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

Abstract

Extracellular Ca²⁺ (Ca²⁺_o) is required for various physiological and developmental processes in animals and plants^1,2,3. 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.