Light-sensitive membrane potentials in onion guard cells

Abstract

INTRACELLULAR electrical recordings in onion guard cells show that they maintain a membrane potential difference (MPD), inside negative. The MPD is light-sensitive; cells subjected to short light and dark cycles depolarise in the dark and hyperpolarise in the light. The swiftness of the electrical changes makes them among the fastest known stomatal responses, suggesting a causal relationship between the reception of light, the changes in membrane potential and the ion fluxes known to be associated with stomatal movement. The magnitude and electrical sign of the responses are consistent with the activity of a light-sensitive proton pump which would provide the driving force for ion transport across the cell membrane. Guard cells, the specialised structures that control gas exchange in the leaves of higher plants, use potassium to modulate their intracellular osmotic potential and perform the mechanical work needed to open and close the stomatal pore¹. When guard cells accumulate K⁺ (with intracellular concentrations reaching up to 0.5 M (refs 2 and 3)), water content increases and swelling occurs, causing the pore to open. During the reverse process, K⁺ is extruded (intracellular concentration diminishes to 0.1 M (refs 2 and 3)), leading to water loss and pore closure. Control of intracellular K⁺ concentrations is a universal feature of living organisms. Nerve cells sore energy used to generate electrical signals by maintaining concentration gradients of Na⁺ and K⁺ (ref. 4); bacterial cells selectively accumulate K⁺ (ref. 5) and, in plants, cells expand by increasing their turgor through uptake of K⁺ and its counter-ions⁶. While the cellular basis of K⁺ uptake in stomatal guard cells is poorly understood, one might expect common mechanisms for K⁺ transport even in widely divergent species. Two distinct mechanisms for K⁺ transport have been defined. In one, common in animal cells, K⁺ uptake is driven by ATP through a Na⁺/K⁺ ATPase⁷; in the second system, demonstrated in bacteria, ion transport is conducted by a proton-motive force⁸ generated by pH and electrical potential gradients across the membrane⁹. In showing that wall-less guard cell protoplasts swell when illuminated, we previously argued that a proton-motive force could drive K⁺ uptake into the guard cells¹⁰. The hypothesis generates the basic prediction of a light-sensitive, inside negative, MPD across the guard cell membrane. We report here the experimental verification of that prediction, through intracellular recordings in guard cells of Allium cepa.