Carbon monoxide (CO) is a hazardous gaseous molecule, whose concentration in atmosphere is recently rising. CO also is an endogenous regulator of a variety of biological processes in animals and plants. However, whether CO regulates plant adaptation to Hg-contaminated environments is unknown. In this study, we investigated the effect of CO on biological responses of Indian mustard (Brassica juncea), a plant species frequently used for heavy metal accumulation, to mercury (Hg) toxicity. Exposure of B. juncea to Hg(II) triggered production of O-2(-) and H2O2 as well as peroxides. However, such an effect can be reversed by CO exposure. Plants treated with 0.2 mM CO accumulated less amounts of Hg and had improved root elongation. Treatment with CO reduced activities of superoxide dismutase and increased activities of catalase, ascorbate peroxidase and guaiacol peroxidase in Hg-treated plants. CO-mediated alleviation of Hg toxicity was closely related to the accumulated proline, an antioxidant and reduced non-protein thiols, a sulfhydryl-containing compound that has strong capability for chelating heavy metals. These results indicate that CO plays a crucial role in preventing the plant from Hg toxicity. (C) 2010 Elsevier B.V. All rights reserved.