Toxicity And Underlying Molecular Mechanism Of Lanthanum And Cerium On Wheat

With the increasing demand for rare earth elements(REEs)all over the world,REEs have been inevitably released into water and soil during the improper use and disposal relevant products process,posing potential risk to the environmental safety and even the human health.However,it is unclear whether REEs are toxic in the presence of environmental factors,and no relevant environmental standards for REEs have been set yet.Therefore,it is urgent to investigate the bioavailability and toxicity of REEs and explore the possible mechanisms in order to provide theoretical basis for the development of environmental standards and ecological risk assessment.In this study,the toxicity of two REEs,lanthanum(La)and cerium(Ce),was investigated by using wheat(Triticum aestivum)as model organism.It was conducted in simulative soil pore water,combined with laboratory chemical analysis,model methods and high-throughput metabolomics technology.The main research contents and conclusions are as follows:(1)The effects of major environmental factors,p H,Ca and nitrilotriacetic acid(NTA),on the single and mixture toxicity of La and Ce were invistigated,and a quantitative model for predicting toxicity of REEs was developed.The results showed that,when La and Ce were expressed as the total dissolved concentration,p H had little effect on the toxicity of REEs,while Ca and NTA could significantly alleviate their adverse effects.However,if they were expressed as free ion activities,p H and NTA had no influence on the EC50 of La and Ce,but Ca showed protective effect.NTA could reduce the toxic effect of REEs by reducing their free ion activities while the alleviatation with Ca was for different mechanism.After taking into account the impacts of different environmental factors,two bioavailability-based models(biological ligand model,BLM and electrostatic interaction model,ETM)were successfully developed.They could accurately predict the toxic effects of La and Ce under different exposure conditions and explain more than 76%of the toxicity variations.InBLM,the binding constants of Ca,La and Ce to biotic ligands are log K_CaBL=4.14,log K_LaBL=6.67,log K_CeBL=6.59,respectively.(2)This research further attempted to reveal the toxic mechanisms of La and Ce on wheat with the physiological and biochemical indicators.The effects of single and mixture REEs on wheat biomass,photosynthetic intensity and antioxidant system were explored.It was found that the growth of root was inhibited more severely than the shoots,because the REEs mainly accumulated in the root of wheat and rarely transferred to the shoots.Under the effect of small amounts of REEs,the chlorophyll content in shoots of wheat increased with the increase of REEs,which improved the photosynthetic capacity.After investigating the antioxidant system of wheat,it was found that both the root and shoots were suffered from oxidative damage,but for the root and shoots,there were different changes on the peroxidase,catalase and antioxidant glutathione,which were for the clearance of hydrogen peroxide,as well as superoxide dismutase for the clearance of superoxide radical.In the root,most of these antioxidant enzymes and glutathione were inhibited by REEs.In the shoots,the activities of the three enzymes and the glutathione were not affected by the treatments of single La and La-Ce mixture,but could be significantly improved when the concentration range of Ce was 1?M to 2?M.By analyzing the effects of mixture treatments on physiological and biochemical indexes of wheat,it was found that the interaction types of La and Ce at low concentrations(?1?M)were mainly additive or synergistic,while at high concentrations(>1?M)were significant antagonistic.(3)In order to further elucidate the molecular toxic effect of La and Ce on wheat,the effects of La and Ce on the metabolism of wheat were analyzed by high-throughput metabolomics.Results indicated that single and mixture treatments of La and Ce in roots could affect the carbohydrate metabolism and nucleotide metabolism of wheat.It could interfere with the normal utilization of carbohydrate substances,and cause the accumulation of carbohydrate substances.Meanwhile,it colud also affect the metabolism of purines and pyrimidines,which might interfere with the synthesis of DNA and RNA.In the shoots,La and Ce single and mixture treatments influenced the carbohydrate and amino acid metabolism of wheat.The increase of REEs concentration caused the accumulation of carbohydrate,decreasing small molecule acid content,and increasing types of upregulated amino acid,which could alleviate the oxidative stress to some extent.By analyzing the interactions between La and Ce in metabolism,it could be found that,at the overall metabolic level,there was an additive effect.In root,La and Ce showed synergistic and additive effects in carbohydrate metabolism and nucleotide metabolism,but in amino acid metabolism,they showed antagonistic effects.In the shoots,the interaction types of La and Ce in carbohydrate metabolism and amino acid metabolism were both synergistic,but antagonistic was observed in nucleotide metabolism.Our results will provide basic data and new ideas for explaining the interaction mechanism,bioavailability and toxicity of REEs,and will serve to build up a solid theoretical basis for the ecological risk assessment framework and environmental quality standard system for REEs.