Regulation of external calcium-induced stomatal closure by chloroplast localized CAS

Abstract

Ca²⁺ plays a key role as a second messenger in intracellular signaling in stomatal movements. Stomatal movements are regulated by various environmental and endogenous signals, and oscillations in cytosolic Ca²⁺ concentration ([Ca²⁺]_cyt) are a fundamental component of stomatal closure. [Ca²⁺]_cyt oscillations are mediated by Ca²⁺ flux from extracellular space and internal stores, such as vacuoles and ER. However, little is known about roles of chloroplasts in guard cell signaling. CAS is a Ca²⁺ binding protein that had been previously reported as a Ca²⁺ sensor in plasma membrane. We examined the subcellular localization of CAS and found that CAS is localized to the chloroplast thylakoid membrane. CAS inactivation resulted in significantly reduced [Ca²⁺]_cyt increase and inhibition of stomatal closure induced by external Ca²⁺, while overexpression of CAS promoted stomatal closure. These results provide the first evidence that chloroplasts plays an essential role in the regulation of [Ca²⁺]_cyt oscillations and stomatal closure.