Making sense out of Ca2+ signals: Their role in regulating stomatal movements

Abstract

Plant cells maintain high Ca2+ concentration gradients between the cytosol and the extracellular matrix, as well as intracellular compartments. During evolution, the regulatory mechanisms, maintaining low cytosolic free Ca2+ concentrations, most likely provided the backbone for the development of Ca2+-dependent signalling pathways. In this review, the current understanding of molecular mechanisms involved in Ca2+ homeostasis of plants cells is evaluated. The question is addressed to which extent the mechanisms, controlling the cytosolic Ca2+ concentration, are linked to Ca2+-based signalling. A large number of environmental stimuli can evoke Ca2+ signals, but the Ca2+-induced responses are likely to differ depending on the stimulus applied. Two mechanisms are put forward to explain signal specificity of Ca2+-dependent responses. A signal may evoke a specific Ca2+ signature that is recognized by downstream signalling components. Alternatively, Ca2+ signals are accompanied by Ca2+-independent signalling events that determine the specificity of the response. The existence of such parallel-acting pathways explains why guard cell responses to abscisic acid (ABA) can occur in the absence, as well as in the presence, of Ca2+ signals. Future research may shed new light on the relation between parallel acting Ca2+-dependent and -independent events, and may provide insights in their evolutionary origin. © 2009 Blackwell Publishing Ltd.