Alleviation of cadmium toxicity in Lemna minor by exogenous salicylic acid

Multivariate characterization of salicylic acid and potassium induced physio-biochemical and phytoremediation responses in quinoa exposed to lead and cadmium contamination

The levels of soils pollutants such as lead (Pb) and cadmium (Cd) have significantly increased recently resulting in ecological disturbances and threatening crop production. Various amendments have been employed to enhance the tolerance of crops to withstand Cd and Pb stresses. However, the role of combined application of potassium (K) and of salicylic acid (SA) for Cd and Pb stress mitigation and phytoremediation by quinoa (Chenopodium quinoa Willd) has not been comprehended well. In the present study, the effect of 10 mM K and 0.1 mM SA was tested on the quinoa plants subjected to 250 μM Pb and/or 100 μM Cd. The Pb and Cd treatments were applied separately or together. Phytotoxicity induced by Pb and Cd resulted in drastic decrease (>60%) in chlorophyll contents, stomatal conductance, and plant biomass. The collective treatment of Pb and Cd induced an increase in the concentration of hydrogen peroxide (13-fold) and lipid peroxidation (16-fold) that resulted in a 61% reduction in membrane stability. The application of 10 mM K and/or 0.1 mM SA was remarkable in mitigating the adverse effect of Pb and Cd. The reduction in plant biomass was 17% when 10 mM K and 0.1 mM SA were applied together under the combined treatment of both the metals. The simultaneous application of K and SA effectively mitigated oxidative stress by enhancing the activities of superoxide dismutase, peroxidase, ascorbate peroxidase, and catalase by 12, 10, 7 and 10-folds respectively. The positive effect of K and SA on these attributes resulted in a remarkable reduction in metal accumulation and translocation and lipid peroxidation. The stressed plants supplemented with K and SA exhibited a significant improvement in the membrane stability index, chlorophyll content, and stomatal conductance. This study concluded that the combined application of K and SA could be a good approach for reducing Pb and Cd phytotoxicity in quinoa and enhancing their phytostabilization potential in the contaminated soils.

Efficacy of Fe-Mg-bimetallic biochar in stabilization of multiple heavy metals-contaminated soil and attenuation of toxicity in spinach (Spinacia oleracea L.)

Globally, soil contamination with heavy metals (HMs) pose serious threats to soil health, crop productivity, and human health. The present investigation involved synthesis and analysis of biochar with bimetallic combination of iron and magnesium (Fe-Mg-BC). Our study evaluated how Fe-Mg-BC affects the absorption of cadmium (Cd), lead (Pb), and copper (Cu) in spinach (Spinacia oleracea L.) and remediation of soil contaminated with multiple HMs. Results demonstrated the successful loading of iron (Fe) and magnesium (Mg) onto pristine biochar (BC) derived from peanut shells. The addition of Fe-Mg-BC (3%) notably increased spinach biomass, enhancing photosynthesis, transpiration, stomatal conductance, and intercellular CO2 levels by 22%, 21%, 103%, and 15.3%, respectively. Compared to control, Fe-Mg-BC (3%) suppressed metal-induced oxidative stress by boosting levels of superoxide dismutase (SOD), ascorbate peroxidase (APX) and catalase (CAT) in roots by 40.9%, 57%, 54.8 %, and in shoots by 55.5%, 65.5%, and 37.4% in shoots, respectively. The Fe-Mg-BC effectively reduced the uptake of Cd, Pb, and Cu in spinach tissues by transforming their bioavailable fractions to non-bioavailable forms. The Fe-Mg-BC (3%) significantly reduced the mobility of Cd, Pb and Cu in soil and limited the concentration of Cd, Pb, and Cu in plant roots by 34.1%, 79.2%, 47%, and shoots by 56.3%, 43.3%, and 54.1%, respectively, compared to control. These findings underscore the potential of Fe-Mg-BC as a promising amendment for reclaiming soils contaminated with variety of HMs, thereby making a significant contribution to the promotion of safer food production.

Biomonitoring of heavy metals and their phytoremediation by duckweeds: Advances and prospects

Heavy metals (HMs) contamination of water bodies severely threatens human and ecosystem health. There is growing interest in the use of duckweeds for HMs biomonitoring and phytoremediation due to their fast growth, low cultivation costs, and excellent HM uptake efficiency. In this review, we summarize the current state of knowledge on duckweeds and their suitability for HM biomonitoring and phytoremediation. Duckweeds have been used for phytotoxicity assays since the 1930s. Some toxicity tests based on duckweeds have been listed in international guidelines. Duckweeds have also been recognized for their ability to facilitate HM phytoremediation in aquatic environments. Large-scale screening of duckweed germplasm optimized for HM biomonitoring and phytoremediation is still essential. We further discuss the morphological, physiological, and molecular effects of HMs on duckweeds. However, the existing data are clearly insufficient, especially in regard to dissection of the transcriptome, metabolome, proteome responses and molecular mechanisms of duckweeds under HM stresses. We also evaluate the influence of environmental factors, exogenous substances, duckweed community composition, and HM interactions on their HM sensitivity and HM accumulation, which need to be considered in practical application scenarios. Finally, we identify challenges and propose approaches for improving the effectiveness of duckweeds for bioremediation from the aspects of selection of duckweed strain, cultivation optimization, engineered duckweeds. We foresee great promise for duckweeds as phytoremediation agents, providing environmentally safe and economically efficient means for HM removal. However, the primary limiting issue is that so few researchers have recognized the outstanding advantages of duckweeds. We hope that this review can pique the interest and attention of more researchers.

Eco-friendly approach to decrease the harmful effects of untreated wastewater on growth, yield, biochemical constituents, and heavy metal contents of carrot (Daucus carota L.)

Here, the impact of irrigation using untreated wastewater (WW) on carrots (Daucus carota L.) was examined. We hypothesized that the addition of ethylenediaminetetraacetic acid (EDTA), dry algal powder (Spirulina platensis or Chlorella vulgaris), and Salix alba leaves powder would function as chelators for harmful contaminants in wastewater. The findings showed that irrigation of carrot plants with the sampled untreated wastewater led to significant decreases in the shoot lengths, fresh, dry weights of shoots and roots at stage I, the diameter of roots, pigment content, carotenoids, total soluble carbohydrate content, and soluble protein content. Furthermore, a significantly increased level of proline, total phenols, and the activities of polyphenol oxidase (PPO), peroxidase (POX), superoxide dismutase (SOD), and catalase (CAT) was identified in stage I samples. In contrast to the stage I, the length of the roots, the number of leaves on each plant, wet and dry weights of the stage II roots were all greatly enhanced. In spite of the increased yield due to the wastewater irrigation, carrot roots irrigated with wastewater had significantly more cadmium (Cd), nickel (Ni), cobalt (Co), and lead (Pb) than is considered safe. Our data clearly show that the application of Spirulina platensis, Chlorella vulgaris, EDTA, and leaves powder of salix was able to alleviate the toxicity of wastewater on carrot plants. For example, we recorded a significant decrease in the accumulation of carrot's Cd, Ni, Co, and Pb contents. We conclude that the treatments with Spirulina platensis and Chlorella vulgaris can be utilized as eco-friendly tools to lessen the damaging effects of wastewater irrigation on carrot plants.

Salicylic acid alleviates Zn-induced inhibition of growth via enhancing antioxidant system and glutathione metabolism in alfalfa

Zinc (Zn) is considered as one of the heavy metal pollutants in soil affecting agriculture. Salicylic acid (SA) is an important phytohormone that can mitigate effects against various abiotic stresses in plants, however, its exploration to improve Zn stress tolerance in alfalfa plants is still elusive. Thus, in the present study, exogenous SA treatment was conducted on alfalfa plants under Zn stress. The effects of exogenous SA on the physiological effects of alfalfa plants and the expression levels related genes were studied. This study tested the biomass, relative water content, chlorophyll levels, photosynthetic capacity, proline and soluble sugar contents, detected the activity of antioxidant enzymes (such as peroxidase and superoxide dismutase), glutathione biosynthesis, and endogenous SA levels, and quantified the genes associated with the antioxidant system and glutathione metabolism-mediated Zn stress. The results showed that exogenous SA could elevate the physiological adaptability of alfalfa plants through enhancing photosynthesis, proline and soluble sugar levels, stimulating antioxidant system and glutathione metabolism, and inducing the transcription level of related genes, thereby diminishing oxidative stress, inhibiting excessive Zn accumulation of alfalfa plants, increasing tolerance to Zn stress, and reducing the toxicity of Zn. Collectively, the application of SA alleviates Zn toxicity in alfalfa plants. The findings gave first insights into the regulatory mechanism of the Zn stress tolerance of alfalfa by exogenous SA and this might have positive implications for managing other plants which are suffering Zn stress.

Pseudomonas stutzeri improves the tolerance of Lemna minor to Cu(OH)2 nanopesticide by regulating the uptake of copper, antioxidant defense mechanisms, and the expression of metacaspase-1, chlorophyllase, and stress-responsive genes

This study investigated the effect of Pseudomonas stutzeri inoculation on Lemna minor treated with Cu(OH)2 nanopesticide (NP). The results showed that P. stutzeri inoculation increased the relative growth rate (RGR) in NP-treated plants. Although chlorophyll and carotenoid contents decreased significantly in NP-treated plants, P. stutzeri inoculation led to an increase in chlorophyll and carotenoid contents in NP-treated plants. Copper (Cu) content increased with increasing NP concentration, but it decreased significantly in the presence of P. stutzeri. NP treatment caused increased H2O2 and TBARS levels, as well as proline levels. However, P. stutzeri inoculation led to decreased H2O2 and TBARS levels and increased SOD, POX, GST, GR, GPX, and DHAR activities. The expression of genes encoding SOD, GST, metacaspase-1, and chlorophyllase was upregulated by NP treatment alone. Additionally, when plants were inoculated with P. stutzeri, the expression of these genes was further enhanced. In conclusion, P. stutzeri inoculation had a positive effect on the growth and antioxidant system of L. minor treated with NP as it enhanced RGR, increased chlorophyll and carotenoid contents, and decreased Cu content and oxidative stress. These findings suggested that P. stutzeri has the potential to promote aquatic plant growth and counteract the negative impacts of NP on these plants.

Foliar application of salicylic acid inhibits the cadmium uptake and accumulation in lettuce (Lactuca sativa L.)

Introduction

Salicylic acid (SA) is a multi-functional endogenous phytohormone implicated in the growth, development, and metabolism of many plant species.

Methods

This study evaluated the effects of different concentrations of SA (0, 25, 100, 200, and 500 mg/L) on the growth and cadmium (Cd) content of lettuce (Lactuca sativa L.) under Cd stress. The different concentrations of SA treatments were administered through foliar application.

Results

Our results showed that 100-200 mg/L SA significantly increased the plant height and biomass of lettuce under Cd stress. When SA concentration was 200 mg/L, the plant height and root length of lettuce increased by 19.42% and 22.77%, respectively, compared with Cd treatment alone. Moreover, 200 mg/L and 500mg/L SA concentrations could reduce peroxidase (POD) and superoxide dismutase (SOD) activities caused by Cd stress. When the concentration of exogenous SA was 500 mg/L, the POD and SOD activities of lettuce leaves decreased by 15.51% and 19.91%, respectively, compared with Cd treatment. A certain concentration of SA reduced the uptake of Cd by the lettuce root system and the transport of Cd from the lettuce root system to shoots by down-regulating the expression of Nramp5, HMA4, and SAMT, thus reducing the Cd content of lettuce shoots. When the concentration of SA was 100 mg/L, 200 mg/L, and 500 mg/L, the Cd contents of lettuce shoots were 11.28%, 22.70%, and 18.16%, respectively, lower than that of Cd treatment alone. Furthermore, principal component and correlation analyses showed that the Cd content of lettuce shoots was correlated with plant height, root length, biomass, antioxidant enzymes, and the expression level of genes related to Cd uptake.

Discussion

In general, these results provide a reference for the mechanism by which SA reduces the Cd accumulation in vegetables and a theoretical basis for developing heavy metal blockers with SA components.

Remediation of soil polluted with Pb and Cd and alleviation of oxidative stress in Brassica rapa plant using nanoscale zerovalent iron supported with coconut-husk biochar

Accumulation of toxic elements by plants from polluted soil can induce the excessive formation of reactive oxygen species (ROS), thereby causing retarded plants' physiological attributes. Several researchers have remediated soil using various forms of zerovalent iron; however, their residual impacts on oxidative stress indicators and health risks in leafy vegetables have not yet been investigated. In this research, nanoscale zerovalent iron supported with coconut-husk biochar (nZVI-CHB) was synthesized through carbothermal reduction process using Fe2O3 and coconut-husk. The stabilization effects of varying concentrations of nZVI-CHB and CHB (250 and 500 mg/kg) on cadmium (Cd) and lead (Pb) in soil were analyzed, and their effects on toxic metals induced oxidative stress, physiological properties, and antioxidant defence systems of the Brassica rapa plant were also checked. The results revealed that the immobilization of Pb and Cd in soil treated with CHB was low, leading to a higher accumulation of metals in plants grown. However, nZVI-CHB could significantly immobilize Pb (57.5-62.12%) and Cd (64.1-75.9%) in the soil, leading to their lower accumulation in plants below recommended safe limits and eventually reduced carcinogenic risk (CR) and hazard quotient (HQ) for both Pb and Cd in children and adults below the recommended tolerable range of <1 for HQ and 10-6 - 10-4 for CR. Also, a low dose of nZVI-CHB significantly mitigated toxic metal-induced oxidative stress in the vegetable plant by inhibiting the toxic metals uptake and increasing antioxidant enzyme activities. Thus, this study provided another insightful way of converting environmental wastes to sustainable adsorbents for soil remediation and proved that a low-dose of nZVI-CHB can effectively improve soil quality, plant physiological attributes and reduce the toxic metals exposure health risk below the tolerable range.

Transcriptomic and Functional Analyses of Two Cadmium Hyper-Enriched Duckweed Strains Reveal Putative Cadmium Tolerance Mechanisms

Cadmium (Cd) is one of the most toxic metals in the environment and exerts deleterious effects on plant growth and production. Duckweed has been reported as a promising candidate for Cd phytoremediation. In this study, the growth, Cd enrichment, and antioxidant enzyme activity of duckweed were investigated. We found that both high-Cd-tolerance duckweed (HCD) and low-Cd-tolerance duckweed (LCD) strains exposed to Cd were hyper-enriched with Cd. To further explore the underlying molecular mechanisms, a genome-wide transcriptome analysis was performed. The results showed that the growth rate, chlorophyll content, and antioxidant enzyme activities of duckweed were significantly affected by Cd stress and differed between the two strains. In the genome-wide transcriptome analysis, the RNA-seq library generated 544,347,670 clean reads, and 1608 and 2045 differentially expressed genes were identified between HCD and LCD, respectively. The antioxidant system was significantly expressed during ribosomal biosynthesis in HCD but not in LCD. Fatty acid metabolism and ethanol production were significantly increased in LCD. Alpha-linolenic acid metabolism likely plays an important role in Cd detoxification in duckweed. These findings contribute to the understanding of Cd tolerance mechanisms in hyperaccumulator plants and lay the foundation for future phytoremediation studies.

Restoration effect of sulfhydryl-modified sepiolite on cadmium in contaminated soil and its effect on the growth of spinach (Spinacia oleracea L)

Coal mining has produced a large amount of coal gangue. It makes the soil around the mining area seriously polluted by heavy metals, affects the growth of crops, and endangers human health. Therefore, there is an urgent need to develop new materials for remediation of Cd in soil. In this study, mercaptosilane-modified sepiolite (Q-Sep) was used as a basic passivator, and it was pretreated with acid (H-Q-Sep) and high temperature (R-Q-Sep) respectively. By analyzing the forms of Cd and pH values in soil after adding modified sepiolite, we compared the remediation effects of two modified methods on Cd in soil. The enrichment of spinach (Spinacia oleracea L) to Cd and changes in physiological and biochemical indexes of spinach were determined, and the effect of modified sepiolite on the growth of spinach was judged. The experimental results showed that the addition of modified sepiolite could significantly increase the soil pH values (p < 0.05); the content of exchangeable Cd in soil decreased by 60.4%; and the maximum increase of residual state was 32.9%. The absorption of Cd in soil by spinach decreased, and root length, plant height, and biomass of spinach all increased. It was proved that the addition of modified sepiolite can improve the productivity of soil, reduce toxicity of heavy metals in soil, and promote growth of plants. As a result, the addition of H-Q-Sep and R-Q-Sep can effectively repair Cd in gangue filled soil, which provides a certain theoretical basis for the passivation remediation of Cd in soil.

Reduction in the cadmium (Cd) accumulation and toxicity in pearl millet (Pennisetum glaucum L.) by regulating physio-biochemical and antioxidant defense system via soil and foliar application of melatonin

Cadmium (Cd) is among the toxic pollutants that harms the both animals and plants. The natural antioxidant, melatonin can improve Cd-stress tolerance but its potential role in reducing Cd stress and resilience mechanisms in pearl millet (Pennisetum glaucum L.) is remain unclear. The present study suggests that Cd causes severe oxidative damage by decreasing photosynthesis, and increasing reactive oxygen species (ROS), malondialdehyde content (MDA), and Cd content in different parts of pearl millet. However, exogenous melatonin (soil application and foliar treatment) mitigated the Cd toxicity and enhanced the growth, antioxidant defense system, and differentially regulated the expression of antioxidant-responsive genes i. e superoxide dismutase SOD-[Fe] 2, Fe-superoxide dismutase, Peroxiredoxin 2C, and L-ascorbate peroxidase-6. The results showed that foliar melatonin at F-200/50 significantly increased the plant height, chlorophyll a, b, a+b and carotenoids by 128%, 121%, 150%, 122%, and 69% over the Cd treatment, respectively. The soil and foliar melatonin at S-100/50 and F-100/50 reduced the ROS by 36%, and 44%, and MDA by 42% and 51% over the Cd treatment, respectively. Moreover, F200/50 significantly boosted the activities of antioxidant enzymes i. e SOD by 141%, CAT 298%, POD 117%, and APX 155% over the Cd treatment. Similarly, a significant reduction in Cd content in root, stem, and leaf was found on exposure to higher concentrations of exogenous melatonin. These findings suggest that exogenous melatonin may significantly and differentially improve the tolerance to Cd stress in crop plants. However, field applications, type of plant species, concentration of dose, and type of stress may vary with the degree of tolerance in crop plants.

Effects of cadmium on transcription, physiology, and ultrastructure of two tobacco cultivars

Cadmium (Cd) is one of the most toxic heavy metal pollutants worldwide. Tobacco is an important cash crop; however, the accumulation of Cd in its biomass is very high. Cadmium may enter the body of smokers with contaminated tobacco and the surrounding environment via smoke. Therefore, it is important to understand the mechanisms of Cd accumulation and tolerance in tobacco plants, especially in the leaves. In this study, the effects of Cd on the growth, accumulation, and biochemical indices of two tobacco varieties, K326 (Cd resistant) and NC55 (Cd sensitive), were studied through transcriptomic and physiological experiments. Transcriptome and physiological analyses showed differences in the expression of Cd transport and Cd resistance related genes between NC55 and K326 under Cd stress. The root meristem cells of NC55 were more severely damaged. The antioxidant enzyme activity, ABA and ZT content, chlorophyll content, photosynthetic rate, and nitrogen metabolism enzyme activity in K326 leaves were higher than in NC55. These data elucidate the mechanisms of low Cd accumulation and high Cd tolerance in K326 leaves and provide a theoretical basis for cultivating tobacco varieties with low Cd accumulation and high Cd resistance.

Rare earth element scandium mitigates the chromium toxicity in Lemna minor by regulating photosynthetic performance, hormonal balance and antioxidant machinery

Chromium (Cr) toxicity is a serious problem that threatens the health of living organisms and especially agricultural production. The presence of excess Cr leads to biomass loss by causing the imbalance of biochemical metabolism and inhibiting photosynthetic activity. A new critical approach to cope with Cr toxicity is the use of the rare earth elements (REEs) as an antioxidant defence system enhancer in plants. However, the effect of scandium (Sc), which is one of the REEs, is not clear enough in Lemna minor exposed to Cr toxicity. For this purpose, the photosynthetic and biochemical effects of scandium (50 μM and 200 μM Sc) treatments were investigated in Lemna minor under Cr stress (100 μM, 200 μM and 500 μM Cr). Parameters related to photosynthesis (F_v/F_m, F_v/F_o) were suppressed under Cr stress. Stress altered antioxidant enzymes activities and hormone contents. Sc applications against stress increased the activities of superoxide dismutase (SOD), NADPH oxidase (NOX), ascorbate peroxidase (APX), glutathione reductase (GR), monodehydroascorbate reductase (MDHAR), and glutathione S-transferase (GST). In addition to the antioxidant system, the contents of indole-3-acetic acid (IAA), abscisic acid (ABA) and jasmonic acid (JA) were also rearranged. However, in all treatment groups, with the provision of ascorbate (AsA) regeneration and effective hormone signaling, reactive oxygen species (ROS) retention which result in high hydrogen peroxide (H₂O₂) content and lipid peroxidation (TBARS) were effectively removed. Sc promoted the maintenance of cellular redox state by regulating antioxidant pathways included in the AsA-GSH cycle. Our results showed that Sc has great potential to confer tolerance to duckweed by reducing Cr induced oxidative damage, protecting the biochemical reactions of photosynthesis, and improving hormone signaling.

Exogenous acetone O-(4-chlorophenylsulfonyl) oxime alleviates Cd stress-induced photosynthetic damage and oxidative stress by regulating the antioxidant defense mechanism in Zea mays

Cadmium (Cd) toxicity in leaves decreases their photosynthetic efficiency by degrading photosynthetic pigments, reducing the activity of gas exchange parameters and photosystem II (PSII), and producing reactive oxygen species. Although acetone O-(4-chlorophenylsulfonyl) oxime (AO) alleviates stress due to heavy metals in plants, its effects on the photosynthetic apparatus and redox balance under Cd stress are not clear. Herein, the role of AO in modulating the relationship between the antioxidant defense system and photosynthetic performance including chlorophyll fluorescence and gas exchange in mitigating the stress damage caused by Cd in maize seedlings was investigated. Three-week-old maize seedlings were pre-treated with AO (0.66 mM) and exposed to 100 µM Cd stress. Our findings indicated that AO application increased Cd accumulation, thiobarbituric acid-reactive substances (TBARS), photosynthetic rate, hydrogen peroxide (H2O2), total chlorophyll and carotenoid, transpiration, stomatal conductance, maximum photochemical efficiency of PSII (Fv/Fm), effective quantum yield of PSII (ΦPSII), intercellular CO2 concentration, photochemical quenching (qP), superoxide dismutase, electron transport rate, proline, ascorbate peroxidase, catalase, guaiacol peroxidase, 4-hydroxybenzoic acid, catechol, and cinnamic acid in maize seedling under Cd stress. Conversely, AO significantly reduced oxidative damage levels (H2O2, TBARS). It was concluded that exogenous AO can overcome Cd-mediated oxidative damage and hence protect the photosynthetic machinery by providing stress tolerance and regulating the antioxidant defense mechanism, which includes proline, phenolic compounds, and antioxidant enzyme activities.

Supplementary Information

The online version contains supplementary material available at 10.1007/s12298-022-01258-5.

Duckweed: a potential phytosensor for heavy metals

Globally, heavy metal (HM) contamination is one of the primary causes of environmental pollution leading to decreased quality of life for those affected. In particular, HM contamination in groundwater poses a serious risk to human health and the potential for destabilization of aquatic ecosystems. At present, strategies to remove HM contamination from wastewater are inefficient, costly, laborious, and often the removal poses as much risk to the environment as the initial contamination. Phytoremediation, plant-based removal of contaminants from soil or water, has long been viewed as an economical and sustainable solution to remove toxic metals from the environment. However, to date, phytoremediation has demonstrated limited successes despite a large volume of literature supporting its potential. A key aspect for achieving robust and meaningful phytoremediation is the selection of a plant species that is well suited to the task. For the removal of pollutants from wastewater, hydrophytes, like duckweed, exhibit significant potential due to their rapid growth on nutrient-rich water, ease of collection, and ability to survive in various ecosystems. As a model for ecotoxicity studies, duckweed is an ideal candidate, as it is easy to cultivate under controlled and even sterile conditions, and the rapid growth enables multi-generational studies. Similarly, recent advances in the genetic engineering and genome-editing of duckweed will enable the transition from fundamental ecotoxicity studies to engineered solutions for phytoremediation of HMs. This review will provide insight into the suitability of duckweeds for phytoremediation of HMs and strategies for engineering next-generation duckweed to provide real-world environmental solutions.

Salicylic Acid Enhances Cadmium Tolerance and Reduces Its Shoot Accumulation in Fagopyrum tataricum Seedlings by Promoting Root Cadmium Retention and Mitigating Oxidative Stress

Soil cadmium (Cd) contamination seriously reduces the production and product quality of Tartary buckwheat (Fagopyrum tataricum), and strategies are urgently needed to mitigate these adverse influences. Herein, we investigated the effect of salicylic acid (SA) on Tartary buckwheat seedlings grown in Cd-contaminated soil in terms of Cd tolerance and accumulation. The results showed that 75-100 µmol L^-1 SA treatment enhanced the Cd tolerance of Tartary buckwheat, as reflected by the significant increase in plant height and root and shoot biomass, as well as largely mitigated oxidative stress. Moreover, 100 µmol L^-1 SA considerably reduced the stem and leaf Cd concentration by 60% and 47%, respectively, which is a consequence of increased root biomass and root Cd retention with promoted Cd partitioning into cell wall and immobile chemical forms. Transcriptome analysis also revealed the upregulation of the genes responsible for cell wall biosynthesis and antioxidative activities in roots, especially secondary cell wall synthesis. The present study determines that 100 µmol L^-1 is the best SA concentration for reducing Cd accumulation and toxicity in Tartary buckwheat and indicates the important role of root in Cd stress in this species.

Increasing Heavy Metal Tolerance by the Exogenous Application of Organic Acids

Several metals belong to a group of non-biodegradable inorganic constituents that, at low concentrations, play fundamental roles as essential micronutrients for the growth and development of plants. However, in high concentrations they can have toxic and/or mutagenic effects, which can be counteracted by natural chemical compounds called chelators. Chelators have a diversity of chemical structures; many are organic acids, including carboxylic acids and cyclic phenolic acids. The exogenous application of such compounds is a non-genetic approach, which is proving to be a successful strategy to reduce damage caused by heavy metal toxicity. In this review, we will present the latest literature on the exogenous addition of both carboxylic acids, including the Kreb's Cycle intermediates citric and malic acid, as well as oxalic acid, lipoic acid, and phenolic acids (gallic and caffeic acid). The use of two non-traditional organic acids, the phytohormones jasmonic and salicylic acids, is also discussed. We place particular emphasis on physiological and molecular responses, and their impact in increasing heavy metal tolerance, especially in crop species.

Exogenous Application of Salicylic Acid and Hydrogen Peroxide Ameliorate Cadmium Stress in Milk Thistle by Enhancing Morpho-Physiological Attributes Grown at Two Different Altitudes

Cadmium (Cd+2) is a potential and widespread toxic environmental pollutant, mainly derived from a rapid industrial process that has inhibitory effects on growth, physiological, and biochemical attributes of various plant species, including medicinal plants such as Silybum marianum L. Gaertn commonly known as milk thistle. Plant signaling molecules, when applied exogenously, help to enhance/activate endogenous biosynthesis of potentially important signaling molecules and antioxidants that boost tolerance against various abiotic stresses, e.g., heavy metal stress. The present study documented the protective role of salicylic acid (SA;0.25 μM) and hydrogen peroxide (H2O2; 10 μM) priming, foliar spray, and combinational treatments in reducing Cd+2 toxicity (500 μM) in milk thistle grown at two diverse ecological zones of Balochistan Province of Pakistan i.e., Quetta (Qta) and Turbat (Tbt). The morpho-physiological and biochemical attributes of milk thistle were significantly affected by Cd+2 toxicity; however, priming and foliar spray of SA and H2O2 significantly improved the growth attributes (root/shoot length, leaf area, and root/shoot fresh and dry weight), photosynthetic pigments (Chl a, b, and carotenoids) and secondary metabolites (Anthocyanin, Soluble phenolics, and Tannins) at both altitudes by suppressing the negative impact of Cd+2. However, the oxidative damage parameters, i.e., MDA and H2O2, decreased astonishingly under the treatment of signaling molecules, thereby protecting membrane integrity under Cd+2 stress. The morphological variations were profound at the low altitude (Tbt) as compared to the high altitude (Qta). Interestingly, the physiological and biochemical attributes at both altitudes improved under SA and H2O2 treatments, thus hampered the toxic effect of Cd+2. These signaling compounds enhanced tolerance of plants under heavy metal stress conditions with the consideration of altitudinal, and ambient temperature variations remain to be the key concerns.

Effect of zinc imbalance and salicylic acid co-supply on Arabidopsis response to fungal pathogens with different lifestyles

In higher plants, Zn nutritional imbalance can affect growth, physiology and response to stress, with effect variable depending on host-pathogen interaction. Mechanisms through which Zn operates are not yet well known. The hormone salicylic acid (SA) can affect plant ion uptake, transport and defence responses. Thus, in this study the impact of Zn imbalance and SA co-supply on severity of infection with the necrotrophic fungal pathogen B. cinerea or the biotroph G. cichoracearum was assessed in A. thaliana Col-0. Spectrophotometric assays for pigments and malondialdehyde (MDA) content as a marker of lipid peroxidation, plant defensin 1.2 gene expression by semi-quantitative PCR, callose visualization by fluorescence microscopy and diseases evaluation by macro- and microscopic observations were carried out. Zinc plant concentration varied with the supplied dose. In comparison with the control, Zn-deficit or Zn-excess led to reduced chlorophyll content and PDF 1.2 transcripts induction. In Zn-deficient plants, where MDA increased, also the susceptibility to B. cinerea increased, whereas MDA decreased in G. cichoracearum. Zinc excess increased susceptibility to both pathogens. Co-administration of SA positively affected MDA level, callose deposition, PDF 1.2 transcripts and plant response to the two pathogens. The increased susceptibility to B. cinerea in both Zn-deficient and Zn-excess plants could be related to lack of induction of PDF 1.2 transcripts; oxidative stress could explain higher susceptibility to the necrotroph and lower susceptibility to the biotroph in Zn-deficient plants. This research shows that an appropriate evaluation of Zn supply according to the prevalent stress factor is desirable for plants.

Role of Nramp transporter genes of Spirodela polyrhiza in cadmium accumulation

As a pollutant, Cd causes severe impact to the environment and damages living organisms. It can be uptaken from the environment by the natural resistance-associated macrophage protein (Nramp) in plants. However, the ion absorption function of Nramp transporter genes in Spirodela polyrhiza has not been reported. In this study, SpNramp1, SpNramp2, and SpNramp3 from S. polyrhiza were cloned and their functions were analyzed in S. polyrhiza and yeast. Growth parameters and physicochemical indices of wild-type and transgenic lines were measured under Cd stress. Results revealed that SpNramp1, SpNramp2, and SpNramp3 were identified as plasma membrane-localized transporters, and their roles in transporting Cd were verified in yeast. In S. polyrhiza, SpNramp1 overexpression significantly increased the content of Cd, Fe, Mn, and fresh weight. SpNramp2 overexpression increased Mn and Cd. SpNramp3 overexpression increased Fe and Mn concentrations. These results indicate that SpNramp1, SpNramp2, and SpNramp3 had a different preference for ion absorption. Two S. polyrhiza transgenic lines (OE1 and OE3) were obtained. One of them (OE1) showed a stronger accumulation ability, and the other one (OE3) exhibited tolerance capacity to Cd. This study provides new insight into the functions of SpNramp1, SpNramp2, and SpNramp3 and obtains important enrichment lines (OE1) for manipulating Cd accumulation, phytoremediation, and ecological safety.

Modulation of the toxic effects of zinc oxide nanoparticles by exogenous salicylic acid pretreatment in Chenopodium murale L

Due to many uses of zinc oxide nanoparticles (ZnO NPs) in various industries, the release of these particles in the environment and their effects on living organisms is inevitable. In this study, the role of salicylic acid (SA) pretreatments in modulating the toxicity of ZnO NPs was investigated using a hydroponic system. After pretreatment with different concentrations of SA (0, 25, 75, and 150 μM), Chenopodium murale plants were exposed to ZnO NPs (50 mg L^-1). The results showed that exogenous SA increased the length, weight, chlorophyll, proline, starch, and soluble sugars in the plants. Besides, SA pretreatments improved water status in the plants treated with ZnO NPs. In SA-pretreated plants, increased activity of catalase (CAT), guaiacol peroxidase (GPX), and superoxide dismutase (SOD) was associated with a decline in electrolyte leakage (EL %) and membrane peroxidation. Under NPs stress, SA pretreatments increased the content of phenolic compounds by increasing the activity of phenylalanine ammonia-lyase (PAL). Exogenous SA reduced the translocation of larger amounts of Zn to the shoots, with more accumulation in the roots. This result can be used to produce healthy food from plants grown in environments contaminated with nanoparticles. It seems that all concentrations of SA reduced the symptoms of ZnO NPs toxicity in the plant by strengthening the function of the antioxidant system and increasing the content of some metabolites. Findings also suggest that SA pretreatment can compensate for the growth reduction caused by ZnO NPs.

Salicylic acid reduces cadmium (Cd) accumulation in rice (Oryza sativa L.) by regulating root cell wall composition via nitric oxide signaling

The effects of salicylic acid (SA) on cadmium (Cd) accumulation, Cd subcellular distribution, cell wall composition and Cd adsorption in Cd-stressed rice seedlings were examined. The interaction between SA and nitric oxide (NO) signaling in regulating cell wall composition under Cd exposure was also investigated. Our results showed that 5 μmol·L^-1 Cd treatment significantly decreased plant height, root length and plant dry weight by 40.1%, 46.1% and 21.3% (p < 0.05), respectively, and the inhibitory effects of Cd on the growth parameters were alleviated by exogenous SA. Application of SA remarkably decreased Cd concentrations in roots and shoots of rice seedlings by 48.0% and 19.6%, respectively, and increased the distribution ratio of Cd in the root cell wall fraction (from 35.7% to 40.6%) compared with Cd treatment alone. The reduced Cd accumulation in rice plants could be attributed to that SA application promoted pectin synthesis and demethylesterification, thereby increasing Cd deposition in the root cell wall. Moreover, SA application promoted lignin biosynthesis to strengthen the cell wall and prevent Cd from entering the root cells. In addition, NO might be involved in SA-induced pectin synthesis, pectin demethylesterification and lignin biosynthesis as a downstream signaling molecule, contributing to reduced Cd accumulation in Cd-stressed rice seedlings. The results provide deep insights into the mechanisms of exogenous SA action in reducing Cd accumulation in rice plants.

Salicylic acid alleviates chromium (VI) toxicity by restricting its uptake, improving photosynthesis and augmenting antioxidant defense in Solanum lycopersicum L

Contamination of agricultural soil by chromium (Cr) is a serious menace to environmental safety and global food security. Although potential of salicylic acid (SA) in mitigating heavy metal (HM) toxicity in plants is well recognized, detailed physiological mechanisms behind such beneficial effects under Cr-stress in tomato (Solanum lycopersicum L.) plant are far from being completely unravelled. The present study evaluated the efficacy of exogenously applied SA, in alleviating Cr-mediated alterations on photosynthesis and antioxidant defense in tomato exposed to three different concentrations of Cr(VI) [0, 50, and 100 mg Cr(VI) kg^-1 soil]. Exposure of tomato plants to Cr resulted in increased Cr-accumulation and oxidative damage, as signposted by high Cr concentration in root as well as shoot, augmented malondialdehyde (MDA) and superoxides levels, and inhibition in enzymes of ascorbate-glutathione (AsA-GSH) cycle. Furthermore, a significant (P ≤ 0.05) reduction in photosynthetic pigments and gas exchange parameters was also evident in Cr-stressed tomato plants. Findings of the present study showed that exogenous application of 0.5 mM SA not only promoted plant growth subjected to Cr, but also restored Cr-mediated disturbances in plant physiology. A significant (P ≤ 0.05) decrease in Cr acquisition and translocation as evidenced by improved growth and photosynthesis in SA-treated plants was observed. Additionally, exogenous SA application by virtue of its positive effect on efficient antioxidant system ameliorated the Cr-mediated oxidative stress in tomato plants as signposted by lower MDA and superoxide levels and improved AsA-GSH cycle. Overall, current study advocates the potential of exogenous SA application in amelioration of Cr-mediated physiological disturbances in tomato plant.

Phytohormones: Key players in the modulation of heavy metal stress tolerance in plants

Heavy metal (HM) stress in plants has received considerable global attention as it threatens sustainable growth in agriculture worldwide. Hence, desperate efforts have been undertaken for combating the effects of this stress in plants. Interestingly, the use of phytohormones in reducing the impact of HM toxicity has gained much momentum in the recent past. Phytohormones act as chemical messengers that improve the HM stress resistance in plants, thus allowing them to retain their growth and developmental plasticity. Their exogenous application as well as manipulation of endogenous levels through precise targeting of their biosynthesis/signaling components is a promising approach for providing a protective shield against HM stress in plants. However, for the successful use of phytohormones for field plants exposed to HM toxicity, in-depth knowledge of the key pathways regulated by them is of prime importance. Hence, the present review mainly summarizes the key conceptual developments on the involvement of phytohormones in the mitigation of HM stress in plants. The role of various genes, proteins, and signaling components involved in phytohormones associated HM stress tolerance and their modulation has also been discussed. Thus, this update will pave the way for improving HM stress tolerance in plants with the advent of phytohormones for sustainable agriculture growth in the future.

Gibberellic acid application on biomass, oxidative stress response, and photosynthesis in spinach (Spinacia oleracea L.) seedlings under copper stress

The mechanism of Cu tolerance in plants and its control measures are of considerable significance for the remediation of Cu-contaminated soils. Gibberellic acid (GA₃) is involved in plant growth and development and in the response to heavy metal stress. In the present study, changes in the biomass, oxidative stress response responses, and photosynthesis of spinach seedlings were examined under Cu stress with exogenous GA₃ applied at concentrations of 0, 3, 5, 10, 20, 40, 60, or 80 mg L^-1. Under Cu stress, the plant Cu concentration and oxidative damage were greater, photosynthetic parameters and biomass declined, and antioxidant enzyme activities and the proline concentration increased. However, spinach growth did not terminate, indicating that spinach seedlings had strong Cu tolerance. When low concentrations of GA₃ (3-5 mg L^-1) were added to Cu-stressed spinach seedlings, the damage caused by Cu stress to spinach seedlings was reduced, and the Cu tolerance of spinach seedlings was enhanced, which mainly manifested as reduced oxidation damage, an increased proline concentration, elevated antioxidant enzyme activities, decreased Cu concentration in leaves, and increased Cu concentration in roots, increased photosynthetic parameters, and an increased in the total biomass. In contrast, additions of GA₃ at concentrations higher than 40 mg L^-1 intensified oxidative damage and decreased the activities of antioxidant enzymes, photosynthetic parameters, and biomass. Additionally, the Cu concentration increased in leaves and decreased Cu concentration in roots, indicating that high concentrations of GA₃ aggravated stress damage and severely influenced physiological functions in spinach seedlings. In summary, the application of 3-5 mg L^-1 GA₃ to spinach seedlings in Cu-contaminated soil can be used to reduce Cu toxicity to plants and increase Cu tolerance.

Nitrate reductase is required for salicylic acid-induced water stress tolerance of pepper by upraising the AsA-GSH pathway and glyoxalase system

A trial was conducted to evaluate whether nitrate reductase (NR) participates in salicylic acid (SA)-improved water stress (WS) tolerance in pepper (Capsicum annuum L.) plants. Before starting WS treatment, 0.5 mM SA was applied to half of the well-watered (WW) plants as well as to WS-plants as a foliar spray once a day for a week. The soil water holding capacity was maintained at 40 and 80% of the full water storing capacity for WS and and well-watered (WW) plants, respectively. Water stress caused substantial decreases in total plant dry weight, F_v /F_m , chlorophyll a and b, relative water content, leaf water potential (ΨI) by 53, 37, 49, 21, 36 and 33%, respectively relative to control, but significant increases in malondialdehyde (MDA), hydrogen peroxide (H₂ O₂ ), electrolyte leakage (EL), methylglyoxal (MG), proline, key antioxidant enzymes' activities, NO and NR activity. The SA reduced oxidative stress, but improved antioxidant defence system, ascorbate-glutathione (AsA-GSH) cycle enzymes, glyoxalase system-related enzymes, glyoxalase I (Gly I) and glyoxalase II (Gly II), plant growth, photosynthetic traits, NO, NR and proline. SA-induced WS tolerance was further improved by supplementation of sodium nitroprusside (SNP), a donor of NO. NR inhibitor, sodium tungstate (ST) was applied in conjunction with SA and SA + SNP to the WW and WS-plants to assess whether NR contributes to SA-improved WS tolerance. ST abolished the beneficial effects of SA by reducing NO and NR activity in WS-pepper, but the application of SNP along with SA + ST reversed negative effects of ST, showing that NO and NR are jointly needed for SA-induced WS tolerance of pepper plants.

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.

Lemna minor, a hyperaccumulator shows elevated levels of Cd accumulation and genomic template stability in binary application of Cd and Ni: a physiological and genetic approach

In this study, to determine whether having potential to be used as hyperaccumulator for Cd and Ni, numerous experiments were designed for conducting assessments for physiological and genotoxic changes along with defining possible alterations on mineral nutrient status of Lemna minor L. by applying Cd-Ni binary treatments (0, 100, 200 and 400 µM). Our study revealed that there were increases in the concentrations of B, Cr, Fe, K, Mg, and Mn whereas decreases were noticed in the concentrations of Na and Zn and the levels of Ca were inversely proportional to Cd-Ni applications showing tendency to increase at the low concentration and to decrease at the high concentration. Randomly Amplified Polymorphic DNA (RAPD) and Inter Simple Sequence Repeat (ISSR) analyses revealed that rather than band losses and new band formations, mostly intensity changes in the band profiles, and low polymorphism and high genomic template stability (GTS) were observed. Although, to date, L. minor was defined as an efficient hyperaccumulator/potential accumulator or competent phytoremedial agent by researchers. Our research revealed that L. minor showing high accumulation capability for Cd and having low polymorphism rate and high genomic template stability is a versatile hyperaccumulator, especially for Cd; therefore, highly recommended by us for decontamination of water polluted with Cd. NOVELTY STATEMENTMany studies have been focused on the effects of individual metal ions. However, heavy metal contaminants usually exist as their mixtures in natural aquatic environments. Especially, Cd and Ni coexist in industrial wastes.In this study, the accumulation properties of Lemna minor for both Cd and Ni were investigated and the effects of Cd and Ni on the bioaccumulation of B, Ca, Cu, Fe, Mg, K, Mn, Na, Pb and Zn in L. minor were also determined. This study furthermore aimed to assess the genotoxic effects of Cd and Ni found in being extended concentrations on DNA using the Randomly Amplified Polymorphic DNA-Polymerase Chain Reaction (RAPD-PCR) method.

Identification, Phylogeny, and Comparative Expression of the Lipoxygenase Gene Family of the Aquatic Duckweed, Spirodela polyrhiza, during Growth and in Response to Methyl Jasmonate and Salt

Lipoxygenases (LOXs) (EC 1.13.11.12) catalyze the oxygenation of fatty acids and produce oxylipins, including the plant hormone jasmonic acid (JA) and its methyl ester, methyl jasmonate (MeJA). Little information is available about the LOX gene family in aquatic plants. We identified a novel LOX gene family comprising nine LOX genes in the aquatic plant Spirodela polyrhiza (greater duckweed). The reduced anatomy of S. polyrhiza did not lead to a reduction in LOX family genes. The 13-LOX subfamily, with seven genes, predominates, while the 9-LOX subfamily is reduced to two genes, an opposite trend from known LOX families of other plant species. As the 13-LOX subfamily is associated with the synthesis of JA/MeJA, its predominance in the Spirodela genome raises the possibility of a higher requirement for the hormone in the aquatic plant. JA-/MeJA-based feedback regulation during culture aging as well as the induction of LOX gene family members within 6 h of salt exposure are demonstrated.

Enhanced Cd accumulation by Graphene oxide (GO) under Cd stress in duckweed

Effective phytoremediation by aquatic plant such as duckweed could be applied to solve Cd pollution. In the present study, the impact of Graphene oxide (GO) on the accumulation of Cd in duckweed has been studied. The response of duckweed was also investigated, concluding growth, Cd²⁺ flux, and gene expression response. Results showed that GO promoted the accumulation of Cd in duckweed. After 6 h of Cd enrichment in duckweed, Cd content was about 1.4 times that of the control group at fronds and 1.25 times that of the control group at roots, meanwhile, Cd content in the water system was 0.67 times that of the control group. The Cd²⁺ influx increased significantly. 4471 genes were up-regulated and 3230 genes were down-regulated significantly as duckweed treated with GO under Cd treatment. Moreover, phagosome pathway was downregulated, some key proteins: Stx7, Rab7 and Tubastatin B (TUBB) were significantly downregulated with GO addition under Cd stress. Scanning electron microscope (SEM) observation showed that GO and Cd were attached on the cell surface of duckweed as white crystal. GO could be applied in phytoremediation by duckweed of Cd in aquatic system.

Salicylic acid mediated reduction in grain cadmium accumulation and amelioration of toxicity in Oryza sativa L. cv Bandana

Contamination of agricultural fields with Cadmium (Cd) due to several agricultural practices is increasing worldwide. The rice plants can easily take up Cd and accumulate it into different parts, including the grains, posing a threat to human health even at low concentration exposure. Several phytohormones, including Salicylic acid (SA) have been investigated since long for its alleviating properties under various biotic and abiotic stress conditions. In the present study, 100 μM SA application to ameliorate 25 μM Cd stress was studied for 72 h in hydroponics in Oryza sativa cv. Bandana seedlings. Pot experiments were done with same treatment condition and plants were grown till maturity. SA application to Cd exposed rice seedlings alleviated the stress condition, which was established by several physiological, biochemical, histochemical and gene expression analysis. SA treatment to Cd stressed seedlings showed elevated photosynthetic pigment content, on-protein thiol content and relieved the Cd induced growth inhibition considerably. It lowered the accumulation of ROS like, O2^- and H₂O₂ with a regulated antioxidative enzymatic activity. SA application in Cd exposed rice seedlings had upregulated expression of OsHMA3 and OsPCS1 whereasOsNRAMP2 gene was downregulated. Co-application of SA and Cd led to higher yield and improved agronomic traits in comparison to only Cd exposed plants under pot experimentation. Daily intake of Cd and Carcinogenic risk were also reduced by 99.75% and 99.99% respectively in the SA treated Cd stressed plants. SA positively affected the growth and tolerance of rice seedlings to Cd stress. Hence, SA addition to Cd contaminated soil can ensure rice cultivation without posing health risk to consumers.

A study on the mixture repairing effect of biochar and nano iron oxide on toxicity of Cd toward muskmelon

Soil contamination with cadmium (Cd) has become a serious problem, adversely affecting food safety and human health. Effective methods are urgently needed to alleviate toxicity of Cd in plants. In this study, a nine-week continuous pot experiments was conducted to explore the effectiveness of the different nano iron oxide (α-Fe₂O₃, γ-Fe₂O₃, Fe₃O₄) alone and combined with biochar in muskmelon grown on a Cd-contaminated soil. The antioxidant system, chlorophyll, soluble protein, other physiological indexes of muskmelon leaves and the distribution of Cd in matrix soil, leaves and fruit were detected. The results showed that Cd was readily absorbed by plants and caused oxidative stress on plants, while biochar, α-Fe₂O₃ nanoparticles (NPs) and their mixture group (BFe1 group) could significantly improve it. Specifically, the three treatments reduced the Cd content of the fruit by 19.51-78.86%, reduced the Cd content of leaves by 15.44-36.23% and 22.36-31.77% in weeks 3 and 5, respectively. For the activity of enzymes, three treatments decreased superoxide dismutase (SOD) activity and catalase (CAT) activity by 3.41-38.57% and 24.27-30.33% in week 7, respectively. So BFe1 group application immobilized Cd in soil and reduced Cd partitioning in the aboveground tissues. Overall the combination of biochar and α-Fe₂O₃ NPs can alleviate Cd toxicity in muskmelon and can protect human beings from Cd exposure.

Single and combined effects of the drugs salicylic acid and acetazolamide: Adverse changes in physiological parameters of the freshwater macrophyte, Lemna gibba

Pharmaceutical drugs are among the most used chemicals, for human and veterinary medicines, aquaculture and agriculture. Pharmaceuticals are biologically active molecules, having also environmental persistence, thereby exerting biological effects on non-target species. Among the most used pharmaceuticals, one may find salicylic acid (SA), a non-steroid anti-inflammatory drugs (NSAIDs), and acetazolamide (ACZ), a diuretic drug that acts by inhibiting the activity of carbonic anhydrase (CA). In this work, single and combined effects of SA and ACZ were assessed in the aquatic macrophyte Lemna gibba L., focusing on physiological parameters, namely photosynthetic pigments, (chlorophyll a, b and total (Chl a, b and TChl) as well as carotenoids (Car)). In addition, chemical biomarkers, namely, glutathione S-transferases (GSTs), catalase (CAT) and carbonic anhydrase (CA) activities, were also determined. The highest concentrations of ACZ, caused a decrease in the contents of all chlorophylls; this effect was however reverted by SA exposure. Both ACZ and SA levels caused a decrease in CA activity. Nevertheless, when in combination, this inhibition was not observed in plants exposed to the lowest concentration of these drugs. In conclusion, both pharmaceuticals have the capacity to cause alterations in L. gibba enzymatic activity and photosynthetic pigments content. Additionally, SA seems to exert a protective effect on this species against deleterious effects caused by ACZ.

Salicylic acid antagonizes selenium phytotoxicity in rice: selenium homeostasis, oxidative stress metabolism and methylglyoxal detoxification

We investigated the mechanistic consequences of selenium (Se)-toxicity, and its possible mitigation using salicylic acid (SA) in rice. In comparison with control, sodium selenate-exposed 'Se1' (0.5 mM) and 'Se2' (1.0 mM) plants showed accumulation of Se by 190.63 and 288.00 % in roots, 2359.42 and 2054.35 % in leaf sheaths, and 7869.91 and 9063.72 % in leaves, respectively, resulting in severe toxicity symptoms, such as growth inhibition, chlorosis, burning of leaves, and oxidative stress. In contrast, SA addition to Se-stressed plants significantly alleviated the Se-toxicity symptoms, and radically improved shoot height (28.88 %), dry biomass (34.00 %), total chlorophyll (37.51 %), soluble sugar (17.31 %) and leaf water contents (22.31 %) in 'SA + Se2' plants over 'Se2' plants. Notably, SA maintained Se-homeostasis, and decreased 'Se2'-induced oxidative stress by enhancing ascorbate level (67.75 %) and the activities of antioxidant enzymes like superoxide dismutase (20.99 %), catalase (40.97 %), glutathione peroxidase (12.26 %), and glutathione reductase (32.58 %) relative to that in 'Se2' plants. Additionally, SA protected rice plants from the deleterious effects of methylglyoxal by stimulating the activities of glyoxalase enzymes. Furthermore, SA upregulated several genes associated with reactive oxygen species (e.g. OsCuZnSOD1, OsCATB, OsGPX1 and OsAPX2) and methylglyoxal (e.g. OsGLYI-1) detoxifications. These findings unravel a decisive role of SA in alleviating Se-phytotoxicity in rice.

Phytohormones-induced senescence efficiently promotes the transport of cadmium from roots into shoots of plants: A novel strategy for strengthening of phytoremediation

Due to the long growth period of plants, phytoremediation is time costly. Improving the accumulation of cadmium (Cd) in shoots of plants will promote the efficiency of phytoremediation. In this study, two senescence-relative phytohormones, abscisic acid (ABA) and salicylic acid (SA), were applied to strengthening phytoremediation of Cd by tall fescue (Festuca arundinacea S.). Under hydroponic culture, phytohormones treatment increased the Cd content of shoots 11.4-fold over the control, reaching 316.3 mg/kg (dry weight). Phytohormones-induced senescence contributes to the transport of heavy metals, and HMA3 was found to play a key role in this process. Additionally, this strategy could strengthen the accumulation of Cu and Zn in tall fescue shoots. Moreover, in soil pot culture, the strategy increased shoot Cd contents 2.56-fold over the control in tall fescue, and 2.55-fold over the control in Indian mustard (Brassica juncea L.), indicating its comprehensive adaptability and potential use in the field. In summary, senescence-induced heavy metal transport is developed as a novel strategy to strengthen phytoremediation. The strategy could be applied at the end of phytoremediation with an additional short duration (7 days) with comprehensive adaptability, and markedly strengthen the phytoremediation in the field.

The role of endogenous nitric oxide in salicylic acid-induced up-regulation of ascorbate-glutathione cycle involved in salinity tolerance of pepper (Capsicum annuum L.) plants

An experimentation was carried out to appraise whether or not nitric oxide (NO) contributes to salicylic acid (SA)-induced salinity tolerance particularly by regulating ascorbate-glutathione (AsA-GSH) cycle. Before starting salinity stress (SS), SA (0.5 mM) was sprayed to the foliage of plants once every other day for a week and then seedlings were grown under control or SS (100 mM NaCl), for five weeks. Salinity stress enhanced the AsA-GSH cycle-related enzymes, glutathione reductase (GR), ascorbate peroxidase (APX), and dehydroascorbate reductase (DHAR), and monodehydroascorbate reductase (MDHAR). Furthermore, SS caused substantial decreases in plant physiological-related traits such as leaf potassium (K) contents, K⁺/Na⁺ ratio, the ratios of reduced ascorbate/dehydroascorbic acid (AsA/DHA) and reduced glutathione/oxidized glutathione (GSH/GSSG), but in contrast, significant increases occurred in leaf hydrogen peroxide, malondialdehyde, electron leakage, proline, the premier antioxidant enzymes' activities, Na⁺ and NO. SA reduced leaf Na⁺ content and oxidative stress-related traits, but improved all earlier-mentioned traits compared with those in plants treated with SS alone. All positive effects of SA were eliminated by NO scavenger, 0.1 mM 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1- oxyl-3-oxide (c-PTIO) by reducing NO, suggesting that NO produced by SA up-regulated the activities of AsA-GSH cycle and antioxidant enzymes, so it could play a central function as a signal molecule in salt tolerance of pepper plants.

The Role of Salicylic Acid in Plants Exposed to Heavy Metals

Salicylic acid (SA) is a very simple phenolic compound (a C₇H₆O₃ compound composed of an aromatic ring, one carboxylic and a hydroxyl group) and this simplicity contrasts with its high versatility and the involvement of SA in several plant processes either in optimal conditions or in plants facing environmental cues, including heavy metal (HM) stress. Nowadays, a huge body of evidence has unveiled that SA plays a pivotal role as plant growth regulator and influences intra- and inter-plant communication attributable to its methyl ester form, methyl salicylate, which is highly volatile. Under stress, including HM stress, SA interacts with other plant hormones (e.g., auxins, abscisic acid, gibberellin) and promotes the stimulation of antioxidant compounds and enzymes thereby alerting HM-treated plants and helping in counteracting HM stress. The present literature survey reviews recent literature concerning the roles of SA in plants suffering from HM stress with the aim of providing a comprehensive picture about SA and HM, in order to orientate the direction of future research on this topic.

The Role of Salicylic Acid in Plants Exposed to Heavy Metals

Salicylic acid (SA) is a very simple phenolic compound (a C7H6O3 compound composed of an aromatic ring, one carboxylic and a hydroxyl group) and this simplicity contrasts with its high versatility and the involvement of SA in several plant processes either in optimal conditions or in plants facing environmental cues, including heavy metal (HM) stress. Nowadays, a huge body of evidence has unveiled that SA plays a pivotal role as plant growth regulator and influences intra- and inter-plant communication attributable to its methyl ester form, methyl salicylate, which is highly volatile. Under stress, including HM stress, SA interacts with other plant hormones (e.g., auxins, abscisic acid, gibberellin) and promotes the stimulation of antioxidant compounds and enzymes thereby alerting HM-treated plants and helping in counteracting HM stress. The present literature survey reviews recent literature concerning the roles of SA in plants suffering from HM stress with the aim of providing a comprehensive picture about SA and HM, in order to orientate the direction of future research on this topic.

Interactive Effects of Salicylic Acid and Nitric Oxide in Enhancing Rice Tolerance to Cadmium Stress

Cadmium (Cd) is one of the prominent environmental hazards, affecting plant productivity and posing human health risks worldwide. Although salicylic acid (SA) and nitric oxide (NO) are known to have stress mitigating roles, little was explored on how they work together against Cd-toxicity in rice. This study evaluated the individual and combined effects of SA and sodium nitroprusside (SNP), a precursor of NO, on Cd-stress tolerance in rice. Results revealed that Cd at toxic concentrations caused rice biomass reduction, which was linked to enhanced accumulation of Cd in roots and leaves, reduced photosynthetic pigment contents, and decreased leaf water status. Cd also potentiated its phytotoxicity by triggering reactive oxygen species (ROS) generation and depleting several non-enzymatic and enzymatic components in rice leaves. In contrast, SA and/or SNP supplementation with Cd resulted in growth recovery, as evidenced by greater biomass content, improved leaf water content, and protection of photosynthetic pigments. These signaling molecules were particularly effective in restricting Cd uptake and accumulation, with the highest effect being observed in “SA + SNP + Cd” plants. SA and/or SNP alleviated Cd-induced oxidative damage by reducing ROS accumulation and malondialdehyde production through the maintenance of ascorbate and glutathione levels, and redox status, as well as the better activities of antioxidant enzymes superoxide dismutase, catalase, glutathione S-transferase, and monodehydroascorbate reductase. Combined effects of SA and SNP were observed to be more prominent in Cd-stress mitigation than the individual effects of SA followed by that of SNP, suggesting that SA and NO in combination more efficiently boosted physiological and biochemical responses to alleviate Cd-toxicity than either SA or NO alone. This finding signifies a cooperative action of SA and NO in mitigating Cd-induced adverse effects in rice, and perhaps in other crop plants.

Declined cadmium accumulation in Na+/H+ antiporter (NHX1) transgenic duckweed under cadmium stress

Cadmium (Cd) is a serious threat to plants health. Though some genes have been reported to get involved in the regulation of tolerance to Cd, the mechanisms underlying this process are not fully understood. Na⁺/H⁺ antiporter (NHX1) plays an important role in Na⁺/H⁺ trafficking. The salt and cadmium stress tolerance were found to be enhanced by NHX1 in duckweed according to our previous study, however, its function in Cd²⁺ flux under Cd stress has not been studied. Here we explored the Cd²⁺ flux in wild type (WT) and NHX1 transgenic duckweed (NHX1) under Cd stress. We found that the Cd²⁺ influx in NHX1 duckweed was significantly declined, followed by an increased Cd²⁺ efflux after 20 min treatment of Cd, which resulted a less accumulation of Cd in NHX1. Reversely, inhibition of NHX1 by amiloride treatment, enhanced Cd²⁺ influx in NHX1 duckweed, subsequently delayed Cd²⁺ efflux in both genotypes of duckweed under Cd²⁺ shock. H⁺ efflux in NHX1 duckweed was lower compare with that in WT with 20 min Cd²⁺ shock. NHX1 also increased the pH value with Cd²⁺ stress in the transgenic rhizoid. These finding suggested a new function of NHX1 in regulation of Cd²⁺ and H⁺ flow during short-term Cd²⁺ shock.

The co-expression of circRNA and mRNA in the thymuses of chickens exposed to ammonia

Ammonia (NH₃) is one of major air pollutants in intensive poultry houses, affecting chicken health. Circular RNA (circRNA) is a novel type of RNA that can regulate gene expression and be associated with various biological activities. However, the changes of circRNA caused by excess NH₃ in chickens have not been investigated. We found differentially expressed genes and morphological changes in the thymuses of chickens exposed to NH₃ on day 42. We used a combination of RNA deep sequencing, qRT-PCR, and bioinformatic analysis to explore regulatory mechanism of circRNA and mRNA. Transcriptional profiling results showed that 5 circRNA genes and 100 mRNA genes were significantly dyregulated by high NH₃. The results from GO items showed that immune response and the regulation of cytokine production were involved in the mechanisms of chickens exposed to NH₃. Co-expression analysis found that circRNA-mRNA network was correlated with oxidative stress and inflammation. NH₃ exposure decreased mRNA expression of antioxidant-related genes (GPx and GST4) and increased the mRNA expression of inflammation-related genes (IL-1β, IL-6, IL-8, and iNOS) in chicken thymuses. Histopathologic analysis demonstrated that NH₃ caused inflammatory injury in chicken thymuses. In conclusion, the co-expression of circRNA and mRNA took part in chicken thymus inflammatory injury caused by NH₃. Our study further enriches the mechanism of NH₃ toxicity on chickens, which may be valuable for human and animal health protection.

Improved cadmium resistance and removal capacity in Pichia kudriavzevii A16 by sucrose preincubation

Removal of heavy metals from food material by growing micro-organisms is limited by the toxicity to cells. In this study, different preincubation treatments were investigated to analyze their effects on cadmium resistance and removal ability of Pichia kudriavzevii A16 and Saccharomyces cerevisiae CICC1211. Sucrose preincubation improved the cadmium resistance of both yeast cells and increased the cadmium-removal rate of P. kudriavzevii A16. An evident decrease of intracellular and cell-surface cadmium accumulation was observed after sucrose preincubation, which may be the primary reason responsible for the improved cadmium resistance. Flow cytometry assay showed that sucrose significantly reduced the production of reactive oxygen species (ROS) and cell death rate of both yeasts under cadmium compared with those normally cultured cells. Under cadmium stress, the content of both protein carbonyls and malonyldialdehyde were also reduced by the addition of sucrose, the results were in accordance with the tendency of ROS, exhibiting a defending function of sucrose. Osmotic regulators as proline and trehalose were increased by sucrose preincubation in P. kudriavzevii A16 in the presence of cadmium. The results suggested that sucrose preincubation could be applied to improve cadmium resistance and removal rate of yeasts.

Exogenous application of ascorbic acid mitigates cadmium toxicity and uptake in Maize (Zea mays L.)

Cadmium (Cd) contamination in agricultural soils is a prevalent environmental issue and poses potential threats to food security. Foliar ascorbic acid might prove a potent tool to alleviate toxicity of Cd toxicity in maize. An experiment was conducted with objectives to study exogenous ascorbic acid-modulated improvements in physiochemical attributes of maize under Cd toxicity. The experiment was conducted under completely randomized design. Treatments were comprised of varying concentrations of foliar ascorbic acid viz. 0.0, 0.1, 0.3, and 0.5 mM of AsA. Toxicity of Cd decreased the maize growth, increased lipid peroxidation, disturbed protein metabolism, and reduced the antioxidant defense capabilities compared with the control. However, foliar AsA significantly improved maize growth and development, photosynthetic capabilities, and protein concentrations in Cd-stressed maize plants. Meanwhile, the malondialdehyde contents and hydrogen peroxide accumulation levels in Cd-stressed maize plants decreased remarkably with increasing AsA concentrations. Furthermore, the combined treatments conspicuously boosted activities of superoxide dismutase, peroxidase, catalase, and glutathione reductase under the Cd stress alone. In addition, the application of AsA reduced the Cd uptake by 10.3-12.3% in grains. Conclusively, foliar ascorbic acid alleviated the negative effects of Cd stress in maize and improved photosynthetic processes, osmolytes, and antioxidant defense systems.

Salicylic Acid Signals Plant Defence against Cadmium Toxicity

Salicylic acid (SA), as an enigmatic signalling molecule in plants, has been intensively studied to elucidate its role in defence against biotic and abiotic stresses. This review focuses on recent research on the role of the SA signalling pathway in regulating cadmium (Cd) tolerance in plants under various SA exposure methods, including pre-soaking, hydroponic exposure, and spraying. Pretreatment with appropriate levels of SA showed a mitigating effect on Cd damage, whereas an excessive dose of exogenous SA aggravated the toxic effects of Cd. SA signalling mechanisms are mainly associated with modification of reactive oxygen species (ROS) levels in plant tissues. Then, ROS, as second messengers, regulate a series of physiological and genetic adaptive responses, including remodelling cell wall construction, balancing the uptake of Cd and other ions, refining the antioxidant defence system, and regulating photosynthesis, glutathione synthesis and senescence. These findings together elucidate the expanding role of SA in phytotoxicology.

The hypersensitive induced reaction 3 (HIR3) gene contributes to plant basal resistance via an EDS1 and salicylic acid-dependent pathway

The hypersensitive-induced reaction (HIR) gene family is associated with the hypersensitive response (HR) that is a part of the plant defense system against bacterial and fungal pathogens. The involvement of HIR genes in response to viral pathogens has not yet been studied. We now report that the HIR3 genes of Nicotiana benthamiana and Oryza sativa (rice) were upregulated following rice stripe virus (RSV) infection. Silencing of HIR3s in N. benthamiana resulted in an increased accumulation of RSV RNAs, whereas overexpression of HIR3s in N. benthamiana or rice reduced the expression of RSV RNAs and decreased symptom severity, while also conferring resistance to Turnip mosaic virus, Potato virus X, and the bacterial pathogens Pseudomonas syringae and Xanthomonas oryzae. Silencing of HIR3 genes in N. benthamiana reduced the content of salicylic acid (SA) and was accompanied by the downregulated expression of genes in the SA pathway. Transient expression of the two HIR3 gene homologs from N. benthamiana or the rice HIR3 gene in N. benthamiana leaves caused cell death and an accumulation of SA, but did not do so in EDS1-silenced plants or in plants expressing NahG. The results indicate that HIR3 contributes to plant basal resistance via an EDS1- and SA-dependent pathway.

Foliar application of salicylic acid alleviate the cadmium toxicity by modulation the reactive oxygen species in potato

Heavy metal toxicity is one of the main factors that limit crop growth and yield in the world. Salicylic acid (SA) is thought to be a plant hormone that plays an important role in plant growth, development, and resistance to abiotic stresses. To uncover the toxic alleviation effects of SA on potato plants to cadmium (Cd) stress, the morphological, physiological, and biochemical indexes including antioxidant defense system were assayed in potato plants under 200 μM Cd stress in 1/2 Hoagland solution with foliar application of 600 μM SA concentration (10 ml/plant). Interestingly, exogenous SA treatment mitigated Cd toxicity by increasing the relative water content (RWC), chlorophyll, proline, and endogenous SA contents along with decline in malondialdehyde (MDA), hydrogen peroxide (H₂O₂), and superoxide anion radicals (O₂^-). Correspondingly, our study also proved that SA may stimulate the antioxidant enzymatic mechanism pathway including superoxide dismutase (SOD, EC 1.15.1.1), catalase (CAT, EC 1.11.1.6), ascorbate peroxidase (APX, EC 1.11.1.11), and glutathione reductase (GR, EC 1.6.4.2) in potato plants subjected to Cd stress. Moreover, the expression level of selected genes relate to SA and reactive oxygen species (ROS) metabolism (StSABP2, StSOD and StAPX) were enhanced in SA-treated potato plants under Cd stress, indicating that SA treatment regulated the expression of these genes, which in turn enhanced potato tolerance to Cd stress. Taken together, our results indicated that exogenous SA can play a positive regulatory role in alleviating Cd toxicity in potato plants.

Duckweed biomarkers for identifying toxic water contaminants?

Surface or ground waters can be contaminated with numerous toxic substances. The duckweeds Lemna minor and Lemna gibba are widely used for assaying waterborne toxicity to higher plants in terms of growth inhibition and photosynthetic pigment reduction. These tests cannot, however, in themselves determine the nature of the agents responsible for toxicity. Morphological, developmental, physiological, biochemical, and genetic responses of duckweeds to exposure to toxic water contaminants constitute biomarkers of toxic effect. In principle, the very detection of these biomarkers should enable the contaminants having elicited them (and being responsible for the toxicity) to be identified. However, in practice, this is severely compromised by insufficient specificity of biomarkers for their corresponding toxicants and by the lack of documentation of biomarker/toxin relationships. The present contribution illustrates the difficulties of using known water contaminant-related duckweed biomarkers to identify toxins, and discusses possibilities for achieving this goal.

Application of common duckweed (Lemna minor) in phytoremediation of chemicals in the environment: State and future perspective

Over the past 50 years, different strategies have been developed for the remediation of polluted air, land and water. Driven by public opinion and regulatory bottlenecks, ecological based strategies are preferable than conventional methods in the treatments of chemical effluents. Ecological systems with the application of microbes, fungi, earthworms, plants, enzymes, electrode and nanoparticles have been applied to varying degrees in different media for the remediation of various categories of pollutants. Aquatic macrophytes have been used extensively for the remediation of pollutants in wastewater effluents and aquatic environment over the past 30 years with the common duckweed (L. minor) as one of the most effective macrophytes that have been applied for remediation studies. Duckweed has shown strong potentials for the phytoremediation of organic pollutants, heavy metals, agrochemicals, pharmaceuticals and personal care products, radioactive waste, nanomaterials, petroleum hydrocarbons, dyes, toxins, and related pollutants. This review covers the state of duckweed application for the remediation of diverse aquatic pollutants and identifies gaps that are necessary for further studies as we find pragmatic and sound ecological solutions for the remediation of polluted environment for sustainable development.

Effects of copper on the growth, antioxidant enzymes and photosynthesis of spinach seedlings

Examination of plants with strong Cu tolerance and an understanding of their Cu-tolerance mechanisms are of considerable significance for the remediation of Cu-contaminated soil. Although spinach may be a plant with strong Cu tolerance, the threshold of Cu tolerance in this plant and its physiological response mechanisms to Cu are still unclear. In this study, we examined that the effects of different Cu concentrations on the growth parameters, antioxidant enzyme activities, and photosynthesis of spinach seedlings. The results showed that when treated with a low Cu concentration (100 mg L^-1 CuSO₄), the biomass of spinach seedlings increased, whereas the MDA content, the activities of antioxidant enzymes, P_n, g_s and T_r were not significantly different from those in the control (P > 0.05), and Y(II), qP reached their maximum values, indicating that a low Cu concentration (100 mg L^-1 CuSO₄) had minimal negative effects on the life activities of spinach seedlings. In contrast, when treated with high Cu concentrations (800-1000 mg L^-1 CuSO₄), the total biomass of spinach seedlings was markedly decreased, the MDA contents increased, antioxidant enzyme activities initially increased and then decreased to varying degrees, the contents of chlorophyll, P_n, T_r, Fv/Fm, qP_, NPQ, and Y(II) were all decreased. However the growth of spinach did not terminate, implying that the lethal threshold concentration of Cu for spinach is greater than 1000 mg L^-1 CuSO₄ used in this study. In summary, spinach exhibits a high tolerance to Cu and can be considered as an alternative plant for the remediation of Cu-contaminated soils.

Salicylic acid and jasmonic acid restrains nickel toxicity by ameliorating antioxidant defense system in shoots of metallicolous and non-metallicolous Alyssum inflatum Náyr. Populations

The presence of heavy metals in the soils is undoubtedly one of the prime abiotic stresses in the world. There are a considerable amount of plant yield losses because of heavy metal stress. The goal of this study was to assess the morphological, physiological and biochemical changes in Alyssum inflatum Nyár. populations upon exposure to different levels of nickel (Ni) (0, 100, 200, 400) μM, salicylic acid (SA) (0, 50, 200) μM and jasmonic acid (JA) (0, 5, 10) μM. Results showed that there were no considerable interpopulation differences, including the shoot Ni concentrations. Reversing the effects of Ni, SA and JA decreased due to Ni accumulation in both populations. By increasing the levels of Ni stress, the fresh weight (FW) of shoot decreased, whereas the application of SA + JA elevated the FW of the shoot in NM plants. Also, SA + JA mitigated Ni oxidative effects by reducing H₂O₂ concentration in both populations. The results revealed that the exposure of both M and NM plants to high Ni concentration increased superoxide dismutase (SOD), peroxidase (POD), catalase (CAT) and ascorbate peroxidase (APX) activities compared to control in both populations. Conversely, APX activity was inhibited in NM plants. Furthermore, SA and JA treatments reversed the detrimental effects of Ni on carotenoid content and reduced the content of proline in plants exposed to Ni stress. All the above suggests that SA and JA confer tolerance to Ni stress in two population of A. inflatum via several mechanisms.