Involvement Of A Chloroplast-localized Protein CAS In The External Ca²⁺-induced Stomatal Closure.

Abstract

External Ca²⁺ promotes stomatal closure by triggering Ca²⁺ oscillation in guard cells. Elevation of cytosolic Ca²⁺ concentration is dependent on Ca²⁺ entry from extracellular space and/or inner Ca²⁺ stores, such as ER and vacuole. We found that chloroplast localized CAS (Calcium sensing receptor) is involved in the external Ca²⁺-induced stomatal closure in Arabidopsis. CAS is a plant-specific putative Ca²⁺ binding protein that has a single transmembrane domain. CAS carries a potential chloroplast transit peptide at N termini. To confirm its subcellular localization, stable transgenic plants that express CAS-GFP fusion proteins were examined with confocal laser microscopy. The CAS-GFP fusion proteins are localized in chloroplasts in all tissues. Furthermore, CAS is identified in several chloroplast proteomics studies. Ca²⁺-induced stomatal closure was abolished in CAS knock out plants, suggesting that chloroplast localized CAS might be involved in the regulation of stomatal movement.