Calcium: silver bullet in signaling

Accumulating evidence suggests that Ca2+ serves as a messenger in many normal growth and developmental process and in plant responses to biotic and abiotic stresses. Numerous signals have been shown to induce transient elevation of [Ca2+]cyt in plants. Genetic, biochemical, molecular and cell biological approaches in recent years have resulted in significant progress in identifying several Ca2+-sensing proteins in plants and in understanding the function of some of these Ca2+-regulated proteins at the cellular and whole plant level. As more and more Ca2+-sensing proteins are identified it is becoming apparent that plants have several unique Ca2+-sensing proteins and that the downstream components of Ca2+ signaling in plants have novel features and regulatory mechanisms. Although the mechanisms by which Ca2+ regulates diverse biochemical and molecular processes and eventually physiological processes in response to diverse signals are beginning to be understood, recent studies have raised many interesting questions. Despite the fact that Ca2+ sensing proteins are being identified at a rapid pace, progress on the function(s) of many of them is limited. Studies on plant ‘signalome’ — the identification of all signaling components in all messengers mediated transduction pathways, analysis of their function and regulation, and cross talk among these components — should help in understanding the inner workings of plant cell responses to diverse signals. New functional genomics approaches such as reverse genetics, microarray analyses coupled with in vivo protein–protein interaction studies and proteomics should not only permit functional analysis of various components in Ca2+ signaling but also enable identification of a complex network of interactions.