Regulation of channel activity underlying cell volume and polarity signals in Fucus

Polarized zygotes of the marine alga Fucus have been used to investigate the spatial control of Ca2+ signals in a plant cell during apical growth and cell volume regulation in response to external osmotic fluctuations. UV laser microsurgery has been refined to enable plasma membrane patch clamp recordings from localized regions of the polarizing or polarized zygote. A plasma membrane cation channel that is mechanosensitive and significantly permeable to Ca2+ was characterized in cell-attached and excised patch configurations. Parallel measurements of intracellular Ca2+ using ratio photometric and imaging techniques were used to monitor temporal and spatial changes in cytoplasmic Ca2+ (Ca-cyt(2+)) in response to activation of these ion channels by osmotic swelling of the rhizoid. In polarized rhizoid cells spatial regulation of voltage- and mechanosensitive-Ca2+ channels in the plasma membrane underlie changes in Ca2+ that are crucial in signal-response transduction. Direct mechanical stimulation of channels in the plasma membrane of isolated sub-protoplasts from the apex of rhizoid cells can elicit changes in Ca2+ in the underlying cytosol.