Four universal modules and their combinations for decoding amplitude and frequency of Ca2+ signals in plants

Calcium signals are fundamental for plants and play crucial roles in developmental processes and in rapid responses to environmental stimuli and stress factors. Nevertheless, the mechanisms underlying the decoding of calcium signals in plants are still largely unclear. We have addressed the question of Ca2+ signal decoding in a bottom-up modelling approach starting with the thermodynamics of Ca2+ binding to EF hands (EF: Glutamic acid-Phenylalanine) in Ca2+-binding proteins (CBPs). On this basis, we identified four universal, fundamental Ca2+ decoding modules, in which a CBP modulates the activity of a target protein (TP). Representatives of these basic modules are omnipresent in plant cells. Their Ca2+ dependent properties are mainly determined by the interaction of the CBP with the TP rather than the Ca2+-binding properties of the EF hands. Combinations of the modules generated complexes that are also observed in plants, which decode not only the amplitude but also the frequency of Ca2+ signals by setting specific ‘frequency islands’ in which a TP is active. The modelling approach presented here addresses the decoding of Ca2+ signals from the microscopic to the systemic level and could help to find the ‘Rosetta Stone’ for Ca2+ signalling in plants.