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

Extracellular Ca2+ (Ca-o(2+)) is required for various physiological and developmental processes in animals and plants(1-3). In response to varied Ca-o(2+) levels, plants maintain relatively constant internal Ca2+ content, suggesting a precise regulatory mechanism for Ca2+ homeostasis(4). However, little is known about how plants monitor Ca-o(2+) status and whether Ca-o(2+)-sensing receptors exist. The effects of Ca-o(2+) on guard cells in promoting stomatal closure by inducing increases in the concentration of cytosolic Ca2+ ([Ca2+](i))(5-8) provide a clue to Ca-o(2+) sensing. Here we have used a functional screening assay in mammalian cells 9 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 Ca-o(2+)-induced [Ca2+](i) increases. CAS is expressed predominantly in the shoot, including guard cells. Repression of CAS disrupts Ca-o(2+) 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 Ca-o(2+) in plants.