An oxidative stress response to polycyclic aromatic hydrocarbon exposure is rapid and complex in Arabidopsis thaliana

Phytoremediation is the use of plants to remove pollutants from contaminated environments. Polycyclic aromatic hydrocarbons (PAHs), a class of widely distributed organic carcinogenic pollutants, are attractive candidates for phytoremediation. To better understand the biochemical, physiological, and molecular responses to PAHs in plants, Arabidopsis thaliana seedlings were treated with phenanthrene, a three-ringed PAH, to elucidate mechanistic details of oxidative stress. Activity of the antioxidant enzymes superoxide dismutase (SOD), peroxidase (POD), catalase (CAT), and ascorbate peroxidase (APX), as well as H2O2, the redox buffer glutathione (GSH), and lipid oxidation product malondialdehyde (MDA), were measured in leaf tissue after 30 d of treatment at five phenanthrene levels between 0.25 and 1.25 mM. SOD enzyme activity increased monotonically over this treatment range, while CAT activity remained relatively unaffected. POD and APX exhibited peak enzyme activities on 0.25 mM phenanthrene and declined at higher concentrations. H2O2, GSH, and MDA increased with phenanthrene levels, and DAB staining indicated dose-dependent H2O2 accumulation. APX1 and CAT2 mRNA levels were measured at six time points during 72 h of 1 mM phenanthrene treatment, with APX1 peaking at nearly fivefold after 48 h, and CAT2 mRNA becoming minuscule before 12 h. Chlorophyll a and b levels fell with increasing phenanthrene concentration. Transmission electron microscopy revealed that chloroplast and mitochondria in treated plants underwent gross deformation, and cellular structures had collapsed. Taken together, these results support the hypothesis that oxidative stress is an important component of the PAH response in plants.