Calcium signatures and signaling events orchestrate plant-microbe interactions

•Microbe perception by PRRs directly activates Ca2+ channels/pumps and Ca2+ influx.•Activated PRRs indirectly activate Ca2+ channels/pumps via ROS or cNMP.•Elevated [Ca2+]cyt and oxidative bursts constitute a signaling amplification loop.•Ca2+ signals could be fed into transcriptional controls via Ca2+/CaM-regulated TFs.•Ca2+ signaling controls defense both positively and negatively at multiple stages.

Calcium (Ca2+) acts as an essential second messenger connecting the perception of microbe signals to the establishment of appropriate immune and symbiotic responses in plants. Accumulating evidence suggests that plants distinguish different microorganisms through plasma membrane-localized pattern recognition receptors. The particular recognition events are encoded into Ca2+ signatures, which are sensed by diverse intracellular Ca2+ binding proteins. The Ca2+ signatures are eventually decoded to distinct downstream responses through transcriptional reprogramming of the defense or symbiosis-related genes. Recent observations further reveal that Ca2+-mediated signaling is also involved in negative regulation of plant immunity. This review is intended as an overview of Ca2+ signaling during immunity and symbiosis, including Ca2+ responses in the nucleus and cytosol.