Phytochrome Mediates a Reduction of the Surface Charge of Mesotaenium Cells

Abstract

Four-week-old cultures of the unicellular green alga Mesotaenium caldariorum were irradiated with red light or far-red light for 30 min. After irradiation the mobility of the cells was measured in an electric field. From the velocity of migration in the electric field the zeta potential of the cells was calculated. At all pH values investigated, i.e. from pH 5 to pH 9, the zeta potential of the cells was always significantly more negative after far-red irradiation than after red light exposure. The greatest difference was measured at pH 7, with −22.5±0.8 mV after far-red light and −13.8±1.7mV after red light irradiation. The reducing effect of red light on the zeta potential was maintained during inhibition of photosynthesis. In media with very low Ca2+ concentration or after adding the Ca" channel blocker Verapamil, the effect of red light on the zeta potential disappeared. In plasmolyzed cells, too, red light irradiation lost its effect on the surface charge. Experiments with alternating red and far-red light irradiation showed that the effect of red light was reversible by far-red light, and vice versa. Simultaneous red and far-red light irradiation inhibited the zeta potential-reducing effect of red light. These findings suggest that phytochrome and a Ca2+-influx are involved in reducing the surface charge of Mesotaenium cells. The decrease in surface charge might be the consequence of a calcium-induced depolarization of the plasmamembrane and/or a calcium-stimulated release of material to the cell surface that is less negatively charged.