Spatial Organization of Calcium Signaling Involved in Cell Volume Control in the Fucus Rhizoid

Subprotoplasts prepared from different regions of rhizoid and thallus cells of Fucus zygotes displayed mechanosensitive plasma membrane channels in cell-attached patch-clamp experiments by using laser microsurgery. In excised patches, this channel was found to be voltage gated, carrying K+ outward and Ca2+ inward, with a relative permeability of Ca2+/K+ of 0.35 to 0.5, and an increased open probability at membrane potentials more positive than -80 mV. No significant difference was found in the density of this channel type from different regions of rhizoid or thallus cells. Hypoosmotic treatment of intact zygotes induced dramatic transient elevations of cytoplasmic Ca2+, initiating at the rhizoid apex and propagating in a wavelike manner to subapical regions. Localized initiation of the Ca2+ transient correlated with greater osmotic swelling at the rhizoid apex compared with other regions of the zygote. Ca2+ transients exhibited a refractory period between successive hypoosmotic shocks, during which additional transients could not be elicited and the ability to osmoregulate was impaired. Buffering the Ca2+ transients with microinjected Br2BAPTA similarly reduced the ability of rhizoid cells to osmoregulate. Ca2+ influx was associated with the initiation of the Ca2+ transient in apical regions, whereas intracellular sources contributed to its propagation. Thus, localized signal transduction is patterned by interactions of the cell wall, plasma membrane, and intracellular Ca2+ stores.