The exposure of gadolinium at environmental relevant levels induced genotoxic effects in Arabidopsis thaliana (L.)

Rare Earth Elements (REEs) are increasingly being used in agriculture and are also used to produce high end technological devices, thereby increasing their anthropogenic presence in the environment. However, the ecotoxicological mechanism of REEs on organisms is not fully understood. In this study, the effects of gadolinium (Gd) addition on Arabidopsis thaliana (L.) were investigated at both physiological and molecular levels. Four treatments (0, 10, 50 and 200 μmol·L^-1 Gd) were used in the exposure tests. Biomass, root length and chlorophyll content in shoots/roots were measured to investigate the plant's physiological response to Gd stress. Random amplified polymorphic (RAPD)-Polymerase Chain Reaction (PCR) and methylation sensitive arbitrarily primed (MSAP)-PCR were used to investigate changes in genetic variation and DNA methylation of A. thaliana when exposed to Gd. At the physiological level, it was found that low concentration of Gd (10 μmol·L^-1) could significantly increase the plant biomass and root length, while the growth of A. thaliana was significantly inhibited when exposed to 200 μmol·L^-1 of Gd, yet the total soluble protein content in aerial plant parts increased significantly by 24.2% when compared to the control group. Among the 12 primers considered in the RAPD assessment, at the molecular level, only four primers revealed different patterns in their genomic DNA. Compared to the control group, the treatment with 50 μmol·L^-1 of Gd was associated with lower polymorphism, while the treatment with 200 μmol·L^-1 of Gd was associated with higher polymorphism. The polymorphism frequencies for the 50 μmol·L^-1 of Gd and the 200 μmol·L^-1 of Gd were 4.67% and 20.33%, respectively. The MSAP analysis revealed that the demethylation (D) type of Arabidopsis genomic DNA increased significantly under 10 and 50 μmol·L^-1 of Gd, while the methylation (M) type was also significantly increased under 200 μmol·L^-1 of Gd. Generally, the total methylation polymorphism (D+M) increased with an increase of Gd concentration. It was found that high concentrations of Gd appeared to cause DNA damage, but low concentrations of Gd (as low as 10 μmol·L^-1) were associated with DNA methylation change. Further, it was verified by Real time Reverse Transcription PCR (RT-PCR) on the bands detected by the MSAP analysis, that the genes relative to processes including cell cycle, oxidative stress and apoptosis, appeared to be regulated by methylation under Gd stress. These findings reveal new insight regarding ecotoxicity mechanisms of REEs on plants.

Early-Life Benzo[a]Pyrene Exposure Causes Neurodegenerative Syndromes in Adult Zebrafish (Danio rerio) and the Mechanism Involved

There is increasing recognition of the importance of early-life environmental exposures in health disorders at later-life stages. The aim of this study was to evaluate whether early-life exposure to benzo[a]pyrene (BaP) could induce neurodegenerative syndromes at later-life stages in zebrafish. Embryos were exposed to BaP at doses of 0, 0.05, 0.5, 5, and 50 nM from early embryogenesis to 96 h post-fertilization (hpf), then transferred to clean water and maintained for 365 days. We found that BaP decreased locomotor and cognitive ability, neurotransmitter levels of dopamine, 3,4-dihydroxyphenylacetic acid and norepinephrine; and induced loss of dopaminergic neurons and resulted in neurodegeneration. Additionally, BaP increased amyloid β protein and cell apoptosis in the adult zebrafish brain. Further, DNA methyltransferase 1 (DNMT1) and DNMT3a were up-regulated in 96 hpf larvae and the adult brain. MeDIP-sequencing data of the 96 hpf larvae identified 235 differentially methylated genes in promoter, with the fold change > 1.5. Guanylate cyclase 2F (gucy2f) and dopamine receptor D4 related sequence (drd4-rs) were hypermethylation promoters, whereas zinc finger C4H2 domain (zc4h2) was a hypomethylation promoter in 96 hpf larvae and the adult brain. The mRNA levels of gucy2f and drd4-rs were down-regulated, and zc4h2 was up-regulated. Our findings suggested that the lasting modifications of DNA methylation were associated with neurodegenerative syndromes in adult zebrafish as a result of early-life BaP exposure.