Effects of magnesium availability on the activity of plasma membrane ion transporters and light-induced responses from broad bean leaf mesophyll.

Considering the physiological significance of Mg homeostasis in plants, surprisingly little is known about the molecular and ionic mechanisms mediating Mg transport across the plasma membrane and the impact of Mg availability on transport processes at the plasmalemma. In this study, a non-invasive ion-selective microelectrode technique (MIFE) was used to characterize the effects of Mg availability on the activity of plasma membrane H+, K+, Ca2+, and Mg2+ transporters in the mesophyll cells of broad bean (Vicia faba L.) plants. Based on the stoichiometry of ion-flux changes and results of pharmacological experiments, we suggest that at least two mechanisms are involved in Mg2+ uptake across the plasma membrane of bean mesophyll cells. One of them is a non-selective cation channel, also permeable to K+ and Ca2+. The other mechanism, operating at concentrations below 30 microM, was speculated to be an H+/Mg+ exchanger. Experiments performed on leaves grown at different levels of Mg availability (from deficient to excessive) showed that Mg availability has a significant impact on the activity of plasma-membrane transporters for Ca2+, K+, and H+. We discuss the physiological significance of Mg-induced changes in leaf electrophysiological responses to light and the ionic mechanisms underlying this process.