Calcium decoding mechanisms in plants

Ca2+ is a crucial second messenger that is involved in mediating responses to various biotic and abiotic environmental cues and in the regulation of many developmental processes in plants. Intracellular Ca2+ signals are realized by spatially and temporally defined changes in Ca2+ concentration that represent stimulus-specific Ca2+ signatures. These Ca2+ signatures are sensed, decoded and transmitted to downstream responses by a complex tool kit of Ca2+ binding proteins that function as Ca2+ sensors. Plants possess an extensive and complex array of such Ca2+ sensors that convey the information presented in the Ca2+ signatures into phosphorylation events, changes in protein–protein interactions or regulation of gene expression. Prominent Ca2+ sensors like, Calmodulins (CaM), Calmodulin-like proteins (CMLs), calcium dependent protein kinases (CDPKs), Calcineurin B-like proteins (CBLs) and their interacting kinases (CIPKs) exist in complex gene families and form intricate signaling networks in plants that are capable of robust and flexible information processing. In this review we reflect on the recently gained knowledge about the mechanistic principles of these Ca2+ sensors, their biochemical properties, physiological functions and newly identified targets proteins. These aspects will be discussed in the context of emerging functional principles that govern the information processing via these signaling modules.

► In plant cells, calcium signals are represented by stimulus-specific signatures.
► Diversely evolved calcium sensor proteins enable complex information processing.
► We review the mechanisms how calcium signals are sensed and transduced.