Identification of K+ channels in the plasma membrane of maize subsidiary cells

The stomatal complex of Zea mays consists of two guard cells with the pore in between them and two flanking subsidiary cells. Both guard cells and subsidiary cells are important elements for stoma physiology because a well coordinated transmembrane shuttle transport of potassium and chloride ions occurs between these cells during stomatal movement. To shed light upon the corresponding transport systems from subsidiary cells, subsidiary cell protoplasts were enzymatically isolated and in turn, analyzed with the patch-clamp technique. Thereby, two K-selective channel types were identified in the plasma membrane of subsidiary cells. With regard to their voltage-dependent gating behavior, they may act as hyperpolarization-dependent K+ uptake and depolarization-activated K+ release channels during stomatal movement. Interestingly, the K+ channels from subsidiary cells and guard cells similarly responded to membrane voltage as well as to changes in the W gradient. Further, the inward- and outward-rectifying K+ current amplitude decreased upon a rise in the intracellular free Ca2+ level from 2 nM to the muM-range. The results indicate that the plasma membrane of subsidiary cells and guard cells has to be inversely polarized in order to achieve the anti-parallel direction of K+ fluxes between these cell types during stomatal movement.