Identification and Characterization of a Transient Receptor Potential Ion Channel (TRP2) Involved in Acclimation to Low CO2 Conditions in Chlamydomonas reinhardtii

The CO2 concentrating mechanism (CCM) is a key feature of green algal cells, induced in response to limiting CO2 conditions. Calcium-dependent signaling has been shown to play a role in this acclimation process to low inorganic carbon in Chlamydomonas reinhardtii, but the molecular players have not been characterized. One type of Ca2+ channel, the transient receptor potential (TRP) family of ion channels is specific to the algal members of the green lineage and not found in land plants. TRP channels generally mediate the flux of Ca2+ ions in response to environmental perturbations, and one recent study has revealed the role of Ca2+ signaling in acclimation to limiting CO2 in the green alga, C. reinhardtii. In this study, the gene Trp2, encoding a Ca2+ ion channel was identified through bioinformatics analyses as having a role in acclimation of algal cells to limiting CO2. Transcript abundance levels for this gene are significantly upregulated when cells are cultured in limiting CO2 and mutant cells missing the TRP2 protein show an impaired growth phenotype. We show that a calcium binding protein CAS, is downregulated in the trp2 mutant along with other CCM genes under the control of CAS. The results suggest that the TRP2 ion channel is involved in the acclimation of C. reinhardtii cells to limiting CO2.