Cation-induced superoxide generation in tobacco cell suspension culture is dependent on ion valence

Abstract

There have been many reports suggesting the involvement of reactive oxygen species (ROS), including superoxide anion (O2-), in salt stress. Herein, direct evidence that treatments of cell suspension culture of tobacco (Nicotiana tabacum L.; cell line, BY-2) with various salts of trivalent, divalent and monovalent metals stimulate the immediate production of O2- is reported. Among the salts tested, LaCI3 and GdCI3 induced the greatest responses in O2production, whereas CaCI2 and MgCI2 showed only moderate effects; salts of monovalent metals such as KCI and NaCI induced much lower responses, indicating that there is a strong relationship between the valence of metals and the level of O2- production. As the valence of the added metals increased from monovalent to divalent and trivalent, the concentrations required for maximal responses were lowered. Although O2- production by NaCI and KCI required high concentrations associated with hyperosmolarity, the O2- generation induced by NaCI and KCI was significantly greater than that induced simply by hyperosmolarity. Since an NADPH oxidase inhibitor, diphenyleneiodonium chloride, showed a strong inhibitory effect on the trivalent and divalent cation-induced generation of O2-, it is likely that cation treatments activate the O2--generating activity of NADPH oxidase.