Phytochrome-mediated Uptake of Calcium in Mougeotia Cells

Ca(2+) is proposed to function as a messenger in such phytochrome-mediated responses as localized cell growth, intracellular movements, and control of plasma membrane properties. To test this hypothesis, the uptake of Ca(2+) in irradiated and non-irradiated regions of individual threads of the green alga Mougeotia was studied with the aid of (45)Ca(2+) and low temperature autoradiography: 10-20 cells within 40-60 cell-long threads were irradiated for up to 1 min, transferred to darkness for 3 to 10 min, submersed in a radioactive medium for 1 min, washed in an unlabelled medium for 30 min, and then autoradiographed at-80° C for several days.The autoradiographs show that those cells which had been pre-irradiated with red light did take up 2-10 times more Ca(2+) than the adjacent non-irradiated cells of the same thread. Cells pre-irradiated with farred light or red light followed by far-red light showed no enhanced uptake of Ca(2+). These results might be interpreted to indicate, firstly, that phytochrome-Pfr is involved in the enhanced uptake of Ca(2+) and secondly, that the accumulation of radioactive Ca(2+) in red light irradiated cells is an expression of an increased intracellular concentration of Ca(2+). This interpretation is based on the data that (i) the dark interval between irradiation and labelling precluded the involvement of photosynthesis, (ii) the effect of red light was reversible with far-red light, and (iii) the accumulation of Ca(2+) persisted during the long wash-out period. We speculate, that the red light-enhanced accumulation of Ca(2+) in Mougeotia cells is caused by a Pfr-mediated increase of the Ca-permeability of the plasma membrane, and perhaps by a Pfr-impeding of an active Ca(2+)-extrusion.