Insight into mechanism of lanthanum (III) induced damage to plant photosynthesis

Impact of Nickel Toxicity on Growth, Fruit Quality and Antioxidant Response in Zucchini Squash (Cucurbita pepo L.)

The impact of trace metal elements (TMEs) on plants is one current pollution problem, the severity of which is increasing with industrial development, population growth and inappropriate agricultural practices. The latter can have irreversible effects on ecosystems, including species extinction, trophic chain contamination and altered human health, particularly in the case of consumed plants such as zucchini squash (Cucurbita pepo L.). This study aims to investigate the effects of nickel on various physiological and biochemical parameters of zucchini growth, with a particular focus on how this toxic metal impacts the quality of fruit that is consumed by humans. To achieve this, plants aged 45 days were grown for one month on solid media loaded with different concentrations of Ni (0, 100, 300 and 500 µM). The results showed that exposure of plants to Ni resulted in significantly altered growth and higher accumulation of Ni in the shoots (1314 µg·g-1 DW) than in roots and fruits. Concerning non-enzymatic antioxidants, the results showed that Ni toxicity significantly increased total polyphenols, especially in shoots at 300 µM Ni, while flavonoid content decreased in the roots and shoots in response to Ni treatment. Our results also show that nickel tolerance in C. pepo is ensured by a combination of several mechanisms such as an increase in the content of proline. This species can survive and tolerate, to different degrees, toxic cations at concentrations up to 500 µM but with visible symptoms of toxicity such as chlorosis of the leaves. Indeed, based on thresholds of hyperaccumulation, we can qualify Cucurbita pepo as a hyperaccumulator species of nickel.

Addressing lanthanum toxicity in plants: Sources, uptake, accumulation, and mitigation strategies

Lanthanum (La), the second most abundant rare earth element (REE) is emerging as an environmental issue, with the potential to impact ecosystems and human health. Major sources of soil contamination by La include agricultural, and industrial activities. Lanthanum is non-essential for plant growth but accumulates in various plant parts. The uptake of La by plants is intricately influenced by various factors such as soil pH, redox potential, cation exchange capacity, presence of organic acids and rhizosphere composition. These factors significantly impact the availability and absorption of La ions. Lanthanum impact on plants depends on soil characteristics, cultivated species, developmental stage, La concentration, treatment period, and growth conditions. Excessive La concentrations affect cell division, DNA structure, nutrient uptake, and photosynthesis and induce toxicity symptoms. Plants employ detoxification mechanisms like vacuolar sequestration, osmolyte synthesis, and antioxidant defense system. However, higher concentrations of La can overwhelm these defense mechanisms, leading to adverse effects on plant growth and development. Further, accumulation of La in plants increases the risk for human exposure. Strategies to mitigate La toxicity are, therefore, vital for ecosystem protection. The application of phytoremediation, supplementation, chelation, amendments, and biosorption techniques contributes to the mitigation of La toxicity. This review provides insights into La sources, uptake, toxicity, and alleviation strategies in plants. Identifying research gaps and discussing advancements aims to foster a holistic understanding and develop effective strategies for protecting plant health and ecosystem resilience against La contamination.

The mixed blessings of rare earth element supplements for tomatoes and ferns

Rare earth elements (REEs) constitute a key group of critical minerals that are strategic for the global low-carbon economy and several United Nations Sustainable Development Goals. Their expected escalating emissions into the environment from emerging anthropogenic sources can negatively affect natural ecosystems. However, their hormetic effects make these elements effective fertilizers to promote crop production. Here, we investigate the response of tomatoes and ferns to REE exposure (La, Gd, Yb). While ferns were unresponsive to REEs, these elements promote evident benefits in tomatoes, e.g., elevating nutrient uptake, higher photosynthetic capacity and phytohormone enhancement to allocate energy to green tissue and root development. Nevertheless, the non-selective cation uptake incurs risks of accumulating non-essential elements in edible tissues. These evident benefits of REEs on crops support applications in agricultural production systems, create added value to the global distribution and promote better material flow management of REEs as strategic and critical resources.

Toxic effects of lanthanum(III) on photosynthetic performance of rice seedlings: Combined chlorophyll fluorescence, chloroplast structure and thylakoid membrane protein assessment

Rare earth elements (REEs) are emerging as an anticipated pollution in the environment due to their active use in many areas. However, the effects of REEs on the photosynthesis of rice have not been thoroughly explored. Therefore, this study emphasizes how high levels of La(III) affect the thylakoid membrane of rice seedlings, thereby inhibiting photosynthesis and growth. Here, we reported that rice plants treated with La(III) exhibited an increase in La accumulation in the leaves, accompanied by a decrease in chlorophyll content and photosynthetic capacity. La(III) exposure decreased Mg content in leaves, but possibly increased other nutrients including Cu, Mn, and Zn through systemic endocytosis. K-band and L-band appeared in the fluorescence OJIP transients, indicating La(III) stress destroyed the donor and receptor sides of photosystem II (PSII). Numerous reaction centers (RC/CSm) were inactivated by La(III) treatment, which resulted in a reduction in electron transport capacity (decreased ETo/RC and ETo/CSm) and an increase in the dissipation of the excess excitation energy by heat (increased DIo/RC and DIo/CSm). The BN-PAGE analysis of thylakoid membrane protein complexes showed that La(III) induced the degradation of supercomplexes, PSII core, LHCII, PSI core, LHCI, and F1-ATPase binding Cyt b6f complex. Collectively, this study revealed that La(III) causes significant degradation of thylakoid membrane proteins, thereby promoting the decomposition of photosynthetic complexes, ultimately destroying the chloroplast structure and reducing the photosynthetic performance of rice seedlings.

Molecular insights into rare earth element (REE)-mediated phytotoxicity and its impact on human health

Rare earth elements (REEs) that include 15 lanthanides, scandium, and yttrium are a special class of elements due to their remarkable qualities such as magnetism, corrosion resistance, luminescence, and electroconductivity. Over the last few decades, the implication of REEs in agriculture has increased substantially, which was driven by rare earth element (REE)-based fertilizers to increase crop growth and yield. REEs regulate different physiological processes by modulating the cellular Ca2+ level, chlorophyll activities, and photosynthetic rate, promote the protective role of cell membranes, and increase the plant's ability to withstand various stresses and other environmental factors. However, the use of REEs in agriculture is not always beneficial because REEs regulate plant growth and development in dose-dependent manner and excessive usage of them negatively affects plants and agricultural yield. Moreover, increasing applications of REEs together with technological advancement is also a rising concern as they adversely impact all living organisms and disturb different ecosystems. Several animals, plants, microbes, and aquatic and terrestrial organisms are subject to acute and long-term ecotoxicological impacts of various REEs. This concise overview of REEs' phytotoxic effects and implications on human health offers a context for continuing to sew fabric scraps to this incomplete quilt's many layers and colors. This review deals with the applications of REEs in different fields, specifically agriculture, the molecular basis of REE-mediated phytotoxicity, and the consequences for human health.

Lanthanum Supplementation Alleviates Tomato Root Growth Suppression under Low Light Stress

Supplementation with rare earth elements (REEs) such as lanthanum and cerium has been shown to promote plant elongation and/or increase crop yields. On the other hand, there are reports that REE supplementation of plants has no such effect. The appropriate modes for REE utilization and the underlying mechanism are not fully understood. In this study, we investigated how REE supplementation of plants under low light stress affects plant growth and gene expression. Under low light stress conditions, tomato root elongation was observed to be reduced by about half. This suppression of root elongation was found to be considerably alleviated by 20 mM lanthanum ion supplementation. This effect was plant-species-dependent and nutrient-condition-dependent. Under low light stress, the expression of the genes for phytochrome-interacting factor, which induces auxin synthesis, and several auxin-synthesis-related proteins were markedly upregulated by lanthanum ion supplementation. Thus, we speculate that REE supplementation of plants results in auxin-induced cell elongation and alleviates growth suppression under stress conditions.

The microalgae-based wastewater treatment system coupled with Cerium: A potential way for energy saving and microalgae boost

The microalgae-based system attracts more attention in wastewater treatment for high quality effluent, low carbon emission, and resource utilization. Light is the key factor for algae growth, but the light masking in sewage will cause low efficiency of the system. This study designed laboratory scale experiments with Chlorella to investigate the influence of cerium on the nutrient removal by algae wastewater treatment system under different light intensities. The best removal rates of NH₄-N, TP, and COD were 72.43%, 88.87%, and 68.08% under 50 µmol/(m ²·s) light intensity and 1 mg/L Ce. Low concentration of Ce could activate protein synthesis, electron transfer, and antioxidase, while excessive Ce might cause toxicity which could be relieved by strong light for energy supply and further activating superoxide dismutase (SOD) and catalase (CAT). Comparing to other similar experiences, this system reached an equal or greater performance on nutrients removal with better efficiency in light utilization. It might provide a new idea for microalgae-based system development.

Phytotoxicity effect concentrations (ECx) for Ce, Nd and Eu added to soil relative to total and bioaccessible soil REE concentrations, and tissue REE accumulations

Toxicity thresholds (EC_x) for radish, tomato, and durum wheat growth endpoints (shoot length, shoot mass, root length) to Ce, Nd or Eu added to a black organic soil were determined from 14-day dose-response growth assays. EC₁₀ expressed as total soil [REE] had a more than twenty-fold range, from 337 mg/kg to more than >8000 mg/kg. Averaged over all REEs and endpoints, durum wheat was more tolerant than radish and tomato; and averaged over all endpoints, Eu appeared to be the most phytotoxic of the three REEs. Bioaccessibility of each REE was determined by extraction with 0.01 M CaCl₂, which for all three REEs in this soil was quite low, <0.10% of total. However, bioaccessibility of Eu was five or six times greater than that for Ce and Nd, and thus could explain its apparently greater toxicity, namely that Eu was more likely to be accumulated at the site of toxic action in the plant. To discern inherent toxicity from enhanced bioaccumulation, concentration of each REE in root and shoot tissues was determined, for a tissue-residue approach to toxicity assessment. The EC₁₀ expressed as tissue concentration was lower for Nd than for Ce and Eu, thus the most toxic of the three REEs. As for many cationic inorganic elements, toxicity varies with the chemistry of the exposure medium due to its effects on bioaccessibility. Simple methods to harmonize toxicity thresholds from different media enables greater integration into regulatory standards. When EC₂₅ from this and other studies were normalized to CaCl₂-extractable REE in their respective media, the range in Ce EC₂₅ was reduced from 20-fold to 2.5-fold, and the range for Eu EC₂₅ was reduced from 25-fold to 3-fold. This novel and low-input approach to meta-analysis of toxicity thresholds demonstrates the value of considering soil physico-chemical properties as modifiers of soil REE toxicity.

Synthesis of two different zinc oxide nanoflowers and comparison of antioxidant and photocatalytic activity

Nanoflowers are a newly developed class of nanoparticles that show flower-like structures and attract much attention due to their simple preparation methods, high stability, and increased efficiency. The aim of the study is to investigate a strong alternative to reduce the severity of infection and increase the treatment of wastewater by exhibiting biofilm inhibition in medical and environmental applications of the ZnO-NFs with two different shapes. ZnO-NFs were synthesized by two different processes hydrothermal method (named ZnO-NF1) and the precipitation method (named ZnO-NF2). ZnO-NFs produced by two different synthesis methods were compared for the photocatalytic and antioxidant efficiency. The effects of Reactive Red 180 (RR180) and Basic Red 18 (BR18) dyes concentration, photocatalyst amount, and reaction time were investigated on dye removal efficiency for photocatalytic experiments. The color was completely removed for 25 mg/L BR18 and RR180 dyes for 75 min and 90 min, respectively, using 1.5 g/L photocatalyst amount using ZnO-NF1. However, 59.18% dye removal efficiency was obtained for 90 min by using a 1.5 g/L ZnO-NF2 photocatalyst for 25 mg/L BR18 dye removal, while a dye removal efficiency of 90.00% was detected for 90 min using 2 g/L ZnO-NF2 for 25 mg/L RR180 dye. Then, comparison of general properties such as antibacterial, antibiofilm, microbial cell viability, DNA fragmentation, antioxidant activities, and antimicrobial photodynamic therapy of ZnO-NFs were investigated. The antioxidant activity of ZnO-NF2 was found to be higher than ZnO-NF1 at each concentration (82.32% and 87.18% for ZnO-NF1 and ZnO-NF2, respectively, at 200 mg/mL).

Assessment of Tolerance to Lanthanum and Cerium in Helianthus Annuus Plant: Effect on Growth, Mineral Nutrition, and Secondary Metabolism

Rare earth elements (REEs) present a group of nonessential metals for the growth and development of plants. At high concentrations, they can induce internal stress and disturb the physiological and biochemical mechanisms in plants. The potential uptake of lanthanum (La) and cerium (Ce) by the horticultural plant Helianthus annuus and the effect of these elements on its growth, its absorption of macroelements, and the contents of phenolic compounds and flavonoids were assessed. The plants were exposed to 0, 1, 2.5, 5, and 10 µM of La and Ce for 14 days. The results showed a remarkable accumulation of the two REEs, especially in the roots, which was found to be positively correlated with the total phenolic compound and flavonoid content in the plant shoots and roots. The plant's growth parameter patterns (such as dry weight and water content); the levels of potassium, calcium, and magnesium; and the tolerance index varied with the concentrations of the two studied elements. According to the tolerance index values, H. annuus had more affinity to La than to Ce. Although these metals were accumulated in H. annuus tissues, this Asteraceae plant cannot be considered as a hyperaccumulator species of these two REEs, since the obtained REE content in the plant's upper parts was less than 1000 mg·Kg^-1 DW.

Physiological and Biochemical Behaviours and Antioxidant Response of Helianthus annuus under Lanthanum and Cerium Stress

The continuous progress of global manufacturing and anthropogenic activities has resulted in excessive environmental metallic pollution, particularly with rare earth elements (REEs) which have become a prevalent issue of global concern due to their high toxicity and widespread existence. REEs-contaminated soils could ruin agriculture by inducing plant physiology disturbances in various crops that are considered the principal link of the human food chain. The main purpose of the present work is to assess the phytotoxicity of two light REEs, lanthanum (La) and cerium (Ce), in Helianthus annuus after 14 days of exposure to different concentrations of La and Ce (0, 1, 2.5, 5, and 10 µM). Plants showed different variations in shoot and root lengths at the end of the trial period. The accumulation of photosynthetic pigments, such as chlorophylls and carotenoids, as well as the photosynthetic efficiency, the non-photochemical quenching, the photosynthetically active radiation, and the electron transport rate, increased in the two REE treatments. Hydrogen peroxide significantly increased in all applied concentrations of La and Ce. A significant increase in malondialdehyde content was noticed only when plants were exposed to 2.5 µM La and 10 µM Ce. Results also demonstrated that La and Ce induced an increase in the activity of superoxide dismutase, peroxidase, and catalase (only the highest concentration of La decreased catalase activity). The exposure to different REE concentrations induced the accumulation of La and Ce in the plants, mainly in roots. Helianthus annuus showed an effective resistance behaviour facing La- and Ce-induced stresses.

A new mechanism by which environmental hazardous substances enhance their toxicities to plants

The coexistence of hazardous substances enhances their toxicities to plants, but its mechanism is still unclear due to the unknown cytochemical behavior of hazardous substance in plants. In this study, by using interdisciplinary methods, we observed the cytochemical behavior of coexisting hazardous substances {terbium [Tb(III)], benzo(a)pyrene (BaP) and cadmium [Cd(II)] in environments} in plants and thus identified a new mechanism by which coexisting hazardous substances in environments enhance their toxicities to plants. First, Tb(III) at environmental exposure level (1.70 × 10^-10 g/L) breaks the inert rule of clathrin-mediated endocytosis (CME) in leaf cells. Specifically, Tb(III) binds to its receptor [FASCICLIN-like arabinogalactan protein 17 (FLA17)] on the plasma membrane of leaf cells and then docks to an intracellular adaptor protein [adaptor protein 2 (AP2)] to form ternary complex [Tb(III)-FLA17-AP2], which finally initiates CME pathway in leaf cells. Second, coexisting Tb(III), BaP and Cd(II) in environments are simultaneously transported into leaf cells via Tb(III)-initiated CME pathway, leading to the accumulation of them in leaf cells. Finally, these accumulated hazardous substances simultaneously poison plant leaf cells. These results provide theoretical and experimental bases for elucidating the mechanisms of hazardous substances in environments poisoning plants, evaluating their risks, and protecting ecosystems.

Experimental and Theoretical Study of the Effects of Rare Earth Elements on Growth and Chlorophyll of Alfalfa (Medicago sativa L.) Seedling

Rare earth elements (REEs) of low concentration are usually beneficial to plant growth, while they are toxic at high concentrations. The effects of treatment with lanthanum (La) (10 and 20 μM), cerium (Ce) (10 and 20 μM), and terbium (Tb) (10 and 20 μM) on seedling growth of alfalfa (Medicago sativa L.), which is one of the most important perennial leguminous forages in the world, were studied. The results showed that all three REE treatments quickened the germination of seeds. The length of shoot under La (20 μM) treatment was significantly shortened (P < 0.05). In addition, treatment with La, Ce, and Tb had a "hormesis effect" on root length. There was a significant decrease in chlorophyll content on treatment with the three REEs, and the degree of decline was in the order of La < Ce < Tb, under the same concentration. In vitro experiments and quantum chemical calculations were further performed to explain why the treatments with REEs reduced the chlorophyll content. In vitro experiments showed that La, Ce, and Tb treatments reduced the absorbance of chlorophyll, and the decrease followed in the order of La > Ce > Tb. Quantum chemical calculations predicted that the decrease in absorption intensity was caused by the reactions between La, Ce, Tb, and chlorophyll, which formed lanthanides-chlorophyll; and there were five types of stable lanthanides-chlorophyll. In conclusion, the decrease in chlorophyll content on treatment with REEs was caused by the change in chlorophyll structure.

Effect of nanomolar concentrations of lanthanum on Desmodesmus quadricauda cultivated under environmentally relevant conditions

Toxicity of lanthanides is generally regarded as low, and they even have been suggested to be beneficial at low concentrations. This research was conducted to investigate effects of Lanthanum (La) on Desmodesmus quadricauda, a freshwater green microalga. The algal cultures were treated with nanomolar La concentrations under controlled environmentally relevant conditions. Intracellular localization of La was analyzed with μXRF tomography in frozen-hydrated samples. At sublethal concentration (128 nM) La was in hotspots inside the cells, while at lethal 1387 nM that led to release of other ions (K, Zn) from the cells, La filled most of the cells. La had no clear positive effects on growth or photosynthetic parameters, but increasing concentrations led to a dramatic decrease in cell counts. Chlorophyll fluorescence kinetic measurements showed that La led to the inhibition of photosynthesis. Maximal photochemical quantum yield of the PSII reaction center in dark-adapted state (F_v/F_m) decreased at > 4.3 nM La during the 2nd week of treatment. Minimum dark-adapted fluorescence quantum yield (F₀) increased at > 13.5 nM La during the 2nd week of treatment except for control (0.2 nM La, baseline from chemicals) and 0.3 nM La. NPQ at the beginning of the actinic light phase showed significant increase for all the treatments. Metalloproteomics by HPLC-ICPMS showed that La binds to a >500 kDa soluble protein complex already in the sub-nM range of La treatments, in the low nM range to a small-sized (3 kDa) soluble peptide, and at >100 nM La additionally binds to a 1.5 kDa ligand.

La, Ce and Nd in the soil-plant system in a vegetation experiment with barley (Hordeum vulgare L.)

Rare earth elements (REEs) have received enormous attention in recent years. However, there are many gaps in the understanding of their behavior in the soil-plant system. The aim of this study is to investigate the behavior of three most common REEs (La, Ce, Nd) in the soil-plant system directly on soil samples using barley (Hordeum vulgare L.) in a vegetation experiment. We attribute the absence of significant changes in plant biomass and photosynthetic pigment content to the reduced availability of REEs in soil samples. The concentration of water-soluble forms of La, Ce and Nd didn't exceed 1 mg/kg, while the concentration of exchangeable forms varied and decreased in a row La > Ce > Nd. The transfer factor (TF) from soil to above-ground biomass was low for all three elements (<1). The stem-to-leaf TF increased with the increase in REEs concentration in soil. The concentration in plant material increased in the row Ce < Nd < La. REEs concentrations in barley leaves didn't exceed 1-3% of the corresponding element concentration in soil samples. REEs concentration in plant tissues is in close direct correlation with the REEs total concentration in soil, water-soluble and exchange forms. REEs concentration in barley leaves is 3-4 times higher than in the stems and for the group with extraneous concentration of 200 mg/kg for La, Ce and Nd was 6.20 ± 1.48, 2.10 ± 0.51, 6.90 ± 3.00 mg/kg, respectively. We show that there were no major changes in barley plants, but further study is needed of the relationship between the absorption of lanthanides by plants and the content of various forms of lanthanides in the soil.

Effects of exogenous lanthanum(III) exposure on the positive interaction between mutually beneficial populations

Rare earth elements (REEs) are widely used in various fields, and their accumulation has been reported to pose environmental risks. Most studies confirmed the damage of excessive REE exposure to individual plants; however, little attention has been given to their effects on plant populations. A positive interaction indicates a mutually beneficial relationship between two populations, which is beneficial to the survival and growth of the populations. However, it remains unknown whether exogenous REEs affect the positive interactions between populations. This study investigated the effects of exogenous lanthanum(III) [La(III)] exposure on the positive interaction between soybean (Glycine max L.) and wheat (Triticum aestivum L.) populations by their modules. At normal nutrient level (½-strength Hoagland), the inhibition of excessive La(III) on population modules decreased with increasing population density. Decreases of 39.26 to 1.05% for soybean and 41.45 to 2.41% for wheat indicated the inhibition of La(III) on the positive interaction of both populations weakened with increasing population density. At low nutrient level (¼-strength Hoagland), the inhibition of excessive La(III) on population modules increased with increasing population density. Decreases of 5.82-57.14% for soybean and 4.22-59.04% for wheat indicated the inhibition of La(III) on the positive interaction of both population was strengthened with increasing population density. In summary, the inhibitory effects of exogenous La(III) exposure on the positive interaction between populations vary with both nutrient level and population density. This is a new factor that needs to be considered when evaluating the safety risks of REEs in the environment.

A living plant cell-based biosensor for real-time monitoring invisible damage of plant cells under heavy metal stress

Heavy metals inevitably cause invisible or visible damage to plants, leading to significant economic losses. Therefore, it is necessary to develop a method for timely monitoring the damage of plants under the stress of heavy metals. Here, vitronectin-like proteins (VN) on the surface of plant cells is as an important biomarker for monitoring damage of plants under the stress of heavy metals. A living plant cell-based biosensor is constructed to monitor invisible damage of plant cells induced by cadmium [Cd(II)] or lead [Pb(II)]. To fabricate this sensor, l-cysteine was first modified on the glassy carbon electrode followed by the modification of anti-IgG-Au antibody. Then, the living plant cells, incubated with the anti-VN, were modified onto the electrode. The sensor worked by determining the change in electrochemical impedance. Cd(II) and Pb(II) was detected in the linear dynamic range of 45-210 and 120-360 μmol·L^-1, respectively. And the detection limit of Cd(II) and Pb(II) of this biosensor was 18.5 nmol·L^-1 [with confidence interval (95%) 18.4-18.6 nmol·L^-1] and 25.6 nmol·L^-1 [with confidence interval (95%) 25.4-25.8 nmol·L^-1], respectively. In both Arabidopsis and soybean, when the content of VN increased by about 20 times under the stress of Cd(II) or Pb(II), which means when the electron-transfer resistance increased by 35%, chlorophyll content showed significant decrease about 17%. Therefore, by establishing a quantitative relationship among the content of biomarker, the electron-transfer resistance and chlorophyll content in plant cells, the invisible damage of plants under the stress of heavy metals was detected. These results can provide a reference method for early-onset warning systems for heavy metal pollution in the environment.

The effect of lanthanum (III) on the activity of xylanase by Penicillium and its influence on brightness in the paper pulp bleaching

Xylanase is widely used in pulp and paper bleaching. In this study, the effects of rare earth ions (La³⁺, Ce³⁺, Er³⁺ and Gd³⁺) on the activity of xylanase produced by Penicillium are investigated and the application of a xylanase solution containing La³⁺ in paper bleaching is presented. Our results indicate that the bleaching effect of the enzyme solution containing La³⁺ was markedly better when the concentration of La³⁺ was 10^-8 g/L after 4 days of incubation. The mechanism of lanthanum on the improvement of xylanase activity was revealed through electrical conductivity, atomic absorption spectrometer, infrared spectroscopy and fluorescence microscopy analyses. The PCR result clearly demonstrates that a low concentration of La³⁺ led to the transversions of three base pair of gene sequences. Our experiment also reveals that the La³⁺ may have been involved in the cellular metabolic processes of Penicillium and intervened in the base pairing and DNA replication. This research may provide new insights into the improvement of enzymatic activity by lanthanum (III) and its application in paper pulp bleaching.

Evaluation of several phosphate amendments on rare earth element concentrations in rice plant and soil solution by X-ray diffraction

The exploitation and smelting of rare earth resources lead to serious pollution of rare earth elements (REEs) in farmland around mining area. The influence of four kinds of phosphate amendments-phosphate rock (PR), superphosphate (SSP), bone char (BC), and calcium magnesium phosphate (CMP)-on the bioavailability of REEs and the uptake and accumulation of 15 types of REE in rice were conducted in this study. Soil solutions were collected at tillering stage, heading stage and maturing stage, and rice was harvested at maturing stage. The mechanism of phosphate amendments reducing the bioavailability of REEs was studied by X-Ray diffraction and ICP-MS. PR treatment inhibited rice growth, but SSP, BC and CMP treatments all promoted rice growth, improved biomass of roots, shoots and grains, and promoted the uptake of phosphorous in rice. When compared with the CK, SSP, BC and CMP reduced the total REE concentrations in rice roots by 82.2%, 67.9% and 89.6%, shoots by 75.4%, 40.1% and 65.5%, grains by 23.8%, 29.0% and 29.3%, respectively. PR, SSP, BC and CMP significantly reduced the concentrations of REEs in the soil solution at three stages of rice growth. Analytic results of X-ray diffraction shows that adding PR, SSP, BC and CMP can lead to the formation of rare earth phosphate in the soil, thus reduce the activity of the REEs in the soil. Because SSP releases H+ during its dissolution, which has the risk of activating REEs in soil, CMP and BC are potential materials for remediation of REE-contaminated soil.

Potential roles of melatonin and sulfur in alleviation of lanthanum toxicity in tomato seedlings

Owing to the active use of rare-earth elements in many areas, it is necessary to study their behavior in the environment and their biological impact on plants. Despite the role of melatonin and sulfur in plant growth, development and abiotic stress tolerance; it is still not clear how they have a strong regulatory influence and synergistic effect on growth, physiological and biochemical characteristics of plants under different environmental stresses. Therefore, this study highlights how melatonin and sulfur together potentially involved in a reversal of lanthanum-inhibited photosynthetic and growth responses in tomato seedlings. Here, we reported that seedlings grown in a medium containing 150 μM lanthanum exhibited increased overproduction of reactive oxygen species (ROS) and lipid peroxidation together with increased Chlorophyll degradation, and activity of chlorophyllase, proline dehydrogenase and glycolate oxidase (GOx), and decreased photosynthesis and growth. However, the application of melatonin and sulfur showed significant responses on tomato seedlings, although the response of their combined treatment was more effective by further increasing photosynthesis and growth under lanthanum toxicity. Melatonin supplied with sulfur suppressed ROS formation, lipid peroxidation and activity of GOx, and increased photosynthesis by upregulating activities of carbonic anhydrase and ribulose-1,5-bisphosphate carboxylase/oxygenase. Also, sulfur supplementation with melatonin to seedlings resulted in an elevation in the accumulation of Chl and proline by increasing δ-aminolevulinic acid and activity of δ-aminolevulinic acid dehydratase and Δ¹-pyrroline-5-carboxylate synthetase activity. The administration of melatonin with sulfur substantially induced upregulation of enzymes (superoxide dismutase, ascorbate peroxidase, monodehydroascorbate reductase, dehydroascorbate reductase and glutathione reductase) activities involved in the antioxidant system, thereby mitigating ROS-induced oxidative damage. Thus, this study provides strong evidence that melatonin and sulfur have strong regulatory influence and synergistic role in alleviating the adverse effect of lanthanum-toxicity by increasing photosynthesis and growth.

Rare earth elements regulate the endocytosis and DNA methylation in root cells of Arabidopsis thaliana

With increasing application of rare earth elements (REEs), the resulting environmental safety has attracted extensive attention. When REEs act on plant leaves, REEs can initiate endocytosis in leaf cells, causing more REEs enter plant cells and then severe damage to plants. But when REEs directly act on plant roots, whether and how REEs affect the endocytosis in root cells remain unknown. Here, we characterized effects of lanthanum [La(III)], a REE with high accumulation in environment, on the endocytosis in root cells of Arabidopsis thaliana, and revealed effect mechanism from the perspective of DNA methylation. We found that La(III) enhanced the endocytosis in root cells and the extent of enhancement depended on the dose and time of La(III) exposure: 160 μM > 80 μM >30 μM (12 h); 80 μM > 30 μM >160 μM (24 h); 24 h  > 12 h. La(III)-enhanced endocytosis in root cells resulted from DNA methylation, which was closely related to the expression level of genes encoding DNA methylases/demethylases: CMT3, DRM2 and DNMT2 for 12 h, MET1, CMT1, CMT2, CMT3, DRM2, DNMT2, ROS1, DME, DML2, DML5a, and DML5b for 24 h. Conversely, enhanced endocytosis also promoted the expression level of genes encoding DNA methylases/demethylases. Our findings provide references for understanding the mechanisms by which REEs impact plants.

Analysis for effects of lanthanum (III) on the aboveground modules and respiration of soybean populations

The accumulation of rare earth elements (REEs) in the environment has become an environmental safety issue that cannot be ignored. However, previous studies on the environmental risks of REEs have mostly been performed at the individual level. In this work, to explore the effects of REE pollution at the population level, the effects of lanthanum (III) [La(III)] on the aboveground modules of soybean (Glycine max L) populations at different planting densities were investigated by simulating La(III) pollution, and the underlying mechanism was revealed on the physiological and biochemical levels of respiration. The results showed that the addition of 0.4 and 1.2 mM La(III) decreased the aboveground module growth parameters of the soybean populations, and this effect was more evident in the 1.2 mM La(III) treatment. At a certain dose of La(III), the effects of La(III) on the aboveground module growth parameters decreased with increasing plant densities. In addition, the effects of La(III) on the aboveground module growth parameters of soybean plants at different planting densities were related to plant respiration, in particular, to changes in the activities of respiratory key enzymes. The results indicated that the inhibitory effects of La(III) depended on the dose and on the planting density. This finding could provide a novel perspective and a basis for the objective assessment of potential environmental risks of REEs. ONE SENTENCE SUMMARY: La(III) pollution effects on the aboveground modules of soybean populations are related to the changes of the population respiration and the respiratory key enzymes; moreover, these effects are restricted by the population density.

Hormetic dose responses induced by lanthanum in plants

Rare earth elements (REEs) have recently received particular attention due to their accumulation in the environment. Such heightened recognition prompted our evaluation of the possible occurrence of La-induced plant hormesis in the peer-reviewed literature. This study revealed 703 La-induced hormetic concentration/dose responses in plants, which were quantitatively and qualitatively assessed. The maximum (MAX) biological response to low La concentrations/doses is commonly below 150% of control response, with a geometric mean of 142% at 56 μM (geometric mean). The geometric mean concentration of the no-observed-adverse-effect-level (NOAEL) was 249 μM. The MAX:NOAEL distance was commonly below 5-fold, with a geometric mean of 4.5-fold. Hormetic concentration/dose responses varied as per the growth substrate pH, number of concentrations/doses below the NOAEL, and time window. These results provide a unique insight into the effects of low doses of La on plant growth, as well as offer means for improving experimental designs to assess low dose effects.

A detection method in living plant cells for rapidly monitoring the response of plants to exogenous lanthanum

The pollution of rare earth elements (REEs) in ecosystem is becoming more and more serious, so it is urgent to establish methods for monitoring the pollution of REEs. Monitoring environmental pollution via the response of plants to pollutants has become the most stable and accurate method compared with traditional methods, but scientists still need to find the primary response of plants to pollutants to improve the sensitivity and speed of this method. Based on the facts that the initiation of endocytosis is the primary cellular response of the plant leaf cells to REEs and the detection of endocytosis is complex and expensive, we constructed a detection method in living plant cells for rapidly monitoring the response of plants to exogenous lanthanum [La(III), a representative of REEs] by designing a new immuno-electrochemical method for detecting the content change in extracellular vitronectin-like protein (VN) that are closely related to endocytosis. Results showed that when 30 μM La(III) initiated a small amount of endocytosis, the content of extracellular VN increased by 5.46 times, but the structure and function of plasma membrane were not interfered by La(III); when 80 μM La(III) strongly initiated a large amount of endocytosis, the content of extracellular VN increased by 119 times, meanwhile, the structure and function of plasma membrane were damaged. In summary, the detection method can reflect the response of plants to La(III) via detecting the content change in extracellular VN, which provides an effective and convenient way to monitor the response of plants to exogenous REEs.

The rare earth element (REE) lanthanum (La) induces hormesis in plants

Lanthanum is a rare earth element (REE) which has been extensively studied due to its wide application in numerous fields with a potential accumulation in the environment. It has long been known for its potential to stimulate plant growth within a hormetic-biphasic dose response framework. This article provides evidence from a series of high resolution studies published within the last two decades demonstrating a substantial and significant occurrence of lanthanum-induced hormesis in plants. These findings suggest that hormetic responses should be built into the study design of hazard assessment study protocols and included in the risk assessment process. Hormesis also offers the opportunity to substantially improve cost benefit estimates for environmental contaminants, which have the potential to induce beneficial/desirable effects at low doses.

Effects of lanthanum and silicon stress on bio-sequestration of lanthanum in phytoliths in rice seedlings

Plant phytoliths are important for silicon (Si) cycling in natural ecosystems; however, their role in lanthanum (La) sequestration in plants is still unclear. In the present study, we elucidated the mechanism of La-induced damage to the growth of rice (Oryza sativa L.) seedling from the viewpoint of the La sequestration by phytoliths (PhytLa). The phytoliths were extracted by using the microwave digestion method. La concentrations within the plants and phytoliths were determined by a modified lithium metaborate fusion method. Analysis showed that pretreatment with low La concentration not only promoted photosynthesis and transpiration in rice but also enhanced the sequestration ability of phytoliths on La. Conversely, high La concentration inhibited photosynthesis and transpiration in rice and the ability of phytoliths to sequester La. Moreover, high Si concentrations promoted the sequestration ability of phytoliths during these processes. Promotion of combined stress of La and Si on the ability of rice seedling was stronger than that of the single La stress. The sequestration ability of phytoliths in different parts of rice varied significantly, following the order: stem > leaf > root. This pattern could be attributed to factors such as the production of various phytolith morphotypes (such as tubes) and PhytLa, PhytLa efficiency, La accumulation, and the rate of photosynthesis and transpiration in different parts of rice seedlings. This study demonstrated that La uptake in rice seedlings was affected by the presence of Si in the medium, and phytolith played a crucial role in the bio-sequestration of La and assuaged the damage caused by La in rice seedlings. Graphical abstract ᅟ.

Response of Spirodela polyrhiza to cerium: subcellular distribution, growth and biochemical changes

Rare earth elements are new and emerging contaminants in freshwater systems. Greater duckweed (Spirodela polyrhiza L.) is a common aquatic plant widely used in phytotoxicity tests for xenobiotic substances. In this study, the cerium (Ce) accumulation potential, the distribution of Ce in bio-molecules, and ensuing biochemical responses were investigated in greater duckweed fronds when they were exposed to Ce (0, 10, 20, 40, and 60μM). There was a concentration dependent increase in Ce accumulation, which reached a maximum of 67mgg^-1 of dry weight (DW) at 60μM Ce after 14 d. The Ce concentrations in bio-macromolecules followed the order: cellulose and pectin > proteins > polysaccharides > lipids. In response to Ce exposure, significant chlorosis; declines in growth, photosynthetic pigment and protein contents; and cell death were noted at the highest Ce concentration. Photosystem II inhibition, degradation of the reaction center protein D1, and damage to chloroplast ultrastructure were observed in Ce treated S. polyrhiza fronds, as revealed by chlorophyll a fluorescence transients, immunoblotting, and transmission electron microscopy (TEM). O₂^.- accumulation and malondialdehyde (MDA) content in the treated fronds increased in a concentration dependent manner, which indicated that oxidative stress and unsaturated fatty acids (C18:3) were specifically affected by Ce exposure. These results suggest Ce exerts its toxic effects on photosynthesis, with a primary effect on PS II, through oxidative stress.

Lanthanum Affects Bell Pepper Seedling Quality Depending on the Genotype and Time of Exposure by Differentially Modifying Plant Height, Stem Diameter and Concentrations of Chlorophylls, Sugars, Amino Acids, and Proteins

Lanthanum (La) is considered a beneficial element, capable of inducing hormesis. Hormesis is a dose-response relationship phenomenon characterized by low-dose stimulation and high-dose inhibition. Herein we tested the effect of 0 and 10 μM La on growth and biomolecule concentrations of seedlings of four sweet bell pepper (Capsicum annuum L.) varieties, namely Sven, Sympathy, Yolo Wonder, and Zidenka. Seedling evaluations were performed 15 and 30 days after treatment applications (dat) under hydroponic greenhouse conditions. Seedling height was significantly increased by La, growing 20% taller in Yolo Wonder plants, in comparison to the control. Similarly, La significantly enhanced shoot diameter, with increases of 9 and 9.8% in measurements performed 15 and 30 dat, respectively, as compared to the control. Likewise, La-treated seedlings had a higher number of flower buds than the control. An increase in the number of leaves because of La application was observed in Yolo Wonder seedlings, both 15 and 30 dat, while leaf area was augmented in this variety only 30 dat. Nevertheless, La did not affect dry biomass accumulation. La effects on biomolecule concentration were differential over time. In all varieties, La stimulated the biosynthesis of chlorophyll a, b and total 15 dat, though 30 dat only the varieties Sympathy and Yolo Wonder showed enhanced concentrations of these molecules because of La. Total soluble sugars increased in La-treated seedlings 30 dat. Interestingly, while most varieties exposed to La showed a reduction in amino acid concentration 15 dat, the opposite trend was observed 30 dat. Importantly, in all varieties evaluated, La stimulated soluble protein concentration 30 dat. It is important to note that while chlorophyll concentrations increased in all varieties exposed to La, both 15 and 30 dat, those of soluble sugars and proteins consistently increased only 30 dat, but not 15 dat. Our results confirm that La may improve seedling quality by enhancing some growth parameters and biomolecule concentrations, depending on the genotype, and time of exposure.