Blue light activates electrogenic ion pumping in guard cell protoplasts of Vicia faba

Abstract

Transmembrane ion exchange and blue-light dependent responses are central processes in plants in general^1,2 and stomatal guard cells in particular³. Stimuli, including light, cause H⁺ extrusion by stomatal guard cells^4,5, K⁺ uptake, osmotic swelling and enlarged stomatal pore apertures, resulting in increased leaf conductance for CO₂ influx and water efflux⁶. The mechanisms under-lying ion movement and the specific response of stomata to blue light^7–9 remain, however, unknown. We have now investigated these phenomena using gas-exchange techniques on leaves and whole-cell patch clamping^10,11 of protoplasts. Blue light-stimulated stomatal opening in the leaf is correlated with activation of an electrogenic pump in the guard cell plasmalemma which results in hyperpolarizations as large as 45 mV and outward currents as large as 5.5 pA. The pump functions in the absence of K⁺; in the presence of K⁺ an increase in voltage noise upon light-induced hyperpolarization more negative than the K⁺ equilibrium potential implies K⁺ flow through K⁺-selective channels¹². The data are in accord with a chemiosmotic model of stomatal response¹³ whereby active H⁺ extrusion creates an electrochemical gradient for passive ion fluxes. A 25–35-s delay in the onset of hyperpolarization, and its peak several minutes after application of a blue light pulse suggest that blue light functions as an activator, rather than a direct energy source, for ion pumping.