Calcium-dependent and calcium-independent K+ mobilization channels in Vicia faba guard cell vacuoles

Abstract

Decrease in stomatal aperture is accompanied by volume and turgor loss in stomatal guard cells bordering the pore. This turgor loss is achieved through net K + loss from guard cells. In turgid guard cells, the vacuole is the dominant intracellular compartment and, therefore, a high proportion of the K + released on closure must originate from the vacuolar lumen. This vacuolar K + release must occur independently of the identity of the closing stimuli and signal transduction pathway, a point exemplified for ABA-induced closure which can be mediated by Ca 2+ -dependent or Ca 2+ -independent (pH cyt -dependent) signalling. Vacuolar K + channels are involved in K + release and in this study two K + -selective channels were identified in Vicia faba guard cell vacuoles that differ in their regulation by cytosolic free Ca 2+ ([Ca 2+ ] cyt ) and cytosolic pH (pH cyt ). At low (10 nM) [Ca 2+ ] cyt , whole vacuole currents were typical of Fast Vacuolar (FV) currents. The currents were instantaneous, larger at positive potentials and displayed reduced conductance over negative potentials between -20 and -60 mV. The single channel currents identified in the presence of 10 nM [Ca 2+ ] cyt had an inward conductance of 6.4 pS, a permeability ratio of K + to Cl - (P K :P Cl ) of ∼150:1 and a voltage-dependent open probability that reflected the whole vacuole FV currents. The activities of both whole vacuole and single channel FV currents were stimulated by alkaline pH cyt , with an optimum of pH 7.3 for whole vacuole currents. At higher (1 μM) [Ca 2+ ] cyt , the whole vacuole currents are non-rectifying but remain instantaneous, which is typical of Vacuolar K + -selective (VK) currents. Single channels identified at 1 μM [Ca 2+ ] cyt were Ca 2+ -activated, had an inward conductance of ∼90 pS, a P K :P c , of ∼16:1 and a voltage-independent open probability. The activity of both whole vacuole and single channel VK currents was stimulated by acidic pH cyt , with an optimum at pH 6.4 for whole vacuole currents. The FV and VK channels will clearly show distinct patterns of activation during [Ca 2+ ] cyt - or pH cyt -based signalling. Therefore channel-mediated vacuolar K + release appears to be a convergence point for disparate signalling pathways. The interaction of K + channels with signalling pathways is discussed, with particular reference to Ca 2+ -dependent and Ca 2+ -independent signalling pathways activated in guard cells by ABA.