Rapid alteration of cellular redox homeostasis upon exposure to cadmium and mercury in alfalfa seedlings

• Here, the kinetics of oxidative stress responses of alfalfa (Medicago sativa) seedlings to cadmium (Cd) and mercury (Hg) (0, 3, 10 and 30 μm) exposure, expanding from a few minutes to 24 h, were studied. • Intracellular oxidative stress was analysed using 2′,7′- dichlorofluorescin diacetate and extracellular hydrogen peroxide (H 2O2) production was studied with Amplex Red. Growth inhibition, concentrations of ascorbate, glutathione (GSH), homoglutathione (hGSH), Cd and Hg, ascorbate peroxidase (APX) activity, and expression of genes related to GSH metabolism were also determined. • Both Cd and Hg increased cellular reactive oxygen species (ROS) production and extracellular H 2O2 formation, but in different ways. The increase was mild and slow with Cd, but more rapid and transient with Hg. Hg treatments also caused a higher cell death rate, significant oxidation of hGSH, as well as increased APX activity and transient overexpression of glutathione reductase 2, glutamylcysteinyl synthetase, and homoglutathione synthetase genes. However, Cd caused minor alterations. Hg accumulation was one order of magnitude higher than Cd accumulation. • The different kinetics of early physiological responses in vivo to Cd and Hg might be relevant to the characterization of their mechanisms of toxicity. Thus, high accumulation of Hg might explain the metabolism poisoning observed in Hg-treated seedlings.The authors gratefully acknowledge the financial support of the Fundación Ramón Areces, which made possible this work, specially the PhD grant received by COV. This work was also supported by the Spanish Ministry of Education and Science through the projects REUSA (CTM2005-04809/TECNO) and AGL2004-00191. RRA was supported by a FPI PhD grant from the Spanish Ministry of Education and Science.Peer Reviewe