A cell surface receptor mediates extracellular Ca2+ sensing in guard cells

Extracellular Ca2+ (Ca2+o) is required for various physiological and developmental processes in animals and plants1,2,3. In response to varied Ca2+o levels, plants maintain relatively constant internal Ca2+ content, suggesting a precise regulatory mechanism for Ca2+ homeostasis4. However, little is known about how plants monitor Ca2+o status and whether Ca2+o-sensing receptors exist. The effects of Ca2+o on guard cells in promoting stomatal closure by inducing increases in the concentration of cytosolic Ca2+ ([Ca2+]i)5,6,7,8 provide a clue to Ca2+o sensing. Here we have used a functional screening assay in mammalian cells9 to isolate an Arabidopsis complementary DNA clone encoding a Ca2+-sensing receptor, CAS. CAS is localized to the plasma membrane, exhibits low-affinity/high-capacity Ca2+ binding, and mediates Ca2+o-induced [Ca2+]i increases. CAS is expressed predominantly in the shoot, including guard cells. Repression of CAS disrupts Ca2+o signalling in guard cells, and impairs bolting (swift upward growth at the transition to seed production) in response to Ca2+ deficiency, so we conclude that CAS may be a primary transducer of Ca2+o in plants.