Trivalent cations such as those of Al, La, and Gd are phytotoxic. Our previous works showed that addition of LaCl3 or GdCl3 to tobacco cells triggers the generation of superoxide (O-2(.-)). Here, we show that AlCl3 at normal physiological pH (5.8) induces much 2 greater production of O-2(.-) (detected with a specific chemiluminescence probe), indicating that these trivalent cations similarly induce the oxidative bursts. It was shown that NADPH oxidase is involved in the generation of O-2(.-) and the yield of O-2(.-) was dose-dependent (ca. 6 mM Al, optimal). Following the acute spike of O-2(.-), a gradual increase in cytosolic-free Ca2+ concentration ([Ca2+](c)) was detected with the luminescence of recombinant aequorin over-expressed in the cytosol. Interestingly, a O-2(.-) scavenger and a Ca2+ chelator significantly lowered the level of [Ca2+], increase, indicating that the Al-induced O-2(.-) stimulates the influx of Ca2+. Compared to the induction of O-2(.-) generation, the [Ca2+], elevation was shown to be maximal (340 nM) at relatively lower Al concentrations (ca. 1.25 mM). Thus, the Al concentration optimal for O-2(.-) is too much (inhibitory) for [Ca2+],. In addition, high concentrations of Al were shown to be inhibitory to the H2O2-induced Ca2+ influx. This explains the ineffectiveness of high Al concentration in the oxidative burst-mediated induction of [Ca2+], increase. It is likely that Al-induced [Ca2+], elevation is manifested from the finely geared balance between the O-2(.-) mediated driving force and the channel inhibition-mediated brake. Furthermore, it is note-worthy that Al (less than or equal to10 mM) showed no inhibitory effect on the hypo-osmolarity-induced Ca2+ influx, implying that Al may be a selective inhibitor of redox-responsive Ca2+ channels. Possible target channels of Al actions are discussed. (C) 2003 Elsevier Inc. All rights reserved.