Permeability Of Ca (2 +) And Plasma Membrane Ca (2 +) Channels Are Involved In The Blu-ray-induced Arabidopsis Hypocotyl Response To Light Of Experimental Evidence

UV-A and blue light (BL) regulate a number of physiological processes in higher plants. These include phototropism, light-driven chloroplast movements and stomatal movements. The PHOT I gene of Arabidopsis encodes an autophosphosphorylating protein kinase that functions as a photoreceptor for phototropism in response to low-intensity BL. Up to now. little is known about the signaling cascade, which link phototropins to the physiological responses downstream of BL perception. Physiological studies have suggested mat Ca2+ in the cytoplasm plays a pivotal role in these BL-induced responses. A photl-mediated increase in cytosolic Ca2+ was reported in deetiolated seedling of Arabidopsis thaliana. In Arabidopsis mutant defective in photl (photl), hypocotyls lose their ability to bend toward weak BL. In seedling from photl-5, these blue-light-induced calcium signals were reduced, raising the question about the mechanism of BL -dependent Ca2+ influx. It has been proposed that the intracellular [Ca2+] increase induced by BL may result from Ca2+-channel in the plasma membranes (PM). So far proposed Ca2+-channels in the PM of hypocotyls protoplasts have not been identified and characterized. In addition, whether and how the BL receptor photl regulate Ca2+-channels in the PM has not been investigated. In present study we aimed to identify Ca2+ channels involved in BL signaling cascade. The patchclamp technique was applied to record the channels on the hypocotyl protoplasma of wild type and photoreceptor mutant of Arabidopsis thaliana.First, we studied the role of exogenous Ca2+ ion in the hypocotyl phototropism. The curvature of hypocotyls was strongly depended on extracelluar Ca2+ concentration. In the medium with low Ca2+ (< 200 nM), hypocotyl curvature was significantly inhibited. In addition, application of Ca2+-channel blocker (La3+) significantly inhibited hypocotyls curvature even in the medium with higher Ca2+. However, the photoreceptor mutant phot 1-5 had no curvature toward the BL in all different Ca2+ concentrations in the medium.Patch-clamp technique was applied to investigate the inward Ca24-permeable-channels in the PM of hypocotyl protoplasts, when Ba2+ was employed as the charge-carrying cation and set as 10 mM and 50 mM in the pipette and bath solution respectively, a kind of hyperpolarization-activated channels with the conductance as 5.69 pS was recorded. In the photoreceptor mutant photl-5, the hyperpolarization-activated calcium current was much smaller than that of wild ones. The results supported the hypothesis that the hyperpolariztion-dependent calcium-permeable channel on the protoplasma of Arabidopsis thaliana was under the regulation of phototropins.It is concluded that the Ca2+ influx from the apoplast through the Ca2+ channel in the PM plays pivotal role in the hypocotyl curvature toward low-fluence BL. And we can give a further hypothesis that phototropin may act as the upstream regulator of calcium channel in the PM, from which the Ca2+ influx and form the Ca2+ singling as the second messenger in the cytoplasma.