Coupling diurnal cytosolic Ca2+ oscillations to the CAS-IP3 pathway in Arabidopsis

Various signaling pathways rely on changes in cytosolic calcium ion concentration ([Ca2+](i)). In plants, resting [Ca2+](i) oscillates diurnally. We show that in Arabidopsis thaliana, [Ca2+](i) oscillations are synchronized to extracellular Ca2+ concentration ([Ca2+](o)) oscillations largely through the Ca2+- sensing receptor CAS. CAS regulates concentrations of inositol 1,4,5- trisphosphate (IP3), which in turn directs release of Ca2+ from internal stores. The oscillating amplitudes of [Ca2+](o) and [Ca2+](i) are controlled by soil Ca2+ concentrations and transpiration rates. The phase and period of oscillations are likely determined by stomatal conductance. Thus, the internal concentration of Ca2+ in plant cells is constantly being actively revised.