Effects of Polychlorinated Biphenyls (PCBs) and Their Hydroxylated Metabolites (OH-PCBs) on Arabidopsis thaliana

Abstract

Plants metabolize polychlorinated biphenyls (PCBs) into hydroxylated derivatives (OH-PCBs), which are sometimes more toxic than the parent PCBs. The objective of this research was to compare the toxicity of a suite of PCBs and OH-PCBs toward the model plant, Arabidopsis thaliana. While parent PCBs and higher-chlorinated OH-PCBs exhibited a low or nondetectable toxicity, lower-chlorinated OH-PCBs significantly inhibited the germination rate and plant growth, with inhibition concentration 50% (IC₅₀) ranging from 1.6 to 12.0 mg L^-1. The transcriptomic response of A. thaliana to 2,5-dichlorobiphenyl (2,5-DCB), and its OH metabolite, 4'-OH-2,5-DCB, was then examined using whole-genome expression microarrays (Affymetrix). Exposure to 2,5-DCB and 4'-OH-2,5-DCB resulted in different expression patterns, with the former leading to enrichment of genes involved in response to toxic stress and detoxification functions. Exposure to 2,5-DCB induced multiple xenobiotic response genes, such as cytochrome P-450 and glutathione S-transferases, potentially involved in the PCB metabolism. On the contrary, exposure to both compounds resulted in the down-regulation of genes involved in stresses not directly related to toxicity. Unlike its OH derivative, 2,5-DCB was shown to induce a transcriptomic profile similar to plant safeners, which are nontoxic chemicals stimulating detoxification pathways in plants. The differentiated induction of detoxification enzymes by 2,5-DCB may explain its lower phytotoxicity compared to 4'-OH-2,5-DCB.

Figure 1

Dose–response curves of A. thaliana exposed to PCBs (panels A and C) and OH-PCBs (panels B and D) as determined through germination tests (panels A and B) and growth tests (panels C and D). Solid lines for significantly toxic compounds (e.g., 4-CB, 4′-OH-4-CB, 4′-OH-2,5-DCB, and 4′-OH-2,4,6-TCB) were obtained by fitting to a three-parameter sigmoidal inhibition model (Prism 5.04, Graphpad). Solid lines for other, less toxic compounds were obtained by second-order smoothing of experimental data (Prism 5.04, Graphpad). The error bars represent standard deviations between biological replicates.

Figure 2

Chemical structure of fenclorim, 4-chloro-6-methyl-2-phenylpyrimidine (CMP), and sulfomethuron methyl.