Electrophysiological Identification and Activity Analyses of Plasma Membrane K+ Channels in Maize Guard Cells

Stomatal movement, which plays an essential role in plant transpiration and photosynthesis, is controlled by ion channels that mediate K+ and anion fluxes across the plasma membrane (PM) of guard cells. These channels in dicots are accurately regulated by various physiological factors, such as pH, abscisic acid (ABA) and Ca2+; however, the data in monocots are limited. Here the whole-cell patch-clamping technique was applied to analyze the properties and regulations of PM K+ channels in maize guard cells. The results indicated that the hyperpolarization-activated inward-rectifying channels were highly K+-selective. These inward K+ (Kin) channels were sensitive to extracellular K+. Their slope factor (S) decreased when the apoplastic K+ concentration decline, causing a positive shift of the half-activation potential (V1/2). Their activities were promoted by apoplastic acidification but inhibited by apoplastic and cytosolic alkalization. Nevertheless, the outward K+ (Kout) channel activities were uniquely promoted by cytosolic alkalization. Both apoplastic and cytosolic ABA inhibited Kin channels independent of cytosolic Ca2+ ([Ca2+]cyt). And two Ca2+-dependent mechanisms with different Ca2+ affinities may mediate resting- and high-[Ca2+]cyt-induced inhibition on Kin channels, respectively. However, resting [Ca2+]cyt impaired the inhibition of Kin channels induced by apoplastic ABA, not cytosolic ABA. Furthermore, the result that high [Ca2+]cyt attenuated ABA-induced inhibition highlighted the importance of [Ca2+]cyt for Kin channel regulation. There may exist a Ca2+-dependent regulation of the Ca2+-independent ABA signaling pathways for Kin channel inhibition. These results provided an electrophysiological view of the multiple level regulations of PM K+ channel activities and kinetics in maize guard cells.