| Summary: | Calcium (Ca2+) is a ubiquitous ion with signalling function that forms part of a plethora of signal transduction pathways which allow plants to respond to environmental stimuli in a stress-specific manner. Although we know a certain amount about the shape of Ca2+ signals, and the downstream processes regulated by Ca2+ signalling, the role of specific Ca2+ transporters in modulating these signals is yet to be fully understood. Plant Ca2+/H+ exchangers (CAX) are of strong interest to study in terms of Ca2+ signalling due to their high capacity sequestration of Ca2+, as well as their localisation at the vacuolar membrane, with the vacuole being a major Ca2+ store in plant cells. Due to the high gene redundancy observed in plants, however, it has been difficult to gauge their precise role. In order to better understand the role that CAX transporters play, the unicellular green alga Chlamydomonas reinhardtii was used as a model to explore Ca2+ signalling mechanisms in higher plants, in particular the role of CAX in maintaining ion homeostasis and modulating Ca2+ signals. The data presented here shows, for the first time, that C. reinhardtii cells respond to environmental stimuli such as NaCl and CdCl2 with rapid, transient Ca2+ elevations. The NaCl-induced Ca2+ elevations were also found to increase in magnitude with increased stimulus. Analysis of CAX1 and CAX2 knockdowns of C. reinhardtii suggest that CAX function is necessary to maintain a balance of nutrients including Ca, Cu, Fe, K, S, Mg, Mn and Zn. CAX1 and CAX2 knockdowns showed opposing phenotypes in response to ion stress, with CAX1 knockdowns showing reduced growth on CaCl2 and NaCl but CAX2 knockdowns showing increased growth. Initial Ca2+ signalling experiments suggest also that CAX1 of C. reinhardtii is important in maintaining an intracellular store of Ca2+ prior to NaCl-induced Ca2+ release.
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