CALCIUM-DEPENDENT AND CALCIUM-INDEPENDENT K-FABA GUARD-CELL VACUOLES(MOBILIZATION CHANNELS IN VICIA)

Decrease in stomatal aperture is accompanied by volume and turgor loss in stomatal guard cells bordering the pore. This turgor loss is achie ved through net K+ loss from guard cells. In turgid guard cells, the v acuole is the dominant intracellular compartment and, therefore, a hig h proportion of the K+ released on closure must originate from the vac uolar lumen. This vacuolar K+ release must occur independently of the identity of the closing stimuli and signal transduction pathway, a poi nt exemplified for ABA-induced closure which can be mediated by Ca2+-d ependent or Ca2+-independent (pH(cyt)-dependent) signalling. Vacuolar K+ channels are involved in K+ release and in this study two K+-select ive channels were identified in Vicia faba guard cell vacuoles that di ffer in their regulation by cytosolic free Ca2+ ([Ca2+](cyt)) and cyto solic pH (pH(cyt)). At low (10 nM) [Ca2+](cyt), whole vacuole currents were typical of Fast Vacuolar (FV) currents. The currents were instan taneous, larger at positive potentials and displayed reduced conductan ce over negative potentials between -20 and -60 mV. The single channel currents identified in the presence of 10 nM [Ca2+](cyt) had an inwar d conductance of 6.4 pS, a permeability ratio of K+ to Cl- (P-K:P-Cl) of similar to 150:1 and a voltage-dependent open probability that refl ected the whole vacuole FV currents. The activities of both whole vacu ole 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 m u M) [Ca2+](cyt), the whole vacuole currents are non-rectifying but re main instantaneous, which is typical of Vacuolar K+-selective (VK) cur rents. Single channels identified at 1 mu M [Ca2+](cyt) were Ca2+-acti vated, had an inward conductance of similar to 90 pS, a P-K:P-Cl of si milar to 16:1 and a voltage-independent open probability. The activity of both whole vacuole and single channel VK currents was stimulated b y 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 activa tion during [Ca2+](cyt)- or pH(cyt)-based signalling. Therefore channe l-mediated vacuolar K+ release appears to be a convergence point for d isparate signalling pathways. The interaction of K+ channels with sign alling pathways is discussed, with particular reference to Ca2+-depend ent and Ca2+-independent signalling pathways activated in guard cells by ABA.