Hydroxylated polychlorinated biphenyls (OH-PCBs) are toxic contaminants produced via biotic or abiotic transformation of PCBs. The toxicity of a suite of OH-PCBs and their parent compounds was determined using the model plant, Arabidopsis thaliana. All lower-chlorinated OH-PCBs (1-3 chlorine atoms) were toxic for the plants, with inhibition concentration 50% (IC50) ranging from 2 to 11 mg L-1. On the other hand, parent PCBs and higher-chlorinated OH-PCBs (4 chlorine atoms) exhibited low or non-detectable toxicity. The higher toxicity of OH-PCBs for Arabidopsis was then investigated using transcriptomic analysis (expression microarrays, Affymetrix) conducted on plants exposed to 2,5-dichlorobipehnyl (2,5-DCB) and three of its hydroxylated isomers (2'-OH-, 3'-OH-, and 4'-OH-2,5-DCB). All compounds caused strong enrichment in genes involved in response to biotic and abiotic stresses, indicating deleterious effects to plants. Exposure to 2,5-DCB and its OH-derivatives also resulted in upregulation of multiple xenobiotic response genes (XRGs) potentially involved in the metabolic transformation of PCBs (e.g., cytochrome P-450 monooxygenases, glutathione S-transferases). Searching an Arabidopsis transcriptomic database (Genevestigator) revealed that 2,5-DCB – although not the OH-2,5-DCBs – induced an expression profile similar to plant safeners, including chloro-6-methyl-2-phenylpyrimidine (CMP), fenclorim, isoxadifen, and menadione sodium bisulphite (MSB). Safeners are non-toxic chemicals inducing XRGs and protecting plants against pesticides. This observation may explain the lower toxicity of 2,5-DCB as compared with the OH-2,5-DCBs. Unlike the parent compound, the three OH-derivatives also induced expression profiles similar to inhibitors of brassinosteroid synthesis, including brassinazole, paclobutrazole, propiconazole, and uniconazole. Consistently with these observations, exposure to the OH-derivatives resulted in strong downregulation of several genes responsive to brassinosteroid stimulus or involved in brassinosteroid synthesis. Intriguingly, brassinosteroids have been shown to alleviate the toxicity of xenobiotic compounds (including pesticides and PCBs) to plants. The inhibition of brassinosteroid synthesis by the OH-PCBs is therefore expected to increase the toxicity for the plants. These results increase our understanding of the effects of PCBs and OH-PCBs on plants, which may have important implications for human health and the environment.

Understanding the toxicity of hydroxylated PCBs for plants: A transcriptomic analysis

2019 Spring National Meeting

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