ASPECTS OF CA2+ HOMEOSTASIS IN RICCIA-FLUITANS - REACTIONS TO PERTURBATIONS IN CYTOSOLIC-FREE CA2+

Ca2+-selective microelectrodes have been used to test the reaction of Riccia fluitans rhizoid cells to sudden perturbations of their cytosolic-free Ca2+. Under control conditions, i.e. [Ca2+]o = 0.1 mM, [K+]o = 0.01 mM, pH(o) = 7.3, the electrochemical Ca2+ gradient (DELTA-mu-Ca2+/F) across the plasma membrane of R. fluitans is around -690 mV. In the presence of such a steep Ca2+ gradient a sudden increase in external Ca2+ by a factor 100 triggers a rapid Ca2+ entry which transiently changes cytosolic-free Ca2+ by 0.3-0.5 pCa. The cells react instantaneously and eliminate these Ca2+ shifts within 30-60 s, which indicates a tight regulation of the free cytosolic Ca2+. These transients can be partly depressed by treatment with the Ca2+ channel blocker La3+ and by high external K+. This points to a channel used jointly by Ca2+ and K+. In an attempt to analyze the factors which control Ca2+ entry and recovery, respectively, experiments in the presence of cyanide have been carried out: (a) the basic free cytosolic Ca2+ does not respond significantly to cyanide; (b) Ca2+ entry is severely inhibited by cyanide. Since this inhibition depends on the cyanide concentration, an ATP-controlled Ca2+ channel is proposed. It is concluded that the high electrochemical Ca2+ gradient across the plasma membrane only allows a low probability of channel opening. Together with the observation of rapid Ca2+ recovery, this is in agreement with the basic requirements for Ca2+ to act as cellular messenger in plant signal transduction.