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A central question is how specificity in cellular responses to the eukaryotic second messenger Ca²⁺ is achieved. Plant guard cells, that form stomatal pores for gas exchange, provide a powerful system for in depth investigation of Ca²⁺-signaling specificity in plants. In intact guard cells, abscisic acid (ABA) enhances (primes) the Ca²⁺-sensitivity of downstream signaling events that result in activation of S-type anion channels during stomatal closure, providing a specificity mechanism in Ca²⁺-signaling. However, the underlying genetic and biochemical mechanisms remain unknown. Here we show impairment of ABA signal transduction in stomata of calcium-dependent protein kinase quadruple mutant plants. Interestingly, protein phosphatase 2Cs prevent non-specific Ca²⁺-signaling. Moreover, we demonstrate an unexpected interdependence of the Ca²⁺-dependent and Ca²⁺-independent ABA-signaling branches and the in planta requirement of simultaneous phosphorylation at two key phosphorylation sites in SLAC1. We identify novel mechanisms ensuring specificity and robustness within stomatal Ca²⁺-signaling on a cellular, genetic, and biochemical level.

DOI: http://dx.doi.org/10.7554/eLife.03599.001