Toxicity And Translocation Of Graphene Oxide In Arabidopsis

Nanotechnology includes fabrication and use of different engineered nanomaterials(ENMs),such as metal nanoparticles(NPs),quantum dots(QDs),and carbon nanomaterials.The increased production and use of ENMs for industrial and medical applications have raised concerns about their impacts on both the health and the environment.Graphene oxide(GO),an important member of grapheme family,has the great potential to be used for both industry and biomedicine due to its unique properties and 2D structure.Both in vitro and in vivo data have suggested the adverse effects of GO on organisms.With the increased production and uses,the plants may take up ENMs from the environment and further transport them through the vascular system to various tissues.Meanwhile,a series of evidence have been raised that long-term exposure to GO or exposure to high concentrations of GO can cause the adverse effects on organisms,and result in the deposition of GO in cells or organisms.However,little is known about phytotoxicity of GO at environmentally relevant concentrations,and there are still no systematic studies on GO toxicity in plants under the stress condition.Herein,we evaluated the phytotoxicity of GO at environmentally relevant concentrations on Arabidopsis under both the normal and the stress condition.The main objectives for this study were to examine the possible safety property and the translocation of GO with the aid of Arabidopsis as the plant assay system.Considering the fact most of the ENMs released into the environment may be in the range of ng/L or μg/L,we assessed the toxicity of GO in the range of μg/L in Arabidopsis.Firstly,we examine the possible effect of GO under the normal condition.Our data demonstrate that under the normal condition,exposure to 10-1000 μg/L of GO did not influence the germination rate and development of seeds,could not obviously affect the shoot and root development,did not alter the flowering process,and could not influenced the development.Also,GO at the examined concentrations would not alter the activity of antioxidant enzymes such as SOD and CAT in Arabidopsis.We also did not detect the obvious change of MDA content.Therefore,Arabidopsis plants exposed to the examined concentrations of GO may still have the ability to sustain their normal physiological conditions.Moreover,we did not detect the noticeable alterations of expression patterns for genes required for germination and photomorphogenesis,development and function of root,transition from vegetative to reproductive development.That is,no potential developmental alteration would be formed in Arabidopsis plants exposed to the examined concentrations of GO.In Arabidopsis,we did not detect the obvious accumulation of GO in both the mesophyll cells and the sieve element in leaves.We only observed a small amount of GO distributed in the cytosol of parenchyma cells in stem and in sieve element in the root.Nevertheless,we observed the accumulation of a large amount of GO in root hair and the parenchyma cells in root of Arabidopsis.These data suggested that GO may be easily absorbed by Arabidopsis plants through the root hairs and then be deposited in the parenchyma cells in root.However,GO may be somewhat difficult to be translocated into stem or the leaves.Besides these,we further examined the possible effect of GO under the stress conditions.Two widely studied and commonly happened stresses(drought stress and salt stress)were selected for the combinational exposure assay.In this study,20%PEG6000 and 200 mmolL-1 NaCl were used.The reason to select these concentrations is that exposure to PEG 6000(20%)or NaCl(200 mmolL-1)would induce the moderate but adverse effects on Arabidopsis plants,which would be helpful for our observations of more severe adverse effects on plants from the combinational exposure between GO and PEG6000 or NaCl.Very different from the phenotypes under the normal condition,the severe adverse effects were found in GO-exposed Arabidopsis seedlings under the stress conditions.The combinational exposure to 1000 ug/L GO and PEG6000(20%)or NaCl(200 mmolL-1)caused the more severe adverse effects on morphology,fresh weight,root length,root-to-shoot ratio,resulted in more severe increase in H2O2 content and induction of ROS production of Arabidopsis seedlings compared with exposure to PEG6000(20%)or NaCl(200 mmolL-1)alone.Our results suggested that the rmolecular basis for development was altered.The combinational exposure to PEG6000(20%)or NaCl(200 mmolL-1 with 1000 μg L-1 of GO caused more severe alterations in the expression patterns of genes required for root development and abiotic stress compared with exposure to PEG6000(20%)or NaCl(200 mmol L/-)alone.For the cellular basis of stress-induced GO toxicity,we found the more pronounced distribution of GO in the roots,the leaves and stoma of Arabidopsis seedlings exposed to PEG 6000(20%)or NaCl(200 mmol L-1)compared with that in GO exposed Arabidopsis seedlings.We hypothesize that,under the stress conditions,GO may induce oxidative stress and membrane ion leakage,which may in turn induce GO translocation from the roots to the leaves.Our results will be useful for understanding toxicity and translocation of GO under different environmental conditions.