Antioxidative enzymes activity and thiol metabolism in three leafy vegetables under Cd stress

Differences in the accumulation of pentachloronitrobenzene and cadmium in vegetables grown in contaminated soils

The capability of different vegetable species to accumulate Pentachloronitrobenzene (PCNB) and cadmium (Cd) in soils varies significantly. Investigating these characteristics can guide the rational use of farmland contaminated with PCNB and Cd. The growth of five common vegetables (three vegetable species and three varieties of one species) in PCNB and Cd co-contaminated soils in Southwest China was investigated through a 100-day simulated contamination pot experiment. Interspecific and intervariety differences in the uptake and accumulation of PCNB and Cd were also examined. These vegetables included leafy types such as Lactuca sativa (CL), Lactuca sativa var. longifolia (RL), and Brassica rapa subsp. chinensis (BC), and root types such as Red Raphanus sativus (RR) and Lactuca sativa var. angustata (AL). Results showed that light to medium PCNB contamination (0.44-6.74 mg kg-1) promoted the growth of leafy vegetables, while severe contamination (9.88-9.96 mg kg-1) inhibited their growth. Root vegetables were inhibited by PCNB. Soil Cd contamination reduced the biomass of all five vegetables. In co-contamination soil (PCNB: 0.47-9.88 mg kg-1; Cd: 0.46-1.63 mg kg-1), vegetable growth was affected by the interaction between PCNB and Cd. In severely PCNB-contaminated soil, PCNB contents of CL, RL, BC, and AL leaves exceeded food safety limits, while those in RR and AL stems did not. The five vegetables showed varying Cd contamination, with AL leaves being the most contaminated, exceeding the standard by 60 times. PCNB accumulation followed the order: AL leaves > BC > AL stems > RL > CL > RR. Cd accumulation was highest in AL leaves, followed by stems, RR, BC, CL, and lowest in RL, with significant differences (P < 0.05). Co-contaminated soil did not promote PCNB and Cd uptake in vegetables. CL and RL, with low PCNB and Cd accumulation capacities, could be considered low-accumulation varieties for lightly contaminated soils.

Impact of Cd and Pb on the photosynthetic and antioxidant systems of Hemerocallis citrina Baroni as revealed by physiological and transcriptomic analyses

KEY MESSAGE

Cd induces photosynthetic inhibition and oxidative stress damage in H. citrina, which mobilizes the antioxidant system and regulates the expression of corresponding genes to adapt to Cd and Pb stress. Cd and Pb are heavy metals that cause severe pollution and are highly hazardous to organisms. Physiological measurements and transcriptomic analysis were combined to investigate the effect of 5 mM Cd or Pb on Hemerocallis citrina Baroni. Cd significantly inhibited H. citrina growth, while Pb had a minimal impact. Both Cd and Pb suppressed the expression levels of key chlorophyll synthesis genes, resulting in decreased chlorophyll content. At the same time, Cd accelerated chlorophyll degradation. It reduced the maximum photochemical efficiency of photosystem (PS) II, damaging the oxygen-evolving complex and leading to thylakoid dissociation. In contrast, no such phenomena were observed under Pb stress. Cd also inhibited the Calvin cycle by down-regulating the expression of Rubisco and SBPase genes, ultimately disrupting the photosynthetic process. Cd impacted the light reaction processes by damaging the antenna proteins, PS II and PS I activities, and electron transfer rate, while the impact of Pb was weaker. Cd significantly increased reactive oxygen species and malondialdehyde accumulation, and inhibited the activities of antioxidant enzymes and the expression levels of the corresponding genes. However, H. citrina adapted to Pb stress by the recruitment of antioxidant enzymes and the up-regulation of their corresponding genes. In summary, Cd and Pb inhibited chlorophyll synthesis and hindered the light capture and electron transfer processes, with Cd exerting great toxicity than Pb. These results elucidate the physiological and molecular mechanisms by which H. citrina responds to Cd and Pb stress and provide a solid basis for the potential utilization of H. citrina in the greening of heavy metal-polluted lands.

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

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

Cadmium Stress Signaling Pathways in Plants: Molecular Responses and Mechanisms

Heavy metal (HM) pollution, specifically cadmium (Cd) contamination, is a worldwide concern for its consequences for plant health and ecosystem stability. This review sheds light on the intricate mechanisms underlying Cd toxicity in plants and the various strategies employed by these organisms to mitigate its adverse effects. From molecular responses to physiological adaptations, plants have evolved sophisticated defense mechanisms to counteract Cd stress. We highlighted the role of phytochelatins (PCn) in plant detoxification, which chelate and sequester Cd ions to prevent their accumulation and minimize toxicity. Additionally, we explored the involvement of glutathione (GSH) in mitigating oxidative damage caused by Cd exposure and discussed the regulatory mechanisms governing GSH biosynthesis. We highlighted the role of transporter proteins, such as ATP-binding cassette transporters (ABCs) and heavy metal ATPases (HMAs), in mediating the uptake, sequestration, and detoxification of Cd in plants. Overall, this work offered valuable insights into the physiological, molecular, and biochemical mechanisms underlying plant responses to Cd stress, providing a basis for strategies to alleviate the unfavorable effects of HM pollution on plant health and ecosystem resilience.

Rhodopseudomonas palustris shapes bacterial community, reduces Cd bioavailability in Cd contaminated flooding paddy soil, and improves rice performance

Photosynthetic bacteria (PSB) are suitable to live and remediate cadmium (Cd) in the slightly oxygenated or anaerobic flooding paddy field. However, there is currently limited study on the inhibition of Cd accumulation in rice by PSB, and the relevant mechanisms has yet to be elucidated. In the current study, we firstly used Rhodopseudomonas palustris SC06 (a typical PSB) as research target and combined physiology, biochemistry, microbiome and metabolome to evaluate the mechanisms of remeding Cd pollution in paddy field and inhibiting Cd accumulation in rice. Microbiome analysis results revealed that intensive inoculation with R. palustris SC06 successfully survived and multiplied in flooding paddy soil, and significantly increased the relatively abundance of anaerobic bacteria including Desulfobacterota, Anaerolineaceae, Geobacteraceae, and Gemmatimonadaceae by 46.40 %, 45.00 %, 50.12 %, and 21.30 %, respectively. Simultaneously, the structure of microbial community was regulated to maintain relative stability in the rhizosphere soil of rice under Cd stress. In turn, these bacteria communities reduced bioavailable Cd and enhanced residual Cd in soil, and induced the upregulation of sugar and organic acids in the rice roots, which further inhibited Cd uptake in rice seedlings, and dramatically improved the photosynthetic efficiency in the leaves and the activities of antioxidative enzymes in the roots. Finally, Cd content of the roots, stems, leaves, and grains significantly decreased by 38.14 %, 69.10 %, 83.40 %, and 37.24 % comparing with the control, respectively. This study provides a new strategy for the remediation of Cd-contaminated flooding paddy fields and the safe production of rice.

Mechanisms of low cadmium accumulation in crops: A comprehensive overview from rhizosphere soil to edible parts

Cadmium (Cd) is a toxic heavy metal often found in soil and agricultural products. Due to its high mobility, Cd poses a significant health risk when absorbed by crops, a crucial component of the human diet. This absorption primarily occurs through roots and leaves, leading to Cd accumulation in edible parts of the plant. Our research aimed to understand the mechanisms behind the reduced Cd accumulation in certain crop cultivars through an extensive review of the literature. Crops employ various strategies to limit Cd influx from the soil, including rhizosphere microbial fixation and altering root cell metabolism. Additional mechanisms include membrane efflux, specific transport, chelation, and detoxification, facilitated by metalloproteins such as the natural resistance-associated macrophage protein (Nramp) family, heavy metal P-type ATPases (HMA), zinc-iron permease (ZIP), and ATP-binding cassette (ABC) transporters. This paper synthesizes differences in Cd accumulation among plant varieties, presents methods for identifying cultivars with low Cd accumulation, and explores the unique molecular biology of Cd accumulation. Overall, this review provides a comprehensive resource for managing agricultural lands with lower contamination levels and supports the development of crops engineered to accumulate minimal amounts of Cd.

Phytochelatin-Mediated Cultivar-Dependent Cd Accumulations of Lactuca sativa and Implication for Cd Pollution-Safe Cultivars Screening

Cd pollution-safe cultivar (Cd-PSC) is a feasible strategy to minimize Cd contamination in leafy vegetables. The shoot Cd concentrations of 23 Lactuca sativa cultivars under Cd stress ranged from 0.124 to 2.155 mg·kg-1 with a maximum cultivar difference of 8 folds. Typical Cd-PSC C16 (L) and high-Cd-accumulating cultivar C13 (H) were screened to investigate the mechanisms of Cd accumulations in L. sativa through determining Cd concentrations, Cd subcellular distributions, phytochelatin profiles, and phytochelatin biosynthesis-related genes' expressions. Higher Cd distribution in a heat stable fraction in C13 (H) indicated that the high Cd accumulation trait of C13 (H) mainly depended on the Cd-phytochelatin complexes. Root phytochelatin concentrations were significantly elevated in C13 (H) (5.83 folds) than in C16 (L) (2.69 folds) (p < 0.05) under Cd stress. Significantly downregulated expressions of glutathione S-transferase rather than the regulation of phytochelatin synthesis genes in the root of C13 (H) might be responsible for sufficient glutathione supply for phytochelatins synthesis. These findings suggested that phytochelatin elevation in C13 (H) would favor the Cd root to shoot transportation, which provides new insights into the phytochelatin-related cultivar-dependent Cd accumulating characteristic in L. sativa.

Biochar derived from tobacco waste significantly reduces the accumulations of cadmium and copper in edible parts of two vegetables: an in-situ field study

Biochar, as a soil amendment, can be applied to remediate heavy metal (HM) contaminated farmland. However, there is little research on the effect of tobacco biochar (TB) derived from tobacco waste on HM controlling in edible parts of vegetables. In this study, the impact of two TB levels on the plant growth, copper (Cu) and cadmium (Cd) accumulation in the edible parts of lettuce and chrysanthemum, and on Cu and Cd bioavailability of rhizosphere soil was investigated through in-situ field experiments. The results showed that TB has rich oxygen containing functional groups, high porosity, high nitrogen adsorption capacity. The addition of 5 t ha-1 and 10 t ha-1 TB significantly increased the shoot biomass of chrysanthemum, but had no effect on the growth of lettuce. Two levels of TB significantly increased the pH value, but decreased the available Cu and Cd concentrations of rhizosphere soil, thereby reducing the Cu and Cd accumulations in the edible parts of lettuce and chrysanthemum. The findings provided effective evidences that TB derived from tobacco waste is an efficient strategy for controlling Cu and Cd accumulation in the edible parts of vegetables to ensure agri-product safety production in HM-polluted farmland.

Assessment of Shoot Priming Efficiency to Counteract Complex Metal Stress in Halotolerant Lobularia maritima

The study investigated whether short-term priming supports plant defense against complex metal stress and multiple stress (metals and salinity) in halophyte Lobularia maritima (L.) Desv. Plants were pre-treated with ectoine (Ect), nitric oxide donor-sodium nitroprusside (SNP), or hydrogen sulfide donor-GYY4137 for 7 days, and were transferred onto medium containing a mixture of metal ions: Zn, Pb, and Cd. To test the effect of priming agents in multiple stress conditions, shoots were also subjected to low salinity (20 mM NaCl), applied alone, or combined with metals. Hydropriming was a control priming treatment. Stress impact was evaluated on a basis of growth parameters, whereas defense responses were on a basis of the detoxification activity of glutathione S-transferase (GST), radical scavenging activity, and accumulation of thiols and phenolic compounds. Exposure to metals reduced shoot biomass and height but had no impact on the formation of new shoots. Priming with nitric oxide annihilated the toxic effects of metals. It was related to a sharp increase in GST activity, glutathione accumulation, and boosted radical scavenging activity. In NO-treated shoots level of total phenolic compounds (TPC) and flavonoids remained unaffected, in contrast to other metal-treated shoots. Under combined metal stress and salinity, NO and H₂S were capable of restoring or improving growth parameters, as they stimulated radical scavenging activity. Ect and H₂S did not exert any effect on metal-treated shoots in comparison to hydropriming. The results revealed the stimulatory role of nitric oxide and low doses of NaCl in combating the toxic effects of complex metal stress in L. maritima. Both NO and NaCl interfered with thiol metabolism and antioxidant activity, whereas NaCl also contributed to the accumulation of phenolic compounds.

Assessment of health risk, genotoxicity, and thiol compounds in Trigonella foenum-graecum (Fenugreek) under arsenic stress

Arsenic (As) traces have been reported worldwide in vegetables and crops cultivated in As-polluted soils. Being carcinogenic, the presence of As in edibles is of great concern as it ultimately reaches humans and animals through the food chain. Besides, As toxicity adversely affects the growth, physiology, metabolism, and productivity of crops. In the present study, Trigonella foenum-graecum (Fenugreek) was exposed to the As stress (0, 50, 100, and 150 μM sodium arsenate) for a week. Further, evaluation of As accumulation in roots and shoots, magnitude and visualization of oxyradicals, and thiol-based defence offered by Fenugreek was assessed. The root and leaf accumulated 258-453 μg g^-1 dry wt (DW) and 81.4-102.1 μg g^-1 DW of As, respectively. An arsenic-mediated decline in the growth index and increase in oxidative stress was noted. Arsenic stress modulated the content of thiol compounds; especially cysteine content increased from 0.36 to 0.43 µmole g^-1 FW protein was noted. Random Amplified Polymorphic DNA (RAPD)-based analysis showed DNA damage in As-treated plants. Health risk assessment parameters showed that As concentration in the consumable plant shoot was below the critical hazard level (hazard quotient < 1). Moreover, T. foenum-graecum showed varied responses to As-induced oxidative stress with applied concentrations (150 μM being more toxic than lower concentrations). In addition, the RAPD profile and level of thiol compounds were proved significant biomarkers to assess the As toxicity in plants. The conclusion of this study will help users of fenugreek to have a clue and create awareness regarding the consumption.

State-of-the-art OMICS strategies against toxic effects of heavy metals in plants: A review

Environmental pollution of heavy metals (HMs), mainly due to anthropogenic activities, has received growing attention in recent decades. HMs, especially the non-essential carcinogenic ones, including chromium (Cr), cadmium (Cd), mercury (Hg), aluminum (Al), lead (Pb), and arsenic (As), have appeared as the most significant air, water, and soil pollutants, which adversely affect the quantity, quality, and security of plant-based food all over the world. Plants exposed to HMs could experience significant decline in growth and yield. To avoid or tolerate the toxic effects of HMs, plants have developed complicated defense mechanisms, including absorption and accumulation of HMs in cell organelles, immobilization by forming complexes with organic chelates, extraction by using numerous transporters, ion channels, signalling cascades, and transcription elements, among others. OMICS strategies have developed significantly to understand the mechanisms of plant transcriptomics, genomics, proteomics, metabolomics, and ionomics to counter HM-mediated stress stimuli. These strategies have been considered to be reliable and feasible for investigating the roles of genomics (genomes), transcriptomic (coding), mRNA transcripts (non-coding), metabolomics (metabolites), and ionomics (metal ions) to enhance stress resistance or tolerance in plants. The recent developments in the mechanistic understandings of the HMs-plant interaction in terms of their absorption, translocation, and toxicity invasions at the molecular and cellular levels, as well as plants' response and adaptation strategies against these stressors, are summarized in the present review. Transcriptomics, genomics, metabolomics, proteomics, and ionomics for plants against HMs toxicities are reviewed, while challenges and future recommendations are also discussed.

Knowledge Mapping of the Phytoremediation of Cadmium-Contaminated Soil: A Bibliometric Analysis from 1994 to 2021

Cadmium pollution of soil threatens the environmental quality and human health. Phytoremediation of cadmium-contaminated soil has attracted global attention in recent decades. This study aimed to conduct a comprehensive and systematic review of the literature on phytoremediation of cadmium-contaminated soil based on bibliometric analysis. A total of 5494 articles published between 1994 and 2021 were retrieved from the Web of Science Core Collection. Our knowledge mapping presented the authors, journals, countries, institutions, and other basic information to understand the development status of phytoremediation of cadmium-contaminated soil. Based on a keyword cluster analysis, the identified major research domains were "biochar", "Thlaspi caerulescens", "endophytic bacteria", "oxidative stress", "EDTA", and "bioconcentration factor". Overall, this study provided a detailed summary of research trends and hotspots. Based on the keyword co-occurrence and burst analysis, the core concepts and basic theories of this field were completed in 2011. However, the pace of theoretical development has been relatively slow. Finally, future research trends/frontiers were proposed, such as biochar addition, rhizosphere bacterial community manipulation, cadmium subcellular distribution, and health risk assessment.

Effect of strontium on nutrient uptake, physiological parameters, and strontium localization in lettuce

Human activities increase the risk of stable and radioactive strontium (Sr) isotopes entering the environment and food chain. In this study, the effects of Sr on the nutrient uptake and physiological responses of lettuce under different "Sr treatment" concentrations (0, control, 1, 2, 3, 4, and 5 mM) and "times" (7, 14, and 21 day) were studied in a hydroponic system. In addition, the distribution of Sr on the surfaces and cross-sections of lettuce leaves was revealed by scanning electron microscopy with energy-dispersive X-ray (SEM-EDX) analysis. A two-way analysis of variance (ANOVA) method was used to analyze the significance of "Sr treatment," "time," and their "interaction." The results showed that an increase in Sr uptake in lettuce could significantly reduce the uptake of calcium (Ca). The contents of sulfur (S), potassium (K), and iron (Fe) in lettuce leaves showed significant differences with the sampling day. Similarly, the fresh weight of lettuce leaves and roots as well as the photosynthetic pigment contents of lettuce leaves was also significantly different with the sampling day. The activities of antioxidant enzymes (superoxide dismutase (SOD), catalase (CAT), and peroxidase (POD)) showed significant differences with the sampling day. The activities of SOD and CAT decreased significantly with the sampling day, while POD increased significantly. The MDA content increased significantly with increasing hydroponic Sr concentration on the 21st day. SEM-EDX analysis showed that the weight percentage of Sr in the vascular bundle sheath in the cross-section of lettuce leaves was relatively higher than that in the mesophyll. This study aids our understanding of the distribution of Sr in lettuce leaf tissues and the effect of Sr on lettuce physiology.

Arsenic Accumulation and Physiological Response of Three Leafy Vegetable Varieties to As Stress

Arsenic (As) in leafy vegetables may harm humans. Herein, we assessed As accumulation in leafy vegetables and the associated physiological resistance mechanisms using soil pot and hydroponic experiments. Garland chrysanthemum (Chrysanthemum coronarium L.), spinach (Spinacia oleracea L.), and lettuce (Lactuca sativa L.) were tested, and the soil As safety threshold values of the tested leafy vegetables were 91.7, 76.2, and 80.7 mg kg−1, respectively, i.e., higher than the soil environmental quality standard of China. According to growth indicators and oxidative stress markers (malondialdehyde, the ratio of reduced glutathione to oxidized glutathione, and soluble protein), the order of As tolerance was: GC > SP > LE. The high tolerance of GC was due to the low transport factor of As from the roots to the shoots; the high activity of superoxide dismutase, glutathione peroxidase, and catalase; and the high content of phytochelatin in the roots. Results of this work shed light on the use of As-contaminated soils and plant tolerance of As stress.

Accumulation Mechanism and Risk Assessment of Artemisia selengensis Seedling In Vitro with the Hydroponic Culture under Cadmium Pressure

Artemisia selengensis is a perennial herb of the Compositae with therapeutic and economic value in China. The cadmium (Cd) accumulation mechanism and healthy risk evaluation of A. selengensis were investigated in this study. Tissue culture seedlings were obtained by plant tissue culture in vitro, and the effect of Cd stress (Cd concentration of 0.5, 1, 5, 10, 25, 50 and 100 μM) on A. selengensis was studied under hydroponic conditions. The results showed that low-Cd (0.5–1 μM) stress caused a rare effect on the growth of A. selengensis seedlings, which regularly grew below the 10 μM Cd treatment concentration. The biomass growth rate of the 0.5, 1, and 5 μM treatment groups reached 105.8%, 96.6%, and 84.8% after 40 days of cultivation, respectively. In addition, when the concentration of Cd was greater than 10 μM, the plant growth was obviously inhibited, i.e., chlorosis of leaves, blackening roots, destroyed cell ultrastructure, and increased malondialdehyde (MDA) content. The root could be the main location of metal uptake, 57.8–70.8% of the Cd was concentrated in the root after 40 days of cultivation. Furthermore, the root cell wall was involved in the fixation of 49–71% Cd by subcellular extraction, and the involvement of the participating functional groups of the cell wall, such as -COOH, -OH, and -NH₂, in metal uptake was assessed by FTIR analysis. Target hazard quotient (THQ) was used to assess the health risk of A. selengensis, and it was found that the edible part had no health risk only under low-Cd stress (0.5 to 1 μM) and short-term treatment (less than 20 days).

Plasma membrane-localized protein BcHIPP16 promotes the uptake of copper and cadmium in planta

Cadmium (Cd) is one of the toxic heavy metals in soil, which not only suppresses crop production but also threatens human health. In this study, we aim to clarify the biological function of Cd-related gene BcHIPP16, so as to provide potential genetic solutions to decrease the Cd levels of pak choi. Tissue expression analysis showed that BcHIPP16 expressed in almost all the plant bodies. The transcriptional level of BcHIPP16 in roots was higher than that in shoots, which was significantly induced by copper (Cu) deficiency and Cd exposure conditions. Subcellular localization revealed that BcHIPP16 localized in plasma membrane. Expressing BcHIPP16 in yeast cells improved the sensitivity to Cu and Cd and improved their accumulation in yeast. Furthermore, the Cu and Cd content of Arabidopsis seedlings were increased and complemented, respectively when expressing BcHIPP16 in wild type (WT) and hip16 mutants. Non-invasive Micro-test Technology (NMT) was used to measure the real-time Cd²⁺ influx from the root surface of BcHIPP16 transgenic Arabidopsis lines, and the result demonstrated that BcHIPP16 promoted Cd²⁺ influx into Arabidopsis root cells. Taken together, our study showed that BcHIPP16 contributed to absorbing nutrient metal Cu and heavy metal Cd in planta.

Photosynthetic Parameters and Growth of Rice, Lettuce, Sunflower and Tomato in an Entisol as Affected by Soil Acidity and Bioaccumulation of Ba, Cd, Cu, Ni, and Zn

The bioaccumulation of trace elements (TEs) in crops consumed by humans can reduce food production as a consequence of photosynthetic damage in plants and cause several diseases in humans. Liming is a soil management strategy designed to alleviate soil acidity and mitigating these problems by reducing the TE bioavailability. In this study, we evaluated the effect of liming on photosynthesis, growth, and bioaccumulation of barium (Ba), cadmium (Cd), copper (Cu), nickel (Ni), or zinc (Zn) in lettuce (Lactuca sativa L.), rice (Oryza sativa L.), sunflower (Helianthus annuus L.), and tomato (Solanum lycopersicum L.) grown in a sandy Entisol. The crops were grown in either uncontaminated or contaminated Entisol, at two base saturation (BS%) ratios: 30% for all crops or 50% for rice and 70% for lettuce, sunflower, and tomato. The photosynthesis-related parameters varied depending on the metal and the crop, but in general, increasing BS% did not attenuate photosynthetic damage induced by Ba, Cd, Cu, Ni, and Zn in the crops. There was no strong correlation between the photosynthetic parameters and biomass production, which indicates that the suppression of biomass induced by Ba, Cd, Cu, Ni, or Zn is related to other metabolic disorders in addition to the impairment of CO2 assimilation or chlorophyll synthesis in the crops assayed, with the exception of Ni and Zn in lettuce. In conclusion, increasing BS% was not consistent in reducing Ba, Cd, Cu, Ni, and Zn accumulation in the edible parts of lettuce, rice, sunflower, and tomato grown in the sandy soil, which is probably related to the low capacity of this soil to control TE bioavailability.

Heterologous expression of cyanobacterial PCS confers augmented arsenic and cadmium stress tolerance and higher artemisinin in Artemisia annua hairy roots

The present study provides the first report of heterologous expression of phytochelatin synthase from Anabaena PCC 7120 (anaPCS) into the hairy roots of Artemisia annua. Transformed hairy roots of A. annua expressing anaPCS gene showed better tolerance to heavy metals, viz., arsenic (As) and cadmium (Cd) owing to 143 and 191% more As- and Cd-accumulation, respectively, as compared to normal roots with a bioconcentration factor (BCF) of 9.7 and 21.1 for As and Cd, respectively. Under As and Cd stresses, transformed hairy roots possessed significantly higher amounts of phytochelatins and thiols probably due to the presence of both AaPCS (Artemisia annua PCS) and anaPCS. In addition, artemisinin synthesis was also induced in transformed hairy roots under heavy metals stresses. In-silico analysis revealed the presence of conserved motifs in both AaPCS and anaPCS sequences as well as structural modelling of PCS functional domain was conducted. Interaction of AaPCS and anaPCS proteins with CdCl₂ and sodium arsenate gene ontology analysis gave insights to anaPCS functioning in transformed hairy roots of A. annua. The study provides transformed hairy roots of A. annua as an efficient tool for effective phytoremediation with added advantages of artemisinin extraction from hairy roots used for phytoremediation.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s11816-021-00682-5.

Distribution of phytochelatins, metal-binding compounds, in plant foods: A survey of commonly consumed fruits, vegetables, grains and legumes

Phytochelatins (PyCs) are metal-binding compounds produced by plants. PyCs may reduce bioavailability of dietary toxic metals such as cadmium. However, the PyC concentrations in foods are unknown. The objective of this study was to analyze PyC contents in a subset of commonly consumed plant foods. Foods (20) across five groups were analyzed and PyCs quantified using liquid chromatography-mass spectrometry (LC-MS/MS). The impact of factors such as food processing were also explored. PyCs were in all 20 foods. Five PyC types were detected with PyC2-Gly, PyC3-Gly and PyC2-Ala at quantifiable concentrations. PyC2-Gly was found at the highest concentrations and most widely distributed. PyC2-Gly concentrations were highest in fruits and root vegetables. Foods with increased processing tended to have reduced PyC concentrations. This survey of commonly consumed plant foods in the United States demonstrates PyCs are widely distributed and provides a foundation for understanding their concentrations and impact in the human diet.

Abscisic acid modulates differential physiological and biochemical responses of roots, stems, and leaves in mung bean seedlings to cadmium stress

Experiments were conducted to determine how exogenous abscisic acid (ABA) mediates the tolerance of plants to cadmium (Cd) exposure. Cd stress strongly reduced all the growth parameters of mung bean seedlings. Cd significantly increased ascorbate peroxidase (APX) and catalase (CAT) activities in roots and stems, and peroxidase (POD) activities in roots, stems, and leaves of mung bean seedlings. Cd caused remarkable increases in the levels of leaf chlorophyll and carotenoid, root polyphenols, and malondialdehyde (MDA) and proline in the three organs. However, Cd greatly decreased leaf CAT activity, root and leaf ascorbic acid (AsA) levels, and stem and leaf polyphenol levels. Foliar application of ABA partially alleviated Cd toxicity on the seedlings. ABA could restore most of the changed biochemical parameters caused by Cd, suggesting that ABA played roles in the protection of membrane lipid peroxidation and the modulation of antioxidative defense systems in response to Cd stress. Our results also implied the differential physiological and biochemical responsive patterns of roots, stems, and leaves to Cd and ABA in mung bean seedlings. The great changes in many biochemical parameters in roots suggested that roots were the first to be affected by Cd and play pivotal roles in response to Cd, especially in chelating Cd and reducing Cd absorption.

Enhanced vacuole compartmentalization of cadmium in root cells contributes to glutathione-induced reduction of cadmium translocation from roots to shoots in pakchoi (Brassica chinensis L.)

Our previous studies showed that exogenous glutathione (GSH) decreased cadmium (Cd) concentration in shoots and alleviated the growth inhibition in pakchoi (Brassica chinensis L.) under Cd stress. Nevertheless, it is largely unknown how GSH decreases Cd accumulation in edible parts of pakchoi. This experiment mainly explored the mechanisms of GSH-induced reduction of Cd accumulation in shoot of pakchoi. The results showed that compared with sole Cd treatment, Cd + GSH treatment remarkably increased the expression of BcIRT1 and BcIRT2, and further enhanced the concentrations of Cd and Fe in root. By contrast, GSH application declined the concentration of Cd in the xylem sap. However, these results were not caused by xylem loading process because the expression of BcHMA2 and BcHMA4 had not significant difference between sole Cd treatment and Cd + GSH treatment. In addition, exogenous GSH significantly enhanced the expression of BcPCS1 and promoted the synthesis of PC₂, PC₃ and PC₄ under Cd stress. At the same time, exogenous GSH also significantly improved the expression of BcABCC1 and BcABCC2 in the roots of seedling under Cd stress, suggesting that more PCs-Cd complexes may be sequestrated into vacuoles by ABCC1 and ABCC2 transporters. The results showed that exogenous GSH could up-regulate the expression of BcIRT1/2 to increase the Cd accumulation in root, and the improvement of PCs contents and the expression of BcABCC1/2 enhanced the compartmentalization of Cd in root vacuole of pakchoi under Cd stress. To sum up, exogenous GSH reduce the concentration of free Cd²⁺ in the cytoplast of root cells and then dropped the loading of Cd into the xylem, which eventually given rise to the reduction of Cd accumulation in edible portion of pakchoi.

Physiological and genetic effects of cadmium and copper mixtures on carrot under greenhouse cultivation

The exposure to combinations of heavy metals can affect the genes of vegetables and heavy metals would accumulate in vegetables and thereby indirectly affecting human health. Exploring the links between genetic changes and phenotypic changes of carrot under the combined pollution of Cd and Cu is of great significance for studying the mechanism of heavy metal pollution. Therefore, this study examined the effects of mixtures of cadmium (Cd) and copper (Cu) on physiological measures (malondialdehyde (MDA), proline, and antioxidant enzyme) and expression of growth-related genes (gibberellin gene, carotene gene, and glycogene) in carrot under greenhouse cultivation. The results showed in the additions with mixtures of Cd and Cu at higher concentration, the MDA content increased significantly (p < 0.05), whereas the proline content was not significantly different from those in the control. In the mixed treatments with high Cd concentrations, the activity of superoxide dismutase (SOD) was significantly lower than that in the control (p < 0.05); whereas the activity of peroxidase (POD) increased to different degrees compared to the control. In the additions with mixtures of Cd and Cu, compared with the control, the expression of the gibberellin gene was downregulated from 1.97 to 20.35 times (not including the 0.2 mg kg^-1 Cd and 20 mg kg^-1 Cu mixture, the expression of gibberellin gene in this treatment was upregulated 1.29 times), which lead to decreases in the length and dry weight of carrots. The expression of the carotene gene in mixed treatments downregulated more than that in single treatments, which could reduce the ability of carrots to resist oxidative damage, as suggested by the significant increase in the MDA content. In the addition with mixtures of Cd and Cu, compared with the control, the expression of the glycogene was downregulated by 1.42-59.40 times, which can cause a significant reduction in the sugar content in carrots and possibly further reduce their ability to resist heavy metal damage. A cluster analysis showed that in the additions with mixtures of Cd and Cu, the plant phenotype was affected first, and then with increases in the added concentration, the expression of genes was also affected. In summary, in the additions with mixtures of Cd and Cu, plants were damaged as Cd and Cu concentrations increased.

Effects of electric fields on Cd accumulation and photosynthesis in Zea mays seedlings

Phytoremediation enhanced by electrokinetic has been considered as a potential technology for remediating contaminated soils. However, the effects of electric fields on Cd accumulation and photosynthesis in Zea mays (as a cathode) is still unclear. In the present study, Zea mays seedlings were exposed to various doses of Cd²⁺ (10, 50, 100 μM) to explore the impact of electric fields on Cd accumulation and photosynthesis of Zea mays. Results showed that upon exposure to a concentration of 100 μM Cd, electric fields significantly altered the Cd contents in maize shoots, whereas the concentration of 50 μM Cd increased the Cd contents in maize roots as well as affected the Cd transport from roots to shoots. Uptake index (UI) increased by 1.34%-66.16% with the application of electric fields. The variation of photosynthetic rates attributed to the open or closure of stoma was similar to the change of shoot fresh weight, particularly in maize exposed to high Cd stress. This study proposes a new technology in Cd phytoremediation and provides important information on physiological processes in maize when exposed to Cd stress and electric fields.

Effects of Zn in sludge-derived biochar on Cd immobilization and biological uptake by lettuce

Considering the high Zn content of municipal sewage sludge and its competition with Cd during plant uptake due to their similar properties, the presence of Zn in sludge-derived biochar (SDBC) may affect Cd immobilization and uptake by plants. To confirm this, SDBC samples with different Zn contents were prepared and characterized. Their Cd immobilization behavior was studied by conducting batch sorption experiments, and their effects on Cd uptake by lettuce were explored by conducting hydroponic experiments. The results reveal that some Zn contained in the sewage sludge was transformed into ZnO during pyrolysis. The Brunauer-Emmett-Teller (BET) surface area of the SDBC samples containing 2324 mg kg-1 Zn (BC-2324) was 18.3 m2 g-1, which was 132% larger than that of the samples containing 1438 mg kg-1 Zn (BC-1438). The SDBC samples containing 1901 mg kg-1 (BC-1901) exhibited the highest Langmuir sorption capacity of 3476 mg kg-1, which is 115% higher than that of SB-1438. Furthermore, the lettuce remedied with SB-1901 exhibited 44% more biomass; lower peroxidase, catalase, and malondialdehyde activity; and 18.4% less Cd in the leaves of the lettuce than the lettuce remedied with BC-1438, suggesting the potential benefits of using Zn-rich SDBC for soil amendment.

Differences in cadmium accumulation between indica and japonica rice cultivars in the reproductive stage

Excessive cadmium (Cd) in rice grains is of great concern worldwide, particularly in southern China where heavy metal pollution in the soil is widespread. Much work has been done regarding the key genes responsible for Cd absorption, transport, and accumulation in rice, but little is known about the differences of Cd accumulation between indica and japonica rice cultivars during the reproductive stage. Furthermore, physiological parameters, such as nonstructural carbohydrate content, involved in Cd accumulation have not been fully elucidated. We studied several indica and japonica cultivars under three different Cd treatment levels and harvested them at different periods after heading. Differences in Cd accumulation between subspecies mainly were generated during the reproductive stage. An increase in the Cd pollution level caused the average absorption rate of Cd in the aerial parts of the indica cultivars in the reproductive stage to be 6.17, 4.52, and 3.89 times greater than that of the japonica cultivars across the three Cd treatments. The contribution of Cd absorption by shoots to Cd accumulation at the pre- or postheading stages was 33.8% and 66.2% in indica, and 44.9% and 55.1% in japonica. We found a significant negative correlation between Cd content in the rice grains and the content of nonstructural carbohydrates in the sheath (P < 0.05). Cd translocation from sheath to grain occurred along with sugar transfer in the indica cultivars. The Cd content of the indica cultivar grain was 1.84-4.14 times higher than that of the japonica cultivars (P < 0.05). The japonica cultivars thus met the cereal Cd limits of China (0.2 mg kg^-1) under low and moderate soil Cd pollution. These findings are helpful for the selection of proper cultivars and field management practices to alleviate Cd exposure risk in rice production.

Potential Mechanisms of Abiotic Stress Tolerance in Crop Plants Induced by Thiourea

Abiotic stresses, such as temperature extremes, drought, salinity, and heavy metals are major factors limiting crop productivity and sustainability worldwide. Abiotic stresses disturb plant growth and yield formation. Several chemical compounds, known as plant growth regulators (PGRs), modulate plant responses to biotic and abiotic stresses at the cellular, tissue, and organ levels. Thiourea (TU) is an important synthetic PGR containing nitrogen (36%) and sulfur (42%) that has gained wide attention for its role in plant stress tolerance. Tolerance against abiotic stresses is a complex phenomenon involving an array of mechanisms, and TU may modulate several of these. An understanding of TU-induced tolerance mechanisms may help improve crop yield under stress conditions. However, the potential mechanisms involved in TU-induced plant stress tolerance are still elusive. In this review, we discuss the essential role of TU-induced tolerance in improving performance of plants growing under abiotic stresses and potential mechanisms underlying TU-induced stress tolerance. We also highlight exploitation of new avenues critical in TU-induced stress tolerance.