• calcium;
  • chloroplast;
  • Ca2+-sensing receptor (CAS);
  • guard cells;
  • signal transduction;
  • stomatal movement.


  • • 
    Guard cell movements are regulated by environmental cues including, for example, elevations in extracellular Ca2+ concentration. Here, the subcellular localization and physiological function of the Ca2+-sensing receptor (CAS) protein was investigated.
  • • 
    CAS protein localization was ascertained by microscopic analyses of green fluorescent protein (GFP) fusion proteins and biochemical fractionation assays. Comparative guard cell movement investigations were performed in wild-type and cas loss-of-function mutant lines of Arabidopsis thaliana. Cytoplasmic Ca2+ dynamics were addressed in plants expressing the yellow cameleon reporter protein YC3.6.
  • • 
    This study identified CAS as a chloroplast-localized protein that is crucial for proper stomatal regulation in response to elevations of external Ca2+. CAS fulfils this role through modulation of the cytoplasmic Ca2+ concentration.
  • • 
    This work reveals a novel role of the chloroplast in cellular Ca2+ signal transduction.