Visualizing Ca²⁺ signatures in plants

Ca²⁺ is a key player in an astonishing variety of plant signal transduction pathways where transient, spiking or oscillatory changes in cytosolic Ca²⁺ levels help to couple environmental or developmental cues to appropriate cellular responses. Understanding whether and how much Ca²⁺ signaling contributes to defining stimulus-response specificity has long been a challenge, but recent work has provided strong evidence that specific information can indeed be encoded in the spatiotemporal characteristics of Ca²⁺ signals. Identification of the Ca²⁺ binding proteins that transduce Ca²⁺ signals by regulating downstream effector proteins has revealed a complex network of Ca²⁺ sensor families, of which some members show distinct patterns of expression and subcellular localization. By utilizing genetically encoded fluorescent Ca²⁺ probes to monitor Ca²⁺ changes at high spatiotemporal resolution, it is now possible to explore whether such spatial heterogeneities in Ca²⁺ sensor distribution are coordinated with subcellular microdomains of Ca²⁺ signaling. Such visualization of Ca²⁺ signaling will also help to address which cellular compartments and transporters contribute to mobilizing and sequestering Ca²⁺ and thus define stimulus-specific Ca²⁺ signatures.