Phot1 and phot2 mediate blue light-induced transient increases in cytosolic Ca2+ differently in Arabidopsis leaves.

Phototropins (phot1 and phot2) are blue light (BL) receptors that mediate phototropism, chloroplast movements, and stomatal opening in Arabidopsis thaliana. Physiological studies have suggested that Ca2+ in the cytoplasm plays a pivotal role in these BL-induced responses. A phot1-mediated increase in cytosolic Ca2+ was reported in deetiolated seedlings of A. thaliana; however, the contribution of phot2 remains unknown. We examined a BL-induced transient increase in cytosolic free Ca2+ in leaves of transgenic A. thaliana of WT plants, phot1 and phot2 mutants, and phot1 phot2 double mutants expressing the Ca2+-sensitive luminescent protein aequorin. phot1 and phot2 had different photosensitivities: phot1 increased cytosolic Ca2+ at lower fluence rates (0.1-50 micromol x m-2 x s-1) and phot2 increased it at higher fluence rates (1-250 micromol x m-2 x s-1). By using Ca2+ channel blockers, Ca2+ chelating agents, and inhibitors of phospholipase C, we further demonstrated that both phot1 and phot2 could induce Ca2+ influx from the apoplast through the Ca2+ channel in the plasma membrane, whereas phot2 alone induced phospholipase C-mediated phosphoinositide signaling, which might result in Ca2+ release from internal Ca2+ stores. These results suggest that phot1 and phot2 mediate the BL-induced increase in cytosolic free Ca2+ differently.