Possible involvement of CAS in the regulation of dark-induced stromal Ca²⁺ elevation in chloroplast

Abstract

Although chloroplasts contain high concentrations (4-23 mM) of total Ca²⁺, stroma free Ca²⁺ concentration is maintained at sub-micro M. Sai and Johnson (2002 Plant Cell) previously demonstrated that the stromal Ca²⁺ levels were transiently elevated in response to the light-to-dark transition. Interestingly, dark-induced stromal Ca²⁺ elevation precedes the generation of cytosolic Ca²⁺ transients, suggesting the presence of Ca²⁺ communication between chloroplasts and cytoplasm. However, the underlying molecular mechanisms are largely unknown. We recently demonstrated that thylakoid membrane-localized CAS is involved in external Ca²⁺-induced cytosolic Ca²⁺ elevation and stomatal closure (Nomura et al., 2007 Plant J). Here we provide evidence that CAS is essential for dark-induced stromal Ca²⁺ elevation in chloroplasts. These results suggest that thylakoid-localized CAS plays a crucial role in the regulation of cytoplasmic Ca²⁺ levels by chloroplasts.