Ca2+ and Mg2+ ions counteract the reduction by fosetyl-Al (aluminum tris[ethyl phosphonate]) of peroxidase activity fromsuspension-cultured grapevine cells

Grapevine (Vitis vinifera cv. Monastrell) cell suspension cultures were treated with 1.5 mM fosetyl-A1, a frequently used systemic fungicide for grapevine diseases caused by oomycetes. These cells showed a reduction in the level of peroxidase activity secreted into the culture media when compared to non-treated cells, the effect being mainly related to a decrease in the level of the basic B1 peroxidase isozyme. The effect of fosetyl-A1 on peroxidase was analogous to that observed with the Ca2+-channel blockers Co2+, Cd2+ and La3+, and was counteracted by Ca2+ ions, but was not reversed when the Ca2+-ionophore A23187 was added to the culture media. Moreover, the effect of fosetyl-A1 on peroxidase activity and peroxidase isozymes was also partially reversed by Mg2+ ions but not by Sr2+, and was accentuated by Ba2+ ions. These results suggested that Ca2+ and Mg2+ ions specifically overcome the inhibitory effect of fosetyl-A1 on peroxidase. In this context, an apoplastic Ca2+/Mg2+-displacement hypothesis is proposed for the mechanism of action of fosetyl-A1 on peroxidase from grapevine cells.