Chloroplast-mediated regulation of CO ₂ -concentrating mechanism by Ca ²⁺ -binding protein CAS in the green alga Chlamydomonas reinhardtii

Significance Ca 2+ and CO 2 are fundamental biological signaling molecules in microbes, animals, and plants. Although Ca 2+ was proposed to act as a second messenger in CO 2 signaling in guard cells of terrestrial plants, the role of Ca 2+ in CO 2 signal transduction pathways in aquatic photosynthetic organisms remains largely unknown. We show here that a chloroplast Ca 2+ -binding protein, CAS, changes its localization in response to environmental CO 2 conditions and regulates the expression of nuclear-encoded limiting-CO 2 –induced genes, including two key bicarbonate transporters. These findings led us to propose a model for the participation of Ca 2+ signals in chloroplast-regulated CO 2 signal transduction of aquatic photosynthetic organisms and help us to further understand the role of Ca 2+ in CO 2 signal transduction in eukaryotes.