Sensing Extracellular Calcium

Abstract

In plants, various physiological processes are regulated by extracellular calcium (Ca 2+ o ). For instance, Ca 2+ o triggers increases and oscillations in the intracellular Ca 2+ concentration ([Ca 2+ ] i ) of guard cells and thereby promotes stomatal closure. However, the mechanisms whereby plants sense and respond to changes in Ca 2+ o remain unclear. Han et al. used a functional screening approach, in which Arabidopsis cDNA was expressed in human embryonic kidney (HEK 293) cells, and the intracellular response to changes in Ca 2+ o was monitored with the Ca 2+ indicator Fura-2, to clone a protein that mediated Ca 2+ o -induced [Ca 2+ ] i increase (CICI). The protein, which the authors named CAS, was predicted to have a single transmembrane domain with an extracellular N terminus that did not resemble any functional domains known. The authors used a combination of 45 Ca blotting of purified N and C termini, equilibrium dialysis, and time-course analysis of CAS activation and deactivation to determine that the N terminus showed low-affinity, high-capacity Ca 2+ binding. Northern analysis indicated that CAS was mostly localized in shoots, and experiments with gene reporters, as well as immunolocalization, indicated that it was localized in the plasma membrane of guard cells. Reducing CAS expression with antisense disrupted CICI signaling in guard cells and inhibited bolting (rapid upward growth at the initiation of seed production) in response to Ca 2+ deficiency. Thus, CAS appears to represent a novel Ca 2+ o sensor with physiological functions relevant to stomata closure and plant development. S. Han, R. Tang, L. K. Anderson, T. E. Woerner, Z.-M. Pei, A cell surface receptor mediates extracellular Ca 2+ sensing in guard cells. Nature 425 , 196-200 (2003). [Online Journal]