Free Ca2+and cytoplasmic streaming in the alga Chara

Plant cells, like those of animals, contain the Ca2+-binding protein calmodulin1–4. By analogy with animal cells it has thus been suggested that the intracellular free Ca2+concentration may have an important role in the regulation of plant cell activities. This suggestions has been supported by various physiological experiments, but so far direct evidence, invol ving measurements of intracellular Ca2+levels, has not been obtained. We describe here measurements of intracellular Ca2+in the giant alga Chara by microinjection of the protein aequorin, which emits blue light in proportion to Ca2+concentration. Chara exhibit an ATP-dependent cytoplasmic streaming shown to be inhibited by Ca2+(refs 5, 6). We report that Chara cells have a low free Ca2+concentration, comparable with those of animal cells, and that action potentials which inhibit cytoplasmic streaming7increase this Ca2+concentration substantially.