Stress-induced transient elevation of stromal Ca⁺² levels in chloroplasts

Abstract

Calcium plays an important role as a second messenger in biotic and abiotic stress responses in plants. We recently found that chloroplasts are involved in the generation of cytosolic Ca⁺² signals and subsequent stomatal closure (Nomura et al., 2007 Plant J). However, little is known about stromal Ca⁺² responses. Here we provide direct evidence that stromal Ca⁺² levels are up-regulated by H₂O₂ or elicitor treatments. Stromal free Ca⁺² concentration is usually maintained at sub-μM. H₂O₂ treatment caused a rapid elevation of cytosolic Ca⁺² concentration within 1-2 min. Subsequently, slower increases in stromal Ca⁺² levels were observed within 20-30 min. Similarly, elicitor treatments caused a slow increase in stromal Ca⁺² concentration, preceded by a rapid cytosolic Ca⁺² elevation. These results suggest that Ca⁺² communication between the cytosol and chloroplasts plays important roles in stresses and pathogen responses.