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), whichinturndirects 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.