Electrocoupling of Ion Transporters in Plants: Interaction with Internal Ion Concentrations
There are five major electroenzymes in the plasmalemma of plant cells: a driving electrogenic pump, inward and outward rectifying K+ channels, a Cl−-2H+ symporter, and Cl−-channels. It has been demonstrated previously (Gradmann, Blatt & Thiel 1993, J. Membrane Biol.136:327–332) how voltage-gating of these electroenzymes causes oscillations of the transmembrane voltage (V) at constant substrate concentrations. The purpose of this study is to examine the interaction of the same transporter ensemble with cytoplasmic concentrations of K+ and Cl−. The former model system has been extended to account for changing internal concentrations. Constant-field theory has been applied to describe the influence of ion concentrations on current-voltage relationships of the active channels. The extended model is investigated using a reference set of model parameters. In this configuration, the system converges to stable slow oscillations with intrinsic changes in cytoplasmic K+ and Cl− concentrations. These slow oscillations reflect alternation between a state of salt uptake at steady negative values of V and a state of net salt loss at rapidly oscillating V, the latter being analogous to the previously reported oscillations. By switching off either concentration changes or gating, it is demonstrated that the fast oscillations are mostly due to the gating properties of the Cl− channel, whereas the slow oscillations are controlled by the effect of the Cl− concentration on the current. The sensitivity of output results y (e.g., frequency of oscillations) to changes of the model parameters x (e.g., maximum Cl− conductance) has been investigated for the reference system. Further examples are presented where some larger changes of specific model parameters cause fundamentally different behavior, e.g., convergence towards a stable state of only the fast oscillations without intrinsic concentration changes, or to a steady-state without any oscillations. The main and general result of this study is that the osmotic status of a plant cell is stabilized by the ensemble of familiar electroenzymes through oscillatory interactions with the internal concentrations of the most abundant ions. This convergent behavior of the stand-alone system is an important prerequisite for osmotic regulation by means of other physiological mechanisms, like second messengers and gating modifiers.