Calmodulin as a versatile calcium signal transducer in plants

The complexity of Ca²⁺ patterns observed in eukaryotic cells, including plants, has led to the hypothesis that specific patterns of Ca²⁺ propagation, termed Ca²⁺ signatures, encode information and relay it to downstream elements (effectors) for translation into appropriate cellular responses. Ca²⁺-binding proteins (sensors) play a key role in decoding Ca²⁺ signatures and transducing signals by activating specific targets and pathways. Calmodulin is a Ca²⁺ sensor known to modulate the activity of many mammalian proteins, whose targets in plants are now being actively characterized. Plants possess an interesting and rapidly growing list of calmodulin targets with a variety of cellular roles. Nevertheless, many targets appear to be unique to plants and remain uncharacterized, calling for a concerted effort to elucidate their functions. Moreover, the extended family of calmodulin-related proteins in plants consists of evolutionarily divergent members, mostly of unknown function, although some have recently been implicated in stress responses. It is hoped that advances in functional genomics, and the research tools it generates, will help to explain the multiplicity of calmodulin genes in plants, and to identify their downstream effectors. This review summarizes current knowledge of the Ca²⁺-calmodulin messenger system in plants and presents suggestions for future areas of research.