The effects of the ionophore A23187 on pattern formation in the alga Micrasterias.

We have used the divalent cation ionophore A23187 to investigate the hypothesis that cytoplasmic localization of Ca2+ is responsible for localized growth in the alga Micrasterias. In a preliminary study we found that, of the major salts contained in the cell's medium, only CaCl2 was needed for normal development. In cells developing in the presence of A23187 and extracellular Ca2+, we postulated that the ionophore would induce a spatially uniform influx of Ca2+ that would overwhelm endogenous Ca2+ gradients. When developing cells were treated with A23187 and 2 mM CaCl2, we observed a delocalization of the cell's normal pattern of wall deposition. This effect was less pronounced when cells were exposed to A23187 and 2 mM MgCl2. These results support the hypothesis that localized regions of high Ca2+ concentration normally mediate localized expansion of tip-growing lobes in Micrasterias.