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

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