Calcium/Calmodulin-Mediated Defense Signaling: What Is Looming on the Horizon for AtSR1/CAMTA3-Mediated Signaling in Plant Immunity

Calcium (Ca(2+)) signaling in plant cells is an essential and early event during plant-microbe interactions. The recognition of microbe-derived molecules activates Ca(2+) channels or Ca(2+) pumps that trigger a transient increase in Ca(2+) in the cytoplasm. The Ca(2+) binding proteins (such as CBL, CPK, CaM, and CML), known as Ca(2+) sensors, relay the Ca(2+) signal into down-stream signaling events, e.g., activating transcription factors in the nucleus. For example, CaM and CML decode the Ca(2+) signals to the CaM/CML-binding protein, especially CaM-binding transcription factors (AtSRs/CAMTAs), to induce the expressions of immune-related genes. In this review, we discuss the recent breakthroughs in down-stream Ca(2+) signaling as a dynamic process, subjected to continuous variation and gradual change. AtSR1/CAMTA3 is a CaM-mediated transcription factor that represses plant immunity in non-stressful environments. Stress-triggered Ca(2+) spikes impact the Ca(2+)-CaM-AtSR1 complex to control plant immune response. We also discuss other regulatory mechanisms in which Ca(2+) signaling activates CPKs and MAPKs cascades followed by regulating the function of AtSR1 by changing its stability, phosphorylation status, and subcellular localization during plant defense.