CAX‐ing a wide net: Cation/H+ transporters in metal remediation and abiotic stress signalling

Abstract

Cation/proton exchangers (CAX s) are a class of secondary energised ion transporter that are being implicated in an increasing range of cellular and physiological functions. CAX s are primarily Ca 2+ efflux transporters that mediate the sequestration of Ca 2+ from the cytosol, usually into the vacuole. Some CAX isoforms have broad substrate specificity, providing the ability to transport trace metal ions such as Mn 2+ and Cd 2+, as well as Ca 2+. In recent years, genomic analyses have begun to uncover the expansion of CAX s within the green lineage and their presence within non‐plant species. Although there appears to be significant conservation in tertiary structure of CAX proteins, there is diversity in function of CAX s between species and individual isoforms. For example, in halophytic plants, CAX s have been recruited to play a role in salt tolerance, while in metal hyperaccumulator plants CAX s are implicated in cadmium transport and tolerance. CAX proteins are involved in various abiotic stress response pathways, in some cases as a modulator of cytosolic Ca 2+ signalling, but in some situations there is evidence of CAX s acting as a pH regulator. The metal transport and abiotic stress tolerance functions of CAX s make them attractive targets for biotechnology, whether to provide mineral nutrient biofortification or toxic metal bioremediation. The study of non‐plant CAX s may also provide insight into both conserved and novel transport mechanisms and functions.