The Toxicity Mechanism Of Silver Nanoparticles(AgNPs) In Arabidopsis Thaliana

Silver nanoparticles(AgNPs)are increasingly used in daily life due to their unique physicochemical and antibacterial properties.However,they are released into the environment during production,transportation and use inevitably.Therefore,the impacts on the environment and human health has attracted people's attention gradually.The effects of AgNPs on the genes and physiological level of higher terrestrial plant Arabidopsis thaliana were studied.After treated with 30 mg L-1 AgNPs and Ag+ for six days,the silver contents,transcriptome,photosynthesis and ROS were detected.Meanwhile,the effects of complex toxicity of AgNPs and Cd2+ on the growth of Arabidopsis thaliana were also studied.After treated with different concentrations of AgNPs and Cd2+ for one week,the main roots length of Arabidopsis seedlings,the number of root hairs,the contents of photosynthetic pigments and soluble proteins and antioxidant enzymes in leaves.The results are as follows:1.The contents of Ag+ in root and leaf tissues of Arabidopsis increased significantly after exposure with 30 mg L-1 AgNPs and Ag+,and the silver absorbed in roots were higher than leaves significantly.For roots,silver content of 4.2 mg g-1 after treatment with 30 mg L-1 AgNPs were higher than 3.2 mg g-1 exposure to 30 mg L-1 Ag+ indicated that silver nanoparticles may be easier to enter roots.However,AgNPs and Ag+ at the same concentration in the leaves have no difference indicated that the toxicity of silver nanoparticles on the leaves could be attributed to the Ag+ on plants toxicity.However,there was no significant difference of the silver content in roots and leaves between 0.12 mg L-1 Ag+ and control indicated that the phytotoxicity of silver nanoparticles were not dependent on the silver ions released from nanoparticles only.2.After six days exposure to 30 mg L-1 AgNPs and Ag+,the transcriptome of Arabidopsis thaliana seedlings were sequenced.The results showed that 626 genes and 767 genes were differentially expressed exposure to AgNPs and Ag+,respectively.Among them,470 genes were up-regulated and 156 genes were down-regulated.Ag+ can cause 514 genes up-regulated,253 genes down-regulated.AgNPs and Ag+ can cause 302 genes and 443 genes differentially expressed specifically,respectively.324 differentially expressed genes were overlapped exposure to AgNPs and Ag+.GO function analysis revealed that the differentially expressed genes were mainly accumulated in the biological processes like stress response,stimulus response and photosynthesis after the exposure to AgNPs and Ag+.3.After 30 mg L-1 AgNPs and Ag+ exposure,the changes of photosynthesis of Arabidopsis thaliana seedlings were determined.The results showed that AgNPs can reduce the chlorophyll a,carotenoid content and the actual photochemical efficiency significantly.The photochemical quenching coefficient qP gradually decreased with PAR increasing and the degree of decreased were higher than control.The non-photochemical quenching coefficient NPQ and the photochemical quenching coefficient Y(NPQ)increased with PAR increasing but the degree of increased were lower than control.And the non-photochemical quenching Y(NO)induced by non-photochemical increased gradually with the PAR increasing,but the degree of increase were higher than that of the control.AgNPs can also significantly reduce the relative electron transfer rate of Arabidopsis leaves,rETRmax and Ek significantly lower than control.The above results show that the AgNPs and Ag+ can affect the photosynthesis of Arabidopsis leaves significantly,the self-protection ability of plants under high light intensity were weakened.4.The high concentration(5 mg L-1)of Cd2+ can inhibit root elongation and increase the protein content and SOD,POD activity significantly.After adding AgNPs in high concentration(5 mg L-1)of Cd2+ treatment,the inhibition effect of AgNPs on Arabidopsis thaliana root hairs was significantly decreased.After adding AgNPs in low concentration(0.1 and 1 mg L-1)of Cd2+ treatment,the inhibition effect of Cd2+ on Arabidopsis thaliana photosynthetic pigment content were significantly increased.In Cd2+ treatment,SOD,POD activity reached the highest when the seedling exposure to 1 mg L-1 AgNPs.Above results showed that Cd2+ and AgNPs have interaction effects on root morphology and leaves physiological indexes of Arabidopsis thaliana.In summary,the same concentration of AgNPs and Ag+can significantly increased the silver concent.in the Arabidopsis tissue,changed transcriptome level,reduced photosynthetic capacity,increased degree of oxidative stress,photosynthesis and oxidative stress-related genes differential expressed.These results showed that the effects of AgNPs on Arabidopsis leaves photosynthesis were likely to be caused by the release of Ag+.However,AgNPs can also cause gene difference expressed specificly showed that AgNPs also has its unique particle effects.Moreover,this study not only could be used to provide data support for exploring combined toxicities of nanomaterials and heavy metals on plant,also provide certain theoretical basis for evaluating biological safety of AgNPs which interacted with heavy metals.