Modulates cytoplasmic Ca(2+) concentration and is crucial for proper stomatal regulation in response to elevated levels of external Ca(2+). May function by regulating concentrations of inositol 1,4,5-trisphosphate (IP3), which in turn triggers release of Ca(2+) from internal stores. May play a role in de-etiolation. Phosphorylated in both bundle sheath and mesophyll cells, under both low and high light regimes (70 vs 900 umol photons/m-2/s).