The Role of Calmodulin and Related Proteins in Plant Cell Function: An Ever-Thickening Plot

Abstract

Stimuli-induced fluctuations in intracellular free calcium (Ca2+) serve as secondary messenger signals that regulate diverse biochemical processes in eukaryotic cells, such as developmental transitions and responses to biotic and abiotic stresses. Stimuli-specific Ca2+ signals are manifested as spatially and temporally defined differential Ca2+ signatures that are sensed, decoded, and transduced to elicit distal responses via an array of Ca2+ binding proteins (CBPs) that function as intracellular Ca2+ sensors. Calmodulin (CaM), the most important eukaryotic CBP, senses and responds to fluctuations in intracellular Ca2+ levels by binding to this ubiquitous second messenger, and transducing given Ca2+ signatures that differentially activate distal effector (target) proteins regulating a broad range of biochemical responses. Ca2+/CaM targets include an increasing number of proteins whose functions continue to be elucidated. Hundreds of reports have highlighted the importance of CaM, and other CBPs, in the transduction of Ca2+-mediated signals involved in transcriptional regulation, protein phosphorylation/dephosphorylation, and metabolic shifts. Other Ca2+-binding proteins are known to play significant functional roles in plant cells as well. This review is primarily focused on the role of CaM in some key plant processes, and discusses recent advances in understanding the pivotal role of CaM in an ever-increasing number of plant cell functions and biochemical responses. We also discuss recent work highlighting the emerging importance of CaM in nuclear and organellar signaling.