Imaging of plant calcium-sensor kinase conformation monitors real time calcium decoding in planta
Changes in cytosolic calcium concentration are among the earliest reactions to a multitude of stress cues. Whereas a plethora of calcium-permeable channels may generate distinct calcium signatures and contribute to response specificities, the mechanisms by which calcium signatures are decoded is poorly understood. Here we develop a genetically encoded, FRET-based reporter that visualizes the conformational change of calcium-dependent protein kinases (CDPKs/CPKs), preceding kinase activation, for calcium-dependent AtCPK21 and calcium-independent AtCPK23. In pollen tubes, naturally displaying a physiological calcium range, CPK21-FRET, but not CPK23-FRET, report activity oscillations with similar features to cytosolic calcium, suggesting an isoform-specific calcium dependency and reversibility of the conformational change. In guard cells CPK21-FRET identifies CPK21 as a decoder of signal-specific calcium signatures in response to ABA and flg22. Based on this data, CDPK-FRET stands as a novel approach for tackling real-time live-cell calcium decoding in a multitude of plant developmental and stress responses.