Four universal modules and their combinations for decoding amplitude and frequency of Ca 2+ signals in plants

Summary Calcium signals are fundamental for plants and play a crucial role in long-term processes such as growth and development, as well as in rapid responses to environmental stimuli and stress factors. Nevertheless, the mechanisms involved in decoding calcium signal in plants are still largely unclear. Here, we have addressed the question of calcium signal decoding in a bottom-up modelling approach. We started with the thermodynamics of Ca 2+ binding to a Ca 2+ binding protein (CBP), e.g. via EF hands. Remarkably, Ca 2+ binding properties of the EF hands do not coincide with the Ca 2+ sensitivity of the protein containing these EF hands. In analysing the next levels of complexity, we identified six universal fundamental Ca 2+ -decoding modules, in which Ca 2+ either interacts directly with a target protein (TP) or modulates its activity via a CBP. These modules are the basic units that enable the amplitude and frequency of Ca 2+ signals to be decoded. Representatives of these modules are omnipresent in plant cells. They straightforwardly explain the puzzling finding that Ca 2+ -dependent kinases exhibit different Ca 2+ -sensitivities when tested with different substrates. In-depth analysis of the properties of the modules provides a fundamental theoretical basis for understanding Ca 2+ signal decoding and may contribute to finding the “Rosetta Stone” for Ca 2+ signals in plants.