Measurement of the Cytosolic Ca²⁺ Concentration in Response to Blue Light in Arabidopsis Guard Cells

Abstract

Arabidopsis phototropins (phot1, phot2) are blue light (BL) receptors that mediate stomatal opening. BL-induced membrane hyperpolarization through the activation of plasma membrane H⁺-ATPase drives K⁺ uptake and elicits stomatal opening. Previous studies suggested the involvement of Ca²⁺ in BL-induced activation of plasma membrane H⁺-ATPase. No evidence is, however, obtained whether phototropins mediate Ca²⁺ increase in guard cells. Here we determined the cytosolic Ca²⁺ concentration in guard cell protoplasts from Arabidopsis transformed with aequorin. We found a transient increase in cytosolic Ca²⁺ in response to BL, which was almost suppressed by DCMU, an inhibitor of photosynthetic electron transport. When BL was superimposed on a background strong red light, which saturated photosynthesis, the BL induced a slight Ca²⁺ increase in wild type but not in phot1 phot2 double mutant. From these results, we concluded that BL induced the Ca²⁺ increase that was dependent on both photosynthesis and phototropins in guard cells.