Biotic and abiotic stress-induced chloroplast Ca²⁺ elevation in Arabidopsis

Abstract

The calcium ion (Ca²⁺) plays a major role in stress signal transduction in plant cells. However, little is known about stress- and pathogen-induced Ca²⁺ oscillation in chloroplasts. Here we provide the first direct evidence that stromal Ca²⁺ levels are elevated by various biotic and abiotic stresses, including pathogen-derived elicitors, H₂O₂, high salinity and high osmolarity. For example, bacterial elicitor (flg22) induced a rapid elevation of cytoplasmic Ca²⁺ concentration within a min and subsequent slower and long lasting stromal Ca²⁺ elevation. We recently found that chloroplast protein CAS is involved in the generation of cytosolic Ca²⁺ signals and subsequent stomatal closure (Nomura et al., 2008 Plant J). Flg22-induced stromal Ca²⁺ elevation was significantly suppressed in the CAS KO mutants. These results suggest that CAS plays a crucial role in the generation of stress- and pathogen-induced stromal Ca²⁺ signals.