Effects of uranium stress on physiological and biochemical characteristics in seedlings of six common edible vegetables

Does exposure to weathered coal ash with an enhanced content of uranium-series radionuclides affect flora? Changes in the physiological indicators of five referent plant species

Coal ash deposited in open landfills is a potential source of environmental pollutants due to the contained toxic element content. The weathered coal ash used in this study additionally contains enhanced activity concentrations of ²³⁸U series radionuclides. This study aimed to determine the physiological effects of enhanced ionizing radiation and toxic elements on five plant species (smilo grass, sticky fleabane, blackberry, mastic and pine tree) inhabiting the coal ash disposal site. Among the potentially toxic measured elements, contents of Sb, As and especially V significantly exceeded their respective levels at the control site, as well as the content of ²³⁸U and its progenies. Significant changes in photosynthetic pigments were recorded following chronic exposure to the plants growing on the coal ash site. Different responses were also observed in the plant species regarding the activity of catalase and glutathione-S-transferase (GST). The level of lipid peroxidation markedly increased in plants from the disposal site, except in blackberry, wherein GST activity was the strongest, indicating an important role of that enzyme in the adaptation to coal ash pollutants. The results of this study suggest that the modulation of the studied biochemical parameters in plants growing on coal ash is primarily species-dependent.

Metal chelating and anti-radical activity of Salvia officinalis in the ameliorative effects against uranium toxicity

Uranium is a highly radioactive heavy metal that is toxic to living things. In this study, physiological, cytogenetic, biochemical and anatomical toxicity caused by uranium and the protective role of sage (Salvia officinalis L.) leaf extract against this toxicity were investigated with the help of Allium test. Germination percentage, root length, weight gain, mitotic index (MI), micronucleus (MN) formation, chromosomal aberrations (CAs), superoxide dismutase (SOD) and catalase (CAT) enzyme activities, malondialdehyde (MDA) levels and changes in root meristem cells were used as indicators of toxicity. In the experimental stage, a total of six groups, one of which was the control, were formed. Group I was treated with tap water, while group II and III were treated only with sage (190 mg/L and 380 mg/L). Groups IV, V and VI were germinated with uranyl acetate dihydrate (0.1 mg/mL), uranyl acetate dihydrate + 190 mg/L sage and uranyl acetate dihydrate + 380 mg/L sage, respectively. Allium cepa L. bulbs of each group were germinated for 72 h, and at the end of the period, routine preparation techniques were applied and physiological, cytogenetic, biochemical and anatomical analyzes were performed. As a result, uranium application caused a significant decrease (p < 0.05) in all physiological parameters and MI values. MN, CAs numbers, SOD and CAT enzyme activities and MDA levels increased significantly (p < 0.05) with uranium application. Uranium promoted CAs in the root tip cells in the form of fragment, vagrant chromosome, sticky chromosome, bridge and unequal distribution of chromatin. In addition, it caused anatomical damages such as epidermis cell damage, cortex cell damage and flattened cell nucleus in root tip meristem cells. Sage application together with uranium caused significant (p < 0.05) increases in physiological parameters and MI values and significant decreases in MN, CAs, SOD and CAT activities and MDA levels. In addition, the application of sage resulted in improvement in the severity of anatomical damages induced by uranium. It was determined that the protective role of sage observed for all parameters investigated was even more pronounced at dose of 380 mg/L. The protective role of sage against uranium toxicity is related to its antioxidant activity, and sage has 82.8% metal chelating and 72.9% DPPH removal activity. As a result, uranyl acetate exhibited versatile toxicity in A. cepa, caused cytotoxicity by decreasing the MI rate, and genotoxicity by increasing the frequencies of MN and CAs. And also, Sage acted as a toxicity-reducing agent by displaying a dose-dependent protective role against the toxic effects induced by uranyl acetate.

Calcium affects uranium responses in Arabidopsis thaliana: From distribution to toxicity

Uranium, a heavy metal and primordial radionuclide, is present in surface waters and soils both naturally and due to industrial activities. Uranium is known to be toxic to plants and its uptake and toxicity can be influenced by multiple factors such as pH and the presence of different ions. However, the precise role of the different ions in uranium uptake is not yet known. Here we investigated whether calcium influences uranium uptake and toxicity in the terrestrial plant Arabidopsis thaliana. To this end, A. thaliana plants were exposed to different calcium and uranium concentrations and furthermore, calcium channels were blocked using the calcium channel blocker lanthanum chloride (LaCl₃). Fresh weight, relative growth rate, concentration of nutrients and uranium and gene expression of oxidative stress-related genes and calcium transporters were determined in roots and shoots. Calcium affected plant growth and oxidative stress in both control (no uranium) and uranium-exposed plants. In shoots, this was influenced by the total calcium concentration, but not by the different tested uranium concentrations. Uranium in turn did influence calcium uptake and distribution. Uranium-exposed plants grown in a medium with a higher calcium concentration showed an increase in gene expression of NADPH oxidases RBOHC and RBOHE and calcium transporter CAX7 after uranium exposure. In roots, these calcium-dependent responses in gene expression were not observed. This indicates that calcium indeed affects uranium toxicity, but only in shoots. In addition, a clear influence of uranium and LaCl₃ (separately and combined) on the expression of calcium transporters was observed.

Effects of U on the growth, reactive oxygen metabolism and osmotic regulation in radish (Raphanus sativus L.)

Uranium (U) is a non-essential and toxic element, so it is necessary to study the physiological mechanism of plant response to U stress. The present study evaluated the growth status, reactive oxygen metabolism and osmotic regulation system in radish (Raphanus sativus) under U stress (0, 25, 50 and 100 μM). The results showed that U had no significant effect on the germination of radish seeds but inhibited the growth of seedlings, such as reduced root activity and increased plasma membrane permeability. U is mainly distributed in radish roots, so it poisons the roots more than the aboveground parts. When U concentration was 25 μM, superoxide dismutase (SOD), catalase (CAT) and peroxidase (POD) activities in radish were increased to cope with the oxidative stress caused by U stress, and the accumulation of proline and soluble sugar was increased to maintain cell turgor. However, under high concentration (100 μM), the damage of radish root was serious; thus, the SOD, CAT and soluble sugar could not respond to U stress. In conclusion, the identification and characterization of U-stress responses in genuine U-tolerant plants would improve our knowledge on the detoxification of this radionuclide.

Chelating Agents in Assisting Phytoremediation of Uranium-Contaminated Soils: A Review

Massive stockpiles of uranium (U) mine tailings have resulted in soil contamination with U. Plants for soil remediation have low extraction efficiency of U. Chelating agents can mobilize U in soils and, hence, enhance phytoextraction of U from the soil. However, the rapid mobilization rate of soil U by chelating agents in a short period than plant uptake rate could increase the risk of groundwater contamination with soluble U leaching down the soil profile. This review summarizes recent progresses in synthesis and application of chelating agents for assisting phytoremediation of U-contaminated soils. In detail, the interactions between chelating agents and U ions are initially elucidated. Subsequently, the mechanisms of phytoextraction and effectiveness of different chelating agents for phytoremediation of U-contaminated soils are given. Moreover, the potential risks associated with chelating agents are discussed. Finally, the synthesis and application of slow-release chelating agents for slowing down metal mobilization in soils are presented. The application of slow-release chelating agents for enhancing phytoextraction of soil U is still scarce. Hence, we propose the preparation of slow-release biodegradable chelating agents, which can control the release speed of chelating agent into the soil in order to match the mobilization rate of soil U with plant uptake rate, while diminishing the risk of residual chelating agent leaching to groundwater.

Uranium (U) source, speciation, uptake, toxicity and bioremediation strategies in soil-plant system: A review

Uranium(U), a highly toxic radionuclide, is becoming a great threat to soil health development, as returning nuclear waste containing U into the soil systems is increased. Numerous studies have focused on: i) tracing the source in U contaminated soils; ii) exploring U geochemistry; and iii) assessing U phyto-uptake and its toxicity to plants. Yet, there are few literature reviews that systematically summarized the U in soil-plant system in past decade. Thus, we present its source, geochemical behavior, uptake, toxicity, detoxification, and bioremediation strategies based on available data, especially published from 2018 to 2021. In this review, we examine processes that can lead to the soil U contamination, indicating that mining activities are currently the main sources. We discuss the relationship between U bioavailability in the soil-plant system and soil conditions including redox potential, soil pH, organic matter, and microorganisms. We then review the soil-plant transfer of U, finding that U mainly accumulates in roots with a quite limited translocation. However, plants such as willow, water lily, and sesban are reported to translocate high U levels from roots to aerial parts. Indeed, U does not possess any identified biological role, but provokes numerous deleterious effects such as reducing seed germination, inhibiting plant growth, depressing photosynthesis, interfering with nutrient uptake, as well as oxidative damage and genotoxicity. Yet, plants tolerate U toxicity via various defense strategies including antioxidant enzymes, compartmentalization, and phytochelatin. Moreover, we review two biological remediation strategies for U-contaminated soil: (i) phytoremediation and (ii) microbial remediation. They are quite low-cost and eco-friendly compared with traditional physical or chemical remediation technologies. Finally, we conclude some promising research challenges regarding U biogeochemical behavior in soil-plant systems. This review, thus, further indicates that the combined application of U low accumulators and microbial inoculants may be an effective strategy for the bioremediation of U-contaminated soils.

The response of the accumulator plants Noccaea caerulescens, Noccaea goesingense and Plantago major towards the uranium

Uranium (U) is a naturally occurring metal; its environmental levels can be increased due to processes in the nuclear industry and fertilizer production. The transfer of U in the food chain from plants is associated with deleterious chemical and radiation effects. To date, limited information is available about U toxicity on plant physiology. This study investigates the responses of metal-accumulating plants to different concentrations of U. The plants Noccaea caerulescens and Noccaea goesingense are known as metal hyperaccumulators and therefore could serve as candidates for the phytoremediation of radioactive hotspots; Plantago major is a widely used pharmaceutical plant that pioneers polluted grounds and therefore should not contain high concentrations of toxic elements. The experimental plants were grown hydroponically at U concentrations between 1 μM and 10 mM. The content of U and essential elements was analyzed in roots and leaves by ICP-MS. The amount of accumulated U was influenced by its concentration in the hydroponics. Roots contained most of the metal, whereas less was transported up to the leaves, with the exception of N. goesingense in a medium concentration of U. U also influenced the nutrient profile of the plants. We localized the U in plant tissues using EDX in the SEM. U was evenly distributed in roots and leaves of Noccaea species, with one exception in the roots of N. goesingense, where the central cylinder contained more U than the cortex. The toxicity of U was assessed by measuring growth and photosynthetic parameters. While root biomass of N. caerulescens was not affected by U, root biomass of N. goesingense decreased significantly at high U concentrations of 0.1 and 10 mM and root biomass of P. major decreased at 10 mM U. Dry weight of leaves was decreased at different U concentrations in the three plant species; a promotive effect was observed in N. caerulescens at lowest concentration offered. Chlorophyll a fluorescence was not affected or negatively affected by U in both Noccaea species, whereas in Plantago also positive effects were observed. Our results show that the impact of U on Plantago and Noccaea relates to its external concentration and to the plant species. When growing in contaminated areas, P. major should not be used for medicinal purpose. Noccaea species and P. major could immobilize U in their rhizosphere in hotspots contaminated by U, and they could extract limited amounts of U into their leaves.

Unraveling response mechanism of photosynthetic metabolism and respiratory metabolism to uranium-exposure in Vicia faba

As a natural radionuclide, uranium (U) has obvious phytotoxicity, the purpose of this study is to unravel the response mechanism of U on photosynthetic and respiratory metabolism in plants. Therefore, 14-day-old Vicia faba seedlings were exposed to 0-25 μM U during 72 h. U effects on growth parameters, physiological parameters of plants, and potential phytotoxicity mechanism were investigated by physiological analysis, and metabolome and transcriptome data. U significantly inhibited photosynthesis and respiration of plants. In metabolome analysis, 53 metabolites related to carbohydrate metabolism were identified (13 up-regulated, 12 down-regulated). In transcriptome analysis, U significantly inhibited the expression of photoreactive electron transport chain (up: 0; down: 31), Calvin cycle (up: 0; down: 12) and photorespiration pathway genes (up: 0; down: 8). U significantly inhibited the expression of cellular energy metabolic pathways genes (e.g., glycolysis, TCA cycle, and oxidative phosphorylation pathways) (up 8, down 18). We concluded that U inhibited the expression of genes involved in the photosynthetic metabolic pathway, which caused the decrease of photosynthetic rate. Meanwhile, U inhibited the expression of the electron transport chain genes in the mitochondrial oxidative phosphorylation pathway, which leads to the abnormal energy supply of cells and the inhibition of root respiration rate.

Advances on the toxicity of uranium to different organisms

The extensive application of radioactive element uranium (U) and its compounds in the nuclear industry has significantly increased the risk of exposure to the environment. Therefore, research on the safety risks and toxicity mechanisms of U exposure has received increasing attention. This paper reviews the toxic effects of U on different species under different conditions, and summarizes the potential toxicity mechanisms. Under the exposure of U, reactive oxygen species (ROS) produced in cells will damage membrane structure in cells, and inhibit respiratory chain reaction by reducing the production of NADH and ATP. It also induce the expression of apoptosis factors such as Bcl-2, Bid, Bax, and caspase family to cause apoptosis cascade reaction, leading to DNA degradation and cell death. We innovatively list some methods to reduce the toxicity of U because some microorganisms can precipitate uranyl ions through biomineralization or reduction processes. Our work provides a solid foundation for further risk assessment of U.