Polychlorinated biphenyls (PCBs) are toxic and persistent contaminants that are widely dispersed in the environment. Higher plants are known to metabolize PCBs into hydroxylated derivatives (OH-PCBs), which are often considered more toxic than the parent compounds. As plants are the basis of the terrestrial food web, understanding the response of plants to toxic contaminants, such as PCBs and OH-PCBs, is essential for assessing the risk that these compounds may impose on the environment and human health. The objective of this research was to compare the toxicity of PCBs and their hydroxylated metabolites toward the model plant, Arabidopsis thaliana. Although the parent PCBs exhibited low or non-detectable toxicity, all lower-chlorinated OH-PCBs (with one to three chlorine atoms) significantly reduced the plant germination, with inhibition concentration 50% (IC50) ranging from 2.2 to 10.7 mg L-1. The toxicity of PCBs and OH-PCBs generally decreased when the degree of chlorination increased, which may be explained by the lower solubility and bioavailability generally associated with higher hydrophobicity. Whole-genome expression microarrays (Affymetrix) were then used to investigate the transcriptomic response in A. thaliana exposed to the model PCB, 2,5-dichlorobiphenyl (2,5-DCB), and its major OH-metabolite (4'-OH-2,5-DCB), showing differential expression of many genes involved in response to stress and biotic/abiotic stimuli (approx. 10% induced by 2,5-DCB and 4'-OH-2,5-DCB vs. approx. 5% in the Arabidopsis genome), reflecting the deleterious effects of both compounds on plants. Exposure to 2,5-DCB and 4'-OH-2,5-DCB also resulted in upregulation of multiple so-called xenobiotic response genes (XRGs) potentially involved in the metabolic transformation of PCBs (e.g., cytochrome P-450 monooxygenases, glutathione S-transferases), suggesting a detoxification mechanism following the three phases of the green liver model (i.e., activation, conjugation, and sequestration). Searching the transcriptomic signatures induced by the two compounds against an Arabidopsis microarray database revealed that 2,5-DCB, but not 4'-OH-2,5-DCB, induced an expression profile similar to plant 'safeners' (e.g., non-toxic chemicals protecting plants against pesticides by inducing XRGs), which may explain the lower toxicity of 2,5-DCB as compared with 4'-OH-2,5-DCB.

Differential response of Arabidopsis thaliana to Polychlorinated Biphenyls (PCBs) and their hydroxylated metabolites (OH-PCBs)

253rd National Meeting (2017)

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