The pH strongly influences the uranium-induced effects on the photosynthetic apparatus of Arabidopsis thaliana plants

Mitigation of uranium toxicity in rice by Sphingopyxis sp. YF1: Evidence from growth, ultrastructure, subcellular distribution, and physiological characteristics

Uranium (U) contamination of rice is an urgent ecological and agricultural problem whose effective alleviation is in great demand. Sphingopyxis genus has been shown to remediate heavy metal-contaminated soils. Rare research delves into the mitigation of uranium (U) toxicity to rice by Sphingopyxis genus. In this study, we exposed rice seedlings for 7 days at U concentrations of 0, 10, 20, 40, and 80 mg L-1 with or without the Sphingopyxis sp. YF1 in the rice nutrient solution. Here, we firstly found YF1 colonized on the root of rice seedlings, significantly mitigated the growth inhibition, and counteracted the chlorophyll content reduction in leaves induced by U. When treated with 1.1 × 107 CFU mL-1 YF1 with the amendment of 10 mg L-1 U, the decrease of U accumulation in rice seedling roots and shoots was the largest among all treatments; reduced by 39.3% and 32.1%, respectively. This was associated with the redistribution of the U proportions in different organelle parts, leading to the alleviation of the U damage to the morphology and structure of rice root. Interestingly, we found YF1 significantly weakens the expression of antioxidant enzymes genes (CuZnSOD,CATA,POD), promotes the up-regulation of metal-transporters genes (OsHMA3 and OsHMA2), and reduces the lipid peroxidation damage induced by U in rice seedlings. In summary, YF1 is a plant-probiotic with potential applications for U-contaminated rice, benefiting producers and consumers.

The plasma membrane-associated cation-binding protein PCaP1 of Arabidopsis thaliana is a uranyl-binding protein

Uranium (U) is a naturally-occurring radionuclide that is toxic to living organisms. Given that proteins are primary targets of U(VI), their identification is an essential step towards understanding the mechanisms of radionuclide toxicity, and possibly detoxification. Here, we implemented a chromatographic strategy including immobilized metal affinity chromatography to trap protein targets of uranyl in Arabidopsis thaliana. This procedure allowed the identification of 38 uranyl-binding proteins (UraBPs) from root and shoot extracts. Among them, UraBP25, previously identified as plasma membrane-associated cation-binding protein 1 (PCaP1), was further characterized as a protein interacting in vitro with U(VI) and other metals using spectroscopic and structural approaches, and in planta through analyses of the fate of U(VI) in Arabidopsis lines with altered PCaP1 gene expression. Our results showed that recombinant PCaP1 binds U(VI) in vitro with affinity in the nM range, as well as Cu(II) and Fe(III) in high proportions, and that Ca(II) competes with U(VI) for binding. U(VI) induces PCaP1 oligomerization through binding at the monomer interface, at both the N-terminal structured domain and the C-terminal flexible region. Finally, U(VI) translocation in Arabidopsis shoots was affected in pcap1 null-mutant, suggesting a role for this protein in ion trafficking in planta.

Ultrafast laser filament-induced fluorescence for detecting uranium stress in Chlamydomonas reinhardtii

Plants and other photosynthetic organisms have been suggested as potential pervasive biosensors for nuclear nonproliferation monitoring. We demonstrate that ultrafast laser filament-induced fluorescence of chlorophyll in the green alga Chlamydomonas reinhardtii is a promising method for remote, in-field detection of stress from exposure to nuclear materials. This method holds an advantage over broad-area surveillance, such as solar-induced fluorescence monitoring, when targeting excitation of a specific plant would improve the detectability, for example when local biota density is low. After exposing C. reinhardtii to uranium, we find that the concentration of chlorophyll a, chlorophyll fluorescence lifetime, and carotenoid content increase. The increased fluorescence lifetime signifies a decrease in non-photochemical quenching. The simultaneous increase in carotenoid content implies oxidative stress, further confirmed by the production of radical oxygen species evidence in the steady-state absorption spectrum. This is potentially a unique signature of uranium, as previous work finds that heavy metal stress generally increases non-photochemical quenching. We identify the temporal profile of the chlorophyll fluorescence to be a distinguishing feature between uranium-exposed and unexposed algae. Discrimination of uranium-exposed samples is possible at a distance of [Formula: see text]35 m with a single laser shot and a modest collection system, as determined through a combination of experiment and simulation of distance-scaled uncertainty in discriminating the temporal profiles. Illustrating the potential for remote detection, detection over 125 m would require 100 laser shots, commensurate with the detection time on the order of 1 s.

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.

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.

Uranium accumulation and its phytotoxicity symptoms in Pisum sativum L

Environmental contamination by uranium (U) and other radionuclides is a serious problem worldwide, especially due to, e.g. mining activities. Ultimate accumulation of released U in aquatic systems and soils represent an escalating problem for all living organisms. In order to investigate U uptake and its toxic effects on Pisum sativum L., pea plantlets were hydroponically grown and treated with different concentrations of U. Five days after exposure to 25 and 50 μM U, P. sativum roots accumulated 2327.5 and 5559.16 mg kg-1 of U, respectively, while in shoots concentrations were 11.16 and 12.16 mg kg-1, respectively. Plants exposed to both U concentrations showed reduced biomass of shoots and reduced content of photosynthetic pigments (total chlorophyll and carotenoids) relative to control. As a biomarker of oxidative stress, lipid peroxidation (LPO) levels were determined, while antioxidative response was determined by catalase (CAT) and glutathione reductase (GR) activities as well as cysteine (Cys) and non-protein thiol (NP-SH) concentrations, both in roots and shoots. Both U treatments significantly increased LPO levels in roots and shoots, with the highest level recorded at 50 μM U, 50.38% in shoots and 59.9% in roots relative to control. U treatment reduced GR activity in shoots, while CAT activity was increased only in roots upon treatment with 25 μM U. In pea roots, cysteine content was significantly increased upon treatment with both U concentrations, for 19.8 and 25.5%, respectively, compared to control plants, while NP-SH content was not affected by the applied U. This study showed significant impact of U on biomass production and biochemical markers of phytotoxicity in P. sativum, indicating presence of oxidative stress and cellular redox imbalance in roots and shoots. Obtained tissue-specific response to U treatment showed higher sensitivity of shoots compared to roots. Much higher accumulation of U in pea roots compared to shoots implies potential role of this species in phytoremediation process.

Development of a metalloproteomic approach to analyse the response of Arabidopsis cells to uranium stress

Elaboration of a top-down proteomic, biochemical and ionoproteomic toolbox to gain insights into the impact of uranyl (U) on Arabidopsis cells.

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.

Calcium in Carbonate Water Facilitates the Transport of U(VI) in Brassica juncea Roots and Enables Root-to-Shoot Translocation

The role of calcium (Ca) on the cellular distribution of U(VI) in Brassica juncea roots and root-to-shoot translocation was investigated using hydroponic experiments, microscopy, and spectroscopy. Uranium accumulated mainly in the roots (727-9376 mg kg-1) after 30 days of exposure to 80 μM dissolved U in water containing 1 mM HCO3 - at different Ca concentrations (0-6 mM) at pH 7.5. However, the concentration of U in the shoots increased 22 times in experiments with 6 mM Ca compared to 0 mM Ca. In the Ca control experiment, transmission electron microscopy-energy-dispersive spectroscopy analyses detected U-P-bearing precipitates in the cortical apoplast of parenchyma cells. In experiments with 0.3 mM Ca, U-P-bearing precipitates were detected in the cortical apoplast and the bordered pits of xylem cells. In experiments with 6 mM Ca, U-P-bearing precipitates aggregated in the xylem with no apoplastic precipitation. These results indicate that Ca in carbonate water inhibits the transport and precipitation of U in the root cortical apoplast and facilitates the symplastic transport and translocation toward shoots. These findings reveal the considerable role of Ca in the presence of carbonate in facilitating the transport of U in plants and present new insights for future assessment and phytoremediation strategies.

Effect of Calcium on the Bioavailability of Dissolved Uranium(VI) in Plant Roots under Circumneutral pH

We integrated field measurements, hydroponic experiments, microscopy, and spectroscopy to investigate the effect of Ca(II) on dissolved U(VI) uptake by plants in 1 mM HCO₃^- solutions at circumneutral pH. The accumulation of U in plants (3.1-21.3 mg kg^-1) from the stream bank of the Rio Paguate, Jackpile Mine, New Mexico served as a motivation for this study. Brassica juncea was the model plant used for the laboratory experiments conducted over a range of U (30-700 μg L^-1) and Ca (0-240 mg L^-1) concentrations. The initial U uptake followed pseudo-second-order kinetics. The initial U uptake rate ( V₀) ranged from 4.4 to 62 μg g^-1 h^-1 in experiments with no added Ca and from 0.73 to 2.07 μg g^-1 h^-1 in experiments with 12 mg L^-1 Ca. No measurable U uptake over time was detected for experiments with 240 mg L^-1 Ca. Ternary Ca-U-CO₃ complexes may affect the decrease in U bioavailability observed in this study. Elemental X-ray mapping using scanning transmission electron microscopy-energy-dispersive spectrometry detected U-P-bearing precipitates within root cell walls in water free of Ca. These results suggest that root interactions with Ca and carbonate in solution affect the bioavailability of U in plants. This study contributes relevant information to applications related to U transport and remediation of contaminated sites.

Arabidopsis thaliana plants challenged with uranium reveal new insights into iron and phosphate homeostasis

Uranium (U) is a naturally occurring radionuclide that is toxic to plants. It is known to interfere with phosphate nutrition and to modify the expression of iron (Fe)-responsive genes. The transporters involved in the uptake of U from the environment are unknown. Here, we addressed whether IRT1, a high-affinity Fe²⁺ transporter, could contribute to U uptake in Arabidopsis thaliana. An irt1 null mutant was grown hydroponically in different conditions of Fe bioavailability and phosphate supply, and challenged with uranyl. Several physiological parameters (fitness, photosynthesis) were measured to evaluate the response to U treatment. We found that IRT1 is not a major route for U uptake in our experimental conditions. However, the analysis of irt1 indicated that uranyl interferes with Fe and phosphate homeostasis at different levels. In phosphate-sufficient conditions, the absence of the cation chelator EDTA in the medium has drastic consequences on the physiology of irt1, with important symptoms of Fe deficiency in chloroplasts. These effects are counterbalanced by U, probably because the radionuclide competes with Fe for complexation with phosphate and thus releases active Fe for metabolic and biogenic processes. Our study reveals that challenging plants with U is useful to decipher the complex interplay between Fe and phosphate.

Thorium impact on tobacco root transcriptome

Thorium is natural actinide metal with potential use in nuclear energetics. Contamination by thorium, originated from mining activities or spills, represents environmental risk due to its radioactivity and chemical toxicity. A promising approach for cleaning of contaminated areas is phytoremediation, which need to be based, however, on detail understanding of the thorium effects on plants. In this study we investigated transcriptomic response of tobacco roots exposed to 200μM thorium for one week. Thorium application resulted in up-regulation of 152 and down-regulation of 100 genes (p-value <0.01, fold change ≥2). The stimulated genes were involved in components of jasmonic acid and salicylic acid signaling pathways and various abiotic (e.g. oxidative stress) and biotic stress (e.g. pathogens, wounding) responsive genes. Further, up-regulation of phosphate starvation genes and down-regulation of genes involved in phytic acid biosynthesis indicated that thorium disturbed phosphate uptake or signaling. Also expression of iron responsive genes was influenced. Negative regulation of several aquaporins indicated disturbance of water homeostasis. Genes potentially involved in thorium transport could be zinc-induced facilitator ZIF2, plant cadmium resistance PCR2, and ABC transporter ABCG40. This study provides the first insight at the processes in plants exposed to thorium.

The functional role of the photosynthetic apparatus in the recovery of Brassica napus plants from pre-emergent metazachlor exposure

Metazachlor is a chloroacetamide herbicide, frequently used in Brassica napus cultivations around the world. Its primary target is the inhibition of very long chain fatty acid biosynthesis. This study included a morphological and physiological screening of hydroponically grown B. napus, exposed to a concentration range of 0, 0.25, 0.50, 0.75 and 1.0kg metazachlor per hectare. The results indicate that within a month after application, growth and development of B. napus are severely affected by low metazachlor doses. At intermediate metazachlor concentrations, loss of phosphorous and potassium from the plant tissues suggests destabilisation of cellular membranes, which may be a direct consequence of metazachlor application. This membrane instability could be indirectly linked with alterations of electron transport and a reduction of carbon assimilation. At increased metazachlor doses of 0.75kga.i.ha(-1), pigment concentrations are strongly reduced. However, chlorophyll fluorescence parameters seem to remain unaffected at metazachlor doses up to 0.75kga.i.ha(-1). At a metazachlor concentration of 1.0kga.i.ha(-1), negative effects are observed on all tested parameters, resulting in limited survival. The results indicate photosynthesis is assured at intermediate metazachlor concentrations for the cost of growth and development. It is clear that photosynthesis plays a key role in the survival strategy of young plants to overcome initially induced chemical stress.