A gradient of cytoplasmic free calcium in growing rhizoid cells of Fucus serratus

It is becoming increasingly clear that cytoplasmic Ca 2+ (Ca 2+ cyt .) has an important role in the regulation of plant cell functions as well as in animal cells 1–3 . However, there is an acute lack of measurements of the cytoplasmic Ca 2+ concentration ([Ca 2+ ] cyt ) in plants. Direct measurements have so far been limited to Chara and Nitella , using aequorin 4 , and to Haemanthus endosperm cells, using Quin-2 (ref. 5). The latter study demonstrated a gradient of [Ca 2+ ] cyt in dividing cells. Here we evaluate the use of Ca 2+ -selective microelectrodes and the fluorescent indicator Quin-2 to measure [Ca 2+ ] cyt in rhizoids of germinating Fucus serratus zygotes. This preparation is particularly attractive for such an investigation as the Fucus zygote is a well-studied developmental system and the relatively large polarized rhizoid cells are predominantly cytoplasmic with no large vacuoles. We demonstrate the presence of a longitudinal gradient of [Ca 2+ ] cyt in the rhizoid cell. This gradient appears to be maintained by preferential Ca 2+ influx in the region of the growing tip. The significance of the gradient for cell polarity is discussed.