The ins and outs of cellular Ca2+ transport

The cytoplasmic Ca2+ signals that participate in nearly all aspects of plant growth and development encode information as binary switches or information-rich signatures. They are the result of influx (thermodynamically passive) and efflux (thermodynamically active) activities mediated by membrane transport proteins. On the influx side, confirming the molecular identities of Ca2+-permeable channels is still a major research topic. Cyclic nucleotide-gated channels and glutamate receptor-like channels are candidates well supported by evidence. On the efflux side, CAX antiporters and P-type ATPase pumps are the principal molecular entities. Both of these active transporters load Ca2+ into specific compartments and have the potential to reduce the magnitude and duration of a Ca2+ transient. Recent studies indicate calmodulin-activated Ca2+ pumps in endomembrane systems can dampen the magnitude and duration of a Ca2+ transient that could otherwise grow into a Ca2+ cell death signature. An important challenge following molecular characterization of the influx and efflux pathways is to understand how they are coordinately regulated to produce a Ca2+ switch or encode specific information into a Ca2+ signature.

► Heterologous expression and in planta mutant data best define a transport function. ► Each Ca2+ influx candidate requires more experimental testing. ► Regulation of influx and efflux pathways control Ca2+ transients. ► Endomembrane Ca2+-pumps can function to suppress a Ca2+ cell death signature.