Reassessing roles for Ca2+ in guard cell signalling

The complexity of cellular signals related to cytosolic free Ca2+ concentra tion is well-illustrated in stomatal guard cells. Guard cells are unique as a higher plant cell model, because of the depth of knowledge to hand about ion channels at the plasma membrane and their regulation. A large body of data indicates actions of Ca2+ in controlling the gating of K+ and Cl- chan nels, and there is now growing evidence for transients and oscillations in cytosolic free Ca2+ concentration based on a mechanism with some similariti es to Ca2+-induced Ca2+ release in animals. While these new discoveries off er a possible explanation for the specificity and diversity of Ca2+-depende nt signalling, they raise a question about the immediate role(s) for Ca2+ a s a primary signal intermediate in some instances.