Abstract
Despite the increasing understanding of the crucial roles of glutathione (GSH) in cellular defense against heavy metal stress as well as oxidative stress, little is known about the functional role of exogenous GSH in mercury (Hg) tolerance in plants. Here, we provide compelling evidence that GSH contributes to Hg tolerance in diverse plants. Exogenous GSH did not mitigate the toxicity of cadmium (Cd), copper (Cu), or zinc (Zn), whereas application of exogenous GSH significantly promoted Hg tolerance during seed germination and seedling growth of Arabidopsis thaliana, tobacco, and pepper. By contrast, addition of buthionine sulfoximine, an inhibitor of GSH biosynthesis, severely retarded seed germination and seedling growth of the plants in the presence of Hg. The effect of exogenous GSH on Hg specific tolerance was also evident in the presence of other heavy metals, such as Cd, Cu, and Zn, together with Hg. GSH treatment significantly decreased H2O2 and O2- levels and lipid peroxidation, but increased chlorophyll content in the presence of Hg. Importantly, GSH treatment resulted in significantly less accumulation of Hg in Arabidopsis plants, and thin layer chromatography and nuclear magnetic resonance analysis revealed that GSH had much stronger binding affinity to Hg than to Cd, Cu, or Zn, suggesting that tight binding of GSH to Hg impedes Hg uptake, leading to low Hg accumulation in plant cells. Collectively, the present findings reveal that GSH is a potent molecule capable of conferring Hg tolerance by inhibiting Hg accumulation in plants.
Highlights
Heavy metal pollution is an environmental threat that affects ecological systems and living organisms, including plants, animals, and humans
To evaluate the effects of GSH on seed germination and seedling growth, Arabidopsis seeds were sown on half-strength MS medium supplemented with various concentrations of GSH, as well as buthionine sulfoximine (BSO) that inhibits a key enzyme necessary for GSH biosynthesis and decreases cellular GSH levels (Griffith and Meister, 1979; Howden et al, 1995a,b)
To assess whether exogenous GSH contributes to heavy metal tolerance, seed germination, and seedling growth of Arabidopsis plants were evaluated on MS medium containing various concentrations of heavy metals with or without GSH
Summary
Heavy metal pollution is an environmental threat that affects ecological systems and living organisms, including plants, animals, and humans. Excess heavy metal accumulation in plants causes physiological disorders and inhibits plant growth and productivity. Since plants are the major source of human food, efforts to develop edible plants with increased tolerance to and reduced accumulation of heavy metals are important for food safety and avoidance of the risk of heavy metal toxicity. To increase heavy metal tolerance and minimize the accumulation or toxicity of heavy metals in plants, it is necessary to understand the mechanisms of uptake, detoxification, and tolerance. To survive and adapt to heavy metal stress, plants have evolved complex defense strategies, such as reduction in cellular free metal content (e.g., metal exclusion, cell wall binding, chelation, and sequestration) and scavenging of reactive oxygen species (ROS) that are generated by heavy metal exposure (Hall, 2002). Enhancement of Hg Tolerance by Exogenous GSH
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