The release of silver nanoparticles (AgNPs) in the environment has raised concerns about their effects on living organisms, including plants. In this study, changes in gene expression in Arabidopsis thaliana exposed to polyvinylpyrrolidone-coated AgNPs and silver ions (Ag(+)) were analyzed using Affymetrix expression microarrays. Exposure to 5 mg/L AgNPs (20 nm) for 10 days resulted in upregulation of 286 genes and downregulation of 81 genes by reference to nonexposed plants. Exposure to 5 mg/L Ag(+) for 10 days resulted in upregulation of 84 genes and downregulation of 53 genes by reference to nonexposed plants. Many genes differentially expressed by AgNPs and Ag(+) were found to be involved in the response of plants to various stresses: upregulated genes were primarily associated with the response to metals and oxidative stress (e.g., vacuolar cation/proton exchanger, superoxide dismutase, cytochrome P450-dependent oxidase, and peroxidase), while downregulated genes were more associated with response to pathogens and hormonal stimuli [e.g., auxin-regulated gene involved in organ size (ARGOS), ethylene signaling pathway, and systemic acquired resistance (SAR) against fungi and bacteria]. A significant overlap was observed between genes differentially expressed in response to AgNPs and Ag(+) (13 and 21% of total up- and downregulated genes, respectively), suggesting that AgNP-induced stress originates partly from silver toxicity and partly from nanoparticle-specific effects. Three highly upregulated genes in the presence of AgNPs, but not Ag(+), belong to the thalianol biosynthetic pathway, which is thought to be involved in the plant defense system. Results from this study provide insights into the molecular mechanisms of the response of plants to AgNPs and Ag(+).

Hydroxylated polychlorinated biphenyls (OH-PCBs) are produced in the environment by the oxidation of PCBs through a variety of mechanisms, including metabolic transformation in living organisms and abiotic reactions with hydroxyl radicals. As a consequence, OH-PCBs have been detected in a wide range of environmental samples, including animal tissues, water, and sediments. OH-PCBs have recently raised serious environmental concerns because they exert a variety of toxic effects at lower doses than the parent PCBs and they are disruptors of the endocrine system. Although evidence has accumulated about the widespread dispersion of OH-PCBs in various compartments of the ecosystem, little is currently known about their biodegradation and behavior in the environment. OH-PCBs are today increasingly considered as a new class of environmental contaminants that possess specific chemical, physical, and biological properties not shared with the parent PCBs. This article reviews recent findings regarding the sources, fate, and toxicities of OH-PCBs in the environment.

Hydroxylated polychlorinated biphenyls (HO-PCBs) are toxic contaminants which are produced in the environment by biological or abiotic oxidation of PCBs. The toxicity of a suite of 23 mono-hydroxylated derivatives of PCBs and 12 parent PCBs was determined using the bacterial bioluminescent assay Microtox®. All HO-PCBs tested exhibited higher toxicity than the corresponding parent PCB, with effect concentration 50% (EC50) ranging from 0.07 to 133 mg L−1. The highest toxicities were recorded with 4-hydroxylated derivatives of di-chlorinated biphenyls (EC50 = 0.07 to 0.36 mg L−1) and 2-hydroxylated derivatives of tri-chlorinated biphenyls carrying a chlorine substituent on the phenolic ring (EC50 = 0.34 to 0.48 mg L−1). The toxicity of HO-PCBs generally decreased when the degree of chlorination increased. Consistently with this observation, a significant positive correlation was measured between toxicity (measured by EC50) and octanol-water partition coefficient (pKow) for the HO-PCBs under study (Pearson’s correlation coefficient, r = 0.74), which may be explained by the lower solubility and bioavailability generally associated with higher hydrophobicity. This study is the first one which assessed the toxicity of a suite of PCBs and HO-PCBs using the bioluminescent assay Microtox®, showing an inverse correlation between toxicity and hydrophobicity.

Three hydroxylated derivatives of PCBs, including 2′-hydroxy-4-chlorobiphenyl (2′-OH-4-CB), 3′-hydroxy-4-chlorobiphenyl (3′-OH-4-CB), and 4′-hydroxy-4-chlorobiphenyl (4′-OH-4-CB), were transformed by the PCB degrader, Burkholderia xenovorans LB400. When the bacterium was growing on biphenyl (biphenyl pathway-inducing conditions), all three hydroxylated isomers were significantly transformed. On the contrary, only 2′-OH-4-CB was transformed by the bacterium growing on succinate (conditions non-inductive of the biphenyl pathway). Gene expression analyses showed a strong induction of key genes of the biphenyl pathway (bph) when cells were grown on biphenyl, which is consistent with the transformation of the three isomers by biphenyl-grown cells. When cells were grown on succinate, only exposure to 2′-OH-4-CB resulted in expression of biphenyl pathway genes, which suggests that this isomer was capable of inducing the biphenyl pathway. These results provide the first evidence that bacteria are able to metabolize PCB derivatives hydroxylated on the non-chlorinated ring.