Stress Signaling II: Calcium Sensing and Signaling

Calcium is an essential second messenger in plant signaling networks. Many environmental and developmental stimuli induce an increase in cytosolic calcium to trigger different physiological responses. The specificity of Ca2+ signaling is achieved by a combination of distinct calcium signatures that are generated by specific calcium channels, pumps and transporters, and diverse calcium sensors that differ by their expression pattern, sub-cellular localization, substrate specificities and calcium sensitivities. Calcium binding modifies the structural conformation or enzymatic activity of the calcium sensors, which subsequently regulate downstream targets. Calmodulin is the most important Ca2+ transducer in eukaryotes and regulates numerous proteins with diverse cellular functions, including protein kinases. Plants also possess specific multigene families of protein kinases that play crucial roles in mediating calcium signaling. The multiplicity and diversity of plant calcium sensors, as well as the interconnections between various signal transduction pathways, constitute a tightly regulated signaling network that induces specific stress responses to improve plant survival.