PHYTOCHROME- and BLUE PIGMENT-MEDIATED LEAF MOVEMENTS: EXPERIMENTAL APPROACH IN THE SIGNAL TRANSDUCTION

Abstract— Leaflet movements of Cassia fasciculata are induced by transferring leaves from light to darkness or from darkness to light. Phytochrome mediates the dark-induced closure whereas a blue and far red light absorbing pigment (cryptochrome?) is the photoreceptor triggering the light-induced opening. These movements are the result of reversible turgor variation driven by ionic migrations (H+, K+, Cl^?) in cortical parenchyma cells of motor organs (“pulvini”) localized at the leaflet base. Calcium plays a predominant role in the regulation of the movements as shown by the inhibitory effects of chelators (EDTA, EGTA), intracellular antagonist TMB-8 and by the promoting effect of ionophore A 23187. Compounds known as calcium channel blockers (LaCl₃, verapamil and nifedipine) inhibited whereas Bay K 8644, a calcium channel activator, promoted the phytochrome-mediated movement. In contrast, all these calcium channel modulators had no effect on the blue pigment-mediated movement. From these results, it is suggested that calcium is not mobilized in the same manner in the two types of movements: possibly from external stores in the phytochrome-mediated response and from internal stores in the blue pigment-mediated response. Calcium acts possibly through calmodulin as suggested by a modification in the kinetics of the movements induced by inhibitors of calmodulin action (trifluoperazine, R 24571, W-7). The unexpected promotion of the movements by these inhibitors shows that calmodulin action on the ion migrations is not simple and direct. Experimental observations suggested that regulation might be done through cAMP metabolism. db-cAMP promoted the movements. Compounds known either to activate adenylate cyclase (prostaglandins, forskolin) or to inhibit phosphodiesterase (imidazolidinones, ICI 58301) induced the same modifications as db-cAMP. By contrast, a phosphodiesterase activator (imidazole) inhibited the movements.