Cadmium toxicity-induced proline accumulation is coupled to iron depletion

Saltbuch extract: a bio-solutionfor cadmium stress sorghum plants in germination and maturation

Cadmium (Cd) is one of the dangerous factors that have negative impacts on plants and human health. Recently, many researchers have been looking for biostimulants to use as bioprotectants that can help or ameliorate plants' tolerance against abiotic stress, including Cd. To test the dangerousness of Cd accumulated in the soil, 200 µM of the latter was applied to sorghum seeds at germination and maturation stages. At the same time, Atriplex halimus water extract (0.1%, 0.25%, 0.5%) was applied to test its efficacy on Cd alleviation in sorghum plants. The obtained results showed that the tested concentrations enhanced the tolerance of sorghum to Cd by enhancing the germination indexes parameters such as germination percentage (GP), seedling vigor index (SVI), and reducing the mean germination time (MGT) of sorghum seeds grown under cadmium stress. On the other hand, the morphological parameters (height and weight) as well as the physiological parameters (chlorophyll and carotenoid) were stimulated in treated maturated sorghum plants under Cd stress. In addition, 0.5% and 0.25% of Atriplex halimus extract (AHE) stimulated the antioxidant enzymes, including superoxide dismutase, catalase, glutathione peroxidase, glutathione-s-transferase, and glutathione reductase. In the same time, an increase in carbon-nitrogen enzymes was recorded in the case of AHE treatment; phosphoenol pyruvate carboxylase, glutamine synthase, glutamate dehydrogenase, and amino acid transferase were all upregulated. These results suggest that using AHE as a biostimulant could be a better strategy to enhance the tolerance of sorghum plants to Cd stress.

Cadmium toxicity in medicinal plants: An overview of the tolerance strategies, biotechnological and omics approaches to alleviate metal stress

Medicinal plants, an important source of herbal medicine, are gaining more demand with the growing human needs in recent times. However, these medicinal plants have been recognized as one of the possible sources of heavy metal toxicity in humans as these medicinal plants are exposed to cadmium-rich soil and water because of extensive industrial and agricultural operations. Cadmium (Cd) is an extremely hazardous metal that has a deleterious impact on plant development and productivity. These plants uptake Cd by symplastic, apoplastic, or via specialized transporters such as HMA, MTPs, NRAMP, ZIP, and ZRT-IRT-like proteins. Cd exerts its effect by producing reactive oxygen species (ROS) and interfere with a range of metabolic and physiological pathways. Studies have shown that it has detrimental effects on various plant growth stages like germination, vegetative and reproductive stages by analyzing the anatomical, morphological and biochemical changes (changes in photosynthetic machinery and membrane permeability). Also, plants respond to Cd toxicity by using various enzymatic and non-enzymatic antioxidant systems. Furthermore, the ROS generated due to the heavy metal stress alters the genes that are actively involved in signal transduction. Thus, the biosynthetic pathway of the important secondary metabolite is altered thereby affecting the synthesis of secondary metabolites either by enhancing or suppressing the metabolite production. The present review discusses the abundance of Cd and its incorporation, accumulation and translocation by plants, phytotoxic implications, and morphological, physiological, biochemical and molecular responses of medicinal plants to Cd toxicity. It explains the Cd detoxification mechanisms exhibited by the medicinal plants and further discusses the omics and biotechnological strategies such as genetic engineering and gene editing CRISPR- Cas 9 approach to ameliorate the Cd stress.

Modulation of osmoprotection and antioxidant defense by exogenously applied acetate enhances cadmium stress tolerance in lentil seedlings

To examine the potential role of acetate in conferring cadmium (Cd) stress tolerance in lentil (Lens culinaris), several phenotypical and physio-biochemical properties have been examined in Cd-stressed lentil seedlings following acetate applications. Acetate treatment inhibited the translocation of Cd from roots to shoots, which resulted in a minimal reduction in photosynthetic pigment contents. Additionally, acetate-treated lentil showed higher shoot (1.1 and 11.72%) and root (4.98 and 30.64%) dry weights compared with acetate-non-treated plants under low-Cd and high-Cd concentrations, respectively. Concurrently, acetate treatments increase osmoprotection under low-Cd stress through proline accumulation (24.69%), as well as enhancement of antioxidant defense by increasing ascorbic acid content (239.13%) and catalase activity (148.51%) under high-Cd stress. Acetate-induced antioxidant defense resulted in a significant diminution in hydrogen peroxide, malondialdehyde and electrolyte leakage in Cd-stressed lentil seedlings. Our results indicated that acetate application mitigated oxidative stress-induced damage by modulating antioxidant defense and osmoprotection, and reducing root-to-shoot Cd transport. These findings indicate an important contribution of acetate in mitigating the Cd toxicity during growth and development of lentil seedlings, and suggest that the exogenous applications of acetate could be an economical and new avenue for controlling heavy metal-caused damage in lentil, and potentially in many other crops.

Insight into Cistus salviifolius extract for potential biostimulant effects in modulating cadmium-induced stress in sorghum plant

The main aim of the current study was to investigate the role of Cistus salviifolius leaves extract (CSE) in alleviating the toxic effect of cadmium (Cd) in sorghum (Sorghum bicolor) plants. The plants exposed to Cd (200 µM) exhibited limited growth, reduced biomass, and chlorophyll content compared to unstressed ones. Nevertheless, supplementation of CSE restored the negative effect of Cd and increased biomass and pigment content. CSE also increased the activities of antioxidant enzymes such as superoxide dismutase (SOD), isocitrate dehydrogenase (ICDH), glutathione peroxidase (GPx), glutathione reductase (GR), and Glutathione-S-Transferase (GST). Furthermore, supplementation of CSE decreased lipid peroxidation and further increased the content of soluble sugar and amino acid. We also found that CSE has a promising effect in modulating the perturbations of carbon and nitrogen metabolism in sorghum plants under Cd stress by examining several carbon-nitrogen enzyme activities: phosphoenolpyruvate carboxylase (PEPC), malate dehydrogenase (NAD-MDH), glutamine synthase (GS), glutamate dehydrogenase (GDH), and aspartate aminotransferase (AAT). Overall, our results confirm that the application of CSE can be a promising mechanism to overcome the negative effects of Cd stress in sorghum plants.

Phytochemical analysis reveals an antioxidant defense response in Lonicera japonica to cadmium-induced oxidative stress

Cadmium (Cd), though potentially beneficial at lower levels to some plant species, at higher levels is a toxic metal that is detrimental to plant growth and development. Cd is also a carcinogen to humans and other contaminated plant consumers, affecting the kidneys and reducing bone strength. In this study we investigated responses of growth, chlorophyll content, reactive oxygen species levels, and antioxidant responses to Cd in honeysuckle leaves (Lonicera japonica Thunb.), a potential Cd hyperaccumulator. Results indicated that plant height, dry weight, leaf area, and chlorophyll content increased when honeysuckle was exposed to 10 mg kg^-1 or 30 mg kg^-1 Cd (low concentration). However, in response to 150 mg kg^-1 or 200 mg kg^-1 Cd (high concentration) these growth parameters and chlorophyll content significantly decreased relative to untreated control plant groups. Higher levels of superoxide radical (O₂^·-) and hydrogen peroxide (H₂O₂) were observed in high concentration Cd groups. The activities of ascorbate peroxidase (APX), monodehydroascorbate reductase (MDHAR), dehydroascorbate reductase (DHAR), and glutathione reductase were enhanced with exposure to increasing levels of Cd. Additionally, the Ascorbate-Glutathione (AsA-GSH) cycle was activated for the removal of H₂O₂ in honeysuckle in response to elevated Cd. The Pearson correlation analysis, a redundancy analysis, and a permutation test indicated that proline and APX were dominant antioxidants for removing O₂^·- and H₂O₂. The antioxidants GSH and non-protein thiols (NPTs) also increased as the concentration of Cd increased.

Alfalfa (Medicago sativa) Sprouts Respiratory Responses to Cadmium Stress Using IR LPAS

Cadmium (Cd) is not considered a fundamental element for plants inducing general growth disturbances and inhibition in many species of plants. The purpose of our research was to examine the ethylene (C₂H₄) and ammonia (NH₃), emissions in alfalfa sprouts with or without Cd, using infrared laser photoacoustic spectroscopy (IR LPAS), in order to suggest new markers that may add a better knowledge of Cd effect. The responses of alfalfa sprouts to C₂H₄ and NH₃ may fluctuate, depending on tissue sensitivity and the phase of plant development. From the determinations of this study, the C₂H₄ was established to be inhibited, while NH₃ was established to be in a higher concentration with the amount of Cd added to the alfalfa seeds for growth when the values were correlated to the control and BIOalfalfa sprouts (Sonnentor).

Targeting Cd coping mechanisms for stress tolerance in Brassica napus under spiked-substrate system: from physiology to remediation perspective

Cadmium (Cd) is a prevalent, non-essential, carcinogenic, and hazardous heavy metal that reduces plant productivity and capacity of arable land area around the globe. In the present substrate-based pot study, seedlings of Brassica napus 180015 were grown equidistantly in the spiked-substrate medium for 60 days under increasing concentrations of Cd (0, 10, 20, 30, 40, 50 mg kg^-1). Following harvest, the morpho-physio-biochemical, antioxidative, and Cd-induced tolerance responses were evaluated in B. napus under an increasing Cd stress regime. Additionally, these parameters were also investigated to select the plant's threshold tolerance limit for Cd under the spiked-substrate system. B. napus showed dynamic behavior regarding morpho-physio-biochemical attributes, including agronomic features, biomass, photosynthetic pigments, relative water content under increased Cd toxicity. Cd stress-induced hydrogen peroxide (H₂O₂) production with high MDA contents and passive EL, followed by the orchestration of both enzymatic (SOD, POD, APX, CAT, and GR) and non-enzymatic antioxidants (flavonoids, TPC, TPA, proline, and total soluble protein) up to a certain limit. In addition, Cd-induced stress upregulated transcriptional levels of antioxidative enzyme SOD, POD, APX, GR, and MT encoded genes in B. napus. The increasing trend of Cd accumulation in different tissues at the highest Cd concentration was as follows: root > leaf > stem. In spiked substrate system, B. napus demonstrated improved metal extractability performance and a high potential for phyto-management of low to moderate Cd contamination, implying that this study could be used for integrative breeding programs and decontaminating heavy metals in real contaminated scenarios.Novelty statementThis study provides an insight into Cd-coping mechanisms of oilseed rape involved in alleviating toxicity and simultaneous phyto-management of increasing Cd concentration under spiked substrate system. The current study is the first scientific evidence of using a Cd-spiked soilless substrate medium. The present study will further strengthen our understanding of Cd-instigated positive responses in B. napus. Furthermore, it will provide a useful basis for integrative breeding programs and decontaminating heavy metals in real contaminated scenarios.

Phytoremediation of Cadmium Contaminated Soil Using Brassica juncea: Influence on PSII Activity, Leaf Gaseous Exchange, Carbohydrate Metabolism, Redox and Elemental Status

Phytoremediation is an ecologically and economically feasible technique to remove heavy metal from soil. The aim of the study was to examine cadmium (Cd) toxicity and phytoremediation aptitude of Brassica juncea. In the present study, plants survived when exposed to different levels of Cd (0, 25, 50 and 100 mg/kg soil) and accumulated a large amount of Cd in its root and shoot. Translocation factor (TF) of Cd from root to shoot was > 1 at both 45 and 60-day stage of growth suggesting that B. juncea is a hyperaccumulator and strong candidate for phytoextraction of Cd. Alongside, Cd impaired photolysis of water, PSII activity, nutrient uptake, photosynthesis and sugar accumulation in the plant. Cd-generated oxidative stress restricts the growth of B. juncea. The toxic effect of Cd was more pronounced at 45-day stage of growth signifying the drifting of plant towards acquirement of exclusion strategy.

Nanoscale Zero-Valent Iron Has Minimum Toxicological Risk on the Germination and Early Growth of Two Grass Species with Potential for Phytostabilization

Two Poaceae species, Agrostis capillaris and Festuca rubra, were selected for their potential as phytostabilizing plants in multicontaminated soils. These species are resistant to contamination and maintain high concentrations of contaminants at the root level. Nanoscale zero-valent iron (nZVI) is an engineered nanomaterial with the ability to stabilize metal(loid)s in soils; its potential toxicological effects in the selected species were studied in a germination test using: (i) control variant without soil; (ii) soil contaminated with Pb and Zn; and (iii) contaminated soil amended with 1% nZVI, as well as in an hydroponic experiment with the addition of nZVI 0, 25, 50 and 100 mg L⁻¹. nZVI had no negative effects on seed germination or seedling growth, but was associated with an increase in shoot growth and reduction of the elongation inhibition rate (root-dependent) of F. rubra seedlings. However, applications of nZVI in the hydroponic solution had no effects on F. rubra but A. capillaris developed longer roots and more biomass. Increasing nZVI concentrations in the growing solution increased Mg and Fe uptake and reduced the Fe translocation factor. Our results indicate that nZVI has few toxic effects on the studied species.

Tolerance mechanism of cadmium in Ceratopteris pteridoides: Translocation and subcellular distribution

Hydroponic experiment was conducted to investigate the biochemical responses and accumulation behaviour of cadmium (Cd) in aquatic fern, Ceratopteris pteridoides, under four different levels of exposure. Plants were grown in 10 μM (CdT1), 20 μM (CdT2), 40 μM (CdT3) and 60 μM (CdT4) concentrations of Cd for 12 consecutive days and Cd accumulation in different plant parts, cell levels and growth medium was estimated. In C. pteridoides, Cd removal kinetics was best described by pseudo-second-order kinetic model. Increased accumulation of Cd in the plants was detected in a concentration dependent manner with maximum under 60 μM of Cd (CdT4) exposure (191.38 mg kg^-1, 186.19 mg kg^-1 and 1316.34 mg kg^-1 in leaves, stems and roots, respectively). Cell wall of C. pteridoides is identified as crucial Cd storage site with the highest (28-69%) accumulation followed by organelles (14-44%) and soluble fraction (6-46%). Increased leaf proline, malondialdehyde (MDA) and protein content with significant reduction (P < 0.05) in chlorophyll concentration and upregulation of antioxidant enzymes catalase (CAT), guaiacol peroxidase (POD) and superoxide dismutase (SOD) reveals the presence of Cd resistance mechanism in C. pteridoides. Calculated higher (>1) bioconcentration factor (BCF) and lower (<1) translocation factor (TF) values evinced the suitability of C. pteridoides in Cd phytostabilization rather than phytoextraction.

Specific H+ level is crucial for accurate phosphate quantification using ascorbate as a reductant

Owing to its essentiality for cellular metabolism, phosphate (PO₄^3-) plays a pivotal role in ecosystem dynamics. Frequent testing of phosphate levels is necessary to monitor ecosystem health. Present investigations were aimed to identify the key factors that are essential for proper quantification of PO₄^3-. Primarily, H⁺ levels played a critical role in the development of molybdenum blue complex by ammonium molybdate and PO₄^3- with ascorbic acid as a reductant. Molybdenum blue complex formed in the presence of 8 to 12 mmol of H⁺ in 3 ml reaction mixture remained stable even after 72 h. Of different concentrations of ammonium molybdate and ascorbic acid tested, best molybdenum blue complex was formed when their concentrations were 24.3 and 5.68 μmol, respectively. More or less similar intensity of molybdenum blue complex (due to reduction of phosphomolybdic acid and not molybdic acid) was formed in the presence of H⁺ at levels ranging from 8 to 10 mmol in 3 ml reaction mixture. Our findings unequivocally demonstrated that (i) the reaction mixture containing 3% ammonium molybdate, 0.1% ascorbic acid and 5 M H₂SO₄ in the ratio of 1:1:1 is ideal for PO₄^3- quantification; (ii) antimony (Sb) significantly curbs the formation of molybdenum blue under these ideal conditions; (iii) this fine-tuned protocol for PO₄^3- quantification could be extended without any problem for determining the level of PO₄^3- both in plant as well as soil samples; and (iv) Azotobacter possesses potential to enhance levels of total PO₄^3- in leaves and grains and soluble/active PO₄^3- in rhizosphere soils of wheat.

Crosstalk among polyamines, phytohormones, hydrogen peroxide, and phenylethanoid glycosides responses in Scrophularia striata to Cd stress

Plants respond to Cadmium (Cd) as a hazardous heavy metal through various mechanisms depending on their available metabolite resources. In this research, the physiological and signaling pathways mediating the responses to Cd stress in Scrophularia striata seedlings were characterized after they were exposed to different Cd concentrations at different time periods. The results showed that the polyamines (PAs), Abscisic acid (ABA) and hydrogen peroxide (H₂O₂) contents were significantly enhanced at 48 h. Moreover, the enzyme activity of phenylalanine ammonia-lyase (PAL) and tyrosine ammonia-lyase (TAL) as regulator enzymes in the phenylpropanoid pathway was increased, related to the reinforcement of phenolic compounds such as phenylethanoid glycosides (as a special compound of this plant). This metabolic profiling indicates that the signal transduction of Cd stress increased the activity of different enzymes (PAL and TAL) by regulating the PAs metabolism, the modulation of ABA, and the H₂O₂ content. As a result, it caused the accumulation of phenolic compounds, especially echinacoside and acteoside, both of which are required to improve the response of Cd stress in S. striata.

Estimating tomato tolerance to heavy metal toxicity: cadmium as study case

This work aimed to develop a reliable and fast approach to estimate the plant tolerance degree to heavy metal (HM) phytotoxicity. Two independent experiments were carried out using tomato accessions, with contrasting morphological features, that were grown in a hydroponic solution containing different CdCl₂ concentrations for 7 days. Plant dry weight and chlorophyll content (SPAD units) were evaluated, and tolerance degree to Cd toxicity was estimated according to the tolerance index (TI), which is a new mathematical formula based on plant biomass proposed in this study. Although with different magnitudes, tomato exhibited reductions in their dry weight concurrently with the increasing CdCl₂ concentration. By contrast, chlorophyll content presented no standard response, decreasing and even increasing according to CdCl₂ concentrations, indicating that only under certain conditions (particularly, at CdCl₂ 50 μM), this parameter can be used to estimate plant tolerance to Cd toxicity. TI was efficiently able to segregate tomato cultivars with similar performance (based on the total dry weight of plants), and such segregation was optimized when the hydroponic solution contained from 25 to 50 μM CdCl₂. Within this range, data pointed at 35 μM CdCl₂ as the best concentration to be employed in studies related to the tomato tolerance/sensitivity to Cd toxicity. In conclusion, TI proved to be a reliable estimator of tolerance degree to Cd exposure in genetically distinct tomato accessions. Moreover, TI can be used for this same purpose in plants under other HM-induced stresses.

Cadmium phytoremediation potential of Brassica crop species: A review

Cadmium (Cd) is a highly toxic metal released into the environment through anthropogenic activities. Phytoremediation is a green technology used for the stabilization or remediation of Cd-contaminated soils. Brassica crop species can produce high biomass under a range of climatic and growing conditions, allowing for considerable uptake and accumulation of Cd, depending on species. These crop species can tolerate Cd stress via different mechanisms, including the stimulation of the antioxidant defense system, chelation, compartmentation of Cd into metabolically inactive parts, and accumulation of total amino-acids and osmoprotectants. A higher Cd-stress level, however, overcomes the defense system and may cause oxidative stress in Brassica species due to overproduction of reactive oxygen species and lipid peroxidation. Therefore, numerous approaches have been followed to decrease Cd toxicity in Brassica species, including selection of Cd-tolerant cultivars, the use of inorganic and organic amendments, exogenous application of soil organisms, and employment of plant-growth regulators. Furthermore, the coupling of genetic engineering with cropping may also help to alleviate Cd toxicity in Brassica species. However, several field studies demonstrated contrasting results. This review suggests that the combination of Cd-tolerant Brassica cultivars and the application of soil amendments, along with proper agricultural practices, may be the most efficient means of the soil Cd phytoattenuation. Breeding and selection of Cd-tolerant species, as well as species with higher biomass production, might be needed in the future when aiming to use Brassica species for phytoremediation.

Cadmium exposure triggers genotype-dependent changes in seed vigor and germination of tomato offspring

Although negative effects on the offspring fitness can be triggered by the mother-plant exposure to environmental stresses, some plants are able to "remember" past incidents and enhance the progeny tolerance. Here, the mineral profile, cytogenetic modifications, and physiological potential of seeds from two tomato cultivars, with contrasting tolerance degrees to cadmium (Cd) toxicity, were evaluated after plant exposure to this metal. Both cultivars exhibited high Cd translocation to the seeds; however, the tolerant tomato accumulated more Cd than did the sensitive one. As a consequence of the Cd accumulation, reductions in the Mn concentration in Cd-challenged plants were detected. Surprisingly, seed germination and vigor were increased in the tolerant tomato cultivar after Cd exposure, despite increases in the chromosomal abnormalities. By contrast, seeds from the sensitive cultivar exhibited no changes in their physiological potential after Cd exposure, despite Cd-induced reductions in the mitotic index. Moreover, bunch position exerted effects on the vigor and type of chromosomal abnormality. The results show that maternal plant exposure to Cd can affect tomato offspring by changing the seed physiological potential, and such effect can be partially explained by alterations in the seed-derived elements (essential and non-essential) and genotype-dependent tolerance mechanisms.

PtrA/NINV, an alkaline/neutral invertase gene of Poncirus trifoliata, confers enhanced tolerance to multiple abiotic stresses by modulating ROS levels and maintaining photosynthetic efficiency

BACKGROUND

Alkaline/neutral invertase (A/N-INV), an enzyme that hydrolyzes sucrose irreversibly into glucose and fructose, is essential for normal plant growth,development, and stress tolerance. However, the physiological and/or molecular mechanism underpinning the role of A/N-INV in abiotic stress tolerance is poorly understood.

RESULTS

In this report, an A/N-INV gene (PtrA/NINV) was isolated from Poncirus trifoliata, a cold-hardy relative of citrus, and functionally characterized. PtrA/NINV expression levels were induced by cold, salt, dehydration, sucrose, and ABA, but decreased by glucose. PtrA/NINV was found to localize in both chloroplasts and mitochondria. Overexpression of PtrA/NINV conferred enhanced tolerance to multiple stresses, including cold, high salinity, and drought, as supported by lower levels of reactive oxygen species (ROS), reduced oxidative damages, decreased water loss rate, and increased photosynthesis efficiency, relative to wild-type (WT). The transgenic plants exhibited higher A/N-INV activity and greater reducing sugar content under normal and stress conditions.

CONCLUSIONS

PtrA/NINV is an important gene implicated in sucrose decomposition, and plays a positive role in abiotic stress tolerance by promoting osmotic adjustment, ROS detoxification and photosynthesis efficiency. Thus, PtrA/NINV has great potential to be used in transgenic breeding for improvement of stress tolerance.