evaluated in hydroponic experiments. Vetiveria zizanioides, E. crassipes, and P. stratiotes experienced reduced growth performance in the presence of Cd as determined from biomass production, survival rate, and crown root number (CN); however, they accumulated high quantities of metals in their tissues, particularly in roots. Root accumulation is considered a key characteristic of so-called excluder species. In this study, only E. crassipes and P. stratiotes had bioconcentration factors and translocation factors (> 1000 and < 1, respectively) suitable for high phytostabilization of Cd. Furthermore, V. zizanioides and P. stratiotes showed the highest percent metal uptake from solution and removal capacity for Zn (~70% and ~2 mg d^-1 g^-1, respectively). Emergent aquatic species (particularly C. alternifolius and T. geniculate) adapted and lived well in Cd- and Zn-contaminated solution and took up high quantities of Cd and Zn in roots, and are therefore considered strong excluders. Beneficial uses of such species in contaminated wetlands include stabilizing toxic metals and limiting erosion. Plant tissue can be applied to other uses, including as a biomass fuel. In field situations, the candidate species may work best when grown together, since each plant genotype possesses a different potential to control Cd and Zn.

Efficiency of the formulated plant-growth promoting Pseudomonas fluorescens MC46 inoculant on triclocarban treatment in soil and its effect on Vigna radiata growth and soil enzyme activities

For bioaugmentation-based treatment of triclocarban (TCC), an emerging soil pollutant that is recalcitrant to biodegradation and phytotransformation, efficient TCC-degrading bacteria with an effective soil-delivering means are required. This work developed the formulated bacterial inoculant, and successfully demonstrated its TCC removal and detoxification performance in pot soil experiment with Vigna radiata plants. The soil bacterium Pseudomonas fluorescens MC46 was isolated as TCC-degrading, plant-growth promoting bacterium. The characterizations were conducted in vitro revealing that it could utilize TCC as a sole carbon source, and at a wide and higher concentration range from 1.6-31.6mgkg^-1 than those previously reported, while the detoxification was assessed by cytogenotoxicity and phytotoxicity tests. The developed sawdust-based inoculant formula combined with molasses (5% w/w), and either PEG or CMC-starch blend (1% w/w) could maintain a 20-week shelf-life inoculant stability in terms of cell viability, and TCC-degrading activity. Bioaugmentation of the formulated inoculants into TCC-contaminated soil efficiently removed TCC up to 74-76% of the initial concentration, mitigated toxicity, restored plant growth and health, and enhanced soil enzyme activities. This work is the first to demonstrate potential application of the formulated plant-growth promoting bacterial inoculant for the treatment and detoxification of a persistent TCC contaminated in soil.

A protocol for combining fluorescent proteins with histological stains for diverse cell wall components

Higher plant function is contingent upon the complex three-dimensional (3D) architecture of plant tissues, yet severe light scattering renders deep, 3D tissue imaging very problematic. Although efforts to 'clear' tissues have been ongoing for over a century, many innovations have been made in recent years. Among them, a protocol called ClearSee efficiently clears tissues and diminishes chlorophyll autofluorescence while maintaining fluorescent proteins - thereby allowing analysis of gene expression and protein localisation in cleared samples. To further increase the usefulness of this protocol, we have developed a ClearSee-based toolbox in which a number of classical histological stains for lignin, suberin and other cell wall components can be used in conjunction with fluorescent reporter lines. We found that a number of classical dyes are highly soluble in ClearSee solution, allowing the old staining protocols to be enormously simplified; these additionally have been unsuitable for co-visualisation with fluorescent markers due to harsh fixation and clearing. Consecutive staining with several dyes allows 3D co-visualisation of distinct cell wall modifications with fluorescent proteins - used as transcriptional reporters or protein localisation tools - deep within tissues. Moreover, the protocol is easily applied on hand sections of different organs. In combination with confocal microscopy, this improves image quality while decreasing the time and cost of embedding/sectioning. It thus provides a low-cost, efficient method for studying thick plant tissues which are usually cumbersome to visualise. Our ClearSee-adapted protocols significantly improve and speed up anatomical and developmental investigations in numerous plant species, and we hope they will contribute to new discoveries in many areas of plant research.

Heavy metal accumulation in leaves and beans of cacao (Theobroma cacao L.) in major cacao growing regions in Peru

Peru is one of the leading exporters of organic cacao beans in the world. However, the accumulation of heavy metals in cacao beans represents a problem for cocoa bean export and chocolate quality. The aim of this study was to investigate the distribution and accumulation of heavy metals in cacao leaves and cocoa beans in three major cacao growing regions of Peru. The study was conducted in cacao plantations of 10 to 15years old in three regions of Peru: North (Regions of Tumbes, Piura, Cajamarca, and Amazonas); Center (Regions of Huánuco and San Martin) and South (Junin and Cuzco). Samples of leaf and cacao beans were collected from 70 cacao plantations, and the nature of cacao clone or genotype sampled was recorded. The concentrations of heavy metals such as Cd, Cr, Cu, Fe, Mn, Ni, Pb and Zn in leaves and beans were determined using atomic absorption spectrophotometer. Overall, concentrations of heavy metals were below the critical limits; however, the presence of high levels of Cd in cacao grown in Amazonas, Piura, and Tumbes regions is of primary concern. Plantations of cacao with different cacao clones show differences in Cd accumulation both in leaves and cocoa beans. Therefore, it is promising to screen low Cd accumulator cacao genotypes for safe production of cacao on lightly to moderately Cd contaminated soils. Also, synergism between Zn and Cd present both in plant and soil suggests that Zn has a direct effect on Cd accumulation in cacao.

Uptake, Accumulation, and in Planta Distribution of Coexisting Cerium Oxide Nanoparticles and Cadmium in Glycine max (L.) Merr.

Agricultural soils are likely to be polluted by both conventional and emerging contaminants at the same time. Understanding the interactions of coexisting engineered nanoparticles (ENPs) and trace elements (a common source of abiotic stress) is critical to gaining insights into the accumulation of these two groups of chemicals by plants. The objectives of this study were to determine the uptake and accumulation of coexisting ENPs and trace elements by soybeans and to gain insights into the physiological mechanisms resulting in different plant accumulation of these materials. The combinations of three cadmium levels (0 [control] and 0.25 and 1 milligrams per kilogram of dry soil) and two CeO2 NPs concentrations (0 [control] and 500 milligrams per kilogram of dry soil) were investigated. Measurements of the plant biomass and physiological parameters indicated that CeO2 NPs led to higher variable fluorescence to maximum fluorescence ratio, suggesting that CeO2 NPs enhanced the plant light energy use efficiency by photosystem II. In addition, the presence of CeO2 NPs did not affect Cd accumulation in soybean, but Cd significantly increased the accumulation of Ce in plant tissues, especially in roots and older leaves. The altered Ce in planta distribution was partially associated with the formation of root apoplastic barriers in the co-presence of Cd and CeO2 NPs.